A new hypothesis for full stage deformation of cement-based materials mixed with MgO expansive agent: Generation and filling of cavity

Abstract

MgO expansive agent (MEA) is an effective admixture for compensating for shrinkage and minimizing cracking in concrete. This study reexamined prevailing explanations of its expansion mechanism, with a particular focus on the synergetic influence on early-age compressive strength. Full-stage deformations and compressive strength of cement pastes with different reactivities and dosages of MEAs were investigated. Four deformation stages were uncovered, and the correlation among deformations and pore structures was systematically analyzed from the standpoint of expansion driving forces. Particular attention was given to the generation of cavity and swelling pressure due to the unique physicochemical properties of fine brucite for water adsorption. Swelling pressure was used to explain the sharp increase in expansion and decrease in strength of cement pastes at early ages, and crystallization pressure was used to account for shrinkage compensation and strength enhancement of MEA at late ages. A new hypothesis, based on the generation and filling process of cavity varying with the reactivity and dosage of MEA, was proposed to describe the expansion mechanism of cement pastes mixed with MEA.