New analytical possibilities for monitoring the phase development during the production of autoclaved aerated concrete

This paper is focused on developing innovative and novel formulations for autoclaved aerated concrete (AAC) that could pave the way especially for medium-size manufactures toward the production of low-sulfate AAC. The aim was to significantly reduce the sulfate concentrations in eluate and, as a consequence, to successfully cope with the existing and upcoming regulatory restrictions on the recyclability of construction waste from AAC. This should, in parallel, satisfy European demands for a reduction in the volume of waste and an increased recyclability of building materials. For achieving this goal, the AAC formulations required a cement content as low as possible and an appropriate alternative to the sulfate carriers used to date. The tasks of these two mentioned components (cement and sulfate carrier) should be taken over by calcined clay. Tests yielded promising results for the complete replacement of the sulfate carrier and a substitution of 20 wt.% of the cement by either of two types of calcined clay in the AAC formulation. The amount of leachable sulfate ions was reduced by up to 70% compared to the reference samples. The amount of crystalline tobermorite remained constant or was even higher in the samples with calcined clay.The amount of crystalline tobermorite can serve as an indicator of the mechanical strength and shrinkage behavior. However, the mechanical characteristics of autoclaved aerated concrete specimens with calcined clay will be determined in a subsequent project. In this follow-up project, the formulations developed under laboratory conditions are to be implemented on an industrial scale.

Section: General Backgroundmentioning

confidence: 99%

Improving the recyclability, environmental compatibility, and CO₂ balance of autoclaved aerated concrete by replacing sulfate carrier and cement with calcined clays

Chucholowski

Müller²,

Thienel

2018

“…In order to visualize the approach to domains of portlandite and tobermorite, the models [5] used for the respective structure are shown in Figure 2) lead to formation of C-S-H phase, ettringite, and further portlandite. It can be seen that a major amount of the hydration products are formed during the first 140 min, an average duration of green cake hydration on an industrial scale.…”

Section: Resultsmentioning

confidence: 99%

XRD analysis of phase development and peak broadening during the production of AAC

Schreiner¹,

Jansen²,

Ectors³

et al. 2018

The production process of autoclaved aerated concrete (AAC) is driven by the transformation of mineral phases. During green cake step, strength is developed by the formation of hydrate phases such as ettringite and in larger quantity portlandite. In respective XRD plots, portlandite often shows anisotropic peak broadening due to anisotropic domain morphology. A new implementation in Rietveld refinement allows to refine the peak broadening calculating anisotropic domainsizes. An appropriate model is used to approximate the domains of portlandite. In this way, it is possible to track the mechanism of portlandite growth during hydration and the quantification of phase development during the green cake hydration is performed more precisely. In autoclaved samples, the main phase is tobermorite, which shows anisotropic peak broadening as well. The application of the described approach in a similar way to portlandite enables an advanced refinement. To approximate the domains of tobermorite a triaxial cylinder model is used, so that tobermorite is characterized more accurately. In addition, this method allows to investigate the influence of variations in the chemical composition of raw materials on domain morphology of tobermorite, as exchanged ions like Al 3+ make a significant impact. Consequently, a new possibility to analyze the link between mechanical and thermal properties and phase characteristics is given. K E Y W O R D SAAC, anisotropic peak broadening, phase development, Rietveld refinement, XRD analysis

“…There are some disadvantages of using an agate mortar for sample preparation, which is why the use of a McCrone mill is suggested . Alternatively, completely automated preparation procedures which are already applied in the cement industry for many years could also be implemented in the AAC production process.…”

Section: Methodsmentioning

confidence: 99%

Quality control of autoclaved aerated concrete by means of X‐ray diffraction

Paul

2018

X-ray diffraction is a very powerful tool for the characterization of crystalline phases in multiphase materials. In recent years, this method has been established in numerous industrial branches for quality control in processes where materials consisting of crystalline phases are produced. For instance, cementitious materials exhibit up to 20 different phases of which each is responsible for certain physical and chemical properties of the product. Knowing and understanding the phase assemblage of a material and the effects on its behavior allows the optimization of a product with respect to properties, quality, and production costs. The cement industry is using this knowledge by characterizing the materials along the production chain from raw materials to the final products by means of X-ray diffraction. Autoclaved aerated concretes (AACs) consist mainly of crystalline and non-crystalline solids, and for that reason, they have been subject to many crystallographic studies using X-ray diffraction. Although the phase assemblage is regularly investigated, only little knowledge gained from these studies is transferred to production sites in order to monitor the manufacturing process. The crystallographic phase assemblage of autoclaved aerated concrete could be examined by X-ray diffraction on a routine basis in order to maintain constant product quality and, thus, even material properties. Another beneficial aspect of this method is that it enables choosing more economic raw materials and optimizing process parameters. K E Y W O R D Sphase assemblage, quality control, X-ray diffraction