This paper reports the results of a study conducted to investigate the effects of rice husk powder (RHP) on the rheological properties and the fluidity of grout mixtures. The experiments were carried out with the grout mixtures including of 4, 8, 12, 16 and 20% content of RHP and 0.75, 1.00, 1.25 and 1.50 ratios of water–cement. The purpose of the study is to provide a new knowledge about using RHP as a filler in cement grouts in order to improve the grout methods and services, and achieve a good product performance. Test results showed that the increase in RHP content in the mix increases the plastic viscosity with all water–cement ratios. At 0.75, 1.00 and 1.25 ratios of water–cement, the grout mixtures showed shear-thinning behavior. On the other hand, this behavior changes to shear-thickening behavior at $$w/c = 1.50$$ at all RHP percentages. Experimental results indicated that this waste product can be utilized as a filler in cement grout for geotechnical application such as filling large voids.