We are currently using blast furnace slag as the only additive to our new product of masonry cement but we are failing to control the compressive strengths within the desired levels. We are getting on average 26MPa at seven days and we are adding 70 per cent slag. What’s the way forward and please tell me more about masonry cement as this a new area for me.

Masonry cement is usually made with Portland cement clinker, gypsum, up to 30 per cent limestone addition and organic additives such as lignosulphonates to increase the workability of the mortar produced with the cement. High strength is not the property that is important with masonry cement. More important is high workability and adhesion to allow the mortar to bind masonry together or to be rendered onto the surface of masonry.

I wish to know which are the problems we may face if we start using phosphogypsum as a set retarder in manufacturing OPC. Currently, we are adding 3.5 per cent of natural gypsum in the finish mill.

I recommend that you conduct trials with the phosphogypsum replacing natural gypsum, however there is reported experience from India and South Africa that this can be done. I would recommend researching the work conducted by the University of Pretoria in conjunction with Pretoria Portland Cement in South Africa.