Dual use of pumice in lime mortars

The use of natural pumice as pozzolanic materials in lime mortars has been known since ancient times. Current professional works are more concerned with the effects of coarse natural pumice used as an aggregate in cement mortars or self-compacting concrete, than with replacing the binder with ground pumice. The use of a combination of finely ground natural pumice as a binder substitute and at the same time the lightening of lime or cement mortars with coarse natural pumice has not yet been the subject of any research. For this reason, the aim of this paper is to describe the effects of the partial replacement of lime with finely ground natural pumice on the mechanical, microstructural and durability properties of air-lime mortars lightweight with coarse natural pumice, which meet the functional and technical criteria imposed on repair mortars. Pumice is lightweight amorphous volcanic material with porous structure created by entrapment of gasses during the rapid cooling of lava. It shows similar pozzolanic activity to trass or natural zeolite predicting an improvement in mechanical properties and durability of lime-pumice mortars. The fine pumice addition to lime mortars led to decrease in amount of kneading water required for the preparation of the mortars with the same workability. The use of lightweight pumice aggregate in the composition of mortars led to their higher porosity, lower density, improved capillary transmission capability and water absorption, and increased salt crystallization resistance, while the replacement of lime with fine pumice resulted in an increase in the strengths and frost resistance of the prepared mortars. The resistance of the mortars to the salt crystallization of NH4NO3, Na2SO4, and NaCl was improved, although the mortars had relatively low resistance to the reaction of NH4NO3 and Na2SO4, where nitrocalcite, cesanite, and calcium sulfoaluminates were formed, breaking the structure of the mortars. Considering the compatibility, functional and technical criteria, lime-pumice lightweight mortars could be classified as repair mortars.