Investigating the standard design and production procedure of heritage mortars for compatible and durable masonry restoration

Abstract

Historic buildings ensure generational knowledge of past events, milestones, construction developments and evolution of materials, architectural designs, and practices throughout the centuries. It is indisputable that heritage buildings' survival against deterioration factors has proven the use of durable materials for their construction. However, they suffer inevitable decay due to ageing. Therefore, restoring the monuments to their original appearance and strength is always necessary for long-term survival. This paper discusses solutions for the design and development methods of new compatible restoration mortars for the architectural heritage, covering four significant aspects, namely: i) visual analysis of the heritage building in question, ii) experimental analysis of the original mortar samples for their physical, mineralogical and chemical properties, iii) characterization of the potential raw materials (available in the study area) that are close to the original, and iv) assessment of the new mortar durability. The mortars collected from the Castle of Good Hope, an important and ancient colonial edifice in the Western Cape Province (South Africa), were earth (samples SK7 to SK9) and hydraulic lime-based (SK1 to SK6), with 21–38 % porosity. The raw materials used on this monument include feldspar aggregates, possibly from the West Coast (Cape Town) and hydraulic lime for SK1, SK3 and SK5 mortars. For the restoration of the lime mortars (SK1, SK3 and, SK5), a hydrated lime-based mortar with a binder-to-aggregate ratio of 1:3, made of west coast sea sand and 5 % seashell additives, with a porosity of 24 %, has proved to be the most durable. The aesthetics for all the restoration mortars M1 to M9 is difficult to achieve considering the original material ageing factor, thus, the use of color-enhancing pigments is recommended.