Phosphate-modified calcium aluminate cement for radioactive waste encapsulation

The present study is part of a wider investigation to develop an alternative cementing system for the encapsulation of problematic low and intermediate level radioactive waste. It has been suggested that alternative cementing systems, with lower internal pH than conventional Portland cement-based composite cements may reduce the corrosion of some reactive metals and may be beneficial for the long-term durability of wasteforms. A potential alternative is an acid-base cementing system, based on mixing calcium aluminate cement with acidic phosphate solutions. Although these systems have been studied previously, there has been no systematic investigation to identify phosphates for producing suitable matrices for application in radioactive waste encapsulation. In the current study monophosphate-modified calcium aluminate cement formulations did not set or develop significant strength, whereas polyphosphate-modified calcium aluminate cement formulations exhibited rapid setting and strength development. It is proposed that polyphosphate-modified systems form amorphous reaction products, which act as binders between the partially- and un-reacted calcium aluminate cement particles, and were responsible for high strength development. Thermogravimetric analysis and scanning electron microscopy results suggest that this binding matrix consists of amorphous calcium phosphate and alumina gel. The results presented in this investigation suggest that polyphosphate-modified calcium aluminate cement has potential as an alternative cementing system for radioactive waste encapsulation.