Red ceramic waste as supplementary cementitious material to mitigate the external sodium sulfate attack

External sulfate attack is one of the main durability concerns affecting cementitious materials. This study evaluates the potential of red ceramic waste (RCW) as a supplementary cementitious material (SCM) to mitigate sodium sulfate attack by reducing reactive alumina availability in cementitious systems. Mortar bars containing 30% RCW as partial replacement of Portland cement were exposed to a 10% sodium sulfate solution for 245 days. Mineralogical characterization, pozzolanic reactivity, alkaline solubility, and linear expansion were investigated. The RCW exhibited low pozzolanic activity and low alumina solubility under alkalinity conditions representative of hydrated cementitious matrices. The RCW increased the total alumina content of the cementitious system, but most of the aluminum remained insoluble and did not promote secondary ettringite formation. The pozzolanic activity of the RCW reduced the residual portlandite content in the hydrated matrix and mitigated gypsum precipitation, a precursor to secondary ettringite formation. The incorporation of RCW reduced the linear expansion of mortars up to 42 days. Long-term results up to 161 days demonstrated that RCW-containing systems maintained low expansion levels, outperforming a commercial sulfate-resistant cement. The mitigation effect was associated with reduced dissolved aluminum availability for secondary ettringite formation. The results demonstrate that RCW showed potential as an SCM to enhance sulfate resistance under the investigated conditions, while simultaneously supporting the valorization of ceramic waste in cement-based materials.