Globally, 1-4 wt%1 of produced ready-mix concrete is returned to the supplier (e.g., due to over-ordering, or due to the concrete not meeting the client requirements). This returned concrete is commonly dumped in a concrete yard, left to hydrate, roughly broken-up, and then either stored at the surface, buried in landfills, or lightly crushed (~30 mm down) to be sold as construction aggregates (e.g., for road base/subbase, driveways).
Milled retuned concrete is being investigated as a potential liming alternative for crops, as it can raise soil pH, while also contributing to atmospheric carbon dioxide removal.2
Carbon capture accounting for this application will have to take in account any pre-existing CO2 uptake by spontaneous carbonation 1) while the concrete is being stored in piles in the yards, and 2) in the baseline scenarios where returned concrete is buried or being used as construction aggregate.
We present here the project goals, methods, and preliminary results obtained from field work and experiments in Ireland and the USA.
(1) Correia, S. L.; Souza, F. L.; Dienstmann, G.; Segadães, A. M. Assessment of the Recycling Potential of Fresh Concrete Waste Using a Factorial Design of Experiments. Waste Management 2009, 29 (11), 2886–2891. https://doi.org/10.1016/j.wasman.2009.06.014.
(2) In 1st trial of its kind in the U.S., Irish startup spreads concrete dust on Illinois farm field to remove CO2 from the air. Chicago Tribune. https://www.chicagotribune.com/2023/11/26/in-1st-trial-of-its-kind-in-the-us-irish-startup-spreads-concrete-dust-on-illinois-farm-field-to-remove-co2-from-the-air/ (accessed 2024-04-05).

The use of concrete has been growing continuously in the 20^th and 21^st century¹, leading to
an increase of its environmental footprint. The current global warming crisis and the significant
consumption of natural reserves have pushed the building industry to search for new and sustainable solutions. New breakthrough technologies for lowering the CO₂ footprint include a circular utilization of demolished concrete. Recent developments have shown that most of the CO₂ originally released by limestone calcination during clinker production can be sequestered by carbonation of the recycled cement paste ². This technology, currently under development by the cement industry, consists in the carbonation of a part of recycled concrete in a humid/aqueous medium, leading to the formation of carbonate minerals and therefore to the permanent storage of the CO₂ in solid form.

A successful deployment of this technology at large scale needs a good understanding and,
eventually, a control of the CaCO₃ polymorphism. Here, we will show a characterization of the
carbonation reactions using both laboratory and synchrotron X-ray scattering experiments. The effect that different ions from cement hydrates and different organics used as modifiers of the hydration kinetics have on the calcium carbonate crystallization kinetics will be presented and discussed in the framework of existing theories of mineral formation.

1) Favier, A.; De Wolf, C.; Scrivener, K.; Habert, G. A Sustainable Future for the European Cement
and Concrete Industry. Technology Assessment for Full Decarbonisation of the Industry by
2050; Zurich, 2018. https://doi.org/10.3929/ethz-b-000301843.

2) Skocek, J.; Zajac, M.; Ben Haha, M. Carbon Capture and Utilization by Mineralization of Cement
Pastes Derived from Recycled Concrete. Sci. Rep. 2020, 10 (1), 1–12.
https://doi.org/10.1038/s41598-020-62503-z.

Recent studies have shown that enforced wet carbonation of recycled cement paste (RCP) is able to provide these materials with a pozzolanic reactivity, enabling their application as SCMs in new cement or concrete. This pozzolanic behavior results from a Si-Al-gel created during the carbonation reaction of hydrated cement paste. The original composition of the hydrated cement paste has a major effect on the composition of these gels and their structure, which also affects the pozzolanic reactivity of such carbonated materials.

This study investigates the effect of the composition and structure of the Si-Al-gel formed during enforced carbonation on its pozzolanic reactivity. For this, various cement pastes that contained ground granulated blast furnace slag in various amounts were prepared. The samples were hydrated, sealed for at least 120 days, and consequently carbonated immersed in a 0.1 mol NaOH solution through which gaseous CO2 was bubbled. After carbonation, the samples were dried in an aerated oven at 40 °C.

