Compressive strength development of steam cured cement paste with high volume wet-grinded coal fly ash

In this study, superfine fly ash slurry [WGFA] was prepared by wet grinding from the coal fly ash. Cement-fly ash binder with 30%, 50%, and 70% dosage of fly ash was designed, with a water/binder ratio of 0.5. The samples were firstly cured for 20 hours under steam curing at 60°C and then cooled to room temperature within 4 hours, followed by further curing to 28 days of age under standard curing condition. The compressive strength at 1 day and 28 days were examined, and the results were discussed in terms of hydration process and pore structure, which were evaluated by XRD, TG, SEM, NMR, and MIP. The results showed that 1 day compressive strength of the system with up to 50% WGFA was higher than that of the cement system. The reason was due to the high reaction activity of WGFA under steam curing and the excellent filling effect of fine particles. However, further curing to 28 days, the strength increase was surprisingly unacceptable, with an increase by 18% at 30% dosage, by 10% at 50% dosage, and by 11% at 70% dosage. These increases were much less than that in raw fly ash system. One reason was due to the denser structure which hindered the transportation of ions and water molecules for further hydration; the other reason was because of very little calcium hydroxide in the system, to activate the pozzolanic reaction of fly ash.