Suspended hydration and loss of freezable water in cement pastes

exposed to 90% relative humidity

by

K.A. Snyder and D.P. Bentz

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899 USA

Reprinted from Cement and Concrete Research, Vol. 34, No. 11, 2045-2056, November

2004.

NOTE: This paper is a contribution of the National Institute of Standards and

Technology and is not subject to copyright.
Available online at www.sciencedirect.com

CEMENT AND

CONCRETE

RESEARCH Pergamon SCIENCE@OIRECT8

Cement and Concrete Research 34 (2004) 2045-2056

Suspended hydration and loss of freezable water in cement pastes exposed

to 90% relative humidity

* K.A. Snyder, D.P. Bentz

Materials and Construction Research Division, Building and Fire Research Laboratory, National Institute of Standards and Technology,

Gaithersburg, MD 20899, USA

Received 12 November 2003; accepted 3 March 2004

Abstract

Degree of hydration (DOH) and differential scanning calorimetry (DSC) measurements are used to characterize the effect of early

exposure to a 90% relative humidity (RH) environment on cement paste hydration. Early exposure to a 90% RH environment can lead t9 the

consumption of freezable water and altered microstructural development. The minimum duration of 100% RHcuring required to eliminate

the effects of an unsaturated environment on microstructural development coincides with the appearance of a DSC peak near -30 .C that

occurs in the range 1-14 days for the pastes studied. The Jennings colloidal microstructural model is used to argue that the -30 .C peak

coincides with the cessation of capillary pore percolation. Alternatively, all samples cured under 100% RH conditions for 7 days prior to 90%

RH exposure hydrated at the same rate as those continuously exposed to 100% RH. The application of these results to the formulation of

separate curing practices for durability and strength is discussed.

Published by Elsevier Ltd.

Keywords; Calorimetry; Curing; Degree of Hydration (Nominated); Humidity; Microstructure

I. Introduction

Cement paste, the binding phase of concrete, is primarily

a calcium silicate hydrate (C-S-H) that develops its

interconnected solid structure from a chemical reaction that

consumes approximately 0.25 g water per g cement, with an

additional 0.15 g water per g cement incorporated into the

hydrated gel structure [1]. Although the majority of the

hydration reaction takes place during the first month, the

hydration during the first few days has the greatest effect on

the overall performance of the concrete. It is during this

initial hydration that the availability of water is critical to

proper C-S- H formation. Most concrete elements are made

with sufficient water for complete hydration and can therefore hydrate sufficiently under sealed conditions. When

evaporation from the surface occurs, however, moisture

transport through the concrete may be insufficient to replace

lost water from either the environment or the interior of the

concrete. Failure to maintain saturation at the surface can

have detrimental effects on concrete surface properties and,

possibly, on the overall concrete structural element perf ormance. Industrial curing practices incorporate a number of

methods for either maintaining a humid environment at the

concrete surface or using chemical or physical barriers to

minimize moisture loss [2].

Because proper curing practices constitute a financial

burden that is proportional to their duration, and because

curing beyond a certain period will have no effect upon

concrete performance, an optimum curing practice is

sought. The ideal curing regimen maintains saturation.

within the concrete surface for the shortest period of time,

such that additional effort would have negligible effects on

the desired concrete properties. The ACI Manual of Practice [2] recommends 7 days curing for concretes made

with Type 1 cement, regardless of the water-cement ratio.

Moreover, "Natural curing from rain, mist, high humidity,

...regarded as sufficient to provide ample curing..."

(ACI 308-92, Section 2.10) may be considered, although

sufficiently high humidity is neither defined nor a lower

limit established.

The more important question, however, is the combined

effects of curing duration and the 1evel of concrete saturation required for normal hydration with respecto saturated

conditions (100% relative humidity (RH)).. The minimum

sufficient level of saturation is not known because thc

* Corresponding author.

E-mail address: kenneth.snyder@nist.gov (K.A. Snyder).

0008-8846/$ -see front matter. Published by Elsevier LId.

doi:] 0.] 0] 6/i.cemconres.2004.03.007
2046 K.A. Snydel; D.? Bentz / Cement and Concrete Research ,j4 (2004) 2045-2056

effects of exposure to environments below 100% RH have

not been sufficiently quantified, inhibiting the development

of rational curing procedures.

A number of previous studies have attempted to quantify

the effect of environmental RH on portland cement paste

hyd:.ation. In the experiment reported by Powers [3], which

quo:ed previous results of Gause and Tucker [4], fresh

cement paste was placed into sealed glass jars. Periodically,

the .jars were opened, and the RH was measured in the air

space over the paste. The measured RH decreased monotoni~ally, terminating at a value in the range of 80-90%,

depc:nding upon the initial water-cement (w/c) mass ratio.

As c:ementpaste hydrated, the chemical reaction consumed

the available free water. When all the surface water had

disappeared, continued hydratjon reduced the RH inside the

jar. At the same time, the reaction products were filling the

initi~lIy water-filled pore space, decreasing the effective

pore diameters and reducing the RH at which the pores

cou]d still remain saturated via the Kelvin-Laplace effect

[5]. Within a sealed system, there is competition between

the decreasing RH due to water consumption and the

decrease in the equilibrium RH due to pore size red~ction.

Bas4:d on the results. reported by Powers [3], the rate of RH

redt;ctiondue to reaction was greater than the reduction in

equilibrium RH environment due to pore size reduction, and

the reaction terminated because the pores were unable to

rem,lin saturated.

