The whole world is facing a critical environmental challenge due to the extensive release of CO₂ and the construction sector plays an important role. Carbonation of waste concrete could address the CO₂ release issue. Our recent research has shown that water plays an important role in facilitating gas dissolution, ion species transformation, and the formation of carbonation products in all carbonation reactions. Based on the role of water, three different advanced carbonation technologies to achieve the total recycling of concrete waste. Advanced technologies including pressurized semi-dry carbonation, flow-through semi-wet carbonation and wet carbonation methods were developed to treat concrete wastes with different sizes. Recent results indicated that both carbonation efficiency and carbonation products were significantly influenced by water content, and waste concrete can be turned into low-carbon products, including carbonated coarse aggregate (CRCA), carbonated fine aggregates (CRFA) and functional carbonated recycled concrete powder (CRCP). In detail, the performance of CRCA and CRFA can be significantly improved with after semi-dry carbonation and semi-wet carbonation for a short period, which can be used to replace natural aggregates. In addition, different kinds of CRCP including calcite-rich, micro-fibre-rich and superfine powders were produced. This CRCP not only was a highly active pozzolanic material but also an effective accelerator affecting the early hydration kinetics of OPC paste, thereby leading to an increased early compressive strength and a comparable 28-d compressive strength, even when 20% OPC was replaced by the CRCP replacement ratio. Furthermore, the high-performance concrete incorporating these carbonated concrete wastes can be produced, which not only promote the recycling of wastes but also sequestrate a large amount CO₂. Overall, the results presented in this study suggested the provided carbonation technologies of concrete waste can make great contribution to the waste recycling and reduction of CO₂ emission of construction sector.

Waste concrete has been recycled and crushed to produce recycled concrete fines (RCFs) as sustainable alternatives to natural sand. Accelerated carbonation has been integrated with the recycling of RCFs to improve their qualities and to sequestrate CO₂. However, RCFs derived from plain concrete were the main focus of previous investigations. Precise ideas about the carbonation behaviours of RCFs originating from blended concrete with supplementary cementitious materials (SCMs) have been scarce.

Therefore, in this study, five types of simulated RCFs made from blended cement paste were prepared based on three traditional SCMs at varying ratios. The RCFs were carbonated and their differences in carbonation efficiency, microstructure, mineralogy, and physical properties were examined. The results demonstrated that the behaviours of RCFs towards carbonation were both SCM- and dosage-dependent, and they were intrinsically associated with the initial amount of portlandite, the Ca/Si ratios of C-S-H, the presence of aluminate hydrates, as well as the compactness of initial microstructures. Specifically, RCFs derived from fly ash (FA) and silica fume (SF) blended cement paste have higher carbonation rate of up to 62.4% than plain RCFs. Despite such an improvement, increasing FA dosages (i.e., 30%) caused decreasing benefits and even adverse impacts on RCFs microstructural densification. This result was attributed to the silica- and alumina-rich nature of FA that depleted portlandite, causing the predominant decalcification on low Ca/Si C-S-H and AFt/AFm during carbonation which were more likely to cause solid volume decrease. Generally, the enhanced access of carbonate species to the inner RCFs matrices due to the deterioration of microstructure during carbonation was considered as both the consequence and driving force for the poor behaviour of RCFs containing FA. By contrast, RCFs derived from ground granulated blast furnace slag (GS) blended cement paste exhibited an opposite trend i.e., lower carbonation rate and degree as compared to plain RCFs. It was critically owing to the compact initial microstructure of these RCFs, thus possessing a high resistance to carbonate species. Carbonation on these RCFs induced limited change in both mineralogy and microstructure. In terms of surface morphologies, vaterite formation was favoured in the presence of FA, the size of calcite grains was restrained by SF, and vesuvianite formed when GS/FA that contained a rich amount of alumina was present. Although mineralogical and microstructural changes were different, the water absorption of all RCFs were found to reduce, possibly due to the densification of microstructures or the formation of the dense Cc deposition rim.

