The core of this study is the detailed investigation of early age cement hydration by ATR-FTIR spectroscopy. Spectra were obtained in situ on a CEM I 52.5R during the first 24 h of hydration. The data is discussed in conjunction with the quantitative phase contents obtained by in situ and ex situ X-ray diffraction. Cement pastes with water-to-cement ratios of 0.3, 0.37, and 0.45 are compared with regard to suitability for the method and the influence of water content on reaction kinetics. In addition, different data evaluation methods are compared with respect to outcome and effort. Whereas the determination of characteristic integral areas proves to be sufficient for qualitative tracking of water consumption and the formation of hydration products like C-S-H and ettringite (AFt), detailed peak fitting provides additional information on the variation of overlaying peaks during raw material consumption and product formation and reveals structural changes within individual phases.

Concrete production is highly resource-intensive, with natural aggregates comprising roughly 70% of its volume. Replacing these natural materials with recycled aggregates like crushed concrete and masonry offers a sustainable alternative. However, the utilization of recycled aggregates requires rigorous quality control. Currently, this inspection remains a time-consuming, manual process with human error, creating a bottleneck for broader industry adoption. This paper proposes a deep learning solution using transfer learning to identify the composition of recycled aggregates in real-time and in an automated system, which effectively translates the lab-based classification of recycled aggregates in an automated production process. This solution relies on a new labeled database of recycled aggregate images classified in nine categories. In this study, recycled aggregates are obtained firstly by crushing demolition waste. Then, nine different combinations of the obtained material were prepared in the lab. In the next step, three pre-trained CNN models were examined, including EfficientNetB0, ResNet50, and VGG16, resulting in validation accuracy of 87%, 81%, and 73%. Accordingly, the EfficientNetB0 model was selected for further performance validation/improvement through implementing cross-validation for generalizability and fine-tuning the top 30 layers of the core model for better feature learning. Finally, we gained a validation accuracy of 91% in recycled aggregates classification. Moreover, EfficientNetB0 had a small size of 23.47 MB and an inference speed of 87.76 ms. Hence, the model was run successfully on an edge device that consisted of a Raspberry Pi 5 for real-time classification. Therefore, automated aggregate classification can be implemented in the concrete production plant.

Recycled belite cement clinker (RC-BCC) can partially replace conventional Portland cement to produce low-carbon concrete. We determine the optimal design of a low-carbon concrete production plant and analyze its demand response potential through process scheduling under time-varying electricity prices. To this end, we formulate and optimize an integrated design and scheduling model, combining different technologies for rotary kiln heating, carbon capture and utilization (CCU), and fuel switching. Compared to concrete production from 100% Portland cement, the global warming impact (GWI) is reduced by 27-65% at a 50% RC-BCC replacement level, depending on the kiln heating technology, fuel type, and carbon intensity of the electricity mix, highlighting the strong mitigation potential of material substitution and CCU. Across the investigated temporal aggregations of electricity price time series data, flexible operation reduces electricity costs by 2.2-2.8% for the electrically-heated rotary kiln and by 13.4-18.2% for the oxyfuel-fired rotary kiln with biomass. The corresponding load shifting amounts to 3.7-5.3% and 25.0-30.6%, respectively. However, the total annualized cost (TAC) remains similar to the non-flexible process due to the additional capital investment requirements. The results show that RC-BCC is a promising technology for greenhouse gas (GHG) reduction and material circularity in cement production.

This article explains the vibrational modes in the Si-O stretching range in the IR spectra of synthetic C-S-H phases with varying C/S ratios. These are compared with selected in situ spectra of hydrates of OPC, and those of synthetic crystalline hydrates. The assignments were supported by 29Si NMR and trimethylsilylation (TMS) data. IR and Raman polarized spectra of oriented crystals of 14 & Aring; tobermorite, jennite and jaffeite enabled direct observation of the Si-O vibrational modes. They were successfully resolved based on the involvement of specific silicon (paired, bridging) and oxygen (bridging, non-bridging) atoms, and were compared with existing theoretical data. The resemblance between the IR spectra of synthetic C-S-H and those formed upon hydration of OPC, proves the suitability of model C-S-H phases for understanding hydration processes. Some uncertainties in the assignment of the C-S-H bands observed in existing in situ IR experiments are discussed, and potential sources of error identified.

