Reference

NSS

Pubii - cations

NBSIR 78-1495

A111DM E37SM4

Factors Affecting the Durability

of Adobe Structures

Paul Wencil Brown 1 Carl R. Robbins2 James R. Clifton 1

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

2 Center for Materials Research

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

July 1978

J repared for

’ QC ““"Mational Park Service

]_QQ Washington, D.C. 20240

. 1)56

78-1495

Jj$]Joaal Bureau of Standards APR 1? i@?|

oof [\CC •-

NBSIR 78-1495

« *

* FACTORS AFFECTING THE DURABILSTY

OF ADOBE STRUCTURES

Paul Wencii Brown 1 Carl R. Robbins2 James R. Clifton 1

’Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D C. 20234

2 Center for Materials Research

National Measurement Laboratory

National Bureau of Standards

Washington, D.C. 20234

July 1978

Prepared for National Park Service

Washington, D.C. 20240

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
.
TABLE OF CONTENTS

Page

Abstract. . . . , iv

1. Introduction 1

2. Selection of Materials 1

3. Analytical Results 2

3. 1 Escalante Ruin 2 3.1.1 General Observations and Visual Analysis 2 3.1.2 Soluble Salt Analysis 3 3.1.3 X-ray Diffraction Analysis 3 3.1.4 Particle Size Distribution Analysis 3 3.1.5 Summary 5

3.2 Fort Bowie National Historic Site 5 3.2.1 General Observations and Visual Analysis 5 3.2.2 Soluble Salt Analysis 6 3.2.3 X-ray Diffraction and Particle Size Distribution

Analyses 6 3.2.4 Summary. 8

3.3 Tumacacori National Monument 8 3.3.1 General Observations and Visual Analysis 8 3.3.2 X-ray Diffraction Analysis 9 3.3.3 Particle Size Distribution Analysis 11

3.3.4 Soluble Salt Analysis 15

3.3.5 Summary 17

4. Porosity Analyses of Tumacacori, Fort Bowie, and Escalante 17

5. Summary and Conclusions..... 19

6. References..., 21

7 . Figures 22

8. Acknowledgment 36

i

iii
ABSTRACT

*

Adobe samples from three structures of historic interest in the state

of Arizona were analyzed to determine their mineral assemblages, particle

size distributions, soluble salt contents, and porosities. These analyses

were accompanied by microscopic observations of polished sections and thin

sections. These data were correlated with the weathering observed and it was

found that soluble salt action was primarily responsible for the deterioration

of the adobe from one of the sites The nature of the particle size distribution has contributed to the rapid deterioration of the adobe from a second

site. The adobe from a third site was found to be well consolidated due to

the presence of large amounts of calcite.

IV
FACTORS AFFECTING THE DURABILITY OF ADOBE STRUCTURES

by

Paul Wencil Brown

Carl R. Robbins

James R. Clifton

1. INTRODUCTION

A large number of adobe and rammed earth structures have been built

in the southwestern United States over a period of approximately 10 centuries.

Due to the scarcity of more conventional building materials and the climatic

conditions prevalent in the region, adobe was the primary material of construction employed. Adobe is a mixture of sand, silt, and clay, however, straw is

usually added and gravel is usually present. Sand may also be added to an adobe soil to achieve the desired proportions [ 1] — . When mixed with water,

adobe can be cast into brick or block, cast in-place, or used as mortar. Adobe

exhibits excellent stability in climates of extreme dryness as is evidenced by

the antiquity of Middle Eastern adobe structures such as the Ziggurat of Agar

Quf. However, exposure of an adobe structure to moisture, whether from rain,

rising ground water or high humidity, causes deterioration generally called

weathering. The rate at which adobe weathers is affected by its mineralogical

composition, particle size distribution, porosity distribution and soluble

salt content as well as its moisture content. Major weathering processes

include erosion or leaching of the silt-clay matrix, soluble salt action,

dimensional instability associated with cyclic wetting and drying, and freezethaw damage [ 1]

.

It is the purpose of this paper to examine the factors which affect

the weathering of adobe.

2. SELECTION OF MATERIALS

Adobe material was obtained from three sites in the state of Arizona.

These sites are Tumacacori National Monument, Fort Bowie National Historic Site,

and Escalante Ruin. Tumacacori National Monument is a mission church, San Jos£

de Tumacacori, and other buildings constructed by Franciscan priests in the late

18th century. The church was abandoned in 1848 and fell into disrepair until 1921.

Since then, repair work has been essentially limited to preserving the existing

structure. Fort Bowie was a cavalry fort first constructed in 1866. A second

fort was built in 1868 and was in use for the next 16 years and then abandoned.

Escalante Ruin is the remains of a prehistoric farming community built along

the Gila River. This site was occupied for several centuries and many of the

artifacts excavated date from A . D . 1150-1450; however, portions of the site

date as early as A . D . 900.

—{lumber: in brackets refer to references at the end of this report.

1
The samples of adobe taken from Tumacacori ranged from well consolidated to severely deteriorated. A sample of soil from the site was also obtained

Three samples were taken from various locations along an interior wall of the nave of the church and showed varying degrees of deterioration. One sample,

considered to be representative of unweathered adobe, was taken from an interior

wall of the sanctuary. In addition other samples were taken from the top of an exposed wall, and from the exterior wall of the nave. The sample of adobe from

Fort Bowie was taken from a severely deteriorated corral wall. The sample from

the Escalante Ruin was taken from the wall of a dwelling unit and was well consolidated. The appearance of a typical polished section of each adobe included

in this investigation is shown in Figure 1.

3. ANALYTICAL RESULTS

3.1 Escalante Ruin

3.1.1 General Observations and Visual Analysis

Escalanate adobe is well consolidated, pinkish gray in color (Munsell

soil chart notation 7.5 yr 7/2) [2] and appears to contain a high percentage of

fine fraction relative to coarse aggregate. The latter consists primarily of

angular to subrounded quartz grains and rounded residual fragments of highly

weathered igneous rock (granites) in which the feldspar has been altered to

clays. The light color of the specimen suggested the presence of a carbonate

which was confirmed by reaction with dilute hydrochloric acid. The carbonate,

found to be calcite, is present in considerable concentration and is distributed

throughout the sample.

