Production Of Autoclaved Aerated Concrete Research Articles

Preparing autoclaved aerated concrete using molybdenum tailings

Due to the background of the difficulty in molybdenum tailings disposal and the shortage of siliceous sand for autoclaved aerated concrete (AAC), molybdenum tailings were used for AAC production in this study. The main process parameters including the quicklime-cement ratio (Q/C), the molar calcium-silicon ratio (C/S), the water-solid materials ratio (W/S), the aluminum powder dosage, the autoclaved temperature, and the autoclaved time, on the basic physical and mechanical properties, such as slurry fluidity, foaming curves, dry density, and mechanical strength, were investigated. Moreover, the hydration production and microstructure were analyzed by XRD, thermal analysis, and SEM. The results showed that molybdenum tailings AAC (MAAC) with a dry density and compressive strength of 641.3 kg/m3 and 3.89 MPa, respectively, were successfully obtained under the following optimal conditions: molybdenum tailings, 60.13 %; quicklime 13.02 %; cement 26.03 %; gypsum 1 %; W/S 0.52; aluminum powder 0.07 %; autoclaved temperature 190 °C; autoclaved time 8h. The main mineral components of quartz and feldspar in the molybdenum tailings participated in the autoclaved reaction, while biotite did not. The main hydration products in the MAAC were flocculent C–S–H gel, plate-like, narrow plate-like, and grass-like tobermorite. The C–S–H gel and tobermorite bond each other and made the pore walls much denser, and were the main contributors to the MAAC strength.

Preperation and Performance Comparision of Autoclaved Aerated Concrete by Using Ceramic and Glass Wastes Instead of Silica

This study aimed to produce autoclaved aerated concrete (AAC) by using glass and ceramic waste in 10%, 20%, 30%, 40%, and 50% proportions as a substitute material for quartzite, and samples were produced under low pressure and heat (2.3 bar and 135 ºC). The microstructural properties were investigated by employing scanning electron microscopy (SEM) analysis. Unit weight, porosity, water absorption, ultrasonic wave velocity, compressive strength, and thermal conductivity parameters were experimentally investigated and compared to a control sample produced without waste. Test results showed that waste addition leads to decreasing porosity, water absorption, and increasing unit weight. Additionally, uniaxial compressive strength, thermal conductivity, and ultrasonic wave velocity values were increased by adding waste. The test results showed that glass and ceramic waste can be used as a quartzite sand replacement in the production of AAC and the optimum replacement proportions for the waste materials was 10%.

Transformation pathway to a green lime offer

AbstractThe Autoclaved Aerated Concrete (AAC) industry has announced an ambitious decarbonization roadmap to 2050 to keep its place as a reference product in the future market of building materials. AAC raw materials are contributing to a large share of the CO2 footprint of AAC and need to be carefully assessed. Lime is today an essential raw material in the production of AAC and will remain so in the future as, unlike some other raw materials, it cannot be interchanged by a less carbon‐intensive alternative. Lime is essential in the formation of Tobermorite and provides the alkaline conditions required for the formation of the hydrogen bubbles structuring the mineral foam. Lime is produced by the calcination of limestone (CaCO3) releasing carbon dioxide (CO2). The CO2 footprint of lime is considerable, and the production process is considered hard‐to‐abate. Today roughly two‐thirds of the CO2 is inherent to the production process while only one‐third of the CO2 is due to the required fuel. Lhoist has developed a Carbon Action 2030 plan with a range of low‐carbon and carbon‐neutral lime solutions. The approach is based on the principle of three pillars: Avoiding, Reducing, and Compensating with a pipeline of effective carbon reduction initiatives behind each pillar. The paper will provide a status update on the key projects in Belgium, France, and Germany. The transformation to low‐carbon lime has triggered projects in energy improvement, sustainable fuels, the usage of hydrogen, kiln electrification, oxyfuel kilns, carbon capture, and storage as well as CO2 utilization. Once the CO2 is captured during the production of lime, the carbonation of the AAC will lead to a carbon‐negative building material. Our roadmap will position Lhoist as a preferred partner for the AAC industry in the future. Our lime product portfolio ranging from Level|Blue™ to Level|Green™ will support the AAC industry to consolidate and develop its referential position in the building materials industry.

