Slag (ggbs) on the Strength and Swelling Properties of Lime-stabilized Kaolinite in the Presence of Sulphates

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. I N T R O D U C T I O N 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