Alternative binders / “Eco-concrete”

Against the background of the current climate debate, the cement industry is more than ever in the focus of public perception with regard to its poor CO2 balance, which is essentially driven by the process of clinker production. In the future, economic efficiency could be significantly worsened by a monetary assessment of CO2 emissions. For this reason, efforts have been underway for many years to use industrial by-products (fly ash, granulated blast furnace slag, etc.) as clinker substitutes.

Beyond reinforced concrete applications, there are numerous binder systems today which do not require any clinker at all, for example geopolymers, as well as alkali activated binders based on combinations of fly ash, granulated blast furnace slag, metakaolins or calcined clays, etc. Due to the lack of clinker content, these products generally have a significantly better CO2 balance than conventional binders.

The alkalis necessary for the formation of the mechanical properties are added here in the form of alkali metal hydroxide or silicate solutions. For this purpose, numerous findings on the mechanical properties (compressive strength, modulus of elasticity, creep and shrinkage behaviour, acid resistance, etc.) are available. An industrial-scale application for non-reinforced concrete products is already taking place in numerous cases.

However, with the exception of individual pilot applications, no systematic commercial applications have been documented for reinforced concrete products. The reasons for this are the unclear durability of steel reinforcement against corrosion by chlorides, loss of alkalinity or even by individual ion species contained in the binders themselves. In contrast to classical binders, passivation of the steel reinforcement cannot be assumed in principle. The composition of the individual alkali activated binders available on the market varies greatly. A systematic scientific basis for assessing the durability of a steel reinforcement solely on the basis of its chemical composition is currently not possible and is the subject of basic research. The Institute of Building Materials Research is currently carrying out the DFG-funded project “Influence of alternative binder systems on the corrosion behaviour of steel reinforcement”.

Within the scope of this consortium project, commercially available alkali activated binders without or with only a small clinker content, which are available in sufficient quantities and in reproducible quality, are to be identified, which have the potential to protect an embedded steel reinforcement against corrosion. Here, the durability has to be evaluated against the background of the intended exposure (dry interior, marine infrastructure). The aim is to make the economic and marketing advantages of “eco concrete” technically usable for selected products.

The project was successfully completed.