Our in-house procedures for the petrographic examination of concrete generally include a petrographic measurement of cement, water and aggregate contents where the sample provided is of sufficient size. The procedure is based on the measurement of the volume proportions of aggregate and cement paste by the point counting of large-area polished plates using a binocular microscope in accordance with ASTM C457. The measured volume proportions are converted to weight fractions using petrographically determined aggregate densities and water/cement ratios. Petrographically determined cement contents are generally at least as accurate as cement contents determined by conventional wet chemical analysis.
Water/cement ratio is assessed petrographically by making a comparison of the abundance of residual cement grains in the unknown sample with control concretes made with known water/cement ratios.
Whilst PFA and GGBS particles can be readily identified using thin sections, their amounts cannot be quantified petrographically. PFA and GGBS contents are quantified at Geomaterials by chemical analysis of a polished concrete surface using an electron microscope fitted with an energy dispersive X-ray microanalysis system.
The cement type in hardened concrete is determined following the method given in BS 1881 but using the electron microprobe. This technique has the advantage that even at very high water/cement ratios where few unhydrated cement grains remain, it is still possible to make a quantitative assessment of the proportions of C3A and C4AF based on the chemical composition of the cement paste (excluding aggregate and dust).