This adobe is predominantly fine grained and dense, with an even distri

bution of relatively large pores (1-2 mm diameter) . Subsamples of a larger

Escalante specimen showed considerable variation in their resistance to the

destructive effects of water. Although ^75% of these subsamples were quite resis

tant , the remainder dissociated readily when immersed in water, reflecting a significant variation within a relatively small sample volume.

Examination of an epoxy impregnated polished section of this adobe

in reflected white light shows an aggregate fraction ^5mm in its largest dimension (Figure 2) . This size fraction consists mainly of rounded to subangular

clear and milky quartz grains, and grains of feldspars. In the finer fractions

the angularity of the aggregate rapidly increases with decreasing aggregate

size. In addition to quartz and feldspars, amphibole, chlorite, and titanite

are observed. The calcium carbonate occurs as a finely crystalline white

material deposited between the finer grains, as a filling in the smaller pores,

and as a lining of the larger pores. It is both a cementing agent and a pigment, whose presence results in a light colored adobe.

2
A petrographic thin section, prepared after impregnating the adobe

with epoxy, was examined in polarized light [2]. The outstanding feature of

the material is its microcrystalline calcite-clay matrix which surrounds the

larger grains and serves as the cementing agent of the adobe (Figure 3) . A

variety of quartz grains are present. These range from small angular individual fragments to large aggregate material which is frequently strained, polycrystalline and rounded in habit. Both alkaline (microcline, orthoclase) and

calcic feldspars are abundant. Much of the feldspar is chemically unaltered,

but some of the plagioclase has been extensively weathered to clay. In addition,

amphibole, chlorite and the iron oxides hematite and magnetite are present in

minor amounts.

3.1.2 Soluble Salt Analysis

Qualitative elemental analysis of the soluble salt fraction was obtained

with a scanning electron microscope (SEM) equipped with an energy dispersive x-ray

analysis (EDXA) attachment. The soluble salt fraction was obtained by dispersing

the adobe in distilled water, filtering the liquid, evaporating it to dryness and

collecting the salt as the residue [2]. The elements identified by this method in

apparent order of abundance are: Ca, Cl, S, Si, and trace amounts of Mg, K, and

Al. The energy distribution spectrum of this fraction obtained by EDXA is shown

in Figure 4. This spectrum typifies the EDXA data obtained in the analyses of

soluble salts present in other samples.

3.1.3 X-ray Diffraction Analysis

A 100 gram subsample of the Escalante specimen was analyzed using

x-ray dif f ractometry in order to obtain an estimate of the relative abundance

of the various minerals present in the bulk sample[2]. The following minerals,

listed in apparent order of abundance, were identified as calcite, quartz,

microcline, plagioclase, orthoclase and illite.

3.1.4 Particle Size Distribution Analysis

The particle size distribution in a 260 gram sample was determined

using sieving and sedimentation techniques [ 2] . The soluble salt content determination was carried out on a companion sample. These data along with x-ray

diffraction analysis data for the various size fractions are reported in Table 1.

The minerals described in this table are listed in decreasing order of abundance.

Quartz, feldspars, calcite, and mica are the major components of the gravel and

sand fractions while the major components of the silt and clay size fractions

are calcite, illite, and quartz. Calcite, because of its insolubility and

cementing properties, is not sensitive to the usual dispersion techniques and

as a consequence, was observed in virtually all size fractions.

The coarse fractions (4-50 mesh) of the adobe specimen from Escalante

Ruin consist primarily of a complex of igneous mineral and rock fragments

3
Table 1 . Summary of Analysis

Escalante Adobe

of

Size Fraction

Mesh Per Ince

(U.S. Standard)

Percent

of Sample

Weight X-ray Analysis Remarks

Gravel

+ 4 4.35

% Subangular to rounded

grains of quartz, very

altered granite, 2-

feldspar biotite granite (little altered)

,

quartzite, schist

-4 +8 4.93 II II

Sand:

-8 +16 4.62

>

H II

-16 +30 4.59 Quartz, feldspars,

calcite, mica

Flakes of chemically

altered biotite mica

observed

-30 +50 6.21 n it II II

+50 +100 10.00 Quartz, feldspars,

calcite, mica (trace)

Quartz and feldspars are

the major minerals

-100 +200 11.55 Quartz, feldspars,

calcite, mica

•l II

-200 +270 11.28

' II II II II

-270 +400 6.38 Quartz, calcite,

feldspars, trace of

illite clay-mica

Quartz is the major

mineral and most

angular of the group

-400 + 20pm 13.18 Quartz, calcite, feld'

spars, illite claymica

- Quartz and calcite are

the major minerals

Silt

(-20pm +2pm) 15.37 Calcite, mixed layer

clay minerals, predominantly illite,

quartz, feldspars

Calcite/illite ratio

much higher than in

clay fraction

Clay

(<2pm) 6.08 Calcite, mixed layer

clay minerals, predominately illite,

quartz

Calcite is the major

mineral

Organic Nil

Soluble

Salts 1.6 Primarily calcium

chloride and gypsum

4
(granites) . The morphology of the grains in this fraction ranges from angular

to rounded. Quartz is prominent and occurs as rose, milky or colorless grains.

Individual fragments of chemically unaltered feldspar crystals are present as are angular fragments of a granite. Grains of a second type of granite are rounded and its feldspars are highly weathered to clays. Occasional rounded

grains of quartzite and schist are observed along with grains of calcite. Minor

minerals present are an amphibole, chlorite, weathered biotite mica, and titanite.

Below 50 mesh, the rock fragments become less prominent and from -50 to +270 mesh,

the mineral assemblage, in order of decreasing abundance, consists of quartz,

feldspars, microcrystalline calcite grains and mica. From -270 mesh to + 2um,

calcite replaces the feldspars as the second most abundant mineral and small

amounts of illite clay-mica appear in the x-ray dif fractograms . Calcite is the

major mineral in the silt fraction and is present mainly in microcrystalline form.

