Clay Minerals (1993) 28, 555-567

S U L P H A T E E X P A N S I O N OF L I M E - S T A B I L I Z E D K A O L I N I T E : I. P H Y S I C A L C H A R A C T E R I S T I C S

M. R. A B D I AND S. W I L D

University of Glamorgan, Department of Civil Engineering and Building, Pontypridd, Mid Glamorgan CF371DL, UK

(Received 25 June 1992; revised 4 January 1993)

ABSTRACT: The effect of gypsum additions on the physical performance of lime-stabilized kaolinite has been determined. Kaolinite containing different amounts of lime (i.e. 6 and 14 wt%) and gypsum (i.e. 2, 4, 6 and 8 wt%) was compacted into cylinders and moist cured at 30~ and 100% r.h. for periods from 2 days up to 20 weeks. Unconfined compressive strength, expansion during curing and subsequent soaking, and water absorption and swelling pressure were determined. The addition of lime and subsequent moist curing was found to reduce substantially the water absorption, linear expansion and swelling pressure of the kaolinite. Although small amounts of gypsum further reduced these parameters, higher gypsum levels (up to 8 wt%) produced substantial water absorption, extreme expansion and high swelling pressures. This excessive volume instability when in contact with water was found, for a particular lime content, to be very sensitive to both the initial moist curing time and the gypsum content. The results indicate that the overriding expansion mechanism operating is imbibition of water or transfer of water by osmosis. The question of what drives this process is the subject of Part II of this paper.

A n immedia te benefit obta ined by the lime t rea tment of clays is the reduct ion or complete el iminat ion of swelling potent ia l and swelling pressures (Mitchell & Hoope r , 1961; Ingles & Metcalf , 1972; Diamond & Kinter , 1964; Bell & Tyrer , 1987; L u n d & Ramsey, 1959). These modified characterist ics are a t t r ibuted to substi tution of the clay cations by calcium and subsequent format ion of calcium silicate and aluminate hydrates. The reduction in swelling results from decreased affinity for water of the Ca-satura ted clay and the format ion of a cementi t ious matr ix which resists volumetr ic expansion.

The react ion of lime with clays produces C-S-H (Diamond et al., 1964) or C-A-S-H (Wild et al., 1989) gels together with, in some cases, crystalline calcium aluminate hydrate (Diamond et al., 1964) or calcium silicate aluminate hydrate (Croft, 1964) phases. (In cement terminology, the following abbreviat ions are used: C: CaO; A: A1203; S: SIO2; S: SO3; H: H 2 0 . ) Where carbonat ion occurs, carbo-aluminates may also form (De Silva & Glasser , 1990). If sulphates which are quite common in clay soils are present , then both the react ion mechanism and the react ion products are modified (Mitchell & Dermatos , 1990). Normal ly the calcium sulpho-aluminate phase ettr ingite (C3A.3CS.H32) is formed and at low sulphate concentrat ions the metas table phase (C3A.CS.H12) may also be observed. In addit ion, at low tempera tures (<15~ where carbonate is present , thaumasi te occurs. Et tr ingi te and thaumasi te form two end-members of the " A F t " phase in which silicate can replace aluminate and carbonate can replace sulphate in an extensive solid-solution. The format ion of this phase has a significant effect on the behaviour of the material .

One common feature of sulphate-containing hydrat ing cementi t ious systems, is that they are subject to expansion. Such expansive behaviour has been repor ted for ordinary

�9 1993 The Mineralogical Society

556 M. R. Abd i and S. Wild

Portland cement, for which there are numerous examples in the literature. These include expansive cements (Taylor, 1990), lime-pfa (fly ash) systems (Wild et al., 1990), cement stabilized colliery shale (Thomas etal. , 1989) and in particular lime-stabilized soils (Mitchell, 1986; Hunter, 1988). This expansion can cause severe disruption and ultimately failure of the cemented material.

