Comparative Laboratory Evaluation of Natural Hydraulic Lime Mortars

APT Student Scholar
Over the past decade, natural hydraulic lime (NHL) has become a popular binder in restoration mortars used for the conservation of historic masonry buildings in North America. The most obvious advantage of the NHL is a more rapid setting as compared with non-hydraulic hydrated lime (and lime putty) mortars. At the same time, NHL mortars are said to have the favorable attributes of most lime-based formulations, for example, low-to-medium 28-day strength, and relatively high-water vapor transmissivity (WVT).

The longer-term performance of NHL mortars, however, is difficult to predict, as they are produced from impure limestone quarried at various geographical locations in Western Europe. As the mineralogy of the source rock varies, so does the chemistry of the individual hydraulic limes manufacture. Moreover, the scarcity of manufacturers’ data and of independent laboratory-based literature on NHL mortars is a very significant issue. The concept of pointing mortar as a building component that is sacrificial and “compatible” –as often discussed for NHLs as for other lime mortars--is difficult to translate into conservation practice in the absence of scientific data. Of course, mortar testing data is only one aspect of those issues, as the behavior of the masonry units would need to be studied on a building-by-building basis, and in a detailed way.

The goal of this research is the examination and comparative evaluation of some fundamental properties of NHL mortars. Eleven NHL binders from four different manufactures were selected, all of them available in the North American market. More than 200 specimens were prepared with a volumetric (1: 2.25) binder-to-sand ratio, based on a common mix design for restoration mortars used in the field, and the specimens were tested at 28-days, 90-days, and 180 days. For comparison purposes, Type-O mortars with a volumetric (1 PC: 2.5 hydrated-lime: 8 aggregate) mix-ratio were prepared under the same condition. Experimental programs were created to study three crucial parameters: water vapor transmission, water absorption by capillary uptake, and compressive strength.