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AUG 6 l-T

BUILDING SCIENCE SERIES '

Causes of Variation

In Cliemical Analyses

And Physical Tests

Of Portland Cement

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UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Causes of Variation in Chemical Analyses

and Physical Tests of Portland Cement

B. Leonard Bean and John R. Dise

Building Research Division

Institute for Applied Technology

National Bureau of Standards Washington, D.C. 20234

Building Science Series 17

Issued March 1969

(Supersedes NBS Monograph 28)

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 40 cents
RELATED PUBLICATIONS

NBS Monograph 28 — Causes of Variation in Chemical Analyses and

Physical Tests of Portland Cement — 25 cents*

NBS Monograph 43 — Chemistry of Cement Proceedings of the Fourth

International Symposium — Washington 1960. Presented in two

volumes. Volume I — $6.50. Volume II — $6.25. The two volumes

at $12.75 a set. (Originally issued September 1962, and reprinted

February 1964.)

NBS Building Science Series 2 and 5 — Interrelations between Cement

and Concrete Properties. Part 1 — 35 cents. Section 1, Materials

and Techniques. Section 2, Water Requirements of Portland Cement.

Section 3, Occurrence of Minor and Trace Elements in Portland

Cement. Part 2 — 35 cents. Section 4, Variables associated with

expansion in the potential sulfate expansion test. Section 5, Heat

of hydration of portland cement. Section 6, Variables associated with

small autoclave expansion values of portland cements. Section 7, Compressive Strength of Test Mortars. Section 8, Compressive

Strength of Steam-Cured Portland Cement Mortars. Part 3 — 55 cents.

*Order publications from Superintendent of Documents, Government Printing Office, Washington, D.C. 20402. (For foreign mailing, add one-fourth of the price of the

publication.)

Library of Congress Catalog Card Number: 68-62784

II
Preface

Reference to this publication may help a cement testing laboratory engaged

in comparative testing to identify and eliminate the causes of erroneous test results. The compilation should also be helpful to laboratories that consider

it advisable to conduct general inspections of equipment and procedures at regular intervals.

Ill
Contents

Page Preface Ill

1. Introduction 1 Part I. Possible causes of error in cement

analysis 3

2. General precautions 3

2.1. The weighing operation 3

2.2. Ignition procedures 3

2.3. Desiccation 4

2.4. Additional precautions 5 Individual determinations 5

3. Silicon dioxide 5

3.1. Causes of variations 5

4. Ammonium hydroxide group (R2O3) 6

4.1. Causes of variations 6

5. Total iron as ferric oxide 7

5.1. Causes of variations 7

6. Aluminum oxide 7

7. Calcium oxide 7

7.1. Causes of variations 7

8. Magnesium oxide 8

8.1. Causes of variations 8

9. Sulfur trioxide 9

9.1. Causes of variations 9

9.1.1. Turbidimeter method 9

9.1.2. Gravimetric method 9

10. Sulfide sulfur 10

11. Loss on ignition 10

11.1. Causes of variations 10

12. Sodium oxide and potassium oxide 10

12.1. Causes of variations 10

13. Phosphorus pentoxide 11

14. Manganic oxide 11

14.1. Causes of variations 11

15. Insoluble residue 11 Concluding remarks 11 Part II. Possible causes of variation in physical tests of Portland cements 13

16. General considerations in the physical

testing of cements 13

16.1. Laboratory 13

16.2. Treatment of samples 13

16.3. Mixing water 13

16.4. Measurement of mixing water 13

16.5. Standard sands 14

16.6. Weighings 14

16.7. Timing device 14

16.8. Oihng of molds 14

16.9. Specimen storage facilities 14

16.10. Storage period for molded

specimens 14

16.11. Removal of specimens from

molds 15

16.12. Marking specimens 15

16.13. Trowels 15

16.14. Rubber gloves 15

16.15. Care and cleaning of apparatus... 15

17. The determination of fineness 15

17.1. Common causes of variations 15

17.2. The turbidimeter fineness test... 16

17.2.1. Causes of variations 16

17.3. The air permeability fineness

test 18

17.3.1. Causes of variations 18

18. Consistency 19

18.1. Mechanical mixing of pastes and

mortars 19

18.1.1. Causes of variations 20

18.2. The normal consistency deter- mination 20

18.2.1. Causes of variations 21

18.3. The flow table test 21

18.3.1. Causes of variations 21

19. The autoclave soundness test 22

19.1. Causes of variations 22

20. Time of setting 23

20.1. Common causes of variations 23

20.2. The Vicat time of setting test 24

20.2.1. Causes of variations 24

20.3. The Gillmore time of setting

test 24

20.3.1. Causes of variations 24

21. The heat of hydration test 25

22. The false set test 25

22.1. Causes of variations 25

23. Air content of mortar 26

23.1. Causes of variations 26

24. Strength of mortars 26

24.1. Common causes of variations 26

24.2. The compressive strength test... 27

24.2.1. Causes of variations 27

24.3. The tensile strength test 28

24.3.1. Causes of variations 28

25. Closure 29

26. References 29

IV
Causes of Variation in Chemical Analyses and Physical Tests of

Portland Cement

B. Leonard Bean and John R. Dise

Variations in testing that could lead to the rejection of a material fully conforming to specification requirements, or the acceptance of a material with undesirable chemical or physical properties, are apparent in the results reported by laboratories participating in comparative tests of Portland cements. Many of the causes for variation in chemical analyses and physical test results are listed in this dis- cussion, and remedies for some of the more frequently encountered deficiences in apparatus and methods are suggested. Particular consideration is given to problems which do not seem to have been

covered in sufficient detail in previous discussions of cement testing procedures. Literature references

are given for additional information. This publication supersedes NBS Monograph 28.

Key Words: Portland cement; chemical analyses; physical tests.

1. Introduction

Many of the organizations engaged in the testing

of Portland cements have found it advantageous to use frequent interlaboratory tests on carefully

prepared samples of the material to evaluate the

performance of the laboratories involved. A report on a comprehensive effort of this kind was submitted to the American Society for Testing Materials at its 1959 Annual Meeting in Atlantic City,

New Jersey [1].* All too frequently, variations in testing that could

lead to the rejection of a portland cement fuUy

conforming to specification requirements, or the

acceptance of one with undesirable properties, are apparent in the results reported by the participants

in these programs. This is due in large measure to the fact that many steps are involved in making

the standard chemical analyses and physical tests.

* Figures in brackets indicate the literature references at the end of this paper.

and that the results are sometimes greatly in- fluenced by seemingly minor variations in equipment and procedures. Many of these influences

have been taken into account in the preparation

of the standard methods of test. However, it is not uncommon to find that inadequate attention has

been given to certain important specification re- quirements because their significance is not fuUy

understood, or that essential considerations are overlooked simply because they are not readily

apparent. For these reasons it is believed that a discussion of the causes of variation, in which

particular attention is given to matters not already

covered in detail in current publications, might

prove helpful to laboratories participating in comparative test programs such as the Cement Ref- erence Sample Program of NBS.

1

Part I. Possible Causes of Error in Cement Analysis

B. Leonard Bean*

In the analysis of portland cement to determine compliance with specification requirements,

the following constituents are usually reported (expressed generally as oxides, but it is not inferred

that they are necessarily present in that form in cement): Si02, Fe203, AI2O3, CaO, MgO, SO.i. Deter- minations of the loss on ignition and insoluble residue are also required. If specified by the purchaser

or otherwise required by the purchase contract, determinations of sulfide sulfur, sodium oxide, potas- sium oxide, titanium dioxide, phosphorus pentoxide, manganic oxide, chloroform soluble organic substances, and free calcium oxide may also be required. The suggestion was made that this author compile information on analytical techniques, both from personal experience and from the literature, which might aid analysts in securing uniform and reliable results. The suggestions which follow, it is hoped, will benefit one who is seeking some means to bring his own results into line with the average

values reported by all chemists taking part in a cooperative study.

Key Words: Portland cement; chemical anab

\ 2. General

j 2.1. The Weighing Operation

The weights used in interlaboratory analyses

should conform to Class S-1 requirements as commonly shown in equipment catalogs. The

weights of 1 g and larger units should preferably

I be made of stainless steel or other corrosion and

i abrasion-resistant alloy not requiring protective

I coating. Weights which are used for regular work

I should frequently be checked against a reference

! set of standard weights which conform to Class S

(or Class M) requirements. This reference set should

j contain weights from 50 g to 10 mg. Whenever a

i weight from a working set is known to have been

dropped, it should be cleaned and its weight

' checked carefully against weights from a reference

[ set before further use. When weights from a refer-

" ence set are involved in an accident such as being

dropped to the floor, the calibration of the weights

I involved in the accident must be considered to be

in doubt. Such weights can be recalibrated and/or

adjusted by the manufacturer, the local State's

i Weights and Measures officials, or an appropriate

1 testing laboratory.

j Balances are now sold with many added features

I to speed the weighing process: damping devices,

I notched beams to accommodate poises, chains,

! projected direct-reading scales, and sets of built-in

! weights. These can save weighing time. Tests in accordance with ASTM E 319-67 (Methods for Testing Balances) must be made occasionally to

il assure freedom from significant errors,

j Electrostatic effects must be guarded against

I' in all weighing operations. The moisture that may

condense on a dry crucible on a balance pan,

I due to humid atmospheres, can be compensated

J to some extent by placing a tare crucible on the

I other balance pan; or by appropriate choice of

] weighing methods with a balance of the single-arm

type with only one pan.

*Research Chemist, Materials Division, Bureau of Public Roads, Federal Highway

Administration, U.S. Department of Transportation, Washington, D.C. 20591.

Precautions

It may be necessary to return a balance or set of weights to the manufacturer for adjustment or

repair. However, local service by manufacturers'

recommended service agencies will correct most

troubles. In some cases annual service contracts may be desirable.

2.2. Ignition Procedures

Where platinum crucibles are used, they should,

in general, be made of pure platinum, that is, not

intentionally alloyed with other metals. At the

temperatures of 1,100 and 1,200 °C used for certain

ignitions in cement analysis, other alloying metals

are liable to volatize slowly and vitiate the results obtained. Such alloyed crucibles or lids are sold and used for some purposes because they are stiffer and do not deform as readily as pure platinum.

Some manufacturers have equipped pure platinum

crucibles with alloyed lids to obviate sticking of crucible and lid. Such practice may be permissible

at lower temperatures than those used in cement

analysis. Any empty crucible and lid which consistently loses more than 0.2 mg per hr at 1,200 °C

should be suspected [2]. Use of an empty alloyed

crucible and correction for the loss it sustains

applied to the crucibles containing precipitates is liable to lead to erroneous results. The alloying

metal covered with precipitate will probably not

volatize at the same rate as in the empty crucible.

Pure platinum crucibles may be purchased which

have reinforced rims and bottoms. This minimizes

deformation without the use of other alloyed metals.

The beginning of the ignition of a precipitate

contained in a wet filter paper should be made at a rate slow enough so that no spattering, popping,

or flaming occurs. After the moisture is expelled

the paper should be "smoked off"." Then, when all danger of flaming is past, the crucible may be

placed in a muffle furnace which is at the proper

temperature for final ignition.

Some check on the temperatures of the muffle

furnaces is advisable from time to time. Con-
lamination of thermocouples may occur resulting

in erroneous temperature readings. A standard

thermocouple and portable potentiometer may be

used to check the accuracy of the thermocouples

and temperature-indicating devices ordinarily used.

The National Bureau of Standard issues a copper

mehing-point standard. No. 45c, which mehs at 1,083.3 °C. Use of pyrometric cones may be helpful

in checking temperatures [3]. E. T. Carlson [4] recommends a modification of

Roberts' technique [5] utilizing the mehing point

of potassium sulfate (1,069.1 °C) as a check on

muffle temperatures. Carlson breaks up the K2SO4

with a mortar and pestle and places about a gram

in each of several covered platinum crucibles.

These are placed at several points in the furnace

with the indicated temperature somewhat below

1,069 °C. They are removed and inspected at about

15 min intervals. The indicated temperature of the

muffle is raised by 10 °C intervals. In this manner

the temperature of the various zones in the furnace can be estimated. Almost invariably the K2SO4 in the back of the furnace will melt first and that at the front last. One must not be confused by an

inversion that takes place in K2SO4 at about

583 °C, but without mehing. The K2SO4 is first crushed to avoid the spattering that takes place on inversion.

When igniting precipitates in a number of crucibles, six, for example, it is well to reverse the

order of the crucibles when about half the ignition

period has elapsed. Thus crucible No. 1 which may

at first have been at the front of the furnace in the

coolest position will spend the remainder of the

time in the back of the furnace which is usually

the hottest zone. (In some furnaces, the middle

portion may be hottest.) It should be borne in mind

also that the temperature shown by the indicating

device is the temperature of the end of the thermocouple. If the thermocouple is covered by a protective tube, as is usually the case, the temperature

of any one crucible on or near the floor of the

furnace will probably differ somewhat from the

thermocouple temperature. The position of the

thermocouple is important. It should be near the

middle of the heated zone. If it is accidentally

bumped while placing crucibles in the furnace,

it may be so misplaced that it no longer indicates

the average furnace temperature. The thermocouple

position should be checked every time the muffle

furnace is used.

2.3. Desiccation

No absolute rules can be given for desiccation.

The time and technique which will give satisfactory

desiccation while the crucibles are cooling to room

temperature so that they can be weighed, may be

a function of many factors: (1) Size of the desiccator,

(2) the number of crucibles, their weight, and the

weight of the contents, (3) the character or composition of the contents, (4) the material of which

the desiccator is constructed, (5) the temperature

and relative humidity in the room (6) the desiccant

used, and (7) the degree to which the lid fits the

desiccator. An excellent discussion of the subject

is given by Hillebrand, Lundell, Bright, and Hoff- man [6]. In general, sulfuric acid or anhydrous

magnesium perchlorate are the preferred desiccants

for cement analysis. Either is satisfactory in glass

desiccators; obviously, sulfuric acid should not be

used in an aluminum desiccator. The fid should be

ground to fit the desiccator on which it is used.

Generally no difficulty is encountered in purchasing

glass desiccators whose fids fit properly. A well- constructed aluminum desiccator has the advantage

over glass of speeding the coohng of ignited precipitates, but in the writer's experience, the lids rarely fit properly and require regrinding before use.

It has been claimed that the cost of such grinding

by the manufacturer would be prohibitive. Ordinary

sihcone stop cock grease (not High-Vac) is a good

sealing and lubricating material for aluminum

desiccators if the lids fit. Petrolatum is satisfactory

for glass desiccators.

When the lid of a desiccator fits and is lubricated

properly, air will be heard to rush in when the

cooling period (usually 30 to 60 min) is over and the

lid is opened slightly by cautiously sliding or by

opening a valve, if the desiccator is so equipped.

If this sound is not heard one is always uncertain

as to how much moisture-laden room air may have

entered the desiccator during the cooling period.

