Strength and microstructure development in Bangkok clay stabilized with calcium carbide residue and biomass ash

Abstract

A mixture of biomass ash (BA) and calcium carbide residual (CCR) was used as a cementing agent for stabilization of soft Bangkok clay. The improvement in unconfined compressive strength of stabilized clay depends on the initial soil water content, binder content (B), CCR:BA ratio, and curing time. The strength improvement can be classified into two zones: active and inert. In the active zone (B = 5-15%), the Ca(OH)2 content from the CCR is high and there are insufficient natural pozzolanic materials in the clay for the reaction. The input of BA increases the SiO2 content and induces the strength development. In the inert zone, (high binder content, B > 15%), the free lime causes unsoundness and results in insignificant strength development. The hydration products from the pozzolanic reaction (ettringite and a calcium aluminate silicate hydrate compound, gismondine) are identified by microstructural analysis via X-ray diffraction (XRD), scanning electron microscope with energy dispersive X-ray spectroscopy (SEM with EDS), and X-ray fluorescence. SEM images show the compact morphology of the stabilized clay as the result of the increase in curing time and binder content. Over time, the cementitious products fill in the pore space, causing denser morphology and higher cementation bond strength between the clay clusters. The clay stabilized by XRD showed no reduction in the intensity of the reflection of the both quartz and kaolinite throughout the curing times. This implies that the amorphous silica from the glass of the BA is more reactive in dissolution in the CCR than the crystalline phase of quartz and kaolin in clay.