Blast furnace slag fine powder with specific surface areas of 6000 and 4000 cm2/g was substituted for regular cement in proportions ranging from 0% to 70% with the aim of producing environmentally friendly concrete. Three different silicate surface treatments with different molar ratios ([SiO₂]/[Na₂O]) were impregnated into the mortar samples. To clarify the modification effect of these treatments, the Vickers hardness test was used to evaluate them by the change in surface hardness. Regardless of the substitution ratio, the improvement in Vickers hardness varied with each molar ratio, reaching a peak at ratio 2.1. Furthermore, the increase in Vickers hardness was less pronounced when blast furnace slag powder with a smaller specific surface area was used.