Effect of Sodium Thiocyanate on the Dissolution of Fly Ash and the Hydration and Strength of Portland Cement–Fly Ash Systems

Abstract Fly ash (FA), a solid waste produced by coal-fired plants, was widely used in cementitious materials for its advantages of improving workability, optimizing durability, and reducing carbon emission. However, incorporation of FA would significantly lower the early age strength, which limits its extensive application. The reason for that is mainly attributed to the low hydration activity of FA. In this study, sodium thiocyanate (NaSCN) was used to accelerate the hydration of FA and thus increase the early strength of portland cement–fly ash (PC-FA) binder, and different curing temperatures (5°C, 20°C, and 60°C) were also considered. The behind mechanism was uncovered by analyzing the compressive strength, hydration heat, and microstructure of hardened paste. Results showed that 0.3% NaSCN increased 1 day and 28 day compressive strength of mortar cured at 20°C by 66.72% and 28.72%, and that for 5°C was improved by 96.08% and 60.90%. The reason for this phenomenon was attributed to the improvement on degree of hydration of binder by incorporating NaSCN, especially for FA. At the early age of hydration, the enhancement in activity of FA directly facilitated the pozzolanic reaction and thus contributing to the strength. The mechanism was mainly that NaSCN facilitated the dissolution of ions in FA and these dissolved ions were chelated by SCN − to form stable complexes, and this inhibited the precipitation on the surface of FA and avoided the formation of coating on the surface of FA particles, thereby promoting the hydration of FA. These findings are expected to provide a potential solution for slow strength development of high volume FA binder, promoting the wide application of FA.