Influence of Phosphorus Doping on Metastable Structure and Properties of Alite

The influence of phosphorus doping on the crystal structure, phase transformation, microstructure and hydration properties of alite was investigated by X-ray powder diffraction with the Rietveld method, differential scanning calorimetry and Fourier transform infrared spectroscopy, scanning electron microscopy and isothermal calorimetry, respectively. The results show that the P₂O₅ addition of 1% (in mass fraction, the same below) can stabilize R-type alite. However, it is unstable at 650-850℃, which transforms into lower temperature forms. P₂O₅ at a lower content (i.e., ≤0.2%) does not affect the hydration characteristics and strength properties of alite. When the content of P₂O₅ is close to the morphotropic phase boundary, alite shows the highest early hydration reactivity. The length-diameter ratio of alite and the early hydration reactivity decrease, the main hydration slows down, but the rate of hydration increases during the deceleration period when the amount of P₂O₅ increases. The stabilization of R-type alite can greatly improve the compressive strength of alite.