Mitchell (1986), in a study of the failure of lime-stabilized pavement bases in Las Vegas (USA) noted that the first signs of distress to the pavement occurred two and a half years after completion, and that in the failed zones, the lime-treated soil had a much lower density and a much higher moisture content than in the unfailed zones. Also, significant amounts of ettringite and thaumasite were observed in both failed and unfailed zones, and in particular, samples taken from unfailed zones were found to swell when placed in water. The swelling was attributed to the expansive nature of ettringite and thaumasite which, when given access to water, produced swell greatly in excess of that exhibited by the untreated soil. In a later paper, Mitchell & Dermatos (1990) reported the results of laboratory tests on 20~ cured lime-kaolinite-sand and lime-montmorillonite-sand compacts containing gypsum in which they attributed the swelling on soaking to ettringite formation.

A later report by Hunter (1988) on the heave of a lime-treated soil sub-base also in Las Vegas (USA) came to conclusions similar to those of Mitchell. The areas of major damage were almost always found adjacent to an obvious source of water and it was concluded that the availability of pore-water is the single most important factor controlling lime-induced heave. An abundance of the mineral thaumasite was identified in the damaged sub-base but as the author points out, the thaumasite may itself not have been the necessary factor inducing the heave, as initially, ettringite would have formed and the transition to thaumasite may have occurred after the heave had taken place.

A more recently reported example of sulphate induced swelling (Snedker & Temporal, 1990) is in Oxfordshire (UK) where serious problems with heave (up to 150 mm in places) arose on certain sections of the carriageway of the newly completed but then unopened M40 motorway around Banbury. It was subsequently established that the lime-stabilized element of the capping layer of the pavement had been subject to sulphate attack and contained clear evidence of ettringite formation.

Until very recently reports in the literature relating to sulphate expansion have mainly been restricted to systems containing Portland cement. Although, as in the above examples, expansion is clearly associated with ettringite (C3A.3CS.H32) formation, the exact manner in which sulphates produce expansion in these hydrating cementitious systems is not clearly understood. Several hypotheses have been advanced to explain the mechanism of expansion associated with ettringite formation. These can be divided into three major categories which are: (i) volume increase due to the reaction product (crystalline ettringite) exhibiting a lower density and therefore a higher volume than the initial reactants; (ii) expansive forces exerted by the anisotropic growth of ettringite crystals or by morphological changes in crystal habit and crystal growth; and (iii) swelling due either to imbibition of water or transfer of water by osmosis. In this case expansion is associated with formation of a colloidal sulpho-aluminate product.

An example of the first category is the reaction of C3A with gypsum and water to form ettringite which in fact gives an overall volume reduction of 7.5% (Mehta, 1973). However, the water consumed in this reaction may be drawn in from outside and therefore its volume would not be included with the other reactants. In this case the change in volume on reaction would give a volume increase of 140-4%. Even if the water needed for the reaction

Sulphate expansion of kaolinite: 1 557

is drawn in from outside, if within the solid mass there is sufficient pore-space to accommodate the increased volume, expansion still need not occur. Only if the reaction product is deposited in such a way that it forces particles apart and disrupts the structure will expansion occur.

The second category advances the theory that it is the growth of ettringite crystals (Ish- Shalom & Bentur, 1974; Budnikov & Kravchenko, 1968; Hansen, 1963) that forces apart the cementing particles. However, the formation of ettringite crystals does not necessarily lead to expansion. Heller & Ben-Yair (1968) reported similar specimens showing formation of equivalent amounts of ettringite but which show widely different levels of expansion. In supersulphated cements, very large quantities of ettringite are often formed (Schroder, 1968; Kondo & Ohsawa, 1968; Midgely & Pettifer, 1971) but expansion is insignificant. Therefore, if the pressure of crystal growth hypothesis is valid, the ettringite crystals must have to grow in a particular way. Direct crystal thrust by anisotropic growth of ettringite crystals has been advanced as a possible mechanism (Ogawa & Roy, 1982; Budnikov & Kravchenko, 1968; Kalousek & Benton, 1970). This type of mechanism is reported to be responsible for expansion of plaster of Paris on setting.

