Abstract <p>The objects of study are metallurgical iron-containing slag, substandard clay material, and ceramic brick. In the chemical technology of ceramic materials, particular importance is given to the physicochemical processes occurring during brick firing, as they primarily determine the operational properties of ceramic materials. It has been established that bricks produced from substandard clay material without the use of tempering agents do not meet the GOST requirements due to the number of cracks without destruction, the presence of through cracks, and multiple chippings resulting from the high sensitivity of the raw mixture to drying. It has been found that the introduction of metallurgical slag into the ceramic mass based on substandard clay, even in an amount of 10%, reduces crack formation, increases compressive and flexural strength, and improves frost resistance. The optimal composition for ceramic brick production contains 20% slag, with the resulting brick meeting the M125 grade and exhibiting no defects. Further increases in metallurgical slag content do not reduce the brick grade but lead to various defects, such as a black core. Studies have shown that the introduction of metallurgical iron-containing slag into ceramic masses promotes the appearance of anorthite, mullite, diopside, fayalite, magnesium ferrite, pigeonite, and forsterite in X-ray diffraction patterns, which contribute to increased brick strength.</p>