Mitigation of Drying and Carbonation Shrinkage of Cement Paste using Magnesia

Abstract

In this work, the effect of magnesia (MgO) on the hydrated phase assemblage, microstructure, as well as drying and carbonation shrinkage of hardened Portland cement paste is studied. By means of X-ray diffraction, scanning electron microscopy (SEM/EDS), and thermodynamic simulation, it shows that MgO hydration does not substantially alter the phase assemblage and microstructure of hardened cement paste, except the formation of brucite. The 5% MgO addition reduces the drying shrinkage of cement by about half, which cannot be simply explained by the molar volumetric increase due to MgO to brucite conversion. The results suggest that besides generating early-age expansion offsetting subsequent drying shrinkage, the formed brucite at early stages can enhance the matrix stiffness probably due to crystal formation with reinforcing potentials. In addition, the MgO addition improves the carbonation resistance of hardened cement due to the formation of magnesium-containing calcium carbonates. However, the effectiveness of MgO in mitigating carbonation shrinkage depends on the drying and carbonation sequence.