Evolution of the microstructure of lime based mortars and influence on the mechanical behaviour: The role of the aggregates

The pore structure of air lime mortars, formulated with aggregates of different mineralogy and grain size distribution, was evaluated with the aim of studying the influence of sand type on the microstructural modification over time and of establishing the relationship between these modifications and the mechanical behaviour. The microstructure of the mortars was determined by mercury intrusion porosimetry at 28, 90 and 360 days, and the results were compared with its mechanical characteristics, namely compressive strength, flexural strength and modulus of elasticity. A remarkable increase of mechanical strength over time was found for all mortars. An optimized grain size distribution of the sand (optimized compactness) proved to be favourable to air lime mortars compared with grain size distribution defined by standard EN 196-1, showing a good behaviour of the interlocked structure but also adequate porosity that favours lime carbonation. A relationship between the mechanical properties and the porous structure was also established: large volumes in the range of macropores tend to reduce the mechanical strength while gel pores, related with the presence of hydraulic compounds, are generally linked to an increase in strength. Overall, the assessment of the microstructure indicates that the packing density and the shape of the aggregates are the dominant parameters influencing the pore size distribution of macropores; however, the mineralogy and related characteristics also have an important impact on capillary porosity. (C) 2018 Elsevier Ltd. All rights reserved.