Thermogravimetric analysis and phase characterizations of Portland fly ash limestone cements

Abstract

In this work, 50% of the Portland cement was replaced with fly ash and limestone powder by mass. Fly ash to limestone powder ratios of 50:0, 45:5, 40:10, and 30:20 were used. Thermogravimetric analysis (TGA), X-ray diffraction and scanning electron microscopy of the paste mixes were carried out after water curing for 7 and 28 days to determine the hydration and pozzolanic reaction that occurred during the early age and after the standard 28-day curing. In addition, the compressive strength and physical properties of the corresponding mortar mixes were also investigated. At 7 days, the use of limestone with fly ash led to an increase in compressive strength when compared to that for a fly ash mix without limestone (50% FA). This was attributed to the filler effect of the limestone rather than the reaction of the limestone powder with C3A as there was no hemicarboaluminate detected; this was possibly due to a decreased amount of C3A being available since 50% of the Portland cement was used. At 28 days, the 45% FA and 5% LS mixture showed an increased compressive strength, a decreased water absorption and a decreased number of voids. Again, this can be attributed to the filler effect of the limestone. Interestingly, at 28 days, monocarboaluminate was detected by TGA and XRD when the limestone powder was added.