Silica-alumina Gels Research Articles

Effect of temperature on carbon dioxide mineralisation in recycled cement paste

In this work, an approach towards a carbon-neutral cement industry using carbon dioxide mineralisation of recycled concrete paste is investigated. It focuses on the effect of temperature on the enforced carbonation of cement paste. The enforced carbonation is a rapid process at ambient temperature, which is further accelerated at elevated temperatures. Moreover, the extent of the reaction is increased when the temperature rises. The carbonation reaction is divided into two kinetic stages. During the first stage, the carbonation kinetics is controlled by the availability of carbon dioxide that increases at higher temperatures. During the second stage, the reaction kinetics is controlled by the dissolution of the hydrates. Increased temperature accelerates this process by increasing the undersaturation level of the dissolving phases. The main carbonation products are calcium carbonate and an alumina–silica gel. The increasing temperature has a limited impact on these phases, and the differences come mainly from the different degrees of carbonation.

Assessment of seasonal variation in distribution, source identification, and risk of polycyclic aromatic hydrocarbon (PAH)-contaminated sediment of Ikpoba River, South-South Nigeria.

The study aims to assess the seasonal variation in distribution, source identification, and risk of 20 polycyclic aromatic hydrocarbons (20 PAHs) in the sediment of the Ikpoba River, south-south Nigeria. The PAHs were extracted in an ultrasonic bath with a mixture of n-hexane and dichloromethane (1:1 v/v). The extract was cleaned by silica-alumina gel mixed with anhydrous Na2SO4 in a chromatography column, eluted by n-hexane, and analysed by gas chromatography-mass spectrometry. The range of the average PAHs in mg.kg-dw was 0.15 (Nap)-0.54 (Acy) and 0.13 (D.al.P)-0.99 (Acy) in wet and dry periods correspondingly, indicating an increase in concentration from wet to dry period. However, the rings of the average concentration of the PAHs show 6 and 3 rings to be the highest values during the wet and dry seasons, respectively. Based on the human health risk analysis, the hazard quotient (HQ) and hazard index (HI), and carcinogenic risk indices showed low non-carcinogenic and carcinogenic risk for both seasons. The ecological risk analysis showed the mean effect range median quotient (mERMQ) recorded a medium-low effect on the biota of the locations, except in AS3 during the wet season and also in WS8 and WS9 during the dry season. The minimum value of the toxic equivalent quotient (TEQ) was > 0.2mg/kg, which indicated a recommendation for the clean-up of the Ikpoba River. The isomer ratio and the principal component analysis (PCA) revealed the sources of the PAHs to be majorly combustion, followed by pyrolytic and petrogenic sources for both seasons.

IDENTIFICATION AND CHARACTERIZATION OF D-LIMONENE AND ROSMARINIC ACID BY HPTLC ANALYSIS OF BLACK CARAWAY OIL WITH ANTIOXIDANT AND ANTIBACTERIAL ACTIVITIES

Due to their enticing citrus scent, the phenolic chemicals D-limonene (DL) and Rosmarinic (RA) acid have been widely employed in food flavoring, soaps, and perfumes in addition to the treatment of many disorders. Using the essential oils extracted from Black caraway oil, a simple and straightforward high-performance thin-layer chromatography (HPTLC) method was created for the simultaneous detection of d-limonene and rosmarinic acid. Hexane, ethyl acetate, and formic acid, in the ratio of 20:19:1, v/v/v, were used as the mobile phase to generate the chromatogram on a silica gel aluminum plate 60 F254 (20 x 10 cm). The concurrent detection of DL and RA in commercial essential oils was successfully accomplished using the devised HPTLC approach. Infrared spectroscopy (IR) and mass spectrometry (MS) experiments were conducted on samples of d-limonene and rosmarinic acid in order to obtain the functional group and mass spectral information. The study data from the mass study were used to pattern the entire mass fragmentation process of DL and RA. The best and most effective molecular formula was used to suggest structures for each fragment. The antibacterial activity was studied with gram-positive bacteria; Bacillus subtilis (B. subtilis), Staphylococcus aureus (S. coccus), and antioxidant activity with that of methanol.

