Chalcedonite aggregate in lime mortars: Assessment of strength, microstructure, and durability

The constructions of historic buildings are on ground and sub-ground level often affected by various salts leeching in the constructions from surrounding environment due to non-existent or non-functional ground-water insulation. The salts dissolved in water are transported through capillary system of masonry and cause severe problems at air-construction interface where the water evaporates into the environment and salts accumulate in the outer-most layer destroying it in the process by the crystallization. To improve the durability of repair mortars and their ability to accumulate salts lightweight porous aggregate is used. This study focuses on the use of chalcedonite aggregate in such materials. Mortars based on aerial and natural hydraulic lime are prepared and the siliceous aggregate is replaced by chalcedonite sand. Fresh state properties of mortars likely to affect microstructure and/or durability e.g. air content in fresh mortar are determined. Flexural and compressive strength are studied at the ages of 28, 56, 90, 180, and 365 days, additionally at 90 days microstructural characteristics, durability to freeze–thaw cycles and salt crystallization resistance are studied. The use of highly porous aggregate leads to significant increase in amount of mixing water needed to achieve the same workability, while air content in fresh mortar and water retention value are only mildly affected. The substitution of aggregate leads to improvement in flexural strength however, compressive strength is decreased. The incorporation of lightweight porous aggregate causes significant increase in total open porosity of the mortars, their water absorption as well as increase in capillary water transport. Even though the significant increase in hygric properties, the durability to freeze–thaw cycling also increases. The improvement in salt crystallization resistance is observed mainly in the case of sulfate attack with only minor improvements in the solutions of nitrates and chlorides. The mortars with chalcedonite aggregate show results favourable for their utility as repair mortars, mainly in damp buildings, where the fast capillary water transport is highly beneficial.