The third category recognizes that in these systems the presence of water has a significant effect on the degree of expansion, and Mehta & Wang (1982) have shown that ettringite itself expands when subjected to water. Also the degree of expansion is related to the crystallite size of the ettringite and to surface water adsorption, and fine crystals thus show significantly greater expansion than coarse crystals. Mehta's work on the water adsorption of crystalline ettringite has been complemented with further work by that author (1983) and by other workers (Negro & Bachiorrini, 1982; Odler & Gasser, 1988) on amorphous or "colloidal ettringite". Mehta concludes that for "colloidal ettringite" to cause large expansion, it must be in contact with an outside source of water.

The association of swelling with the formation of colloidal products has also been reported by Bailey & Chescoe (1980). They reported the development of hollow, tubular amorphous calcium sulpho-aluminate hydrate filaments early in the hydration of C3A when gypsum is present. They suggested that these were formed as a result of the "osmotic" swelling of amorphous calcium sulpho-aluminate layers initially surrounding the C3A particles. The formation of a similar colloidal C-A-S-S-H product in cured pfa-lime mixtures containing gypsum has also been shown by Wild et al. (1990) to be associated with expansion.

The formation of colloidal material can induce swelling and expansion in the presence of water in two ways, either by osmosis or by imbibition. Both require a variation in chemical potential (Dent Glasser & Kataoka, 1981) and an absence of free mixing. In osmosis, free mixing of the solution is prevented by a membrane permeable to the solvent and impermeable to the solute. The solvent moves spontaneously into the solution in which its chemical potential is the lowest, that is the one containing the highest concentration of solute. In imbibition, no semi-permeable membrane is necessary because the relative rigidity of the solid prevents free mixing. However, the solid must be porous enough to imbibe water, be sufficiently flexible to swell appreciably, be relatively insoluble, and be sufficiently polar to attract water molecules. However, the distinction between imbibition and osmosis in relation to colloidal materials and gels is not clearly differentiated as they are both manifestations of the same effect.

Careful examination of the data and observations reported in the literature would therefore suggest that all three of the proposed mechanisms discussed above are possible.

558 M. R. Abdi and S. Wild

However, the conditions under which a particular mechanism will operate or predominate is not known. This is especially the case in lime-stabilized clay systems where it is only very recently that practical cases of sulphate expansion have been reported and where very little research has yet been carried out. The research which has been carried out suggests, in agreement with the previous work on other cement systems, that ettringite is in some way implicated in the expansion process, and in particular the work with clays has clearly shown that for substantial heave to occur there must be access to a source of water. The mechanism in this case may therefore be a modification of the previously suggested mechanisms, or a combination of these mechanisms, or possibly an entirely new mechanism.

It is one of the principal objects of this research project to identify this mechanism (or mechanisms) for lime-stabilized kaolinite and hence predict the conditions under which sulphate expansion will take place in sulphate-containing kaolinite clay soils.

E X P E R I M E N T A L M E T H O D S

Raw materials

The kaolinite used was a Standard Porcelain supplied by ECC International Ltd, St Austell, Cornwall; its basic engineering properties determined by the current authors in accordance with standard tests specified in BS 1377, 1975 and BS 1924, 1975 are presented in Table 1. The hydrated lime Ca(OH)2 employed was produced and supplied by ICI (UK), and is commercially available under the trade name Limbux. According to the manufacturers it contains 96.8% Ca(OH)2. Precautions were taken to prevent carbonation, and the level of carbonation was monitored using thermogravimetric analysis. The gypsum used was calcium sulphate di-hydrate (CaSOa.2H20) supplied at 98% purity by Aldrich Chemicals Co. Ltd.

