Microstructure evolution of cement mortar containing MgO-CaO blended expansive agent and temperature rising inhibitor under multiple curing temperatures

Abstract

Autogenous shrinkage (AS) and thermal shrinkage often lead to cracking in high performance concrete (HPC) under restraint conditions. MgO-CaO blended expansive agent (MC) and temperature rising inhibitor (TRI) has been adopted to reduce the AS and thermal shrinkage of HPC. The macroscopic properties of HPC strongly depend on the pore structure. Therefore, this paper studied the effects of MC and TRI on the pore structure evolution of HPC under multiple curing temperatures using low-field nuclear magnetic resonance spectroscopy (LF-NMR). Experimental results showed that the most probable pore diameter (MPPD) and porosity of mortar increased as the MC content increased due to the crystallization pressure by the hydration of MC. But MPPD and porosity decreased as the TRI content increased owing to a better filling effect of hydration products. Meanwhile, the mortar with the combined use of MC and TRI exhibited notably smaller MPPD and porosity than the plain mortar. In addition, as the curing temperature increased, the porosity of mortar containing both TRI and MC first decreased and then increased, while MPPD increased continuously. MPPD and porosity of mortar with both MC and TRI addition cured at 20 °C and 40 °C continued to decrease with age. However, MPPD and porosity dramatically increased with age at 60 °C as a result of the over-accelerated hydration of MgO.