EFFECTS OF GROUND GRANULATED BLAST FURNACE SLAG

Clay Minerals (1996) 31,423--433

E F F E C T S OF G R O U N D G R A N U L A T E D B L A S T F U R N A C E S L A G ( G G B S ) ON THE S T R E N G T H A N D S W E L L I N G P R O P E R T I E S OF L I M E - S T A B I L I Z E D K A O L I N I T E IN THE P R E S E N C E OF S U L P H A T E S S. W I L D ,

J. M . K I N U T H I A ,

R. B. ROBINSON

AND I. H U M P H R E Y S

Department of Civil Engineering and Building, Universityof Glamorgan, Pontypridd, Mid Glamorgan,South Wales, CF371DL UK (Received 13 July 1995; revised 4 December 1995)

A B S T R A C T : The use of ground granulated blast furnace slag (ggbs) is well established in many cement applications where it provides enhanced durability, including high resistance to chloride penetration, resistance to sulphate attack and protection against alkali silica reaction (ASR). The use of ggbs in soil stabilization is, however, still a novel process in the UK although it has been used in South Africa. This paper reports on efforts to extend the use of ggbs to highway and other foundation layers by determining the beneficial effect which it has on the reduction of expansion due to the presence of sulphates. The paper describes the results of laboratory tests on lime-stabilized kaolinite containing different levels of added sulphate to which different amounts of ggbs have been added. The tests determine the strength development of compacted cylinders, moist cured in a humid environment at 30~ and the linear expansion of these moist cured cylinders on soaking in water. The results illustrate that small additions of ggbs to sulphate containing clays which are stabilized with lime reduce substantially their expansion when exposed to water and have no significant deleterious effect on strength development.

INTRODUCTION Lime (Ca(OH)2) stabilization of clay soil has been widely used in highway and foundation layers as an economic method of providing suitable pavement and fill material whilst avoiding the generally more expensive process of transporting in large quantities of granular fill. In a number of reported cases, however, (Mitchell, 1986; Hunter, 1988; Snedker & Temporal, 1990), serious problems of swelling and heave have occurred where sulphates were present in the soil and this swelling has been associated with ettringite formation. Mitchell (1986) and Hunter (1988) reported very large amounts of heave, in excess of 100%, leading to pavement failure in the Stewart Avenue lime treated subbase in Las Vegas, Nevada, USA. Snedker & Temporal (1990), on the other hand, reported at least 60% heaving on the Banbury IV contract, part of the M40 motorway between Oxford and Birmingham. In both cases, the heave-induced failures were shown to be related to sulphate reactions when

water gained access to the treated sulphate-bearing clay, leading to the formation of highly crystalline materials - - ettringite and thaumasite. In the case of lime-treated kaolinite, in the presence of gypsum, a swelling mechanism has been proposed by Wild et al. (1993) which involves an osmotic process associated with initial nucleation and growth of ettringite crystals on the surface of clay particles. Sulphate expansion associated with ettringite formation can also cause problems in Portland cement-based mortar and concretes and one method widely employed to prevent sulphate expansion in these systems is to replace the cement by ground granulated blast furnace Slag (ggbs). Use of ggbs as a cementitious material blended with Portland cement is based on its activation with alkalis (mainly Ca(OH)E) released from hydration of the Portland cement (Gjorv, 1989; Hakkinen, 1993). In addition, over recent years, there has been renewed interest in activation of ggbs cements with alkalis other than those from Portland cement, e.g. NaOH, NaECO3 or Na4SiO4 (Gjorv, 1989).

