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 substantiallythe 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 swellingpressures. This excessivevolume instabilitywhen 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 i m m e d i a t e benefit o b t a i n e d by the lime t r e a t m e n t of clays is the reduction or complete elimination of swelling potential and swelling pressures (Mitchell & H o o p e r , 1961; Ingles & Metcalf, 1972; D i a m o n d & Kinter, 1964; Bell & Tyrer, 1987; L u n d & R a m s e y , 1959). These modified characteristics are attributed to substitution of the clay cations by calcium and subsequent formation of calcium silicate and aluminate hydrates. The reduction in swelling results from decreased affinity for water of the Ca-saturated clay and the formation of a cementitious matrix which resists volumetric expansion. The reaction of lime with clays produces C-S-H ( D i a m o n d et al., 1964) or C - A - S - H (Wild et al., 1989) gels together with, in some cases, crystalline calcium aluminate hydrate ( D i a m o n d et al., 1964) or calcium silicate aluminate hydrate (Croft, 1964) phases. (In cement terminology, the following abbreviations are used: C: CaO; A : A1203; S: SIO2; S: SO3; H: H 2 0 . ) W h e r e carbonation occurs, carbo-aluminates m a y also form (De Silva & Glasser, 1990). If sulphates which are quite c o m m o n in clay soils are present, then b o t h the reaction mechanism and the reaction products are modified (Mitchell & D e r m a t o s , 1990). Normally the calcium sulpho-aluminate phase ettringite (C3A.3CS.H32) is formed and at low sulphate concentrations the metastable phase (C3A.CS.H12) m a y also be observed. In addition, at low t e m p e r a t u r e s (
