Application of the Rietveld method for the investigation of mortars: a case study on the archaeological site of Thamusida (Morocco)

Abstract

This study presents a multi-technique approach to the investigations of mortars, involving the application of the Rietveld method. The quantitative mineralogical analysis of samples was aimed at (1) verifying its significance in relation to results obtained through other techniques and (2) evaluating its potential in mortars characterisation. Six type of mortars used in the Roman settlement of Thamusida (50 km north of Rabat, Morocco) were investigated through X-ray powder-diffraction and Rietveld refinement, optical microscopy, image analysis, scanning electron microscopy, electron microprobe analysis, X-ray fluorescence (XRF) and combined differential thermal, and thermogravimetric analysis (DT–TGA). All the adopted techniques yielded unique results and complementary data; however, the Rietveld method can actually substitute thermal analysis accurately. CaO oxide contents obtained by DT/TGA and Rietveld are in good agreement, but both were only partly comparable to XRF results. In our case, XRF and Rietveld results diverge when binders contain more than 67 wt.% CaO. The discrepancies can mainly be ascribed to the presence of amorphous material and to the obtainable accuracy by QPA-Rietveld method; conversely, the nonstoichiometry of phases does not affect our results, because phases with variable compositions are present in small amounts. Results further showed that the adopted methods were suitable for mortar characterisation. All studied mortars (1) have weak hydraulic properties; (2) employed binders characterised by highly variable CaO contents and by significant amounts of SiO2; (3) used coarse to fine quartz–carbonate-rich sands as the aggregate; and (4) selectively mixed 2 or 3 parts of aggregate with 1 part of binder, depending on the destination of the mortar.