Efficient insulation for residential buildings is one of the keys for implementing an energy transition. The technology used for exterior walls is usually made by combining load‐bearing and difficult to recycle insulating components. For this reason, Celitement GmbH, Karlsruhe Institute of Technology and Xella Technology and Research have joined their forces to develop a high insulating AAC block, which is also resource efficient using the main components quick lime, sand, and water. The thermal conductivity of the AAC was reduced. Macropores formed in the conventional production process by hydrogen gas, diameter of approximately 1 mm, could be partially replaced by lightweight aggregates, with an average pore diameter of 50 nm. A process for producing lightweight aggregate, which is based on optimized, hydraulically active calcium hydro silicate binders (Celitement), was developed. The aggregate quantity in AAC could be raised to 40% by weight through optimizing formulation and mixing. This project, sponsored by the German Federal Ministry of Education and Research, ended in 2017. Based on the results so far, it is worth pursuing this approach in the future, whereby the material properties of the CSH granules could be further improved to obtain the sought final AAC product.

Durability of autoclaved aerated concrete under high CO2 impact. Ordinary autoclaved aerated concrete (AAC) (P2-0,35) was stored under different humidity conditions and a constant CO2 atmosphere of 1 % by volume. Additionally, AAC samples with a good and poor phase formation were produced and included into the weathering procedure. In addition to the artificial CO2 weathering, ordinary AAC was stored under rain protection outdoor for a period of 3 years. After storing the samples under different conditions and periods, the material was measured for its raw density, compressive strength and mineral phase content. The outdoor stored AAC was tested after 3 years for its thermal conductivity. Boundary conditions such as material moisture and drying behaviour of the AAC as well as ambient moisture, influence the material properties within the weathering procedure. Artificial CO2 weathering leads to other effects than natural weathering. AAC with well crystalline tobermorite does not change its compressive strength or thermal conductivity under natural climate conditions. A negative long-term development of the material properties can be excluded.

New laws are likely which will make putting autoclaved aerated concrete (AAC) into landfills more difficult in the future. Consequently, a project was launched at Xella to reduce the sulphate content in AAC to almost zero. Positive side effects are no risk of thaumasite, agglomeration residue of lime and grey stains. The biggest source of sulphate besides cement is the pure calcium sulphate, which is added either as gypsum or anhydrite to the mixture. Additional calcium sulphate has been used in AAC to improve its material properties for many years. The reduction of calcium sulphate in ordinary cementitious AAC recipes leads to high shrinkage and less compressive strength of the material.Sulphate free AAC with a low bulk density is not yet being made in mass production. Due to the retarding effect on of the hydraulic binder cement, completely sulphate free recipes could not be handled in technologies like Durox, Hebel or Ytong yet. Cement and sulphate free AAC cakes tend to collapse either in the demoulding or autoclaving process, and especially low densities are difficult to process.Technological solutions and recipes were found to produce cement and sulphate free AAC with low bulk densities in moulds with the size of 5.4 m(3). The material meets the requirements for the German bulk density class PP2/0.35 with lambda 0.09 W/(mK). The shrinkage tests show values under 0.20 mm/m according to DIN EN 680. Finally, it was shown that cement free recipes lead to fewer transportation damages due to less brittle surfaces. Further research is currently in progress.

The authors have systematically investigated the role of synthesis conditions upon the structure and morphology of xonotlite. Starting with a mechanochemically prepared, semicrystalline phase with Ca/Si=1, the authors have prepared a series of xonotlite samples hydrothermally, at temperatures between 200 and 250 degrees C. Analysis in each case was by X-ray photoelectron spectroscopy, environmental scanning electron microscopy and X-ray diffraction. The authors' use of a much lower water/solid ratio has indirectly confirmed the 'through solution' mechanism of xonotlite formation, where silicate dissolution is a key precursor of xonotlite formation. Concerning the role of temperature, too low a temperature (< 200 degrees C) fails to yield xonotlite or leads to increased number of structural defects in the silicate chains of xonotlite and too high a temperature (>= 250 degrees C) leads to degradation of the xonotlite structure, through leaching of interchain calcium. Synthesis duration meanwhile leads to increased silicate polymerisation due to diminishing of the defects in the silicate chains and more perfect crystal morphologies.

We have investigated the incorporation of zinc into both nanocrystalline and crystalline calcium silicate hydrates with starting C/S ratios of 2/3 (0.66). Zinc was added replacing calcium in the starting mixtures [Zn/(Zn+Ca)=0–1/4; 0–10 wt.% Zn], and the resultant phases were characterised using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential thermal analysis–thermogravimetry (DTA–TG) and environmental scanning electron microscopy (ESEM).In both groups of samples, increasing zinc content led to gradual structural changes, until eventually a second phase was formed. Zinc was incorporated to similar limits in both sets of samples. The thermal stability of the structures increased to a certain zinc content, beyond which there was structural destabilisation. Zinc incorporation is possible up to ∼6 wt.%. Our observations strongly indicate similar zinc incorporation mechanisms in both sample series, namely incorporation of zinc into the interlayer of C-S-H(I) and the X-sheet of gyrolite for nanocrystalline and crystalline samples, respectively.

We have synthesised 11-Å tobermorite hydrothermally, both pure and with increasing isomorphic substitution of aluminium for silicon. The samples were analysed by X-ray photoelectron spectroscopy (XPS). Aluminium was found, on the basis of its Al 2p binding energies, to be tetrahedrally coordinated. We observed no changes in Ca/(Si+Al) ratio upon aluminium substitution, implying that charge balancing does not occur via the incorporation of additional calcium into the tobermorite structure. Aluminium substitution into the silicate structure led to a decrease in Si 2p binding energies. This implies one of two alternatives. Firstly, that charge balancing occurs via substitution of OH− for O2− in the tobermorite structure. Secondly, the presence of aluminium in the tobermorite structure may negatively influence the degree of silicate polymerisation. Further work is required to determine which of these possibilities is the case.

We have used X-ray photoelectron spectroscopy (XPS) to investigate both tricalcium silicate (Ca3SiO5, C3S) and β-dicalcium silicate (Ca2SiO4, β-C2S), the principal components of cement clinkers. In addition to showing how the two phases may be characterised and differentiated, we show how the sensitivity of these phases to atmospheric carbon dioxide and moisture may, as a result of improper sample preparation, lead to erroneous results. The observed alteration processes of the clinker minerals shed light upon the aging process of cement clinker during storage.