Phosphates and Portland Cement

Sodium triphosphate is sometimes used in the wet process for making portland cement, since it minimises the amount of water needed to make the initial clay/limestone slurry. This reduces the cost of water evaporation prior to furnace input [36]. 

Although sodium triphosphate can be used as a dispersing and deflocculating agent, the presence of more than about 0.5% P2O5 in the raw material input to a portland canent furnace (e.g. in some phosphatic limestones) will decrease the yield of the most cementitious component of the product, that is, 3CaO Si02 ( C3S ). 

Portland cement clinker normally contains not more than about 0.2% P2O5, and further small additions to anhydrous portland cement before hydration and setting are reported to inaease the hardening rate and the ultimate strength. Larger additions (certainly 1.0%) have a retarding effect and can have disastrous results, but the subject is somewhat controversial [37,38]. 

Sodium polyphosphate solutions when brought into contact with set and hardened portland cement concrete will cause severe disintegration. On the other hand, sodium phosphorofluoridate, NaPOsF, when added to the mixing water for concrete, will inhibit the corrosion of steel reinforcement [39]. 

The incorporation of phosphates in formulations with calcium aluminate cements (high-alumina cements) has not met with any marked success, but they may have application as biomaterials (Section 12.14). 

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