Blast furnace slag cement

Blast furnace slag cement

This cement is also known as ground granulated blast furnace slag (GGBFS). It is produced by blending OPC clinkers with blast furnace slag in suitable proportion (generally 25-65%) and grinding together. A small quantity of gypsum is also added which acts as a retarder. The slag can be separately grinded and mixed with OPC in the mixer during concreting also. But in the conventional drum mixers, it is difficult to achieve proper mixing.

The slag is a waste product in the manufacture of steel from open hearth blast furnace. Earlier this waste was thrown outside the plants and huge stacks of slag were lying unused creating disposal problem. It was later found that the constituents of slag were similar to that of cement and the so called “waste product” was rechristened as “by-product”. Its extensive use now has resulted in saving of enormous energy and raw mineral in addition to prevention of pollution.

Limitations of slag cement

 The only word of caution while using this cement is that the early strength is less as compared to OPC. Therefore the form-work removal should be adjusted accordingly. The early strength of blended cement is due to OPC fraction only and the slag fraction joins in strength contribution later on.

Earlier there was a considerable resistance and suspicion in the minds of the users in using the slag cement mainly because the good quality of slag was not available. In fact earlier the thrown out air-cooled slag was being used which is not considered good quality for concrete. Instead of air cooling if the slag is rapidly cooled by pouring water over it, then the process of crystallization is prevented and it solidifies as granules. This slag is ideal for cement. That is the reason why it is now called granulated slag.

 Physical properties of slag cement are similar to those of OPC in terms of fineness, setting time, soundness and strength etc, but the initial reaction with water is slower due to which the heat of hydration is low. It is a better resistant to sulphate, alkali and acid also.

 Recommended use of Slag cement

 (a)In sewage disposal works

(b)In water treatment plants

(c)Where low heat cement is required

(d)Where high chloride and sulphate attack is there