Engineering properties of low-carbon limestone-calcined clay-cement binder treated expansive clay: Macro-mechanical and micro-mechanism

Limestone calcined clay cement (LC3) has emerged as an environmentally friendly alternative capable of replacing up to 45% of conventional cement in infrastructures application, while its application for stabilizing expansive soils is limited. In this study, laboratory tests, encompassing both micro- and macro- perspectives, were undertaken to assess the efficacy of LC3 as a binder (4%, 8%, and 12%) in stabilizing expansive soils cured for different periods up to 56 days. The results demonstrate that the expansive soil exhibits an increase in unconfined compressive strength (UCS) ranging from 150 kPa to 500 kPa after treatment with varying dosages of LC3 over a 7-day curing period. However, a subsequent decline in UCS of 14.5% to 20% was observed at 56 days. Water stability tests revealed a substantial reduction in UCS, approximately 90% to 95%, for treated specimens after saturation in water. Hydraulic conductivity of treated soil lied between 4.69 ×10 -6 to 1.65 ×10 -5 cm/s. The microscopic analysis (i.e., XRD, MIP, and SEM-EDS) reveals that the key hydration products formed during stabilization were calcium silicate hydrate (C-S-H) and calcium aluminium silicate hydrate (C-A-S-H), leading to an augmentation in the proportion of micro-pores within the soil matrix, and a concurrent reduction in macro-pores and soil porosity. Notably, MIP analysis reveals that the extra-large pores increase with higher LC3 dosages and extended curing durations, likely due to micro-cracking induced by the swelling of expansive soil during the curing process. This study shed light on the underlying mechanism of expansive clay stabilization using low carbon LC3 binder in the application of transportation engineering scenario.