Water Absorption and Sulphuric Acid Resistance of Metakaolin–Sisal Fibre Concrete

This study investigated water absorption and sulphuric-acid resistance of metakaolin (MK) and alkali-treated sisal fibre (SF) concrete using a Grade 50 reference mixture. Nineteen experimental runs represented 14 unique formulations and five repeated design points, covering 0–30% MK replacement and 0–3% SF addition by binder mass at a constant water-to-binder ratio of 0.28. Three specimens per formulation were assigned to each acid-exposure period. The 28-day pre-exposure compressive strengths ranged from 15.60 to 52.04 MPa across the formulations. Water absorption was determined at 28 days using a 24-h room-temperature immersion procedure based on ASTM C642-13, while acid resistance was evaluated after 28, 56 and 112 days of exposure to 5% (v/v) H₂SO₄ using mass loss, residual compressive strength and percentage strength loss. Water absorption increased from 3.90% in the control to 6.67% for MK30-SF3. MK7.5-SF0.75 recorded the highest residual compressive strengths at 28 and 56 days, with values of 44.40 and 38.40 MPa, respectively, whereas MK0-SF3 recorded the highest 112-day residual strength at 28.24 MPa. Although MK30-SF3 recorded a relatively low 112-day mass loss of 7.08%, its residual strength was only 8.00 MPa. The results show that durability ranking depended on mixture composition, exposure duration and the indicator considered under the tested acid-exposure conditions. Mass loss and residual compressive strength should therefore be considered together when assessing prolonged sulphuric-acid exposure.