Overall, the results presented in this study suggested that the carbonation behaviours of RCFs from different origins had dramatic differences. The improvement of carbonated RCFs as compared to raw RCFs became less significant and even slightly adverse due to the presence of some SCMs in the parent materials. As such, the understanding of this study could provide a foundation for the strategic adjustment and systematic development of carbonation treatment for RCFs.

Japan has joined the global initiative and set an objective of reducing CO₂ and achieving a carbon-neutral society by the year 2050. This paper presents the technology for the production of Carbon Capture and Utilization (CCU) materials for use in the construction industry derived from waste concrete sludge. As illustrated in Figure 1, the waste concrete sludge is first diluted to promote leaching of calcium ions into the liquid following which it is filtered using a filter press. Boiler exhaust gas is bubbled into this filtrate, producing fine particles of calcium carbonate called "Ecotancal” and preventing boiler gas from escaping into the atmosphere. The remaining solid by-product also contains calcium ions in the form of cement hydration products, which on crushing and carbonation yields a fine powder called “DACS^®”. In this report, the effect of adding both “Ecotancal” and “DACS^®” as mineral admixtures in concrete is discussed.

Non-Dispersive Infrared analysis of Ecotancal shows that it consists of over 90% by mass of calcium carbonate (CaCO₃), and it has a BET-specific surface area six times that of Portland cement. Overall, the CO₂ absorbed by the filtrate outweighs the CO₂ emitted during the production of Ecotancal, making it carbon negative by -390 kg-CO₂/t. Since Ecotancal is primarily calcium carbonate, it can be used to enhance the cohesiveness of Self-Compacting Concrete (SCC). The effect of replacing a part of cement with Ecotancal on the plastic viscosity of mortar was investigated using a Rheometer. The results shown in Figure 2 indicate that as the replacement ratio increases, the plastic viscosity increases indicating the addition of Ecotancal enhances the cohesiveness of mortar.

The production volume of DACS^® is about 40 times that of Ecotankal and TG-DTA substantiates that it can contain up to 35% by mass of calcium carbonate after carbonation. To investigate the effect of adding DACS^®, compressive strength tests were conducted on mortar specimens with 50 kg/m³ and 100 kg/m³ of DACS^® as a replacement for Sand. The results shown in Figure 3 indicate that replacing sand with DACS^® gives higher 7-day and 28-day compressive strength.

In conclusion, this report presents the technology for the production of two types of CCU materials from waste concrete sludge and highlights the benefits of adding them to mortar.

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.

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.

A method for thermally activating waste mortar powder to produce reactivated cementitious materials (RCM) was investigated. The process involved heating the precursor to 1000°C and varying the calcium-silicon ratio by adding limestone in proportions of 0wt%, 20wt%, 40wt% and 60wt%. After thermal activation, paste composed of the RCMs were prepared and carbonation-cured. Phase assemblages of RCMs, and the phase composition, microstructure, and mechanical properties of the carbonation-hardened samples were analyzed. The results showed that the dominant formation of β-C₂S, CS, gehlenite and amorphous in the SCMs after thermal activation. An increase in limestone proportion led to a decrease in amorphous phases and sand content, with the highest C₂S content (45.5%) observed in RCMs with 20% limestone addition. Microstructural analysis revealed the C₂S crystals were formed around the sand due to hot state reaction between residual sand and limestone. After carbonation curing, calcite formed in all the samples, while vaterite crystals appeared only in the RCMs paste with 0% and 20% limestone. Additionally, rhombohedral calcite crystals and calcium-modified silica gel were observed around sand in the carbonated hardened pastes. A trend was noted where the compressive strength of carbonated pastes initially increased first and then decreased with higher limestone proportions. The optimal compressive strength (36.5 MPa) was achieved in pastes prepared with 20% limestone.