Insights into the role of calcination temperature and NaOH dosage in producing supplementary cementitious materials from low-grade clay via alkali-thermal-fusion

This study explored alkali thermal fusion, also known as co-calcination, of low-grade impure clay with different NaOH dosages at different calcination temperatures with a specific objective of finding out the optimum calcination temperature and NaOH dosages for the production of supplementary cementitious materials (SCMs). 0–10% by wt. dosages of NaOH were added to the low-grade clay and calcined at four different temperatures: 400 °C, 500 °C, 600 °C, and 850 °C. X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) Spectroscopy were used to observe the effect of co-calcination on the aluminosilicate phase transformations. Inductively coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), and modified strength activity index (SAI) were employed to assess the pozzolanic properties. Finally, differential thermal analyses were carried out to quantify the energy required to calcine clay along with NaOH of different dosages. The results revealed that the co-calcination method was effective at higher temperatures, enabling increased Na ₂O incorporation and modifying the aluminosilicate phase composition. Al and Si dissolution improved with increasing calcination temperature and NaOH dosages, but at 850 °C, dissolution decreased with higher NaOH dosage due to the aluminosilicate recrystallization. Additionally, mortar samples were prepared by replacing 20% of the Ordinary portland cement (OPC) with calcined clays for strength measurements. The maximum compressive strength of 40 MPa was achieved with 2.5% NaOH calcination at 600 °C, and this batch showed 20% less energy consumption during calcination compared to the one calcined without NaOH. Considering pozzolanic properties and efficiency in leaching of free Na ₂O, compressive strength, and energy requirement, the alkali fusion was optimized at 600 °C with 2.5% NaOH for calcining low-grade clay utilized in this study.