For industrial CO2 utilization, the supply of concentrated CO2 within a continuous, high-volume stream at high temperatures remains a substantial requirement. Membrane processes offer a simple and efficient method to provide CO2 in this form. While several organo-silica-based membranes have been developed for CO2/N2 separation under these conditions, there is no standardized framework guiding comparability and optimization. Therefore, we present these membranes in a Robeson-like plot across various temperatures. Utilizing a standard 1,2-bis(triethoxysilyl)-ethane (BTESE) precursor and a simplified sol–gel method, we prepared a microporous membrane layer and characterized it for an exemplary comparison. This characterization includes key parameters for mixed-gas applications: (1) temperature-dependent single- and mixed-gas permeances to observe interactions, (2) the impact of the driving forces in mixtures (vacuum and concentration) to distinguish between permselectivity and the separation factor clearly, and (3) influence of the support structure to enable permeability calculations at elevated temperatures. Furthermore, a quick interpretation method for assessing the membrane’s microstructure is presented. A qualitative microstructure assessment can be achieved by analyzing the temperature dependencies of the three major diffusion mechanisms that simultaneously occur—Knudsen, surface, and activated diffusion.

Increasing post-demolition autoclaved aerated concrete (pd-AAC) waste is mainly landfilled due to its physical properties and lacking recycling processes. A promising technology is the production of recycled belite cement clinker, which can partially substitute Portland cement clinker. This paper presents experimental data of recycled belite cement clinker production from pd-AAC that has been successfully demonstrated on technology readiness level 4-5 and its associated lifecycle assessment. Different supply chains for pd-AAC and energy are examined. The closed-loop pd-AAC recycling via the belite route that aims for Portland cement clinker substitution shows significant potential savings in environmental impacts. These savings could reach 0.77 kg CO2-Eq/kg pd-AAC compared to the status quo (landfilling) by using renewable electricity, and 0.34 kg CO2-Eq/kg pd-AAC by using natural gas. The gained reduction of around 13.5 % is significant considering that it is the result of substituting only 15.5 % of the overall input material.

This study introduces an innovative approach to designing membranes capable of separating CO2 from industrial gas streams at higher temperatures. The novel membrane design seeks to leverage a well-researched, high-temperature CO2 adsorbent, hydrotalcite, by transforming it into a membrane. This was achieved by combining it with an amorphous organo-silica-based matrix, extending the polymer-based mixed-matrix membrane concept to inorganic compounds. Following the membrane material preparation and investigation of the individual membrane in Part 1 of this study, we examine its permeation and selectivity here. The pure 200 nm thick hydrotalcite membrane exhibits Knudsen behavior due to large intercrystalline pores. In contrast, the organo-silica membrane demonstrates an ideal selectivity of 13.5 and permeance for CO2 of 1.3 × 10−7 mol m−2 s−1 Pa−1 at 25 °C, and at 150 °C, the selectivity is reduced to 4.3. Combining both components results in a hybrid microstructure, featuring selective surface diffusion in the microporous regions and unselective Knudsen diffusion in the mesoporous regions. Further attempts to bridge both components to form a purely microporous microstructure are outlined.

Hydrotalcite exhibits the capability to adsorb CO2 at elevated temperatures. High surface area and favorable coating properties are essential to harness its potential for practical applications. Stable alcohol-based dispersions are needed for thin film applications of mixed membranes containing hydrotalcite. Currently, producing such dispersions without the need for delamination and dispersing agents is a challenging task. This work introduces, for the first time, a manufacturing approach to overcoming the drawbacks mentioned above. It includes a synthesis of hydrotalcite nanoparticles, followed by agent-free delamination of their layers and final dispersion into alcohol without dispersing agents. Further, the hydrotalcite-derived sorption agent is dispersed in a matrix based on organo-silica gels derived from 1,2-bis(triethoxysilyl)ethane (BTESE). The analytical results indicate that the interconnection between hydrotalcite and BTESE-derived gel occurs via forming a strong hydrogen bonding system between the interlayer species (OH groups, CO32−) of hydrotalcite and oxygen and silanol active gel centers. These findings lay the foundation for applications involving incorporating hydrotalcite-like compounds into silica matrices, ultimately enabling the development of materials with exceptional mass transfer properties. In part 2 of this study, the gas separation performance of the organo-silica and the hydrotalcite-like materials and their combined form will be investigated.