9 1996 The Mineralogical Society

424

S. Wild et al.

TABLE 1. Engineering properties of kaolinite. Test

Value

Specific gravity pH value Liquid limit Plastic limit Plasticity index Maximum dry density Optimum moisture content

2.57 4.6 61% 32% 29% 1.56 mg/m3 24.0%

TABLE 2. Chemical and physical properties of lime (Buxton Lime Industries Ltd.). Trade Name

The reaction of ggbs with Portland cement and water is a complex process. When Portland cement hydrates, the principal hydration products are Ca(OH)E and C-S-H gel. The ggbs, due to its high alumina and silica content, produces somewhat more complex hydrates than ordinary Portland cement (OPC). Precipitates of calcium silicate hydrates and calcium aluminate hydrates result from the hydration reaction that ensues. This hydration reaction which is slower than the hydration of Portland cement has a 'pore-blocking' effect resulting in increased long-term hardening of the cement paste. The blocking of pores leads to higher strength and lower permeability (Macphee et al., 1989) which, besides other improved binding and adsorptive effects, enhances resistance of ggbs concrete to attack from sulphates. The formation of ettringite in such systems does not necessarily result in expansion and swelling. For example, expansion is not a problem in supersulphated cement in which 80-85% of the ggbs is blended with 10-15% of anhydrite and Portland cement is included as an activator, although ettringite is a principal hydration product and a substantial quantity of sulphate is present in the system. The well established sulphate-resisting properties imparted to cements by blending them with ggbs suggests that by blending the lime with ggbs, the ggbs may impart similar sulphate-resisting properties to lime-stabilized clays. In both of these systems (i.e. hydrated sulphate-containing limeclays and Portland cements), the principal phases present (i.e. portlandite, ettringite and C-S-H or C-A-S-H gels) are similar. The object of this work is to establish whether replacement of lime by ggbs in lime-stabilized clay provides increased sulphate resistance whilst maintaining or enhancing strength.

'Limbux'

Chemical name

Hydrated lime or calcium hydroxide Dry white powder Ca(OH)2 580~

Physical form Molecular formula Melting/decomposition temperature Bulk density Specific gravity Solubility in water

480 kg/m3 2.3 (water = 1 at 4~ 1.76 g/l sat. sol. at IO~

EXPERIMENTAL

PROCEDURE

Materials used

Hydrated lime and kaolinite (Trade names 'Limbux' and 'Standard Porcelain' respectively) were supplied by Buxton Lime Industries Ltd., Buxton, Derbyshire, UK and by ECC International, St. Austell, Cornwall, UK, respectively. The kaolinite supplied as 'Standard Porcelain' consisted of 84% kaolinite, 13% mica, 1% feldspar and 2% other minerals. The chemical and physical properties of kaolinite and lime are shown in Tables 1 and 2. Ground granulated blast furnace slag was supplied by Civil and Marine Slag Cement Ltd., Llanwern, Newport, UK. Table 3 shows its chemical composition and physical properties together with a typical ordinary Portland cement composition for comparison. Gypsum was supplied by Philip Harris Scientific Ltd. of Pentwyn, Cardiff.

TABLE 3. Chemical composition and physical properties of ggbs and Portland cement from Civil & Marine Slag Cement Ltd., UK. Oxide

Composition GGBS Portland cement

CaO 41 SiO2 35 A1203 11 Fe203 1 Insoluble residue 0.3 Relative density 2.9 Bulk density, kg/m3 1200 Colour Off-white

63 20 6 3 0.5 3.15 1400 Grey

Effects of furnace slag on kaolinite Consistency limits and compaction tests Consistency limits tests and standard Proctor compaction tests were performed on kaolinite, kaolinite-lime, kaolinite-lime-gypsum and on kaolinite-lime-gypsum-ggbs systems in order to characterize material behaviour in the presence of water (Table 4).