Compaction, mixing and pressing

The first stage of the research involved a detailed investigation (i.e. Proctor Test 3, BS 1924, 1975) of the compaction characteristics of kaolinite containing different lime contents, in order to obtain the optimum moisture content required to give the maximum dry density. The mixtures investigated consisted of kaolinite containing 0, 6, 14 and 20 wt% lime expressed as a percentage of the dry weight of the kaolinite. Additional tests were performed to establish whether the optimum moisture content and the maximum dry

TABLE 1. Engineering properties of kaolinite.

Test Value

Specific gravity 2.57 kg/m 3 Moisture content 2.5% pH value 4.6 Liquid limit 59% Plastic limit 29% Plasticity index 30% Dry density 1.56 mg/m 3 Optimum moisture content 23-8%

Sulphate expansion of kaolinite: I 559

density were influenced by gypsum additions. The mixtures investigated contained 6 wt% lime and 2, 4 and 6 wt% gypsum again expressed as a percentage by weight of the kaolinite. Mixing was performed using a Kenwood ){eel Chef variable-speed mixer and a very thorough mixing procedure was adopted with a total mixing period of 18 rain. Compacted specimens were produced in accordance with Test 10, BS 1924, 1975 using standard cylindrical 50 mm diameter, 100 mm long moulds. Standard measures were taken to ensure even compaction and to facilitate extrusion after compaction. The amount of mixture placed in each cylinder (425 g) was that required to achieve the previously determined maximum dry density. The mixture was compressed by means of a manually operated hydraulic jack until fully compacted. After extrusion, the heights and weights of the compacted cylinders were recorded.

Curing environment and dimensional change

The cylinders were moist, cured at a temperature of 30~ under controlled environments for different periods. Desiccators were used as curing chambers in which the desiccant was replaced with water at a level just below a metal mesh support on which the compacted specimens rested. At the end of each designated curing period, the specimens were weighed and their heights measured. Figure 1 shows the apparatus designed to monitor the linear changes experienced by the specimens during moist curing at 30~ and 100% r.h. in a CO2- free atmosphere. The equipment (see Fig. 1) consisted of a glass tank with an aluminium cover and base plate, an internal aluminium stand to hold the specimens, perspex end-caps to locate the specimens and dial gauges (sensitive to 0.01 mm) to detect specimen movement. The aluminium covers and stands were anodized to minimize the risk of corrosion in the warm humid environment of the tank.

The changes in the lengths of the cylindrical specimens were monitored on a daily basis during the curing period until equilibrium had been established and no further changes could be observed. This system was also used to examine the reversible nature of the expansion by replacing the water in the tanks with silica gel and determining the shrinkage during drying.

Dial-gauge

--~'L-~, ~ Alurnlnlum cover ~ Rubber gaskett

Perspex cap Specimen

8! Bolt "L ~. ~ Gloss tank

o Perspex cop

=~ ~ Aluminium stand ';-I~ ~I ~ Wate~

Front elevation

F16.1. Curing chamber designed to provide 100% r.h. and a CO2-free atmosphere for continuously monitoring linear changes during moist curing (all dimensions in mm).

560 M. R. Abdi and S. Wild

In addition to observing the expansion of specimens cured in 100% r.h. at 30~ specimens which had been moist cured in this way for specific curing times were also subsequently placed in contact with liquid water to a depth of 10 mm and their expansions and weights were again monitored at weekly intervals until no further changes were observed. This soaking procedure is termed "wet curing".

Pressure measurement

A system was also designed (see Fig. 2), to measure the longitudinal pressure generated by the moist-cured kaolinite-lime-gypsum cylinders when they were wet cured under conditions of restraint.

A proving ring (1100 N with a sensitivity of 0.72246 N per 0.002 mm division) was used to monitor pressure. This technique allows slight movement in order to measure the pressure, and in doing so allows some relaxation of the specimen. However, the amount of movement experienced was only of the order of a few percent of the total unrestrained movement of the sample and this was therefore considered to have only a minor effect on the value of the pressure recorded.

