Chemistry and performance of blended cements and backfills for use in radioactive waste disposal

The ability of NaCl and MgSO₄ to impair the performance of Portland cement, blended cements containing slag and fly ash and of a permeable backfill have been measured. Performance is determined by decrease in pH, changes in mineralogy and loss of physical coherence. Experiments have been made at 25°, 55° and 85 °C and extensively backed up by chemical models of cement performance. NaCl, up to 1.5 M, has a comparatively slight impact on performance but MgSO₄ rapidly and almost quantitatively reacts, lowering system pH's to <10, conditioned by mixtures of Mg(OH)₂ and magnesium silicates with gypsum.