Belite cement clinker from autoclaved aerated concrete waste fines with high sulfate content

AbstractThe processing of belite cement clinker in a rotary kiln at about 1000°C in a CO2 atmosphere is a new recycling option for Autoclaved Aerated Concrete (AAC) waste that otherwise must be landfilled. Waste fine fractions from a sorting facility enriched in sulfate due to intermixing with waste plaster have been processed. During clinkering the cement clinker phase belite (Ca2SiO4) besides technical ellestadite, (Ca10(SiO4)3(SO4)3Cl2), or ternesite, (Ca5(SiO4)2SO4), are formed, depending on the addition of flux minerals. However, not all phases of the novel clinker react hydraulically. Whereas ternesite reacts with water, ellestadite forms complex solid solution series (Ca/Pb, SO4/PO4), which may be used as an insoluble reservoir mineral for undesirable constituents, such as phosphates and chlorides. The produced clinker has been successfully used to partially substitute OPC in AAC production in technical trials. Waste quantities and landfill costs are minimized, while at the same time, CO2 emissions and the primary resource consumption of AAC production are reduced. Joint work with industrial companies is underway to increase technology readiness. Particularly large reduction effects on CO2 emissions can be achieved through electrical heating of the rotary kiln.

Future ready limes for the AAC industry

AbstractLime 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.

The modern Autoclaved Aerated Concrete (AAC) plant of today for a sustainable tomorrow

AbstractAAC, already produced since the 1920's, has been subject to continuous innovation and evolution over the history of its product life. The same applies to the AAC production process. Recently, the need for innovation has become more apparent than ever, driven by a serious energy crisis in Europe, combined with an increasing shortage of available labor and an increasing awareness of workers safety. Since the 70's, Aircrete Europe founding roots were highly involved in driving innovation in the AAC industry, with an emphasis on AAC panels. In every project designed by Aircrete top‐class innovation and the highest level of sustainability are always a top priority and are considered already from the very early design phase. As a result of this, Aircrete factories can be operated with the highest level of automation and energy‐saving initiatives to ensure higher operator safety, larger operating efficiency, and a minimized carbon footprint. This paper outlines how decades of experience from being involved in its own AAC operations result in how Aircrete can design a factory that is not only able to meet today's highest safety, regulatory, and environmental standards but is also future‐proof for the anticipated global trends in the industrial manufacturing and construction markets.

AAC life cycle: How long can autoclaved aerated concrete buildings be used

AbstractDue to its very good properties, autoclaved aerated concrete (AAC) is a very popular construction material which is a first choice as a masonry unit in a number of countries. Aerated concrete has been mass‐produced for almost 100 years. Experience and dozens of research programs prove it is a good and durable material, eco‐friendly on top of that. Unfortunately, in many countries, the history of AAC production is shorter. This research program tested different AAC blocks in various density classes, which were stored for many years in natural conditions and regularly tested for all properties. Furthermore, the research program included tests of AAC blocks from demolished buildings and blocks tested in climate chambers with CO2 in the lab. The age of those buildings ranged from 25 to 60 years. The analysis of phase composition and microstructure in all samples allowed the research team to determine the moment when the C‐S‐H phase, mainly tobermorite, will become nearly 100% re‐carbonated. Those calculations can be used to determine the life span of buildings erected using autoclaved aerated concrete.

Designing a new recycling network for post-demolition autoclaved aerated concrete (AAC) in Europe

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.

Utilization of coarse non-ferrous fraction of incineration bottom ash as aerating agent in autoclaved aerated concrete

Past studies by the authors have demonstrated the feasibility of utilizing municipal solid waste (MSW) incineration bottom ash (IBA) as an aerating agent to produce autoclaved aerated concrete (AAC). However, the gas generation potential of as-received IBA (General IBA) is significantly poorer than conventional aerating agents such as pure aluminum powder. The classification of IBA showed that coarse non-ferrous IBA fraction with particle size greater than 1.18 mm (CNF-IBA) contains a higher concentration of metallic aluminum. This study proposes specifically utilizing CNF- IBA fraction as an aerating agent to produce AAC. Two other AACs with pure Al powder and General IBA as aerating agents were investigated for systematic comparison. The aeration kinetics, fresh, and hardened properties were experimentally investigated. A novel approach is presented to study the influence of coordination between aeration extent and yield stress development on volume expansion capacity. The developed AAC with CNF-IBA exhibited the highest specific compressive strength. The leaching analysis showed that the usage of IBA as an aerating agent in AAC production is safe during the service life of AAC, and its demolished samples at the end of service time can be landfilled as inert material.

