Valorisation of a waste derived from the decontamination of phosphogypsum leachate as a clay replacement in ceramics

Abstract

This study presents the first valorisation of a waste generated from the cleaning process of phosphogypsum leachate (HApW) as a clay substitute in ceramics, contributing to the development of cleaner materials through industrial waste reuse. The waste is mainly composed of hydroxyapatite but contains non-negligible concentrations of heavy metals such as As, Zn, Cu, Ni, and V, and exhibits a 238U activity concentration of approximately 750 Bq·kg−1. Ceramics were prepared with varying clay replacement by HApW (20, 40, 60, and 80 wt%). Two commercial ceramic clays with distinct chemical and mineralogical compositions (referred to as clay A and clay B) were used. The thermal behaviour, sintering responses and the resulting microstructural and mechanical properties were systematically evaluated to determine the suitability of HApW as a sustainable clay substitute. Ceramic compositions were further assessed for porosity, flexural strength, and environmental safety via leaching tests. Results indicate that high HApW compositions exhibit a mineralogy dominated by HApW derived phases, including substituted apatite (Ca5(Na,Mg)(PO4)3) and complex calcium phosphate (Ca9MgNa(PO4)7), formed through thermal decomposition. HApW induces intense sintering and high final shrinkage, and clay would act as a skeleton that moderates this process. The flexural strength of the ceramic compositions ranges from 7 to 47 MPa and decreases linearly with increasing HApW content; however, this behaviour depends not only on total porosity but also on pore size. Microstructural analysis showed that clay A led to mostly amorphous matrices, while clay B produced denser structures with integrated apatite and acicular mullite crystals, enhancing densification and mechanical performance. Environmental assessment showed that heavy metal release remains below regulatory limits and that the radiological impact is negligible (indicative dose < 0.1 mSv·year−1). Compositional evaluation against ISO 13006 ceramic tile requirements showed that formulations 50A and 60A meet the specifications for BIIa indoor floor tiles, whereas 70B and 80B comply with BIb requirements, identifying these compositions as the optimal ranges for practical application. These findings demonstrate the viability of HApW as a sustainable clay replacement in ceramic production within the framework of cleaner material development.