1]1 a related experiment, PalTott et al. [6] reported

con1inued hydration for 0.59 w/c pastes at relative humidities as low as 55%. Their specimens were 3-mm thick

slice s cut from the middle of a prism after two days of

hyd]'ation. Coming from the middle of the prism, the

sam}les were initially able to draw water from the surrourding paste. As such, the samples represented (virtually) ~;aturated samples at the time of exposure. The degree

of hydration (DOH) was measured after 14 and 90 days of

exp(lsure to various relative humidities, and the values

wert: compared with the DOH of continuously saturated

specimens. The ratio R of the DOH for a sample hydrating

at a relative humidity Q> to the DOH of a sample hydrating

undc:r saturated conditions was approximated by a power law relationship [6] .

R= (1)

specimen to reach equilibrium with the unsaturated environment. A young specimen having large capillary pores

and exposed to an unsaturated environment could lose

sufficient free water through evaporation that the rate of

hydration at the surface could virtually cease. Conversely, a

specimen initially hydrating under saturated conditions for a

sufficiently long time would contain pores small enough

that water would not evaporate into an unsaturated environment, and the specimen could continue to hydrate in an

unsaturated environment at a Tate nearly equal to saturated

conditions. Therefore, the rate of hydration for a specimen

exposed to an unsaturated environment should depend, in

P!lrt, upon the duration of the initial period of hydration

under saturated conditions.

To test this theory, an ~xperimenta]p]an was dev~]oped

to study the hydration and microstructural development in

specimens exposed to an unsaturated environment. In the

experiment, the unsaturated environment consisted of 90%

RH air at 25 °C. A 90% RH environment was chosen

because the common assumption, based largely on Powers

[3], is that hydration continues indefinitely in a 90% RH

environment. Moreover, 90% RH could be considered as

sufficiently high humidity for use as natura] curing.

Thin cement paste specimens were initially exposed to a

]00% RH environment. After either 60r 12 h, or 3 or 7 days

of exposure to a 100% RH, each specimen was moved to a

90% RH environment. DOH measurements from these

exposed specimens, as a function of time, were compared

to with corresponding values from samples continuously

exposed to 100% RH. Differentia] scanning calorimetry

(DSC) measurements were used to detect freezable water

within a specimen asa function of time.

Arguably, a 100% RH exposure is not identical to moist

curing. A 100% RH environment can, however, supply

water to surface pores as needed, provided that the rate of

vapor transport through the atmosphere is faster than the rate

of water consumption due to hydration. Furthermore, due to

the ionic strength of the pore fluid, the pore fluid is likely in

equilibrium with a 95% to 97% RH environment. Therefore,

it is likely that the 100% RH environment will form a film

of]iquid over the surface ofthepaste~Moreover, results will

show that the rate of hydration for the specimens exposed to

100% RH was identical, within experimental error, to the

independent measurements made on cement pastes in contact with water.

A similar experi~ent with this one has been performed

by Bager [7]. In that experiment, specimens were exposed to

58% RH after water curing for 2, 3, 7, and 10 days. Bager

[7] used mercury intrusion porosimetry to characterize the

microstructure, DSC to determine the total freezab]e water,

and electrical conductivity and permeability measurements

to characterize the effect of exposure on transport. The

experiment reported herein extends the work of Bager [7]

by using the temperature-dependent DSC data to characterize the developing microstructure and the DOH to characterize the macroscopic effect~, both as a function of time. ]n

The ratio R for values of 4> less than 0.55 is, by

defillition, zero. Using Eq. (I), the rate of hydration for

expcisure to a 90% RH environment is 0.37 times that for a

continuously saturated specimen.

l nfortunately, neither experiment demonstrates the full

complexity of the effects that an unsaturated environment

« ] 00% RH) during initial hydration can have on microstructural development. The rate of hydration in an un saturatt:d environment will depend upon the age of the cement

pastc: at the' time of exposure, and the time required for the
K.A. Snydel; D.P. Bentz I Cement and Concrete Research 34 (2004) 2045-2056 2047

this way, the time-dependent effects of early exposure can

be 1;tudied in more detail.

2. I:xperiment

~[1}e molds for the cement paste specimens were made

from 1.6~mm-thick (nominally 1/16 in.) polytetrafluoroethylene (pTFE) sheets. For each mold, two pieces, each

150 x 160 mrn, were cut from thePTFE sheet. A mechanical punch was used to create twenty-three 25-mm diameter

holl~s through one of the pieces. The two pieces were then

adhered to one another using a room temperature vulcanizing (RTV) adhesive, forming a mold for 23 cement paste

dis1: specimens, each 1.6-mrn thick. A mold was assembled

for each initial 100% RH curing period, plus one mold for

the specimens remaining indefinitely im the 100% RH

environment.

Cement paste specimens having w/c mass ratios of 0.30,

0.40, and 0.50 were prepared from the Cement and Concrete

Reference Laboratory (CCRL) proficiency sample portland

cement number 116 [8] and distilled water. Table 1 shows

the cement chemical composition. For each experiment, at a

givl~n w/c, the cement and water were equilibrated at 25 °C,

then combined in a resealable plastic bag and kneaded by

hand, with the aid of a vibration table, for a period of 5 min.

The mixed paste was spread over each PTFE mold using a

small strip of PTFE as a trowel. The cast specimens were

plac:ed onto a shelf inside a sealed glass container, the

bot1om of which contained distilled water to maintain a

100% RH environment. The glass container was located in

an I~nvironmental chamber maintained at 25 °C. To minimize the effect of bleeding, the 0.5 w/c samples were

invl~rted every 15 min for the first hour, every 30 min for

the second hour, and every hour for the next 4 h.

