Preparation of calcium oxide–C2S–C4A3S¯ clinkers for autoclaved aerated concrete

Effect of recycled concrete fine powder after calcination on the properties of autoclaved aerated concrete

The question of durability in the construction industry applies to building materials, components, and structures. In general, building materials, components, and structures are considered durable if their properties remain stable. Durability characteristics depend on a number of complex factors and conditions and may not always be effectively determined using simple standardized methods. Despite ongoing discussions and research, the question of durability and its measuring methods remains elusive. Studies revealed, for example, that shrinkage, phase composition, and durability of autoclaved aerated concrete (AAC) are interrelated. The durability of a building or structure, as discussed in research papers, involves the durability of products and the (designed and anticipated) useful life. Studies on the durability of AAC have been prompted by the increasing production and use of AAC in the construction industry in Poland, specifically in wall constructions. Researches began in early 1960s and focused on AAC materials in industrial production and AAC partitions in experimental conditions and real‐world residential buildings. The studies examined the levels and distribution of moisture in concrete walls; conclusions have been formulated concerning the drying time, the effects of indoor and outdoor conditions, and the partition orientation relative to the points of the compass. Extended research on AAC durability co‐funded by AAC production plants and the Institute of Ceramics and Building Materials (ICiMB, formerly COBRPB CEBET) were resumed in 1999. The studies examined AAC made with fly ash and sand sampled at 5 cm intervals from partitions of 20—35, 40‐year‐old buildings and AAC stored outdoors under extreme conditions for the period of 21 and 40 years (protected with a tarpaper top cover), exposed to snow during winter. The building's technical condition was evaluated when the concrete samples were collected. The following research questions were addressed: operating moisture levels in partitions made of AAC produced using various production technologies (based on fly ash, sand); mineral composition and porosity structure of AAC after a long period of use (as compared to the mineral composition of “fresh” AAC produced using the same technologies); and whether changes in the tested parameters over time may contribute to AAC destruction, resulting in loss or significant reduction of AAC basic qualities, including compressive strength and frost resistance. The results of these studies will be discussed in this article. An analysis of the results of studies confirms the durability profile of AAC, which should be considered in formulating conclusions about the possible uses of AAC. Further research is needed to determine the limit values of AAC properties in the context of real‐world applications.

Low-sulfate autoclaved aerated concrete (AAC): A recyclable AAC with calcined clay

Autoclaved aerated concrete (AAC) is used as masonry blocks and prefabricated reinforced elements preferably in residential buildings. Due to its porous structure and mineral composition, it combines low thermal conductivity and fire resistance properties. Consequently, the popularity of AAC increases. However, due to significant AAC production volumes in many European countries since the 1960s and 1970s and given building lifetimes, strongly increasing post-demolition AAC waste volumes can be expected in the following decades. Recycling these post-demolition AAC wastes could protect primary resources and landfill capacities and reduce greenhouse gas emissions. But, recycling of post-demolition AAC is not yet established. The majority of the waste is landfilled even though landfill capacities have decreased and the legal framework conditions in Europe regarding a circular economy are becoming stricter. Therefore, new recycling options are needed. Current research approaches propose different open-loop recycling routes for post-demolition AAC, e.g. lightweight aggregate concrete, lightweight mortar, no-fines concrete, floor screed, animal bedding, oil- and chemical binders, and insulating fills for voids and interstitial spaces. Additionally, closed-loop recycling is possible and under research. Finely ground post-demolition AAC powder can be directly used in AAC production or can be chemically converted to belite (C2S) clinker to substitute primary cement in AAC production. These promising recycling options are compared regarding environmental and economic aspects. We find that the resource consumption is lower in all recycling options since post-demolition AAC helps to save primary resources. Furthermore, greenhouse gas emissions associated with the substituted primary resources are saved - especially when substituting primary cement in closed-loop recycling. In economic terms, increasing landfill costs could be avoided, which leaves a considerable margin for the cost of pre-processing, transport and recycling. The results can help decision-makers to implement circular management for AAC by fostering post-demolition AAC recycling and reducing its landfilling.

Lime is an essential raw material for the production of Autoclaved Aerated Concrete (AAC). Its functionality is triple in this application: The mineral phase giving rise to the mechanical strength of AAC (tobermorite) is formed by the reaction of quicklime (calcium oxide, CaO) with silica (silicon dioxide, SiO2) under hydrothermal curing. Lime is also providing the alkaline conditions required for the formation of the hydrogen bubbles structuring the mineral foam. Finally, the exothermic hydration (“slaking”) reaction with water is providing the required energy input in the system. Lime is produced by the calcination of limestone (CaCO3) releasing carbon dioxide (CO2). Today's limes appreciated for the production of AAC are showing slow reactivity, with t60 values typically higher than 8 and up to 16 min and more. Those limes are obtained by severe calcination, so‐called “hardburning,” which requires a higher amount of energy compared to standard high reactive limes, releasing more CO2 accordingly. Seeking for improving the carbon footprint of its product portfolio, Lhoist investigates different pathways to produce limes suitable for the production of AAC. This paper presents new lime qualities under development, their slaking behavior, and impact on the typical AAC process steps. The new limes show smooth while modified slaking profiles. The use of such lime qualities will allow for a progressive shift from today's formulation technologies towards formulations with highly improved environmental profile.