In context of carbon capture and storage in cement and concrete industry, there is a strong demand for fast, reliable, and low-cost CO2 quantification methods. Attenuated total reflection infrared spectroscopy (ATR-IR) in conjunction with multivariate calibration via partial-least-squares regression was applied to quantify CaCO3 in carbonated hardened Portland cement pastes, as this method shows great potential in the field of process control. Thermogravimetric analysis coupled with infrared spectrometry for the detection of the evolving gases was used as a reference for quantification. Three methods for the quantitative analysis with different partial-least-squares parameters were developed on a series of ground physical mixtures of slightly carbonated and highly carbonated hydrated cement pastes that had absorbed up to 77% of the theoretical capacity for CO2. Additional samples for optimization and validation of the method were prepared by accelerated carbonation of cylindrical slices of hardened cement paste as a function of exposure time. In these experiments, the major CO2 uptake occurs in the first 60 min until the formation of CaCO3 layers limits the diffusion of CO2 and Ca2+ ions. The developed partial-least-squares models provided low estimation errors of max. 1.5 wt% and high correlation coefficients above 99.5%. The validation covers a concentration range of 20-48 wt% of CaCO3. Limitations of the method are discussed. image

The processing of belite cement clinker in a rotary kiln at about 1000°C in a CO 2 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 (Ca 2 SiO 4 ) besides technical ellestadite, (Ca 10 (SiO 4 ) 3 (SO 4 ) 3 Cl 2 ), or ternesite, (Ca 5 (SiO 4 ) 2 SO 4 ), 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, SO 4 /PO 4 ), 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, CO 2 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 CO 2 emissions can be achieved through electrical heating of the rotary kiln.

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.

The suitability of CaCl2 as a mineralizing agent in the synthesis of a low-temperature C2S-cement clinker from wastes of autoclaved aerated concrete was investigated. As chlorellestadite is a potential host mineral for the immobilization of chlorine, the formation conditions for the highest joint content of chlorellestadite and C2S were studied in samples with different sulfate contents. Oven experiments were conducted at temperatures between 700 and 1200 °C. The samples were analyzed by X-ray diffraction in combination with chemical and thermal analysis and Raman spectroscopy. Calculation of the yield of C2S and ellestadite for all samples proves the optimum temperature range for the C2S-ellestadite clinker from 950 to 1000 °C. At lower temperatures, the formation of a carbonate-rich halogenide melt promotes the crystallization of a significant amount of spurrite at the expense of C2S. Ellestadite formation mainly depends on the sulfate content and to a lesser extent on the synthesis temperature. However, at higher temperatures, with ternesite another sulfate coexists in sulfate-rich samples at the expense of ellestadite. In addition, distinct evidence for non-stoichiometry and carbonate substitution in the structure of low-temperature ellestadite was found. Low sulfate content leads to the crystallization of Ca10[Si2O7]3Cl2 at higher temperatures. In all samples treated at temperatures above 1000 °C chlorine loss starts. Its extent decreases with increasing sulfate content.

The synthesis of low-temperature belite (C2S) clinker from wastes of autoclaved aerated concrete and limestone was studied in the presence of CaCl2 as a mineralizing agent. Synthetic chlorellestadite (SCE; Ca10(SiO4)3(SO4)3Cl2) forms in experiments at temperatures between 700 and 1200 °C. Samples were investigated by X-ray diffraction and Raman spectroscopy. In general, the amount of SCE depends mainly on the sulfate content and to a lesser extent on the synthesis temperature. At lower temperatures of formation, a non-stoichiometric SCE seems to crystallize in a monoclinic symmetry similar to hydroxylellestadite. Rietveld refinements revealed the presence of chlorine and calcium vacancies. Raman spectroscopy proved the partial substitution of sulfate by CO32− groups in ellestadites formed at 800 °C and 900 °C in air. Incorporation of CO3 results in a shorter unit cell parameters and smaller cell volume similar to CO3−apatite. At low temperatures, SCE coexists with spurrite intermixed on a very fine nm scale. At temperatures above 900 °C in air, ellestadite is carbonate-free and above 1000 °C chlorine loss starts in all samples.

The processing of belite cement clinker in a rotary kiln at about 1000 o C 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 CO 2 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 CO 2 emissions can be achieved through the electrical heating of the rotary kiln and the coupled sequestration of the released CO 2 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.