The hollow perspex cylinder containing the sample was lightly oiled to reduce skin friction. This reduced restraint of the small longitudinal movement of the sample which absorbed water through a series of small holes drilled in the cylinder slightly above the base.

The longitudinal force produced was transferred to the proving ring via a perspex piston and a brass rod seated on a ball-bearing to ensure self-alignment. The pressure generated was monitored with time.

Top plate

�9 Bo l t

- Dial gauge

- Prov|ng ring

Boll bearing

Brass plunger

- Perspex cap

- Perforated perspex

mould ( S m m th i ck )

�9 Sample

Water

-Glass water bath

5ram Diameter holes

FIG. 2. Apparatus designed to measure the magnitude of the swelling pressure generated under restraint.

Sulphate expansion o f kaolinite: I 561

R E S U L T S

Compaction

The results of compaction (i.e Proctor) tests performed on the kaolinite-lime mixtures are presented in Figs. 3 and 4, respectively. The addition of gypsum did not result in marked or systematic changes in these parameters although there was a tendency for the optimum moisture content to be slightly greater when gypsum was present. The moisture content and dry density of all mixes were fixed at 29% and 1-45 mg/m 3 respectively.

Linear expansion during curing

Figure 5 shows the linear expansion of the kaolinite-6 wt% lime-gypsum cylinders during moist curing at 30~ and 100% r.h. for up to 75 days, at which time expansion had ceased. Figure 6 shows the linear changes of the moist cured kaolinite-6 wt% lime-gypsum cylinders during drying with silica gel at 30~ until no further shrinkage occurred.

The expansion experienced by the specimens during moist curing was not fully reversible on drying and the degree of irreversibility or permanent set increased with increase in the gypsum content. This is illustrated in Fig. 7 which shows the total percentage linear dimensional change of the cylinders vs. gypsum content after completing the curing and drying cycles. A second set of specimens containing 14 wt% lime and different percentages of gypsum as above, were also tested in the same way and these showed very similar

1.56 r~

.~E 1.52

'~ 1 . 4 8

"0 1,44

E

�9 ~ 1.40 r4 '8 t r

12 16 Lime content (%)

20

Fl6. 3. Maximum dry density vs. lime content for compacted kaolinite-lime mixes.

g:: - . . . . . . . . . . . . .

o.o K - - ' - - ' ~ - - - " " : Z : " : " "-<~-rz~-=u~--I I

~0@~

l - 1 . 0 i i i r

20 40 60 80

Curing t ime (Doys)

FIG. 5. Linear expansion vs. curing time for kaolinite- 6 wt% lime-gypsum cylinders, moist cured at 30~ and

100% r.h.

3O

v

28 o

28 "5

1

J / 15 I I

10 15 20 Lime Content (%)

FIG. 4. Optimum moisture content vs. lime content for compacted kaolinite-lime mixes.

0.2 ~, o.o ~

-.$ ~ ~ gypsum c "'3 -.e

5 10 1,5 20 25 30 35

Dr~ng t lme (Days)

FiG. 6. Linear shrinkage vs. drying time for 75 day moist cured kaolinite-6 wt% lime-gypsum cylinders.

562 M. R. Abdi and S. Wild

behaviour. These results establish that the equilibrium expansion achieved during moist curing increases with increase in gypsum content and there is also a general but not wholly consistent trend for the drying shrinkage to decrease with increase in gypsum content. Thus, on completion of the full curing and drying cycle, there is a systematic increase in the total linear dimensional changes of the cylinders with increase in gypsum content. These observations suggest that the expansion is associated with the formation of products which inhibit shrinkage during drying.

Linear expansion o f cured specimens during soaking

In order to determine the contribution which wet curing makes to the overall expansion process, specimens were first moist cured for limited periods and then stood in contact with water as described earlier. The initial moist curing period chosen prior to soaking was one week at 30~ and 100% r.h. Figures 8 and 9 compare the plots of percentage linear expansion and percentage weight increase, respectively, vs. soaking time, for kaolinite-6 wt% lime cylinders with various gypsum contents.

