Effects of chemical nature of polyvinyl alcohol on early hydration of Portland cement

Abstract

Polyvinyl alcohols (PVAs) are used commonly in polymer-modified mortars to manipulate their properties. The present work reports the kinetics of hydration of off-white Portland cement pastes modified by PVA (PVA/cement = 0.5 mass%) as a function of the degree of hydrolysis (DH) and the degree of polymerisation (DP). To this end, the hydrolysis of PVAs in extracted cement pore water and in cement paste was analysed using titration method, the heat flow from the hydration of unmodified and PVA-modified cement pastes were monitored over the first 24 h following mixing using isothermal calorimetry, and the Ca(OH)2 (CH) contents of these cement pastes were calculated from thermogravimetric analysis. The results show that PVAs underwent near-complete hydrolysis within the first hour following the mixing of the aqueous PVA solution with cement, i.e., the original PVA was present in the pore solution as fully hydrolysed PVA chains and released acetate groups. Addition of PVA retarded the hydration of C3S to form C–S–H and CH and also the hydration reaction of C3A and gypsum to form ettringite; however, the addition of PVA had no effect on the formation of monosulphate except for the case of samples with high DP and fully hydrolysed PVA was added which resulted in the accelerated formation of monosulphate. The DH and DP of PVA have an important effect on the hydration of Portland cement. The effects of DH resulted from the variations in the amounts of fully hydrolysed PVA and the released acetate groups.