Understanding the effect of sulfate ions on the hydrothermal curing of autoclaved aerated concrete

The addition of calcium sulfate in the preparation of autoclaved aerated concrete (AAC) leads to improved mechanical properties but the precise mechanism is not well constrained. To study this, the hydrothermal curing process was monitored quantitatively with and without sulfate addition using ex situ and in situ X-ray diffraction. Higher concentrations of sulfate ions delayed the formation of the intermediate C-(A)-S-H phase and the dissolution of quartz. Once the additional sulfate ions were depleted by hydroxylellestadite formation, C-(A)-S-H formation and quartz dissolution started simultaneously at higher rates. This delayed reaction is suggested to enhance the mechanical strength by i) improving the diffusion of silicate ions in hydrothermal solution, ii) improving the distribution of C-(A)-S-H and tobermorite crystallites, and iii) closing larger capillary pores in the range of 1–5 μm. This could explain some of the ambiguous results from previous studies and provide an opportunity to optimize the AAC production process.

Use of untreated phosphogypsum as a raw material for autoclaved aerated concrete preparation

Phosphogypsum (PG) is a common industrial waste from the production of phosphoric acid by the wet process. Usually, it requires pretreatment before use due to the presence of impurities. Developing a method that can be directly used to reutilize untreated PG in building materials is thus of great significance. Autoclaved aerated concrete (AAC) has a load-bearing and filling structure, the use of solid wastes as substitutes for siliceous and calcareous materials in the production of AAC has been widely demonstrated. Herein, a new type of AAC was developed by using untreated PG, lime, Al powder, fly ash and cement. The influence of untreated PG content on the foaming process, mechanical properties and hydration products of the final AAC product was investigated through X-ray Diffraction (XRD), Thermogravimetric analysis (TG-DTA), Fourier Transform Infrared spectroscopy (FTIR), Mercury intrusion porosimetry (MIP) and Scanning Electron Microscope (SEM). The results show that the sample with 15% PG content (PG4) exhibited the best performance, with the values of the bulk density and compressive strength being 715 kg/m3 and 7.5 MPa, respectively. In addition, the density range meets the B07 level and the compressive strength exceeds the standard 2.5 MPa (GB/T11968-2020). This study described the effect of untreated PG on the performance of the AAC product, which provides a new approach to use of untreated PG and the production of AAC.

Life cycle assessment of post-demolition autoclaved aerated concrete (AAC) recycling options

Autoclaved aerated concrete (AAC) is a widely used building material for masonry units, prefabricated reinforced components, and lightweight mineral insulation boards. Its low thermal conductivity and good fire resistance increase its popularity in residential buildings. Thus, post-demolition wastes are expected to increase in the future. However, post-demolition AAC (pd-AAC) is mainly disposed in landfills while landfill capacities decrease and legal framework conditions in Europe are tightening. This study performed life cycle assessments (LCA) of different pd-AAC recycling options and compared them to each other and to current landfilling to identify the best end-of-life handling of pd-AAC from an ecological perspective. The functional unit was 1 kg pd-AAC, and the system boundaries included pd-AAC at the demolition site, transports, pd-AAC treatment, and secondary production processes. Final products of the recycling process gained environmental credits/rewards for avoiding primary production using system expansion. Providing primary resources, primary production, and use phase were not in the scope of this study. Results show that especially closed-loop recycling of pd-AAC in AAC production has a high potential of improving environmental impacts. In the best recycling option (high substitution in AAC-0.35), potential savings per kg pd-AAC compared to landfilling reach up to 0.5 kg CO2-Eq, 7 MJ fossil resources, 0.005 mol H+-Eq (acidification), 0.17 CTU (freshwater ecotoxicity), 0.2 g P-Eq (freshwater eutrophication), 5.2 × 10-9 CTUh (carcinogenic effects), 4.4 × 10-8 CTUh (non-carcinogenic effects), 2.5 × 10-5 g CFC-11-Eq (ozone layer depletion), and 1.6 g NMVOC-Eq (photochemical ozone creation). Despite data uncertainties, recycling of pd-AAC is advantageous for several recycling options, including the production of AAC, light mortar, lightweight aggregate concrete, and shuttering blocks made from concrete without fine fractions (no-fines concrete). In Germany, up to 280,000 t CO2-Eq could have been saved in 2022 by pd-AAC recycling using different recycling options instead of landfilling.