In each case the changes are compared with those for kaolinite-only cylinders of the same initial moisture content. An equivalent set of curves was also obtained for the kaolinite-14 wt% lime-gypsum cylinders which also exhibited very similar behaviour, although the equilibrium expansions and weight increases were rather less at the higher lime content (Abdi, 1992).

It is clear from these results that the expansion of the wet cured specimens is a direct

2,(i

~, 1 . 5

1.0

o 0..=

'-=- - 0 . ~

- i . o - - 1 . 5 i I I

o 2 4 (z% 8 Gypsum con ten t

FIG. 7. Total linear change vs. gypsum content for kaolinite-6 wt% lime-gypsum cylinders after completion of moist curing and drying cycle.

16

m 12

8

b 4

0

30

E 2s

2O

15

5

0

1 2 3 4 5 6 7 8 Sookin 9 time (Weeks)

FIG. 8. Linear expansion vs. soaking time for kaolinite- 6 wt% lime-gypsum cylinders initially moist cured at 30~ and 100% r.h. for one week and soaked in distilled

water.

_ ~ ~ _ 8% g~surn

4% gypsum

kaolinite

- O~ gypsum

1 2 3 4 5 6 7

Sooking time (Weeks)

FIG. 9. Weight change vs. soaking time for kaolinite- 6 wt% lime-gypsum cylinders initially moist cured at 30~ and 100% r.h. for one week and soaked in distilled

water.

Sulphate expansion of kaolinite: I 563

result of water absorption. This is illustrated effectively by Figs. 10 and 11 which show the percentage linear expansion of the soaked cylinders vs. their percentage weight increase for compositions of kaolinite-6 wt% lime-gypsum and kaolinite-14 wt% lime-gypsum, respectively. Both curves have the same gradient which is 0.8% expansion per 1% increase in weight. The results establish that combinations of lime with kaolinite, or lime and gypsum with kaolinite, have a profound effect on the kaolinite's ability to absorb water and expand, in some cases inhibiting the expansion and in other cases enhancing it, depending on the gypsum content. In addition to the tests described above a whole series of identical tests was carried out on specimens of the same compositions which had been subjected to a range of different initial periods of moist curing of from 1 to 20 weeks.

The effect of initial moist curing time at different gypsum contents on the expansion and weight increase of specimens on soaking is illustrated in Figs. 12 and 13. The phenomenon of excessive expansion coupled with high water absorption is critically dependent on both the gypsum content and the initial moist curing period. For low gypsum contents, high expansion on soaking only occurs if the initial moist curing periods are very short, whereas for high gypsum contents very extended initial moist curing periods still result in substantial expansion on soaking. This observation indicates that the expansion process is in some way controlled by the initial chemical reactions occurring between the gypsum, lime and

20

16

~ 8

b 4

5 10 15 20 25 WeicJ~t chonge (~;)

F]6. 10. Linear expansion v s . weight change for one week moist cured (30~ 100% r.h.) kaolinite-6 wt% lime-gypsum cylinders, during soaking until equilibrium

had been established.

2O

.~ 12

~8

o o 5 lO 5 20 25 ~o

Wekjtlt change (Y~)

FIG. l l . Linear expansion v s . weight change for one week moist cured (30~ 100% r.h.) kaolinite-14 wt% lime-gypsum cylinders, during soaking until equilibrium

had been established.

22 2O

" - ' 1 6

g 14

~ 8

.~ 4- I J 2

0

~ 1 1 - - O ~ OX gypaum

\ , \ = : : = ---e--- 8,~ gypsJm

I I I I 1 2 3 4

Curing time before soaking (Weeks)

FIG. 12. Linear expansion after soaking v s . initial moist curing time for kaolinite-6 wt% lime-gypsum cylinders.

