Avaliação da aderência dos revestimentos argamassados: uma contribuição à identificação do sistema de aderência mecânico

Abstract

The evaluation of finishing coat of mortar in the construction practice is commonly carried out by a macroscopic approach. However, for a better understanding of the several mechanisms related to mortar characterization and performance a microstructure assessment on these materials is necessary. The adherence of a mortar to the substrate proceeds by mechanical and/or chemical means and the former is allegedly found in mortar coverings. Since the mechanical bonding takes place by the migration of the cement paste toward the substrates pores followed by the interlocking with the hydration products, this work evaluates the paste penetration and tries to correlate it to the bond strength, by means of pull-off tests. In this research approach, three mortar types have been employed: an industry-produced, a 1:6 cement-sand ratio (by volume) and a 1:1:6 cement-hydrated lime-sand ratio (by volume); these mortars were applied over ceramic block (brick) masonry and structural concrete block masonry. Different surface conditions were simulated: wet, dry, dust covered, gravel, greased and watersaturated (very wet); the following additions were also employed: styrene-butadiene resin and nylon fibers. Considering the properties evaluated under the plastic condition (flow table value, water retention and retained air), the best performances were observed for the additive-containing mortars (industry-produced and styrene-butadiene resin mortars) and lime-containing mortars. Concerning the properties evaluated in the hardened condition (compression resistance and bending strength), the best results corresponded to those for a particular mix with high cement ratio (1:6 cement-sand ratio followed by the 1:1:6 cement-hydrated lime-sand ratio type). Regarding the hardened condition properties associated with the elastic behavior and fissure-formationpossibility (elastic module and drying shrinkage), the best results were obtained for the additive-containing mortars (industry-produced and styrene-butadiene resin mortars) and lime-containing mortars. Taking into account the substrate surface conditions, the highest pull-off values corresponded to the suitable surface preparation situations. However, the lowest in situ water permeability results were obtained for the 1:6 cementsand ratio mortar. The mineralogical and microstructural analyses carried out on the mortar-substrate interface by means of X-ray fluorescence spectrometry, scanning electron microscope with attached EDS microanalyser and X-ray diffraction showed the evidences of paste-derived phases penetrating the ceramic block minute pores (such as portlandite, CSH and etringite). Analyses on the penetration profile and relative concentration of the chemical element calcium inside the pores network have demonstrated its important role in the bonding, when correlated with the pull-off strength values; usually, the best results could be associated with the highest relative calcium concentration within the initial 500km in depth, from the mortar-brick interface.