Influence of S-type Magnesia Expansive Agent on Micro-structure, Shrinkage and Durability of Concrete

Incorporating magnesia expansive agents (MEA) into concrete is one of the effective ways to solve the deterioration of concrete performance due to cracking caused by shrinkage. S-type MEA can provide a stable micro-expansive to compensate for shrinkage of concrete over a long period of time because of the low activity of hydration, which is widely used in engineering. In this paper, S-type MEA was mixed with concrete at different MEA contents (i.e., 4%, 5% and 6%, mass fraction). The effect of MEA on the microstructure and macroscopic properties (i.e., mechanical, shrinkage and durability) of concrete was investigated. The results show that the crystal growth of the hydration product magnesium hydroxide expands the microscopic pores of concrete with MEA, and the porosity of concrete increases from 7.93% to 10.63% at 28 d when the MEA content is 5%. Therefore, the concrete strength before 28 d and the resistance to chloride ion penetration are reduced. Also, the content of pores in the range of 10–50 nm decreases, thus improving the shrinkage performance of the concrete. The autogenous shrinkage is reduced by 25.66%, 18.58% and 46.02% when the MEA content is 4%, 5%, and 6%, respectively. As the MEA hydrates continuously, the gradual filling effect of magnesium hydroxide reduces the porosity and improves the strength of concrete with MEA. However, swelling from un-hydrated MEA is coupled with the effect of either freeze-thaw cycles or sulfate attack with increasing the MEA content, thus having a negative effect on the durability of concrete. In addition, the shrinkage model and chloride ion permeability coefficient model of concrete with MEA were also proposed to predict the long-term performance of concrete, thus having a certain guiding value on the engineering applications. © 2023 Chinese Ceramic Society. All rights reserved.