Effect of Solution Concentration on the Properties of a Cementitious Grout Wasteform for Low-Level Nuclear Waste

Abstract

A simulated low-level nuclear waste solution was studied for possible solidification in a cement-based matrix. The waste composition was based on an alkaline mixture of Na₃(PO₄)·12H₂O, NaNO₂, Na₂CO₃ and Al(NO₃)₃·9H₂O, and the binder composition was cement (21%), fly ash (68%), and attapulgite clay (11%). The materials were mixed at a high solution-to-binder ratio of 1.0 l/kg, and curing temperatures varied from 45 to 90°C. The effect of changes in solution concentration was studied. Solution concentration ranged from a dilution to 5.5% (designed to simulate a possible off-gas condensate obtained during vitrification of the waste) to the full concentration of the simulated waste. Compressive strength and early age heat development increased as the concentration was increased up to 67%, but at higher concentrations both compressive strength and heat development decreased. X-ray diffraction and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy pointed to a high degree of reaction of the fly ash in the mixes and formation of zeolites at the higher concentrations. Na-P1 zeolite formed in increasing quantities as the concentration was raised to 67%, but at the highest concentrations the zeolite formed was sodalite.