Composition of test samples Generally, 1 - 3 wt% lime is needed for modifying soil properties and 2 - 8 wt% lime for stabilization, depending on the clay content. As the current work was carried out on 'pure' clay, a reference value of 6 wt% lime was chosen as a sensible addition to the kaolinite clay. Gypsum was included to monitor the effect of sulphate on swelling behaviour, and ggbs was included to establish whether incorporation of this material reduced swelling. Previous work by Wild et al. (1993) has shown that substantial swelling is experienced by compacted moist cured kaolinite6 wt% lime mixes during subsequent soaking, when gypsum is incorporated in the mix at amounts in excess of 2 wt%. Hence, in the current work, a level of 4 wt% gypsum was taken as a reference value, for purposes of comparison. Using the kaolinite-6 wt% lime and the kaolinite-6 wt% l i m e - 4 wt% gypsum mixes as controls, 4 wt% ggbs was also included to determine its effect on

425

the swelling properties. In addition, to indicate the relative effect of each component of the lime-ggbs combination, mixes were made, varying the relative proportion of lime to ggbs but maintaining an overall total amount of 10 wt%.

Preparation of test samples and curing Dry materials, e n o u g h to p r o d u c e three compacted cylindrical test samples which are 50 mm in diameter and 100 mm in length, were thoroughly mixed in a variable speed Kenwood Chef Excell mixer for 2 min before slowly adding the calculated amount of water. Intermittent hand mixing with palette knives was necessary to achieve an homogeneous mix. A steel mould fitted with a collar so as to accommodate all the material required for one sample, was used to compress the cylinders to maximum dry density at optimum moisture content. The pre-fabricated mould ensured that the material was not over compressed. The cylinders were extruded using a steel plunger, trimmed, cleaned of releasing oil, weighed, measured and wrapped in several runs of cling film. The cylinders were labelled and placed in polythene bags before being placed on a platform in sealed plastic containers. Water was always maintained below the platform to ensure that there was no evaporation from the samples. The plastic containers were then placed in an environmental chamber capable of maintaining temperatures to

TABLE 4. Proctor and consistency limits results for kaolinite-lime-gypsum-ggbs systems. % Slag

Control Control Sample Sample Sample Control Sample Sample Sample

1 2 3 4 5 6

0 0 4 6 8 0 4 6 8

- Not Determined

% Lime % Gypsum

0 6 6 4 2 6 6 4 2

0 0 0 0 0 4 4 4 4

Liquid limit (EL)

Plastic limit (PL)

(%)

(%)

61 73 83 81 79 65 87 85 83

32 41 41 38 35 29 42 39 35

Plasticity Linear Maximum Optimum index shrinkage dry moisture (PI) (LS) density content (%) (%) (MDD) (OMC) (Mg/m 3) (%) 29 32 42 43 44 36 45 46 48

6.0 10.4 10.2 10.0 5.1 8.5 8.4 8.3

1.560 1.490 1.382 1.394 1.405 1.520 1.394 1.404 1.419

24 25 29 30 28 25 31 30 30

S. Wild et al.

426

Gauge

Dial

Aluminium cover

~

~

~

"1 "7 7-

~

U

I

II II

J ----

L

5 mm thick perspexcap Bolt

~~Specmen

~

-Aluminiumstand

FRONT ELEVATION

Water FIG. 1. Chamber employed to monitor the linear expansion during soaking of the previously moist-cured cylinders. (Not to scale; dimensions in mm.)

_ 0.1~ and humidity to + 2% relative humidity. The samples were moist cured for curing periods of 7 and 28 days at 30~ and 100% relative humidity.

Testing At the end of each of the moist curing periods, three samples were removed from the environmental chamber. Any surface moisture on the cylinders was removed with paper tissue, prior to their being weighed. At this stage two cylinders were subjected to unconfined compressive strength (UCS) tests and one to swelling tests. The end surfaces of samples to be tested for UCS were gently abraded using a fiat wire brush to ensure good contact with the testing rig platens. A special self-levelling device was used to ensure a uniaxial load application. An M30K J J Instruments testing machine capable of loading up to 30 kN was used to apply the load at a compression rate of 1 mm/ min. After testing, a small quantity of material was taken from the interior of the tested sample for moisture content determination to establish the moisture condition at the end of each curing period. Previous work by Abdi & Wild (1993) on kaolinite-6 wt% l i m e - 4 wt% gypsum mixes using identical materials and curing environment has

