A 3D study of mortar degradation by X-ray computed microtomography

Prediction of long term performance and durability of concrete is an essential issue for many practical problems. In various situations, the cement paste is attacked by dissolved elements present in the water surrounding the structure and more or less deeply penetrating it. In this work the consequences of leaching, accelerated by ammonium nitrate, on samples of a standard mortar have been characterised in 3D by X-ray computed microtomography (XCMT). This non-destructive method provides quantitative 3D maps of the sample's X-ray absorption coefficient mu. At each voxel (cubic volume element composing the 3D map), the value of mu is the average of the absorption coefficients of the different solids and fluids contained into it at the moment of acquisition. Considered samples are cylinders of 8 mm in diameter and 3 cm in height that have been cored into larger samples of a standard mortar. Microtomography data acquisitions have been performed on the beam line ID19 at the European Synchrotron Radiation Facility (Grenoble, France). At regular intervals during the leaching process, 900 radiographs of the sample have been acquired at different angular positions using the CCD FRELON camera (2048 x 2048 pixels). With the selected optical set up, the physical size of a pixel is 7.46 x 7.46 mum(2). After numerical reconstruction, the 3D final images are composed of about 1100 x 1100 x 1000 cubic voxels. For each sample, four 3D images corresponding to the initial state, and the states after 24 hours, 48 hours, and 61 hours of leaching, have been registered giving for each voxel an estimation of its chemical evolution. Preliminary treatments put into evidence the macro porosity evolution, the 3D displacement of the reaction front, and allowed a first quantification of calcium release as a function of time and space.