Care must be exercised to avoid blowing precipitates out of the crucibles by this inrushing air. If a desiccator has been found not to have been

kept tightly closed overnight, it is good practice

to recharge with fresh desiccant.

Hot crucibles should never be placed in a desic- cator at quitting time and then weighed in the

morning. Such weights will seldom agree with

those obtained after the usual cooling period.

Weighings made just prior to quitting time should,

be viewed with suspicion if there has been any

tendency to shorten the usual desiccating period

in favor of leaving at a given hour. Crucibles and

contents which appear to gain weight on the

balance during weighing may have been left in the desiccator for too short a period. Slightly warm

crucibles cause convection currents in the balance^,

and affect it in other ways which generally resultlj in a weight which is too low and somewhat unstable!,

during the weighing period. This latter phenomenoni

may be superimposed on the uncertainty involved

in weighing hydroscopic materials which have^

been properly cooled for a sufficient length of time

in a good desiccator.

Ignited sihca, ammonium hydroxide group (R203),i

and calcium oxide should be treated as hydroscopic

materials, or potentially so, and kept tightly covered

in the crucible during desiccation and weighing.

Calcium oxide should be desiccated over mag-
nesium perchlorate or preferably, phosphorus

pentoxide and weighed as rapidly as possible.

It is not easy to know when a desiccator should

be recharged with fresh desiccant. G. Boehm [6,

p. 33, note 22] describes a technique ^ for checking

the effectiveness of sulfuric acid as a drying agent,

by using acid containing 18 g of BaSO^ dissolved

in each liter. Some analysts have found it useful

to mix a few pieces of indicating Drierite with

magnesium perchlorate in a desiccator; when the

blue color changes to pink the desiccant should be

discarded. In fact, it probably should have been

discarded prior to that. Most laboratories do not have the vacuum equipment required to reactivate

magnesium perchlorate and prefer to discard the

used material and recharge the desiccator with

fresh material.

2.4. Additional Precautions

Filter paper should be visually inspected before

folding and, of course, discarded if evidence of a weak spot or a hole is detected.

A piece of glass broken from a wash bottle tip

is an occasional cause of high results. Failure to decompose the sample of cement

completely in the initial hydrochloric acid treatment can lead to erroneous results for almost all subsequent determinations.

Silicone stopcock grease is not recommended

for use in burettes. It may tend partially to water- proof the inner surface of the burette. Silicone

grease can be removed from a burette by use of

fuming sulfuric acid. Extreme caution must be

exercised in using fuming sulfuric to avoid injury.

Individual Determinations

Many of the statements which follow are intended

to apply to comparative analyses in which, pre- sumably, the utmost in accuracy is sought. Less

rigorous techniques may be satisfactory for routine

analyses, where conformance to specification re- quirements is not in doubt.

3. Silicon Dioxide

This determination is based on either of two

methods:

Referee. A double evaporation is made of the

hydrochloric acid solution of the cement to dryness

to convert Si02 to the insoluble form. The solution

is filtered and the insoluble siliceous residue is volatiHzed by hydrofluoric acid and the loss of

weight is reported as pure SiO-z. Optional. In this method silicon dioxide is deter- mined gravimetrically as in the referee method

except that ammonium chloride is added and the

solution is not evaporated to dryness. The method

was developed for the analysis of portland cement

and is not recommended for other types of hydrauhc

cements.^

3.1. Causes of Variations

High Results. For the determination of silica,

ignition temperature of 1,200 °C for at least 1 hr

is recommended, with subsequent heating periods

of 30 min at the same temperature until constant weight within 0.2 mg has been established [8, also 6, p. 679].

Recovery of residual silica from the ignited

ammonium hydroxide precipitate together with a

^ ". . . no precipitate will form in sulfuric acid containing 18 g of dissolved barium

sulfate per liter until enough water has been taken up to reduce the concentration

of acid to about 93 percent. Between 93 and 84 percent, needlelike crystals of BaS04 • 2H2S04 H2O separate freely. On further dilution by absorbed water, the

needlelike crystals change to fine crystals of BaSO<, at which point the acid is no

longer effective as a drying ageiit."

''The brief summaries of methods in this discussion contain much of the same material that is in the summaries given in ASTM C114-67 [7].

correction for the silica recovered from a reagent

blank can lead to high results. This is discussed on page 682 of [6], where the importance is stressed

of adding as much iron and alunlina to the reagent

blank as is contained in the sample being analyzed.

This is time-consuming and makes necessary the use of two blanks, if silica is to be recovered: one containing nothing but reagents in order to correct the R2O3 for filter paper ash; the other

with added iron and alumina to carry down silica from the reagents and glassware. At the present

time, neither ASTM [7, pp. 97-99] nor Federal

[8, Method 1101, pars. 3.1.1 and 3.1.2] methods

require recovery of silica from R2O3 because

many chemists believe the amount involved is likely to be almost insignificant, the probable

errors of recovery high, and the time required

excessive. The same reasoning may not be valid

concerning the effect on alumina values obtained

if no correction is made for residual sihca in R2O3.

Low Results. In careful work, the HF residue can

generally be kept below 1 mg in cement analysis.

If the referee (double dehydration) method is used

this residue should seldom be more than half a

milligram. Whenever the HF residue is much higher

than these values, uncertainty in results for silica exists, usually tending to give low silica values. (Subsequent determinations of other constituents

may also be affected.) Impurities may be weighed

as chlorides during the ignition of the crude silica and as sulfates or oxides after treatment with HF

and H2SO4. The error can be minimized by follow- ing the usual dilute HCl (1:99) and water washings

323-071 O - 69 - 2
of the precipitated silica with one or two washings

with dilute H2SO4 (1:4) or by moistening the combined papers containing the silica with a few drops

of dilute sulfuric acid before drying and igniting

[9, also 6, p. 863]. Thorough washing of silica to remove contaminants is the best insurance against

high HF residues. The higher the HF residue the more error can be introduced if the contaminants

are not present in the same form before and after treatment with HF and H2SO4 [6, p. 681]. Special

precautions need to be taken in washing when the

optional ammonium-chloride method of silica separation is used. The bulk of the gelatinous silicic acid should be transferred to the funnel without

dilution with water and the small volume of con- centrated acid solution allowed to drain through.

Thorough washing with hot HC1(1 : 99) and then hot water follow. It is useful to tear off small pieces of

filter paper and use them together with a rubber

policeman to scrub the inside of the beaker thor- oughly in order to remove all adhering silica. This

use of filter paper should be delayed until no more

ferric chloride color can be seen. If the scraps of

filter paper are added to the funnel too soon, their

presence impedes the washing procedure and

causes high HF residue.

Where use of a steam bath is recommended for dehydration of silica, by either the double-dehydration or the ammonium chloride method, incomplete

decomposition of the sample may occur if the

water in the steam bath is not hot enough. The

method of heating the steam bath should be so regulated that the water in the steam bath is maintained at a rolling boil. If the temperature

of the water is below boiling, some silica may

escape dehydration, causing low values to be obtained for silica, and high values for alumina,

calcium oxide, and possibly magnesium oxide.

Low results for silica can be caused by baking

the second dehydration at temperatures over 110 °C

or for longer than 1 hr. Such treatment can cause

recombination of silica and magnesia, resulting

in formation of an acid soluble siUcate [6, p. 678].

4. Ammonium Hydroxide Group (R2O3)

In this method aluminum, iron, titanium, and

phosphorus are precipitated from the filtrate, after SiO^ removal, by means of ammonium hydroxide. With care, little if any manganese will be precipitated. The precipitate is ignited and

weighed as the oxides.

4.1. Causes of Variations

High Results. In the determination of the am- monium hydroxide group ignition of R2O3 at 1,100

°C for 1 hr is recommended, followed by subsequent

30-min ignition periods until constancy of weight

within 0.2 mg is indicated.

If 10 to 15 ml of HCl is not present before neutralization with NH4OH, coprecipitation of some magnesium hydroxide with the R2O3 may occur. The presence of that quantity of HCl insures the

formation of a sufficient amount of NH4CI so that the ionization of NH4OH is depressed to a point

where insufficient hydroxyl ion is present to exceed

the solubihty product of Mg(0H)2. Ammonium

chloride also tends to keep magnesium in solution

by formation of an ammonia complex, assists in buff"ering the solution, and aids in coagulation of

the precipitate.

Use of ammonium hydroxide which contains

carbon dioxide can cause coprecipitation of calcium

and magnesium carbonates. Reagent ammonium

hydroxide bottles should be kept stoppered to prevent contamination. Digestion of the ammoniacal

solution and precipitate should not be prolonged

beyond 10 min. Otherwise CO2 may be absorbed

from the air.

The results for R2O3 may be affected by the

completeness of recovery of silica in the previous

step. Any silica which has escaped the double

dehydration or ammonium-chloride separation will cause high results for R2O3 unless a careful recovery

of residual silica is made. This would then cause

high results to be reported for aluminum oxide. In careful analyses, all residual silica recovered

from the R2O3 (whether from the cement, the

reagents, or the glassware) should be subtracted

from the R2O3 weight. It is certainly not AI2O3.

For this purpose, the residual silica should not be

corrected for the residual silica found in the blank

to which iron and alumina had been added to sweep it down. In addition another correction

should be made to the R2O3 weight, by subtracting

the reagent blank, to correct for the impurities

in the reagents (other than silica) and for the

filter paper ash. This blank will generally amount

to about 0.5 mg. Thus the total correction to be

subtracted may be on the order of 1. mg, the

equivalent of 0.2 percent AI2O3. This may cause

a significant diff"erence in the calculated tricalcium

aluminate content. Low Results. Failure to neutralize with ammonia

to the exact end point will cause low results. Stopping short of the methyl-red end point will leave some hydroxides unprecipitated. Going

past the end point of methyl red by more than the one drop of 1:1 NH4OH prescribed, will redissolve

alumina and cause low results. The presence of

any iron in the ferrous condition will cause results

to be low because ferrous hydroxide is not quantitatively precipitated at this pH. Whenever ferrous

iron is suspected, such as by a blackened precipi6
tate, redissolve with hydrochloric acid, add four

or five drops of concentrated nitric acid, and

boil for a few minutes to oxidize aU the iron. Then

precipitate with NH4OH as before. It may be necessary to add more methyl-red indicator from time to time because of the effect of the nitric acid on the

indicator. In the event the first precipitation of R2O3 has

been completed at the end of the working day, the

filter paper together with its precipitate should not be treated with acid, pulped up, and allowed to

stand overnight in this condition. It may even be

better not to do this over a lunch hour. In a strongly

acidic environment organic matter from the filter paper pulp tends to complex some of the aluminum

and prevent completeness of precipitation on subsequent treatment with NII4OH [6, pp. 66 and

98]. In this respect filter paper may act a httle like tartaric or citric acid. Use of filter paper for this separat'on which is less retentive than Whatman

#40 is not recommended because some Fe(0H)3

might pass through.

5. Total Iron as Ferric Oxide

In this method, the FeaOs content of the cement

is determined on a separate portion of the cement

by reducing the iron to the ferrous state with stannous chloride (SnCL) and titrating with a standard solution of potassium dichromate (K2Cr207).

This determination is not affected by any titanium

or vanadium that may be present in the cement.

5.1. Causes of Variation

In the determination of iron, the reduction process

with stannous chloride is critical. If less than the

required amount is added, results will be low. An

excess of one drop after the yellow color of ferric ions disappears, will give satisfactory results pro- vided the specified concentration of stannous

chloride is used. This solution has a tendency to become more concentrated on standing in the

presence of metallic tin. After cooling to room

temperature and adding the mercuric chloride

solution, a small amount of white silky precipitate

should be in evidence. If none appears, insufficient stannous chloride was used and the determination

should be repeated. The presence of much mer- curous chloride is also objectionable because it may become oxidized. If the precipitate is gray,

it indicates the presence of finely divided elemental

mercury because too much stannous chloride had

been used. In the latter event, also repeat the

determination because the results will be high

[9, pp. 573 and 580].

After reduction with stannous chloride, the

solution should be cooled in a cool water bath and

the remainder of the determination completed

immediately thereafter. The reduced solution

should not be allowed to stand overnight before

titrating; air oxidation could cause low results.

6. Aluminum Oxide

Alumina is not actually determined in cement

but is calculated by diff^erence. The ammonium

hydroxide precipitate in cement is assumed to consist of AI2O3, Fe203, Ti02, P2O5, and residual

Si02, together with possible traces of Zr02, V2O3,

and Cr203. If, as is usually done, only the percentage

of Fe203 is subtracted in the calculation, the results

for AI2O3 will always be expected to be high by the

amount of the other undetermined oxides. Any

error in the R2O3 or Fe203 determinations will cause

corresponding errors in the AI2O3 percentage

reported. Note the suggestions given in the two

preceding sections.

7. Calcium Oxide

This determination is based on either of two

methods:

Referee, Gravimetric. Manganese is removed from

the filtrate after the determination of the ammonium

hydroxide group. Calcium is then doubly precipi- tated as the oxalate. The precipitate is converted

to CaO by ignition and weighed.

Optional. The calcium oxide (CaO) is determined

volumetrically. After removal of silicon dioxide

and the ammonium hydroxide group, the calcium

is precipitated only once as the oxalate. After

filtering, the oxalate is redissolved and titrated with potassium permanganate (KMn04).

7.1. Causes of Variations

High Results. In the determination of calcium

oxide high results can be caused by failure to remove manganese, or to separate iron or alumina

completely. When the optional method is used, the

results obtained may be either high or low depending

on the degree of washing with hot water. The sug- gestion in the specification method to use about

75 ml of hot water in washing will generally be

satisfactory. However, variations in size of wash

bottle tips, lung power of individual chemists, and

the degree to which the precipitated calcium oxalate

7
is churned up during washing, will cause variations

in the result. It is suggested that each analyst

occasionally analyze a sample of an NBS standard

sample of cement or a comparative sample whose

composition has been established and vary his operations throughout the analysis, particularly

his washing technique, until he is able to get CaO

values close to the certificate value for that cement.

If the final titration in the presence of the added

filter paper is prolonged unduly some of the paper

will be oxidized. The one-hour standing period for precipitation of the oxalate is critical and should

not be prolonged lest coprecipitation of magnesium

result.

It is difficult to ignite CaO to constant weight

for the referee determination. Ignition at 1200 °C

for at least 1 hr is recommended, followed by heat- ing at the same temperature for 30-min periods

until constancy of weight within 0.2 mg is indicated.

The crucibles should have well fitting covers. Magnesium perchlorate is preferable to sulfuric acid

for desiccating CaO. Phosphorus pentoxide is definitely superior to either. The second weighing

of CaO should be made in reverse order from the

first weighing to detect any errors due to standing

in a desiccator already opened. It is especially

necessary to rotate the crucibles in the furnace

for the CaO ignition.