shown that linear expansion during moist curing is small relative to that observed during subsequent soaking. This was confirmed by observations on the current mixes. Therefore detailed expansion measurements were only carried out during soaking. Samples to be tested for swelling were placed on a platform in a glass tank and covered with a lid fitted with dial gauges (Fig. 1). The cylindrical samples were partially immersed in water to a depth of 10 mm above their base. The tank was placed in the environmental chamber where conditions were maintained at 30~ and 100% relative humidity. Linear axial swelling was monitored on a daily basis until no further expansion occurred. Both the moist curing environment and the soaking environment were closed sealed systems to reduce the availability of carbon dioxide and prevent carbonation of the lime. Clearly excessive carbonation of the lime would reduce the amount of lime available for pozzolanic reaction and is, therefore, undesirable. RESULTS It is generally accepted that the addition of small amounts of lime to kaolinite produces a marked increase in the plastic limit. The liquid limit may

Effects of furnace slag on kaolinite 90

427

b

.a

401

85

/

r. 8O

m0% Gypsum "o'4% Gypsum 754(6) 6(4)

8(2)

11"0%Gypsum -o-4% Gypsum 304(6) 6(4)

Slag(Lime) Content %

1 0 % Gypsum "0"4~ Gypsum i 35 (6)' 6(4)

8(2)


8(2)


F~c. 2. Change in (a) liquid limit, (b) plastic limit and (c) plasticity index with composition, kaolinite-10 wt% (slag/lime) mixes with and without 4 wt% gypsum.

3000

2500

2000

E 1500

1000

MIX COMPOSITION

500

4t'K-6%L

-t- K-6%L-4%G ~kK-6%L-4%G-4%GGBS 9 K-6%L-4%GGBS [

I

2

4

Curing period (Weeks) FIG. 3. Unconfined compressive strength vs. curing time for kaolinite-6 wt% lime cylinders containing 4 wt% gypsum and/or 4 wt% ground granulated blast furnace slag.

428

S. Wild et al. 3000

2500

2000

1500

1000

7 DAY S T R E N G T H

+ 0 % Gypsum 500

D 4%

Gypsum

20 DAY S T R E N G T H 9* ' 0 %

Gypsum

90-4 % Gypsum I 6(4)

4 (6)

(2)


Fro. 4. Unconfined corrpressive strength (U.C.S.) vs. composition (slag/lime) for cylinders with and without gypsum (4 wt%) moist cured for 7 and 28 days.

increase or decrease but in such a way that there is an increase in the plasticity index with increasing lime content for lime contents up to 6 wt%. This is confirmed by the data given in Table 4. Figure 2 shows the effect of different lime-slag ratios on the Atterberg limits of kaolinite and kaolinite-4 wt% gypsum. This illustrates that as the lime to slag ratio decreases, there was a small reduction in liquid limit and a more marked decrease in the plastic limit thus producing an increase in the plasticity index. The effect of this decrease in lime to slag ratio on linear shrinkage (Table 4) was relatively minor producing a slight reduction. These trends were maintained when gypsum was present in the kaolinite (Fig. 2) but gypsum has the effect of producing slightly higher liquid limits and plasticity indices, and slightly lower shrinkage. The addition of lime to kaolinite produced a sharp reduction in maximum dry density corresponding to

an increase in optimum moisture content (Table 4). However, a decrease in lime to slag ratio produced a slight increase in maximum dry density and a small but non-systematic variation in optimum moisture content. Similar trends were observed when gypsum was present, the effect of the gypsum being to produce a slightly greater maximum dry density and optimum moisture content. Figure 3 shows compressive strength vs. curing period for compacted and moist-cured kaolinite6 wt% lime cylinders containing 4 wt% gypsum, 4 wt% ggbs and 4 wt% gypsum plus 4 wt% ggbs. The addition of 4 wt% gypsum resulted in a significant increase in compressive strength in specimens both with and without ggbs. This is in agreement with previous reports (Waswa e t al., 1993) of the effect of gypsum on strength. The addition of 4 wt% ggbs produced a small reduction

429

Effects o f furnace slag on kaolinite 12

10

q_K_ao~L.4o/oG.4%GGBS /

-H-K-6% L-4.%GGB$

9 K-6%L

9-

; ; 2

4

6

! !

