Standard protocols for restoring heritage cementing materials

Abstract

The history of the South African construction industry dates as far back as the seventeenth century, with structures such as the Castle of Good Hope, cathedrals, museums and memorials, among many others. Heritage structures represent the history of a country and its development. These structures do not only elaborate on the history, but also embody unique designs and materials used for their construction. This makes historic structures physically distinct from modern constructions. However, heritage structures often show critical signs of deterioration, which threaten their existence thereof. In attempts to rescue these national treasures, the use of Portland cement has been a common practice, but there are problems with using such modern materials. The application of incompatible materials is said to accelerate the deterioration and endanger the authenticity of these monuments. This practice is a result of a misunderstanding of the original material behaviour and has been responsible for the failure of many restoration projects. For developing countries, South Africa in particular, this approach has undoubtedly impacted negatively on the economy, as repeated repairs are necessary. Characterisation of the original cementing materials is the key to compatible restoration of heritage buildings. The research describes the development of an integrated protocol to establish the properties of the original heritage cementing materials. The key objective for the study was to investigate and standardise an analytical procedure whose primary purpose is the identification of major and minor components of heritage cementing materials and binder-to-aggregate ratio, which would be useful for production of repair materials. The results obtained from the experimental investigation into the physical, chemical and mineralogical properties of the original materials from the Castle of Good Hope and Robben Island are presented in this study. The experimental results were analysed for their ability to provide useful data at minimal costs. After that, a standard protocol was developed, incorporating the procedure for sampling as well as preparation of the sample, material analysis and documentation. The standard protocol includes the cohesion test, visual investigation and titration test for analysis of the salts and metal oxides present in the materials. These tests provide relevant data for the search of replica repair materials. The standard protocol will be useful for local heritage authorities, as it could be incorporated into the conservation management plans prior to restoration works. This will ensure compatible and sustainable restoration of historic buildings, not only in South Africa, but around the world. The heritage materials for the two oldest buildings in the Western Cape were made of hydraulic lime-based mortars, seashells, and uniform and well-graded sand. Less than 5% of the materials showed no traces of a binder. The restored areas were restored with cement-based materials. The affordable standard methodology for characterising original heritage cementing materials involved the sample collection that represents the majority of the building, sample preparation, detailed visual investigation and cohesion test, as well as hydraulicity and cementation indices analysis to determine the type of binder. These tests were concluded to be convenient, easy to conduct and cost-effective.