Historic Churches 2019

BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION 33 how and why it failed in order to improve the durability of the masonry, extending its service life. The focus should be on enabling the specification of a mix that will help mitigate the problem. Background information is always important when determining a test schedule, but particularly where failure has occurred. Viewed in conjunction with existing information, laboratory data should provide a fuller understanding of the problem and assist design of a repair mix. Modifications may, for example, involve adjusting aggregate grading or mix composition. Often the approach involves preliminary testing followed by further laboratory work, each phase providing the rationale for subsequent testing, as illustrated by the following two case studies. CASE 1: REBUILDING OF DAMAGED GRADE II LISTED ARCHES – NORTH YORKSHIRE Mortar testing was required to confirm the binder type, mix composition and details of aggregate composition from samples supplied to the laboratory, to enable the mortar in the rebuilt sections to match the existing mortar of the surrounding areas. Procedures included acid digestion of the binder, recovery of the aggregates and grading. Initial review of results noted a relatively high binder content and abundant lime inclusions within the sample. Since acid digestion cannot distinguish between lime in the binder and in the aggregate, further testing was necessary to identify any presence of limestone aggregate, and to allow the mix composition to be adjusted accordingly. A thin section of the mortar sample was therefore prepared for petrographic examination under the microscope. The examination confirmed the binder as a high calcium air lime and provided the following information regarding the aggregate: • the composition was mainly a natural quartz sand with other minor components, including coal, coal ash and clinker as well as limestone fragments • the shape and characteristics of the natural sand particles indicated a river bank, dredged or possible beach source. Thin section microscopy also revealed that while some of the limestone fragments had been burnt (indicating that they were residual fragments of the stone originally used to make the binder), others had not been. The inference was that a quantity of crushed limestone had been purposefully added to the aggregate, probably as a carbonation aid to speed up initial hardening of the mortar. The thin section was therefore subjected to ‘point counting’ to quantify the limestone present within the aggregate, enabling correction of the mix proportions and avoiding over estimation of the binder. This gave an ‘effective’ binder content and the repair mix design could then be adjusted. CASE 2: RESTORATION OF A 13th-CENTURY GRADE I LISTED CHURCH – LINCOLNSHIRE This project included analysis of mortar samples from internal and external locations. Information supplied with the samples confirmed that the church had undergone restoration works in the latter part of the 19th century. The corresponding test schedule included visual examination, indicative tests, XRD, chemical analysis and aggregate separation, examination and grading. Findings confirmed that two different types of mortar had been used, both contained high calcium air lime but one had been modified with pozzolans to impart hydraulic properties, as set out in the table on page 34. Leaching and local re-deposition of binder components were identified within one sample of the air lime mortar, indicating that the corresponding mortar joints had been affected by water percolation. Testing also revealed a difference between two samples of pozzolan- modified mortar. Although each binder was made from magnesian limestone, one sample was friable and weathered, the other relatively hard. Both contained ash but only the harder one contained the pozzolanic components expected, mullite and hydrocalumite. This highlights the effects of weathering as the mortars were Historic mortar samples including natural air lime (centre rear) and a range of pozzolanic mortars Lime extracted into solution for calcium oxide (CaO) determination