Supplementary cementitious materials (SCMs) represent an alternative for the industry to achieve sustainability by reducing cement contents without significant compromises of the mechanical properties and enhancing durability. SCMs play a dual role during hydration: a physical effect promoting nucleation and cement hydration and a chemical effect through pozzolanic activity. Rice husk ash (RI-IA) and natural pozzolans (NP) were evaluated using compressive strength and durability tests in a multi-level experimental program. RHA increased the strength more than NP, which is well explained by its prominent chemical effect (78%) assessed by isothermal calorimetry and its high amorphous silica content. Both RI-IA and NP produced significant reductions in the permeability of the concrete, which is mostly explained by the chemical effect. Decoupling the physical and chemical effects of a SCM allows for optimisation of its manufacturing process. (C) 2015 Elsevier Ltd. All rights reserved.