A study was made of the effects of calcarenitic stone particles, known as pietra gentile, inclusions on the rheological behavior of photopolymerizable siloxane-modified acrylic formulations, intended as protective for the calcarenitic stone structures. Different amounts of stone particles (ranging from 15% wt to 35% wt) were added to the modified acrylic mixture to achieve a natural color matching for calcarenitic stone substrates. The presence of stone particles was expected to modify the rheological behavior of the protective formulation. Therefore, the viscosity of the mixtures was studied at ambient temperature as function of the shear rate and the solid volume fraction. A relationship was obtained to predict the modification of viscosity of each formulation as a consequence of inclusions of different amounts of stone. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 942-947, 2011