The use of locally sourced raw materials has the potential to reduce the environmental impact and cost of cement, especially in those locations where the supply of conventional resources for cement production, such as limestone, is based on massive import. To this aim, this study investigates the utilisation of volcanic ashes sourced in Kenya in both blended and alkali-activated cements. Quantitative X-ray powder diffraction (XRD) phase analysis of the ashes was performed in combination with XRF with the aim of evaluating the amount and chemical composition of the amorphous fraction, as an aid to the proper formulation of the alternative cement mixes. The compressive strength and degree of hydration, and compressive strength and yield stress, were tested for the blended and alkali-activated cements respectively. The results show that the volcanic ashes are characterised by a significant amount of an amorphous glassy phase. However, the relatively low Al/Si ratio of the amorphous fraction leads to different behaviours of the volcanic ashes in blended and alkali-activated cements. The high Si content activates the pozzolanic reaction in blended cements and it is observed that the mechanical properties of ternary blends based on volcanic ashes are comparable to those of control LC3 blends. With respect to the alkali-activated cements, the limited concentration of Al in the amorphous fraction of the volcanic ashes inhibits the precipitation of N-A-S-H, therefore the addition of 37 wt.% metakaolin, based on a Design-of-Experiments (DoE) approach, was found to be suitable for optimising both the mechanical performance and workability.