Design and development of heritage mortar for restoration

Abstract

Historical buildings are a narrative of the countries’ written and unwritten history. Their long-term existence ensures generational knowledge of past events, milestones, construction developments and evolution in materials, designs, concepts, and practices throughout the centuries. It is indisputable that heritage buildings’ survival against human behaviour, natural disasters, and environmental and atmospheric attacks through the years has proven the use of durable materials for their construction. Inevitable decay due to long-term exposure to deterioration factors is a common problem for these monuments. This has resulted in a need to execute restoration works to reinstate the heritage structures to their original appearance, physical state, and strength for extended survival. Several restoration projects in the past and recent times have been performed to reinstate the missing elements on heritage structures, masonry mortars in particular. Unfortunately, many of these restoration interventions on these structures have been erroneous and led to poor results from a restoration standpoint, especially regarding aesthetic criteria. The most common mistake in restoration work is the use of incompatible restoration materials due to a misunderstanding of the materials used on these historic wonders. This has resulted in a waste of resources due to consequential repeated restoration projects and the loss of original structural concepts in terms of appearance and integrity. These common mistakes can potentially threaten the historical significance of heritage structures. The problem extends further to designing and producing mortars that are compatible and their assessment for compatibility and durability. To overcome the problems associated with material incompatibility, researchers and heritage restorers have discovered the applicability of analysing original mortars for their chemical, mineralogical, physical, and mechanical properties preceding the execution of restoration works (ICOMOS, Venice Charter, 1964). The experimental analysis of these mortars is believed to offer promising results for the long-term survival of restored historical masonries. To align the current study with the literature regarding sustainable and compatible restoration of historical mortars, two phases were examined. The initial phase of this study comprised an experimental analysis of the samples extracted from the masonry joints, the floor, plaster, and renders of the ancient colonial edifice in Cape Town, South Africa, Castle of Good Hope built in 1666. The building precincts on Robben Island were constructed between 1700s and 1800s. A total of nine representative samples were carefully extracted from the Castle of Good Hope, twelve from the Pre-primary school building and three samples from the Maximum-security prison on Robben Island for analysis. The sample size was decided based on material availability, considering the restriction associated with causing as little destruction as possible on historical structures. Phase 1 of this doctoral thesis involved the determination of the aesthetic properties of the collected mortars using spectrophotometry, the chemical composition through X-ray fluorescence (XRF), the mineralogy using powder X-ray diffraction (PXRD) and thermogravimetry - differential scanning calorimetry (TGA-DSC), the microtexture by environmental scanning electron microscopy (ESEM) and the analysis of the porous system through mercury intrusion porosimetry (MIP). The original mortars from the Castle were found to be earth and hydraulic lime-based, with a porosity of between 21 and 38%. The Island building's mortars were mostly natural cementbased and hydraulic lime-based, highlighting possible previous restoration works using natural cement. These mortars portrayed a porosity lower than mortars from the Castle (18 to 30%). The raw materials used on these monuments include feldspar aggregates, possibly from the West Coast (Cape Town) and sub-hydraulic lime. Phase 2 of this study involved the designand development of eight different mortar mixes following a unique procedure invented in this research work. The freshly mixed mortars were evaluated for consistency. In contrast, the hardened cubes of 40  40  40 mm and beams of 40  40  160 mm were evaluated for compatibility and durability using destructive (hygric tests, ageing tests through salt crystallisation and freeze-thaw cycles, compressive and centre point loading flexural strength tests) and a non-destructive techniques (ultrasound pulse velocity, UPV). The durability tests aimed to assess the new mortars' performance and verify their long-term existence in restoration practice. The old mortars properties were also assessed against the new ones, and restoration interventions made. For restoration purposes, a hydrated lime-based mortar with a binder-to-aggregate ratio of 1:3 by weight, made of West Coast Sea sand and 5% seashell content with a porosity of 24% proved to be the most durable among the eight produced mortars. Meanwhile, the aggregates of similar sources with the addition of natural cement are proposed for earth mortars. The aesthetics of all the mortars were difficult to achieve, given the ageing factor of the original materials. Thus, the use of colour-enhancing pigments is recommended. A standardised guideline for producing compatible and durable mortars has been documented for restorers to execute the works on historic structures properly. This research confirmed that the compatibility and durability of heritage mortars depends not only on their performance, but also on their ability to match the properties of the existing materials with appropriate application techniques by skilled masons.