Chemical interactions at the concrete/clay interface due to thaumasite form of sulfate attack

Proceedings of the Institution of Civil Engineers

Geotechnical Engineering 166 August 2013 Issue GE4

Pages 408–414 http://dx.doi.org/10.1680/geng.12.00010

Paper 1200010

Received 01/02/2012 Accepted 18/10/2012

Published online 22/02/2013

Keywords: foundations/piles & piling

ICE Publishing: All rights reserved

Geotechnical EngineeringVolume 166 Issue GE4

Chemical interactions at the concrete/clayinterface due to thaumasite form of sulfateattackBrueckner, Williamson and Clark

Chemical interactions at theconcrete/clay interface due tothaumasite form of sulfate attackRene Brueckner Dipl-Ing, PhD, CEng, MICoorMaterials/Corrosion Engineer, Mott MacDonald, Altrincham, UK

Sarah J. Williamson BEng(Hons), PhD, CEng, MIStructEStructural Engineering Leader, Laing O’Rourke, Dartford, UK

Les A. Clark OBE, FREng, BEng, PhD, CEng, FIStructE, FICEEmeritus Professor of Structural Engineering, The University ofBirmingham, Birmingham, UK

The stability of structures is determined by physical interactions between the foundations and the surrounding soil.

Concrete foundations are also subject to chemical interactions at the concrete/clay interface, which can result in

changes in the properties of the adjacent soil. The occurrence of the thaumasite form of sulfate attack particularly

affects the skin friction between the concrete and the ground. In this investigation of the chemical interactions at the

concrete/clay interface it was found that although an increase in moisture content and pH was confirmed, and this

moisture and pH gradient was also observed within the thaumasite layer, no further effects on the adjacent clay due

to thaumasite form of sulfate attack were found.

1. IntroductionBuried concrete foundations need to transfer the load of the

superstructure to the underlying soil formation without over-

stressing the latter (Cernica, 1995). Beside this, the performance

of other structures such as retaining walls depends on interfacial

interactions that are determined by both physical and chemical

effects. In the event of thaumasite sulfate attack (TSA) of buried

concrete changes in skin friction occur, as discussed by Brueck-

ner (2008) and Brueckner et al. (2012). The effect of TSA on

skin friction is generally positive, and therefore it was concluded

that substructures built without consideration of the effects of

TSA should be stable. TSA targets the calcium silicate hydrates

(C-S-H), which are the main binding agent in all Portland-

cement-based binders. Whereas the consumption of calcium

aluminate hydrates (C-A-H) during conventional sulfate attack

causes expansion and, ultimately, cracking of the cement matrix,

TSA leads to a transformation of the cement matrix into an

uncohesive mass from the surface inwards.

Beside the physical interactions, which are mainly responsible for

the stability, there are also chemical interactions between soil and

concrete piles/foundations that occur at the interface, or reach

within the clay. The presence of calcium and hydroxyl ions in the

pore water of the soil due to leaching from the cement paste

causes a sequence of reactions that vary with soil composition,

mineralogy and pore water chemistry, and according to Webster

and Sheary (1962) result in the following changes of the soil

j reduction of the plasticity index

j reduction of volume change

j flocculation of clay particles to make soils more friable

j increase in optimum moisture content, allowing compaction

under wetter conditions (soils dry out more rapidly)

j some increase in strength and stability.

Furthermore, the presence of hydroxyl ions (OH�) increases the

pH of the soil, which favours a solidification process as a result

of pozzolanic reactions in which, according to Sherwood (1993),

dissolution of silicon and aluminium from the clay takes place.

The dissolved components react with the calcium ions present in

the pore water and form C-S-H and C-A-H. According to El-

Rawi and Awad (1981), these reactions result in a significant

long-term increase in shear strength, and may often be combined

with a reduction of permeability. This feature has made the use

of lime and cement successful in the improvement of clay

properties, such as stabilisation.