Low Results. The presence of CO2 during the

precipitation of the ammonium hydroxide group can cause some of the calcium to precipitate there

instead of later as the oxalate. This was discussed more fully under the R2O3 precipitation. Failure

to wash the R2O3 precipitate adequately may leave

calcium absorbed on the gelatinous precipitate.

8. Magnesium Oxide

This determination is based on one of three

methods.

Referee. Magnesium is doubly precipitated as magnesium ammonium phosphate from the filtrate after removal of calcium. The precipitate is ignited

and weighed as magnesium pyrophosphate

(Mg2P207). The MgO equivalent is then calculated.

Optional Method A. Magnesium is precipitated

only once as magnesium ammonium phosphate

from the filtrate after removal of CaO. The precipitate is ignited to Mg2P207 and weighed.

Optional Method B. This method may be used on the filtrate after removal of CaO or when the

determinations of silicon dioxide (Si02), aluminum

oxide (AI2O3), ferric oxide (Fe203), and calcium

oxide (CaO) are omitted. It is a volumetric procedure

based on the precipitation of magnesium oxyquinolate (either in the absence or presence of calcium

oxalate), filtration, and subsequent bromination

with standard potassium bromate-potassium bro- mide solution and back titration of the excess with

standard sodium thiosulfate (Na2S203) solution.

8.1. Causes of Variations

There are many precautions to be heeded in order to obtain correct results for the gravimetric

determination of magnesium. The subject is covered thoroughly in authoritative discussions

by Hoffman and Lundell [10], Kolthoff and Sandell

[9, p. 352-^2], and Hillebrand, Lundell, Bright,

and Hoffman [6, p. 636-40]. It is impossible in a paper of this type to list more than a few precautions

to be observed in the determination of magnesium

by either the phosphate or oxyquinolate method.

High Results. If manganese has not been re- moved—and it is not mandatory — high values

for magnesium may be obtained by the oxyquinolate

(optional) method.

Use of filter paper that is too coarse for filtration

of R2O3 may allow iron to run through and increase

the subsequent MgO values. A green precipitate

of what is supposed to be magnesium oxyquinolate

is evidence of contamination by iron. This may

also be caused by failure to keep all the iron in the ferric state during its precipitation and washing.

The magnesium oxyquinolate precipitate should

be yellow.

Herman found [11] that this method gave high

results even after manganese removal in the case

of cements containing over 2 percent MgO.

When the phosphate (referee) method is used,

any mechanical loss of calcium by creeping over

or passing through the filter paper, will produce

high results for magnesium because of coprecipitation of calcium phosphate. 1 If the ignition of magnesium ammonium phosphate in the presence of filter paper is carried out

too rapidly, incorrect results, either high or low,

may be obtained. The carbon may become partially

fireproofed, rendering it impossible to burn it out

[6, p. 637]. Or, the pyrophosphate may be slowly

reduced and phosphorus volatilized. This latter action can ruin a platinum crucible. It is probably

safer to ignite such precipitates in porcelain. The

final ignition temperature should be 1,100 °C. At

1,200 °C, as permitted in some specification methods, the pyrophosphate slowly loses weight [9, p. 359].

Low Results. Absence of sufficient NH4CI during

the R2O3 precipitation will cause some of the

magnesium to precipitate there and subsequent

values for MgO will be low.

During the calcium precipitation, if the oxalate

is added to an ammoniacal solution, or if the solution and precipitate are boiled, some coprecipitation

of magnesium will result [9, pp. 347-8]. Older

specification methods recommend such procedure

and some laboratories may still be using this tech- nique, causing low magnesium results.

8
Berman [11] reported low results for MgO by

the optional oxyquinolate method at levels less than 1 percent MgO.

In the phosphate (referee) method, supersaturated

solutions of magnesium ammonium phosphate

sometimes prevent complete precipitation in the case of low magnesia contents. This can be remedied by longer standing before filtration. It is generally advisable not to discard the filtrate from

the phosphate separation for at least one day so

that in the event postprecipitation does occur, the

additional precipitate may be filtered off and added

to the main precipitate. The blank should be

treated in the same manner. The amounts of ammonium oxalate called for

in the specification methods for precipitation of calcium, generally do not leave enough excess to interfere with the magnesium determination according to Kolthoff and Sandell [9, p. 360]. In a few

instances, in the author's experience, removal of oxalate and ammonium salts by evaporation with

75 ml of nitric acid [6, p. 133] and [9, p. 370] has

increased the amount of magnesium ammonium

phosphate that could be precipitated from certain

cements. For very careful work, the additional

time consumed may be warranted.

9. Sulfur Trioxide

This determination is based on either of two

methods:

Referee, Gravimetric. Sulfate is precipitated from

an acid solution of the cement with barium chloride

(BaClo). The precipitate is ignited and weighed as barium sulfate (BaS04) and the SO3 equivalent is calculated. Optional, Turbidimetric. A rapid determination

of sulfur trioxide content is made by means of a Wagner turbidimeter. The SO3 content of the

sample under test is read from a calibration curve

established with a cement of known SO3 content. Correct results depend upon a strict adherence to the specified conditions, times and temperatures.

9.1. Causes of Variations

9.1.1. Turbidimeter Method

When the optional, turbidimetric, method is used for the determination of sulfur trioxide, it

is recommended that the operator study Rudy's

paper [12] upon which the specification method was

based. It contains many excellent suggestions and

precautions to be observed: necessity for close

control of temperature, changes of variables

which require preparation of a new calibration curve, and necessity for maintaining fairly uniform

BaCl2 • 2H2O crystal size. In connection with the

latter point, a further precaution might be in order.

It is usual to separate a rather large supply of the

BaCl2 • 2H2O that will pass a No. 40 sieve and

be retained on a No. 80 sieve, because a new cahbration curve must be prepared every time a new batch

is sieved. If. however, the operator should fall into the habit of shaking the bottle before taking

out each scoop of crystals in the mistaken idea

that mixing is desirable, he will actually change the

cr>stal size by such shaking and obtain incorrect

results. If four or five months have elapsed since

the supply of crystals was sieved, it is advisable to resieve the BaCL • 2H2O and prepare a new cahbration curve anyway. The crystal size may have

changed just by standing.

Molded glass sedimentation tanks should be

used for SO3 determinations because hydrochloric

acid solutions are liable to dissolve the material

used to hold the glass together in forming the other

type of tank. Many of the suggestions given under 17.2 in Part II of this Monograph will be helpful in maintaining the turbidimeter in proper operating

condition.

, 9.1.2. Gravimetric Method

From a study of the discussions on the deter- mination of sulfur as barium sulfate in well-known

textbooks such as those bv Kolthoff and Sandell

[9, pp. 322-335] and Hillebrand, Lundell, Bright,

and Hoffman [6, pp. 711—722], one almost reaches

the conclusion that it should be impossible to obtain the correct result by precipitating BaS04

in the presence of all the constituents of portland

cement. Evidently, the reasonably good results generally obtained are because compensating

errors just about balance each other. It will be

noted in the certificate of analysis of NBS Standard

Sample 177 that J. R. Spann, of our NBS laboratory,

reported a value of 1.59 percent SO3 found in that cement. His method of determination included

reduction of iron with hydroxylamine. addition

of BaCL solution rapidly and treatment of the

ignited BaS04 with HF to remove silica [9, pp. 329,

330, 332]. In comparison with this value of 1.59 percent by a refined method, Spann obtained

values of 1.56 percent and 1.63 percent SO3 for the same cement by the ASTM and Federal referee

specification method.

Failure to wash out all the occluded BaCL, or

to burn off all the carbon of the filter paper, are probably the two most common causes of high

results. A digestion time much shorter than 12 hr generally leads to low results. After filtration, the

precipitated BaS04 should be washed thoroughly

with hot water, as stated in the specification method.

The temptation to use dilute HCl solution for washing in order to remove iron stains must be avoided.

Any Fe2(S04)3 present already has a lower molecular

9
weight than its equivalent amount of BaS04 and

will probably lose its SO3 during ignition. Washing

out the Fe2(S04)3 with acid will make the results

still lower. The silica occluded possibly compensates for the lower weight caused by some of th( sulfate coming down as Fe2(S04)3.

Slow ignition with good access of air will avoic reduction of BaS04.

10. Sulfide Sulfur

In this method, sulfide sulfur is determined by

evolution as hydrogen sulfide (H2S) from an acid

solution of the cement into a solution of ammoniacal

zinc sulfate (ZnS04). The sulfide sulfur is then

titrated with a standard solution of potassium iodate

(KIO3). Sulfites, thiosulfates, and other compounds

intermediate between sulfides and sulfates are

assumed to be absent. If such compounds are pres^ ent, they may cause an error in the determination

The evolution method for sulfide sulfur [13] h

not regarded as being the most accurate available

However, it is fairly rapid, and even though all the assumptions upon which it is based may not hole

rigidly, it is widely used throughout industry.

11. Loss on Ignition

In this method, the cement is ignited in a muffle

furnace at a controlled temperature. The loss is assumed to represent the total moisture and carbon

dioxide (CO2) in the cement. This procedure is not

suitable for the determination of the loss on ignition

of Portland blast-furnace slag cement and of slag

cement.

11.1. Causes of Variations

High Results for Portland Cement. If the ignition

temperature is too high, approaching 1,200 °C,

SO3 will be lost and some alkali volatilized. This

will not only cause high results but will make it difficult to attain constant weight. A muffle furnace

with an accurate temperature control is much to be preferred to ignition over a gas burner where

the temperature can only be estimated. A 1 || sample of hot cement takes a long time to cooj. in a desiccator. If it is weighed slightly warm, lowj

weights can be obtained which cause high results,

Generally, at least 1 hr in the desiccator is required.

Sulfide-Containing Cements. Work of severa

investigators has shown that the regular ignitior loss procedure used in the analysis of portland

cement is not applicable to cements containing

sulfide constituents. ASTM and Federal methods^

have been revised to include suitable corrections,

based on an additional sulfate determination per- formed on the ignited material together with

suitable calculations.

Chaiken [14] indicated that a direct ignition

method utilizing a helium atmosphere gave satis^i factory results for blast-furnace slag cements.

12. Sodium Oxide and Potassium Oxide -

The specification methods cover the determination of sodium oxide and potassium oxide by flame

photometry using the direct intensity procedure.

12.1. Causes of Variations

No attempt will be made to discuss the gravimetric determination of sodium oxide and potassium

oxide in cement because it has been rather extensively covered in standard texts [6, pp. 925-930]

and is now seldom used because of the greater ease of using flame photometry for the purpose.

In general, the flame photometric determination

of potassium is rather straightforward and not subject to many sources of error within the accuracy

generally required in cement analysis. The same

cannot be said for the sodium determination. Errors,

if present, generally tend to give slightly low

results. The calcium present emits light in the same

wavelength region, 589 nm (m/u.) at which sodium

is measured. Silica, present in varying amounts

in cement, inhibits the emission of calcium. Because

the standard solutions used do not normally contain

siHca, this leads to somewhat low results for sodium

(by a few hundredths of a percent) unless the silica

is removed [15]. In fact, one or two cements manufactured domestically contain such a low contentj

of Na20 that some flame photometers indicate! a slight negative Na20 content if silica is not removed. It is rather embarrassing to try to report

a negative Na20 content for a sample of cement.' Flame photometers equipped with adjustablewidth slits can be made to minimize this effect by* using a rather narrow slit. Some older flame photometers of the motor-driven chopper type tend to give

low results if the motor mountings become loose.

If the standard solutions are stored in glass bottles for several months, a small amount of sodium'

may be leached from the glass. This makes the

actual sodium content of the solution higher than

its nominal value. This can cause results reported

for sodium oxide to be low by a few hundredths of a

percent. An exchange between constituents of the glass and the solution may occur under some

conditions. The best remedy is to store the standard

solutions in polyethylene bottles. These have

the added advantage of not supplying any chips

of glass to plug up the atomizer.

10
13. Phosphorus Pentoxide

A spectrophotometric method is applicable to the determination of phosphorus pentoxide in Portland cement. Under the conditions of the

test, no constituent normally present in portland

cement will interfere. Such a method has been adopted by ASTM. This

method should solve most of the difficulties in this determination that have bothered chemists in the

past.

A gravimetric method for the determination of

phosphorus pentoxide is being considered as a

Federal method. It is a modified form of the previous

ASTM gravimetric procedure. The changes include

addition of a measured volume of a standard solution

of phosphate to each sample as well as to the blank.

This phosphate is added to minimize the tendency

for formation of supersaturated solutions. Filtrates from the magnesium ammonium phosphate pre- cipitation should not be discarded for several

days. If any postprecipitation occurs, the precipitate

should be filtered off, ignited, and the weight so obtained added to that already found.

14. Manganic Oxide

In this procedure, manganic oxide is determined

volumetrically by titration with sodium arsenite

solution after oxidizing the manganese in the

cement with sodium metabismuthate (NaBiOs).

14.1. Causes of Variations

The bismuthate method for determining manganic

oxide [7, pp. 117-118 and 6, pp. 443-446] is fairly

straightforward. A few precautions might be emphasized. It is important that the 1:33 HNO3 added

after oxidation with NaBiOa, be completely freed

of HNO2 by boiling. In the titration it may take

practice to recognize the end point. This is the

point at which no further color change can be noted on further addition of sodium arsenite. The end

point of the solution should be observed in a strong

light and against a white background. It is suggested

that a spectrophotometric method for determining

manganic oxide in portland cement would be a welcome addition to the specification methods.

15. Insoluble Residue

This method or any other method designed for the estimation of an acid-insoluble substance in any type of cement is empirical, because the amount

obtained depends on the reagents and the time and

temperature of digestion. If the amount is large,

there may be a little variation in duplicate deter- minations. The procedure should be followed

closely in order to reduce the variation to a mini- mum. When the method is used on blended cement,

the decomposition in acid is considered to be

complete when the portland cement chnker is decomposed completely. An ammonium nitrate solution is used in the final washing to prevent

finely ground insoluble material from passing

through the filter paper. Halstead and Chaiken made a thorough study of the various factors affecting results obtained for the insoluble residue determination. Their published [16] recommendations have been used as the basis for recent revisions in ASTM and Federal

specification methods. These modifications became

necessary largely because the previous specification methods were not giving reproducible results

for slag cements. These investigators recommended

more specific requirements for heating equipment,

dilution and digestion techniques, and thorough

washing.

Concluding Remarks

It is generally recognized that analytical chemistry

still is a mixture of science and art. As long as this situation prevails there will be differences in results obtained by different analysts for the same sample

of material. The suggestions given in this paper are presented with the hope that they may be of some

assistance in minimizing these differences in the

analysis of cement.

The author acknowledges advice and assistance

of Paul Pontius, of the Mass and Volume Metrology

Branch of NBS, on material pertaining to weighing

and weighing equipment.

