Academic journal article Heritage Science

Specialist ‘Restoration Mortars’ for Stone Elements: A Comparison of the Physical Properties of Two Stone Repair Materials

Academic journal article Heritage Science

Specialist ‘Restoration Mortars’ for Stone Elements: A Comparison of the Physical Properties of Two Stone Repair Materials

Article excerpt

Authors: Clare Torney (corresponding author) (equal contributor) [1]; Alan M Forster (equal contributor) [2]; Ewan M Szadurski (equal contributor) [2]

Background

Masonry repair

Deterioration of stone masonry is a worldwide problem, which can be addressed by replacement of entire stone units, by consolidating the delaminated surfaces or by plastic repair [1]. The nature of substrate materials and their state of conservation offers almost unlimited permutations in specification and technical repair solutions. Plastic repair using a restoration mortar is one popular method used for the treatment of a number of different building elements [2]; the word ?plastic? denotes the plasticity of the fresh mortar rather than any specific composition [3]. Used within a philosophical framework of repair (Figure 1) this method can, in the short-term, provide a lower cost alternative [1]. The reinstatement of natural stone is often associated with the removal of potentially large quantities of existing masonry and is therefore often viewed as being overly intrusive [1, 2, 4]. Partly as a result of this, ?plastic? repairs appear to have increased in popularity and prevalence since the 1960?s [1, 2]. However, the damage caused to host masonry by using inappropriate repair materials is well documented, and is perhaps most well known in cases of repair using Portland Cement based materials [5, 6, 7]. Less well known is the fact that damage can be caused by other materials of low permeability, including excessively strong (i.e. eminently hydraulic) lime mortars, which may result in entrapment of moisture. This can lead to accelerated deterioration of masonry due to mechanisms such as freeze-thaw cycling and salt sub-florescence [6, 8, 9, 10, 11]. Furthermore, the current and future predicted alteration in rainfall patterns associated with climate change [12] are likely to result in escalation of building performance stress issues associated with moisture management.

Figure 1: Remedial action options for cases of masonry deterioration. Based on current literature [1, 3]. [see PDF for image]

Much research has been undertaken [13] upon the physical, chemical and microstructural properties of Portland Cement-based mortars and renders. Accordingly, an increasing understanding of the associated failure-inducing mechanisms of such products, used in-conjunction with porous building materials is developing (e.g. [5, 7, 14]). This knowledge and experience has led to the reintroduction of lime (Table 1) as a repair material for building conservation work.

Table 1 caption: Compressive strength ranges for natural hydraulic lime mortar samples prepared in accordance with the current British Standard (BSI, 2010) [see PDF for image]

Mikos et al.?s [15] work on restoration of natural stone surfaces highlights primary physical testing methodologies and visual aesthetic considerations (including colour matching and surface finish). At the heart of Mikos et al.?s [15] work is the importance of determining compatibility to ensure the minimisation of harm to host substrates. The established key characteristics find broad agreement with various authors working in this area [15, 16, 17]. The assessment of these characteristics are not necessarily absolute values, but reflect useful, measureable performance trends [15] and therefore must also be seen within a context of the substrate and other practical on site variables. Mikos et al. [15] indicates amongst others, some essential requirements and characteristics for consideration, including; mineralogical and chemical composition, the understanding of the development of physical characteristics (including hygrothermal parameters, capillary water absorption & water vapour permeability), curing regime and application factors.

Mikos et al. [15] continues to emphasise the importance of involving expert assessment in the determination of, and translation of practical recommendations into tangible specification for repair works to natural stone. …

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