Laboratory investigation into the modification of transport properties of high-volume fly ash mortar by chemical admixtures

This work aims to optimize the transport properties of high-volume fly ash (HVFA) mortars that replace Portland cement with a Class C fly ash at 60% by weight. Ten chemical admixtures (five nonpolymers and five polymers) previously reported to improve the early-age strength or water resistance of concrete were investigated for their effects on the HVFA mortars. A total of 26 mortar mixtures were designed and tested, based on a statistical design to explore the potential synergistic effects among the polymeric admixtures and the nonpolymeric admixtures, respectively. At the age of 28 days, the compressive strength, splitting tensile strength, and water sorptivity of all mortar mixtures and the gas permeability of six selected mixtures from each group were tested. In addition, the surface free energy and pore size distribution of the selected mortars were obtained in the effort to correlate with water sorptivity and gas permeability, respectively. The predictive models built on the results of designed experiments suggest that if used individually, tributyl phosphate, hydroxypropyl methyl cellulose, and hydroxyl terminated polysiloxane are the three admixtures that can effectively reduce the water sorptivity of HVFA mortars. For the six selected HVFA mortars, those containing polymeric admixtures exhibited a significantly lower gas permeability coefficient than those containing nonpolymeric admixtures. The water sorptivity of HVFA mortars was mainly affected by the microstructure of mixtures rather than their surface free energy. The nonpolymeric admixtures worked better in refining the pore structure in the HVFA mortars and reducing the critical radius than the polymeric admixtures.