Soaking Period (Days)

FIt. 5. Unconfined linear expansion vs. soaking time for 7-day moist-cured kaolinite-6 wt% lime cylinders containing 4 wt% gypsum and/or 4 wt% ground granulated blast furnace slag.

in strength when gypsum was absent, but resulted in an initial acceleration in strength development when gypsum was present. Figure 4 shows the effect on the 7-day and 28day strengths of changing the relative proportions of lime to slag both with and without added gypsum. Without added gypsum there was a consistent gain in strength as the slag to lime ratio increased. Significantly, when gypsum was added the strengths of the specimens with low lime to slag ratios did not increase greatly whereas specimens with high lime to slag ratios showed a substantial increase in strength. This suggests that gypsum had a much more profound effect on the lime-kaolinite reaction than it did on the hydration of the slag. With regard to sulphate expansion, the stabilized soil might be expected to be in its most vulnerable

state in the early stages of curing when negligible interparticle bonding had developed (Abdi & Wild, 1993). Therefore, swelling measurements during soaking were performed on specimens which had been moist cured for only 7 days and 28 days. Figures 5 and 6 show the percent linear expansion on soaking of 7-day and 28-day moistcured cylinders, respectively, for kaolinite-6 wt% lime compositions containing 4 wt% ggbs, 4 wt% gypsum, and 4 wt% ggbs plus 4 wt% gypsum. For 7-day cured specimens (Fig. 5), the effect of the slag on reducing expansion was dramatic, particularly when gypsum had been added, where expansion was reduced from over 10% to 0.26%. For 28-day cured specimens (Fig. 6) where expansion was very much less, the addition of ggbs still reduced expansion by a significant factor (i.e. from 1.45% to 0.23%). It is also of interest to

430

S. Wild et al. 1.6

1.4

1.2

i

MIX COMPOSITION 9 K-6%L-4%G 0.8

-~- K - 6 % L - 4 % G - 4 % G G B S I K-6%I,-4%GGDS

0.6

0.4

0.2

0 m 0

I 2

I 4

I 6

I 8

I 10

I 12

14


FI~. 6. Unconfined linear expansion vs. soaking time for 28-day moist-cured cylinders containing 4 wt% gypsum and/or 4 wt% ground granulated blast furnace slag.

note that the expansion of the kaolinite-lime specimens containing ggbs was virtually the same regardless of whether gypsum was or was not present, which suggests that the ggbs had in some way eliminated the expansive effect of the gypsum. The effectiveness of the slag in eliminating the effect of gypsum on expansion was also sensitive to the relative amount of slag to lime which was added. Figures 7 and 8 show the effect of different slag to lime ratios on the swelling of 7-day and 28-day cured cylinders, respectively, when gypsum was and was not added. Because slag was present in all specimens the percent expansions were much smaller than the maximum observed in Figs. 5 and 6 at equivalent ages. Without added gypsum, changing the lime to slag ratio over the range Chosen had little effect on the linear swell.