22 2 0 \\

~" 18 �9 ox g ~ . , .

(~ 14 \ �9 , 4x gypsum "~ 12 ~ \ �9 6x ~ m

i 6 X,,~. o -:-____--!~

-- I I I l

0 I 2 3 4 5

Curing time before soaking (Weeks)

F1G. 13. Linear expansion after soaking v s . initial moist curing time for kaolinite-14 wt% lime-gypsum

cylinders.

564 M. R. Abdi and S. Wild

kaolinite and the preliminary reaction products which are formed. If it is assumed that all the added gypsum is consumed in crystalline ettringite formation then, even for the highest gypsum additions used (8 wt%) , the amount of lime required (3-44 wt%) and the free water required (6.42 wt%) are substantially less than the percentages initially added to the kaolinite. Therefore the amount of water taken up during soaking is greatly in excess of that which could be accounted for by crystalline ettringite production alone.

Pressure generated under restraint within an open system

The magnitude of the pressure generated in restraint was measured for the kaolinite- 6 wt% lime-gypsum cylinders. The cylinders were first moist cured for one week under the same conditions as already ment ioned in the previous section, and were then placed in a proving ring as described earlier, in contact with water at room tempera ture (i.e. 20 + I~ The resulting pressures are presented graphically in Fig. 14. For reference purposes a cylinder of compacted kaolinite with no added lime or gypsum was also included�9 Comparison of Fig. 14, which shows the pressure change under restraint vs. soaking time, with Fig. 8, which shows unrestrained linear expansion vs. soaking time reveals a remarkably consistent pat tern in relation to gypsum content. The only anomaly is that specimens containing 6 and 8 wt% gypsum produced virtually the same pressures over the period of observation.

D I S C U S S I O N

The following three principal observations can be made in relation to the results obtained: (i) the susceptibility to extreme expansion and swelling occurs in the early stages of the reaction between the kaolinite, lime and gypsum; (ii) the period over which the kaolinite with a fixed lime content is susceptible to this swelling increases as the gypsum/lime ratio increases; and (iii) the swelling is principally caused by water absorption rather than by

3 0 (

2 5 ( 2

2 0 (

c'4 E

Z ~ 15s

~ 10(

n

5 0

6~ . g y p s u m P r o v i n g r i n g f a c t o r = 0 . 7 2 2 4 6 N / D i v . = ~

A

�9 . A = = A = = A = A A A . = A A y p s u r n ~ - v v v v . - . - . . v v . -

. . . . . . �9 . . . . . . . k o ~ i n T t e

�9 . _= : _ = = . . . �9 m m = m �9 m =

�9 �9 �9 �9 �9 . 2 ~ g y p s u m

0 I I I I I 0 5 1 0 1 5 2 0 2 5 3 0

S o a k i n g t i m e ( D a y s )

Fz6.14. Swelling pressure vs. soaking time for kaolinite-6 wt% lime-gypsum cylinders initially moist cured at 30~ and 100% r.h. for one week, and soaked in distilled water at 21 + I~

Sulphate expansion o f kaolinite: 1 565

direct format ion of solid react ion products al though observat ions of drying shrinkage do signify that solid react ion products may be involved in the overall process.

Mielenz & King (1955) have proposed that two mechanisms are invovled in the swelling of clay solids: (1) a relaxat ion of effective compressive strength re la ted to enlargement of capil lary films, and (2) osmotic imbibi t ion of water by clay minerals part icularly those with an expanding structure such as montmori l loni te . Beyond the first few monolayers , water is thought to be imbibed in response to a concentrat ion gradient . This arises because there are more ions per volume of solution in the vicinity of a clay mineral surface than in the bulk of the solution. Wate r uptake therefore dilutes the double layer solution, the concentrat ion of which is made more similar to that of the solution outside the double layer. It can be easily demons t ra ted (see Append ix) that the average thickness of the water layer a round each clay particle will increase l inearly with increase in moisture content; this means that expansion should show a l inear relat ionship with moisture uptake. This is exactly what is observed in the current work. The results therefore indicate that the overriding mechanism operat ing in sulphate-containing l ime-stabil ized clay is imbibi t ion of water or transfer by osmosis. Whether the process of water absorpt ion is control led solely by changes in ionic concentrat ions at the clay particle surfaces or by the format ion of some compound (such as ettr ingite or thaumasi te ei ther on the clay particle surfaces or in the pore spaces) which st imulates water absorpt ion, is not clear at this stage. This question will be considered in Par t I I of this publ icat ion (Wild et al., 1993).

