Scanning electron microscopy of autoclaved aerated concrete with supplementary raw materials

Microscopy is a key analysis technology for the understanding of the achieved properties of building materials. In the case of Autoclaved Aerated Concrete (AAC) it is even more important due to the phase transformation during the hydrothermal hardening. The incorporation of substitution materials in AAC might lead to a change of properties like strength, shrinkage or colour, but the reason for these changes can in most cases be explained by a difference in microstructure. In the current study various AACs were designed, produced and analysed. The influence of the supplementary materials on all manufacturing steps were evaluated and adjusted to keep the workability.
An industrial by-product (municipal recycling waste glass) and micro-silica are investigated as supplementary materials. The glass addition leads to a less crystalline appearance of tobermorite, the main binding phase. The higher solubility of glass compared to quartz causes a relatively dense layer of hydration products around the glass grains, which is enriched in sodium (and magnesium) as well. Due to the enhanced dissolution of the glass the grain does not have contact with the surrounding phases. The quartz grains show a decreasing reactivity with the increasing glass amount. Micro-silica serves as thickener in AAC during the mixing, which needs to be compensated. However, the increased strength by a rather small addition of micro-silica is reflected by the shape change of the tobermorite crystals, which increase in width.