Improving hydraulic properties of lime–rice husk ash (RHA) binders with metakaolin (MK)

► MK contributed to improve long term mechanical properties of lime-rice husk ash binders. ► MK corrected the reduction in long term strength observed in previous works. ► The pozzolan mixture with 75% RHA and 25% MK gave best hydraulic properties. To improve long-term hydraulic properties of binders from RHA and lime, 25–75% MK was added to RHA. Binders were formulated and properties were compared to that containing RHA or MK as only pozzolans. The lime–pozzolan ratio was 1:3. The properties tested after 7, 28 and 56 days were: absolute density and fineness of the binders, initial setting time, chemical and mineralogical composition of hydrated binders, flexural and compressive strengths and water absorption of mortars. The micrographs of the hardened binder pastes at 56 days permitted to evaluate the densification of different matrixes and the development of pores. From the results obtained, it was concluded that, MK increased the density of mixtures and decreased their grindability. The presence of MK decreased the SiO 2 content of binders and increases their Al 2O 3 and Fe 2O 3 contents. Calcium-silicate hydrates (CSH) gel and gehlenite (C 2ASH 8) were the main phases formed during the pozzolanic reaction in the presence of MK. No reduction in flexural and compressive strengths was observed after 28 days for binders containing MK. The mixture of 25% MK and 75% RHA which is recommended gave flexural and compressive strengths higher than binder with RHA or MK as the only pozzolan. Water absorption of mortars was less than 20%.