ACKNOWLEDGMENTS

The authors would like to thank ICI Lime Division who supplied the lime, and English China Clay Company who supplied the kaolinite. The authors are also grateful to the technical staff of the Dept of Civil Engineering and Building, University of Glamorgan for their assistance and to Prof P. S. Coupe, Head of Department who provided the relevant facilities within his department. The authors are also grateful to Dr A. B. Poole of Queen Mary and Westfield College, University of London, for helpful comments.

REFERENCES

ABDI M.R. (1992) Effect of calcium sulphate on lime-stabilised kaolinite. PhD thesis, Polytechnic of Wales, UK. BAILEY J, & CHESCOE E. (1980) A progress report on analytical electron microscopy studies of the hydration of

tricalcium aluminate. 7th Int. Sym. Chem. Cem. II, 595-598. BELL F.G. & TYRER M.J. (1987) Lime-stabilisation and clay mineralogy. Proc. Foundations Tunnels. lI, 1-7. BENTUR A. & Isn-SHALO~ M. (19"74) Properties of type K expansive cement of pure components. Cem. Conc. Res.

4, 709-721. BUDNIKOV D.B. & KRAVCHENKO I.V. (1968) Expansive cements. Proc. 5th Int. Symp. Chem. Cem. 4, 319-400. COATMAN R.D., THOMAS N.L. & DOUBLE D.D. (1980) Studies of the growth of "silicate gardens" and related

phenomena. J. Mat. Sci. 15, 2017-2026. CROFT J.B. (1964) The processes involved in the lime-stabilisation of clay soils. Aust. Road Res. Board, Proc. 2nd

Conf. Vol. 2, 116%1200. DENT GLASSER L.S. & KATAOKA N. (1981) The chemistry of alkali-aggregate reactions. Proc. Conf. Alkali-

Aggregate Reaction Concrete, Cape Town, South Africa, $252/23. DE SILVA P.S. & GLASSER F.P. (1990) Hydration of cements based on metakaolin; thermochemistry. Adv. Cem.

Res. 3, 167-177. DIAMOND S. & KINTER E.B. (1964) Mechanisms of soil-lime stabilisation: An interpretive review. Highway Res.

Record 92, 83-102. DIAMOND S., WHITE J.L. & DOLEH W.L. (1964) Transformation of clay minerals by calcium hydroxide attack. Clays

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566 M. R. A b d i a n d S. W i l d

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A P P E N D I X

Swelling, water absorption and clay plates

If the clay platelets are being pushed apart by water molecules then the average interparticle spacing 2d will increase. The thickness of the water layer d is equal to

VOlume of water (_m3) ] eight of clay solids (kg) + [Specific surface of clay (m2/kg)]

Vw i.e. d = - -

msS

Sulphate expansion o f kaolinite: I 567

where V~ is the volume of water, ms is the mass of clay, and S is the specific surface of clay. If mw is the mass of water (kg), and Pw is the density of water (kg/m 3) then:

but moisture content of clay

mw d -

p~rns S

Mc mw =--x 100 ms

Therefore, the thickness of the water layer d around each particle is given by:

Mc d =

l OO pwS

This means that as Pw and S are constant, the thickness of the water layer around each particle will increase directly in proportion with increase in moisture content, i.e. the linear expansion should be directly proportional to moisture content.