Lightweight mortars with chalcedonite – Towards functional and durable materials for repair of historical masonry

In the repair of historical masonry and listed buildings, the requirements for compatibility and preservation of the original appearance and cultural and historical value of the rehabilitated structure are often discussed. In particular, technical, technological and material compatibility is often addressed by engineers and conservation professionals and the resulting solution often involves finding a compromise between an ideal, technically functional solution and preserving the historic value of the building for future generations. The material solution of mortars for repair of historical structures is based on the application of a suitable binder and porous filler, which allows achieve their high porosity, water vapor permeability, sufficient strength and durability. Obviously, there is no single ideal material solution for these mortars and it is always necessary to take into account the current state of the reconstructed structure and the composition and properties of the original materials in their design and application. The aim of the research presented here was to design and experimentally assess rendering and plastering mortars based on lime hydrate and natural hydraulic lime partially lightened with chalcedonite as a porous aggregate. The results of the material tests were compared with criteria imposed in WTA recommendation 2-9-03.2020/E. Chalcedonite was used as an alternative filler to silica sand and its application enabled the required values of porosity, compressive strength, water absorption coefficient and water vapor diffusion resistance factor to be achieved. The lightened plasters also showed increased hygroscopicity, low thermal conductivity and increased resistance to salt crystallization. Depending on the binder used and the demanded mechanical strength, the examined mortars thus represent materials applicable in the repair of damp and saline masonry of historical buildings.