The resulting carbonated recycled cement pastes (cRCP) were then investigated for their pozzolanic reactivity according to the R3 test. The results indicate apparent differences between the reactivity of the various cRCP samples. The cRCP obtained from the CEM III showed the highest heat of hydration in the R3 test, while the cRCP from the CEM I showed the lowest results of all cRCPs. This correlates with the fact that the carbonated CEM III showed the highest amorphous content and lowest bulk Ca/Si ratio of all cRCPs, while the carbonated CEM I showed opposite results.

To elucidate these observed differences in reactivity in more detail, the gel structure of the cRCPs is currently investigated by Fourier-transform infrared spectroscopy (FTIR) and ²⁹Si as well as ²⁷Al NMR.

This study helps to identify promising candidates of RCP to be used as SCM after carbonation treatment.

Additives – small amounts of (organic) molecules – are known to be able to control the outcome of a crystallization process, even when present in minor quantities. Outcomes of these additive-mineral interactions are ubiquitous. An example is the cement industry, which highly relies on additives to produce cement with tailored properties (e.g. approximately $20 billion/year are spent on cement additives). But, despite the pivotal role of organic molecules in the formation of industrial materials such as cement, surprisingly little is known about their modus operandi at the nanoscale during the early stages of nucleation and growth. We aim to correlate the physicochemical properties of model organic molecules with their functionality during the crystallization process of portlandite, an important product in the hydration process of cement. To achieve this, we employ in situ time-resolved Pair Distribution Function (PDF) analyses combined with potentiometric titration measurements, as well as complementary in situ Small-Angle X-ray Scattering (SAXS) experiments to monitor the mineralization process in the presence of different types of organic molecules. In particular, our newly devised method for PDF pushes the detection limit of what has been previously achieved in mineral nucleation studies from diluted solution. This study will contribute to a better understanding of the formation of cement and offer new insights for the development of sustainable crystallization additives in industry.

Concrete industry is among the most impactful sectors in terms of global warming potential due to its significant carbon dioxide emissions. Therefore, a strong reduction is needed to achieve the net zero commitment by 2050. Although approximately half of the emissions can be eliminated through energy source replacement with renewables, the remaining part is inherently due to the chemical processes of cement production. Numerous promising alternative cementitious materials arose in the recent years to avoid the emissions associated with Portland cement. However, small quantities of cement are often required to trigger and foster the hydration processes to form concrete. Hence, one of the goals of the concrete industry is to compensate the emissions associated with the calcination process by reabsorbing the carbon dioxide. To do so, different possibilities were investigated in the recent years, taking advantage of the natural predisposition of concrete as carbon sink. As a matter of fact, concrete is naturally subject to carbonation – a process that absorbs and binds carbon dioxide by precipitating calcium carbonate – and, hence, it is possible to implement fast and efficient carbonation processes directly at the production stage. As a result, the emitted carbon dioxide can be reabsorbed in a closed loop before entering the atmosphere. This study proposes an optimization of the already existing concrete carbon mixing processes, implementing the experiences from the studies available in literature to enhance the carbon dioxide uptake inside the material at industrial scale, without hindering the final mechanical and durability properties.

pH buffering in aqeuous solutions are much studied in mineral-water interactions related to groundwaters, ocean acidification and CO₂ storage in deep saline aquifers. In aqeuous mineral carbonation, pH swing methods have been studied to optimize the precipitation of binders such as CaCO3 and MgCO3. Mineral carbonation of calcium silicates have ganered significant attention recently. The buffering mechanism in carbonation of calcium silicate pastes are not well studied.We studied the effect of pH on the buffering mechanism and its effect on carbonation products. Our findings of pH in aqueous calcium silicate carbonation reveal buffering limitations on calcite production in aqueous solutions.We used geochemical modelling coupled with carbonation reactions to gain insights into the buffering behaviour of CO₂-water-silicate interactions. The carbonation products revealed formation of CSH-like gel and calcite.The findings wil enable us to alter the buffering mechanism towards production of CO₂-binders with efficient capturing ability.