Validated thin‐layer chromatography method for simultaneous determination of sulfacetamide sodium and hydrocortisone acetate in the ophthalmic formulation

AbstractHigh‐performance thin‐layer chromatography has the utmost separation efficiency. High‐performance thin‐layer chromatography methods can be used to separate, detect, and estimate the different phytochemicals and drug components along with additives. A simple, accurate, precise, and robust high‐performance thin‐layer chromatography method has been developed and validated for the simultaneous estimation of sulfacetamide sodium and hydrocortisone acetate in eye drops. The separation was carried out on pre‐coated silica gel aluminum plate 60 F254 as a stationary phase using ethyl acetate: toluene: methanol: triethylamine (6:2:2:0.1) as a mobile phase. The densitometric measurement of separated components was performed at 254 nm. The method was validated for accuracy, precision, reproducibility, robustness, the limit of detection, and the limit of quantification as per International Council for Harmonization (Q2[R1]) guidelines. The regression coefficients (r2) for sulfacetamide sodium and hydrocortisone acetate were found to be 0.9998 and 0.9994, respectively. The values for percentage recoveries of sulfacetamide sodium and hydrocortisone acetate were found to be 100.2%–100.9% and 101.1%–101.2%, respectively. The developed thin‐layer chromatography method can be used for the analysis of sulfacetamide sodium and hydrocortisone acetate in eye drops.

Industrial waste silica-alumina gel recycling: Low-temperature synthesis of mullite whiskers for mass production

The increasing demand for mullite whisker-reinforced, toughened ceramic materials and mullite raw materials that meet industrial requirements has prompted the search for new and alternative sources, as well as effective technologies to obtain the target products. In this work, mullite whiskers of high purity were synthesized by a vapor-liquid–solid (V-L-S) process using industrial waste silica-alumina gel and Al2(SO4)3·18H2O as raw materials, with AlF3·3H2O and Na2SO4 as additives. The effects of sintering temperatures on the mullitization reactions and mullite morphology were investigated by XRD, TG-DTA, SEM and so forth. The results suggest that the introduction of AlF3·3H2O and Na2SO4 alters the mullitization reaction path, which leads to an initial mullitization reaction temperature of 720 °C. The SEM results demonstrate that mullite whiskers transformed from secondary growth to anisotropic growth when the sintering temperature was increased from 720 °C to 825 °C. By analyzing the experimental results, the mechanism of AlF3·3H2O-assisted growth of mullite whiskers with Na2SO4 as the liquid phase template is proposed based on the “dissolution-precipitation” process. Herein, a novel and feasible solution for the recycling of silica-rich industrial waste is proposed, which offers new and simple insights into the high value-added recycling of industrial waste, which provides new ideas for the actual mass production of mullite whiskers.

Chemical Distributions of Different Sodium Hydroxide Molarities on Fly Ash/Dolomite-Based Geopolymer.

Geopolymers are an inorganic material in an alkaline environment that is synthesized with alumina–silica gel. The structure of geopolymers consists of an inorganic chain of material and a covalent-bound molecular system. Currently, Ordinary Portland Cement (OPC) has caused carbon dioxide (CO2) emissions which causes greenhouse effects. This analysis investigates the impact on fly ash/dolomite-based-geopolymer with various molarities of sodium hydroxide solutions which are 6 M, 8 M, 10 M, 12 M and 14 M. The samples of fly ash/dolomite-based-geopolymer were prepared with the usage of solid to liquid of 2.0, by mass and alkaline activator ratio of 2.5, by mass. After that, the geopolymer was cast in 50 × 50 × 50 mm molds before testing after 7 days of curing. The samples were tested on compressive strength, density, water absorption, morphology, elemental distributions and phase analysis. From the results, the usage of 8 M of NaOH gave the optimum properties for the fly ash/dolomite-based geopolymer. The elemental distribution analysis exposes the Al, Si, Ca, Fe and Mg chemical distribution of the samples from the selected area. The distribution of the elements is related to the compressive strength and compared with the chemical composition of the fly ash and dolomite.

Semi-dry carbonation of recycled concrete paste

Demolished concrete is a large carbon sink which has not been utilized yet. Carbonation of hydrated cement paste from old concrete can effectively store CO2, and in addition, the resulting carbonated cement paste can be used as a pozzolanic material in new composite cements. This concept reduces the CO2 footprint of cement and promotes circularity and recycling. This work investigates a semi-dry gas-solid process for the carbonation of recycled cement paste. Decreasing gas humidity and hence the content of water adsorbed on the solid cement grains limits the transport of reactants from the dissolving hydrates and the overall reaction kinetics. It also leads to co-precipitation of the carbonation products, i.e., calcium carbonate and the alumina-silica gel, into a single dense matrix within the original grains. A decreasing water availability increases also the complexity and heterogeneity of the microstructure formed, since calcium carbonate precipitates as different polymorphs, including an amorphous form, and because the final phase assemblage comprises intermediate phases such as decalcified C-(A)-S-H, remains of the original hydrates in addition to the final carbonation products.

