Investigation on high-volume fly ash pastes modified with micro-size metakaolin subjected to high temperatures

Abstract

Portland cement (PC) containing high-volume fly ash (HVFA) is usually used to obtain economical and more sustainable merits, but these merits suffer from dramatically low compressive strength especially at early ages. In this work, the possibility of using micro-size metakaolin (MSK) particles to improve the compressive strength of HVFA paste before and after subjecting to high temperatures was studied. To produce HVFA paste, cement was partially substituted with 70% fly ash (FA), by weight. After that, FA was partially substituted with MSK at ratios fluctuating from 5% to 20% with an interval of 5%, by weight. The effect of MSK on the workability of HVFA mixture was measured. After curing, specimens were subjected to different high temperatures fluctuating from 400 to 1000 °C with an interval of 200 °C for 2 h. The results were analyzed by different techniques named X-ray diffraction (XRD), thermogravimetry (TGA) and scanning electron microscopy (SEM). The results showed that the incorporation of MSK particles into HVFA mixture exhibited a negative effect on the workability and a positive effect on the compressive strength before and after firing.