A study of the workability of natural hydraulic lime mortars and its influence on strength

Abstract

Lime mortars are currently used for both repairs to historic buildings and new construction. An important mortar property is workability. Lime mortars are often described as highly workable, however, due to the many variables contributing to workability, this is difficult to quantify. Workability largely depends on water content, and this will greatly affect compressive and flexural strengths, which in turn will determine the overall quality and durability of a mortar. The objective of this paper is to examine the relationship between workability and strength of naturally hydraulic lime (NHL) mortars in order to optimize their properties for a more efficient use in building. To this aim, mortars of several hydraulic strengths were mixed with different water contents in order to vary flow diameters and characterize workability. Compressive and flexural strength were then tested to measure the effect of the initial flow on the strength of the hardened mortar. This paper demonstrates that NHL 3.5 and 5 mortars require a flow value close to that of 185 mm prescribed by the European standards whereas NHL 2 mixes require a significantly lower value. This paper concludes that one universal flow value is inadequate when trying to optimize strength of NHL mortars and that, in order to optimize strength, mortars should be mixed to attain a different flow diameter for each hydraulic strength. However, other properties such as bond strength and water retention need to be considered before unique flow diameters are prescribed to lime mortars of different hydraulic strengths.