Application of an artificial pozzolana (made from the waste product of Yugoslav ferro-silica plants) as an admixture to portland cement resulted in good mechanical strength in spite of an increased water requirement. The most significant result of silica-fume addition was much better resistance of cement/mortar to the chemical attack of acids and sulfates. Effectiveness in preventing alkali-silica expansion was also observed. The above mentioned phenomena were established by means of standard mechanical testing of cement mortars exposed to various external influences. X-ray diffraction and porosity analysis were also used to follow the chemical and structural characteristics of cement gel modified by the presence of silica fume.