Cellulose fibres have already been applied commercially as an alternative to asbestos in fibre-cements composites. In spite of their industrial scale production for more than 20 years, these composites still require much research efforts, which focus mainly on durability aspects. The influence of the most relevant deterioration mechanisms can be minimized if mineral admixtures with high pozzolanic activity replace ordinary Portland cement (OPC). The improvements then achieved are due to the decrease in Ca(OH)(2) content and the more compact matrix and interfaces in the composite. In this respect, rice husk ash (RHA) is one of the most promising materials to be applied as a partial cement replacement in the cellulose-reinforced cement-based composites. This is due to the high active silica content of the ash and the widespread availability of the husks. To assess the influences of different chemical compositions of RHA, and the effects of autoclave curing on the pore characteristics of bamboo-pulp-reinforced cement composites, a comparative study was carried out in which pore characteristics were assessed by mercury intrusion porosimetry (MIP). Complementarily, the effects exerted by changes in the pore structure of the composites on their water permeability are evaluated by analytical and experimental approaches. It was observed that the incorporation of RHA in the composites could cause an extensive pore refinement in the matrix and in the interface layer, thereby decreasing water permeability. The results indicate that partial replacement of cement by RHA can improve the durability characteristics of cellulose-cement composites.