In order to reduce greenhouse gas emissions, Taiheiyo Cement Corporation implemented the Development of Carbon Circulation Technology for the Cement Industry, a project subsidized by the New Energy and Industrial Technology Development Organization (NEDO), from July 2020 to March 2022. In this project, we have developed a series of technologies for carbon recycling from CO₂ capture from cement kiln exhaust gas to effective utilization of the recovered CO₂ in recycled construction waste and cement/concrete products. This paper gives an overview of the subsidized project.

View full abstract

 Taiheiyo Cement Group is working on the development of various technologies for the aim of achieving carbon neutrality throughout its supply chain by 2050.
　Among many CO₂ capture technologies, chemical absorption using amine-based absorbent was adopted in the pilot plant installed at the Kumagaya Plant in fiscal year 2021, and its demonstration test was started to verify the effect of CO₂ capture from cement kiln exhaust gas.
　It was confirmed through the test that CO₂ was separated from cement kiln exhaust gas and recovered in high concentrations in a stable form. New findings were also obtained from laboratory tests conducted on the durability of the amine-based absorbent.

View full abstract

 This study examined an accelerated carbonation process using an externally heated rotary kiln for efficient sequestration of CO₂ separated and captured from cement kiln exhaust gas in waste concrete aggregates. For a scale-up test, demonstration equipment with an input capacity of 500 kg/h was installed in the Taiheiyo Cement Kumagaya Plant. An evaluation was made on the effectiveness of the accelerated carbonation process for waste concrete aggregates, as well as on the environmental safety of the processed products as a subbase pavement material.
　The results showed that two hours of accelerated carbonation could fix 75 kg-CO₂/tcement of CO₂, and the effectiveness of the process was also confirmed by the demonstration tests at a scale of 500 kg/h. Moreover, there was no increase in the amount of hexavalent chromium elution attributable to the accelerated carbonation, indicating no problem the environmental safety. These results suggest that the waste concrete aggregates treated by accelerated carbonation can be recycled as a subbase pavement material.

View full abstract

 This study investigated CO₂ fixation in concrete sludge under very fast reaction conditions and its reaction mechanism for the development of industrial-scale CO₂ fixation. Concrete sludge samples with different amounts of fixed CO₂ were prepared by wet carbonation tests. Analysis on them revealed that the first reacting with CO₂ were Ca(OH) ₂, followed by monosulfate, and finally monocarbonate, hemicarbonate and high Ca/Si C-S-H. High Ca/Si C-S-H changed to low Ca/Si C-S-H and pozzolanic material, which remained until the end of this experiment. These reaction mechanisms were generally consistent with the results of chemical equilibrium calculations in a previous study. This indicated that the reaction would proceed in the order of hydrates with higher reactivities with CO₂, even under the very fast reaction conditions. In addition, the same results were obtained in a CO₂ fixation test using demonstration equipment that was approximately 1000 times larger than the laboratory equipment. Furthermore, the concrete sludge with fixed CO₂ prepared in this study was found to have the same components as those contained in existing mineral admixtures, suggesting the potential of the product for that purpose.

View full abstract

 As more importance is given to CO₂ emissions reduction in the cement industry, technological development efforts are needed also in the area of effective utilization of CO₂. In this study, fixation of CO₂ in ready-mixed concrete and lightweight foamed concrete was investigated. The fixation in ready-mixed concrete was made by mixing an appropriate amount of carbonated cement slurry in the fresh concrete. The results showed that approximately 7.7 kg/m³ (23 kg/t-cement) of CO₂ was immobilized, with the performance of concrete kept at the required level. As compared to ordinary concrete, diffusion of CO₂ was expected to be easier in lightweight foamed concrete which was manufactured by mixing cement milk with air bubbles to form a porous structure when cured. Field tests were performed assuming lightweight embankment construction. The results showed that the amount of CO₂ fixed in the specimens was 256 kg/t-cement in the surface layer, 171 kg/tcement in the middle layer, and 141 kg/t-cement in the bottom layer. From this study, it was found that by sealing the hardened body and injecting CO₂ from the outside, the carbonation was made to proceed inward, immobilizing a large amount of CO₂.

