Investigation of Microstructural Changes and Strength Property Variation of Sandcrete Blocks Produced with Metakaolin-Blended Cement

Abstract

Metakaolin (MK), a highly reactive pozzolanic material, enhances the strength and durability of cement-based composites. However, the widespread production of low-quality sandcrete blocks in Nigeria, often caused by reduced cement content and poor practices, has contributed to structural deficiencies. This study investigates the influence of partially replacing Portland Limestone Cement (PLC) with MK on the compressive strength and microstructure of Hollow Sandcrete Blocks (HSBs). Six mixes were prepared with 0, 5, 10, 15, 20, and 25% MK replacements at a constant water–binder ratio of 0.40. A total of 72 blocks were cast to standard dimensions (450 × 225 × 225 mm) and tested for compressive strength at 3, 7, 14, 21, and 28 days. At 28 days, the 10% MK replacement achieved the highest strength of 3.70 N/mm2, representing approximately 4% improvement over the control. The 10% metakaolin mix recorded the lowest water absorption and porosity values of 8.06 and 14.12%, respectively. Scanning Electron Microscopy (SEM) confirmed a denser microstructure at this level, while EDX analyses highlighted the high silica and alumina content of MK and the improved matrix densification associated with partial replacement. This study identifies 10% MK replacement as the optimum level for structural sandcrete blocks under Nigerian conditions, meeting NIS 87:2007 requirements for load-bearing applications. The findings provide practical insights for improving block quality and ensuring safer, more durable construction practices.