Multi-scale X-ray techniques for assessing recycled concrete aggregate: from XRPD analysis of leftover cement in recycled aggregates to micro-CT imaging of concrete microstructure

The content of leftover cement paste is a crucial parameter for determining recycled aggregates quality. Various methods assess this, including wet techniques (acid dissolution, chemical degradation, water absorption), physical approaches (freeze-thaw cycles, mechanical shredding, oven-dried density, thermal disaggregation), and spectroscopic or microscopy analyses. However, these methods often lack accuracy, are time-consuming, or depend on operator skill. A novel X-ray Powder Diffraction and Rietveld quantitative phase analysis approach is introduced to improve measurements precision. Results are compared with multi-scale analyses (microscopy, X- ray computed tomography, mechanical testing) on recycled aggregate concrete specimens. Findings highlight a strong correlation between leftover cement paste content and key concrete properties, including microstructure, hydration products, and mechanical performance. This study confirms that leftover cement paste content is a decisive factor in recycled concrete aggregate properties, and that the proposed method offers a rapid and reliable approach to control this parameter.