However, when gypsum was present, there was a significant variation in percentage linear swell with changing lime to slag ratio and surprisingly it decreased as the lime to slag ratio increased. To what degree this trend continued is unknown at present, although the results do suggest that only relatively small additions of slag are required to eliminate the effect of sulphate expansion observed in the current work. DISCUSSION When clays such as kaolinite are subjected to moisture they show intercrystalline swelling (Arabi & Wild, 1989). Water, a few monolayers thick, is strongly adsorbed at the negatively charged particle surfaces. Beyond this, an extensive adsorbed layer is formed due to the concentration gradient between

431

Effects o f furnace slag on kaolinite

1.5

MIX COMPOSITION

9 K-4%L-6%GGB$ 9 K-6%L-4%GGBS 9 K-2%L-8%GGBS-4%G 9 K-4%L-6%GGBS-4%G 9 K-6%L-4%GGBS-4%G

i

0.5

0

I

I

I

I

I

2

4

6

8

10

12

14


Fie. 7. Unconfined linear expansion vs. soaking time for 7-day moist-cured cylinders of different slag/lime ratios, with and without gypsum.

the bulk solution and the electrical double layer (consisting of water molecules and exchangeable cations). The addition of lime modifies the electrical double layer, reducing the thickness of the adsorbed water layer and thus reducing the swelling capacity. Flocculation also ocurrs. In addition to the rapid ion exchange reaction, on treatment with lime there is also a slow chemical reaction which produces cementitious C-A-S-H gel and in some cases crystalline calcium aluminate hydrate and calcium silicate aluminate hydrate phases. The cementing effect of these reaction products binds the clay particles together, further resisting swelling. Therefore, lime-clay specimens cured for long periods would be expected to show increased resistance to swelling. When sulphates are present in addition to lime, the reaction at the clay particle surfaces is modified. Ettringite crystals

nucleate and grow on the surface of the kaolinite plates, within a colloidal C-A-S-S-H product, and the swelling capacity shows a dramatic increase. Wild et at. (1993) proposed that an osmotic mechanism, in which concentration gradients are generated within this surface colloidal layer, is responsible for the extreme swelling which occurs when sulphate (i.e gypsum) is present. In the current work the addition of ggbs to lime-stabilized kaolinite (Figs. 5, 6) dramatically reduced the swelling of specimens in contact with water. This, however, is not a result of increased interparticle bonding, as the addition of ggbs tends to produce a slight decrease in strength (Fig. 3) suggesting little or no improvement in interparticle bonding. The hydration of slag in the presence of calcium sulphate is known from work on supersulphated cements (Taylor, 1990) to produce C-S-H gel and

432

S. W i l d

et al.

0.7

0.6

0.5

0.4

0.3

0.2

0.1 ~K-

MIX COMPOSITION 2%L-gGGBS 9 K-4%L-6%GGBS 1K-2%L~O/oGGBS4O/oG OK4O/oL~~

9 K-6%L-4%GGBS &K-6O/o~4%GGBS~%C

I

I

I

I

I

I

2

4

6

8

10

12

14


FIG. 8. Unconfined linear expansion

soaking time for 28-day moist-cured cylinders of different slag/lime ratios, with and without gypsum.

vs.

ettringite. The calcium sulphate is rapidly consumed (within a few days) and the slag particles provide nucleation sites on which well-developed ettringite crystals form. The manner in which the ettringite forms is such that little expansion occurs. In the current work, therefore, there will be competing reactions (i.e. slag hydration activated by lime in the presence of calcium sulphate to give C-S-H gel and ettringite, and kaolinite-lime aqueous reaction in the presence of calcium sulphate to give a colloidal C-A-S-S-H product plus ettdngite) and also competing nucleation sites on the kaolinite particle surfaces and on the slag particle surfaces. Therefore, one possible explanation of the dramatic reduction in expansion, on addition of ggbs, is that the slag hydration reaction becomes the dominant reaction, and nucleation and growth of ettringite

crystals on slag particles starves the kaolinite particles of the available sulphate. Thus, ettringite nucleation on the clay particle surfaces is prevented. The nature of the reaction product which forms on the clay particles is modified and the clay no longer undergoes extreme expansion when saturated with water. When gypsum is not present, increasing the slag to lime ratio but keeping the total slag and lime constant (at 10%) results (see Fig. 4) in a systematic increase in strength. This indicates that the slag hydration reaction, which is activated by lime, is a more rapid reaction than the lime kaolinite reaction. Although for these specimens expansion is extremely small (