11

Part II. Possible Causes of Variation in Physical Tests of

Portland Cements

John R. Disc*

The physical properties of portland cement for which specification requirements have been

developed are Fineness, Soundness, Time of Setting, Heat of Hydration, False Set, Air Content of Mortar, Compressive Strength, and Tensile Strength. Fineness tests are made on the cement itself; soudness, time of setting, heat of hydration and false set tests are made on pastes consisting of cement

and water. Air content, compressive strength, and tensile strength tests are made on mortars consisting

of cement, water, and a natural silica sand. In the following discussion of the possible causes of varia- tion in these tests, particular attention is given to matters not already covered in detail in current

publications. Some background information, including the year in which each method was first published by the American Society for Testing Materials, has been incorporated in the belief that it will foster greater appreciation for the efforts that have been expended in the development of the details of the tests. A brief description of each method has also been included in order that the discussion might have more meaning for a casual reader.

Key Words: Portland cement; physical tests.

16. General Considerations in the Physical Testing of Cements

I Discussions of important considerations which

j are generally applicable to several or all test proI cedures follow. It is intended that the pertinent

1 parts of this section be considered an integral part

I of all subsequent sections.

j 16.1. Laboratory

(a) Temperature and relative humidity. The

atmosphere of a laboratory can have a significant

! effect upon the behavior of mixes and test speci-

' mens. The standard methods of test for cements

I require that the relative humidity be maintained

at not less than 50 percent, and the temperature

j between 20.0 and 27.5 °C. In nearly all instances,

I automatically controlled conditioning equipment

' must be used in order to accomplish this. Drafts

i of air as well as low relative humidity can cause

' rapid evaporation of water from both mixes and

' specimens, and therefore work benches and

testing apparatus should be shielded from excessive

I drafts from air conditioner duct outlets. Shielding

from direct sunlight and local sources of heat such

[as radiators, steam pipes, hot plates, autoclaves,

i and ovens is also necessary.

(b) Lighting. The laboratory should be evenly

i illuminated with natural or artificial light so that

j operators wiU have no difficulty in reading instru- ments and gages or dials on test apparatus.

(c) Arrangement of equipment. Equipment should

jhe so located that all items are readily accessible

to laboratory personnel. Crowding of apparatus

1 is to be avoided because this promotes carelessness

j in testing and creates safety problems.

I 16.2. Treatment of Samples

I Aeration of cement markedly affects its physical

*Manager, Cement and Concrete Reference Laboratory, National Bureau of Standards, Washington, D.C. 20234.

323-071 O - 69 - 3

characteristics. For this reason, samples should be

kept in air-tight, moisture-proof containers from

the time they are taken until all tests are completed.

Before testing, each sample should be passed

through a No. 20 sieve in order to mix the sample,

break up lumps, and remove foreign materials.

Foreign materials and hardened lumps that do not break up on sieving or brushing should be discarded.

An unusual amount of lumpy material may be an

indication that moisture is present in the sample.

16.3. Mixing Water

In general, water fit for drinking is considered

to be satisfactory for use in the testing of cement.

Water from specimen storage tanks should not be

used in the preparation of mixes. In instances where

the suitability of the mixing water is questioned,

duplicate tests using distilled or demineralized

water may provide helpful information.

It is required that the temperature of mixing

water be maintained between 21.3 and 24.7 °C. Since the temperature in outside pipes will vary

over a broad range in any 12-month period, it is usually necessary to make special provisions for bringing the temperature of tap water within the

specified range prior to use. With respect to time of setting and strength

tests, it may be helpful to bear in mind that warm

water hastens the hardening of cement and cold

water retards it.

16.4. Measurement of Mixing Water

Mixing water should be measured in a clean

glass graduate of the capacity and type specified

in the applicable test method. In no case should the

capacity be more than twice the volume required

in the mix. A graduate which is not clean will not

deliver the indicated amount of water, and for that

13
reason frequent cleaning with dilute HCl, or a suit- able glassware cleaning solution, is recommended.

Significant errors can sometimes be attributed

to bad practices in reading graduates. A technique

which avoids a number of possible variations

consists of permitting the filled graduate to come

to rest in a vertical position while it is held lightly

at the top between the tips of two fingers; after which the elevation of the hand is adjusted to bring

the bottom of the meniscus into a horizontal plane

with the line of sight. The reading is made using

the bottom of the meniscus.

The degree of uniformity being attained in these

measurements should be checked periodically by

weighing a series of measured volumes of the same

magnitude on a mix balance. In this check, the

graduate may be used as both the weighing container and the measuring device. It may be of

interest to compare the weights thus obtained with

the theoretical weight of a given volume of water,

at the temperature involved, as derived from a standard volumetric table.

16.5. Standard Sands

The two gradings of natural silica sand from

Ottawa, 111., used in making mortars for test pur- poses are customarily referred to as Standard

Sand and Graded Standard Sand. Because a number of other gradings are also prepared by the

processing plants, it is advisable for a laboratory

to check each new shipment to ascertain that the

proper sand has been received.

Contamination of the sand with an agent capable

of entraining air in mortars sometimes occurs during

processing, shipment, or storage. In many instances,

the first evidence of an accident is a substantial

increase in the air content, or a noticeable decrease

in the strength, of test mortars. The test method

for air-entrainment of standard Ottawa sand developed by DeFore and Corah [17] provides a way in which to determine if a given lot of sand is con- taminated. Another method is to run duplicate

tests with sand considered to be uncontaminated.

16.6. Weighings

(a) Mix balance. The balance should be located on a sturdy table with a rigid, nonskid top. The

top should be level, and the point at which the

balance is placed should be shielded from direct

sunlight and from stray currents of air. The balance

should be capable of weighing loads of 1 to 2 kg

with an accuracy of ± 1 g. (b) Mix weights. A common mistake in weighings

is the use of mix weights which do not conform to the accuracy requirements of the various cement

test methods. The difficulty can usually be attributed

to the loss of slugs from a "two-piece" weight, or surface wear on the bottom of a solid weight. Be- cause of the wear problem, it is recommended that weights be hfted rather than skidded from one

location to another, and that they be stored in a

suitable block when not in use. Every laboratory

should be equipped with a spare set of new mix

weights that can be used for periodic checking of

the weights in daily use.

16.7. Timing Device

One or more timing devices, preferably of the type > equipped with sweep second hands, should be so located that operators can conveniently observe the

time from any point in the laboratory. In general,

stop watches and pocket watches are less suitable

than devices such as wall clocks and bench-type

timers with large faces.

16.8. Oiling of Molds

Several of the test procedures require that the

surfaces of molds be thinly covered with mineral oil. Nondetergent oil is preferable. Animal fats or vegetable oils should not be used in lieu of mineral oils. Reductions in the apparent strengths of mortars of

10 percent or more may be encountered when an

unsatisfactory lubricant is used.

16.9. Specimen Storage Facilities

Well made specimen storage facilities are essential. Some of the important considerations are as

follows:

(a) Temperature. It is required that the temperature of the air in a moist cabinet or moist room, and

the temperature of the water in a storage tank, be

maintained between 21.3 and 24.7 °C. Many cement

testing laboratories have found it necessary to in- stall automatic controls in order to maintain storage

temperatures within these limits.

(b) Relative humidity. It is required that the moist

closet or moist room be so constructed as to provide

storage facilities for test specimens at a relative humidity of not less than 90 percent. Specimens in storage should both look moist and feel moist.

(c) Storage of fiUed molds. Molds containing speci- mens should be so stored that the upper surfaces are

freely and uniformly exposed to the atmosphere of

the moist storage unit. Molds should not be piled one upon the other in such a way that the plates of

the upper molds exclude air from the specimens

beneath. When sprays are used to humidify the

moist storage unit, precautions should be taken to prevent water from falling upon the surfaces of

freshly molded specimens.

16.10. Storage Period for Molded Specimens

Molded specimens are to be stored in a moist

atmosphere until they have attained the age of 24 hr,

at which time they are either tested or placed in water storage. In no case should specimens be

removed from the molds until they are at least 20 hr

old. If removed before 24 hr have elapsed, they

14
should be placed on the shelves of the moist storage

unit until the expiration of the 24-hr period.

16.11. Removal of Specimens From Molds

Specimens which are damaged during mold

stripping operations should be discarded even

though the damage may be considered to be slight.

Damage to test specimens, and also to molds, can be

minimized if a rawhide hammer is used to apply the

gentle taps usually needed to break the bond be- tween mold and specimen. When stripping is completed, care should be taken to ensure that mold

parts are not inadvertently interchanged. In many

instances, the dimensions of molds will not conform

to specification requirements after an interchange

of parts.

16.12. Marking Specimens

Reliable identification marks, preferably numerals, must be made on each test specimen if errors due to mistaken identity are to be avoided. The

marks should be made on paste specimens, such

as the Gilmore pat and the autoclave bar, immediately after they are formed. Identification tags

should be used for freshly molded mortar specimens

such as briquets and cubes. Under no circumstances

should identification of a specimen depend upon its position on a shelf or table, or its proximity to a marked specimen. When mortar specimens, such

as briquets and cubes, are removed from moist

storage, the identification should be marked on the

bottom surfaces with a soft graphite crayon after the plates have been removed, but while the speci- mens are still in the molds. Colored crayons should

not be used because their marks may fade very

quickly after specimens are placed in a water

storage tank. The practice of using a sharp tool to scratch marks in the surfaces of mortar specimens is objectionable.

16.13. Trowels

The specifications require that the trowels used

in making specimens have steel blades, 4 to 6 in. in length, with straight edges. Many operators have

found that a slightly curved blade is preferable for

making briquets. Briquet trowels should not be

used for other purposes. Damaged, blunt, or badly worn implements should not be used in any

operation.

Excessive trowehng of the paste in a Vicat ring

or an autoclave bar mold often occurs after the

surface of the material is torn in the process of sHcing off that which projects above the top edge

of the mold. Torn and uneven surfaces will be

encountered frequently when an implement with

blunt edges is employed, and therefore, it is advantageous to reserve a trowel with specially sharpened

edges for use in the preparation of paste specimens.

16.14. Rubber Gloves

Rubber gloves are used during the preparation of

autoclave bars and mortar briquets, and in several

other testing operations where it is undesirable to have the hands come into contact with pastes or mortars. Comfortable, well-fitting, pliable gloves are necessary for accurate work.

16.15. Care and Cleaning of Apparatus

It is usually difficult, and sometimes impossible,

t,o obtain satisfactory results with testing apparatus

that is not clean, and it is to be concluded that continuous attention should be given the important matter of cleanliness. However, it must be remembered that poor cleaning techniques may create

rather than solve problems. In this connection,

mention is made of the fact that power-driven wire

brushes and brushes with steel bristles are not

suitable for use in cleaning molds. Also, that the use

of kerosene, which may act as an air-detraining

agent, in the cleaning of molds and mixing equipment may cause anomalies in test results. Situations frequently arise wherein a laboratory

finds it necessary to adjust the weight or volume of

test apparatus. A common mistake on these occasions is failure to clean the apparatus prior to start- ing the adjustment. Another is the use of an

inaccurate balance or weight for checking the work.

These mistakes may have a very significant effect on tests employing a Vicat apparatus, the Gillmore

needles, or a 400-ml measure.

17. The Determination of Fineness

Fineness has long been recognized as one of the

important physical properties of a cement. Early

tests were based on the use of sieves and it was not

until 1933 that a satisfactory replacement, the

Turbidimeter Fineness Method (ASTM Designation:

C115), was introduced. Another new test, the Air

Permeability Fineness Method (ASTM Designation:

C204), was introduced in 1943. Currently, both of these procedures are used as standard tests of

cement.

17.1. Common Causes of Variations

Although the two fineness tests are distinctly different in technique and theory, there are a number of considerations which are of importance in either procedure. Some of these are presented

herewith.

(1) Problems with the standard fineness sample.

Although great care is taken in the blending and

packaging of standard fineness samples, an occa15
sional vial is found to be unsuitable for calibration

purposes. It therefore is advisable to check the

calibration of an instrument with a freshly opened

sample whenever an unexpected difficulty is en- countered. The risk of contaminating fresh samples

with glass chips when the containers are opened

may be reduced if opening is accomplished by

inverting the vial and pressing the sides in the empty

sections against a thin, curved, high-resistance wire

mounted between the terminals of a small

transformer.

(2) Exposure of apparatus to a source of variable

heat. The location of a turbidimeter or an air permeability apparatus above, or near, a source of variable

heat such as a radiator, hot plate, oven, or auto- clave, may cause considerable variation in test results.

(3) Errors in calculations. Among the common

errors in calculations are the following: (a) Mistakes

in the use of arithmetic, algebra, or logarithms,

(b) Errors in the transcription of figures, (c) Mistakes in the use of slide rules and other calculating

devices, (d) Selection of the wrong equation for a basic calculation.

(4) Conversion of one test value to another by means of a factor. Since the air permeability value

is frequently about 1.7 times greater than the

turbidimeter value, this factor is sometimes used in converting from one test value to the other. The

practice is unsound because the results in these

tests may be affected by such unpredictable

influences as differences in the ignition loss of a cement from day to day, or differences in the particle

size distribution caused by variations in grinding,

and values obtained in this manner may be seriously

in error.

17.2. The Turbidimeter Fineness Test

The method of test for fineness of portland cement

by the turbidimeter was introduced in a paper

presented by L. A. Wagner in 1933 [18], and

adopted as a standard test for cement in 1934. The

procedure provides for the determination of specific

surfaces by measurement of the turbidities of ce- ments in liquid suspensions. Essentially, the

turbidimeter consists of a source of light of constant

intensity adjusted so that approximately parallel rays of light pass through a suspension of cement

in clear white kerosene and impinge upon the

sensitive plate of a photoelectric cell. The current generated by the cell is measured with a microammeter and the indicated reading is a measure of the turbidity of this suspension. The turbidity is in turn a measure of the surface area of the suspended

cement.

An attractive feature of the test is that it provides

a means for determining the particle size distribu- tion as well as the specific surface of portland

cements. This advantage has been nulhfied to a certain extent, however, by the difficulties a laboratory encounters in maintaining all of the various

elements of the apparatus in satisfactory operating

condition, and in teaching new operators to make the

determination. It is important to bear in mind that the test is not intended for use in determining the

fineness of materials other than portland cement.

17.2.1. Causes of Variations

A. Apparatus

(1) Apparatus improperly mounted. Vibration of

the wall, or other object, to which the shelf supporting the sedimentation tank has been fastened

may disturb the suspension and thereby affect test results. The shelf should be so located that the

indicator on the scale on the side of the case reads

zero when the center of the slot in the photoelectric

cell is at the same elevation as the surface of the

liquid in the tank.

(2) Water cell in poor condition. When the cell

is not completely filled with water, a difference in transmission of light may be encountered above and

below the water line. Also, a wavering film of matter

inside the cell, or dirt and sedimentation fines on the

glass ends of the cell, may influence the transmission of light. Keeping the cell completely filled with

distUled water helps to avoid these problems.

