Variables Affecting the Strength of Masonry Mortars

Masonry cement mortars with additives which entrain various levels of air as well as mortars containing lime were evaluated. A mathematical model was developed describing brick-to-mortar bond strength. Lime mortars, prepared as specified by ASTM with higher proportions of portland cement, provided higher bond strengths and higher compressive strengths compared to masonry cement mortars. When prepared with the level of portland cement typical of masonry cement mortars, lime mortars had comparable bond strengths but weaker compressive strengths at similar air content. A one percent increase in Portland cement content increased the bond strength four times as much as reducing the air content by 1%. Lime did not provide increased bond strength. The higher bond strengths achieved with Type S portland cement/lime mortars over Type N are a direct result of higher portland cement content.