Effect of different expansive agents on the early age structural build-up process of cement paste

CaO-based expansive agent (CEA) and MgO-based expansive agent (MEA) are commonly used expansive agents to control the shrinkage of concrete. Understanding the effect of CEA and MEA on the structural build-up of cement paste is of significance to their application in 3D printing concrete. This study investigates the effect of CEA and MEA on the structural build-up process of cement paste within 120 min. The calorimetric, 1H low-field nuclear magnetic resonance and ion concentration tests were carried out to explore the intrinsic mechanism. The findings suggest that CEA significantly accelerates structural build-up rate by elevating the ionic strength in solution and facilitating cement hydration. The addition of 1%, 3% and 5% CEA increases the 120-min static yield stress of cement paste by 40.6%, 66.8% and 132.9%, respectively. MEA increases the initial static yield stress by decreasing the interparticle distance through its great specific surface area and intensifying interparticle interactions, and the addition of 1%, 3% and 5% MEA increases the initial static yield stress of cement paste by 22.5%, 35.3% and 66.5%, respectively. However, the delaying impact of MEA on hydration offsets its improvement of interparticle interaction, leading to an insignificant impact on the static yield stress evolution. The combination of CEA and MEA takes advantage of their respective benefits in promoting hydration and flocculation, which increases the structural build-up rate while increasing the initial static yield stress of the paste. This study can provide guidance for the rheological controlling of 3D printing concrete containing expansive agent.