The carbon uptake by carbonation of concrete structures – some remarks by perspective of TA

An issue which recently raised remarkable attention in both industry and politics with regard to CO$_{2}$-emissions of the cement industry is the uptake of CO$_{2}$ into concrete structures from the atmosphere due to weathering. Lobbyists urge to implement this overlooked sink of CO$_{2}$ within the global CO$_{2}$-balances. In this paper we examine the question whether the currently proposed methodology and database are sufficient for implementation. The experimental database is assessed to be doubtful due to inadequate testing. No reliable correlation of carbonation and age of buildings exist. Simple models are insufficient to allow a transfer in real buildings. A compensation of today\u27s emissions with carbon that is sequestered by the existing building stock is implausible. The after-service life is assessed to be of higher importance. But the practicability and the economy of new procedures for enhanced carbonation of crushed concrete are uncertain. On the existing basis, the drafting of guidelines for IPCC appears to be very problematic. There are indications that a substantial fraction of CO$_{2}$ once emitted would remain within the atmosphere for thousands of years. Even if CO$_{2}$ is sequestered with a long-term delay, damages caused by extreme events cannot be retroactively reversed. The cement industry finds itself in a tricky situation. In the medium term, there is no serious alternative binder in sight and the realization conditions of CCS are unclear. For this reason, to the author\u27s perspective, reinforced R&D in the field of radical innovations of low-CO$_{2}$-binders is strongly recommended

Zero emission circular concrete

The project Zero Emission Circular Concrete develops a CO2-neutral, high-quality, and resource-efficient concrete cycle starting with end-of-life concrete. A cement clinker with a reduced CO2 footprint is processed from waste concrete fines at a strongly reduced temperature of approximately 1000°C. The main clinker mineral is belite, Ca2SiO4. The residual CO2 is released in concentrated form and used for the technical carbonation of either waste concrete fines as supplementary cementitious material or of coarse crushed waste concrete. The coarse fraction is treated in a new process based on a pressurized autoclave, where hardening by carbonation improves the properties of the recycled aggregate. Both carbonation options are investigated on a laboratory scale. Recycling cement is developed from belite cement clinker, Portland cement clinker, and other substitutes in a joint project with the industry. A 30% substitution rate of Portland cement clinker compared to European cement standards is targeted. Subsequently, formulations for recycling concrete will be developed from recycled cement and recycled aggregate. The processing of concrete products and precast concrete elements will be tested in plant trials. A pilot plant for belite cement clinker is currently under construction to bring its technology readiness level to four

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

The 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

Chlorellestadite (Synth): Formation, Structure, and Carbonate Substitution during Synthesis of Belite Clinker from Wastes in the Presence of CaCl2_2 and CO2_2

The synthesis of low-temperature belite (C$_2$S) clinker from wastes of autoclaved aerated concrete and limestone was studied in the presence of CaCl$_2$ as a mineralizing agent. Synthetic chlorellestadite (SCE; Ca$_{10}$(SiO$_4$)$_3$(SO$_4$)$_3$Cl$_2$) 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 CO$_3$$^{2−}$ groups in ellestadites formed at 800 °C and 900 °C in air. Incorporation of CO$_3$ results in a shorter unit cell parameters and smaller cell volume similar to CO$_{3−}$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

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

The 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

CaCl2 as a Mineralizing Agent in Low-Temperature Recycling of Autoclaved Aerated Concrete: Cl-Immobilization by Formation of Chlorellestadite

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

Accelerated carbonation of hardened cement paste: Quantification of calcium carbonate via ATR infrared spectroscopy

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 physicalmixtures 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

Elastofibroma dorsi – differential diagnosis in chest wall tumours

BACKGROUND: Elastofibromas are benign soft tissue tumours mostly of the infrascapular region between the thoracic wall, the serratus anterior and the latissimus dorsi muscle with a prevalence of up to 24% in the elderly. The pathogenesis of the lesion is still unclear, but repetitive microtrauma by friction between the scapula and the thoracic wall may cause the reactive hyperproliferation of fibroelastic tissue. METHODS: We present a series of seven cases with elastofibroma dorsi with reference to clinical findings, further clinical course and functional results after resection, as well as recurrence. Data were obtained retrospectively by clinical examination, phone calls to the patients' general practitioners and charts review. Follow-up time ranged from four months to nine years and averaged 53 months. RESULTS: The patients presented with swelling of the infrascapular region or snapping scapula. In three cases, the lesion was painful. The ratio men/women was 2/5 with a mean age of 64 years. The tumor sizes ranged from 3 to 13 cm. The typical macroscopic aspect was characterized as poorly defined fibroelastic soft tissue lesion with a white and yellow cut surface caused by intermingled remnants of fatty tissue. Microscopically, the lesions consisted of broad collagenous strands and densely packed enlarged and fragmented elastic fibres with mostly round shapes. In all patients but one, postoperative seroma (which had to be punctuated) occurred after resection; however, at follow-up time, no patient reported any decrease of function or sensation at the shoulder or the arm of the operated side. None of the patients experienced a relapse. CONCLUSION: In differential diagnosis of soft tissue tumors located at this specific site, elastofibroma should be considered as likely diagnosis. Due to its benign behaviour, the tumor should be resected only in symptomatic patients