Untersuchung des Einflusses von Sulfaten auf das System CaO-SiO2-Al2O3-K2O-H2O mittels Wärmeflusskalorimetrie und in-situ Neutronenbeugung unter hydrothermalen Bedingungen

Autoclaved aerated concrete consists of the raw materials sand, cement, quicklime, water and aluminum powder (pore forming material). Additionally sulfate carriers are used to improve the physico-mechanical characteristics such as compressive strength and shrinkage. The aim
of this work was to determine the effect of sulfate and/or different sulfates in the production of autoclaved aerated concrete (AAC). Because of the production process of AAC three sections were investigated:
1. The period before hydrothermal hardening is shaped by the hydration of the quicklime and the cement hydration (which is not completed at the end of this period). The slaking of burned lime is retarded:

with increasing firing temperature and/or burning duration of the lime
with addition of a sulfate carrier (the more soluble the bigger the retardation)
with addition of cement

2. By means of neutron diffraction, phase development could be recorded continuously during hydrothermal hardening.
Sulfates decrease the reaction rate of Ca(OH)2 with SiO2. At first, the hydrothermal
reaction is solution-controlled (amorphous CSH phases are formed) and increasingly
affected by diffusion processes (crystalline CSH phases appear). Sulfates seem to
interrupt the expected continuous transition between amorphous and crystalline CSH
phases for a period of approx. 30 min.
3. Sulfates increase compressive strength, lower shrinking of the autoclaved aerated
concrete product. Apart from these positive influences, it also effects unwanted side
effects: The heat conductivity and the quantity of soluble sulfate rise. A high alkali
content promotes the sulfate solubility from the aerated concrete.