Belite cement clinker from autoclaved aerated concrete waste – A contribution towards CO2‐reduced circular building materials

AbstractThe processing of belite cement clinker in a rotary kiln at about 1000oC is a new recycling option for autoclaved aerated concrete (AAC) waste that otherwise must be landfilled. The clinker produced can partially substitute ordinary portland cement (OPC) in AAC production. Waste quantities and landfill costs are minimized, while at the same time CO2 emissions and the primary resource consumption of AAC production are reduced. The technology is currently under development. New analytical possibilities and modeling have made it possible to optimize the process conditions to such an extent that the use of belite cement clinker in aerated concrete production has already been technically tested. Particularly large effects on CO2 emissions can be achieved through the electrical heating of the rotary kiln and the coupled sequestration of the released CO2 in other secondary products such as recycled aggregate for concrete production from waste concrete. Comparable concepts for the AAC cycle are currently being worked on together with the industry partner Xella. Although decentralized plant concepts would be useful in order to minimize transportation, small plants are currently not economical according to initial estimates. In the long term, emission‐free product cycles are aimed at.

Circular economy's potential to reduce AAC carbon emissions

AbstractEstablishing a circular economy for Xella's building materials – among them Autoclaved Aerated Concrete (AAC) – is and will be one of Xella's key ESG projects for the coming years. In Germany, Xella has made it its goal to duplicate the powder‐input in the main AAC‐products from annually 60,000 t to 120,000 t by 2030.AAC‐powder used today emerges from the processing of AAC production leftovers, cutting residues and leftovers returned from job sites. In order to achieve the 2030 goal, Xella Germany will (1) generate new sinks, and (2) leverage new sources for AAC‐powder. Current production recipes are designed to take up emerging powder‐quantities, but nothing beyond. The present study describes the inspection and evaluation of current powder input shares per product type and options for their full exploitation by recipe amendments. In 2021, prototypes for Ytong PP2‐0,35 and PP4‐0,50 containing 20 m.‐% powder, and Ytong PP4‐0,55 containing 30 m.‐% powder were developed by the Xella Technologie‐ und Forschungsgesellschaft mbH. In order to prospectively supply sufficient quantities of AAC‐powder, Xella Germany aims at leveraging waste‐AAC from building demolition, so called “post‐demolition AAC” or “pd‐AAC”.We combined the “zero burden” and the “avoided burden methodology” to expand the life cycle assessment for the investigated circularity scenarios over the edge of waste generation. Our assessment revealed substitution credits of 0.55 kg CO2e/kg pd‐AAC for Ytong PP2‐0,35, and 0.43 respectively 0.33 kg CO2e/kg pd‐AAC for Ytong PP4‐050 and PP4‐0,55. The present study illustrates that our ambition to duplicate Xella Germany's powder consumption is achievable, when the new formulations are fully and consequently implemented. The recipe amendments have the potential to reduce expenditures for sand, quicklime, cement and gypsum/anhydrite, thereby reducing Xella Germany's annual (Scope 3) AAC carbon emissions up to 36 kt CO2e.

Comparison of different post-demolition autoclaved aerated concrete (AAC) recycling options

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.

Performance Investigation of the Incorporation of Ground Granulated Blast Furnace Slag with Fly Ash in Autoclaved Aerated Concrete

Autoclaved aerated concrete (AAC) is one of the most common types of lightweight cellular concrete, having a density of approximately one-fourth of that of conventional plain cement concrete. The use of industrial waste materials in concrete as a replacement for cement has garnered a lot of attention in recent years as a way to reduce the environmental effect of concrete. In this study, an attempt has been made to study the effect of AAC blocks made of industrial wastes such as fly Ash (FA) and ground granulated blast furnace slag (GGBS). Fly ash, along with different dosages of GGBS, was used as a partial replacement for cement in the production of AAC. For all the different dosages, microstructural analysis was performed using a Scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDAX), and Fourier transform infrared spectroscopy (FTIR). Mechanical performances of AAC were determined by conducting various tests like compressive strength, modulus of rupture, dry density, and water absorption. The results revealed that the dosage of “15% GGBS + 85% cement” has maximum compressive strength, modulus of elasticity, and modulus of rupture made of Class F Fly Ash when compared to Class C Fly Ash based AAC blocks. Besides, the incorporation of GGBS in the manufacturing process would increase the compressive strength of AAC up to 68%. Hence, it is recommended to use 15% GGBS + 85% cement as a potential rate of replacement, to improve the mechanical properties of AAC blocks significantly.

Rice husk ash as a silica source for the production of autoclaved aerated concrete – A chance to save energy and primary resources

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.

