Leveraging Delayed Strength Gains in Supplementary Cementitious Material Concretes: Rethinking Mix Design for Enhanced Cost Efficiency and Sustainability

Engineers commonly use the 28-day characteristic strength of concrete for project calculations, but this may not reflect the full-strength potential, especially in concretes with supplementary cementitious materials (SCMs). SCMs, known for their slow reactivity, often delay optimal strength beyond 28 days, requiring higher cement content to speed up early strength development, thus increasing production costs. This study examined the relationship between concrete age and mechanical strength across eight cement types, including tests for axial compression, water absorption, void index, and specific mass. The findings showed that pozzolan and slag cements gained significant long-term strength due to slow pozzolanic reactions. Conversely, limestone filler mixes had lower initial strength and slower progress, likely due to increased porosity from fine fillers. A correlation was found between higher pozzolan content and improved durability, including reduced water absorption and void index. Cost analysis indicated that optimizing cement mix designs for targeted strength levels could reduce production costs, especially for concretes with high SCM content. Using long-term characteristic strength rather than the traditional 28-day strength resulted in approximately 14% savings, particularly for slag- and pozzolan-based cements. The savings were less significant for other cement types, emphasizing the importance of adjusting mix designs based on both performance and financial considerations.