Determination of Optimum Fly Ash and PVA Fiber Ratio in Cement Mortars ABSTRACT The use of fly ash and fiber in cement mortar significantly improves the physical and mechanical properties of the mortar. In this study, the effect of using fly ash and fiber in cement mortars on the physical and mechanical properties of the mortar was investigated. For this purpose, 0, 10, 15 and 20% fly ash (UK) was used instead of cement in order to reduce the cement cost in the prepared cement mortar. In order to increase the mechanical properties of the fly ash added cement mortar, 1%, 2% and 3% Polyvinyl Alcohol (PVA) fiber was added to the mixture and the free shrinkage, flexural strength and compressive strengths of these mortar samples were examined. As a result of the experiments; It has been observed that as the UK substitution rates increase, the spread diameter increases, whereas in the mixtures with UK, the spread diameter decreases as the PVA rate increases. It was observed that the compressive strength decreases as the UK substitute increases in the mixtures, but when the PVA fiber is added in the mixtures with PVA fiber (UK15 coded mixture), the compressive strength increases by approximately 12%. However, it has been observed that as the addition of PVA fiber increases in UK substituted blends, the bending strength increases and the highest bending strength is in the mixture with 10% fly ash (UK10) substituted with 3% PVA fiber. On the other hand, it has been determined that there is a negative relationship between fiber ratio and length change in mortars and the length change decreases as the fiber amount increases. Keywords: Cement mortar, PVA fiber, fly ash, compressive strength.