Analysis methodology of XCT results for testing ingress of substances in hardened cement paste: Explained with chloride immersion test

X-ray computed tomography (XCT) can non-destructively detect microstructure of cement-based material. However, its raw results involve errors induced by the equipment and external factors. Once studying changes taken place in a sample, such as durability related studies, which need testing on a same sample at intervals, these errors will result in misleading conclusions. At the moment, in literature, no method focusing on effectively and reasonably eliminating or minimising these errors has not been observed. After testing and analysing ten series of XCT results for samples experiencing various types of exposures, authors established a rational data-processing method to minimise the influence of these errors on test results, and is introduced in this paper. To assist readers’ understanding, the method and relevant information are illustrated with a case of immersion of hardened cement pastes in NaCl solution. Obtained results prove that diffusion of substances in the hardened cement paste can be properly revealed with the data processing method introduced; trend and depth of microstructure change caused by the exposure can be determined; the immersion of samples in NaCl solution led to a gradual decrease in density and a further hydration of cement particles, which should be related to leaching of Calcium from the sample into immersion solution; once immersed in same NaCl solution, the change of microstructure in the 0.35 w/c was slower than that in the 0.5 w/c sample and the influence depth had an approximate linear relationship with square root of immersion duration.