A mineralogical study of 350-year-old historical mortars for restoration purposes: the Case of the Castle of Good Hope (Cape Town, South Africa)

Abstract

The restoration and conservation process of structures of historical significance, specifically the mortar, is complex. This is mainly because historic structures symbolize many countries’ backgrounds and historical events. Hence, caution is always necessary to preserve their authenticity. For the ultimate results of historic restoration activities, there are several aspects to consider, such as compatibility, retreatability, reversibility, and durability. These are achieved through original mortar characterization beforehand. The original material analysis has been well received and explored worldwide; however, the African continent is yet to invest more research on this concept for sustainable restoration projects. To address the long-existing challenge and make informed decisions on suitable restoration mortars for future restorers, a reverse engineering approach, by means of physical and mineralogical analysis of original historic mortars from the Castle of Good Hope, a 350-year-old colonial structure located in Cape Town, South Africa was conducted. As part of the preliminary investigation on original mortar characterization for restoration interventions, this paper reports on the aesthetic, physical and mineralogical properties of samples collected from the oldest section of the Castle. A semi-quantitative analysis employing colorimetry, mercury intrusion porosimetry, powder x-ray diffraction and thermogravimetric-differential scanning calorimetry was carried out. The physical and mineralogical analysis show original mortars for this structure to be mainly whitish to cream lime-based with porosity ranging between 20–38%. The samples observed under XRD included, on average 64% of calcium carbonate (calcite) phases, 19% average peaks representing quartz, 3% gypsum and other minerals such as microcline, albite and biotite. The earth mortars showed prevalence of quarts (88%) and kaolinite in the range of 9%.