J.~or each w/c value, four initial 100% RH curing periods

were investigated: 6 and 12 hours and 3 and 7 days. Five

PTI;E molds were filled with cement paste at the same time

and were then placed into the glass container and exposed to

100% RH at 25 °C. When a mold was removed from the

gl~:s container, the two adhered PTFE sheets were pried

apart to expose both sides of the 23 paste disks remaining in

the perforated sheet. The PTFE sheet containing the 23 paste

disks was then placed into a glove box having an atmosphere of25 °C and 90% RH air and equipped with a fan

for circulation. To prevent carbonation, the glove box was

supplied with CO2-free air at a rate of three glove box

volumes per day.

The humidity and temperature within the glove box were

maintained using electronic feedback controllers with thermocouple temperature and semiconductor humidity sensors.

The humidity sensor was standardized using a commercial

two-pressure humidity generator based upon the device

developed at NIST [9]. The electronic controllers maintained the temperature to within 0.5°C and the RH to within

0.5%. However, when the 6- and 12-h specimens were first

placed into the chamber, the RH would increase and

subsequently return to 90% RH after a few hours.

Periodically, specimens in the glove box were removed

from the PTFE mold for measurements of DOH and

freezable water content. The DOH was determined by mass

loss on ignition between 105 and 950 °C. The cement paste

disks were first ground using a mortar and pestle, and then

flushed with a volume of methyl alcohol approximately

equal to five times the sample volume. The ground and

flushed samples were placed into a 105 °C vented oven

overnight and were subsequently heated to 950 °C in a box

furnace for at least 3 h. The DOH was determined from the

ultimate loss on ignition (0.235 grn H2O/grn cement) for a

completely hydrated paste using the technique described by

Powers [10].

A qualitative measure of freezable water was made using

a differential scanning calorimeter (DSC). Samples having

masses ranging from 50 to 90 mg were taken from the cast

paste disks. A DSC scan consisted of three steps: equilibrate

the sample at a temperature of -5 °C, equilibrate the

sample at a temperature of + 5 °C, and scan from + 5 to

-60 °C at a rate of -0.5 °C/min. (Equilibrating at -5°C

ensured repeatable results.) This scan rate was chosen

because it is reasonably slow [11], and the rate of cooling

does not change the estimated volume of supercooled ice

formation [12]. In addition, this rate had the practical

advantage of allowing for multiple measurements in a single

day..

The DSC measurements were used to monitor the presence of water and not as a means of performing cryoporometry. Quantitative cryoporometry relies on the melting

scan [13], and the results can be used to estimate the pore

size distribution using the Gibbs-Thomson equation [14].

Cryoporometry studies of cement paste have revealed a

unimodal pore size distribution (a single wide "hump")

over the entire freezing temperature range [15]. By comparison, during freezing, the ice front penetrates the largest

pores first, then the progressively smaller pores as the

temperature decreases, analogous to mercury intrusion

porosimetry. Therefore, any localized peaks that may appear

during a DSC freezing scan must be due to reservoirs

composed of relatively large pores that are completely

surrounded by passages composed of smaller pores.

Tabl: I

Chemical composition of the CCRL proficiency sample portland cement

number 116
204!: K.A. Snydel; D.P. Bentz / Cement and Concrete Research 34 (2004) 2045-2056

3.llesults

~

~rhe experimental results are presented in two groups: the

DOH and the DSC data. The DOH data are discussed first

because their interpretation and relationship to concrete field

perJ:onnance are straightforward. The DSC data, although

more complex, are only analyzed qualitatively to gain

additional insight into the DOH results and to study microstructural development.

Herein, the samples are referred to by their watercement ratios (w/c) and their duration of initial exposure

to ":he 100% RH environment. For example, the 3-day

samples are those that were exposed to the 100% RH

environment for 3 days prior to 90% RH exposure. Continuous exposure to 100% RH is referred to as a saturated

water vapor exposure (SWVE). Conversely, exposure to

90~~ RH is referred to as an unsaturated water vapor

exposure (UWVE).

Fig. 2. Degree of hydration a versus time t since mixing for 0.40 wlc pastes

exposed to 90% RH after curing at 100% RH for 6 h, 12 h, 3 d, and 7 d. The

solid curve represents the degree of hydration for a continuously saturated

specimen. The horizontal dashed line labeled 20% refers to the capillary

porosity as estimated from Eq. (2). The venical dashed line labeled DSC

denotes the age at which the -30 .C peak first appears. 3.1. Degree afhydration

"[be DOH (IX) data are plotted in Figs. 1-3 for each w/c

value. In each figure, there are four sets of data

coI1:esponding to the four curing periods at 100% RH before

subsequent exposure to 90% RH. Measurements were

per[ormed in duplicate pairs, and each datum is plotted on

the figure. In many cases, the difference between the

meliSured values was so small that the symbols overlap

one another. Additionally, the uncertainty in the individual

meliSurements was sufficiently small, typically less than 2%,

tha1 the presence of error bars in the figure would have been

obscured by the datum symbol.

In addition to the measured DOH, in each figure, there

appears a solid curve, a horizontal dashed line, and a vertical

dashed line. The solid curve is the DOH for the SwVE

specimens. The curve represents a best fit using a parabolic-hyperbolic equation [16-18], and the individual data

have been omitted for clarity. This curve was corroborated

by independent experiments using calorimetry on the same

cement at the same w/c values [19,20]. The horizontal

dashed line labeled "20%" indicates the DOH at which

the estimated capillary porosity is 20%, and the capillary

pores are assumed to be no longer percolated. The criterion

of 20% capillary porosity was chosen as a compromise

tI

Fig. .). Degree of hydration a versus time t since mixing for 0.30 wlc pastes

exposed to 90% RH after curing at 100% RH for 6 h, 12 h, 3 d, and 7 d. The

~()Iid curve represents the degree of hydration for a continuously saturated

~rC( imen. and the dashed curve represents the degree of hydration for a

~~il~d ilpccimcn. The horizontal dashed line labeled 20% refers to the

Chl,illlllY ror()~ily as estimated from Eq. (2). The vertical dashed line

l..t~lcd osr: dCI!()tc!; the age at which the -30 DC peak first appears.