A mixed layer, predominantly illitic, clay is present along with quartz and feldspars. In this size fraction, only the quartz grains are angular. The minerals

observed in the clay size fraction are calcite, which is a major component, a moderate amount of a mixed layer clay showing a broad (001) x-ray diffraction

o peak at ^10.8 A, and a small amount of quartz. The clay present is predominantly

illite which is a dimensionally stable clay.

The clay-silt matrix is the bonding or cementing material of adobes.

The clay which is slowly, formed by chemical weathering, and which is present on the surfaces of all size fractions of feldspars also contributes to bonding. This

is particularly true in the clay-silt size fraction which is composed of clay particles initially present plus those on the surfaces of the tiny grains of feldspars

that are gradually being converted to clays.

3.1.5 Summary

The calcite-clay bonding matrix of Escalante adobe is its distinctive

feature. The calcite-clay ratio is not uniform in the matrix, but ranges on a micro scale from regions rich in clay to those which are nearly pure calcite.

The latter are very resistant to the eroding effects of water and account for

the variability in slaking resistance observed. The presence of calcite accounts

for both the color and the high degree of consolidation generally observed in

this adobe.

3.2 Fort Bowie National Historic Site

3.2.1 General Observations and Visual Analysis

The specimen of adobe from an exterior corral wall of Fort Bowie

National Monument analyzed is yellowish brown (10 yr. 5/4, Munsell soil chart

notation) and highly porous with many fine pores and a high percentage of large

ones ranging from 'ulmm to 3ram in diameter. Microstructural details of the adobe

are obscured by a clay-silt coating of the grains. Gross structural details

which may be observed are fragments of residual organic material such as grass

or straw and a high percentage of coarse aggregate.

5
A distinctive feature of the Fort Bowie adobe observed in' polished

section is the high ratio of aggregate to fine matrix. The aggregate consists

primarily of angular to subrounded grains of quartz and feldspars ranging from

v0.3mm to vl5mm in largest dimension. The general aspect of the material is

shown in Figure 5. Examination of a petrographic thin section in polarized light showed

large mosaic quartz grains. Large feldspar grains including microcline and

orthoclase, both unaltered and altered to clays, and plagioclase grains, both

unaltered and altered to clays, were also observed. The grains were angular

to subrounded in habit and this assemblage persisted over a wide size range.

In addition to the major mineral assemblage, minor amounts of micas, amphibole,

and hematite were observed. The dominant feature observed in thin section is

the high ratio of relatively coarse grains to clay-silt matrix, Figure 6. The

high proportion of large grains seperated and bonded by many narrow regions of

silt-clay matrix is readily observed in this figure.

3.2.2 Soluble Salt Analysis

Only a small amount of soluble salts was found in this sample. Energy

dispersive x-ray analysis was used to obtain a qualitative estimate of the elements

present in the soluble salt fraction. Elements identified by this method, in

apparent order of abundance, are: Ca, Si, K, S, Cl, and Mg.

3.2.3 X-ray Diffraction and Particle Size Distribution Analyses

A subsample of the Fort Bowie specimen was crushed, ground and thoroughly mixed for x-ray analysis of the bulk sample. The following minerals

(Listed in apparent order of abundance) were identified: quartz, plagioclase,

microcline, orthoclase, weathered muscovite, illite, magnetite and a trace of

kaolinite. Table 2 lists the particle size distribution, soluble salt and

mineralogical data for this adobe. The minerals are listed in decreasing

order of abundance. Size distribution analyses revealed a high proportion of coarse aggregate. Over 25% of this material is gravel and nearly 60% is

larger than 50 mesh.

The adobe from the Fort Bowie National Historic Site is composed primarily of fragments of igneous rocks and minerals bonded by a clay-silt matrix.

The coarse aggregate (10-15mm in its longest dimension) is angular and consists

mainly of granite fragments with weathered feldspars, quartz grains and occasional

fragments of quartzite. Amphibole and magnetite are present in minor amounts.

With decreasing grain size the rock fragments are replaced by quartz

and feldspars, and the assemblage of quartz, feldspars, and mica and magnetite

persists through several size fractions to ^20pm. At this point, illite and

Kaolinite are detected by x-ray analysis. The proportion of these minerals

increases with decreasing grain size as the clay size (<2 microns) is approached.

The clay fraction in decreasing order of abundance consists of illite-mica,

quartz, and kaolinite and is dimensionally stable.

6
Table 2 . Summary of Analysis

Fort Bowie Adobe

of

Size Fraction

Mesh Per Inch

(U.S. Standard)

Percent

of Sample

Weight X-ray Analysis Remarks

Gravel

:

+4 13.39 Large angular grains of

igneous rocks (two feldspar granites) . Some of

the feldspars are highly

altered to clays. Amphibole and magnetite present

-4 + 8 12.12 II II

Sand:

-8 + 16 11.99 II II

-16 + 30 11.06 Quartz, feldspars,

magnetite, mica

(trace)

As above with individual

quartz grains more abundant

-30 + 50 9.50 Quartz, feldspar,

mica, magnetite

As above with individual

quartz and feldspar grains

more abundant

-40 + 100 7.08 «l II Rock fragments decreasing.

Quartz and feldspar grains

predominant

-100 + 200 6.47 II II Quartz and feldspar are the

major minerals

-200 + 270 3.20 II II II II

-270 + 400 2.11 II II II II Feldspar grains are angular

to subrounded and show some degree of alteration to

clays. Quartz grains are more angular

-400 + 20ym 7.63 Quartz, feldspars,

illite-mica, kaolinite

(trace) , magnetite

(trace)

Silt

(-20um + 2ym) 8.86 Quartz, illite-mica

feldspars, kaolinite

Clay

(-2um) 6.56 Illite-mica, quartz,

kaolinite

Organic Nil

Soluble

Salts 0.03

7
3 . 2.4 Summary

The large number of pores with diameters in the range of ^1 to 3nn

(Figure 1) and the high ratio of coarse aggregate to bonding matrix (Figure 6 and Table 2) are distinctive features of the adobe from the corrals at Fort

Bowie. These factors account for the severe deterioration of this structure

over a relatively short time span. The high degree of porosity allows the

intrusion of water which reduces the shear resistance of the silt-clay matrix

to the point where it cannot resist the flow of the coarse aggregate which is

present in high proportion.

