Characterization of Ancient Yemeni Qadhad Plaster: A Low-Permeability Lime–Pozzolan Material for Sustainable and Durable Conservation and Construction

Qadhad is a traditional Yemeni lime-based waterproof plaster used for centuries in cisterns, reservoirs, and roofs, yet its scientific characteristics have not been fully documented until now. This study presents the first comprehensive characterization of ancient Qadhad plasters collected from four archaeological sites across Yemen, to evaluate their physical, hydraulic, chemical, mineralogical, and microstructural properties and sustainability potential. Physical and hydraulic testing (density, porosity, permeability, and absorption), combined with XRF, XRD, and SEM–EDS analyses, confirmed that Qadhad is a hydraulic lime–pozzolan composite containing reactive silica and alumina from volcanic aggregates that form C-S-H, C-A-H, and C-N-H phases responsible for waterproofing and durability. The measured permeability values (6.55 × 10⁻ 9 –26.2 × 10⁻ 8 m/s) are significantly lower than those typically reported for plaster materials, confirming its superior hydraulic performance. The low porosity (19.6–25.0%) and dense microstructure further support its exceptional impermeability. Environmentally, Qadhad is sustainable, utilizing local materials, low firing temperatures (≈900 °C), and CO₂ reabsorption, resulting in a low-carbon balance. These findings establish Qadhad as a superior waterproof plaster and a sustainable material for heritage conservation and modern construction.