Fig. 3. Degree of hydration IX versus time t since mixing for 0.50 w/c pastes

exposed to 90%RH after curing at 100% RH for 6 h, 12 h, 3 d, and 7 d. The

solid curve represents the degree of hydration for a continuously saturated

specimen. The horizontal dashed line labeled 20% refers to the capillary

porosity as estimated from Eq. (2). The vertical dashed line labeled DSC

denotes the age at which the -30 °C peak Mt appears.
K.A. Snydel; D.P. Bentz I Cement and Concrete Research 34 (2004) 2045-2056 2049

hydration is less than that of the SWVE specimens. Of

all the 3-day specimens, only the 0.30 w/c paste continued to hydrate at a rate greater than a companion sealed

specimen.

3.2. DSC: 100% RH exposure

Fig. 4. Freezable water, evident as heat flow (W/kg) from the sample, as a

function of age for a 0.30 w/c cement paste cured at ) 00% RH. The data

labels indicate the age of the specimen since the time of casting. Individual

curves are offset for comparison purposes.

The DSC data for the SWVE specimens are considered

first as a basis for analyzing the specimens exposed to 90%

RH. The DSC data for the SWVE specimens are shown in

Figs. 4-6 for the three w/c values. Each figure shows a

series of curves, each with a label indicating the age of the

specimen. The curves are offset vertically from one another

and are plotted on the same ordinate scale to facilitate a

direct comparison of both the relative peak heights and the

corresponding freezing temperature.

Each figure gives a qualitative picture of microstructure

fonnation during hydration under 100% RH conditions,

indicated by the three peaks that may appear during

hydration. Fig. 4 shows that, chronologically, the first,

peak appears (during the first few hours) at ~ 20 °C

and is due to water in the capillary pores. Figures 5 and 6

reveal that the next peak forms (after I-day hydration) at

-45 °C and is due to water freezing in the pores with

entryways only few nanometers wide. The last peak forms

near -30 °C and is due to pores of an intennediate size,

which will be discussed subsequently. For this last peak, at

-30 °C, the time of appearance, the time of subsequent

disappearance, and the freezing temperature appear to be a

function of the w/c value (initial solids fraction). As

mentioned previously, because the pore size distribution

in the cement paste is unimodal [15], the presence of

distinct peaks can only be attributed to the fonnation of

reservoirs, composed of relatively large pores, completely

among published values that range from 18% to over 22%

[21-23]. The corresponding DOH (X at which the porosity (J

equals 0.20 can be estimated using an equation that is based

upon the approach of Young and Hansen [24] and modified

to account for multiple clinker phases:

1 + 1.15cx

1 +3.15(wjc) 0=1- (2)

90

25

Relative Humidity (%)

30 60 65 70 75 80 ~---n:=:~ ~ } '

20

' ;1\ ~

~

0>

~

~ 11'

~

7~

14d ~-

10

""'-

The vertical dashed line labeled "DSC" denotes the age

when the DSC ueezing peak at -30 °C fIrst appears in

SWVE samples.

In Fig. I, for the 0.30 w/c specimens, there is a second

dashed curve just below the solid curve. This second curve

is the DOH for a specimen in which the uesh paste is

immediately placed inside a sealed container with no

additional water (sealed specimen). This curve was generated from data reported elsewhere [19,20]. The curve for the

sealed specimens is not visible for the 0.40 and the 0.50 w/c

in Figs. 2 and 3 because it overlaps the solid curve. At these

larger values of w/c, the initial volume of water within the

specimen is sufficient to hydrate the specimen at a rate

indistinguishable from a saturated specimen for at least 28

days.

A~ can be seen in Figs. 1-3, for all three w/c values, the

6- and 12-h specimens continue to hydrate for the first 2 to 3

days, and then appear to almost cease hydrating. During the

initial 2- to 3-day exposure to 90% RH, the specimens

evaporate free water and come to equilibrium with the 90%

RH environment. This result is dramatic, given that a 90%

RH exposure is not typically considered severe.

The 3- and 7-day specimens exhibit different behaviors

from the 6- and 12-h specimens. For all three values of

w/c, the UWVE 7-day specimens continue to hydrate at a

rate indistinguishable uom SWVE specimen. Although

the 3-day specimens continue to hydrate, their rate of

Fig. 5. Freezable water, evident as heat flow (W/kg) from the sample, as a

function of time for a 0.40 w/c cement paste cured at 100% RH. The data

labels indicate the age of the specimen since the time of casting. Individual

curves are offset for comparison purposes.

~

10

T (oC)
2050 K.A. Snydel; D.P. Bentz / Cement and Concrete Research 3~t (2004) 2045-2056

sUITounded by passageways composed of relatively smaller

pores.

Figs. 4-6 also include estimates for the corresponding

equilibrium Fill, shown along the upper x axis. The relationship between freezing temperature and equilibrium RH

has been calculated by Fagerlund [25]. Based on the

Fagerlund [2:5] derivation, to evaporate water from pores

with a freeziIJlg peak at -30 °C, the external RH at 25 °C

would have to fall below 75% RH. This relation, however,

is only a ph:vsically motivated approximation. Moreover,

because the peaks indicate the size of the poressUITounding

the reservoirs" the RH corresponds to the equilibrium RH for

the smaller pores surrounding the reservoir and not the

equilibrium ~~ for the reservoir itself, which would be a

higher RH.