3.3 Tumacacori National Monument

The locations of the adobe samples taken from Tumacacori National

Monument, their identification codes, and their condition when sampled are indicated in Figure 7. An additional sample, T 015 , is a soil sample taken

from the N.E. corner of the grounds. Three of these specimens To^

g (interior),

To

^ 0 (exterior) and To^ (soil) were size fractionated and the size fractions were studied in detail. The remaining four samples were selected to investigate

other aspects of this adobe.

3.3.1 General Observations and Visual Analysis

A typical specimen of Tumacacori adobe is dark brown (Munsell notation

7.5 yr 4/2). As in the case of the other adobes investigated, most of the structural detail is obscured by the fine matrix which coats the larger grains. This

adobe has many fine pores. It also contains coarse pores ranging up to 4mm in

diameter. Residual fragments of organic material (straw, grass or twigs) are preserved in the finer fraction of the adobe. This adobe is in general wellconsolidated and contains a much lower proportion of coarse aggregate than the

adobe from Fort Bowie. Occasional rounded to sub rounded grains of quartz and

granites are observed but most of the aggregate is not visible. Gmail white pore

fillings of gypsum are present in several of the specimens. Calcite is. also

present.

Epoxy- impregnated polished sections of specimens To^

g

, ?o.^, To^ 7 , To^, and To^

g were prepared for examination in reflected white light at low

magnification. The general aspect of the material is shown in Figure 8 . The

large aggregate consists mainly of grains of quartz and chemically weathered

granites. The angularity of the aggregate grains increases rapidly with decreasing grain size in the finer size fractions. The white material visible

in many of the pores is gypsum.

Although gypsum was observed in all specimens, it was most abundant

in speciment To^ which was taken from a wet area of the structure where ground

water is active. Euhedral crystals of gypsum up to 3mm in their longest dimension were observed in a polished specimen of To^

.

8
These crystals were commonly interconnected by small gypsum filled

veins or channels. The general aspect of the adobe from this sampling site

may be seen in a photomicrograph of an unpolished surface, Figure 9. Examination of a petrographic thin section (To^) , which is typical of Tumacacori adobe,

shows a fabric and mineral assemblage quite different from those of Escalante and

Fort Bowie, and distinctive for this material. The coarse aggregate consists of

both rounded and angular grains of quartz, rounded fragments of quartzite, euhedral

crystals of unaltered alkaline feldspar, and angular to rounded grains of calcic

feldspars, most showing chemical alteration to clay. In addition chlorite, hematite, muscovite, altered amphibole , gypsum and titanite are present in a clay-sil

matrix. A distinctive feature of this adobe is the chemical zoning of the feldspars

which is easily observed in polarized light, Figure 10.

3.3.2 X-ray Diffraction Analysis

3ulk x-ray powder diffraction patterns were taken of samples To^q, To h'

To 12* To15 anc^ To 17* P a tterns were quite similar, differing mainly in the

relative proportions of the various minerals present. These differences, however,

are not large and can probably be attributed to the proportioning of the adobe soil

with sand during the construction of this structure. The nature of this proportioning is indicated by comparison of the particle distribution of the soil sample, To^,.,

with those of samples To^g, and To^

g which were taken from the structure. The percentage weight of the silt-clay fractions in the soil sample is 83.3 while those

of To^

Q

and To^

g are 23.9 and 18.1, respectively. This indicates that the soil to

sand ratio used in proportioning adobe for Tumacacori was about 1 to 4. In addition

the rounded aspect and the general absence of particle angularity of the grains in

the coarser sand and gravel fractions of Tumacacori adobe indicates that the sand

was subjected to abrasive action. Particles of this size tend to become rounded

through the action of running water. This suggests that this material was obtained

from a stream near the site.

The diffraction pattern of To^

2 shows the following minerals, listed in

apparent order of abundance: quartz, alkaline and calcic feldspars, muscovite,

amphibole, biotite, illite, and traces of kaolinite and gypsum. Sample To^

g is

very similar to To^

2 but contains more gypsum, sample To^ contains less muscovite,

alkali feldspar, and amphibole, and sample To^contains more alkali feldspar, less

amphilobe and a much higher percentage of gypsum. Sample To^,- (soil) contains

much less muscovite, feldspar, and gypsum than the samples from the structure but

shows a higher percentage of illite and kaolinite. In addition, the diffraction

O pattern of the soil sample shows a prominent peak at ^14. 7A. This is characteristic of the (001) reflection of the montmorillonite group of clay minerals.

The clay size fraction (<2pm) was separated from five Tumacacori specimens

by sedimentation. Both oriented and unoriented x-ray sample mounts were prepared.

All samples were glycolated to test for the characteristic swelling of minerals of

the montmorillonite group. Analytical results are given in Table 3. The minerals

are listed in decreasing order of abundance.

9
Table 3. Major Minerals in the <2ym

Fractions of Tumacacori Adobe

SPECIMEN

NUMBER

ANALYTICAL

RESULTS

To

10 Quartz, illite, feldspars

T°12 Quartz, illite, feldspars

kaolinite

T°15 Quartz, illite, montmorillonite

feldspars, kaolinite

T°17 Quartz, illite, feldspars,

mica, kaolinite

T°18 Quartz, illite, feldspars,

montmorillonite

10
3.3.3 Particle Size Distribution Analysis

Specimens To^

g

, Toie an<^ To

lS were selected for size fraction analysis.

Descriptions of the mineral and rock fragments of the various size fractions are given in Tables 4, 5 and 6. Components are listed in decreasing apparent order

of abundance. The rock fragments of soil speciment To^,-, are similar to those of

To

1q

and To lg . The organic matter observed in the adobe from Tumacacori consisted of

twigs and what appeared to be fragments of grass or straw. In all instances

there was a larger amount of organic matter present in adobe material from the

structure than from the soil sample. While this is not a conclusive indication

that it was added intentionally, this possibility cannot be excluded.

