Biochar-Engineered Limestone Calcined Clay Cement (LC<sup>3</sup>): Early Strength Evolution and Phase Development

Limestone calcined clay cement (LC3) typically exhibits low early-age strength, limiting its practical use in construction. To address this, the present study performs a comparative assessment of three waste-derived siliceous additives; rice husk ash (RHA), rice husk biochar (RHB), and rice straw biochar (RSB), to enhance early strength development. A comprehensive experimental program evaluated their effects on consistency, compressive strength, ultrasonic pulse velocity (UPV), microstructure, and hydration products using TGA/DTG, FTIR, XRD, and SEM analyses. Results showed that LC3-RHA significantly improved compressive strength at 7 and 28 days. It has been observed that LC3-RSB exhibited the highest early strength gain (3 d), attributed to enhanced pozzolanic activity, pore refinement, and internal curing. The hybrid analytical approach, based on qualitative and quantitative analysis, has been employed and it was observed that LC3-RSB supersede the LC3 strength by 29%, 14% and 19%, at 3, 7, and 28 days, respectively. And have marginal difference form the ordinary Portland cement (OPC) strength. TGA and FTIR analyses confirmed increased carbonate formation, evidenced by mass retention in the 700 to 850 °C range and characteristic bands at 1410 to 1500 cm−1, indicating CO2 capture via physical adsorption and mineralization. RSB outperformed RHB in carbonation potential due to its higher porosity and favorable ash composition. Overall, biochar-modified LC3 offers a sustainable, carbon-negative binder solution by valorizing agro-waste, reducing clinker content, and improving environmental performance.