Mechanism for the modification of Portland cement hydration using polyacrylic acid

Determining ways to increase the dissolution and setting of the calcium silicate phases which make up Portland cement (PC) will be advantageous in producing a dense, strong and ultimately a durable cementitious material. Polymers including polyelectrolytes such as polyacrylic acid (PAA) have been known to modify the dissolution behaviour of PCs. In the present work the hydration of PC paste with PAA is investigated. Considerable changes in microstructure of the hydrated cement pastes were observed in the presence of PAA as compared with the control paste. The physical properties of mortars containing PAA were also studied. The presence of PAA alone in a paste or mortar did not improve the compressive strength. A combination of metakaolin (MK) and PAA significantly improved the properties of the mortar over those of the mortars containing MK only. Furthermore, the combination of MK and PAA much improved the workability of the mortar thus allowing reduction in the water:cement ratio, while aiding its placement and finishing in the moulds as well as considerably increasing the strength developed. The mechanism by which PAA with and without MK alters the hydration of PC is discussed. Possible reasons for the synergistic effect of MK and PAA combination on the cement hydration behaviour are given.