Effects of CaO-based and MgO-based expansion agent, curing temperature and restraint degree on pore structure of early-age mortar

Abstract

High-strength and high-performance concrete (HSHPC) is prone to cracking at early age due to the large thermal shrinkage and autogenous shrinkage under restraint. CaO-based expansion agent (CEA) and MgO-based expansion agent (MEA) are widely adopted to compensate for the shrinkage of HSHPC. The macro-performance of concrete is closely related to the pore structure. However, there were few researches addressing the pore structure of concrete with the combination use of CEA and MEA. In this study, the effects of CEA and MEA content, curing temperature and restraint degree on the pore structure of early-age mortar were investigated by low-field nuclear magnetic resonance spectroscopy. The experimental results show that the CEA and MEA addition, curing temperature and restraint degree had a strong influence on the pore structure development of mortar. The most probable pore diameter (MPPD) of mortar with the combination use of MEA and CEA first decreased and then increased with age, differing from the continuous decrease of mortar with CEA addition only. Meanwhile, the MPPD and porosity decreased with age curing at 20 ℃ but increased with age curing at 60 ℃. In addition, the restraint could improve the pore structure of mortar with CEA and MEA addition. Finally, an analytical bimodal pore structure model was proposed considering the CEA and MEA content, curing temperature and restraint degree. The predicted value of the model shows a good agreement with the present test data.