Effect of the solvent action of saturated sulfate solutions on limestones in the ASTM soundness test

Abstract

The sulfate soundness test (ASTM C88) is widely used to gauge the resistance of concrete aggregates to damage by freezing and thawing. But, saturated Mg and Na sulfate solutions can dissolve limestones, introducing a bias that is not intended in the purpose of the method, which is to estimate thephysical resistance of the stone to forces exerted by crystal growth. Although the method notes the possible dissolving of limestone by the solution and suggests that when limestones are tested a previously used solution be employed—to mitigate the solvent effect—the method does not sufficiently guard against undersaturation with respect to Ca ion. The equilibrium concentration of Ca ion, in saturated MgSO4 and Na2SO4 solutions, was determined by atomic absorption using both minus 74 micron limestone powders and the test sizes (9.5×4.75 mm). It was concluded that the degree of undersaturation could typically still be as high as 40% after 100 hrs exposure of the coarser material. The bias caused by the undersaturation varies with the type of limestone. Homogeneous, dense, crystalline limestones in which the effect of the solvent action be only to produce a slight weight loss, entirely attributable to a small amount of dissolved CaCO3, will not be greatly disadvantaged in the test. However, the same degree of solvent action on the matrix of a biomicrite will undermine the coarser fossil fragments so that they are then vulnerable to the mechanical action of sulfate salt formation, producing a significant weight loss, in a way that would not have occurred without the initial solution and weakening of the micrite phase.