In most studies focusing on the engineering behaviour of rocks exposed to high temperatures, thermal procedure tends to apply a slow and linear heating rate, and constant exposure time. Also, granite, marble, limestone, and sandstone are generally used because many mineralogical features of these rocks such as texture, mineral size and mineral content can be easily determined under polarized microscope. Contrary of literature, engineering behaviours of the highly porous calcareous rocks (calcarenites and chalk) were evaluated considering logarithmic/exponential heating rates and different exposure times as in a real fire in this study. The porous structure of the carbonate rocks depending on the primary voids was observed in SEM images for initial condition. SEM images of the samples exposed to high temperatures indicated that thermal treatment plays an important role on developing of secondary porosity due to the grain clumping in calcarenites and forming of new micro fissures in chalks. Lumping of minerals observed in calcarenite samples due to increasing temperatures is the main reason for the continuous decrease in strength. Decrease in porosity due to the sintering and fusing between clay minerals up to 600 degrees C caused the increase in strength of the chalks unlike coarse grained calcarenites. While high temperatures caused strength decrease in coarse crystalline calcarenites by creating thermal stress in the crystal grains due to expansion, on the contrary, fine crystalline chalks were more resilient to this stress because smaller crystals acted as expansion hinges during heating. Also, exposure time was insignificant in fine crystalline rocks, whereas coarse crystalline rocks showed great thermal damage. Depending on the increased temperatures, considering to the strength of some rocks increases while some decreases, a new thermal treatment coefficient graph was suggested to evaluate the rocks with different behaviours together. Because P-wave velocity tends to decrease in almost all rocks whether the strength increases or decreases, it was proposed to use tensile strength as an input parameter in the suggested approach.