An imporomt feature of these DSC results for 100% RH

exposure spe(:imens is the time-dependent behavior of the

freezing peak at -30 °C. This peak forms after the peak at

-45 "C, indicating that the pore throats that freeze at -30

Fig. 6. Freezable water, evident as heat flow (W/kg) from the sample, as a

function of time for a 0.50 w/c cement paste cured at 100% RH. The data

labels indicate the age of the specimen since the time of casting. Individual

curves are offset for comparison purposes.

Relative Humidity (%)

30 60 65 70 75 80 85 90

, I I ~h II) I

6h.~- 6 h f

Relative Humidity (%)

30 60 65 70 75 80 85 90

, I I I I I I

12 h

25! -- 25

"'""'

/~ 

~_.-N"",-""-""""

20

'Q)

~

~15

CI

10

20

.,2:!--'

~

~15

CI

--~

1d.;..-::::---,, , 7d ~

~

"--~ 

""-~

10 14d ~ ~ ;

~.".., , ","", ",,"--'--"'~-V-~

:EI I I I .I I j I I

~60 -50 -40 -30 -20 -10

T (oC)

Relative Humidity (%)

30, ~o ~5 ~Q ~5 ~O 8,5 99

/"\ 3d

I II I j.l I I I I

iO -fiO -40 -30. ~20 -10

T (oC)

Relative Humidity (%)

30r-¥--~5 ~Q ~5~Q ~5 90

f\ 7d

25 25

~

""'--~--~-- -~

7d \.._~

r.--r--- /'\ "--- --~~- 20

-a~

~15

al10

20i

I

~ '

C) ~ I

~ 151

;:;; 101

\

...J'

~---c_~-~~ _C~CC---

'"

14d ' ;.-- ~

.-'~

""

~--~ ~

I 

14d

"'"

28d ~ ~

28d,./"""l.,.~~- ~---",,-,,--~- ~~

u ' I I I ,I ,I I

-1,0 -SO -40 -30 -20 -10

T (oC)

I 

I 1, I , I, I I

;0 .-50 -40 -30 -20 -10

T (DC)

;;E.-

}'ig.7.. Frcezable water, evident as heat flow (W/kg) from the sample, as a function of time for 0.30 w/c cement paste exposed to 90% RH after curing at 100%

RH j()r h h. 12 h, 3d, ~nd 7. d. Individual curves are offset for comparison purposes.
K.A. SnydeI;D.P. Bentz I Cement and Concrete Research 34 (2004) 2045-2056 2051

peak (coITesponding to -30 DC peak here) for 0.35 and

0.40 w/c pastes at 4 months of age. Similarly, the same peak

remained after 4 months for pastes having a w/c equal to or

greater than 0.45.

3.3. DSC: 90% RH exposure

°C are the result of hydration. Moreover, the time at which

the peak at -30 °C forms and disappears is a function of

the wlc value (initial solids fraction). The appearance of the

-30 °C peak occurs roughly near the age at which the

capillary pores are assumed to be no longer percolated

(approximately 20% capillary porosity). The disappearance

of the -30 °C peak occurs after 2 and 7 days of hydration

for the 0.30 and 0.40 wlc paste, respectively. For the 0.50 wi

c paste, the -30 °C peak formed between 7 and 14 days,

and remained beyond 28 days.

In general, these results are consistent with previous

results of Bager and Sellevold [26,27] on cement pastes

aged for 4 months prior to DSC measurements. In that

experiment, the freezing peaks located at -.25 and -45 °C

correspond to the -30 and -45 °C peaks, respectively,

seen here; the difference is attributed to the slower freezing

rate in the Bager and Sellevold [27J experiment. The Bager

and Sellevold [27] data also show an absence of a -25 °C

The DSC data for the samples exposed to 90% RH are

shown in Figs. 7-9. Each figure is composed of four

graphs, one for each initial exposure time at 100% RH prior

to exposure to 90% RH, and this age is shown at the top

center of the graph.

The 6- and 12-h data are quite similar for all three values

of w/c. The peak at -45 °C either never forms or forms

and subsequently disappears. For those specimens where the

peak does occur, the age at which it appears depends upon

the w/c value. The peak at -30 °C never forms for any 6-

or 12-h specimen, regardless of the w/cvalue.

Fig. 8. Freezable water, evident as heat flow (W/kg) from the sample, as a function of time for 0.40 w/c cement paste exposed to 90% RH after curing at ] 00%

RH for 6 h. ]2 h. 3 d. and 7 d. Iildividual curves are offset for comparison puI]Joses.
2052 K.A. Snydel; D:P. Bentz / Cement and Concrete Resean:h 34 (2004) 2045-2056

Relative Humidity (%)

65 70 75 80 85 90

II I I

60 r

12 h

~

-"

~

'Oi

~

~

Qj

~

~ "'-

-.-"-"' ~ -j 7d~r--""'-'--"'-

10

".--L I ,I ,I I I I

-60 -SO -40 -30 -20 -10

T (oC)

Relative Humidity (%)

30 60 65 70 75 80 85 90

11 I I I I I

1\

7d

~

y7d '

14d'-

I 21d

I

i--

I .