(3) Sedimentation tank in poor condition.

Scratches on the faces of the sedimentation tank

may affect the transmission of fight.

(4) Reflection of light from bright surfaces inside

turbidimeter case. It is required that the interior of

the case, and the exterior surfaces of the shelf,

parabolic reflector, water cell, shield, and photoelectric cell hood be painted with a dull, optical

black paint. In addition, the cabinet should be free of light reflecting dust, light-colored wiring, and

shiny pieces of metal, and the gaskets used to seal

the end of the water cell should be black.

(5) Trouble in the electrical system. Among the

possible causes of difficulty are the following:

(a) Corrosion of electrical contacts and of the

wires in the rheostats.

(b) Broken or loose connecting wires.

(c) A weak storage battery.

(d) Deterioration of the photoelectric cell. Photoelectric cells tend to have different fatigue characteristics, and therefore the cell always constitutes

a potential source of difficulty even though it may

have been in use for only a relatively short time.

(e) Trouble with the light bulb. Malfunctioning of

a part inside the bulb or looseness of the bulb in the

socket can cause erratic microammeter readings.

(f) Looseness of the reflector in its holder.

(g) Improper focusing of the fight beam.

(h) Changes in the color, or intensity, of fight

emitted by a bulb after prolonged usage.

(i) Malfunctioning of voltage regulator. In laboratories where storage batteries have been replaced

with a voltage regulator, malfunctioning of the regulator may constitute a problem.

(6) Trouble with the microammeter. Among the

possible difficulties with the microammeter are the

16
following:

(a) Damage in usage. A bent tip on the indicating

hand may be an indication that the instrument has

been overloaded.

(b) Internal corrosion of parts while instrument is in storage. A microammeter that has not been used

for some time may have to be "exercised" before

it will give uniform readings.

(c) Static charges on the instrument, or on the

person using it. In some instances, it has been found

necessary to ground both the microammeter and the

operator.

(d) Location of the microammeter on a table containing magnetic material, or near some strong

external magnetic field.

(7) Variations in the character of the suspension.

Among the causes of variations in the suspension

are the following:

(a) Appreciable variations in room temperatures.

(b) Use of a suspending medium other than clear,

white kerosene.

(c) Traces of water in the kerosene. Water may

cause foaming inside the tank during agitation of

the suspension.

(d) Changes which often occur in oleic acid or linseed oil with age or exposure to heat and light.

When difficulties are encountered in cahbrating the

apparatus, one of the first corrective actions should

be to obtain fresh supplies of dispersing agent and

kerosene. In general, technical grade oleic acid does

not make a suitable dispersing agent.

(8) Trouble with the timing buret. Lint in the

capillary of the buret, even though practically

invisible, can slow down the flow of kerosene and

thereby , cause an appreciable variation in test results. Accordingly, utmost precautions should be

taken to keep lint out of the buret, and the kerosene.

As a safeguard, a filter made of No. 325 sieve cloth should be placed across the top of the buret whenever kerosene is being added, and the top opening

should always be covered when the buret is not in use. Use of an improperly made or a broken timing

buret is to be avoided.

(9) Problems with the wet sieving apparatus. Consideration should be given to the following:

(a) Improper nozzle design. It is difficult to obtain

good reproducibihty in test results if the angle of incHnation of the intermediate and outer rows of holes does not conform with specification

requirements.

(b) Variations in water pressure. Among the contributing factors are plugging of holes in the spray

nozzle, fluctuations of the pressure in the water line,

and errors in setting the pressure. The latter dif- ficulty may be caused by the use of a badly marked

or inaccurate gage, or by the use of a gage which

does not conform to the scale division, maximum

capacity, and nominal diameter requirements of the

specifications. It should be noted that a gas gage is not suitable for use with water unless it has been

modified for such usage. Fluctuations in Une pressure may be reduced or ehminated by the installa- tion of a pressure regulator in the line back of the gage. The overall performance of the spray assembly

should be checked periodically by collecting and

weighing the amount of water passing through the

nozzle in one minute.

(c) Unsatisfactory sieves. One possible cause of

difficulty in wet sieving operations is the use of lacquered sieve frames. During a washing, the

lacquer tends to flake off and fall onto the screen. This increases the amount of material remaining on the screen at the end of a test. Another possible cause of difficulty is use of a damaged screen. Laboratory personnel must be careful not to injure screens during the blotting operation subsequent to washing or during cleaning. In the event an accident

occurs, the damaged sieve should be retired from

service. Plugging of holes in torn screens with solder

is undesirable. A third possible cause of difficulty

is the use of screens in which the openings have

been enlarged by immersion in aqua regia to change

the sieve correction factor. B. Test Procedure

(1) Improper cleaning of tank prior to starting test. If an oily film has accumulated on the interior faces

of a tank, it should be removed with a "lint-free" wiper before a test is started. A "squeegee" of synthetic rubber makes a good wiping device.

Wiping of a dry tank with a dry cloth is to be

avoided because this may create static charges on

the tank.

(2) Variations in the agitation of suspensions.

Variations in the agitation process include allowing

the cement to slide along the inner faces of the tank

during rotation, and loss of a part of the suspension

during alternate inversions. A number of laboratories have found that the use of a rotatable tank

holder helps to promote uniformity in agitation

operations. Customarily, this holder is provided

with an adjustable screw in the center of the top

to hold the glass Ud of the tank in place, and so designed that it can be mounted to a wall by a pivot

arrangement at its center, and rotated through 180°

by means of a handle located near one end.

(3) Fouling of tank faces during handling of the tank. Care must be taken, especially during the

agitation process, to avoid getting liquid or finger

prints on the faces of the tank which are to be

placed in the path of the light. If it becomes necessary to clean the tank after agitation, the possibihty

that a static charge may be created on the faces of the tank as a result of wiping must be considered.

(4) Variations in the wet sieving operation. Among

the possible causes of difficulty are the following:

(a) Errors in weighing the sample or the residue.

Since the weight of the material retained on the screen is often less than 10 percent of the original

weight of the sample, errors in weighing can have a

significant effect on test results. It therefore is recommended that both the sample and the residue

be weighed to at least 0.001 g using the same pan

17
of the same analytical balance and the same set of

analytical weights.

(b) Improper manipulation of the sieve. Precautions should be taken to avoid loss of cement due to splashing at the beginning of the wet sieving

operation. After washing begins, strict efforts should

be made to keep the sieve in the same horizontal

plane at all times. Such practices as holding it at an angle or moving it up and down preclude repro- ducibihty. In consideration of the importance of this

detail, the use of a small wooden platform to support

the sieve in the proper position throughout the

washing period is strongly recommended. The

sieve should be kept in motion in order to prevent

the accumulation of material around the edge.

(5) Errors in calculation. A common source of

error is the failure to use standard procedures in the

calculation and application of correction factors for No. 325 sieves.

17.3. The Air Permeability Fineness Test

The method of test for fineness of portland cement

by the air permeability apparatus was introduced in a paper presented by R. L. Blaine in 1943 [19], and

adopted as a standard test for cement in 1946. The

procedure provides for the determination of specific

surfaces by measurement of the rates of air flow

through prepared beds of cements of definite

porosities. Essentially, the air permeability ap- paratus consists of a permeability ceU and a U-tube

manometer filled to about the midpoint with a

suitable liquid. The test is accomplished by attach- ing the cell, containing a carefully measured

amount of cement, to the manometer and evacuating

the air in one arm thereof until the liquid rises to a

point just beneath the cell, after which the liquid is released and the time required for the meniscus to

fall through a distance of approximately 5.5 cm

recorded. The specific surface is then calculated

from this time-of-flow determination.

The procedure is favored by many investigators

because it is economical to perform, and because it can be used to determine the fineness of materials

other than portland cements. It is to be noted that the test is not suitable for determining the specific

surface of materials finer than 7,000 cm^ per g, or the specific surface of porous materials such as diatomaceous earth and fly ashes containing porous

cinders. Also, that in testing materials other than

Portland cement, it is generally necessary to use a

specific gravity other than that (3.15) which is customarily assigned to portland cement.

Air permeability fineness determinations are sometimes influenced by factors which are pecuHar

to tests in a single laboratory, or to those in a small

group of laboratories, and it is not uncommon to find that a modification of the standard procedure

has been made in an effort to compensate for such

influences. It is very probable that some of the

variations in comparative test results can be at- tributed to the fact that values reported by several

laboratories were obtained by a modified procedure,

rather than the standard procedure because the operators concerned were not aware of the differ- ence in techniques.

17.3.1. Causes of Variations

A. Apparatus

(1) Nonconformance of permeability cell witli specification requirements. Variations in the volume^

of the test bed may result from wearing of or damage

to the cell and plunger, or nonplaneness of the! perforated disk. When a unit is being used regularly,

the dimensions and the physical condition of the component parts should be checked at least once

in every six months, and the volume of the com- pacted bed should be checked at least once in every twenty-four months. In checking dimensions,

close attention should be given to the clearance

between the plunger and cell; particularly so 11 nonstandard plastic plungers are being used. If during the examination of physical characteristics

the disk is found to be warped, it should either be

replaced or marked in such manner that the operator

will remember to place it in the cell with the same

side up for every test. If one or more parts are re-

'

placed, due thought should be given to the possibility that a new plunger manufactured to specifica

tion requirements might not be suitable for use with an old cell which still meets specification re- quirements, or that a new disk manufactured to|

specification requirements may not be suitable fori use with an old cell and plunger which still meet

'

specification requirements. It should also be borne

in mind that the apparatus must be recalibrated with! a standard sample whenever any part of the cell-j plunger assembly is replaced or altered by repairs.

(2) Uneven disks of filter paper. It is difficult to seat disks of filter paper inside the cell if the edges i are torn or uneven or badly curled from the cutting process. Disks having these characteristics should

not be used.

(3) Unsatisfactory stopcock lubricant. Stopcock

lubricants sometimes foul the upper part of the manometer to the extent that droplets of manometer fluid will cling to the inner surfaces after the liquid has receded. When this occurs, the inside of the manometer should be cleaned, and a check

made to insure that petrolatum, or a petrolatum

base stopcock lubricant, is available for future use.

j One way in which a manometer tube may be cleaned

'

is under the supervision of an expert chemist, to

fill it with an organic solvent to remove all traces of manometer fluid, rinse out the solvent with water,

allow the interior to dry, fill it with a 20-percent

solution of fuming sulfuric acid which is then allowed to stand for a period of from 2 to 12 hr, and

finally rinse with water to remove the acid.

(4) Incorrect amount of fluid in the manometer

The specifications require that the manometer be

filled with fluid to the midpoint. Since the calcu- lated midpoint often falls either above or below the

18
lower mark, in the interest of uniformity, it is customary to consider that the lower mark is at the

midpoint of the tube. When this feature is checked,

it should be remembered that the level of the

liquid will often be 2 to 3 mm below the normal level immediately after a test.

(5) Changes in viscosity of manometer fluid. Use

of a liquid whose viscosity may vary appreciably

with changes in temperature is undesirable.

(6) Inaccurate timer.

B. Test Procedure

(1) Errors in calibration of apparatus. Among the

common errors in calibration are the following:

(a) Incorrect porosity of bed. A loosely compacted

test bed is highly sensitive to sUght bumps whereas

a tightly compacted bed may spring back when the

plunger is released. Since the porosity of bed has

an important relationship to these problems, it is essential that a suitable value be selected. In general, a value greater than 0.500 should be used for materials finer than Type I cement, and a value less than 0.500 should be used for coarser materials.

(b) Failure to fluff the standard fineness sample

prior to use.

(2) Cement not at room temperature at time of test.

(3) Variations in the volume of the test bed. Variations in the volume of the bed may be caused by the

following:

(a) Failure to seat filter papers properly. It is required that a rod slightly smaller than the cell diameter be used to press the edges of the bottom

piece of filter paper against the metal disk.

(b) Failure to level the surface of the bed prior to compaction. It is required that the side of the cell be tapped lightly in order to level the bed of cement.

Too much tapping is undesirable, particularly when

coarse cements are being tested.

(c) Loss of cement around sides of rapidly

descending plunger.

(d) Improper compaction techniques. Among

these is rotation of the plunger after it comes in contact with the top piece of filter paper.

(e) Too rapid removal of plunger after compaction.

(4) Errors in timing. Such errors are frequently

encountered where the operator fails to use the

bottom of the meniscus of the manometer liquid in gaging when to start and stop his timer.

(5) Leakage of air around outside of cell during

test.

(6) Theoretical errors. A common mistake is the

failure of an individual operator to calibrate himself

by determining his own Tg for the standard fineness

sample, or a suitable reference sample. Another is the selection of the wrong equation for calculation

of fineness. The latter problem may be avoided by

using the following composite equation for all calculations:

Vt, p(i-e)

where:

S = specific surface in sq cm per g of the test sample,

Ss — specific surface in sq cm per g of the standard

sample used in calibration of the apparatus,

T = measured time interval in seconds of ma- nometer drop for test sample, = measured time interval in seconds of ma- nometer drop for standard sample used in calibration of the apparatus, n = viscosity of air in poises at the temperature

of test of the test sample,

ns = viscosity of air in poises at the temperature

of test of the standard sample used in cali- bration of the apparatus,

e = porosity of prepared bed of test sample,

Cs = porosity of prepared bed of standard sample

used in calibration of apparatus,

p — specific gravity of test sample (for portland

cement a value of 3.15 shall be used), and

Ps = specific gravity of standard sample used in calibration of apparatus (assumed to be 3.15).

18. Consistency

Both hand mixing and mechanical mixing pro- cedures are used in the preparation of pastes and

• mortars of the consistency required for the standard

I tests of Portland cement. Pastes for heat of hydration tests are first mixed by hand with a spatula, and

' then with an electric stirrer, using the amount of water indicated in the test method. All other pastes

are mixed with a mechanical mixer and their

consistencies measured with the Vicat apparatus.

Standard 1 :3 mortars for tensile strength specimens

are mixed by hand using a calculated amount of

water. All other mortars are mixed with a mechanical

mixer and their consistencies measured with the

10-in flow table.

18.1. Mechanical Mixing of Pastes and

Mortars

Continuing interest in the use of mechanical mixing for the preparation of pastes and mortars led to the adoption of a standard mixing device in 1953. A

method for mechanical mixing of plastic mortars was also put into use at that time. A method for mechanical mixing of pastes of normal consistency was adopted in 1959. The mixer and the two mixing

procedures are described in ASTM Designation:

C305. A separate method for mechanical mixing of

pastes for false set tests was added to the standard

procedures in 1962. This method is described in

19
ASTM Designation: C451. One of the most common

difficulties with these mechanical mixing procedures

has been inadequate mixing of pastes and mortars due to excessive clearance between the end of the

mixing paddle and the bottom of the mixing bowl.

