The influence of natural aggregates on the performance of replacement mortars for ancient buildings: the effects of mineralogy, grading and shape

Abstract

The importance of the selection of the type of aggregates and their effects on the modification of mortar properties is widely recognized. However, despite recent scientific advances on the effect of aggregates on the properties of mortars and concretes, few studies show how the characteristics of aggregates affect the microstructure and consequently, the final quality of mortars. Moreover, in general, existing studies show that there is an approach mostly related to the use of Portland cement, which several researchers have already shown that have adverse effects, when it is used in rehabilitation works of ancient buildings. In the context of rehabilitation, this factor is particularly relevant, since the main objective is to obtain mortars with specific characteristics and performances compatible with those that exist in old masonry. The aggregate, as an integral part of mortars, and in some cases defined as their "skeleton", has a direct influence on the microstructure of mortars, which, in turn, influences the physical, chemical and mechanical properties, as well as the finishing and the final appearance of the renders, especially in the case of lime mortars. Therefore, the present study intends to contribute to a better knowledge of the influence of mineralogical and physical characteristics (grain size distribution and shape) of the natural aggregates on the mortars’ pore structure and, consequently, on their physical-mechanical behaviour and their durability, in particular for the rehabilitation of old buildings. This knowledge will allow, in a substantiated way, optimizing the various parameters and defining repair solutions for different fields of application. To achieve these goals, an extensive laboratory characterization methodology was developed: parameters like the workability and water retentivity of fresh mortars; mechanical strength, adhesion to the substrate and hygric properties of hardened mortars; as well as the performance of the mortars subjected to salt crystallization and freeze/thaw cycles, were used. Furthermore, a chemical, mineralogical and microstructural characterization was also performed in order to relate the physical and mechanical results with the microstructural changes, namely concerning dimensions and volume of pores, during the carbonation and/or hydration processes. The results show that the aggregates characteristics have a relevant influence on the microstructure of mortars, namely in air lime mortars. However, different ranges of pores influence different properties of mortars, regardless the type of binder or aggregate used in their composition. Generally, a reduction in macroporosity leads to higher mechanical strength, namely compressive strength. However, the reduction in the diameter of the macropores can lead to an increase of capillary porosity and microporosity, which tends to compromise the durability.