I,- -'"~ 

=E.- 10

T (oC)

Fig. 9. Freezable water, evident as heat flow (W /kg) from the sample, as a function of time for 0'.50 w/c cement paste exposed to 90% RH after curing at 100%

RH for 6 h, 12 h, 3 d, and 7 d. Individual curves are offset for comparison purposes.

effects, with the peak area decreasing noticeably for exposures below 90% RH and disappearing entirely for exposures belo.w 58% RH exposure.

Regret1rully, the mass of the specimens was not determined to a meaningful precision. Very small pieces of paste

would stic;k to the mold and falloff during handling. Minor

revisions Ito the experimental setup are needed to avoid this

problem.

4. DiscuS!iion

The re:.ults from the 90% RH exposure are sufficiently

unexpected to warrant the development of a conceptual

model (CM) to explain what could be happening within

the microstructure 10 account for the behavior. The CM

developed here is built upon the Jennings Colloidal Model

(JCM; [29]). Because the JCM is so important to the

conceptual model, it will be described briefly.

For the 0.30 and 0.40 w/c pastes, the DSC data for 3 and

7 days of exposure to 90% RH were as expected, based on

the data nom 100% RH exposure specimens. The -30 °C

peak never appears in the 3- or7-day 0.30 w/c paste because

it had already disappeared by this time in the .} 00% RH

exposure specimen. The disappearance of the -30 °Cpeak

in the 3- and 7-day 0.40 w/c pastes is consistent with the

behavior of the SWVE specimens.

The 3- and 7-day exposure data for the 0.50 w/c paste

revealed an effect of exposure on microstructural development. For the SWVEO.5 w/c specimens, the -30 °C peak

appears after 14 days of hydration. Forthe samples exposed

to 90% RH at 3 and 7 days, the peak does not appear before

the end of testing at 28 days.

Although not as dramatic, the peak at -45 °C appears to

diminish and drift to higher temperatures over time for many

of the exposed specimens. This effect is noticeable for the 6-

and 12-:h exposures and is arguable for the 3- and 7.,day

specimens. Bager and Sellevold [27] observed similar

~o~].c:.~Isl

20

~

~15

aI

10
K.A. Snydel; D.P. Bentz I Cement and Concrete Research 34 (2004) 2045-2056 2053

4.1. Jennings Colloidal Model .-

-@

Globule

,

,

I

,

,

,

,

-.

Fig. 10. Schematic of JCM. LD C-S-H is composed of globules, and the

interglobule pores freeze at -4S .C. The Inter LD C-S- H microstructure

is composed of LD C-S-H clusters, and the inter LD C-S-H pores

freeze at -30 .C.

lary spaces, and a schematic of the CM is shown in Fig.

11. Initially, the inter-LD C-S-H thickness is far smaller

than either the capillary pores or the intercapillary spaces.

As a result, for water to freeze within the capillary pores,

the advancing ice front has to penetrate only the intercapillary pore space, manifesting itself as a DSC peak at

-20°C. This situation is depicted in Fig. 11 (a).

Upon further hydration, the inter-LD C-S-H continues

to fill both the capillary pores and the intercapillary spaces.

Eventually, the intercapillary pore space becomes filled

with inter-LD C-S-H, and the capillary pores are no

longer percolated via the intercapillary pore space. For

water to freeze within the existing capillary pores, the

advancing ice front must now pass through the inter-LD

C-S-H pores. As a result, DSC data no longer exhibit a

peak at -20 °C, but at -30 °C instead. This situation is

depicted in Fig. 11 (b). Unfortunately, in this experiment,

measures were not taken to eliminate supercooled surface

water through the use of silver iodide [28]; thus, the

The behavior of both the SWVE and UWVE specimens

can be explained in a manner that is consistent with the

microstructural model for 1- to 100-nm length scales by

Jennings [29] and Jennings and Tennis [30,31]. The JCM

[29] identifies four types of pores, as shown in Table 2. Also

shown in Table 2 are the corresponding DSC peak freezing

temperatures from Valckenborg et al.. [31], Bager and Sellevold [26,27], and this work.

A schematic of the JCM is shown in Fig. 10. According

to the JCM, the basic unit ofC-S-H is a colloidal particle

having a radius on the order of 1.5 nIn. During the middle

period of hydration (4 to 24 h), these smallest units fonD

globules. These globules, in turn, condense to fOnD lowdensity (LD) C-S-H clusters having pores that are emptied

of water only below 20% RH [29]. The globules probably

fonD first and condense to fonD the LD structure during the

period between 10 and 24 h [29].

During the late period of hydration (after 24 h), collections of LD C-S-H clusters aggregate to fonD a microstructure having inter-LD cluster pores. These pores are

characterized as being full of water at 90% RH and empty

by 40% RH [29]. Therefore, the JCM is consistent with the

DSC results for SWVE specimens, assuming that the -45

°C peak corresponds to water within the LD C-.S-H pores

and the -30 °C peak corresponds to water that is only

accessible via the inter-LD pores. These freezing temperatures are denoted in the schematic in Fig. 10.

The JCM fonDS a foundation for understanding the DSC

results for the 100% RH exposure. From the JCM and the

DSC results, a self-consistent CM is developed to explain

the observations. The CM is fITst developed based on the

SWVE specimens and then used to explain the DSC results

for the UWVE specimens.

4.2. CM: 100% RH environment

Here, it is conjectured that the inter-LD C-S-H fonns

on the walls of both the capillary pores and the intercapilTable 2

Summary of pore size classes and approximate DSC pore freezing

temperature T m

(a;b) (b) (c) (d)

Fig. II. Schematic of capillary pores, each lined with Inter-LD C-S-H. (a)

Capillary pores are continuous. (b) Capillary pores are accessible only via

regions filled with Inter-LD C-S-H.

