Effects of Lightly Burnt MgO Expansive Agent on the Deformation and Microstructure of Reinforced Concrete Wall

Abstract

Compensation for shrinkages with three kinds of lightly burnt MgO expansive agent (LBMEA) is used in a reinforced concrete wall poured in the summer. Influences of the internal temperature history on the expansion of concrete and the microstructure of cement paste containing LBMEA were investigated. The results showed that LBMEA exhibited significant expansion around the end of the fall temperature stage; then, the expansion rate declined obviously, and concrete containing LBMEA with low hydration reactivity (140 s and 220 s) showed larger expansion than LBMEA with high hydration reactivity (60 s). Microstructural analysis indicated that brucite preferentially forms in the pores in cement paste containing LBMEA with high reactivity, but brucite mainly grows on the surface of the MgO particles in cement paste containing LBMEA with low reactivity during the early age. Paste containing LBMEA with low reactivity showed a larger volume of single brucite crystal than LBMEA with high reactivity, which further led to larger expansion in the latter than the former. The results revealed the expansion process of LBMEA and can help engineers select suitable LBMEA for application to actual engineering.

Effect of the Water-Binder Ratio on the Autogenous Shrinkage of C50 Mass Concrete Mixed with MgO Expansion Agent

“…According to Chinese standard GB/T 50081-2019 [ 42 ], the concrete was poured into molds whose dimensions were 150 mm × 150 mm × 150 mm. Cured in the variable temperature oven for 24 h, concrete specimens would be removed from molds and wrapped in plastic wrap, after which they continued to be cured.…”

Effect of Mixing Light-Burned MgO with Different Activity on the Expansion of Cement Paste

“…She found that the hydration reaction of light-burned MgO is consistent with the first-order reaction kinetics; that is, the hydration process is controlled by chemical reactions. Jiaping Liu [23] and Changwen Miao [24] found that the effect of increasing temperature on hydration of high activity magnesium oxide expansion agent is more significant. The expansion of high activity magnesium oxide expansion agent at normal temperature is lower than that of low activity magnesium oxide expansion agent, but higher than that of the low activity magnesium oxide expansion agent at high temperature.…”