MINERAL ADDITIVES IN THE TECHNOLOGY OF AUTOCLAVE CONCRETE CONCRETE PRODUCTION

The construction industry is one of the largest consumers of energy and rawmaterials. It is also responsible for excessive greenhouse gas emissions. The share of autoclavedaerated concrete (AAC) in the structure of wall materials in many European countries reaches 50 %and more. Due to low energy consumption of production, high operational and construction-technological properties, relatively low cost, and environmental friendliness, these materials displacetraditional wall materials from the construction market (ceramic and silicate bricks, expanded clayconcrete).The article presents an analysis of modern approaches to reducing the energy consumptionduring AAC production, such as reducing the clinker component in the aerated concrete mixture.Prospects for the use of fly ash, blast furnace granulated slag and other mineral additives that reducecement consumption and improve the performance of AAC are presented.It is shown that the most acceptable technological method of reducing the resource and energysaving technology of AAC production is the transition to the production of low-density material whilemaintaining its physical and mechanical properties.The results of the evaluation of metakaolin additives in the composition of AAC are given.Inexpensive and affordable local effective pozzolanic additives can increase the economic andenvironmental attractiveness of autoclaved aerated concrete. It is established that the addition ofmetakaolin can act as an alternative component in aerated concrete mixtures and replace gypsumadditive.Additives of calcined clay, in particular metakaolin Al2O3 • 2SiO2 is one of the most effectivepozzolanic additives. Laboratory and experimental studies have shown an increase in the physicaland mechanical properties of autoclaved aerated concrete.

An investigation of the influence of various types of cement on the physicochemical properties of autoclaved aerated concrete

Introduction. The article addresses the influence of various types of cement on the physicochemical properties of the aerated concrete mix and finished products that have D600 density and are manufactured according to the impact technology. The study is aimed at improving the rheological properties of the mix and the operational parameters of autoclaved aerated concrete (AAC). The authors study a reduction in the “carbon footprint” of AAC CEM II. The aim of the study is to improve the physicochemical properties of the aerated concrete mix to stabilize the AAC production process and increase the strength-density ratio of the ultimate product.
 
 Materials and methods. To analyze the influence of various types of cement on AAC, trial batches were manufactured. The following grades of cement were used in testing: CEM I 52.5 N, CEM I 42.5B, PC 500-D0-N, CEM II/A-M (S-I) 42.5N, CEM II/A-L 42.5N, CEM II/A-L 42.5B. Chemical and mineralogical compositions of cements were identified as factors affecting the rheological properties of the aerated concrete mix. The properties of the aerated concrete mix were evaluated against the following parameters: consistency identified using the Southard viscosimeter, mix boiling point, boiling time, plastic strength, and cutting temperature. Final properties of products were evaluated against such parameters as strength and density. The flow was measured using the Southard viscosimeter; the value of plastic strength was identified using the penetrometer. The strength and density of ACC were identified pursuant to GOST (State Standard) 10180-2012 and GOST 12730.5-2018.
 
 Results. The authors have identified dependences between changes in the rheological properties of the mix, operational parameters of AAC and the mineralogical composition of cement. The authors present graphs describing dependencies between plastic strength and the C3A content, plastic strength and the MgO content, ultimate strength and the MgO content, strength and the C3S content; density and the C3S + C4AF content. A graph describing the effect of limestone and slag additives on the mix consistency is presented. The authors describe the way the amount of alkaline oxides in cement affects the ultimate strength of AAC. The types of defects caused by particular types of cement have been identified. The cement, having the highest value of the strength-density ratio, has been selected.
 
 Conclusions. It is shown that the cement, containing limestone and slag additives, cannot deteriorate any physicochemical or operational characteristics of AAC. The use of CEM II reduces the cost of production and the “carbon footprint”.

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

Autoclaved aerated concrete (AAC) is a construction material, which is mainly used for exterior walls thanks to its excellent thermal properties. Compared to concrete it has less than one-tenth the thermal conductivity. One main disadvantage is the high sulfate leaching of AAC that normally jeopardizes or even prevents its recyclability. This article introduces calcined clay (CT) as a measure to overcome this drawback and provide significant additional advantages. First, calcined clay is used as a replacement for the sulfate carrier in AAC and in a second step, it is used as supplementary cementitious material replacing up to 30 wt% of the cement in the AAC mixture. This stepwise replacement reduces the leachable sulfate by up to 75 % compared to reference samples. This permits for the first time the AAC to be used pure as recycled construction waste at the end of its lifetime according to German environmental regulations. The suitability of low-sulfate AAC products using calcined clay is verified in full-scale production and analyzed with respect to mechanical properties such as compressive strength and shrinkage, mineral composition and morphology as well as leaching behavior. The results show that the AAC made with calcined clay is suitable for the production of AAC blocks in the density classes 0.4 and 0.5 according to the relevant European standard.