Pre-hydrated calcium silicates

Prehydrated calcium silicates are a new family of hydraulic binder that offer an interesting potential future alternative to Portland cement but with much lower carbon footprint.

Pre-hydrated calcium silicates are a new family of hydraulic binders that are of a similar chemical composition to standard Portland cements (which are also based on calcium silicates) but that require much lower energy inputs and limestone in the raw mix for production. As a result, they have a much lower carbon footprint.

The production of pre-hydrated calcium silicates takes place in a two-stage process. As with Portland cement, the raw materials undergo heating following by grinding. However, the heating stage involves hydrothermal processing in an autoclave at about 200°C and 12 bar, rather than in a rotary kiln at 1400°C, as required for Portland cement.

The result of the hydrothermal process is a stable intermediate product that requires activation by intergrinding with a silica-containing material, such as quartz, glass or feldspar. The end product is a reactive calcium hydrosilicate that can be used as an alternative to Portland cement.

A second production method involves a low-temperature (<550°C) tempering process followed by a soft grinding process and the addition of reactive fillers, such as granulated blastfurnace slag or limestone.

Both approaches to creating pre-hydrated calcium silicate binders are fairly recent, having been developed within the last decade. Both also rely on established process engineering, while the product is comparable with Portland cement, handling and hardening in the same way. The product is also homogenous and is miscible with Portland cement.

Concrete produced using pre-hydrated hydraulic calcium silicates shows good durability with low porosity and high resistance to aggressive environments. It also exhibits an extremely low heat of hydration, reducing the risk that the concrete will crack during curing. At a conventional water-to-binder ratio, however, concrete strength is much lower than other alternative binders and may offer lower protection to steel reinforcement.

As a relatively new binder, pre-hydrated calcium silicates offer interesting potential; however, more research and development are required before they are suitable for the mass market. In particular, practical trials are required, including work on additives and to improve strength values. Meanwhile, the autoclave and intergrinding elements will need to be optimised and scaled-up for industrial use.