Accordingly, the present invention provides an improved process for the production of Portland slag cement using granulated blast furnace slag, which comprises: (viii) forming of cement clinker by known process, (ix) ball-milling of cement clinker for a period ranging between 30-60 minutes in dry condition, (x) reducing size of granulated blast furnace slag by any process to obtain the size in the range between 210 to 100 .mu.m for using as feed for attrition mill, (xi) wet milling of granulated blast furnace slag for a period ranging between 5-15 minutes in an attrition mill using granulated blast furnace slag to water ratio in the range of 1:1 to 1:2 and granulated blast furnace slag to grinding bail ratio in the range of 1:5 to 1:15, (xii) removing water from the slurry obtained after attrition milling by known process, (xiii) drying of the obtained slurry by known process, (xiv) mixing intimately of: attrition milled slag obtain in step (vi) in the range of: 50 to 95% by weight ball-milled clinker obtain in step (ii) in the range of: 05 to 45% by weight gypsum in the range of: 01 to 05% by weight for a period in the range of 15 to 30 minutes.