Investigation of crushed brick-matrix interface in lime-based ancient mortar by microscopy and nanoindentation

Lime-based mortars containing crushed clay bricks were widely used in history and proved to be more durable than other binding materials, especially when used in seismic areas. So far, their enhanced strength and durability have been attributed to the formation of hydration products on the interface between the lime-based matrix and brick fragments. These conclusions have been exclusively reached on the basis of macroscopic mechanical tests and local chemical analyzes of mortar constituents, while the local mechanical analyzes were lacking. The purpose of this paper is to verify the previous measurements and to quantify the elastic properties of the interfacial transition zone (ITZ) between crushed brick fragments and the surrounding matrix by means of nanoindentation, complemented by microscopy investigations. To that goal, an ancient mortar sample from a late Byzantine church was investigated. The elemental analysis revealed an increased amount of silica and alumina in the vicinity of brick fragments in relation to the matrix composition. In addition, an increased stiffness of ITZ compared to the lime matrix in the distance up to 20-30 μm from the grains was encountered by nanoindentation. Therefore, it was confirmed that the interface is stronger than the surrounding matrix, unlike the situation of inert sand aggregates, where ITZ is weakened by shrinkage-induced cracking. The average Young's modulus in ITZ assessed by nanoindentation (18.4 GPa) was comparable with low-density C-S-H phases that appear in cementitious composites. The information contained in this paper contribute to a better understanding of traditional mortars and provide the input data for their further micromechanical modeling.