Integrating wheat straw and silica fume as a balanced mechanical ameliorator for expansive soil: a novel agri-industrial waste solution

Resource utilization of agricultural and industrial wastes with minimal screening is highly desirable in the context of sustainable development and environmental protection. In this regard, the current study proposes a novel solution of integrating milled wheat straw (WS) with minimal screening and silica fume (SF) in the form of composite binary admixture (CBA) for the stabilization of highly expansive soils. The optimum amount of WS and SF to produce CBA was determined based on a series of Atterberg’s limit tests. The mechanical performance of CBA-treated soil was assessed based on the unconfined compression, direct shear, and flexural tests which showed that unconfined compressive strength ( q u ), cohesion ( c ), and flexural strength ( f ) were increased by 94.3%, 65.7%, and 90.7%, respectively, with an addition of 16% of CBA and 28 days of curing. Furthermore, the CBA-treated soil underwent only a 26% reduction in deformability index ( I D ) with an addition of 24% CBA. Furthermore, volumetric change response was assessed based on ID consolidation and swelling tests which showed that compression index ( C c ), recompression index ( C r ), swell potential, free swell index (FSI), and swell pressure were reduced by 72.5%, 47.7%, 59%, 35.8%, and 65%, respectively, with an addition of 16% CBA in the soil and 28 days of curing. In addition, wetting-drying (W-D) cycle tests demonstrated that CBA-treated soil was less vulnerable to W-D seasons as compared to untreated soil. Mineralogical and microstructural tests revealed that the balanced Ca:Si and Ca:Al environment created by CBA within the soil matrix produces cementing compounds, i.e., CSH and CAH, imparts strong bonds, and causes aggregation improving the mechanical response of expansive soil.