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Due to the shortage of high-quality fly ash (FA) in Kenya, the successful construction of the Mombasa–Nairobi railway had to rely on the abundant natural pozzolan resources in the country instead. In the work reported in this article, concrete samples produced using 30% FA, 30% fine tuff (FT), 30% fine volcanic slag (FVS), or 10% FA and 20% FVS to replace cement were tested. The hydration activity of the FT was found to be poor. The mechanical properties and durability of the concrete made with FVS were close to those of the FA concrete and optimal performance was obtained from a mixture of FA and FVS. Mercury intrusion porosimetry, X-ray diffraction and differential thermal analysis were used for microanalysis of cementitious systems with 70% cement and 30% FA, 70% cement and 30% FT, and 70% cement and 30% FVS. The results were promising and suggested that the FVS possessed obvious secondary hydration activity and could improve the microstructure of the concrete matrix. It is therefore feasible to use natural pozzolans instead of FA to produce high-performance concrete in railway projects, which would reduce construction costs and help protect the environment. [ABSTRACT FROM AUTHOR]