Preparation aragonite whisker-rich materials by wet carbonation of cement: Towards yielding micro-fiber reinforced cement and sequestrating CO2

In this study, an innovative wet carbonation process was developed to prepare aragonite whisker-rich materials (AWM) from cement and simulated flue gas, which subsequently served as micro-fiber additions to cement. The parameters including temperature, duration and MgCl2 concentration governing the aragonite formation were investigated, and the formation mechanism was explored. The results showed that needle-like aragonite whisker with a single crystal length of 10–30 μm and diameter of 0.5–2 μm could be synthesized rapidly when the cement suspension containing 0.05 M/L MgCl2 was carbonated at 80 °C. The formation of aragonite was favored in a suspension with a high concentration of Ca2+ and a minimum Mg2+/Ca2+ molar ratio >0.12. Additionally, amorphous phases including alumina-silica gel and silica gel were also detected. By mixing the AWM with cement, a new type of micro-fiber reinforced cement was developed, which showed significant mechanical performance improvement and embodied CO2 content (25.3 %).

Eco-friendly stability-indicating HPTLC micro-determination of the first FDA approved SARS-CoV-2 antiviral prodrug Remdesivir: Study of degradation kinetics and structural elucidation of the degradants using HPTLC-MS

The worldwide spread coronavirus (covid-19) pandemic caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) represents a global health crisis. The world was forced to face a great challenge to control and overcome this health disaster through various containment measures including efficient vaccination side by side with effective medication. Remdesivir (RMD) is the first FDA approved antiviral agent for treatment of covid-19 pandemic and hence regarded as the first-in-class medication of this highly contagious respiratory disease. The current study represents the first stability indicating HPTLC method for the estimation of RMD in bulk form and pharmaceutical formulation. The method employed TLC silica gel aluminum plates 60 F254 as stationary phase and green mobile phase composed of ethyl acetate and ethanol (96: 4, v/v) with densitometric detection at 245 nm. Comprehensive validation of the adopted method was accomplished according to the ICH guidelines regarding linearity, ranges, detection and quantification limits, precision, accuracy and robustness. The developed method offered a neat separation of the drug in presence of pharmaceutical excipients as well as in presence of acidic, alkaline, neutral hydrolytic, oxidative and photolytic degradants. Additionally, structural elucidation of alkaline and hydrolytic oxidation degradation products was carried out using HPTLC-MS. Furthermore, for the first time the acidic and alkaline degradation kinetics of RMD were studied and its degradation rate constants and half-lives were calculated. Moreover, greenness appraisal of the developed method as well as comparison with previously published stability indicating HPLC methods were performed using analytical Eco-scale, GAPI and AGREE metrics.

Comparative study of two versatile multi-analyte chromatographic methods for determination of diacerein together with four non-steroidal anti-inflammatory drugs: Greenness appraisal using Analytical Eco-Scale and AGREE metrics

According to green analytical chemistry (GAC) principles, multi-analyte methods are usually more preferred than methods determining one analyte at a time (principle 8) as they allow saving resources (time, reagents and money) (principle 9) and permit the reduction of generated waste (principle 7) as well as reduction of needed samples (principle 2). The present work herein, describes for the first time the development, validation and comparison of novel green versatile multi-analyte HPLC-DAD and HPTLC methods for the quantitative estimation of five drugs (diacerein, aceclofenac, diclofenac sodium, celecoxib and meloxicam) within a single run in a short analysis time. For HPLC-DAD, separation was performed through using Zorbax SB C18 (4.6 × 250 mm, 5 μm particle size) with the mobile phase composed of 0.05 M phosphate buffer pH 3.0 and acetonitrile (42:58, v/v) at a flow rate of 1 mL/min. The chromatograms were extracted at 258, 276 and 355 nm. In HPTLC procedure, precoated TLC silica gel aluminum plates 60 F254 were used with a mobile phase consisting of mixture of (chloroform: methanol: acetic acid, 92:8:0.25, v/v/v). The developed plates were scanned densitometrically at 258, 280 and 360 nm. Validation of the proposed methods was performed following the ICH guidelines for linearity, ranges, precision, accuracy, robustness, detection and quantification limits. Good linearities were confirmed by the high values of correlation coefficients (r > 0.9992). Appraisal of the greenness of the developed methods and comparison with different reported chromatographic methods was performed using the analytical Eco-scale (AES) approach and the novel Analytical GREEnness (AGREE) metric.

