| Abstract: | SUMMARIES. Grouting aims to address a lack of adhesion occurring within the render layers and support of a wall painting through the addition of an adhesive material with bulking properties. It is often desirable that a grout should set in the absence of air, and one way of achieving this is by using a lime-based grout with the addition of a pozzolana, which will react with the calcium hydroxide to form stable insoluble compounds possessing cementing properties. This paper characterizes a number of pozzolanas currently used for grouting wall paintings on calcareous renders, and assesses the working properties and performance characteristics of grouts made with the pozzolanas. The significance of the method of sample preparation on the properties of the grout is discussed.
CONCLUSION. All of the pozzolana grouts had good working properties, most notably ease of injectability and a rapid rate of set. The research demonstrated the influence of the setting conditions on the properties of the grout, and the difficulty of emulating in situ setting conditions realistically. The consequence of this is that comparisons between grouts set under different conditions are of limited value, and standardized test methodologies are still needed. Given their satisfactory working properties, the grouts should be selected for use on the basis of their performance characteristics. The volume change, density, porosity and water vapour permeability were found to be interrelated, and to be influenced in part by the water content of the grout, but also by the particle size and morphology of the pozzolana, and by the pore structure of the grout. These factors have not been adequately researched and require further investigation. The grouts with the highest uniaxial compressive strengths and moduli of rupture also had the highest Young's moduli, and all of the grouts were stronger than the control under compression. Analysis of the aqueous extractions demonstrated that the soluble ion contents of the grouts could be reduced by washing the pozzolanas, and that more ions could be extracted from grouts set in porous supports than in non-porous supports, possibly because the lime-pozzolana reaction was at a more advanced stage in the non-porous supports. Given that the extractable amounts of alkali ions depend on the length and conditions of the setting period and extraction period, comparisons between different studies may be misleading, and it is difficult to attach qualitative statements to the quantitative values obtained from ion analysis. A standardized methodology for measuring and assessing the soluble ions introduced by grouts is therefore still needed. A more satisfactory method of quantifying the ions released during setting might be to take a sample of grout and allow it to set under water. The brick dust and trass grouts performed most closely to a control made from non-hydraulic lime and fine sand, although the trass grout may have a high soluble ions content. The diatomaceous earth grout shrank considerably on setting, and was much more porous and less dense than the other grouts. The pozzolana grigia and GBFS grouts were both very strong, and the GBFS grout also had low porosity and water vapour permeability, but these pozzolanas are intended to be used as additives for lime and sand grouts, and require further testing in lower proportions. |
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