Rheology modification of flowable mortar with CO2

Abstract

The rheological properties of mortar samples with different additions of CO2 during mixing were determined. Flowable cement-based mortars at three different w/c (0.55, 0.45 and 0.35) were produced and the effects of CO2 gas were examined on the fresh properties of slump flow, yield stress and plastic viscosity. According to the experimental results, the mortar yield stress and plastic viscosity increased faster over time than in a reference system with the increasing addition of CO2 and greater effects were seen at the lowest w/c of 0.35 than at the others. The impact of the CO2 on the yield stress was more pronounced than on the viscosity. In the w/c 0.35 system the properties showed no effect at dosages of 0.06% and 0.13% CO2 by weight of cement. Increasing dosages to 0.19 and 0.25% resulted in an increasing reduction in slump flow, immediate increases in yield stress of 62 and 298%, increases in structural buildup by a factor of 5 and 10, respectively, and increased the initial plastic viscosity by 33 and 148%, respectively. The effects are attributable to the growth of reaction products decreasing the interparticle spacing and increasing the surface area as associated with the CaCO3 and gel phase. CO2 was demonstrated to be a potential yield stress control agent; an appropriate dose can increase rheological properties as measured immediately after and increase their rate of change with time. This aligns with specialized applications that hinge upon changing rheology such as form pressure control or 3D concrete printing.