Influence of Low-Carbon Calcined Clay (Metakaolin) on Normal-Strength Concrete's Mechanical and Durability Properties

Cement production accounts for approximately 8%–10% of anthropogenic CO₂ emissions, and partial replacement of clinker with calcined clay is one of the most practical ways of reducing this burden. However, most published work has used high-purity kaolinitic clays and high-performance concrete; thus, the behavior of locally sourced clays in ordinary structural concrete is not well documented. This study examined the influence of metakaolin, which is produced from a kaolinitic clay deposit in Abeokuta, Ogun State, Nigeria, on the mechanical and durability properties of normal-strength concrete. The clay was air-dried, sieved, and calcined at 750 °C for 120 min, and metakaolin was used to replace a CEM II/B-L 42.5 N Portland limestone cement at 0%, 5%, 10%, 15%, and 20% by mass of binder. Five grade C25 mixes (MK0–MK20) were proportioned at a nominal ratio of 1:2:4 with a water–binder ratio of 0.55 and a constant binder content of 315 kg/m³. The fresh concrete was tested for slump, and the hardened concrete was tested for density, compressive strength at 7, 14, and 28 days, splitting tensile and flexural strength at 28 days, and water absorption and depth of water penetration under pressure at 56 days. All mixes gave a true slump of 95–125 mm and densities of 2405–2525 kg/m³. The control attained a 28-day compressive strength of 26.33 N/mm², and none of the blended mixes reached the design strength within 28 days. The 15% mix performed best among the blends, retaining 90.7% of the control strength (23.88 N/mm²), with splitting tensile and flexural strengths of 2.87 and 2.95 N/mm², respectively. The 20% mix gained 93% in strength between 7 and 28 days, indicating a continuing pozzolanic reaction. Metakaolin markedly improved the transport properties: water absorption decreased from 1.057% to 0.464% at 15% replacement (56.1% reduction), and penetration depth decreased from 4.05 mm to 2.80 mm at 20% replacement (30.9% reduction). The analysis of variance showed that both effects were significant at the 5% level. A 15% replacement level provided the best balance of strength retention, workability, and durability for the materials used