Deterioration mechanisms of lime-pozzolan and NHL mortars due to sulfate attack

Abstract eng:
This study aims to investigate the effect of sulfate salts to the deterioration mechanism and patterns observed in hydraulic mortars. Natural hydraulic lime (NHL) and lime-pozzolan mortars were subjected simultaneously to accelerated aging with sodium sulfate, accompanied by weight monitoring (according to standard test) and macroscopic recording of their condition. Representative samples from each mixture were examined by scanning electron microscopy and X-ray diffraction in order to identify the deterioration products formed at preset time periods, along with any sulfate reaction phases. The microstructure and mineralogy of both reference and deteriorated mixtures were contrasted and compared in order to describe the effect of sulfates in every different mixture. The differences in pore structure as well as the stability of the setting/ hydration products of the mortars were investigated regarding the type of binder, the effect of pozzolanic admixtures and the binder to aggregate ratio. It was concluded that depending on the above parameters, the diversity observed in the intensity and typology of deterioration can be attributed to the initiation and domination of either chemical or physical mechanisms. Therefore, electron microscopy revealed no indications of Na2SO4 crystals inside the pores of the NHL mortars, in contrast to lime-pozzolan whose pore system was abundant with thenardite phases, along with the presence of some secondary formed sulfate phases. The results recorded are finally discussed related to the durability and service life of the mixtures studied.

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