The microstructure of 4-year-old hardened cement-fly ash paste

In this study, the microstructure of 4-year-old hardened cement-fly ash paste containing 50% fly ash has been investigated. 65.44% of Ca(OH)2 produced by cement hydration is consumed by the pozzolanic reaction of fly ash in 4 years and the Ca(OH)2 content of the hardened cement-fly ash paste is only 3.15% at the age of 4 years. Most of the small fly ash particles react heavily but many large fly ash particles remain almost unreacted. 72.7% of fly ash remains after 4 years, which indicates that most of the fly ash takes physical effect during the hardening process of paste. Compared with cement paste, there is a relative more significant improvement of the pore structure of cement-fly ash paste due to the pozzolanic reaction of fly ash from 90 days to 4 years. The mean Ca/Si, Al/Si, and Ca/(Si + Al) atom ratios of the gel in the 4-year-old hardened cement-fly ash paste are 1.52, 0.33 and 1.15 respectively, which indicate that the gel has a lower Ca/Si atom ratio and is richer in Al content than that of Portland cement paste.