The adobe of Tumacacori is composed primarily of residual fragments

of igneous rocks (granites) and their products of chemical weathering. However,

the samples examined showed some variability on both the micro- and macro-scale.

With decreasing grain size the weathered rock fragments are gradually replaced

by individual mineral grains, primarily quartz and feldspars with minor amounts

of mica, amphibole, magnetite, titanite, gypsum and perhaps calcite (-30 +50 mesh)

At -270 +400 mesh, angular quartz is the dominant mineral and remains the major

mineral with decreasing grain size even in the clay size fraction. The feldspar

grains in the -270 +400 mesh fraction are subangular to rounded and chemically

weathered with clay crystals on their surfaces. With further decreases in particle size, the percentages of true clay minerals gradually increase.

Quartz is the dominant phase in the clay size ( <2 pm) fraction of the

five Tumacacori specimens studied (Table 3) . The mineral assemblages are all

quite similar. Traces of kaolinite, micas and montmori llonite are probably

present in all samples but in amounts beyond the limits of detection by x-ray

analysis

The montmori llonite observed in the bulk samples of To^

5 and To^g has

an (001) d-spacing of ^14. 7A which expands to ^17 A with glycolation. It is

present as a minor mineral in sample To^

g but is present in sample To^g in amounts

easily detectable by x-ray analysis, since the bulk soil sample contains a larger

clay fraction than the samples from the structure.

Clay mineralogy affects the stability of adobe because of the marked

effect of clay on the physical properties of soils and soil materials. In particular clay mineralogy affects expansion and variations in cohesiveness and

plasticity of soils as a function of moisture content. These effects result

primarily from the property of clay minerals to attract and hold water directly

on the clay-particle surfaces. In the case of the swelling clays, such as montmorillonite, incorporation of interlayer water is accompanied by a large increase

in volume. The amount of clay in Tumacacori adobe is relatively small (8-12 wt.%)

Montmori llonite was detectable in only one of the adobe specimens from the structure (To^g) and is present in this specimen in only minor amounts. It may be

concluded, therefore, that the effects of the presence of a swelling clay on the

plysical properties of Tumacacori adobe are minor.

11
Table 4. Tumacacori (To^g) Size Fraction Analysis

Size Fraction Percent

Mesh Per Inch of Sample

(U.S. Standard) Weight X-ray Analysis Remarks

Gravel

+ 1.39

-4 + 8 3.79 Angular to subrounded

grains of quartz and of

weathered granites.

Occasional rounded

quartzite grains.

Sand

-8 + 16 5.99

-16 + 30 8.57 Feldspars, quartz, Angular grains of weathered

mica, amphibole granites, many with biotite,

angular to subrounded grains

of clear and milky quartz.

-30 + 50 9.52 II II II Individual grains of feldspars and amphibole also

present. Granite fragments

decreasing in amounts.

-50 +100 8.48 Quartz, feldspars II It II

-100 +200 8.26 Quartz, feldspars,

mica, amphibole

II II II

-200 + 270 6.79 II II It II II II

-270 + 400 6.45 Quartz, feldspars. Angular quartz is the domimica, amphibole nant mineral. Feldspars and

(trace) mica decreasing.

-400 +20um 16.67 Quartz, feldspars. II II II mica, illite amphibole (trace)

Silt

(-20ym +2ym) 12.67

Clay

(<2ym) 11.24

Organic 0.18

Soluble

Salts 1.26

Quartz, feldspars,

mica, and illite

Quartz, illite,

feldspars

Twigs, grass

Primarily CaSO. *2H

2 0 and

CaCl

2 (chlorides and sulfates of Na, K, and Mg

likely)

12
Table 5. Tumacacori (To 1Q ) Size Fraction Analysis

X o

Size

Mesh

(U.S.

Fraction

Per Inch

Standard)

Percent

of Sample

Weight X-ray Analysis

Gravel

+ 4 2.25

-4 + 8 4.35

Sand

-8 +16 9.37

-16 + 30 13.05 Feldspars

,

mica

quartz

-30 + 50 12.05 Feldspars

,

mica

quartz

-50 + 100 8.89 Feldspars

,

mica

quartz

-100 + 200 8.40 Feldspars, quartz

mica, amphibole

-200 + 270 5.91 II II II

-270 +400 4.86 II II II

-400 + 20ym 11.88 Quartz, feldspars

mica, illite, amphibole

Remarks

Rounded to subangular

grains of quartz and

weathered granites.

Occasional rounded

grains of quartzite.

II II II

Quartz grains increasing

relative to rock fragments

Individual feldspar

grains and mica flakes,

angular quartz. Rock

fragments decreasing

II II

Angular quartz is the

dominant mineral

II II II

Silt

(-20um + 2ym) 9.07 Quartz, feldspars,

mica, illite, amphibole

II

Clay

(<2ym) 9.02 Quartz, illite, feldspars, montmorillonite

Organic 0.21 Twigs and grasses

Soluble

Salts 0.37

13
Table 6. Tumacacori Soil Sample (T0 -^) Size Fraction Analysis

Size Fraction Percent

Mesh Per Inch of Sample

(U.S. Standard) Weight X-ray Analysis

Gravel

+ 4 0.18

-4 + 8 0.97

Sand

-8 +16 0.85

-16 + 30 1.06 Feldspars

,

quartz, mica

-30 + 50 1.64 II II II

-50 +100 1.58 II II II

-100 +200 1.68 Feldspars

,

amphibole

quartz, mica

-200 +270 1.59 II II II

-270 + 400 1.10 II II II

-400 + 20ym 5.89 II II II

Silt

(20ym + 2ym) 37.74 Quartz, feldspars, mica,

illite (trace) , montmorillonite (trace)

Clay

(<2ym) 45.54 Quartz, illite, montmotillonite, feldspars, kaolinite

mica

Organic Nil

Soluble

Salts 0.17

14
3.3.4 Soluble Salt Analyses

Soluble salt fractions from four Tumacacori specimens (To^q, To.^,

To^ and To^g) were separated for qualitative elemental analysis by energy

dispersive x-ray analysis. Analytical results are given in Table 7. The

elements are listed in decreasing order of elemental abundance.