Milititsky et al. (1982) showed, in their investigations on soil–

structure interactions, an increase in moisture content of the clay

of up to 4% after 7 days. However, the moisture content increase

falls rapidly away from the interface, as observed by Milititsky et

al. (1982), Chuang and Reese (1969), O’Neill and Reese (1970)

and Yong (1979). Lee (2001) investigated the interaction over a

period of up to 10 months, and observed that the increase in

moisture content decreases with time. The same trend was ob-

served for pH, which reached the natural soil value at a distance of

approximately 25 mm from the interface. This was similar to the

distance where the moisture increase was reduced to zero. The

influence of calcium ions was observed in increased concentrations

in the surrounding clay up to a distance of 75 mm over a period of

10 months, and this was reflected in the plasticity of the clay.

Hill et al. (2003) have started measurements on changes in pH

and chemical composition of the clay surrounding a pile which is

affected by sulfate attack; however, final data have not yet been

published.

408

After the TSA-affected bridge foundations along the M5 motor-

way in Gloucestershire were discovered in 1998, a part of the

subsequent comprehensive investigation included the effects of

TSA on the interface interactions, which were discussed by Slater

et al. (2003). Significant relationships between chemical, miner-

alogical or physical soil parameters and distance from the

concrete or degree of thaumasite attack were not found. However,

several trends were identified. Those relating to pure soil–

structure interactions were

j increase of moisture content towards the concrete

j increase of pH value towards the concrete

j increase of calcium concentration, derived from the leaching

of the calcium hydroxide from the concrete pore water

towards the concrete.

On the other hand, trends were observed in the clay adjacent to

the structure, which related to factors and features of the

thaumasite form of sulfate attack, as follows.

j Water-soluble magnesium decreased closer to the concrete,

and had an inverse relationship with pH.

j Pyrite and indirect sulfide concentration decreased with

increasing attack – that is, most thaumasite attack occurred

where there was most pyrite oxidation.

j Sulfates and total sulfur increased with increasing attack.

j Gypsum values were highest where there was partial attack,

and were depleted where there was full attack.

j The pH value and water-soluble magnesium increased and

decreased respectively with increasing attack.

The trends developed in relation to the availability of the

transport medium: that is, they are dependent on the groundwater

level. No attack was found above the maximum water level, and

full attack was encountered below the minimum water level that

corresponded to permanently wet conditions.

2. Methodology

2.1 General

The chemical interactions at the concrete/clay interface were

determined alongside the investigation of the effects of TSA on

skin friction at the concrete/soil interface reported by Brueckner

et al. (2012) using the same specimens.

2.2 Specimens

The investigation was carried out on specimens simulating a

concrete pile/soil interface at three different depths: 0.5 m, 2.0 m

and 3.5 m. The depth of clay on top of the concrete was 100 mm,

on which a reservoir of a 1.8% sulfate solution was maintained to

accelerate the thaumasite formation at the interface.

Two types of clay were used: a sulfate-containing illitic Lower

Lias Clay (LLC) and an inert kaolinitic English China Clay

(ECC). Three different concrete mixes with varying cement

content (c) and water–cement (w/c) ratio were used in the

investigation, and two of these (mix 1, c ¼ 290 kg/m3, w/c ¼0.75; mix 2, c ¼ 320 kg/m3, w/c ¼ 0.55) correspond to mixes

used by the Building Research Establishment (BRE) in a field

trial as described by Crammond and Halliwell (1998) and

Crammond et al. (2003). Mix 3 consists of 320 kg/m3 cement

with w/c ¼ 0.75. According to BRE Special Digest 1 (BRE,

2005) the design chemical (DC) class of mixes 1 and 3 is DC-1

and of mix 2 is DC-2. The cement type used was a Portland

cement CEM I 42.5 N with a C3A content of 8%. The mix

parameters were chosen to accelerate TSA at the interface.

To simulate underground conditions three different pressures (10,

40, 70 kN/m2) were applied to the interface. Two different casting

faces – top-cast and bottom-cast face – were in contact with the

clay. The top-cast surface represents precast (PC) conditions, and

the bottom cast face represents cast-in situ (CS) conditions,

simulating precast and cast in place piles respectively. In the case

of the PC condition the concrete was cast in the moulds, cured

and, after a curing time of 28 days, backfilled with the weathered

clay, whereas in the CS condition concrete was cast directly

against the clay. The specimens were stored in an environmental

chamber at a constant temperature of 68C for up to 27 months.

