The effect of internal relative humidity on the early age deformation of cement paste containing magnesia

Abstract Magnesia (MgO) is a kind of expansive agent to compensate volume shrinkage and mitigate cracking normally used in mass concrete and high-performance concrete. The hydration of magnesia is sensitive to changes in relative humidity (RH) and previous researches show strong correlation between magnesia’s expansive behavior and curing conditions, but the effect of internal RH on cement paste with magnesia and the mechanism behind the phenomenon is yet uncovered. In this research, the influence of RH is investigated from aspects of deformation, pore structure, mechanical properties and hydration. Results show that specimens under the three curing conditions had RH levels in the order of SAP internal curing, underwater curing and sealed curing form the highest to the lowest. Internal RH has impact on the expansive behavior of magnesia by affecting the shrinkage induced by capillary pressure and the development of the stiffness of cement paste. The deformation is then further interpreted by poro-elastic approach with adoption of capillary pressure and crystallization pressure theory, in which the estimated deformation agrees in trend with the experimental results, and RH level shows influence on the impact of hydration degree of MgO on expansive strains. The crystallization pressure due to Mg(OH)2 growth is a plausible driving force for the expansion of cement paste.