Performance Assessment of Carbon Dioxide Sequestration in Cement Composites with a Granulation Technique.

The cement industry emits a significant amount of carbon dioxide (CO2). Therefore, the cement industry should recycle the emitted CO2. However, sequestration by carbonation in cement composites absorbs a very small amount of CO2. Therefore, a direct way of achieving this is to improve the absorption performance of CO2 in cement composites. In this study, to improve absorption, unlike in existing studies, a granulation technique was applied, and the material used was calcium hydroxide (CH). In addition, granulated CH was coated to prevent a reaction during the curing of cement paste. The coated CH granule (CCHG) was applied to 5% of the cement weight as an additive material, and the specimens were cured for 91 days to wait for the coating of CCHG to fully phase-change. The experiment of CO2 absorption showed an unexpected result, where the use of blast furnace slag (BFS) and fly ash (FA) had a negative effect on CO2 sequestration. This was because BFS and FA had a filler effect in the cement matrix, and the filler effect caused the blocking of the path of CO2. In addition, BFS and FA are well-known pozzolanic materials; the pozzolan reaction caused a reduction in the amount of CH because the pozzolan reaction consumed the CH to produce a calcium silicate hydrate. Therefore, the pozzolan reaction also had a negative effect on the CO2 sequestration performance combined with the filler effect. The CO2 sequestration efficiency was decreased between ordinary cement paste and BFS-applied specimens by 45.45%. In addition, compared to cases of ordinary cement paste and FA-applied specimens, the CO2 sequestration performance was decreased by 63.64%. Comprehensively, CO2 sequestration performance depends on the porosity and amount of CH.

Investigation of Newly Developed PCM/SiC Composite Aggregate to Improve Residual Performance after Exposure to High Temperature.

High temperature conditions, such as fire, are detrimental factors to the mechanical and chemical properties of concrete. In this paper, the authors developed a new type of coarse aggregate, named PCM/SiC composite aggregate (enhanced aggregate: EA), to improve fire-resistance performance. To investigate the validity of EA for construction materials, a compressive strength test, static modulus of elasticity, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were conducted. In addition, this EA has been developed to improve residual performance after exposure to high temperature, with residual compressive strength and internal temperature measurement tested at 1000 °C. Furthermore, chemical properties after heating were investigated by XRD and SEM-EDAX. The results show that the percentage of residual compressive strength of heated concrete with EA is higher than plain concrete. The concrete with EA exhibited primary cement composites such as C-H and C-S-H after exposure to high temperature through XRD and SEM-EDAX. On the other hand, major hydration products could not be observed in plain concrete. PCM and SiC offer an opportunity to delay the increase in concrete temperature. From evaluation of the results, we can see that EA enhanced the residual performance of concrete after exposure to high temperature conditions.

Effects of Thermal Conductive Materials on the Freeze-Thaw Resistance of Concrete.

To solve the problem of black ice, many studies are being carried out. The key in recent days is enhancing the thermal conductivity of concrete. In this study, to improve the thermal conductivity, silicon carbide was used to substitute 50% and 100% of the fine aggregate. In addition, steel fiber is not only for enhancing the mechanical properties but could enhance thermal conductive material. Hence, the arched-type steel fiber was used up to a 1% volume fraction in this study. Furthermore, graphite was used for 5% of the volume fraction for enhancing the thermal conductivity. However, thermal damage would occur due to the difference in thermal conductivity between materials. Therefore, the thermal durability must be verified first. The target application of the concrete in this study was its use as road paving material. To evaluate the thermal durability, freeze-thaw and rapid cyclic thermal attacks were performed. The thermal conductivity of the specimens was increased with the increase in thermal conductive materials. Graphite has already been reported to have a negative effect on mechanical properties, and the results showed that this was the case. However, the steel fiber compensated for the negative effect of graphite, and the silicon carbide provided a filler effect. Graphite also had a negative effect on the freeze-thaw and rapid cyclic thermal attack, but the steel fiber compensated for the reduction in thermal durability. The silicon carbide also helped to improve the thermal durability in the same way as steel fiber. Comprehensively, the steel fiber enhanced all of the properties of the tests. Using 100% silicon carbide was considered the acceptable range, but 50% of silicon carbide was the best. Graphite decreased all the properties except for the thermal conductivity. Therefore, the content of graphite or using other conductive materials used should be carefully considered in further studies.

Utilization of Waste Polysilicon Sludge in Concrete.

Increasing use of cement in the construction industry is causing an alarming increase in carbon dioxide (CO2) emissions, which is a serious environmental threat, it can be reduced by the addition of supplementary cementitious materials (SCMs). The commonly used SCMs like ground granulated blast furnace slag (GGBS), metakaolin (MK) and fly ash (FA) have been successfully used to replace the cement partially or completely. Polysilicon sludge obtained from the photovoltaic industry is also a type of waste material that can be used as SCM because it has high content of reactive SiO2. This study investigates the effects of replacing cement with polysilicon sludge in concrete. Different concrete specimens were made by replacing varying proportions of cement with polysilicon sludge and their properties, such as, fresh properties, compressive strength, heat release, chloride penetration, freeze/thaw resistance and microstructural investigations were determined. The results demonstrate that the polysilicon sludge can be used effectively to replace cement, and environmental threats associated with its disposal can be reduced.

Influence of Coarse Aggregates and Silica Fume on the Mechanical Properties, Durability, and Microstructure of Concrete.

Globally, concrete is the most widely used construction material. The composition of concrete plays an important role in controlling its overall performance. Concrete is composed of approximately 70%-80% aggregates, by volume. Therefore, it is mandatory to investigate the effect of aggregates on the performance of concrete. For this purpose, this study investigated the effect of three different coarse aggregates on the mechanical properties, durability, and microstructure of concrete. Concrete specimens were made using aggregates obtained from three regions with different mineralogies. The specimens were also made by replacing cement with silica fume. The specimens were analyzed in terms of compressive, flexural, and splitting tensile strengths, chloride penetration, carbonation, mercury intrusion porosimetry, and scanning electron microscopy. The results demonstrate that the specimens made with rougher coarse aggregates and silica fume had enhanced performance in comparison to those made with smoother aggregates.

Mechanical Properties and Sulfate Resistance of High Volume Fly Ash Cement Mortars with Air-Cooled Slag as Fine Aggregate and Polypropylene Fibers.

The depletion of natural sand and production of the huge amount of cement in the construction industry are serious threats to the environment, which can be reduced by the utilization of by-products as cement replacement material. In this study, cement was replaced with fly ash up to 45% (by weight). In addition, the natural fine aggregate was replaced with air-cooled blast furnace slag aggregate (here referred to as "slag aggregate") at a level of 50% and 100% (by weight). Polypropylene fiber was also added, at a dosage of 0.25% of binder weight. Mortar specimens were prepared and analyzed using tests for compressive, flexure, and splitting tensile strength, as well as for microhardness, and ultrasonic pulse velocity. In addition, the specimens were exposed to sulfate solution and investigated for changes in length, mass, and compressive strength. Electron microscopy and X-ray diffraction analysis were performed to examine the microstructure and phase changes of mortar specimens exposed to sulfate solution. The results indicate that mortar specimens made with 50% slag aggregate and 0.25 % fiber showed enhanced mechanical properties. The performance of slag aggregate mortars under sulfate attack was improved significantly.