DETERMINATION OF TOLTRAZURIL IN PRESENCE OF COMPLETE ALKALINE DEGRADATION PRODUCT BY RP-HPLC AND TLC-DENSITOMETRIC METHODS

Objective: Toltrazuril is veterinary medicine, which is extensively used as an antiprotozoal drug. This drug can be analyzed by two sensitive chromatographic, accurate, and reproducible methods that have been developed and validated for the determination of toltrazuril in the presence of its alkaline degradation product. Methods: The first method involves an RP–HPLC separation of the two components, successfully achieved using Eclipse XDB-C18 Column (4.6 x 150 mm, 5 µm), using a mixture of acetonitrile and water (60:40, v/v) as a mobile phase at a flow rate of 1.4 ml/min. quantification was done with UV detector at a wavelength 242 nm UV detection. The second method is based on the separation and quantification of toltrazuril and its alkaline degradation product by TLC-densitometry on TLC silica gel aluminum plates GF254, using dichloromethane: methanol (9.5:0.5, v/v) as the developing system followed by densitometric measurement at 242 nm UV detection. Results: For RP-HPLC linearity of toltrazuril over the concentration range of 12.5–75 µg/ml, the components were well resolved from each other with significant diverse Rt values of 6.17 and 8.62 min for toltrazuril and its degradation product, respectively. While in TLC-densitometry method, the linearity of toltrazuril over the concentration range 0.2-6 µg/spot and the studied components were well resolved from each other with significant different Rf values of 0.33 and 0.52 for toltrazuril and its alkaline degradation product, respectively. For both the developed and validated methods the %RSD was found to be less than 2% and the % recovery was found to be between [98-102 %]. Conclusion: Fortunately, effective separation of the drug from its degradation product was established, along with the determination of toltrazuril as a raw material and the marketed pharmaceutical formulation, such as suspension without any interference. Hence it can be used for routine analysis in various pharmaceutical industries.

Upcycling sintering red mud waste for novel superfine composite mineral admixture and CO2 sequestration

To utilize the sintering red mud waste（SRM）on a large scale and sequester CO2, a room temperature in-situ wet carbonation method was proposed for activation of SRM. The wet carbonation reaction process of SRM was investigated, and the evolution of phase composition, microstructure and pore structure during carbonation of the SRM was revealed by XRD, TG/DTG, FT-IR, NMR, SEM and nitrogen adsorption. Moreover, the feasibility of carbonated SRM (C-SRM) as a high value-added mineral admixture was explored. The results showed that the carbonation of SRM produced a large number of fine calcite crystals and silica-alumina gel with high polymerization. After 3 h of carbonation, the SRM achieved a CO2 sequestration rate of 15.3%, calcite (Cc) yield of 86.5% and Na+ leaching rate of 81.8%. The C-SRM possessed a high pozzolanic reactivity and fine particle size, showing its potential as a cement mineral admixture.

Phase assemblance evolution during wet carbonation of recycled concrete fines

In this work, recycled concrete fine (RCF), one of the major products generated from waste concrete recycling process, was carbonated in water. The carbonation mechanism of RCF is investigated, aiming to bring forward the basic understanding of carbonation mechanism and develop value-added products. It was found that the CO2 dissolution and calcite precipitation limited the carbonation process for the initial stage. Afterwards, the kinetics was dominated by the dissolution of RCF. Portlandite was firstly consumed and the decalcification of C-S-H involved three steps associated with pH development of the aqueous environment. The main carbonation products were calcite, amorphous calcium carbonate (CC), alumina-silica gel, silica gel and alumina gel. The formed CC was turned from a poorly-crystalline layer into aggregated calcite grains. Meanwhile, a silica-rich layer was still located at the outermost surface of carbonated RCF. The carbonated RCF had a significantly high pozzolanic reactivity, being reused as supplementary cementitious materials.

