Resistance of Lime-Natural Pozzolan Mortars in Salt-Laden Environments

Salt crystallization belongs to the main decay processes in historic masonry mortars in which lime-pozzolan binders were often used. In this paper, the resistance to salts crystallization, their accumulation, and the microstructural changes of lime mortars with five different natural pozzolans are thoroughly investigated. Natural, fine ground, thermally untreated pozzolans (chalcedonite, pumice, zeolite, spongilite, and lava) were used as a partial replacement for lime in the amount of 10%, 20%, and 40%. The salt crystallization resistance of mortars were determined using Na2SO4, NaCl, and NH4NO3 solutions. The microstructure of the mortars after the testing was monitored with a scanning electron microscope, the accumulation of anions in the mortars was determined by chemical analyses and mass increases, and the formation of new minerals was confirmed using X-ray diffraction analysis. Nitrates were most accumulated in mortars with lava, sulfates in spongilite mortars, and chlorides in mortars with chalcedonite. Both crystalline and amorphous neoplasms were observed in the microstructures of mortars, the most common being nitrocalcite, cesanite, gypsum, ettringite, and hydrocalumite. The results showed that different natural pozzolans are a bit more suited to different saline environments and it should be considered when using them.