Portland Cement-Lime Mortars for Conservation

Abstract

It is currently widely accepted that lime, rather than Portland cement (PC), should be used for the repair of historic fabrics as most historic mortars were lime based and properties of PC mortar such as strength, stiffness and permeability can be incompatible with historic stone and brick, with consequent damage to fabrics. As a result, PC mortars are no longer used in conservation. However, lime addition significantly alters the properties of PC mortars. This paper investigates the variation of PC mortar properties with increasing hydrated lime addition (CL90s), and compares the results with typical values of natural hydraulic lime (NHL) mortars often used in conservation. The results evidence that PC/lime mortars display good workability with a high water retention, low air content and a reliable bond. Their properties are comparable to those of NHL mortars therefore, they are probably compatible with certain historic fabrics. The air content of the PC/lime mortars is low, similar to the typical air content of NHL mortars. Their water retention is high, significantly superior to those of equivalent PC mortars and comparable to typical NHL mortar values. The compressive and flexural strength and elastic modulus of the lower-lime-content mortars (1/0.25/6 and 1/1/6-PC/lime/sand) are superior to the values typically reached by NHL mortars however, their bond strength is similar. The strength, elastic modulus and bond of the PC/lime mortar with higher lime content (1/2/6) are comparable to the values typically reached by NHL2 mortars. Low lime contents produced stronger, stiffer mortars with greater bond, increased flexibility and water retention and lower air content than PC alone.