Durability of ultra-high performance fiber reinforced concrete against internal sulfate attack using gypsum-contaminated aggregates

The scarcity of suitable aggregates, often driven by economic or geographical factors, necessitates the use of sulfate-containing materials in concrete. However, their detrimental impact on durability and mechanical behavior raises doubts about long-term performance. Aggregates in many regions of the Middle East are contaminated with gypsum which highly limits their use in conventional concrete. With its exceptional durability and strength, ultra-high-performance fibre-reinforced concrete may provide a new field of application for such aggregates. The present study addresses the possibility of using gypsum-contaminated sand in ultra-high performance fiber reinforced concrete (UHPFRC) made with binary blends of Portland cement and silica fume, and ternary blends of Portland cement, silica fume, and ground granulated blast furnace slag. The wet cured UHPFRC samples were tested for expansion, compressive strength, porosity and microstructure as a function of sand SO₃ content and curing age. Test results showed minimal expansion not exceeding 0.025 % even at the highest SO₃ levels of 4.5 % due to very low w/b ratio, absence of external free water, and presence of mineral admixtures, thus indicating exceptional durability against internal sulfate attack. The series of ternary blends with 151 MPa compressive strength at 180 days performed better compared to 145 MPa of the series with binary blend. SEM and XRD analyses confirmed the absence of deleterious ettringite formation within the paste and aggregate interfaces associated with denser microstructure reveal enhanced durability of UHPFRC. • UHPFRC showed excellent resistance to internal sulfate attack. • No significant expansion or cracking was observed in sulfate-rich environments. • SEM and XRD analysis confirmed intact microstructure without ettringite damage. • Ternary blends with GGBS improved sulfate resistance and compressive strength. • UHPFRC safely utilized gypsum-contaminated sand.