This paper contains results of tests of samples of mortar picked up in antique objects, which date back to the 2nd and 3rd century AD, i.e., Kom el-Dikka complex in Alexandria. As the tests have proved, the types of mortar are very specific due to uncrystallized silica which oversaturates the samples. Silica is thought to be introduced purposefully to the mortars because of the specific character of the objects where the least penetrability is desired. The condition of the mortars, in spite of exposure to water and after so many centuries, is still good due to their relatively high compressive strength.