Chiang Mai Journal of Science

       Limestone and calcined clay, due to their abundance and sustainability are seen as feasible materials to be used to replace Portland cement in the future. In this work, Portland cement was replaced with calcined clay and limestone at 50 % by weight. In addition to compressive strength, the microstructure and phase characterizations were investigated at 7 and 28 days to determine hydration and pozzolanic reaction of the mixes using thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. When using 20% calcined clay, the 50PC30LS20CC mix showed an increase in the compressive strength by 225% compared to the 50PC50LS mix. Although the compressive strength was still lower than 100 PC control mix, using calcined clay and limestone together lead to a significant reduction in the water absorption compared to 50% LS mix due to the phases formed and the filling effects, and comparable result to 100PC mix was found when calcined clay was used at 20%. In addition to calcium carbonate detected in mixes with limestone, monocarboaluminate phase was also found at 7 days and became clearer at 28 days, especially when there was a combination of calcined clay and limestone in the mix due to the presence of alumina from calcined clay. Moreover, pozzolanic reaction from the use of calcium clay resulted in the increase of C-S-H and C2ASH8 which increased with calcined clay content and corresponds with a reduction in calcium hydroxide and an increase in compressive strength.