Type O and Type N mortars are widely used to repoint historic buildings, often to their detriment. While mortar testing may indicate that a historic mortar was similar to a 1:1:6 Type N or a 1:2:9 Type O proportioned mortar as we have already noted modern Portland cement is typically higher in strength and density than older cement. A modern type O and a historic type O, for example, are not apples to apples for this reason.
Additionally, the actual compressive strength of today’s Type N and Type O are often greatly underestimated. But before addressing real-world mortar strengths we have to differentiate between proportion or property mixed mortars, both of which are allowed under ASTM C270 “Standard Specification for Mortar for Unit Masonry” :
A proportion blend may be mixed by a manufacturer but more commonly it’s made onsite. It must meet the specific proportions as stated in this standard, and since the proportions are explicitly stated there are no applicable performance criteria or specified strength minimums or testing requirements. It is essentially a recipe.
A property blend is designed by the manufacturer who determines the amount of sand, lime, and cement to achieve a mix that meets the performance requirements outlined in the chart below. A property mortar should have test data from a qualified laboratory that indicates the compressive strength and flow, the water retention, the air content, and the aggregate ratio of the mortar tested, as well as certificates of compliance for each raw material used.
As noted in this chart, ASTM C270 establishes minimum 28-day strengths of 350 PSI for Type O property blend mortar and 750 PSI for Type N property blend mortar. These are *minimum* required strengths. Actual 28-day test data for a property blend Type N is generally in the range of 1700 PSI and higher, and Type O can easily be 1200PSI and higher. These strengths are simply too high for most historic masonry buildings. Proportion blends generally achieve even higher strengths, and the mortar blends may vary from batch to batch during field mixing.
Given these considerations, we recommend extreme care when specifying cement lime mortars for historic restoration projects.