Adiabatically cured, alkali-activated cement-based wasteforms containing high levels of fly ash

Abstract

Cementitious binder compositions with high fly ash contents proposed for immobilization of highly alkaline low-level radioactive waste solutions have significantly different chemistry and behavior than normal Portland cement pastes. In this paper, we investigate the mineralogy of a simulated wasteform proposed for the immobilization of the tank wastes stored at the Hanford Reservation, Washington State, USA. These solutions have very high (multimolar) concentrations of sodium, aluminate, phosphate, carbonate, nitrate, and nitrite ions. The cementitious blend proposed to solidify this solution contains a high proportion of fly ash to avoid the large heat output that would be associated with the hydration of a neat Portland cement. In addition to calcium silicate hydrate (C-S-H), the binding phase normally formed by hydrating cements, zeolites also formed during adiabatic curing up to 90°C. The zeolites, sodalite and Na-P1 (gismondine framework), immobilize part of the sodium in the system. Compared to C-S-H in ordinary Portland cement (OPC), the C-S-H in this system, which had a layer spacing of 1.1 nm, was more crystalline, and had a longer chain length and a higher aluminum content. Changing the curing conditions had a large effect on early age mineralogy, for example, traces of chabazite are formed on isothermal curing at 90°C. The long-term mineralogy depended mainly on the final curing temperature.