Autoclaved aerated concrete grains as alternative absorbent and filter media for phosphorus recovery from municipal wastewater: A case study in Hanoi, Vietnam

After introducing SNCR in coal combustion process in power plants, the valuable by-product such as fly ash remains contaminated with amount of ammonia in form of NH4HSO4, (NH4)2SO4 respectively, which became undesirable in AAC technology because the toxic ammonia is released in the air during the mixing process. This paper deals with the effect of varying ammonia content in fly ash after selective non-catalytic reduction (SNCR) on the physical-mechanical properties of the fly ash based autoclaved aerated concrete (AAC) with the main focus on determination of the impact of the various content of ammonium ion in fly ash on the initial consistency of fresh slurry, the residual content of ammonium ion in hardened aerated matrix and also the impact on the bulk density, compressive strength and tobermorite formation after hydrothermal treatment. Seven batches of AAC, made out of fly ash with rising content of ammonium ion from 0 ppm to 250 ppm, were tested and based on the results obtained it was found out that ammonia is released during the mixing process entirely and doesn‘t remain in AAC after autoclaving, moreover it doesn‘t affect the properties of both fresh slurry (no apparent foaming effect noticed) and thermally treated samples of AAC. Formation of tobermorite wasn’t negatively affected.

Study on the structure and properties of autoclaved aerated concrete produced with the stone-sawing mud

The pollutant elimination performance and bacterial communities of unpowered baffle rural sewage reactor filtered with construction wastes

Experimental study of high-performance autoclaved aerated concrete produced with recycled wood fibre and rubber powder

Utilization of iron tailings as substitute in autoclaved aerated concrete: physico-mechanical and microstructure of hydration products

Effect of Ca/Si ratio on the properties of steel slag and deactivated ZSM-5 autoclaved aerated concrete

Tailings use is an important environment program and also society subject. Iron tailing is an environmental pollution resources, but it can be used to produce autoclaved aerated concrete (AAC). The chemical composition of iron tailing and the mineral compositions were analyzed by XRD test. The content of radioactive substance in iron tailing was also tested. The testing results show that the iron tailing meet the requirment for AAC production. Iron tailing was sieved to analyze the gradation and was milled by the ball grinding mill machine to proper fineness. Based on the given material mixing ratio, sample test showed that the density of AAC varies from 438 to 629 kg m-3 with different finenesses of iron tailing, and the height ratio of final height to casting height of AAC changes with the finenesses of iron tailing. The research findings have important lessons for the production of aerated concrete with tailing.

Autoclaved aerated concrete (AAC) is a widely used building material for masonry blocks. Its porous structure and mineral composition lead to low thermal conductivity and fire resistance. European AAC production and usage strongly increased in the 1960s and 1970s. Therefore, assuming limited buildings’ lifetimes, significant post-demolition AAC volumes can be expected in the following decades. However, post-demolition AAC recycling in high-value environmentally friendly applications is still to be established as most post-demolition AAC is currently landfilled. Different recycling options for post-demolition AAC are presently being researched. However, a recycling network to implement these options is neither designed nor established. This contribution focuses on creating a European recycling network, including mathematical modelling, data acquisition, and solving the model. i.e. minimising the total costs. The mathematical modelling uses a capacitated warehouse location problem with multi-sourcing and direct delivery. Results show that recycling plants of smaller capacity (100,000 t input/a) are placed in the recycling networks in 2020 and 2025. With higher waste quantities being expected from 2030 onwards, plants with a larger capacity (200,000 t input/a) are added, especially in Poland, where the highest pd-AAC amount in Europe is expected. The recycling network shows a decentralised structure with numerous recycling plants to keep transport costs low. Most network costs result from variable processing costs, showing the highest cost increases from 2020 to 2050. Fixed costs increase with the higher number of recycling plants and account for the second-largest share of total network costs. Transport costs are comparatively low thanks to the decentralised structure of the network. Overall, waste generation is expected to increase by 226% from 2020 to 2050, while the total costs of the recycling network are expected to rise by 151% only. The results support decision-makers in fostering recycling and implementing a circular economy for post-demolition AAC.

Preparation and properties of autoclaved aerated concrete using coal gangue and iron ore tailings