Celitement is a new type of cement that is based on hydraulic calcium-hydrosilicate (hCHS) that possesses a potential for minimizing the ratio C/S from above 3 in OPC down to 1, which significantly reduces the amount of CO$_2$ released during processing. The reaction kinetics of hCHS differs from that of classical clinker phases due to the presence of highly reactive silicate species, which involve silanol groups instead of pure calcium silicates and aluminates and aluminoferrites. In contrast to Portland cement, no calcium hydroxide is formed during hydration, which otherwise regulates the Ca concentration. Without the buffering role of Ca(OH)$_2$ the concentration of the dissolved species c(Ca$^{2+}$) and c(SiO$_4^{4-}$) and the corresponding pH must be controlled to ensure a reproducible reaction. Pure hCHS reacts isochemically with water, resulting in a C-S-H phase with the same chemical composition as a single hydration product, with a homogeneous distribution of the main elements Ca and Si throughout the sample. Here we study via nanoindentation the mechanical properties of two different types of hardened pastes made out of Celitement (C/S=1.28), with varying amounts of hCHS and variable water to cement ratio. We couple nanoindentation grids with Raman mappings to link the nanoscale mechanical properties to individual microstructural components, yielding in-depth insight into the mechanics of the mineralogical phases constituting the hardened cement paste. We show that we can identify in hardened Celitement paste both fresh C-S-H with varying density, and C-S-H from the raw material using their specific Raman spectra, while simultaneously measuring their mechanical properties. Albeit not suitable for phase identification, EDX measurements provide valuable information about the distribution of alkalis, thus further helping to understand the reaction pattern of hCHS.

A new process for the manufacture of a novel hydraulic binder (Celitement) with a high potential for saving energy and reducing carbon dioxide emissions was developed at the Karlsruhe Institute of Technology. To obtain the ideal process conditions and to monitor the optimal product quality of the binder, it was necessary to develop a measurement system for the online determination of certain quality parameters. A method for the direct calculation of different quality parameters of both precursor and final product from online measured near-infrared (NIR) spectra is presented in this article. A successful transformation between calibration models gained with laboratory and online NIR is also demonstrated. This enables optimisation of the calibration models on a laboratory scale and a seamless transfer to industrial scale due to changes in composition of the raw materials, formulation or processing parameters.

Efficient insulation for residential buildings is one of the keys for implementing an energy transition. The technology used for exterior walls is usually made by combining load‐bearing and difficult to recycle insulating components. For this reason, Celitement GmbH, Karlsruhe Institute of Technology and Xella Technology and Research have joined their forces to develop a high insulating AAC block, which is also resource efficient using the main components quick lime, sand, and water. The thermal conductivity of the AAC was reduced. Macropores formed in the conventional production process by hydrogen gas, diameter of approximately 1 mm, could be partially replaced by lightweight aggregates, with an average pore diameter of 50 nm. A process for producing lightweight aggregate, which is based on optimized, hydraulically active calcium hydro silicate binders (Celitement), was developed. The aggregate quantity in AAC could be raised to 40% by weight through optimizing formulation and mixing. This project, sponsored by the German Federal Ministry of Education and Research, ended in 2017. Based on the results so far, it is worth pursuing this approach in the future, whereby the material properties of the CSH granules could be further improved to obtain the sought final AAC product.

Online measurement of the product quality is a challenging task in cement production, especially in the production of Celitement, a novel environmentally friendly hydraulic binder. The mineralogy and chemical composition of clinker in ordinary Portland cement production is measured by X-ray diffraction (XRD) and X-ray fluorescence (XRF), where only crystalline constituents can be detected. But only a small part of the Celitement components can be measured via XRD, because most constituents have an amorphous structure. This paper describes the development of algorithms suitable for an on-line monitoring of the final processing step of Celitement based on NIR-data. For calibration intermediate products were dried at different temperatures and ground for variable durations. The products were analyzed using XRD and thermogravimetric analyses together with NIR-spectroscopy to investigate the dependency between the drying and the milling processes on one and the NIR-signal on the other side. As a result, different characteristic parameters have been defined. A short overview of the Celitement process and the challenging tasks of the online measurement and evaluation of the product quality will be presented. Subsequently, methods for systematic development of near-infrared calibration models and the determination of the final calibration model will be introduced. The application of the model on experimental data illustrates that NIR-spectroscopy allows for a quick and sufficiently exact determination of crucial process parameters. Keywords—Calibration model, celitement, cementitious material, NIR spectroscopy.