Effect of expansive additives and external restraint on the early age mechanical properties and microstructure of cement paste

The early age expansion of cement paste/concrete due to expansive additives (EA) has a significant impact on the microstructure and early-age mechanical properties of cement paste and concrete. Moreover, external restraints owing to reinforcement and supports play a crucial role in reducing concrete expansion and may also affect the microstructure. In this study, a measurement technique to monitor the development of early age elastic modulus of paste (from casting to hardened stage) focusing on the effect of external restraints was established. The influence of external restraint on the microstructure and elastic modulus was examined using the Elastic Modulus Measurement Ambient Response Method (EMM-ARM) and mercury intrusion porosimetry (MIP). This study also focuses on the effect of water-to-binder (w/b) ratios and amount of EA on the early-age mechanical properties of cement paste. The experimental results revealed that under unrestrained conditions, the addition of EA caused the microstructure of the paste to become porous, resulting in a decrease in mechanical properties. Conversely, under restrained conditions, the elastic modulus of the paste with EA and the level of porosity were comparable to the paste without EA. These findings have significant implications for the utilisation of EA in concrete construction and provide potential avenues for enhancing the durability and performance of concrete structures.