Eco-friendly chromatographic methods for concurrent estimation of Montelukast and Bambuterol with its pharmacopoeial related substance Terbutaline: Greenness appraisal using analytical Eco-scale, GAPI and AGREE metrics

This study represents the development of new eco-friendly HPLC and HPTLC methods for simultaneous quantification of montelukast sodium (MLK), bambuterol hydrochloride (BAM) and terbutaline (TER) as the pharmacopeial related substance of BAM. The HPLC-DAD procedure employed Zorbax eclipse plus C8 column (4.6 × 250 mm, 5 µm particle size) as a stationary phase and a mobile phase consisting of a mixture of 0.003 M phosphate buffer pH 5.0 and acetonitrile eluted in a gradient mode at a constant flow rate of 1.1 mL/min. The chromatograms were recorded at 210 nm for the three analytes. The HPTLC employed precoated HPTLC silica gel aluminum plates 60 F254 as stationary phase with a mobile phase system consisting of ethyl acetate, ethanol and ammonia (78.9: 12: 9.1). The developed plates were scanned densitometrically at 210 nm for the three analytes. Analytical performance of the adopted methods was validated according to the ICH guidelines with respect to linearity, ranges, detection and quantification limits, precision, accuracy, specificity and robustness. The adopted methods were successfully applied for the concurrent analysis of MLK and BAM in their combined tablets. Finally, greenness assessment using various approaches; analytical Eco-Scale, green analytical procedure index (GAPI) and the more recent Analytical GREEnness metric (AGREE) was performed to confirm the greenness of the proposed HPLC and HPTLC methods and to compare them with previously published chromatographic methods.

Validated high performance thin layer chromatographic analysis of leaves and flower extracts of

is a significant therapeutic plant has a place with family apocynaceae contains in excess of 70 distinct sorts of chemotherapeutic agents and alkaloids which help in treating different illnesses. For the most part, it is known as Vincarosea, Ammocallisrosea and Lochnerarosea. There are numerous or more than 400 alkaloids present in plant, which are used as flavor, agrochemicals, pharmaceuticals, fragrance, ingredients, food addictives, and pesticides. To develop a validated high performance thin layer chromatographic method for the analysis of leaves and flower extracts of Sample solutions were applied onto the plates with automatic TLC sampler Linomat V (Camag, Muttenz, Switzerland) and were controlled by WinCATS software. Plates were developed in 10 x 10cm twin trough glass chamber (Camag, Muttenz, Switzerland). A CAMAG TLC scanner was used for scanning the TLC plates. Pre-coated silica gel aluminium plates 60F254. For vincristine, simultaneous estimation of vincristine was performed by HPTLC on a silica gel plate using toluene-methanol-diethylamine (8.75: 0.75: 0.5, v/v/v) as the mobile phase. The method was validated as per the ICH guidelines. The Rf value was found to be 0.76 for flower and 0.80 for leaves at 250 nm which shows the presence of vincristin in . In this research paper, a validated HPTLC Method has been developed for the analysis of leaves and flower extracts

Mechanisms of carbonation hydration hardening in Portland cements

Carbonation hardening of Portland cement concrete can contribute to lower its CO2 footprint and to improve its strength. The reaction mechanisms involved in carbonation curing were investigated on fresh cement pastes. The quantitative results gained allowed to link the curing conditions to the reactions kinetics, phase assemblage and microstructure evolutions.During the early carbonation curing, alite and belite react with dissolved CO2. The main carbonation products are calcium carbonate, an alumina-silica gel and a C-S-H phase with a low Ca/Si ratio. Hydration curing following the carbonation transforms the silica-rich phase into a C-S-H phase typical of hydrated Portland cements. Additionally, anhydrous calcium aluminates from clinker react to form ettringite and monocarbonate phases. The structure and composition of these phases is governed by the local equilibria in the dense microstructure. The carbonation and hydration reactions decrease the pore volume and refine the porosity resembling the microstructure development in the composite cements.

Benign design intumescent flame-retarding aliphatic waterborne polyester coatings modified by precipitated silica aerogel and aluminum powder

The organic-inorganic hybrid intumescent flame retardant coatings (IFRCs) is an efficient and ecological strategy to obtain excellent fireproof performance. The benign design on aliphatic waterborne polyester (AWP)-based IFRCs is modified by silicon-based aerogels and aluminum powder simultaneously, which is characterized by various techniques to explore the flame retardant mechanism. The results show that an appropriate precipitated silica aerogel (0.45 wt%) and aluminum powder (0.8 wt%) impart synergetic flame-retarding and smoke suppression effect to the AWP-based IFRCs, evidenced by the reduced fire growth index of 0.50 from 0.63 kW m−2 s−1, as well as the raised flame retardancy index of 2.52 from 1. Because the labyrinth barrier effect of amorphous silica-alumina gels derives from the in-situ cross-linking between the active Si(OH)4 and AlOOH or Al(OH)3, as well as the enhanced adhesive property between silanols and plywood surface, which facilitates the ceramic-like charring of AWP-based IFRCs, leading to the formation of continuous and tortuous char. It expands the design methods and flame-retarding mechanism of ecological organic-inorganic hybrid IFRCs.

