The effects of admixtures of inorganic hydrates on the hydration hardening of magnesium potassium phosphate cement

Admixtures of Na2B4O7·10H2O (NB), Ca(NO3)2·4H2O (CN) and Na2SO4·10H2O (NS) are studied to discover how they affect the work performance, hydration-induced increases in temperature and mechanical performance of magnesium potassium phosphate cements (MKPCs). Analytical methods such as X-ray diffraction spectroscopy, eight-channel microcalorimetry and thermogravimetric-differential scanning calorimetry are used to investigate the underlying mechanisms of these changes. Incorporation of a single retarder (NB) results in short MKPC hardening times and low early strengths. Admixture with 1·5% or more NB and CN effectively lengthens the hardening time of MKPC, but greatly decreases its fluidity. Incorporation of NB, CN and NS in the ratios 1·5%:1·5%:5% results in an initial hardening time of 37·02 min and the appearance of two temperature peaks in the hydration temperature curve. The resulting cement mix has fluidity of 115 mm and strength of 52·56 MPa after 12 h of hydration. Reaction between NB and CN creates a calcium borate film that envelopes magnesium oxide particles, preventing dissolution of magnesium oxide and generation of the hydrated product MgKPO4·6H2O (MKP), thus slowing down the hydration process. Incorporation of an appropriate quantity of NS endows fluidity of MKPC that can reach 125 mm, owing to crystal waters released during solvation and changes in phase.