The addition of limestone fines (LF) to mortar or concrete can improve the performance of the mortar or concrete. However, the effects of adding LF appear to be dependent on the water content and paste volume, and thus it is not easy to comprehend how LF actually works. Herein, it is postulated that the effects of LF are caused by the corresponding changes in water film thickness (WFT) and paste film thickness (PFT). To study the effects of LF, a series of mortar mixes with different paste volumes and LF contents have been tested for their packing density, flowability, cohesiveness and strength. From the packing density results, the WFT and PFT of the mortar mixes were calculated and the measured properties of the mortar mixes were correlated to the WFT and PFT. It was found that the WFT and PFT are the major factors governing the performance of mortar and the effects of LF are caused by the decrease in WFT and the increase in PFT due to the addition of LF. (C) 2014 Elsevier B.V. All rights reserved.