Recycled concrete fines (0.3-2.36 mm) is the fine fractions of crushed waste concrete. Accelerated carbonation has been widely reported as an effective technique in improving the qualities of RCFs. However, the carbonation rate is generally slow due to the limitation in the gaseous diffusion of CO₂. In this study, an improved aqueous carbonation method, by using alkali for the preparation of initial carbonation solution, was adopted for carbonating RCFs. It aims at enhancing the carbonation efficiency for improving industrial productivity. The rate and amount of calcium carbonate (Cc) precipitations, the morphology of Cc, the advance of the carbonation front, and the evolution of physical properties of the carbonated RCFs were investigated. The results indicated that carbonation efficiency was significantly improved by increasing the rate and total amount of Cc precipitation by up to 100%, and the carbonation depth by two times. After carbonation, the physical properties of these RCFs were enhanced, and could facilitate their recycling and application.

Concrete is an indispensable construction material, but its production uses a large amount of limestone and emits a large amount of CO₂. To fundamentally solve the problems, Ca in concrete is regarded as a potential unused resource capable of capturing CO₂. By developing a technology to regenerate waste concrete and CO₂ in the air as calcium carbonate concrete (CCC), a new resource recycling system called “C⁴S”, Calcium Carbonate Circulation System for Construction will be realized. This poster outlines a status of the project. Efficient methods for crushing waste concrete and capturing CO₂ are being developed. Crushed waste concrete is separated into particles of a certain size range with no distribution and is then repeatedly exposed to moisture supply and drying in the atmosphere to accelerate the carbonation. The carbonated particles are mixed with a calcium bicarbonate solution produced by blowing an atmospheric CO₂ into water in which carbonated particles have been placed. The mixture is then packed densely into a 10 cm diameter cylindrical mold under pressure to harden to some extent, followed by drying in air and curing in the calcium bicarbonate solution to complete the hardened body of CCC with 20 MPa compressive strength. The cylindrical CCCs are connected in layers and several of them are bundled together and then prestressed to form a column member. Currently, a rigid-frame structure consisting of columns and beams and a wall structure is being considered for buildings using CCC, and an exhibition structure is scheduled to be constructed in 2025. In addition, various studies are being conducted to implement C⁴S in society in 2030, including optimum recycling scenario and analysis of LCCO₂ reduction effect. With the realization of C⁴S, concrete could be permanently utilized in the construction and global warming would be greatly suppressed in the future.

The growing demand for sustainable cement production has led to increased exploration of alternative approaches, particularly the utilization of supplementary cementitious materials (SCMs) to mitigate clinker-related environmental impact. Co-reactive, a spin-off from RWTH Aachen University, introduces a large-scale CO2 mineralization process based on olivine, a magnesium silicate-rich natural resource. This innovative approach allows on-site SCM production within cement plants, capitalizing on the imminent availability of purified CO2 at scale and eliminating costly transportation and storage.
Co-reactive is currently focused on validating the patent-pending process design through a semi-continuous prototype, optimizing reaction conditions, and material engineering. Notably, the lab-scale mineralization unit has demonstrated a significant increase in reaction yield through design of experiments considering multiple parameters such as temperature, pressure, and additives. The produced material allows for compressive strength tests to evaluate its substitution capabilities. The favorable reaction conditions at lab scale are then transferred to the prototype for the validation of the innovative process design. Concurrently, this setup allows the production of larger quantities for comprehensive application tests, crucial for achieving milestones towards product certification. This research signifies an important step towards sustainable cement and concrete production, addressing key challenges in the utilization of SCMs and contributing to the broader goals of reducing carbon emissions in the construction industry.

