This study has investigated the effect of replacing limestone with different percentages of limestone powder (5%, 10% and 15%) using the T KS method on the compressive strength of the mortar. The results have shown that the compressive strength of the mortar has decreased with curing time (at day 14) and with an increase in the proportion of limestone powder, contrary to what has been expected. This decrease in strength was attributed to the occurrence of delayed ettringite formation (DEF) in the mortar samples. DEF is a phenomenon that occurs in concrete and mortar when the conditions for ettringite formation are not optimal, which leads to the formation of a more expansive form of ettringite at a later stage. The presence of limestone aggregates in the mortar mix has been shown to contribute to the formation of DEF, resulting in the lower compressive strength values observed. This effect becomes greater the smaller the size of the limestone granules.The results suggest that caution should be exercised when using limestone powder as a replacement in mortar mixes, as it may lead to DEF and reduced compressive strength.