Analytical method development and validation for estimation of chrysin in chrysin loaded phytosomes using high performance thin layer chromatography

Chrysin is a potential flavonoid used in the treatment of various diseases and disorders, some of the major ones are inflammation, gout, HIV-AIDS, and cancer. In the present study, the Chrysin Phytosomes are prepared by applying the anti-solvent procedure and are quantified using an analytical method that is High Performance Thin Layer Chromatography (HP-TLC) which is developed and validated in accordance with International Council for Harmonization guidelines. In the method development for chrysin, silica gel aluminum plate 60 F254 is been used as stationary phase and the mobile phase used is n-hexane:ethyl acetate:methanol:formic acid in the ratio of 8:8:1:0.2 v/v/v/v is used. The method is validated for linearity, range, limit of detection, limit of quantification, precision, and robustness. The developed analytical method was found to be simple, reliable, precise, and robust. The validated High Performance Thin Layer Chromatography method was successfully applied for quantification of chrysin in chrysin-loaded phytosomes.

Recycling and utilization assessment of steel slag in metakaolin based geopolymer from steel slag by-product to green geopolymer

This paper presents an experimental study that aimed to recycle steel slag waste as a supplementary material in metakaolin based geopolymer, resulting in a new type of geopolymer composites. The mechanical properties and microstructure analysis of geopolymer composites were carried out. The metakaolin based geopolymer composite (MGC) was prepared by a three-key parameter (the mole ratios of SiO2/Al2O3, Na2O/SiO2, and H2O/Al2O3) composition design method. The flexural strength and compressive strength test results show that the strength increment of MGC after 3 days is very limited. MGC exhibits the typical early strength property on compressive strength. Then, the steel slag powder (SS) was added to replace metakaolin to synthesize the steel slag-metakaolin based geopolymer composite (SSMGC) with the determined mix proportion. The strength test results indicate that the incorporation of SS contributes to the strength increment at a later age than MGC. The XRD patterns, SEM images, and EDS line scanning analysis indicate that SSMGC undergoes hydration in alkaline solution and accumulates hydrated calcium silicate gel with curing age besides the silica-alumina gel generated during geopolymerization. Due to the low content and low activity of steel slag active components, the degree of hydration and the amount of calcium silicate gel are inferior to the geopolymerization and silica-alumina gel caused by metakaolin, and ultimately lead to the relatively lower strength of SSMGC than that of MGC. However, the negative effect of adding SS to MGC on the mechanical strength becomes very small with curing age.

CO2 mineralization of demolished concrete wastes into a supplementary cementitious material – a new CCU approach for the cement industry

This contribution discusses the carbon capture and utilization (CCU) approach based on CO2 mineralization of cement paste from recycled concrete as new approach to capture CO2 and significantly contribute to the reduction in CO2 emissions associated with cement production. The current literature suggests that all CO2 released from the decomposition of limestone during clinker production can be sequestered by carbonation of the end-of-life cement paste. This carbonation can be achieved in a few hours at ambient temperature and pressure and with a relatively low CO2 concentration (< 10 %) in the gas. The carbonation of cement paste produces calcite and an amorphous alumina-silica gel, the latter being a pozzolanic material that can be utilized as a supplementary cementitious material. The pozzolanic reaction of the alumina-silica gel is very rapid as a result of its high specific surface and amorphous structure. Thus, composite cements containing carbonated cement paste are characterized by a rapid strength gain. The successful implementation of this CCU approach relies also on improved concrete recycling techniques and methods currently under development to separate out the cement paste fines and such. Full concrete recycling will further improve the circular utilization of cement and concrete by using recycled aggregates instead of natural deposits of aggregates. Although the feasibility of the process has already been demonstrated at the industrial scale, there are still several open questions related to optimum carbonation conditions and the performance of carbonated material in novel composite cements.