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Salt weathering can cause substantial deterioration of natural rocks, building stones, masonry materials, monuments, and engineering structures. Nearly two centuries of salt weathering studies, both theoretically and empirically, have manifested its power as well as its complexity. This paper attempts to unite the kinds of literature assess the various theories in the light of the combined information. The theoretical approaches concerning the most cited mechanisms of salt weathering such as crystallization, hydration and thermal expansion of crystalline salts are thoroughly reviewed. It is understood that there is no universally acceptable hard and sound theoretical information on this topic yet. More precise theories should be developed to elucidate the complications of the mechanisms of salt weathering as well as to interpret the results of empirical studies.

Sodium sulphate has been implicated as one of the most destructive weathering agents in many field observations and numerous laboratory studies. We hypothesize however, that sodium sulphate would not be invariably effective on any type of rock. To verify the supposition, a laboratory cyclic impregnation-drying experiment was undertaken. In addition to sodium sulphate, two other destructive hydratable salts, magnesium sulphate and sodium carbonate, were used to attack eight types of rock. In all three salt attacks, rock breakdown occurred only during immersion due to the exertion of higher crystallization pressure driven by the greater supersaturation reached after dissolution of the crystals precipitated during drying. Sodium sulphate was the most destructive salt in six out of the eight rocks tested, and even granite was substantially disintegrated. However, although probability is small, sodium sulphate indeed manifested its impotency against a relatively weak rock (Tago Sandstone). Contrary to its modest damaging power on other rocks, magnesium sulphate destroyed Tago Sandstone which could resist sodium sulphate attack. Sodium carbonate was the least destructive of the three hydratable salts. The general damage mechanism of hydratable salts, the process of damage of Tago Sandstone by magnesium sulphate and the possible reasons behind the impotency of sodium sulphate against Tago Sandstone are all investigated.

A laboratory salt weathering experiment was performed using fi ve salts to attack eight types of rocks to determine the relative signifi cance of rock durability and salt aggressivity to salt crystallization damage. The infl uence of individual rock properties on the salt susceptibility of the rocks was also evaluated. To study the relation between pore characteristics, salt uptake, and damage, the pre-and post-experiment pore size distributions of the rocks were also examined. It is observed that both salt type and rock properties infl uenced the damage pattern. The durability ranking of the rocks became signifi cantly altered with the salt type while the variation in salt effi cacy ranking with rock type was less pronounced. Of the fi ve salts used, sodium chloride and aluminium sulfate were invariably ineffective with all rock types while sodium carbonate, sodium sulfate, and magnesium sulfate, were markedly more effective in damaging most types of rock used. Of the rock properties investigated, the microporosity (of pores smaller than 0·05 or 0·1 µm) showed the most signifi cant infl uence on deterioration of the rocks associated with salt crystallization, whereas microporosity of pores smaller than 5 µm played a more important role in salt uptake. Pore size distribution was thus the key factor controlling salt uptake and damage. Rocks with a large number of pores (<5 µm) and a high proportion of pores (<0·05 or 0·1 µm) were particularly susceptible to salt crystallization damage. However, anomalies arose that could not be explained in terms of rock properties or salt effi cacy alone. Overall, the relative infl uences of salt type/effi cacy and rock type/ properties on salt damage propensity were not clear enough to draw a reasonable conclusion. Salt crystallization damage appears to be infl uenced by the individual interactions between salts and rocks, which could explain the anomalous results.

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