Table 7. Elemental Analysis of

in Tumacacori Adobe

Soluble Salts

Specimen

Number

Soluble Salt

Content as a Percentage of

Sample Weight Analytical Results*

To

10

1.26 S, Ca, Cl, K, Si and trace

amounts of Mg, Al, and Na

Ton 8.78** Ca, Cl, X, S, Mg

To15 0.17 K, Cl, S, Ca, Si, Mg, and

trace amounts of Na, Al , and P

To

16

5.78 Ca, K, Mg, Cl, S and a trace

amount of Si

To

18

0.37 Not analyzed

* Based on three analyses of each specimen.

** As a percent by weight of the -50 mesh fraction

A quantitative analysis of the composition of the salts obtained from

Specimen To^ was carried out to determine the relative amounts of the major

species present. This was accomplished using ion-selective electrodes in com- bination with gravimetric techniques. The results of this analysis are given

in Table 8.

Although the relative amounts of each differed, calcium, sulfur, potassium, and chlorine were observed in all salts, no attempt was made to identify

the compounds present by x-ray analysis. However, CaS0

4 *2H2 0 and CaCl^ were

identified in specimen To^ by SEM-EDXA. The hygroscopic nature of the salts was examined by measuring their weight gains on equilibration with air at 23°C

and 35%r.h. after drying to constant weight at 105°C. The salts from sample

To^

0 exhibited a weight gain of about 21% while those from To.^ and To^g gained

59% and 6 3%, respectively. Sufficient water was taken up by the salts from To^

and To^g to cause them to appear as viscous fluids as illustrated in Figure 11.

15
Table 8. Analysis of Salts Extracted from

Specimen To^

Chemical Weight

Species Percent

1\ 19.5

Na

+ 0.6 ++ . ++

Mg and Ca 16.0

Cl" 12.4

SO

=

4

jt H

2 0 (as water of hydrationr

20.2

31.3

/ Includes weights of species present in trace amounts.

Gypsum was observed in all of the Tumacacori adobe samples. Small

white particles or pore fillings in sample To^, believed to be gypsum, but

too fine to be separated were examined by energy dispersive x-ray analysis.

An SEM Ca~S map confirmed the presence of gypsum. Figure 12 shows the distribution of these fine particles at 10X magnification. When the gypsum just

fills the pores and channels of the adobe, it may leave the matrix undisturbed

and in fact have a cementing effect. This was observed in To^g which was very

well consolidated in spite of being wet. However, when the gypsum crystals

greatly exceed the size of the pores, such as in sample To^, they displace

the clay-silt matrix in their immediate vicinity and may cause a disruption

of the matrix as stresses produced by crystal growth are relieved. Cyclic

wetting and drying exacerbate this effect through the dissolution and regrowth

of these crystals.

The effects of freeze-thaw damage may also be more severe in the

presence of soluble salts. The hygroscopic nature of salts such as MgCl

2

, MgSO^, or CaCl

2 causes the retention of water within the structure. If this

water freezes, disrupture effects can occur dur to ice crystallization. In

addition, the freezing of water facilitates salt crystallization. As a result,

cyclic dissolution and regrowth of salt crystals can occur with changes in

temperature as well as with changes in the absolute moisture content within

an adobe structure.

The soil sample, To^g, and sample To^g were found to contain 0.17

and 0.37 wt% soluble salts, respectively, while samples To^g, To ll'

To

16

contained much higher salt concentrations (see Table 7) . This salt buildup

is indicative of the migration of ground water into parts of the structure.

Elemental analysis of the salt compositions indicated the presence of gypsum.

16
This was not unexpected since the nave walls were originally covered with a gypsum wash. However, the high concentrations of gypsum observed in the bulk

material indicate the movement of water within the body of the structure.

Analyses also revealed the presence of alkalies and chloride ions indicating

that rising ground water is carrying these ions into the structure. As this

water evaporates, the salts are left behind, either at the surface (efflorescence) or immediately below it (subflorescence) . Their presence ultimately

leads to severe disruption of the adobe matrix either through crystal growth,

as in the case with gypsum, or due to their deliquescence, as in the case with

calcium or magnesium chlorides, or both. Both effects occur in certain portions

of Tumacacori and appear to be the major cause of deterioration of portions of

the interior nave wall.

3.3.4 Summary

The degree of consolidation observed in the various adobe samples from

Tumacacori correlated with the concentrations of soluble salts present. Samples

taken from a number of locations in the structure were relatively uniform in

mineralogical composition and particle size distribution. The soil sample,

however, was found to be high in silt and clay, suggesting that the coarser

fractions were intentionally added to achieve the desired proportioning. The

distinctive feature of Tumacacori adobe is the chemical zoning of the feldspars.

4. POROSITY ANALYSES OF TUMACACORI, FORT BOWIE, AND ESCALANTE RUIN

The weathering of adobe is closely related to the effects of moisture;

therefore, the durability of an adobe may be expected to be closely related to

its porosity. As a consequence, density and porosity measurements were carried

out on selected adobe samples. In addition the particle size distributions in

the silt and clay-size fractions were analyzed. Densities were determined by

helium pycnometry while porosities were determined using mercury intrusion

porosimetry. The density and porosity data are summarized in Table 9, and the

pore size distributions are shown in Figure 13. In this figure the percentage

of total porosity is plotted as a function of pore diameter. The porosity data

presented in Table 9 and Figure 13 are confined to pores with diameters between

177 and 0.0035pm. The nature of the distributions of large diameter pores (voids)

in these specimens is mentioned in the preceeding sections.