Aluminium manufacturing industries are encountering challenges in managing the disposal of bauxite residue (red mud), which is generated during the production of aluminium. The existence of heavy metals like iron and titanium in the red mud causes environmental pollution and makes it difficult to utilize in diverse applications. In order to increase the utilization, this study focuses on the feasibility of CO₂ mineralization of red mud. Mineral carbonation is a promising technique to mitigate the atmospheric carbon dioxide concentration by precipitation of stable carbonates. It is estimated that the Carbon Capturing Utilization and Storage technology will cut down global climate change's effect by 15-55% by 2100. Alkaline industrial wastes as feedstock are seeking more attention to carbon dioxide mineralization due to their potential as a carbon sink. The major oxide compositions of red mud are Fe₂O₃, Al₂O₃, SiO₂, Na₂O, TiO₂ , etc. The red mud contains iron oxide primarily in the forms of hematite (Fe₂O₃) and goethite (FeOOH), aluminium oxide predominantly as gibbsite (Al(OH)₃) and boehmite (AlOOH), and sodium in the form of sodium aluminium silicate (Na₂Al₂Si₁₄O₃₂·3H₂O). Major phases of the red mud are non-reactive and crystalline in nature. In this study different temperatures (25 ºC, 45 ºC, and 65 ºC) were used to conduct the mineralization at high pressure (10 bar). The equipment is specially fabricated to conduct the mineralization of samples. The effect of mineralization is characterized by X-ray diffraction (XRD), Thermogravimetry Analysis (TGA), and Scanning Electron Microscope (SEM). The characterization revealed a pre-carbonation in the red mud due to its long-term exposure to an open environment. Compared with the mineralization at three different temperatures, the red mud mineralized at 45 ºC performed better in all the tests. The increased compressive strength of 11.6%, 64%, and 50% was noticed in the specimens cured at 25 ºC, 45 ºC, and 65 ºC, respectively. The microstructure analysis revealed that the hematite phase in the red mud acts as nucleation sites for the carbonate phase and contributes to a compact microstructure. The agglomeration of mineralized phases indicates the marginal increase in true density, resulting in a denser microstructure and an increase in strength after mineralization. Moreover, the morphology of sodalite and gibbsite was identified from SEM analysis and conformed with previous studies.

The research project “CO2crete”, with financial contributions by the Swedish Agency for Economic and Regional Growth and the European Union within the program “Just Transition Fund” is evaluating and validating different carbonation techniques on mineral materials, such as concrete waste for the concrete industry. The high potential for permanent carbon sink and enhanced upcycling by accelerated carbonation on recycled concrete aggregate (RCA), provides a promising enhancer of aggregate value as carbonated recycled aggregate (CRA). In certain recycling processes and industrial actors, the treatment process undergoes an accumulation of RCA in intermediate storage facilities under seasonal climate conditions (sheltered) before utilization in new application. The objectives of this study were to investigate the effective CO₂ binding capacity on RCA (8/16 mm) preconditioned under those seasonal median temperatures (8 °C and 20 °C) and relative humidities (50 ± 5 % and 85 ± 5 %), denoted as 8C-RCA and 20C-RCA respectively, before being exposed to accelerated carbonation (10 % CO₂-vol, 38° C, 97 % RH) under 4 hours, with typical flue gas parameters from the local municipal incineration plant. To quantify the bound CO₂ on such heterogenous material, a sample preparation method was investigated to retrieve a representable subsample using mechanical rotator dividers, followed by crushing to 0/4 mm and isopropanol exchange for hydrate stoppage. The crushed RCA into 0/4 mm was repeatedly subdivided in rotator for greater homogenization to about 50 grams and later milled for analyses. The analysis consisted of the thermogravimetric derivate curve for identification of the dehydrated and decomposition ranges of carbonates, starting from 500 to 800 °C. The decomposition interval was used for assessing the gravimetric measurement points on larger sample sizes (~15g x3) in a temperature-controlled oven. The results showcase the sample preparation having excellent coefficient of variance (C.o.V.) of less than 3% overall. With the overall CO₂ bound on 8C-RCA and 20C-RCA, the values before carbonation are about 1.45 % and 1.25 % respectively, indicating a higher natural carbonation before the accelerated testing on 8C-RCA. After four hours of accelerated carbonation, the CO₂ sequestered under conditions applied in the 20C-RCA case rose from 1.25 % to 2.51 % while in the 8C-RCA case, it rose from 1.45 % to 2.0 %. This difference could potentially be due to kinetics and transport properties from the different preconditioned RCAs prior to accelerated carbonation under those specific parameters.

In the last decades, the cement industry has reduced its environmental impact by replacing clinker with supplementary cementitious materials (SCM). Among the most used are Fly Ash (FA), from coal combustion, and Granulated Blast Furnace Slag (GBFS), from primary steel making. Because of the extensive practice of clinker substitution, the European cement sector is now facing an increasing scarcity of SCMs.
To mitigate this effect, alternative SCMs are explored and evaluated. Among the suitable products to be used for this purpose, mineral carbonated composites (MCC) are promising candidates. MCC is obtained using the wet-enhanced mineralization. Among the advantages, MCC can permanently capture carbon, converting gaseous CO2 into carbonate phases, and highly impacting the environmental footprint of a binder. Additionally, the formation of carbonates induces the generation of amorphous phases, participating in cement hydration.
In this poster, a comparison between the most commonly known SCMs normed by European legislation and an MCC (produced in a laboratory) is provided. FA, GBFS, Limestone, and MCC are used as partial replacements for CEM I 52,5 N. An evaluation of the reactivity and hydration products’ formation is carried out (1,7,28,90 days). Compressive and flexural strengths on concrete are also included in the evaluation at the same curing times.
The MCC has low measured reactivity. However, MCC shows higher pozzolanic reactivity than FA and GBFS, confirmed by the greater portlandite consumption over time, as well as the participation of the hydraulic phases. Mechanical performances are improved compared to FA, ensuring satisfactory performances for a replacement level up to 30%wt.

