Early-stage hydration heat development in blended Portland cement-based binders containing natural zeolite in an amount of up to 40% by mass is investigated using an isothermal heat flow calorimeter. Experimental results show that the presence of natural zeolite in the blended binders accelerates the onset of C3S hydration and boosts the first phase of C(3)A hydration. The second phase of C(3)A hydration gains on importance in the hydration process with the increasing zeolite dosage. For the zeolite contents higher than 30%, the maximum specific hydration heat power in this phase is even higher than the maximum corresponding to the C3S hydration. The effectiveness of the natural zeolite as a binder, as expressed by the difference specific hydration heat per 1 g of zeolite, is found to decrease with its increasing dosage in the blends. The replacement of Portland cement in an amount of 10% by mass by natural zeolite can be considered as a limit for its effective use. Above this limit, a substantial part of zeolite does not directly participate in the hydration process and plays a role of a fine filler instead. The limited zeolite participation in the blended-cement hydration can explain the lower compressive strength of zeolite concretes containing higher amounts of natural zeolite, which was reported before. (C) 2014 Elsevier B.V. All rights reserved.