<p>This experimental study examined the properties of concrete with different amounts converter slag of Esfahan Steel Company. The experiment investigated properties of concrete containing converter slag with micro-SiO₂ and the simultaneous effect of the materials. The results show that the slag has been pozzolanicity to (until) 20% of using in concrete did not have adverse effects on the properties of concrete. Also the effect of the simultaneous use of converter slag and micro-SiO₂ is twice and micro-SiO₂ can cover the defects caused by the use of slag very well. The results indicates that the use of micro-SiO₂ and slag favorable effect on the strength of certain ages.</p>