View full abstract

 CARBOFIX^Ⓡ CEMENT, CO₂-absorbing/cured Cementitious Material, has β-C₂S, one of the minerals in portland cement, as the main mineral component, and also contains aluminate phases. The content of CaO in it is lower than that in ordinary portland cement, which makes it possible to fire clinker at lower temperatures, thereby reducing CO₂ emissions during production. The reduction was calculated with reference to JIS R 0303, etc., and was found to be 140 kg-CO₂/t-cement less than the CO₂ emissions of 824 kg-CO₂/t-cement for portland cement production. CARBOFIX CEMENT reacts with CO₂ to produce calcium carbonate, which fills the voids to develop strength. The CO₂ binding rate of mortars with different water-cement ratios after carbonation curing was 280-325 kg-CO₂/t-cement. In this case, the use of CARBOFIX CEMENT with carbonation curing reduced CO₂ emissions by 420-465 kg-CO₂/t-cement compared to the CO₂ emissions for portland cement production.

View full abstract

 The mechanism of CO₂ binding by carbonation of β-C₂S, the main mineral component of CARBOFIX^Ⓡ CEMENT, was investigated. The carbonated β-C₂S analyzed in this study had constituent phases consisting of calcite, aragonite and silica gel containing a small amount of calcium oxide. Through TG-MS, it was observed that decarbonation started at around 300°C, which is lower than the decarbonation temperature of typical calcium carbonate. This phenomenon was thought to be caused because calcium carbonate and silica gel coexisted in the carbonated β-C₂S, and silica gel acted to lower the decarbonation temperature of calcium carbonate. Because the components of the carbonated β-C₂S were calcium carbonate and silica gel, CO₂ was bound as a form of calcium carbonate in the carbonation of β-C₂S. Therefore, it is expected that once bound, CO₂ is not released into air and remain stable for a long period of time.

View full abstract

 In order to enhance both durability and early age strength, improved early age strength development of fly ash cement (AF) was studied with increasing the amount of alite, f.CaO content and Blaine surface area compared to Ordinary Portland Cement (OPC). In this paper, the early age strength under steam curing condition of AF as precast concrete product was studied. The long-term hydration by selective dissolution method, Quantitative X-Ray Diffraction, and SEM-EDS was investigated. The experimental results showed that the compressive strength of AF just after demolding was about 4N/mm² higher than OPC under the same curing conditions. The reaction of clinker and fly ash were enhanced under steam curing conditions compared to sealed ones at long-term ages. Therefore, it seems that AF shows not only higher early age strength compared to OPC and common fly ash cement but also the resistance to alkali silica reaction about the same as the common fly ash cement under air curing which is the practical use as precast concrete product. Moreover, it was found that Ca/(Si+Al) ratio of C-S-H was similar in both the AF paste and fly ash cement containing OPC paste at 182 days and was about 1.4.

View full abstract

 As global warming is an urgent global issue, cement industry is promoting policies including waste utilization to reduce the emission of greenhouse gases. Various types of plastic wastes are used as alternatives to coal in the cement burning process, including automobile shredder residues, and other difficult-to-recycle plastic wastes which were otherwise usually incinerated and/or disposed of in a landfill. This study aimed to develop a material flow model incorporating both the cement production and plastic waste treatment processes for the estimation of CO₂ emissions based on a life-cycle analysis. CO₂ emissions were compared between two systems: the recovery system, wherein the coal used for firing the cement clinker was partly replaced with plastic waste, and the baseline system, wherein the cement clinker was fired with coal only and plastic waste incinerated separately. Based on actual plant data, the use of 23kg of plastic waste per ton of cement produced was estimated to result in a reduction in CO₂ emissions of 76kg compared with the baseline system. Most of the reduced CO₂ emissions were those accounted for the combustion of coal and plastic waste. Sensitivity analyses showed that the use of plastic waste contributed to calculated emissions reduction, whereas the alternate scenarios of electricity and transportation did not cause much change. These results support continued development of technologies for plastic waste utilizations aiming at CO₂ emissions in the cement industry.

View full abstract

 With the growing demand for reduction of CO₂ emissions, development of new concrete admixtures is becoming an urgent issue in the construction industry. This study investigated the effect of partial metakaolin replacement in cement on the strength and durability of precast concrete. It was found that, when 20% of cement was replaced with metakaolin, i) the compressive strength significantly increased by about 20%, ii) the drying shrinkage strain lightly increased by about 5%，iii) the chloride penetration depth obviously decreased by about 40%, and iv) the carbonation depth slightly increased by about 20%.

View full abstract