TSA-unaffected control specimens were tested at an age of

6 months. The variables investigated are summarised in Table 1.

2.3 Testing

The water content, pH value and mineralogical composition were

determined at distances of 0–10 mm, 10–20 mm, 20–30 mm,

30–40 mm and 40–50 mm from the interface. The water content

and pH value were measured according to BS 1377-3 (BSI, 1990).

The mineralogical composition of the clay samples was deter-

mined using the Siemens D5000 powder X-ray diffractometer

with a silicon powder standard. The main purpose of these tests

Concrete mix Applied pressure: kN/m2 Method of casting Clay type Storage solution

10 40 70 Precast In situ LLC ECC Sulfate Water

1 (290–0.75) 3 3 3 3 3 3 3 3 3

2 (320–0.55) 3 3 3 3 3 3 3 3

3 (320–0.75) 3 3 3 3 3 3 3

Table 1. Variables investigated

409


Chemical interactions at the concrete/clayinterface due to thaumasite form ofsulfate attackBrueckner, Williamson and Clark

was to identify the formation of brucite and thaumasite; changes

in the clay structure did not form part of the testing.

3. Chemical interactions

3.1 Macroscopical observations

Different macroscopic clay structures were observed during

sampling. The first approximately 10 mm of clay showed a friable

structure that was mostly separated by cracks from the main body

(Figures 1 and 2). This zone also appeared drier than the samples

further into the clay, but the moisture content was actually higher.

3.2 Water content

The typical moisture content distribution of the clay adjacent to

the interface is illustrated in Figure 3, where the figure for 5 mm

relates to the 0–10 mm zone of clay. The moisture content at the

interface was significantly increased compared with the relatively consistent moisture content measured away from the surface.

Slight differences in moisture content, caused by variation in

compaction, occurred; however, a direct relationship due to this

or the casting position was not clearly defined.

A slight difference in the interface moisture, depending on the

age of the specimens, was observed in the control specimens. The

interface moisture content of the 6 month old control specimens

ranged from 35% to 44% (average 39%), compared with the

average interface moisture content of 32.5% for the 27 month old

specimens. This confirms the observations made by Lee (2001)

that the interface water content decreases slightly with time.

In the shear strength tests, shearing occurred either at the zone of

clay with increased moisture content or directly at the interface,

which demonstrates the tendency of the failure surface to occur

in the higher-moisture-content, lower-strength clay.

3.3 pH values

The pH value was measured at the same distance from the

interface as the moisture content. It ranged from 9.3 to 11.2 and

from 10.3 to 11.1 at the interface for kaolinitic ECC and illitic

LLC respectively, and was found to vary with distance from the

interface as well as with storage age.

The pH distribution within LLC adjacent to the interface is

illustrated in Figure 4, which was typical for all the specimens

investigated, and no significant differences between the casting

positions occurred. The pH at the interface was high, and reduced

linearly inwards; however, the rate of reduction depended on the

age of the specimen. This is demonstrated in Figures 5 and 6 by

the significantly different pH-affected area observed for the

control specimens (at an age of 6 months) compared with the

18 month old specimens. The original natural pH of the LLC was

7.7 and that of the ECC 5.4.

The reduction of the pH to the natural soil value did not coincide

with the position at which the moisture content attained a

constant value, as was observed by Lee (2001). Both the moisture

Figure 1. Clay structure at clay/concrete interface, LLC

Figure 2. Clay structure at clay/concrete interface, ECC

20

25

30

35

40

45

50

0 10 20 30 40 50

Moi

stur

e co

nten

t: %

Distance from interface: mm

PC 10 kPaPC 40 kPaPC 70 kPaCS 10 kPaCS 40 kPaCS 70 kPa

Figure 3. Change in moisture content with distance from

interface, mix 3-LLC (290–0.75), 27 months

410



and the pH were independent of each other, but both had the

same increasing trend towards the concrete.

The pH at the interface depended on clay type, where the LLC

illitic interface generated a pH range of 10.3–11.1 and at the

ECC kaolinitic interface the range was 9.5–10.0, which is less

favourable for TSA. Despite this, thaumasite formed at both

interfaces, but a higher level of deterioration was observed at the

ECC interface.