18.1.1. Causes of Variation

A. Apparatus

(1) Nonconformance of mixer design with specification requirements. It is required that the mixer be

of the epicyclic type, that the speed be estabhshed

by mechanical means, that the mixing bowl have a nominal capacity of 5 quarts, and that the paddle

and bowl be made of stainless steel. The Model

N-50 Mixer, made by the Hobart Manufacturing

Co., is considered to conform with these require- ments. However, it is to be noted that the bowl and

paddle often supphed as original equipment are not made of stainless steel, and therefore must be

replaced if the mixer is to be used in the testing of

cement. Mixers which do not conform with specification requirements should not be used. Paddles

equipped with wipers should not be used unless

authorized.

(2) Failure to provide correct current for mixer

motor. Mixer motors are customarily designed to operate on 115 V, 60 cycle, alternating current. Speed control problems will be encountered when

efforts are made to operate the conventional motor at a different voltage or frequency.

(3) Failure to maintain correct paddle to bowl

clearances. Incorrect clearance between the end of

the paddle and the bottom of the bowl is a frequent

problem in mechanical mixing procedures. When

the clearance is too great, materials accumulate

around the bottom of the bowl with the result that the mixture obtained is not of the expected consistency. When the clearance is too small, the paddle

grinds up a portion of the standard sand, thereby

changing the consistency of the mix and often caus- ing excessive wear of both the paddle and the bowl.

(a) Adjustment of clearances. Small adjustments can often be made simply by bending the lugs on

the side of the bowl. (Note: In general, the use of

washers beneath the lugs to raise the bowl is unsatisfactory.) Major adjustments can be made by

raising or lowering the motor housing, or by having

a competent machinist rework a given paddle to fit a given bowl. Intermediate adjustments can be

made by means of the prescribed adjustment

bracket.

(b) Marking of bowls and paddles. Since they are seldom, if ever, identical in every small detail,

paddles and bowls, in most instances, cannot be

interchanged at random after clearances have been

adjusted for a given mixer-paddle-bowl combination.

Therefore, if equipment is carefully adjusted in such manner that two paddle-bowl sets can be used with

a single mixer, each set must be match-marked so

that the proper combination can be easily recognized. Where more than one mixer is in use, each

mixer-paddle-bowl set must be match-marked.

(c) Correction for wear. Wearing of the driving

gears or wearing of the slot in the paddle which

holds it to the drive shaft may create play which

permits the paddle to come close to or touch the side of the bowl as it rotates. Replacement of the gears

or paddle, or repair of the slot will usually eliminate

this problem.

(4) Bowl damage. Bowls may be accidentally

damaged by dropping of the bowl, dropping of the paddle into the bowl, or by raising the bowl into the mixing position before it is properly seated in the

frame. Careless or nonprofessional efforts to repair

a dent may result in a change in the form of the bowl

which renders it unsatisfactory for use.

(5) Failure to provide a lid for the bowl. A cover

is needed to prevent evaporation losses during

pauses in the mixing operation. This hd should be

made of material not attacked by the cement, and

should be sufficiently rigid that it will lie flat across the top of the bowl. Rags should not be used in heu

of a lid. (6) Leakage of oil from gear housing into mixing

bowl. A change in the consistency of the mix may

occur if oil falls into the bowl from the gear housing

during a mixing operation. When a mixer has not been used for some time, the bowl should be

checked for oil, and cleaned if necessary, prior to { starting a test. B. Test Procedure

(1) Use of wrong amount of cement and mixing

water; or cement, sand, and mixing water.

(2) Failure to remove all moisture from paddle

and bowl before use.

(3) Failure to introduce materials into the bowl in the prescribed sequence.

(4) Loss of materials as they are being introduced

into the bowl. Accidental losses can be minimized

by the use of a funnel-shaped hopper.

(5) Failure to follow the prescribed mixing

procedure.

18.2. The Normal Consistency Determination

The amount of water to be used in preparing

pastes for time of setting and autoclave expansion

tests, and in preparing mortars for tensile strength

tests, is established by the normal consistency de- termination (ASTM Designation: C190). The ap- I paratus for this determination was developed out of !, equipment used in about 1818 by L. J. Vicat, Chief

Engineer of Bridges and Roads in France [20] , and

the test had been in use for many years in both the United States and Europe prior to its adoption as a standard cement testing procedure in 1904. The

method provides for the determination of the con- sistency of machine-mixed paste by observation of the depth to which the 1-cm diam plunger of a Vicat

apparatus penetrates a ring-confined sample during ^ a 30-s period, and the paste is considered to be of normal consistency when the penetration is be- !'

20
tween 9 and 11 mm. One of the more important

„ variables in this procedure is failure to observe the

J specified time limits on the various operations. It has been found that for normal portland cements two

)' repeat tests in a laboratory can be expected to agree

> within 0.5 percentage points and test results beI tween two laboratories can be expected to agree

within 0.9 percentage points 95 percent of the time.

J 18.2.1. Causes of Variations

a A. Apparatus

; (1) Incorrect plunger weight. Incorrect plunger

i weight may result from loss of shot from the barrel

of the plunger, reduction of weight due to loss of

a metal in a machining operation, accumulation of

; dirt on the plunger, or replacement of the 1-mm

ij needle. The weight may be adjusted by varying the

I amount of shot in the barrel (when construction of

ithe apparatus permits); or by filing material away

|| from, or adding solder to, the customary knurled

!

I knob on the needle assembly. When solder is added,

I it must be determined that the adjustment is

Ij '

i! permanent.

1 (2) Incorrect plunger diameter. The diameter of

I the plunger may be incorrect due to errors in

'! machining or damage in usage. Also, a substantial

fi reduction in diameter of the end of the plunger may

! occur over a period of time from polishing the sides

i with emery cloth.

j (3) End of plunger in poor condition. The condif tion of the end should always be checked after an

' uncontrolled drop, or after it has come in forcible

I contact with a hard object.

; (4) Frictional resistance to fall of plunger. Among

; the conditions which tend to retard the fall of the

I plunger are the following:

I (a) Use of viscous oil or grease on plunger. The

*;! plunger should be oiled at the contact point with the

j frame. Only light oils are suitable for this purpose,

fj (b) Hardened cement or rust on the sides of the

j plunger.

(c) Accumulation of dirt at point where plunger

'II contacts frame. Dirt will accumulate rapidly on the

I portion of the plunger that sHdes through the

I frame of the Vicat apparatus unless the operator

develops the habit of holding the lower end while

making adjustments of the zero setting.

j (5) Vicat apparatus in poor condition. The ap- paratus is judged to be in poor condition if the indiI cator is bent or twisted in such fashion that it does

not point to comparable Hnes on both scales; or if the scale is tarnished and difficult to read.

t B. Test Procedure

(1) Failure to use the mechanical mixing procedure. Some laboratories have been reluctant to discontinue the former standard practice of mixing

pastes by hand. It therefore is noted that the water

', requirement for hand mixing is usually greater than

for mechanical mixing and that in many instances

the difference will be on the order of 0.7 percent.

(2) Retempering to obtain paste of the desired

consistency.

(3) Failure to completely fill the Vicat mold with

paste. Such failures may be minimized if the ball of paste is gently shaken into an ellipsoidal shape

while resting in the palm of the hand prior to insertion of the mass into the larger end of the mold.

(4) Failure to make the proper zero setting. It is a common practice to make the zero setting by adjusting the indicator to read zero when the bottom of the

plunger is resting on the top of the side of the

mold, which in turn is resting on the glass plate used

with the mold. It is to be noted that this is only an

approximation of the place where the surface of the

paste will actually be. Individuals wearing bifocal

glasses sometimes find it difficult to make an

accurate setting.

(5) Improper release of the plunger. The manipulation of the setscrew holding the plunger must be

such that the plunger will be released gently,

quickly and completely.

(6) Failure to release the plunger exactly 30 s after mixing. The time that elapses between completion

of mixing and release of the plunger has an important bearing on test results, particularly when

cement with false setting characteristics, or highly

plastic cements such as Type III or Masonry Cements, are under test.

(7) Vibration of the work table while penetration

tests are in progress.

18.3. The Flow Table Test

A comprehensive specification (ASTM Designation: C230) for the 10-in flow table, based on the

knowledge gained during more than 30 years of

experimentation with and use of this device for measuring the consistency of mortars, was adopted

in 1949. The apparatus consists essentially of a

10-in diam rigid table, with a perpendicular shaft,

mounted on an integrally cast rigid iron frame

fastened to the top of a 300-lb concrete pedestal.

Accessory items are a mold in the shape of the

frustrum of a cone, and a specially marked caliper.

The test is accomplished by leaving the amount of

mortar required to fill the mold on the table while

it is raised and dropped a specified number of times

by means of an electrically driven cam fastened to the frame. The flow is the resulting increase in diameter of the mortar mass expressed as a per- centage of the original base diameter.

18.3.1. Causes of Variations

All of the important requirements for the table are covered in detail in ASTM Specification C230, and

a lengthy check list for use in trouble shooting is available [21]. Since so much information has

already been printed, only a few causes of variations

other than those taken into account in section 16

are presented herewith.

21
A. Apparatus

(1) Nonconformance of apparatus with specification requirements. This is a common problem with

brand new and old apparatus alike. New apparatus

which does not conform to specification requirements should not be accepted. Old apparatus

should be repaired or replaced whenever the follow- ing signs of wear or damage are noted:

(a) Damage to contact surfaces.

(b) Excessive clearances between bore and shaft.

(c) Leakage of water from beneath flow mold due

to damage to bottom edge.

(d) Inaccurate flow measurements due to wearing of caliper.

(e) Curvature in straightedge of trowel used in striking excess material from the flow mold.

(f) Improper aHnement of cam with table shaft due to bending of the threaded end of the cam

shaft.

(g) Viscuous oil on flow table shaft.

(h) Loose mounting bolts.

(i) Unstable pedestal.

(j) Poor contact between plate and pedestal,

(k) Poor concrete in pedestal. All pedestals

should be moist cured for at least 28 days

prior to use.

(2) Failure to provide correct current for flow

table motor.

B. Test Procedure

(1) Failure to dry and clean the table top prior to

use.

(2) Variations in filling the flow mold. Variations

in the amount of material contained within the

19. The Autoclave

Soundness tests have been used for many years

to detect those qualities of a cement which tend to destroy its strength and durabihty, and at least four

procedures employing artificial conditions which

would hasten the development of such defects were either being used or studied in the earlv 1900's. An autoclave test was among these, and ultimately,

in 1940, it was adopted as a standard test for cement

(ASTM Designation: C151). The method of test for autoclave expansion of portland cement provides

for the exposure of a prism (1 by 1 by 10 in) of

hardened cement paste to a steam pressure of 295

psi (gage) within a specially designed autoclave for a period of 3 hr. Changes in length of the prism are measured by means of a suitable length comparator.

19.1. Causes of Variations

A. Apparatus

(1) Mold problems. The mold problems described

below are frequently encountered:

(a) Use of multicompartmented molds. Single or double compartment molds are preferred because

they make it possible to put bars in moist storage soon after molding is completed.

mold may be caused by variations in compacting

mortars into the mold, including the failure to use

the prescribed 20 tamping pressures. The practice

of throwing mortar into the mold also constitutes

a variable. The mold should be held firmly against

the table top during the filUng operation in order to reduce seepage under the bottom edge.

(3) Failure to dry and clean the table top after

filling the mold. Any water that may have seeped

from beneath the mold, and all spilled mortar, should

be wiped away prior to proceeding with the test.

(4) Errors in determining the flow. The flow is the

increase in diameter of the mass of mortar expressed

as a percentage of the original diameter. The flow

caliper specified for use in measuring the flow is graduated to indicate one fourth of actual flow so

that readings of four measurements may be added

to give the flow value. Calipers graduated to read

the flow directly in percent or percentage markings

on table tops usually give less satisfactory values. In using the specified caliper due care should be

exercised to avoid mistakes in readings and mistakes in adding the readings.

(5) Adjustment of flows by application of correction factors. A mixture of ground silica and mineral

oil supplied by the Cement and Concrete Refer- ence Laboratory has been used for a number of years as a means of gaging flow table performance.

Some laboratories have attempted to correct mortar

flow values by adding or subtracting the diff'erence between assigned flow values and actual values

obtained with the silica-oil mixture. Test results are not reliable when adjusted in this manner. Soundness Test

(b) Thin end plates in bar molds. Thin end plates

may permit the gage studs to become so deeply

imbedded in the ends of bars that an accurate measurement of length cannot be obtained.

(c) Damaged gage studs. Use of gage studs with

damaged contact points may cause an error in the

measurement of a bar, particularly if the bar is inverted between the initial and final readings. For

this and other reasons, each bar should be marked

on one end to protect against inversion.

(2) Operational problems with the autoclave. The

operation of the autoclave is an important factor in some soundness tests. Accordingly, all of the follow- ing items merit special consideration.

(a) Leakage of steam through joint between lid and chamber. Leakage may be caused by a defective

gasket or by a failure to tighten the holding bolts in a satisfactory manner.

(b) Leakage of steam through a defective safety

valve.

(c) Carelessness in venting air at the start of a

test. The vent valve should not be closed until a

strong, steady stream of steam emerges.

(d) Low line voltage.

(e) Deterioration of heating elements. The failure

22
of an autoclave to heat at the specified rate, or its failure to maintain the specified test pressure, can

: often be corrected by replacing the heating elements.

I (f) Errors in indication of pressure. In numerous

' instances, gages have been reset by laboratory

' personnel so that the indicated steam pressure

I would be in agreement with the temperature within

the chamber as indicated by a total immersion

thermometer. Such adjustments are usually in error

I because the thermometer well of an autoclave is

I designed for the use of a partial immersion ther- mometer. For this and other reasons, the accuracy

of indication of the pressure gage should be verified periodically on a gage tester.

(g) Improper manipulation of heater controls. The

initial heating rate is significantly affected by the

number of bars in the chamber, and therefore it must be controlled by manipulating the heater

controls in a sequence previously found to be

I satisfactory for the number of bars under test. Only

j one sequence is required if vacancies in the rack are

filled with previously tested bars so that the number

' in the chamber will be the same for every run. Use

j of a voltage regulator may also prove helpful.

' (h) Use of too much water in the chamber. A

I measured quantity of water, which is 7 to 10 percent

the volume of the chamber, should be added prior

to the start of each test. Use of an amount greater

than 10 percent may cause an appreciable change

in the heating characteristics of an autoclave. Any

j water remaining in the chamber at the end of a test should be removed with a siphon and the walls

I wiped dry with a sponge to prevent rusting.

I (i) Improper adjustment of automatic pressure

controls.

i (j) Plugging of connecting pipes. The gradual

i accumulation of material in the connecting pipes

may prevent the pressure regulator, the pressure

j gage, or the safety device from performing in a sat- isfactory manner. Where iron pipe and stainless

I steel pipe are used together in the same connecting

I system, electrolytic action may cause the iron pipe

ji to plug up very rapidly.