(a) Valckenborg et al. [3IJ.

(b) Jennings [29J.

(c) Bager and Sellevold [26,27J.

(d) This work.
2054 K.A. Snydel; D.P. Bentz / Cement and Concrete Research 34 (1004) 2045-2056

corresponding decrease in penI1eability would be one to

two orders of magnitude. Based on this assumption, extremely 10w-penI1eability concretes can only be achieved in

those systems for which the -30 °C peak has disappeared.

Upon studying Figs. 1-3, the disappearance of the -30

°C freezing peak appears to coincide with the age at which

the capillary porosity is approximately equal to 20%. One

could argue that the disappearance of the -30 °C peak

corresponds to a significant reduction in the critical pore

diameter dc and accounts for the dramatic change in penI1eability that Powers et al. [21] based their conclusions on. The

agreement between the age when the peak first appears and

the predicted minimum curing duration by Powers et al. [21]

may have only been coincidental, attributed to the changes in

cement production in the intervening decades. The only

defmitive way to resolve this question would be to perfonI1

simultaneous DSC and penI1eability measurements.

4.3. CM: 90% RH environment

physicar interpretation of the -20 °C peaks cannot be

stated definitively in this experiment.

As seen in Figs. 1-3, the 20% capillary porosity (based on

Eq. (2)) and the appearance of the -30 °C peak give

different estimates for the age at which the capillary pores

are no longer percolated. There are a number of explanations

for this difference. In a pixel-based model, percolation is a

well-defined quantity and has a binary value. By contrast, in a

hydrating cement paste, there is a critical pore diameter dc

[32] that, conceptually, defmes the diameter of the largest

sphere that can pass through the system. For these systems,

there will always be a critical pore diameter, even if the length

scale of dc must approach atomic dimensions; hence, there is

no definite point at which porosity is no longer percolated.

Therefore, in cementitious systems, percolation can only be

defined with respect to a given threshold pore diameter.

In addition, the penneability of the system can be

approximated by dc2 [32]. If at early ages, the criticar

capillary pore diameter has dimensions of micrometers,

th~ permeability has a value that is on the order of 10- 12

m2. When the -30 °C peak appears, the critical pore

diameter has decreased to the range 1-10 nm, and the

permeability has fallen to approximately 10 -18 m2, which

is typical for cementitious systems. This is consistent with

the dramatic change in penneability that Powers et al. [21]

originally observed and attributed to the cessation of capillary pore percolation.

There are three possible explanations for the subsequent

disappearance of the -30 °C peak: loss of water from the

capillary pores, collapse (and densification) of the phase

containing the inter-LD C-S-H pores, or the cessation of

percolation of the inter-LD C-S-H phase. The loss of water

from the capillary pores (due to hydration) is unlikely to

occur within 7 days in a SWVE 0.40 w/c paste (the age at

which the -30 °C peak disappears in this experiment).

Given that the SWVE specimens hydrated at virtually the

same rate as the saturated specimens did, water imbibition

through chemical shrinkage probably occurred at a sufficient

rate to keep the pores saturated with water. Alternatively, a

physical collapse of the inter-LD C-S-H phase would

suggest a freezing peak near -30 °c that slowly moves

toward lower temperatures, but this was not observed here.

The cessation of inter-LD C-S-H pore percolation is

plausible if continued hydration could fill in the inter-LD

pores with a regular microstructure, possibly LD C-S-H,

until the freezable capillary pore water is only accessible via

the LD C-S-H pores, similar with the argument for the

appearance of the -30 °C peak. If this were to happen,

available freezablewater would not freeze until -45 °C, as

was observed. The rate at which this OCCUJ1S would also

depend on the initial solids fraction (w/c), as was also

observed in this experiment.

The disappearance of the -30 °c peak in the SWVE

specimens suggests the critical pore diameter dc decreases

fTom the size of the inter-LD pores (1-10 nm) down to the

size of LD C-S-H pores (approximately 1.2 nm). J1he

The analysis of the UWVE specimens can now be

fonnulated in the context of the CM for SWVE specimens.

For the specimens exposed to 90% RH, the primary difference is the loss of freezable water within the pores.

The water loss was most evident in the specimens

exposed to 90% RH at a young age. For all values of

w/c, the UWVE specimens at 6 and 12 h never exhibited

a freezing peak at -30 °C. Moreover, those 6- and 12-

h specimens that fonned a peak at -45 °c eventually lost

all the water from those pores. The loss of water from

the -45 °c peak indicates that upon consumption of all

the available free water, the chemical driving force for

hydration is sufficient to extract water from the reservoirs

surrounded by LD C-S-H pores.

The 3- and the 7-day data for the 0.3 and 0.4 w/c pastes

were, in general, consistent with the data from the SWVE

specimens. Because the -30 ° C peak had disappeared

prior to 3 days for SWVE 0.3 w/c pastes, the paste was

sufficiently able to retain moisture10 continue hydrating at a

rate equal to that of a sealed specimen. As has been

observed from chemical shrinkage measurements, as impermeable as the SWVE 3-day 0.3 w/c paste is, it is still able to

absorb moisture in a 100% RH environment. This accounts

for the difference in the hydration rate between the 3- and

the 7-day 0.3 w/c pastes.

The 3-day 0.4 w/c paste, having only just fonned the

-30 °c peak, was still able to lose water to the environment (the rate of hydration was slower than that for a sealed

specimen) because a sufficient volume of capillary pores

was not surrounded by inter-LD pores. The 7-day 0.4 w/c

paste (being under similar conditions, with respect to the

-30 °c peak, as the 3-day 0.3 w/c paste) continued to

hydrate at a rate equal to a sealed specimen, which is

identical to SWVE specimen at this w/c value.

