Development of ultra-fine SAP powder for lower-shrinkage and higher-strength cement pastes made with ultra-low water-to-binder ratio

Superabsorbent polymer (SAP) as internal curing agent can effectively mitigate the autogenous shrinkage and promote cement hydration of ultra-high performance concrete (UHPC). However, it has a risk of impairing the mechanical performance of UHPC given the macro pores left. This study developed ultra-fine SAP powder aiming at securing the lower shrinkage and higher strength of UHPC. The influence of SAP powder on (average diameter (ds) of 5.78-60.85 & mu;m) the water absorption/desorption characteristics, strength, shrinkage, and microstructure of cement paste with ultra-low water-to-binder ratio (w/b) was systematically studied. The results showed that the reduction of particle size of SAP can reduce the spacing distance of SAP in cement paste, which is beneficial for the internal curing, such as reducing the autogenous shrinkage, improving hydration of cement, and increasing the compressive strength. When the ds of SAP powder was 11.55 & mu;m, it can reduce autogenous shrinkage by 72.4% without any reduction in the compressive strength compared to paste made without SAP powder. Such SAP powder led to about 5% higher hydration degree and 12.5% lower porosity compared to the conventional SAP. A quantitative approach considering the spatial distance and water absorption of SAP was proposed to describe their effect on internal curing.