I (3) Errors in measurements due to comparator.

The graduations on some dials are very difficult to read, and in some instances the physical condition

j of the dial face is such that accurate readings may

be difficult to obtain. Dials with these deficiencies

should be replaced.

B. Test Procedure

(1) Use of an incorrect amount of cement or mixing

water.

(2) Allowing gage studs to shift during molding of

bars.

(3) Excessive troweling of the surface of bars. Excessive troweling can be avoided if the operator uses a trowel with sharpened edges, and performs

the smoothing operation by drawing the flat side

of this trowel (with the leading edge slightly raised), over three-fourths of the length of the mold, in such

fashion that the surface of the paste behind the

trowel will be smoothed while any excess material

will be "crowded" toward the end of the mold.

Toward the end of the stroke, the leading edge of

the trowel should be lowered to ride along the top

edge of the mold, and the extra material then sliced

away by the sharp edge of the trowel. The entire

operation of smoothing and finishing can be completed in this manner in the course of a few passes

along the mold.

(4<) Allowing bars to dry after molding is completed.

Bars should be put into moist storage immediately

after molding is completed, and should not be dis- turbed until it is time to strip the mold. The pro- cedure of smoothing the surface an hour or two

after the mold has been filled does not conform with

standard practice.

(5) Autoclaving bars which are less than 24 hours

old.

(6) Failure to measure bars immediately after removal from moist storage.

(7) Improper cooling of bars after autoclaving. It is important that bars be cooled to room temperature prior to measurement. A cylindrical container

approximately 8 in in diam and 16 in in height,

equipped with a bail, makes a convenient cooHng

chamber. In use, this container is filled with water

and placed on a hot plate at the end of the 3-hr

autoclaving period. Later, when the autoclave is opened, the rack and bars as a single unit may be

lifted from the autoclave by means of a suitable

metal hook and immersed in the near-boiling water

in the container. The container may then be trans- ferred to a sink, and cold water introduced through

a rubber tube connected to a cold water spigot. The

rate of cooling may be controlled by regulation of

the flow of cold water through the tube.

(8) Errors in calculations. Such errors are usually

mistakes in arithmetic.

20. Time of Setting

Setting properties have been of interest since the 20.1. Common Causes of Variations

very earliest days of cement testing and numerous

procedures for gaging this important quality have The two tests are similar in many respects, and

been developed. Presently, there are two standard therefore, there are a number of considerations

methods of test, namely the Vicat Time of Setting which are of importance in either procedure. Some

Test (ASTM Designation: C191), and the Gillmore of the common causes of variations other than those

Time of Setting Test (ASTM Designation: C266). taken into account in previous sections of this

23
discussion are presented herewith.

(1) Error in the determination of the normal

consistency. It is generally understood that the more

mixing water used, the slower the set. The amount

of mixing water may be incorrect due to an error in the determination of the normal consistency.

(2) Inadequate mixing of paste. It must be re- membered that machine mixing will give unsatisfactory results if the mixer does not conform to all requirements of the applicable specifications.

(3) Regaging of paste. Regaging usually tends to make paste slower setting.

(4) Irregularities in testing. A Vicat apparatus, or

a set of Gillmore needles, should be shielded from

drafts of air and from direct sunhght which might cause evaporation of moisture from specimens

while readings are being taken. Also, the length of

time specimens are kept out of moist storage should

be held to a minimum.

(5) Errors in determining time intervals. Where

errors in reading times or in calculating time

intervals are encountered, the use of a "militarytype" clock with a face showing the hours from

0000 to 2400 may prove helpful.

20.2. The Vicat Time of Setting Test

The current method of test for time of setting of

hydraulic cement by the Vicat needle was developed

out of equipment and procedures used in about

1818 by L. J. Vicat [20]. The evolutionary process

began in Europe and had been underway there and

in the United States for many years before a crude

version of the present procedure was adopted as a standard test for cement in 1904.

20.2.1. Causes of Variations

A. Apparatus

(1) Vicat apparatus in poor condition. In general an apparatus that would not be suitable for use in normal consistency determinations would not be

suitable for use in Vicat time of setting tests. Otherwise, particular attention should be given to the straightness of the needle, the diameter and the

condition of its end, and the possible presence of hardened cement or rust on its sides. The condition

of the needle should always be checked following

a free fall through a mass of paste in a plastic state.

(2) Use of oil on Vicat ring. Use of oil on the ring

to facilitate the removal of hardened paste is not

particularly helpful, and introduces the possibility that the paste might be contaminated with oil during the test. B. Test Procedure

(1) Improper procedure. The paste may not be

suitable for test purposes if it has been used pre- viously in a normal consistency determination.

(2) Failure to completely fill the Vicat ring with

paste. Small holes in the paste may have a significant effect on test results.

20.3. The Gillmore Time of Setting Test

The use of weighted wires (needles) was originally proposed as a test for time of setting of ce- ment by a Committee of the American Society of

Civil Engineers in 1885. The wires were called

"Gillmore wires" as a courtesy to General Gill- more, of the Corps of Engineers, the chairman of

the committee, although they had not been pro- posed by him, but had been suggested by others

[22]. A test based on that early method was adopted

as a standard test for cement in 1916. A set of Gillmore needles consists of an initial needle, weighing V4 lb and equipped with a Vi2-in diam cylindrical tip; and a final needle, weighing 1 lb and equipped with a V24-in diam cylindrical tip. Initial and final setting times are determined by the

application of these needles to the flattened top of a specimen of paste (a pat) 3 in in diam at the bottom

and V2 in thick at the center. The Gillmore test is preferred by many testing agencies because it is simpler, and therefore more economical to per- form, than the Vicat test. Its chief weaknesses are considered to be the failure to obtain a homogenous

mass upon which to make the determination, the

difficulty of applying the needles at right angles to the surface of the specimen, and the personal equation in determining when the needles cease to make

an appreciable identation.

20.3.1. Causes of Variations

A. Apparatus

(1) Incorrect diameters of needle tips. The diameter of a needle tip may be incorrect due to errors

in machining, wear, dropping, or encrustations of

hardened cement.

(2) Incorrect needle weights. The weight of a needle may be incorrect due to the inadvertent loss of shot from the ball, or the addition of excess shot. New needles that do not have a shot chamber should

be rejected.

(3) Failure to provide required frame for needles. The ends of needles stored in wooden blocks, or in drawers, are frequently found to be damaged. The

specified frame helps to preclude such damage as well as facilitating use of the needles. Frames

should be easily dismantled in order that the needles can be removed for inspection. Those not so con- structed should either be corrected or discarded.

New frames that cannot be dismantled should be

rejected.

(4) Bent needle shafts. B. Test Procedure

(1) Use of an incorrect amount of cement or water. Since the specified batch size makes more paste

than is required to make a pat, smaller samples

are sometimes mixed by hand or with a trowel. These methods usually require more mixing water

than that indicated for normal consistency, and

therefore the rate of hardening of the paste is

24
altered. Results obtained in this manner are only

approximations of the time of setting.

(2) Specimens improperly formed. This is fre- quently due to the failure to follow specified pro- cedures. Among the other contributing factors are the following:

(a) Excessive trowehng. If operators seem to be

over-troweling specimens, an effort should be made

to form pats with less than 40 trowehng strokes.

It is preferred that only one appUcation of pressure

be used in flattening the top of the pat.

(b) Slanting or uneven surface on top of pat. The

surface of the pat should be smooth, and it should

be parallel to the surface of the glass plate on which the specimen is formed. A template may be

helpful in preparing specimens which have smooth,

level surfaces, and which are of a uniform height.

This template should not be confused with the so- called "pat mold," a device which is not suitable

for use in the preparation of specimens.

(3) Variation in technique of applying needles.

The needles must always be appMed at right angles

to the surface of the pat.

(4) The personal equation in judging indentations.

Variations in determining the point at which the

pat will bear, without appreciable indentation, the

Gillmore needle, may be due to inadequate lighting

of the test area, and therefore the installation of

special hghts to ensure that surfaces of pats are adequately illuminated may prove helpful. Use of fluorescent hghts or a special hght box is rec- omniended to avoid exposing pats to the "drying

heat" of incandescent hghts.

(5) Too frequent checking of specimens.

(6) Vibration of needles at time of application to specimen. Needles mounted on a shelf affixed to the

side of a moist cabinet are affected by the shocks

caused by the closing of a door of the cabinet and

this disturbance may influence test results if it occurs during the apphcation of a needle to the

surface of a specimen.

21. The Heat of Hydration Test

A method of test for heat of hydration of portland

cement (ASTM Designation: C186); based on

studies conducted by L. Shartsis and E. S. Newman

of the National Bureau of Standards [23], and

others, was adopted in 1944. A number of changes

in apparatus and methods designed to improve the

precision of the determination have been made in subsequent years. The most recent of these changes were adopted in 1968. The test is customarily hsted among the physical

tests of cement and therefore comes within the

sgope of this review. However, no comments about

the related apparatus and methods are being of- fered because they would be of interest to only a few laboratories. For the benefit of anyone who

needs such additional information, it is noted that a discussion of the test and a good bibliography can

be found in "The Chemistry of Portland Cement"

by R. H. Bogue [24].

22. The False Set Test

Development of a method of testing for premature

stiffening (false set) was initiated in 1953. A method

of test for determination of false set in mortar

(ASTM Designation: C359), which is presently

widely regarded as being useful only for research

purposes, was adopted in 1955 (ASTM Designation:

C451). An alternative procedure, using cement

paste and patterned closely after a method that had been part of the Federal Specifications for Hydraulic Cements for many years, was adopted

in 1960. The latter was subsequently used as the

basis for optional specification limits. The paste method provides a means for evaluating

the false set potential of portland cements by observation of the loss in penetration of pastes within a few minutes after mixing. An initial penetration of the plunger of a Vicat apparatus of 34 ±4 mm is used to establish that the correct amount of water

has been incorporated in a machine-mixed paste.

This initial penetration is recorded and a second

penetration reading taken 5 min after completion

of mixing. The percentage of penetration, as ob- tained by dividing the final penetration by the

initial penetration and multiplying the result by

100, indicates the extent to which false set has

occurred.

22.1. Causes of Variations

A. Apparatus

(1) Use of unsuitable sample containers. The

behavior of cements in false set tests is significantly

affected by the protection afforded samples prior

to testing. Use of plastic or metal containers helps

to minimize exposure of samples. Use of paper or

cloth sacks should be avoided.

B Test Procedure

(1) Use of an incorrect amount of cement or water.

If the water requirements for all cements seem to be consistently in error, the mechanical mixer and

Vicat apparatus should be checked thoroughly for conformance with the requirements of applicable

specifications.

(2) Failure to observe specified time limits on test operations. Timing is a highly important factor in false set tests. Particularly so where mixing is con25
cerned as the test tends to become more severe when the period is shortened and less hkely to give an indication of false set when it is extended.

(3) Aeration of test samples. Prolonged exposure

of test samples to air immediately before use may

have a measurable effect on test results. Cement,

for a false set test should be kept in an air-tight

container until the time comes for it to go into the mixing bowl.

(4) Errors in reporting test results.

23. Air Content of Mortar

The need for a rapid physical test whereby the

air-entrainment characteristics of portland cements

treated with air-entraining agents could be regulated led to the adoption, in 1944, of the air content

of mortar determination as a standard test for cement (ASTM Designation: C185). A number of inadequacies in the original equipment and pro- cedures were soon recognized, and these were

corrected in extensive revisions of the method in subsequent years. The current procedure provides for the deter- mination of the air content of machine-mixed

mortar; consisting of 1 part cement, 4 parts standard sand, and the amount of water required to obtain a flow of 80 to 95 percent after 10 drops of the flow table; by comparing the weight of a 400-

ml sample thereof to the theoretical air-free weight

of a similar volume as calculated by the absolute

volume method. In general, the uniformity in re- sults is found to be better in tests of air-entraining

cements than in tests of non-air-entraining cements.

23.1. Causes of Variations

A. Apparatus

(1) Variations in volume of 400-ml measure. It is required that the volume of the measure be between

399 and 401 ml. In numerous instances the volumes

of new containers do not conform to this requirement, and therefore, as a precaution, the volume

of every new measure should be checked, and ad- justed if necessary, prior to acceptance by the

laboratory.

(2) Balance not suitable for test. The desired accuracy in test results may not be attained if the

capacity of the balance is inadequate.

B. Test Procedure

(1) Use of an incorrect amount of cement, sand, or water. If water requirements for all cements seem

to be consistently in error, the flow table and mechanical mixer should be thoroughly checked for conformance with the requirements of applicable

specifications.

(2) Failure to place mortar gently in the measure. The technique of throwing mortar into the measure

does not conform with standard practice.

(3) Failure to observe specified time limits on test operations. The desired accuracy cannot be ob- tained if the time limits on various test operations

are disregarded.

(4) Improper handling of the 400-ml measure. Shaking or jolting of the measure during the filling

operation must be avoided.

(5) Failure to use specified equipment in perform- ance of test. Particular attention is invited to the

advantages of using the specified spoon, spatula,

tapping stick, and straightedge in the filling of the

400-ml measure.

(6) Variations in weighings. In many instances,

balance weights will give more uniform results than the beam and poise with which many mix

balances are now equipped.

(7) Errors in calculation. The air content of mortar

is calculated from a formula given in the related test method. It is important to remember that the value

for P (percentage of mixing water) is always a whole

number. For example, if the weight of the water is 60 percent of the weight of the cement, P would be

equal to 60 and not 0.60.

24. Strength of Mortars

The use of mortar strength tests as a criterion

of the quality of cements was recorded in the liter- ature as early as 1772 [20]. Efforts to develop standard methods for measuring the strength of portland

cement mortars began around 1848 [25] and have

been carried forward by a succession of independent

investigators and specification bodies since that time. Presently, both compression (ASTM Designation: C109) and tensile strength (ASTM Designation:

C190) tests are included in the hst of standard

physical tests for portland cements.

24.1. Common Causes of Variation

Although there are many actual or theoretical

differences between the two strength tests, the com- mon use of mortar specimens introduces a number

of considerations which would be of importance in either determination. Some of these have been

covered in other parts of this review. Some are presented herewith.

(1) Nonconformance of mold design with specification requirements.

(2) Leakage of water from molds. The loss of water

through the joints of a mold, or the joint between a mold and its base plate, may permit specimens to develop strengths which are higher than average.

In this connection, it is important that mold base

plates be of sufficient thickness to preclude bending

26
''during molding and handling of specimens, and that an appropriate sealant be used.

i (3) Unsuitable water storage facilities. Mainte- nance of correct storage temperature is essential. In addition, some investigators have concluded that the pH of the storage water affects test results, and

have recommended that a level of 7 be maintained.

Others who have studied this problem have found

that storage of specimens in plastic bags, along with

' a small amount of curing water, helps to improve

the reproducibility of strength values.

