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If the silica raw material used in autoclaved aerated concrete (AAC) requires a lower level of energy for dissolution, a lower autoclaving temperature could be applied to reach the required product performance. Accordingly, the raw material quartz sand in the AAC mixture was replaced with rice husk ash (RHA) which has a higher solubility of silica with increasing temperatures in alkaline binder systems. AAC samples were produced at autoclaving temperatures of 152 ◦C, 165 ◦C, 175 ◦C, and 192 ◦C with an autoclaving time of 6 h. Quantitative X-ray diffraction (XRD) analysis according to Rietveld refinement, qualitative XRD analysis, and scanning electron microscopy (SEM) were carried out to investigate the microstructure of the hardened AAC samples. Additionally, the compressive strength, dry bulk density, A-value, and total drying shrinkage were determined. In contrast to the quartz-based AAC, the properties of the RHA-based AAC were improved by applying a lower autoclaving temperature than the typical temperature used for industrial production of AAC, i.e. 192 ◦C. For the RHA-based AAC, a maximum increase of 22% in the compressive strength, 132% in the total content of tobermorite, 19% in the A-value, and a maximum decrease of 33% in the total value of drying shrinkage were observed as the applied autoclaving temperature was changed from 192 ◦C to 165 ◦C. Moreover, the material efficiency of the AAC product can be improved by using RHA as an agricultural waste product in the mixture. The RHA-based AAC autoclaved at 165 ◦C exhibited a slightly higher compressive strength to density ratio (A-value) than the quartz-based AAC autoclaved at 192 ◦C. However, a higher total drying shrinkage was observed for the RHA-based AAC autoclaved at 165 ◦C compared to the quartz-based AAC autoclaved at 192 ◦C. From a general point of view, the results showed that the optimum temperature for autoclaving AAC has a direct dependency on the dissolution properties of silica raw materials used in the mixture.

Products of the refractory industry are key for the production of heavy industry goods such as steel and iron, cement, aluminum and glass. Corresponding industries are dependent on thermal processes to manufacture their products, which in turn would not be possible if there were no refractory materials, such as refractory bricks or refractory mixes. For the production of refractory materials, primary raw materials or semi-finished products such as corundum, bauxite or zircon are used. Industrial recycling of refractory raw materials would reduce dependencies, conserve resources and reduce global CO2 emissions. Today, only a small quantity of the refractory materials used can be recycled because raw materials (regenerates) obtained from end-of-life materials are of insufficient quality. In this study, regenerates from different refractory waste products could be obtained using the innovative processing method of electrodynamic fragmentation. It was shown that these regenerates have a high chemical purity and are therefore of high quality. It could be confirmed that the use of these regenerates in refractory materials does not affect the characteristic properties, such as refractoriness and mechanical strength. Thus, electrodynamic fragmentation is a process, which is able to provide high-quality raw materials for the refractory industry from used materials.

This study examines the effects of autoclaving temperature on the mechanical and microstructural properties of autoclaved aerated concrete (AAC) containing calcined diatomaceous earth (C-DE) as the main silica source. The autoclaving process was conducted using an autoclave at various temperatures of T = 134–192 °C and a corresponding steam pressure of P = 2–12 bar for 6 hours. The resultant phases were characterized using quantitative and qualitative X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). The compressive strength of samples containing C-DE as the main silica source increased by 30% when the autoclaving temperature decreased by around 60 °C. This is in complete opposition to samples containing quartz as the only SiO2 source. The compressive strength of the latter decreased by 85% when the autoclaving temperature decreased by approximately 60 °C. For all samples, changes in compressive strength corresponded with changes in tobermorite formation.

This study examines the effects of calcined diatomaceous earth (C-DE) as a silica raw material of a higher solubility than quartz, on the mechanical and microstructural properties of autoclaved aerated concrete (AAC) produced at the steam curing temperature of 192 °C for 6 h. Quartz within an AAC mixture was replaced at various replacement ratios by C-DE. The resultant phases were characterized using quantitative and qualitative X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), and thermogravimetric analysis (TGA). In the mixtures with more than 50% substitution of quartz with C-DE, the autoclaving temperature of 192 °C with a duration of 6 h prevented the crystallization of amorphous calcium silicate hydrate (C-S-H) to tobermorite. The optimum strength was obtained at the substitution ratio of 75% despite having the lowest tobermorite content. By substituting quartz with C-DE for higher than 50%, plate-like tobermorite was changed to grass-like and amorphous C-S-H with low CaO/SiO2.

