Bond-strength performance of hydraulic lime and natural cement mortared sandstone masonry

•Flexural and compressive stress–strain response of NHL and natural cement mortar presented.•Mortar stiffness correlates with increasing hydraulicity of binder.•New data presented on the bond strength of mortared sandstone masonry units.•Influence of pre-wetting time on bond strength of mortared sandstone units presented.•Sandstone masonry bond strength increases linearly with increasing hydraulicity of binder. Flexural bond strength is an important performance characteristic of masonry structures yet there is no guidance for lime-mortared stonework in design codes of practice. This study investigates the bond strength of natural hydraulic lime (NHL) and natural cement mortared sandstone masonry. To this end, the flexural bond strength of masonry couplets, built with mortars of three hydraulic strengths and one natural cement and having a water-content adjusted to achieve a similar consistency, was measured with the bond wrench test. Practical mortar compositions and natural curing conditions were used within the experimental programme. Bond strength was found to be directly related to binder hydraulicity and sandstone pre-wetting time – a positive effect in the case of the former and a negative influence in the case of the latter. Pre-wetting time, however, had a greater influence on the feebly hydraulic lime binder (NHL 2) than on the moderately (NHL 3.5) and eminently hydraulic (NHL 5) lime binders. The results presented will assist in improving our knowledge of lime mortared sandstone masonry and in the development of design guidance.