(4) Improper treatment of specimens after removal

' from storage. It is required that all strength speci- 'Fmens be placed in moist air storage immediately

after molding. Specimens to be tested at 24 hr are taken directly from the moist air storage and tested

' at once. Specimens to be tested at later ages are transferred to water storage for additional curing.

[Air drying of any of these specimens just before

?l testing can cause an increase in compressive

strengths or a reduction in tensile strengths. For

this reason, when several 24-hr specimens are to be

^ tested at one time, it is recommended that they be

\ placed in a pan immediately upon removal from

moist storage, and kept covered with a moist cloth

until ready for test. When two or more specimens,

Pj other than 24-hr specimens, are to be tested at one

time, they should be removed from the water stor- 'jage tanks, placed immediately in a pan of water,

and kept there until actually tested. As immersion

! in water of an appreciably different temperature

than the tank water just prior to testing can have

! a significant influence on results, it is recommended

J, that the water in the pan be obtained from the storI'i age tank.

ij (5) Failure to clean specimens prior to testing.

|l Loose sand grains or incrustations must be removed

1 from the bearing surfaces of specimens before they

^! are inserted in the testing machine. If specimens

\ seem to be excessively coated with a soapy substance, the use of a different mold lubricant should

] be considered.

(6) Failure to observe suitable time tolerances on

j testing. Every effort should be made to maintain

equal intervals between the preparation and testing

of strength specimens. Since it may not always be

possible to do this, some tolerances on time should

. be observed. The following are suggested: 1-day.

I within 1/2 hr; 3-day, within 2 hr; 7-day, within 4 hr.

, and 28-day, within 8 hr.

ij (7) Inaccuracy of testing machines. The accuracy

j of indication of a testing machine should be verified

j following its installation in a laboratory, and at least once in every two years thereafter. When the

I accuracy of a machine is questioned, and it is not

j convenient to make an immediate verification, it

j may prove helpful to compare results obtained with

the machine in question against results obtained

1 with machines in nearby laboratories. In these in- stances the comparative tests should be made on

^ specimens from a single batch, and checks should

\ be made in two properly equipped laboratories.

rather than in just one. It may be helpful to remember that the failure to maintain a testing machine

in proper operating condition is one of the common

causes of errors in strength results.

(8) Failure to load test specimens at specified rate. Appropriate rates for application of loads to strength

specimens have been determined by careful ex- perimentation. Failure to use the specified rate may

have a significant effect on test results, particularly

in compressive strength determinations.

24.2. The Compressive Strength Test

The procedure whereby the strength characteristics of a cement are evaluated by the compressive

strength of 2-in mortar cubes; consisting of 1 part

cement, 2.75 parts Graded Standard Sand, and the

amount of water required to obtain a flow of 100 to 115 percent after 25 drops of the flow table; was

adopted as a standard test for cement in 1934. It is favored by many investigators because the progressive increase in the strength of cubes, with age,

parallels the increase in strength of concrete, with

age, and because the agreement between laboratories using correct techniques and equipment is generally acceptable. Numerous refinements in apparatus and procedures have been made since

1934, i.e., machine mixing has replaced hand mixing

and the instructions for molding of specimens have

been expanded, with the result that the number of laboratories reporting unreasonable compressive

strength values has been substantially reduced.

24.2.1. Causes of Variations

A. Apparatus

(1) Nonplaneness of interior faces of molds. Dif- ferences in strength ranging from 3 percent at 1 day to 8 percent at 28 days were found on one oc- casion where 2-in mortar cubes were tested between convex blocks having a mid-ordinate of 0.004

in [26]. These findings are believed to be indicative

of the influence nonplaneness of mold faces may

have on test results.

(2) Failure to keep corners of molds clean. The

accumulation of hardened cement in the corners of

the pockets in a cube mold reduces the overall size of test specimens, and thereby causes reductions

in compressive strengths. Prompt cleaning of the

molds after stripping will help to avoid this difficulty,

and normally most of the residue can be removed

with a damp cloth if cleaning is undertaken before

the material has had an opportunity to harden in air. (3) Excessive use of oil on molds.

(4) Inadequate sealing of molds. Operators fre- quently fail to follow the instructions for seahng

of cube molds. In numerous instances, the departure

is attributed to difficulty in applying the specified

sealing material. One effective technique is to melt

the material in a conventional oil can with a "cut- down" spout, and to apply the liquid thus obtained

to the mold, as one might apply oil, while holding

the hot can in a hand protected by an asbestos glove.

I 27
This procedure will work with either the paraffinrosin mixture mentioned in ASTM Method C109 or the microcrystalline wax mentioned in Federal Test Method 2001.1.

(5) Problems with spherical blocks on compression

machines. Common problems include the following:

(a) Block frozen in place and therefore unable to adjust to specimen.

(b) Improper design of block.

(c) Damage to spherical block that prevents nor- mal movement.

(d) Use of lubricant other than Hght oil on surface

of sphere.

(e) Contact between sphere and its seat occurring

only at top of sphere.

(6) Nonplaneness offaces of compression machine

bearing blocks. Reductions in compressive strength

as great as 25 percent are known to have occurred

where blocks with very uneven bearing surfaces were being used. All blocks tend to lose their plane- ness with usage, and the change may occur rapidly

if the faces have not been properly hardened. Plane- ness can be checked with a flashlight and a tool- maker's straightedge. If the planeness is within

0.001 in as required, very little, if any, light will show between the straightedge and the surface of the block when the flashhght is held behind the

straightedge. If a more conclusive check is desired,

a piece of cellophane from a cigarette pack can be

used beneath the straightedge as a feeler gage.

(7) Inadequacies in design of testing machine.

Dials on testing machines used in determining the

compressive strengths of mortars should conform

to the requirements for dials set forth in ASTM

Designation C39.

(8) Inadequate maintenance of compression test- ing machine. Every laboratory should be provided

with maintenance literature for the compression

machine it uses, and all special maintenance equipment should be available, i.e., the special wrench

and oil gun needed for machines equipped with a

fluid support. The indicating hands should be

checked periodically for freedom of movement,

with particular attention being given to maximum

hands. The level of oil in all oil reservoirs should

be checked frequently. In the event of mechanical

failure, the services of a competent repairman

should be obtained promptly.

B. Test Procedure

(1) Use of an incorrect amount of cement, sand, or

water. If the water requirements for aU cements seem to be consistently in error, the flow table and

mechanical mixer should be checked thoroughly

for conformance with the requirements of applicable

specifications.

(2) Mixing of mortars by hand. The mixing of

. mortars by hand, rather than with the mechanical

mixer as specified, wiU usually produce lower test results.

(3) Variations in tamping pressures. Nonuniformity in the tamping pressures used in compacting

mortar in the mold pockets is a variable in compressive strength determinations.

(4) Improper positioning of specimens on compression machine bearing blocks. It is important that cubes be centered carefully on the bearing blocks

of the testing machine. The use of a special center- ing template may be helpful.

(5) Failure to zero compression machine properly

at start of test.

(6) Errors in reporting test results. A confusing

error, particularly where low strengths are con- cerned, is the failure to divide the load in pounds

by four to obtain pounds per square inch.

24.3. The Tensile Strength Test

The procedure whereby the strength characteristics of a cement are evaluated by the tensile strength of 1-in thick, figure-8 mortar specimens

(briquets); consisting of 1 part cement, 3 parts

Standard Sand, and the amount of water indicated

by the normal consistency determination; was intro- duced about 1880 [25], and was adopted as a standard test for cement in 1904. The test has been

favored by many investigators because it is simple,

and therefore economical to perform. The retro- gression of strength with age, which is sometimes

encountered, is frequently mentioned as a dis- advantage.

24.3.1. Causes of Variations

A. Apparatus

(1) Unsatisfactory work gloves. Rubber gloves

must be pliable and snug-fitting so that they will not prevent uniform and vigorous movement of the

fingers throughout the mixing period.

(2) Use of worn test apparatus. When strength

values seem to be consistently in error, all apparatus

should be carefully checked for conformance with

the requirements of applicable specifications. A

particular problem in recent years has been the low

strength of specimens produced in molds which

have worn so thin as to no longer comply with

minimum thickness requirements. Another problem has been the leakage of water from molds which

are worn in such fashion that the halves no longer

meet at all points.

(3) Variations in mold design. The molds pres- ently in use vary from the single-pocket to the six- pocket type, and fabrication techniques tend to vary with the type of mold. It would be advantageous

for all laboratories to use the customary three- pocket type.

(4) Inadequate maintenance of testing machine.

From time to time, grips are observed wherein the

rollers are no longer free to turn. This and other

important maintenance problems are discussed in detail in existing publications [21].

B. Test Procedure

(1) Use of an incorrect amount of cement, sand, or

water. Consistent errors in the amount of mixing

28
water used may indicate an error in tiie normal

consistency determination, or an error in calculating

the amount of water from the normal consistency. In this latter calculation, the given percentage of

water is expressed in terms of the weight of cement

and sand, and not in terms of the weight of the

cement alone.

(2) Use of mixing equipment coated with oil or other contaminants.

(3) Use of gloves, trowels, or mixing slabs that

have not been allowed to dry.

(4) Loss of mixing water due to evaporation. Use

of too large an area on the mixing slab during hand

mixing promotes evaporation of water.

(5) Variations in techniques for filling molds.

Where two or more molds are being filled from the same batch, each phase of the fabrication process

should be completed on each mold before the next phase is started, i.e., compaction of the mortar into each pocket in each mold should be completed

before the heaping and smoothing operation is started. The mortar in each pocket should be flush with the top of the mold when smoothing is completed.

(6) Improper compaction of mortar in the mold

pockets. It is recommended that the thumb pres- sures be applied in groups of two per briquet on each of six trips along the mold. Care should be

taken that the thumb pressures are applied entirely

to the surface of the mortar, and not to the surface

of the mold. The specified thumb pressure of be- tween 15 and 20 lb may be measured by molding

the briquets on a platform scale or on other suitable

apparatus that will indicate the pressure exerted.

(7) Variations in techniques of finishing speci- mens. In standard practice the excess material is removed from the molds with a smoothing motion

rather than with a sawing motion. Mastery of the

smoothing operation requires a certain amount of

practice, and therefore many laboratories permit

inexperienced operators to use the simpler sawing

procedure while learning the test. The prescribed

smoothing technique should be adopted at the end

of the apprenticeship period.

(8) Failure to remove all sand grains from face of

mold. Sand grains between the mold and its base

plate promote the loss of mixing water from fin- ished specimens as well as an increase in thickness.

Either of these conditions is capable of causing an

apparent increase in strength.

25. Closure

This discussion of causes of variation in physical

tests of Portland cement h,as been written in such

fashion that it constitutes a master check list with

explanatory comments. If further assistance is needed, complementary check sheets can be ob- tained from the Cement and Concrete Reference

Laboratory, National Bureau of Standards, Washington, D.C. 20234.

The author acknowledges the assistance and

advice received from his associates in the Cement

and Concrete Reference Laboratory.

26. References

[1] Youden, W. J., Statistical aspects of the cement testing pro- gram, ASTM Proc. 59, 1120 (1959). Crandall, J. R., and Blaine, R. L., Statistical evaluation of

interlaboratory cement tests. Ibid., p. 1129.

[2] Bean, L., and Hackney, E. J., A study of some operations involved in cement analysis, ASTM Bull. 197, 44 (Apr.

1954).

[3| Information concerning pyrometric cones may be obtained from The Edward Orton Jr., Ceramic Foundation, 1445 Summit St., Columbus 1, Ohio.

[4| Private communication.

[5| Roberts. H. S., Phys. Rev. 23, 386 (1924).

[6| Hiliebrand, W. F., Lundell, G. E. F., Bright, H. A., and Hoff- man, J. I., Applied Inorganic Analysis, pp. 32-33, 46-51,

503. (Published by John Wiley & Sons, Inc., New York,

N.Y.,2d ed.,1953.)

[7| Book of ASTM Standards, Part 9, C114-67 (1968).

[8| Fed. Test Method Std. No. 158, Sept. 1960: Cements hydraulic: sampling, inspection, and testing; method 1001,

par. 4.2.

[9| Kolthoff,' I. M., and Sandell, E. B., Textbook of Quantitative Inorganic Analysis, p. 391 (Published by the Macmillan Co., New York, N.Y., 3rd ed., 1952).

[lOJ Hoffman, J. I., and Lundell, G. E. F., The precipitation and

ignition of magnesium ammonium phosphate, BS J. Res. 5, 279-93(1930). RP200.

[Ill Berman. H. A.. An improved 8-hydroxyquinoline method for

the determination of magnesium oxide in portland cement, ASTM Bull.. 237, 53 (Apr. 1959).

[12] Rudy, R. B., Determination of sulfuric anhydride in portland cement by means of the Wagner Turbidimeter, J. Res. NBS 16, 555 No. 6 (June 1936), RP893.

[13] Bright, H. A., Determination of sulphur occurring as sul- phide in Portland cement, J. Res. NBS 18, 137 No. 2 (Feb. 1937), RP968.

[14] Chaiken, B.. Determination of ignition loss in portland blast- furnace slag cements, ASTM Bull., No. 238, p. 53 (May

1959).

[15] Diamond, J. J., and Bean, L., Improvements in flame photo- metric determination of sodium in portland cement. Anal. Chem. 25,1825(Dec. 1953).

[16] Halstead, W. J., and Chaiken, B., Insoluble residue deter- mination in Portland and portland-slag cements, ASTM

Bull. No. 229, pp. 60-65 (Apr. 1958).

[17] DeFore, M. R., and Corah, H. J., A test method for air- entrainment of standard Ottawa sand, ASTM Bull. No. 248 (Sept. 1960).

[18] Wagner, L. A., A rapid method for the determination of the

specific surface of portland cement, ASTM Proc. 33, 553

pt. II (1933).

[19] Blaine, R. L., A simplified air permeability fineness ap- paratus, ASTM Bull. No. 123, p. 51 (Aug. 1943).

[20] Gonnerman, H. F., Development of cement performance

tests and requirements, P. C. A. Research Bull. No. 93

(Mar. 1958).

29
[21] Book of ASTM Standards, Part 9, Appendix I. Manual of cement testing.

[22] Data considered by Committee C-1 of the American Society

for Testing Materials in Preparing the Standard Specifications and Tests for Portland Cement (Serial Designation

C 9-17).

[23] Shartsis, L., and Newman, E. S., A study of the heat of solu- tion procedure for determining the heat of hydration of

Portland cement, ASTM Proc. 43, p. 905 (1943).

[24] Bogue, R. H., The Chemistry of Portland Cement, p. 606 (Reinhold Publishing Corp., New York, N.Y.).

[25] Swayze. M. A., The testing of cements for inherent strength ASTM Bull. No. 209 (Oct. 1955).

[26] Dwyer, J. R.. Effect of departure from planeness of bearing surfaces on compressive strength of 2-in. mortar cubes, ASTM Proc. 36, 351 pt. II (1936).

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