Mineral waste is by far the largest waste stream in Europe. Unlike other waste streams, it is rarely recycled and is predominantly disposed of in landfills. The aim of this study is to bring added value to some mineral waste by valorizing them as a partial substitute for cement in mortar composition.

To this end, mineral wastes are first crushed to obtain particle sizes ranging from 0 to 100 μm. Half of the resulting fine particles undergoes a carbonation process. Tests are carried out on fresh mortars in which cement is partially replaced by carbonated or non-carbonated fines, to evaluate their impact on mortars workability. Initial findings indicate that, at a fixed substitution rate, carbonation of the studied fines improves the workability of the mortars by reducing their porosity.

Tests are also conducted on hardened mortar specimens to determine their mechanical strength. Preliminary tests indicate that carbonation treatment of fines contributes to improve the mechanical performances of mortar specimens compared to specimens made of raw fines, although both result in a loss of mechanical properties when substituted to Portland cement. It also helps to reduce the porosity of the hardened mortar and, consequently, improves its durability.

In conclusion, substituting cement with carbonated fines slightly improves the durability of mortars, and reduces the loss of mechanical strength compared to mortars made with non-carbonated fines. Workability is also improved when fines are carbonated.

With a still increasing demand for concrete, not only CO₂ emissions at cement production, but also concrete waste is an important point of interest. Mineral carbonation is one opportunity to reduce the enviromental footprint of the cement industry and to recycle concrete after service life. In this work, carbonation of hydrated cement after different times of curing was investigated. Further, the re-use of the carbonated cement paste (cCP) as supplementary cementitious material (SCM) in ordinary portland cement (OPC) and its influence on the early hydration of OPC was investigated. For cement, three OPCs with different chemical composition and particle fineness were used. For carbonation of cement paste the hydrated cement was crushed and grinded after certain time intervalls (28 d, 56 d, 180 d) and then carbonated for 2 h in a 0.1 M KOH solution, which was saturated with CO₂ beforehand. To get an general idea of particle size, phase assemblage and CO₂ uptake XRD, TGA, BET and lasergranulometry were done with the dried cCP. Comparing carbonated samples cured for 28 d and 56 d, there is no significant difference in phase assemblage or CO₂ uptake. However, when looking at the cCP cured for 180 d carbonation reaction after 2 h is more progressed with more CO₂ uptake and more crystalline calcium carbonate. Additionally, the early hydration of OPC mixed with different amounts of cCP (10 %, 20 % and 30 %) was investigated. The addition of cCP to OPC seems to accelerate the early hydration of OPC. With more added cCP the induction period of the early hydration is significantly shorter. For further investigations of the early hydration of OPC mixed with cCP in-situ XRD and calorimetry was done.

Carbon mineralization or CO₂ mineral sequestration is a promising low-tech approach to bind the CO₂. Material, in our case ash, containing Ca- and Mg-rich minerals reacts with CO₂ to form carbonate minerals such as calcite. Since spontaneous carbonation is insufficient, it is desirable to perform accelerated carbonation, i.e., with an increased concentration or pressure of CO₂^1,2.

The study presented here is part of the EU project AshCycle – Integration of underutilized ashes into material cycles by Industry-Urban symbiosis, in which we have already developed a methodology to assess the CO₂ sequestration capacity of waste ashes³. Using this methodology, different ashes from Slovenia, Croatia, Denmark, Finland, and the Netherlands were exposed to accelerated carbonation conditions in a closed carbonation chamber at different CO₂ concentrations (4% and 10% v/v CO₂), high relative humidity (80-85%) and room temperature (20 °C) up to the maximum CO₂ uptake. Using the calcimetric method and XRF, XRD, and DTA/TG analyses, the amount of CO₂ sequestered was quantified and the carbonation efficiency was calculated. The CO₂ sequestration potential was evaluated on various ashes from wood biomass, co-incineration, sewage sludge, and municipal solid waste incineration. Based on the chemical and mineralogical composition of the ashes, the results obtained so far make it possible to predict which ashes have sequestration potential.

