The effects of natural zeolite on the theological and workability properties of the grout mixtures were studied. Setting times of grouts were also determined as part of the experimental study. For comparison, grout mixtures were also prepared with a commercially available viscosity modifying admixture (VMA). The experimental results show that addition of natural zeolite modifies both the rheological and workability properties of grouts. For a constant superplasticizer (SP) content, an increase in the zeolite amount significantly increases the yield stress, the apparent and plastic viscosity, and reduces the fluidity and deformability. Moreover, an increase in the amount of SP causes a significant reduction in both the yield stress and plastic viscosity of the grouts. It was also observed that, grouts prepared with natural zeolite addition have a pseudo-plastic behavior, and shear-thinning behavior increases with an increase in the zeolite amount. Therefore, it has been shown that using natural zeolite as a VMA it is possible to obtain grouts that have satisfactory theological properties, especially if natural zeolite is used in combination with a superplasticizer. (c) 2008 Elsevier Ltd. All rights reserved.