Compressive strength, characterizations and drying shrinkage mechanism of mortars using calcined clay from different sources

Metakaolin and calcined clay are widely available globally that can be used as supplementary cementitious materials. In this research, the potential of low-grade calcined clays as Portland cement replacement was compared against metakaolin (MK) to develop a more reactive pozzolan for use as supplementary cementitious materials (SCMs). Lampang, Lopburi calcined clays in Thailand and metakaolin products was used at 10% wt. and 20% wt. of Portland replacement. The compressive strength, drying shrinkage and phase characterizations were investigated. X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM) were used in the characterizations analysis. The results showed that compressive strength increased when replacing calcined clays at 10-20% wt. of Portland cement. The compressive strength of Lampang and Lopburi calcined clay replacement had performance to metakaolin, however, Lampang calcined clay was found to be the optimum replacement at 10% wt. It is also interesting to note that in drying shrinkage testing, it was found that the three calcined clay from different sources resulted in different shrinkage mechanisms. The drying shrinkage of calcined clays varies due to chemical and physical factors. MK shows higher autogenous shrinkage at early ages due to smaller particle size and higher surface area which resulted in faster pozzolanic reaction. In contrast, Lampang calcined clay (LAM-CC), with impurities and larger particles, exhibits less autogenous shrinkage at early ages but higher drying shrinkage at later ages due to water loss. Lopburi calcined clay (LOP-CC) shows similar early shrinkage to Portland cement, but its long-term shrinkage is lower due to continuous hydration and water retention, despite its larger particles size than MK and LAM-CC. This study demonstrates the feasibility of Lampang and Lopburi calcined clay for use as SCMs to contribute to sustainable alternatives in cement production.