The use of cementitious composites reinforced with plant fibers in construction elements has emerged as a promising alternative to mitigate the environmental impact of the construction industry. The challenge of chemical incompatibility between plant fibers and the matrix has been addressed through the incorporation of mineral additives, with metakaolin being the most commonly used despite its high energy consumption during production and CO2 generation. An alternative to this industrial additive is rice husk ash (RHA), a pozzolanic material derived from agricultural waste. The research aims to evaluate the influence of a high RHA content replacing cement on the mechanical properties and sustainability indicators of composites reinforced with 4 % and 6 %, by mass, of sisal fibers. Composites incorporating 50 % RHA exhibited higher compressive strength, multiple cracks, and increased toughness under flexion compared to composites with metakaolinite, fly ash, and silica of fume. Sustainability assessments indicated that replacing metakaolin with RHA resulted in reduced CO2 emissions and embodied energy, contributing to the enhanced eco-efficiency of the composites. This improvement was particularly notable in terms of increased compressive strength and toughness.