Determining the characteristics of historical mortars of the Castle of Good Hope (1666–1679) and Robben Island (1700–1800) by cost-effective methods

Abstract

The Castle of Good Hope (1666–1679) is the oldest existing colonial building in South Africa, and was a bastion fort in the 17th century. Robben Island’s 18th century precint, which includes parts of the maximum security prison and pre-primary school, is one of ten UNESCO World Heritage Sites in South Africa. Although more than a century apart in construction, both these surviving buildings are so physically distinct from modern constructions they have become important historical monuments and major tourist attractions. However, most heritage structures, including the Castle of Good Hope and the building precincts on Robben Island, have shown critical signs of deterioration, threatening their existence and requiring restoration. Regrettably, incompatible materials are often used to restore these structures, accelerating the damage and endangering the structures’ authenticity. Our work investigates the characteristics of mortars used on the Castle of Good Hope and selected building precincts on Robben Island for compatible and durable masonry restoration. We describe a cost-effective protocol to identify the major and minor components of the original heritage mortars used. The results indicate different binder to aggregate ratios, the use of hydraulic lime as a binder, sand and seashell aggregates and traces of salts (sulphates, chlorides, nitrites and carbonates) for mortars used in both structures. These components would be useful for designing and producing the repair materials that match the original properties, so improving overall functionality and performance.