17
Table 9. Densities and Porosities of Selected Adobe Samples

Sample

Designation

Density

grams /cc

Net Pore

Volume

cc/grams

Average

Pore Diameter

yrrt

Measured

Total Porosity*

o

To

10

2.56 0.233 4.2 37.4

T°ll 2.56 0.213 14.0 34.5

To

12

2.60 0.199 2.5 33.9

To17 2.46 0.318 4.8 43.0

Fort Bowie 2.68 0.150 3.4 27.9

Escalante 2.80 0.227 2.8 38.2

*Size range 177 - 0.0035ym

The densities of the adobes listed in Table 9 are somewhat variable,

however, densities of the adobe samples from Tumacacori do not vary appreciably,

differing at most by 5.7%. Similarly, the average pore diameters observed in

these samples does not vary greatly, with the exception of To^. A larger average

pore diameter in To^ may be attributable to the disruptive affects of salt

action. Cyclic dissolution and recrystallization of salt and water in the pores

appears to alter the internal porosity distribution; total measured porosity

does not appear to increase and may, in fact, decrease dur to space filling by

salt crystals.

The total measured porosities of the adobes from the three sites cover

a rather broad range from about 28% to 43%. There is not a direct correlation

with the degree of weathering. For example, the Fort Bowie corral is severely

weathered yet this sample shows the smallest total measured porosity. However,

a comparison of the porosities of samples from Tumacacori may be qualitatively

correlated. Sample To^q, from the top of an exposed wall, and To^, from an exterior location, show the highest measured porosity, yet the porosity from

To^

0 which has been subjected to more severe weathering conditions (See Table

10) is lower. It should be pointed out, however, that variations in the original

proportioning of these adobes along with the amount of water used would have

a significant effect on their total porosities.

The particle size distributions of the samples listed in Table 9 were determined using a sedimentation technique. These analyses were limited

to particles which passed a 75 urn sieve and were carried out to examine the

size distribution within the silt and clay size fractions. Table 10 summarizes

the data shown in Figure 14. Figure 14 plots the cumulative mass percent as a function of particle diameter and shows the relative proportions of silt and

18
clay in addition to the distributions of particle sizes. Reference to the

samples from Tumacacori indicates that To 1Q and To^ are deficient in clay

when compared with To^ an^ To^

7

, but contain relatively larger amounts of

silt and fine sand (Table 10) . Figure 14 shows samples To^q and To^ to be

devoid of clay particles with diameters below about 1.1pm. This indicates

that the weathering of these samples is primarily reflected in the loss of

fine clay particles, while their relatively high silt contents may indicate

that particles in this size range are less sensitive to the action of water.

Table 10. Particle Size Distribution by Sedimentation

Sample

Designation

% Sand

-75 +20pm

% Silt

-20pm +2pm

% Clay

-2 pm

To

10

22 71 7

T°ll 10 78 12

T°12 14 61 25

T°17 6 66.5 27.5

Fort Bowie 17 61 22

Escalante 18 55 27

5. SUMMARY AND CONCLUSIONS

Adobe from Escalante Ruin was found to be well consolidated in spite

of its high soluble salt content. The high degree of consolidation along

with the light color of this material is attributable to the presence of

massive amounts of calcite which behaves both as a cementing agent and a pigment.

The adobe from Fort Bowie National Historic Site has a low soluble

salt content but exhibits poor resistance to weathering. This was found

to be the result of the particle size distribution since this adobe contains a large fraction of gravel and coarse sand but is deficient in the fine, bonding

fractions

.

The adobe from Tumacacori National Monument showed varying degrees

of consolidation. A series of samples from an interior wall showed variable

consolidation which was dependent on their soluble salt contents. The presence

of soluble salts is indicative of rising ground water and those samples with

high salt concentrations showed signs of significant deterioration. Other

samples in which less salt was present showed a high degree of consolidation.

One sample from an exterior wall was well consolidated in spite of the fact

that the action of rain had leached out a significant amount of its fine clay

fraction.

19
An analysis of the salts present in one deteriorated sample revealed

the presence of chlorides and sulfates of calcium and magnesium; of these MgCl

2

, MgSO^ and CaCl

2 are hygroscopic salts. The tendency of the salts to retain

water leads to deterioration through several processes including cyclic wetting

and drying, cyclic salt recrystallization, and freeze-thaw damage.

A comparison of the particle size distribution of a soil sample from

the Tumacacori site with those of several samples from the structure suggested

that the adobe was deliberately proportioned for use in construction. The church

at Tumacacori was built with adobe apparently composed of 1 part of soil diluted

with about 4 parts of sand.

Based on the analyses carried out on the adobe from the three sites

investigated, it was possible to correlate the type and severity of weathering

observed with compositional, particle size distribution, porosity, and soluble

salt analyses. Analyses of the mineralogical aspects of the samples studied

allowed an assessment of their variability while indicating that the deterioration observed in these samples did not result from the presence of a swelling clay.

The combination of these analyses also provides a basis for the selection of

adobe repair material which would be representative of that originally used.

20
6 . REFERENCES

1. J. R. Clifton, Preservation of Historic Adobe Structures - A

Status Report, NBS Tech. Note 934, U.S. GPO (1977).

2. J. R. Clifton, P. W. Brown, and C. R. Robbins, Methods for

Characterizing the Physicochemical Properties of Adobe Build

ing Materials, NBS Tech. Note. In press.

21
22

Figure

1.

Epoxy-impregnated

polished

sections

of

adobe

samples

from

Tumacacorl

,

Fort

Bowie,

and

Escalante.

The

general

aspects

of

color

and

void

and

aggregate

distribution

may

be

observed
Figure 2. Epoxy-impregnated polished section of adobe from Escalante showing

the aggregate distribution in the fine matrix. The light color of

the matrix is due to the presence of calcite. 10X magnification.

23
Figure 3. Thin section of adobe from Escalante viewed in transmitted polarized

light showing coarse quartz and feldspar grains in their characteristic

calcite-clay matrix. 100X magnification

24
25

Figure

4.

Energy

distribution

spectrum

of

the

soluble

salts

extracted

from

Escalante

adobe.

Ca

>

Cl

>

S.

Trace

amounts

of

Mg,

K,

and

Al

ar<

also

observed.
Figure 5. Epoxy-impregnated polished section of adobe from Fort Bowie showing

the distribution of aggregate in the matrix. 1 OX magnification.

