Geopolymer stabilization of carbon-negative gasified olive stone biochar as a subgrade construction material

Biochar is a carbon rich substance derived from biomass pyrolysis or gasification that can capture and storage carbon. The utilization of biochar as a host material in geopolymer composites to replace quarry aggregate offers dual benefits: it reduces reliance on natural resources and sequesters carbon within infrastructure. This study proposed a novel design of geopolymer stabilized olive stone biochar (OSB) composites intended for use as a road subgrade construction material. Industrial residues, specifically fly ash (FA) and ground granulated blast furnace slag (S), served as precursors for the geopolymerization process. The strength of these geopolymer composites was assessed using the unconfined compressive strength (UCS) test. Scanning electron microscopy (SEM) was employed to examine how precursor type, dosage, curing period, and temperature affected the UCS performance from a microstructural standpoint. S-based geopolymers generally showed higher UCS values compared to FA-based ones. Curing time and temperature were crucial in the OSB geopolymerization process, with longer curing periods or higher temperatures typically resulting in greater UCS. The peak UCS for OSB using both FA and S-based geopolymers was approximately 6 MPa. Given considerations of cost-effectiveness, feasible curing conditions, and mechanical strength, it is recommended to use FA or S-based geopolymers containing 30 % precursor, cured at 20°C for 7 days, for OSB stabilization. Under these conditions, OSB + 30 % FA and OSB + 30 % S geopolymers achieved UCS values of 1.064 and 2.099 MPa, respectively, exceeding the local requirement of 700 kPa UCS for stabilized road subgrades. California Bearing Ratio (CBR) test results for the optimal blends showed significant enhancements in CBR values with the addition of FA-based or S-based geopolymers, thereby reducing pavement thickness and construction costs. The repeated load triaxial (RLT) test further evaluated the dynamic strength of geopolymer-stabilized OSB composites, demonstrating their potential as substitutes for quarry aggregates in subgrade materials. Samples with 30 % FA or S, cured for 7 days at 20°C, were sufficiently stiff to withstand simulated traffic loads. Economic analysis suggested that for the same volume used, the cost of OSB was comparable to that of natural aggregate. Carbon footprint assessment indicated that geopolymer stabilization of OSB using 30 % FA or S were carbon-negative materials. Incorporating OSB into geopolymers can mitigate the carbon footprint in the construction sector and promote the development of more sustainable subgrade construction materials.