The increase of pH in the clay adjacent to concrete is attributed

to ion diffusion processes in which alkali (Naþ, Kþ) and hydroxyl

ions (OH�), within the concrete and soil pore solution culminate

in the flocculation of clay particles. This increases the friability of

the clay, as was observed on the specimens, and illustrated in

Figures 1 and 2. Furthermore, it appeared that the optimum

moisture content was increased and an increase of the strength

and stability occurred as a result of the presence of hydroxyl ions.

The conditions would favour solidification processes as a result of

pozzolanic reactions where C-S-H and C-A-H form in the clay.

3.4 Mineralogical changes

Mineralogical changes of the clay adjacent to the interface were

identified in the case of the ECC, compared with original clay

samples where traces of brucite were detected: see Figure 7.

However, it is assumed that it is more likely that this brucite was

derived from the thaumasite formation process at the interface

where, according to Crammond and Nixon (1993), it is frequently

observed. Further changes in the mineralogical composition

depending on the distance from the interface were not found.

Significant differences in the mineralogical composition between

the natural LLC and the LLC adjacent to the concrete were not

identified; see Figure 8. Therefore it is concluded that no signifi-

cant changes of the mineralogy of the clay at the interface

occurred using the Siemens D5000 powder X-ray diffractometer

(XRD), although the presence of swelling clays and any changes

within were not included in the XRD testing. Microscopic

investigations at the boundary concrete/thaumasite/clay confirmed

this, and no traces of TSA reaction products were found in the

clay using polarised light microscopy or XRD. Furthermore, no

differences in gypsum and pyrite concentrations were detected

using XRD. This is due to the extensive supply of sulfate ions

through the highly concentrated and aggressive magnesium

sulfate solution.

Mineralogical changes occurred within the thaumasite reaction

product layer adjacent to the clay during the investigation period.

Interfaces that were subject to an extensive thaumasite formation

of up to 24 mm after 27 months showed changes in the composi-

tion of the thaumasite layer, as is discussed in more detail by

Brueckner et al. (2012). The change in composition was visible

in the different appearances of the TSA layer, and is indicated

schematically in Figure 9. The older, reddish reaction layer zone,

which was in contact with the clay, was enriched with gypsum,

whereas the newly formed, greyish reaction products contained

less gypsum (Figure 10).

4. DiscussionThe water content and pH measurements of the clay adjacent to

the concrete confirmed the findings of several researchers,

including Chuang and Reese (1969), Lee (2001), Milititsky et al.

7·0

7·5

8·0

8·5

9·0

9·5

10·0

10·5

11·0

11·5

0 10 20 30 40 50

pH


Mix 3 PC (27 m)Mix 4 PC (27 m)Mix 3 CS (27 m)Mix 4 (27 m)CSMix 5 (27 m)CS

Figure 4. Change in pH value with distance from interface,

PC/CS-LLC

5

6

7

8

9

10

11

12

0 5 10 15 20 3025 40 45 50

pH

Distance from interface: mm35

Mix 3 controlMix 3 (18 m)Mix 3 (27 m)Mix 4 (27 m)


PC-ECC

5

6

7

8

9

10

11

12

0 5 10 15 20 25 30 35 40 45 50

pH


Mix 3 controlMix 3 (18 m)Mix 3 (27 m)Mix 4 (27 m)Mix 5 (27 m)


CS-ECC

411



(1982), O’Neill and Reese (1970) and Yong (1979), that these

two parameters increase towards the concrete/soil interface. How-

ever, the measurements undertaken during this investigation did

not agree with the findings of Lee (2001) that water content and

pH reduce to constant values at approximately the same distance

from the interface. Water content and pH follow different trends,

although a similar trend occurs during the early stages after

construction, when the majority of chemical interactions between

concrete and soil take place. Groundwater becomes enriched with

hydroxyl and alkali ions from the concrete pore solution, which

5 10 15 20 25 30 35 40 45 502-Theta-scale

Pure ECC

Interface ECC

M

K

B

MQ

K

Q M�

C

MM K�

B

K

QM

Lin

(cou

nts)

