The shortage of conventional supplementary cementitious materials opens up the prospect for broader utilization of nontraditional and natural pozzolans (NNPs). The NNPs used in the study included three calcined clays, three volcanic ashes (also referred to as natural pozzolans), three ground bottom ashes, and two fluidized bed combustion ashes. The study focused on the assessment of the performance of these NNPs in portland cement systems by evaluating the following parameters: (a) development of the heat of reaction in cement pastes containing 25, 30, and 35 % of NNPs by weight of cement (bwoc) by means of isothermal calorimetry; (b) measurement of the consumption of calcium hydroxide (CH) in cement pastes containing 0, 25, 30, 35, 40, and 45 % (bwoc) of NNPs after 7, 28, and 56 days of curing using thermogravimetric analysis technique; (c) determination and quantification of the reaction products by quanti- tative x-ray diffraction in cement pastes containing 0 and 25 % (bwoc) of NNPs after 7, 28, and 56 days of curing; and (d) measurement of the 7- and 28-day compressive strength of mortar cubes. In all cases, the cumulative heat values of pastes containing NNPs were higher than those of reference pastes containing inert fillers, confirming that all NNPs were undergoing pozzolanic reaction. For most of the NNPs, the highest amount of CH consumed was observed in systems with 35 % replacement level. The early-age CH consumption corre- lated well with the content of alumina in NNPs whereas the later-age data correlated better with the silica content of the NNPs. In terms of compressive strength development, the highest early-age values were observed in mortars containing calcined clays, whereas other NNPs con- tributed to strength development at latter ages.