This study investigated the effects of the addition of inorganic salt (NaCl, artificial seawater salt or MgSO₄) to calcium bicarbonate solution on the compressive strength of calcium carbonate concrete (CCC) . The CCC is a material composed of mortar powder or carbonated mortar powder and water, which is molded by applying pressure of up to 10 MPa and then treated by cycles of immersion in a calcium bicarbonate solution and furnace drying at 105℃. The CCC specimens in this study were prepared to have a diameter of 1 cm and a height of 2cm or a diameter of 10cm and a height of 20 cm and subjected to compressive strength testing. The Ca ion and total carbonate ion in the liquid phase of the calcium bicarbonate solution were measured by ICP-atomic emission spectrometry and infrared analysis. The amount of calcium carbonate formation in CCC was measured by TG analysis, and the phase composition was analyzed by XRD. The results showed that by adding inorganic salts to the calcium bicarbonate solution, the concentrations of Ca and total carbonate ions in the solution increased, resulting in an increase in the amount of CaCO₃ formation in the CCC treated by cyclic immersion and drying, and an improvement of compressive strength compared to the case without the addition of inorganic salts.
　Furthermore, it was suggested that the formation of both CaCO₃ and hemihydrate gypsum contributed to strength development of the CCC treated by cycles of immersion and drying curing when MgSO₄ was added to calcium bicarbonate solution.

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 Efforts to reduce CO₂ emissions have been accelerated in various industries around the world in order to achieve a decarbonized society. Our company has developed CARBOFIX^Ⓡ cement which has low CO₂ emissions during manufacturing and hardens through a chemical reaction with CO₂ during curing. In this paper, we manufactured splitton blocks, which were immediate demolding products, using CARBOFIX cement at a precast concrete factory, and examined their applicability. The results showed that by performing carbonation curing using a portable carbonation curing equipment after splitting, in addition to the conventional manufacturing process of splitton blocks, it was possible to fix approximately 40 kg/m³ CO₂ in the splitton blocks while ensuring a strength of 18N/mm², the standard design strength, or higher. Furthermore, splitton blocks manufactured using CARBOFIX cement were used on road slopes as part of reinforcement work and have been in service since June 2024.

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 With the aim of reducing CO₂ emissions, the cement industry has been studying the possibility of increasing the amount of minor additional constituents used in ordinary portland cement (OPC) from the current level. In this study, the authors evaluated the properties of fresh and hardened concrete and durability of concrete using the test cement that contained 10% minor additional constituents (10OPC), and compared them with the quality of concrete using the current OPC. The results showed that the fresh concrete properties with 10OPC were equivalent to those with the current OPC, with the setting time being slightly earlier than that with the current OPC. With the fineness of cement increased, 10OPC achieved the same compressive strength of concrete as the current OPC at any test age at ambient temperatures of 10℃, 20℃ and 30℃. These results showed that the properties of concrete using 10OPC were equivalent to those using the current OPC, and that concrete of the same quality as with the current OPC could be produced by optimizing the cement design.

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 A demonstration test was conducted toward the practical application of a technology developed by Taiheiyo Cement Group to effectively utilize CO₂ separated and captured from the exhaust gases generated at cement factories. Specifications of pavement materials manufactured using the CO₂ utilization technology were determined, and test construction of the subgrade, base course and concrete paving was conducted. It was shown that the construction using the materials containing absorbed CO₂ could be carried out in the same way as with conventional materials, with the quality of the completed structure achieving the target values. With the specifications in the current study used, it was estimated that the material-related CO₂ emissions per square meter of pavement would be reduced by 15.4%.

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 This study quantified CO₂ in various concrete binders and aggregates using back titration, thermogravimetric analysis (TG-DTA), and the combustion-infrared absorption method, and examined the consistency among these methods along with error factors related to material characteristics. Back titration is considered the most appropriate method for both binders and aggregates because that directly measures CO₂, though sulfides pose a potential issue. In contrast, the TG-DTA method may underestimate or overestimate CO₂ due to the oxidation of sulfides in blast furnace slag, combustion of unburned carbon in fly ash, and dehydration of clay minerals in aggregates, which coincide with the temperature range for decomposition of calcium carbonate. The combustion-infrared absorption method may also produce inaccurate CO₂ values, as elemental or organic carbon within the aggregate particles can lead to overestimation or underestimation. In blended cement, sulfur compounds may interfere with infrared absorption, further skewing the CO₂ measurement. From these findings, back titration was identified as the most reliable method for CO₂ quantification in concrete materials. Understanding the specific characteristics of each sample and selecting the appropriate method is crucial for accurate CO₂ analysis in concrete materials and concrete structures.

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