Figure 7. Mineralogical composition of pure and interface ECC

clay: B, brucite; C, calcite; K, kaolinite; M, muscovite; Q, quartz

5 10 15 20 25 30 35 40 45 502-Theta-scale

Pure LLC

Interface LLC

GM

MQ

Q M�

A M Q M�P P

QM

Lin

(cou

nts)

C

Figure 8. Mineralogical composition of pure and interface LLC

clay: A, anorthoclase; C, calcite; G, gypsum; M, muscovite;

P, pyrite; Q, quartz

412



increases the pH of the adjacent soil and causes solidification

processes. The ion exchange occurs mainly during the initial

months of the structure, and decreases gradually. This is due to

leaching or decalcification processes in the near-surface zone,

which depend on the bicarbonate content of the groundwater. The

density of the concrete pore structure in the surface layer

increases, and therefore the amounts of available hydroxyl and

alkali ions are reduced. This decreases the ion exchange mechan-

isms between concrete and soil and the pH at the interface is

reduced by the natural pH of the standing or flowing groundwater

due to diffusion.

The formation of thaumasite does not have an observable effect

on these two processes. The outer layer of concrete undergoes

continuous sulfate attack, which requires highly alkaline condi-

tions at the concrete surface to form stable thaumasite. However,

the pH-affected zone reduces gradually in the adjacent soil

towards the concrete/thaumasite/soil interface, and this contri-

butes to the decomposition of the thaumasite as observed at the

kaolinitic ECC interface in the pH value 9.7–10. It is suggested

that the pH gradient observed in the clay is also present within

the TSA layer, because the gypsum content decreased towards the

concrete surface. The pH drops below a threshold where the

stability of thaumasite significantly decreases, and the TSA layer

becomes enriched with gypsum.

The main reaction products found were thaumasite, ettringite,

quartz and calcite. In addition, gypsum was detected at the

boundary of the acidic ECC, whereas no increased amount of

gypsum was found at the LLC interface with a pH of 10.3–11.1.

The decomposition of thaumasite and ettringite in favour of

mainly gypsum within the formed reaction product layer at the

ECC interface (pH 9.7–10) supports the hypothesis of thaumasite

being an intermediate reaction product during TSA.

The formation of thaumasite does not affect the mineralogical

composition of the adjacent clay that could be detected using

XRD. Traces of brucite were found in the zone next to the

interface, but these could be associated with the formation of a

brucite layer at the thaumasite/soil interface. According to

Crammond and Nixon (1993), brucite is frequently observed at

the interface when concrete is exposed to magnesium-containing

solutions or groundwater.

5. ConclusionThe investigation of the chemical interactions at the concrete/clay

interface confirmed several trends reported in the literature, but

also showed disagreement with some previous findings. The

observations are based on findings from a concrete/clay interface

unaffected by TSA, and from an interface consisting of concrete/

thaumasite/clay. The conclusions of the investigations are listed

below.

j The clay within 10 mm of the interface is more friable than

elsewhere, despite having an increased moisture content,

which is caused by flocculation of clay particles.

j Within a distance of 10–15 mm from the concrete the

moisture content increases towards the interface; however, the

increase decreases with time.

j The pH increases towards the interface, but both the zone

affected by increased pH and the increase in pH itself

decrease with time.

j A relationship between moisture content and pH was not

apparent.

j The mineralogical composition of the TSA reaction product

layer changes close to the interface. The older TSA layer

close to the clay is enriched in gypsum, which is due to the

decomposition of thaumasite and ettringite in favour of

mainly gypsum with time.

j The pH gradient continues within the TSA reaction product

layer up to the unattacked concrete surface.

j Changes in the clay mineralogy due to TSA were not detected

within the clay adjacent to the interface.

Gypsum-rich reddish layer

Gypsum-poor greyish layer

Figure 9. Structure of TSA layers

60

50

40

30

20

10

0

Min

eral

con

cent

ratio

n: %

Calcite Quartz Thaumasite Ettringite Gypsum

Greyish

Reddish

Figure 10. Mineralogical composition of TSA layers

413



AcknowledgementsThe senior author is very grateful to the Engineering and Physical

Sciences Research Council (EPSRC), who funded this research

project, and to the University of Birmingham.

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Chemical interactions at the concrete/clay interface due to thaumasite form of sulfate attack