Altbeton wird eine Rohstoffquelle der Zukunft sein und damit zur Nachhaltigkeit beitragen. In welche Richtung dies geschehen kann, wird in diesem Beitrag mithilfe von Okobilanzergebnissen aus dem BMBF-Projekt “ELDYNTON – Elektrodynamische Fragmentierung von Altbeton“ erlautert. Hier wird Altbeton durch ein innovatives Recyclingverfahren ELDYNTON aufbereitet, um ein hochwertiges Recycling (Ruckfuhrung in den Betonkreislauf) der entstandenen Fraktionen zu gewahrleisten. Eine entscheidende Voraussetzung ist die zerstorungsfreie Trennung von Zementstein und Gesteinskornung. Die Methode der elektrodynamischen Fragmentierung beruht auf ultrakurzen Unterwasserentladungen und ermoglicht eine sortenreine Separierung. Unter dem Aspekt der Ressourcenschonung ist das Ruckgewinnen von Rohstoffen via Recycling grundsatzlich wunschenswert, ob es sich jedoch unter dem Gesichtspunkt der Okologie lohnt, muss naher untersucht werden. Hierzu wird die Methode der Okobilanz (ISO 14040/44) herangezogen und die Ergebnisse mit den okologischen Wirkungen der Primarbereitstellung verglichen. Bei gleicher Transportdistanz liegen die Emissionsaquivalente des ELDYNTON-Verfahrens in einigen Kategorien uber denen der primaren Bereitstellung. Mussen die Substitutionsguter jedoch weiter als 125 km gegenuber denen des ELDYNTON-Verfahrens transportiert werden, schneidet das ELDYNTON-Verfahren in den betrachteten Wirkungskategorien besser ab. Old concrete becomes new concrete – an innovative recycling approach. Old concrete will be a future raw material source and is thereby resource-saving. The study underlines this with the results of the conducted life cycle assessment. For this purpose old concrete gets treated by an innovative recycling approach to ensure a high valuable recycling of the prepared fractions (recirculation into the concrete cycle). A vital prerequisite is the non-destructive separation of cement stone and aggregate. The methodology of the electrodynamic fragmentation guarantees this with ultrashort, underwater electrical discharges. The recycling of resources is generally desirable under the target of resource conservation, but the methodology of the electrodynamic fragmentation has to be investigated if it is advantageous under environmental aspects. Therefore the methodology of the life cycle assessment according ISO 14040/44 is used and the results are compared with the environmental impact of the primary provision. The results show that by using waste heat from another suitable location/site the impacts of the innovative approach are lower than the impacts of the primary production. The results are similar by using higher transport distances for the primary products than for the recycled materials.

Die vorliegende Untersuchung zeigt, dass mittels gepulster Hochspannungsimpulse eine selektive Aufbereitung von Schlacke aus Mullverbrennungsanlagen moglich ist. Es fallen, neben einem Feinanteil der als Filterruckstand aus dem Prozesswasser gewonnen wird, qualitativ funf Stoffgruppen an, deren Quantitat je nach Zusammensetzung der Schlacke variiert. Neben Glas, Keramik und Schmelzprodukten sind Eisen und NE-Metalle von grosem Interesse in Bezug auf Recycling. Schadliche Schwermetalle reichern sich im Feinanteil an, Chloride und Sulfate werden grostenteils mit dem Prozesswasser ausgewaschen. Somit kann das zu deponierenden Schlackevolumen auf die Halfte reduziert werden. A promising pulsed power technology for the effective processing of municipal waste incineration bottom ash is presented. After the selective fragmentation, beside a fine fraction extracted from the process water, five groups of material like glass, ceramics or melt products remain. Especially the recycling of metals is efficiently enabled. The quantity varies due to the waste composition. Heavy metals accumulate in the fine fraction whereas chlorides and sulfates are separated by the process water. The recycling rate can be increased by the selective fragmentation and the deposited slag volume can be halved.