Early-age mechanical properties and hydration degrees of magnesium phosphate cement paste in freezing winter of cold regions

Abstract

Magnesium phosphate cement (MPC) paste is a good patching material for concrete repair works. This study focused on early-age mechanical properties and hydration degrees of MPC paste in freezing winter of two types of cold regions, which are highland regions and high-latitude regions. MPC pastes with different mix proportions under different curing modes were experienced a series of experiments, including fluidity and setting time, compressive strength, pore structure characterization, thermogravimetric analysis, X-ray diffraction and electron microscopic scanning. Results showed that for winter applications, MPC pastes in simulated highland temperature conditions performed better than those in true high-latitude temperature conditions, illustrated by pore structure analyses and hydration products analyses. The best M/P ratios for highland applications and high-latitude applications were respectively 4 and 5, which respectively delivered 24 h strengths at about 50 MPa and 25 MPa. Even in severe cold winter, the setting of MPC paste depended on the hydration speed instead of the frozen action. Mixing water temperature was good for early-age hydration of MPC paste which cured for six hours in natural cold environment, however, the effect vanished at 24 h. Both the amounts of struvite and unhydrated phosphate, which could be quantitatively assessed by TG-DSC tests or qualitatively assessed by XRD test, were good indicators of the hydration degree of MPC paste in cold weather. These results provided valuable suggestions for winter applications of MPC paste in different types of cold regions.