Context 164 - May 2020

26 C O N T E X T 1 6 4 : M A Y 2 0 2 0 balanced very high fluidity requirements with lowest possible water content (up to 86 per cent). These were shown to flow many metres through the structure at a ‘runny custard’ consistency. They are complete mortars as they contain very finely ground aggregate fillers (particle sizes not exceeding 0.1mm). Fluid grout was always injected from the bottom of a structure, flowing and settling by gravity only. Progress was made upwards in small stages of one or two stone courses at a time. 2. Modified hydraulic lime grouts with thixo- tropic restricted flow characteristics for ease of placing in situations where only the outer portion of narrow ashlar joints required to be filled to depths of up to 150mm. Again, these are complete mortars, containing finely ground aggregates and very low water content (up to 46 per cent). Restricted flow grouts were applied from the top-down to avoid working over freshly placed grout. Joint finishing and aftercare After initial setting, joint surfaces were finished using conventional lime work practices such as tamping and/or scraping the surface. Conventional good practice lime aftercare procedures were applied in com- mon with all lime work. Results Over the construction phase of 15 months, approximately 8,000 litres of hydraulic lime grout was injected into joints and interstitial voids throughout the monument, equal to about three per cent of its volume. A large proportion of this was delivered through the narrow nozzles via exterior mortar joints, but where opportuni- ties arose where stones were removed, large volumes could be poured directly into the core to dissipate through the void network. We observed environmental conditions improving within the monument immediately after all exterior joints were filled with grout. Despite the masonry being saturated from years of water penetration and from the grouting operation, interior surfaces began to appear drier and salt efflorescence started to occur, indicating net evaporation. A temporary limewash was applied to internal surfaces for sacrificial protection of the finely carved and fragile ashlar surfaces against poten- tially damaging effects of crystallising salts at the surface.The surface is dry-brushed each week to remove these salt blooms. We are continuously monitoring interstitial moisture levels using probes inserted into a network of mortar joints. The probes measure humidity and temperature conditions within the wall, allowing us to monitor and report on the drying-out phase over the next three years and beyond. Kinlay Laidlaw is director of building surveying and heritage consultants Laidlaw Associates Building Surveying. Top to bottom: Restricted-flow grout being placed into a joint Examples of grouting nozzles A finished grouted joint after initial hydraulic set, with scraped finish