Differences in mineralogical and textural evolution during firing of calcareous and non-calcareous bricks are studied and correlated with their behaviour in hygric and weathering tests. Results reveal significant differences in the evolution of vitrification degree, porosity and pore size distribution. Such evolution depends mostly on raw clay composition and firing temperatures. A higher degree of vitrification and of compressive strength is displayed by calcareous rather than non-calcareous bricks at lower firing temperatures of between 700 and 900 degreesC. However, their resistance to salt crystallisation and freezing is not notably improved because of unfavourable pore size distribution and crack development. The latter are caused by the transformation of calcite into calcium oxide at around 800 degreesC, which reacts readily with moisture to form calcium hydroxide, thus leading to a volume increase (lime blowing). This problem can be avoided by closely controlling grain size and content of carbonates in the raw clays. High firing temperatures of 1100 degreesC in the case of calcareous clay and 1000 degreesC in the case of non-calcareous clay are required to produce durable bricks that remain unaltered upon weathering. The improved durability appears to be due to a more favourable pore size distribution and a reduction in porosity. Results from textural and hygric studies of the brick samples indicate that these parameters can to a significant extent be controlled by varying raw clay composition and firing temperature, thus making it possible to fabricate replacement bricks for particular conservation purposes. This paper addresses limitations regarding the interpretation of test results, as well as the lack of a systematic application of existing standards for evaluating the state of conservation of historic bricks and for establishing specifications for replacement bricks. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.
