Concrete is the most used building material in the world with an annual production of 8 billion cubic meters. As major binder for concrete ordinary Portland cement (OPC) has an annual production of 2.9 billion tons. Due to the fact that the production of OPC is very energy consuming and leads to a high output of CO2 it is necessary to develop alternative binder with equal characteristics.
A promising approach is to produce binders which mainly consist of dicalcium silicate (C2S – Belite). We succeeded to make a highly reactive belite binder in a two stepped process. The first step is a hydrothermal treatment of a mixture of Portlandite and almost pure SiO2 (nano silica, micro silica). Trials to optimize the pressure and the duration of the hydrothermal treatment showed that 15 bars and 16 to 20 h are ideal. The resulted precursor, alpha-dicalcium silicate hydrate (α-C2SH), is annealed in a second step. Various attempts to identify the ideal temperature and duration revealed that one hour at 420°C is enough to dehydrate the whole α-C2SH. XRD measurements detected reactive x-C2S and x-ray amorphous material besides inert γ-C2S after the dehydration process.
One remarkable aspect of OPC is the reaction rate and the formation of strength maintaining phases. The reaction rate is examined by means of isothermal heat conduction calorimetry. The results indicate that the alternative binder shows a heat release of 250J/g after 72h which is comparable with normal OPC (200 to 335 J/g after 72h). The hydration products are mainly Portlandite and x-ray amorphous calcium silicate hydrate (C-S-H-phases) which are also formed at the hydration of OPC. Present investigations showed that not only x-C2S, but also the x-ray amorphous material is significantly involved in the strength development.
The next step in development is to identify if industrial by-products can be used to produce a similar belite binder which is also competitive. Possible materials must have a high concentration of CaO and/or SiO2. By-products as Quartz, residues of coal combustion (fly ash) or vitreous by-products of the pig iron production (ground granulated blast furnace slag) seem to be suitable. Non-common materials like rice husk ash could be a proper replacement, too. First trials with the by-products led to a precursor with a minor amount of α-C2SH. Nevertheless, these binders showed after annealing a heat release of 236J/g which indicates a high conversion degree.