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 1.5M, 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.