The size of the aggregate in this adobe as compared to that in Escalante (Figure 2) or Tumacacori (Figure 8) is noteworthy.

26
Figure 6. Thin section of adobe from Fort Bowie viewed in transmitted polarized

light showing large aggregate grains separated by narrow regions of

silt-clay matrix. 50X magnification. This feature is distinctive of

this adobe.

27
28

i

National

Monument

showing

the

sampling

locations
Figure 8. Epoxy-impregnated polished section of adobe from Tumacacori showing

the distribution of aggregate in the fine matrix. 10X magnification.

29
Figure 9. Epoxy-impregnated fracture surface of adobe from Tumacacori showing

deposits of gypsum (white) filling the pores. 10X magnification.

Sampl e To-j

^

.

30
Figure 10, Thin section of adobe from Tumacacori showing a chemically zoned

feldspar crystal in the silt-clay matrix. Such zoning is common in the detrital feldspars of this locality, iransmitted polarize

light. 100X magnification.

31
Figure 11. Soluble salts extracted from Tumacacori specimen To-j,. This salt

sample was dryed at 105°C to constant weight and then exposed to laboratory air (23°C, 35% r.h.) for 30 minutes. 65X magnification.

Transmitted polarized light.

32
Fiaure 12 An energy dispersive x-ray map of the calcium and sulfur distnbu

Figure 12. speclme„ To17

. This shows the distribution of

the gypsum particles (white areas) in this specimen. g

cation

33
001

(N 05 P i

1 P C 05 0 O S tr

Eh

a o p

£l 3

C5 05 P P £5 i—1 P 05 05 0 •p p

Eh P 05 O H+J

Oh i

id) o O g i—1 C/5 Eh fd 0 d) P

&H

p d) 'd

Qj ip

05 gaoi

<u

n

g P 1—1 05 Id 0 a 05 a g a> 3 •p -p p

05 S P o OO0H

•H OQ

O

p 3 >i 0 p) U P o p id p 3 O g 01 o fa 3 0 3 Eh P g 35 0 3 c d) a

gH 3 £5

Eh P c d) dJ *H P> £1 C • tn

05 3 g -P c p a £d 0 id m

•H

O

n >1

-p 05

O P 3 W o pi £3 • C H dj c id P £3 O P POP 05 P P P C P • 35 P r- C g

r-~ iji 3.

!>i ^ »H *H

p

+j

C 05 •H id g

p

05

O (0 3 0 O' P O p pp d) £3 0 o > 3 a Eh 05 fO CD £d C

CD TO

N 3

£5C P 0 £5

Eh Id 05 P P

ro

r—1

CD

S-i 3

Cn

fa

34
r>4

iNSOMSd SSVW SAlivmwnO

ac

UJ'

Ld5<o

UJ

_Jo

)—Qr <

0. P0O

Id >1 u in

QJ IM fl

H

C £ 0 6 U

<d 4-> 3 X U1 H 0) P G C C

•H

O -H

p -H • 4h urn ^ c

rH

C 3 <U >4

h <n x> Q) -H

<d -h

w a) 6 i" P £

«J ,H4-J a) -P O BIX!

01 E"H *H c P >i-H

cu 'O <n -p c <u g -p

0) id > C g -r-i ui a) m • P (D-rl *H CM in *h u

rO rH i—i o *H O O -H 4-1

xt, e-h

p -p <d

4-1 P T3

•r4 ~ <U (0 5 hx a a) rH E-i -Q mo a>

d)H no i—I • 'H 4-1 O ' 01 01 •h o a) c

4-1 rH i—I —- 15 p o a e o

useh e ax

Ch <n cm oi

C0 C0 I 4-1 QJ '—a) OHO) P

C4-H id 5 <d O rH rH

01 OQ

O rHO

•H 4J

r

O Eh P P C O O <d T) O C

fd id O T3 g C a. o

fd CM rH + O

0)O Eh

4-> CM c i o)

id

—' 0)

0)co

•H

4-1 3

XI

•rHp

4-1 cn <d H

TJ

Q)N

•H

01

g Id 41 au

0) 3 OH

g

d & in oi -h id P

EH

w 01 01 44 0p3

Cn •H

(p

35
8 , ACKNOWLEDGMENT

The authors wish to acknowledge the financial support of the Western

Archeological Center of the National Park Service. In particular they wish

to express their sincere thanks to George Cattanach, Dennis Fenn and Anthony

Crosby of the National Park Service for their technical and administrative

assistance.
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 78-14952. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

FACTORS AFFECTING THE DURABILITY OF ADOBE STRUCTURES

5. Publication Date July 1978

6. Performing Organization Code

7. AUTHOR(S) Paul Wencil Brown, Carl R. Robbins, James R. Clifton 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Park Service

Washington, DC 20234

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Adobe samples from three sites of historic interest in the State of Arizona were

analyzed to determine their mineral assemblages, particle size distributions, soluble

salt contents, and porosities. These analyses were accompanied by microscopic

observations of polished sections and thin sections. These data were correlated

with the weathering observed and it was found that soluble salt action was responsible

for the deterioration of the adobe from one of the sites. The nature of the particle

size distribution has resulted in the rapid deterioration of the adobe from a second

site. The adobe from a third site was found to be well consolidated due to the

presence of large amounts of calcite.

. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

] name; separated by semicolons)

Adobe; clay; particle size distribution; soluble salt analysis; weathering;

x-ray analysis.

18. AVAILABILITY kvl Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

1

~

! For Official Distribution. Do Not Release to NTIS

UNCL ASSIFIED

40

1 1 Order From Sup. of Doc., U.S. Government Printing Office Washington, D.C. 20402. SD Cat. No. 0520. SECURITY CLASS 22. Price (THIS PAGE)

1*1 Order From National Technical Information Service (NTIS) $4.50

Springfield, Virginia 22151 UNCLASSIFIED

USCOMM-DC 29042- P 7 4
.

2, fp(

;• Vi

;W

'_0 0\