## 3 Results and Discussion
### Microstructural Analysis  
The compacted \"pure\" kaolinite seems to possess a leaf-like structure as indicated by Figure 8a. A complex set of reactions could occur when kaolin clay comes in contact with water, one of which can involve the formation of an extensive absorbed film resulting from the existence of a concentration gradient composed of water molecules and exchangeable ions. When gypsum is added to kaolinite, depending on the quantity and curing duration, this may lead to a reduction in volume or increased strength, thus acting as a stabilising agent. Soluble gypsum does contain calcium hydroxide, and the process of ionic exchanges (Ca\({}^{2+}\) from gypsum) with the soil (Al\({}^{3+}\) and Si\({}^{+}\)) and the development of the products of hydration (such as calcium silicate hydrate (CSH) or calcium alumino-silicate hydrate (CASH)) would lead to increased strength or volume reduction. Figure 8b, therefore, indicates a well-connected soil matrix with fewer pores made possible by wrapping of  
Figure 7: Behaviour of samples at failure: (**a**) unstabilised soils; (**b**) stabilised soils with 10% gypsum under 100 kPa constant effective pressure; (**c**) stabilised soils with 10% gypsum under 200 kPa constant effective pressure; (**d**) stabilised soils with 20% gypsum under 100 kPa constant effective pressure; (**e**) stabilised soils with 20% gypsum under 200 kPa constant effective pressure; (**f**) stabilised soils with 30% gypsum under 100 kPa constant effective pressure; (**g**) stabilised soils with 30% gypsum under 200 kPa constant effective pressure.  
the fabric by gypsum with formation of polymer clusters. When stabilised by lime, the possibility of ettringite formation was increased (Figure 8c,d). Although they could be deleterious in some respects especially in terms of soil deformation; it should also be noted that ettringite formation is a necessary process during the hydration process for an increase in strength. As indicated previously, for a gypsum-time-GGBS-soil system, complex sets of reactions occurring at the same time are feasible. GGBS hydration with lime as a catalyst or an activator in the presence of gypsum can lead to the formation of C-S-H and ettringite products. Furthermore, soil-line reactions in the presence of gypsum would cause the formation of C-A-S-H and ettringite products including the development of nucleation sites in the soil and GGBS surfaces. GGBS present in greater quantities compared to lime would tend to have more influence and, thus, contribute more to the formation of cementitious gels required for hydration to enhance a reduction in ettringite formation. The lime-GGBS ratio needed for this to occur should be approximately 2 or more [11]. However, because of the higher amounts of sulphates present in the stabilised soil as is the case in this study, the amount of GGBS used (8% by dry weight of soil) compared to that of lime (4% by dry weight of the soil) may not have been sufficient to prevent any potential negative effects of sulphates, resulting in the traces of ettringites suggested in Figure 8e. As also noted previously, the presence of unreacted gypsum and the development of ettringite minerals may disturb the soil matrix, leading to larger voids and subsequent strength reduction at 28 days of curing [38; 39].  
Figure 8: Scanning electron micrograph: (**a**) Pure soil (kaolinite); (**b**) sulphate soil; (**c**) lime-stabilised sulphate soil [36]; (**d**) lime-stabilised sulphate soil [36] (**e**) lime/GGBS-stabilised sulphate soil [15].