The 3- and 7-day 0.5 w/c paste data highlight different

aspects of cement hydration. For both cases, the -30 °c
K.A. Snydel; D.P. Bentz / Cement and Concrete Research 34 (2004) 2045-2056 2055

peak has not fOnI1ed at the time of exposure. Based on

previous arguments, the capillary pores are sufficiently

percolated to lose a large quantity of evaporable water.

Interestingly, both the 3- and 7-day 0.5 w/c pastes continued

hydrating at a measurable rate. Although some capillary

water was probably lost from both systems, it appears as

though that hydration continued by consuming water trapped

in the LD C-S-H pores that freeze at -45 °C (6- and 12-

h DSC data demonstrate that hydration can remove water

from these pores). In fact, the -45 °C peak heights for both

the 3- and 7~day systems appear to decrease. The fact that the

7-day system is able to hydrate at the same rate as an SWVE

system is not altogether surprising because higher w/c pastes

contain greater volumes of water in the LD C-S-H pores

that freeze at -45 °C [33].

a specimen. This recommendation would agree with ACI

308 guidelines.

If the criterion changes from observed DOH to microstructure and durability performance, the fixed 7 -day

curing may be too burdensome for lower w/c pastes and

insufficient for higher w/c pastes. Delaying the exposure to

an unsaturated environment until the -30 °C peak first

appears would ensure, at the very least, proper microstructural development. Based on these experimental

results, the minimum curing at 25 °C for the 0.30; 0.40,

and 0.50 w/c ordinary portland cement pastes are approximately at 1, 3, and 14 days of saturated curing, respectively. Remarkably, these recommendations agree with the

recommendations of Powers et al. [21] that were made

nearly 45 years ago.

4.4. Implications: Microstructure development

5. Conclusion

The effect of an unsaturated water vapor environment on,

the rate of cement paste hydration depends on the duration

of the initial hydration under saturated conditions. The

minimum duration of curing required to meet performance

objectives depends upon the specific performance criterion.

When the performance criterion is a rate of hydration equal

to that of a continuously saturated specimen, the minimum

curing time appears to be 7 days for all wlc values ranging

from 0.3 to 0.5. When the criterion is microstructural

development, the time required for the paste to develop a

sufficiently dense microstructure, to mitigate the effects of

exposure, appears to coincide with the formation of a DSC

freezing peak near -30 °C. For the 0.30, 0.40, and 0.50 wi

c pastes studied here, the -30 °C peak forms at 1, 3, and

14 days, respectively. Arguably, the disappearance of the

-30 °c peak may be a more reliable indicator of microstructural development, but further experiments are needed

to justify this. A colloidal microstructural model is used to

argue that the appearance of the -30 °C freezing peak

coincides with the time at which the capillary pores are no

longer percolated. Consistent with this hypothesis, samples

exposed to 90% RH after the appearance of the -30 °C

freezing peak continued to hydrate at a rate equal to, or

greater than, the rate of hydration under sealed conditions.

As a result, the observation of the DSC freezing peak at

-30 °c in ordinary portland cement concretes is a likely

candidate for the development of a field test for the

minimum duration of curing required to ensure that subsequent exposure to an unsaturated environment will have

little effect on the microstructural development.

The absence of the -30 DC peak on the 3- and 7-day

exposures for the 0.5 .w/c specimen raises questions

regarding microstructure formation under UWVE. Although DOH data exhibit only a minor effect due to the

exposure, the DSC data suggest that the microstructure

might be considerably different. The absence of the freezing peak at -30 DC indicates that either capillary pores

remained percolated or the capillary pores were enclosed

by inter-LD C-S-H, but no water remained within the

capillary pores. Regardless, given evidence that the pastes

hydrated at lower relative humidities exhibited higher

levels of silicate polymerization [34], that the observed

-45 9C peak height decreased, and that the amount of

bound water is proportional to the amount of free water

[26,33], it is quite likely that the final microstructure of the

0.50 w/c 3- and 7-day specimens at 28 days are different

from the corresponding saturated specimens. Evidence to

this effect for 0.4 w/c mortars can be found in the Bager

experiment [7]. Using the diffusion coefficient of 14 days

of curing as a reference, Bager [7] found that the diffusion

coefficient of specimens cured for 2 and 7 days were a

factor of 3 and 2 greater, respectively.

Moreover, simply rewetting a paste that has been exposed prior to the formation of the -30 C C peak might not

negate any detrimental effects. Based on rewetting experiments on 0.40 and 0.60 w/c pastes [27], water loss in both

the -30 and -45 DC peaks is not recovered upon rewetting,

even though the total initial evaporable water content was

nearly fully recovered upon rewetting.

4.5. Implications: Curing

The practical result of this experiment may be a means of

determining the minimum required curing duration, depending upon the performance criterion. Based solely on the

DOH data, one would conclude that the minimum duration

of curing is 7 days, regardless of w/c value, for these are the

only UWYE specimens that hydrated at a rate equivalent to

Acknowledgements

The authors wish to acknowledge the helpful comments

and suggestions by Prof. Hamlin Jennings of Northwestern

University, and for the additional comments and discussion
2056 K.A. Snydel; D;P. Bentz / Cement and Concrete Research 34 (2004) 2045-2056

by Dr. Jeffrey Bullard of the Building and Fire Research

Laboratory (NIST).

The authors also wish to acknowledge the journal

reviewers, whose thoughtful comments contributed to the

quality of the paper.

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