The importance of volumetric w/c for porous supplementary cementitious materials in concrete

Porous supplementary cementitious materials (SCMs) are known to adversely affect the rheology of fresh concrete. When cement is replaced on a weight basis, these effects are often intensified due to the high number of porous particles introduced relative to the volume of cement displaced. In this study, biochar was selected as a representative porous SCM owing to its high porosity. To address this, a volumetric water-to-cementitious materials ratio (w/c) approach was employed, in which biochar replaced cement by volume at levels of 5 %, 10 %, and 20 % and were compared with a conventional 5 wt.% replacement to assess their impact on workability, hydration and mechanical properties. The results demonstrated that weight-based replacement introduces a large number of porous SCM, leading to severe slump loss (69 % reduction at the 5 wt.% level), shortened setting times, decreased moisture retention, and limited internal curing capacity. In contrast, the volumetric replacement method provided a more balanced substitution by incorporating a volume of porous SCM equivalent to that of the cement replaced. It ensured workability of concrete, as indicated by only a 12 % reduction in slump at the 20 vol.% replacement level. Remarkably, due to improved moisture retention, although the 10 vol.% biochar mixture contains 5 % less cement than the 5 wt.% mixture, it still achieved 1 %, 0 %, 3 %, and 6 % higher compressive strength at 3, 7, 28 and 56 days, respectively. This underscores the effectiveness of volumetric replacement for porous SCMs in concrete. • The volumetric water-to-cement (w/c) ratio effectively addressed the workability issue in concrete containing porous supplementary cementitious materials (SCMs). • The volumetric w/c ratio provided better internal curing of porous SCM in cement compared to the weight-based w/c ratio. • The 10 vol.% samples achieved higher strength than the 5 wt.% samples, despite having less cement.