Abstract：To solve the problem of low early stage strength of cemented backfill with cementitious materials developed by acidic slag powders with low activity, the effect of fineness of slag powders on the strength at early stage was researched using the mechanical activation by grinding slag powders in different durations. The results show that relative to the specific surface area of slag powders without grinding of 404.7 m²/kg, the specific surface areas of slag powders respectively increase to 521 and 577.2 m²/kg by grinding the original slag powders for 1 and 1.5 h. The 3 d compression strength of the materials grinding for 1 h increases 33.3%, and the 7 d compression strength has no obvious increase. The 3 and 7 d compression strengthes of the materials grinding for 1.5 h respectively increase 22.2% and 18.1%. The hydration products of slag powders cementitious materials are mainly acicular ettringite and flocculent C-S-H gel, which constitute the materials by dense gel and bond the aggregates together.

Keywords: acidic slag powder; activity; strength at early stage; concentration of slurry; fineness; specific surface area