References:

¹Alturki, A. The Global Carbon Footprint and How New Carbon Mineralization Technologies Can Be Used to Reduce CO₂ Emissions. ChemEngineering 2022, 6, 44.

² Li, L.; Wu, M. An overview of utilizing CO₂ for accelerated carbonation treatment in the concrete industry. J. CO₂ Util. 2022, 60, 102000.

³ Tominc, S.; Ducman, V. Methodology for Evaluating the CO₂ Sequestration Capacity of Waste Ashes. Materials 2023, 16, 5284.

About two-thirds of the total CO2 emitted by cement manufacturing are generated during the decarbonation of limestone. Since limestone is the main raw material component of Portland cement, the CO2 emissions from the decarbonation process are difficult to circumvent in cement production [1]. To avoid those material-related CO2 emissions, alternative binders and sequestration of CO2 can be considered. Mg-based binders can fulfil both requirements. They can be made from magnesium silicates and hardened by carbonation. Thus, CO2 emissions caused by carbonates as raw materials are avoided, and CO2 is bound by the mineral carbonation of the Mg-based material [2]. In order to study the carbonation process of Mg-minerals, the carbonation of Mg(OH)2 was investigated. Mg(OH)2 powder was carbonated in a CO2 saturated aqueous solution of deionized water. The experimental setup was open to the laboratory atmosphere and gas consisting of 100 vol.-% CO2 was continuously bubbled through the slurry [3]. The experiments were conducted at room temperature and at 50°C. During the experiments, the development of pH and conductivity of the mixture were continuously measured. By means of that, the carbonation reaction could be observed more in detail. The solid reaction products were characterized using QXRD and TGA. Under the experimental conditions used, the hydrated magnesium carbonates nesquehonite and hydromagnesite were formed during 1 h of carbonation.

[1] G. Habert, S. A. Miller, V. M. John, J. L. Provis, A. Favier, A. Horvath, K. L. Scrivener, Environmental impacts and decarbonization strategies in the cement and concrete industries, Nature Reviews Earth & Environment. 1 (2020) 559-573.

[2] E. Gartner, T. Sui, Alternative cement clinkers, Cement and Concrete Research. 114 (2018) 27-39.

[3] S. Villmow, A. Mielkau, F. Goetz-Neunhoeffer, J. Neubauer, Wet carbonation of C3A and pre-hydrated C3A, Cement and Concrete Research. 173 (3023) 107259.

Carbonation is a degradation process with conflicting effects on cementitious materials. Limited extent of carbonation helps densify the pore structure and reduce permeability, while prolonged exposure to CO₂ leads to a loss of alkalinity, formation of cracks, and potential disintegration of the cementitious matrix. There has been substantial research undertaken in the attempt to prolong the service life of concrete, ranging from restoring alkalinity (e.g. electromigration) to sealing cracks (e.g. self-healing). However, no known empirical study has focused on exploring the possibility to remedy all the effects of carbonation at once. Recently, some authors have proposed recalcification as a promising technique to restore calcium leached CEM I. Restored alkalinity, pore densification, and depolymerization of silicate chains were observed after a short immersion in a saturated Ca(OH)₂ solution.

The aim of this study was to explore the potential application of recalcification on carbonated CEM I. Changes in alkalinity, mineralogy and microstructure were examined after carbonation and recalcification. We observed that immersion in Ca(OH)₂ restored the alkalinity of the carbonated materials, although new portlandite was not observed. The increase in the Ca/Si ratio at the contact surface was believed to be the result of Ca incorporation into degraded C-S-H, supported by the reduction in Si-O-Si vibrational energy of C-S-H. In terms of microstructure, recalcification did not lead to significant densification of capillary pores but reduced the pore volume in C-S-H. Our data suggest the use of recalcification as a repair method for carbonated cementitious structures, although further studies on a larger scale are required.