Composition of lime-cement and air-entrained cement mortar as a function of distance to the brick-mortar interface: consequences for masonry

The composition of a lime-cement mortar and an air-entrained cement mortar was studied as a function of distance to the brick-mortar interface. Both mortars had the same cement-to-sand ratio and the same water-to-cement ratio; in the lime-cement mortar the binder-to-sand ratio was highest. The measurements indicate that the mortar composition (i.e. the contents of sand, cured binder and voids) and the contents of chemical substances of the cured binder (i.e. the contents of calcite, portlandite and ettringite) change with distance to the brick-mortar interface. For the mortar composition the tendency of these changes is the same, but for the contents of the chemical substances of cured binder for the two mortar types the tendency of these changes is opposite and also the extent of the changes is significantly different. For the air-entrained cement mortar, the observations are explained by the enrichment of binder towards the brick-mortar interface, resulting from the compaction of fresh mortar. In the lime-cement mortar such an enrichment of binder hardly occurs and the observations are explained by the intense carbonation that takes place. As a result, the contents of the chemical substances in the mortars is very much different. In the air-entrained cement mortar, near the brick-mortar interface the enrichment of cement and the low water content (resulting from the low water retentivity of this mortar), lower the water-to-cement ratio and as a consequence the cement is not fully hydrated. In the lime-cement mortar, as the Ca(OH)(2) content and the water content is higher, near the brick-mortar interface, a carbonated zone is formed which is hardly permeable for CO2 (and probably water). This does not occur in the air-entrained cement mortar, it remains permeable.