Developmental study of a low-pH magnesium phosphate cement for environmental applications

Abstract

This paper presents a laboratory investigation into the development of a low-pH magnesium phosphate-based cement for use in in-situ stabilisation/solidification systems, as a more effective alternative to Portland cement. The idea is to develop a cement-based binder with a low enough pH to facilitate biodegradation in combination with stabilisation/solidification processes. Potassium dihydrogen orthophosphate was selected as the phosphate source, dead-burned magnesia as the magnesium source and boric acid as the retarder. The range of mixes were tested primarily on their pH development which was found to be in the range of 6-9.5 for a magnesia to phosphate ratio range of 1:1 to 1:5. The testing revealed a dense microstructure, high early-age strength development and low volume expansion of the developed cement. Observed fracturing of some of the cured cement samples has been related to the curing conditions and the impurities present in the magnesia. On the basis of microstructural examination, observed white crystalline deposits on cured samples are likely to be a reaction product of magnesia and potassium dihydrogen phosphate.