The incorporation of carbon nanotubes (CNTs) in Ultra-High Performance Concrete (UHPC) was investigated with regard to the dispersion, mechanical properties, and electromagnetic shielding ...effectiveness (SE). Dispersed CNT solution (0–2.0 wt%) was prepared by sonication and subsequent shear mixing with superplasticizer to achieve the high flowability for UHPC slurry. The dispersion method was effective to manufacture CNT-UHPC composite up to the critical incorporation concentration (CIC). Conducted experiments revealed that CNTs below the CIC improve the mechanical properties of UHPC through pore filling, bridging, and a denser C–S–H structure. CNTs substantially improve the electrical conductivity and resulting SE up to the percolation threshold. Two methods of ASTM D4935-18 and IEEE-STD-299 with electromagnetic compatibility (EMC) theory verified the result and suggested that larger specimen with sufficient incident area should be fabricated to accurately evaluate the SE of actual structures. Otherwise, the ability of SE of UHPC-CNT composites could be biased.
•Properties of CSAB cement were investigated with different water and gypsum content.•Thermodynamic modeling based on QXRD results was conducted to explore the hydration.•Increasing water promoted ...hydration, but lowered the strength development of pastes.•Gypsum played a role in controlling the hydration of ye’elmite as well as belite.•Strätlingite formation depended on a water content and a kinetic effect of hydration.
The main objective of this study was to investigate the phase and strength development of calcium sulfoaluminate-belite (CSAB) cement pastes with different amounts of gypsum and water. Thermodynamic modeling and a series of experiments including X-ray diffraction (XRD), thermogravimetric analysis, isothermal calorimetry, and compressive strength tests were performed. Decreasing the mixing water increased the strength of CSAB pastes, but decreased the degree of hydration. Interpretation of the early age XRD results and thermodynamic modeling suggested the formation of a meta-stable phase from the hydration of belite, possibly C-(A)-S-H, which transformed into strätlingite at later ages only in the samples with high water content, likely due to easier diffusion of ions at higher w/c. Furthermore, the XRD results and thermodynamic modeling confirmed that the amount of gypsum controls the hydration of ye’elimite as well as belite in the CSAB cements.
A new modified tea-bag method with various cement-based solutions is used to investigate the absorption kinetics of superabsorbent polymer (SAP) in concrete with low water to cement ratio (W/C). It ...includes a centrifugal process to efficiently remove the excess solution that can cause inaccurate absorption measurements. Various cement-based artificial pore solutions are prepared by dissolving ions in cement filtrates and considering the influence of silica fumes and polycarboxylate-ether type superplasticizers. Depending on the monovalent ion concentrations under the given Ca concentration of the filtrate, the absorption kinetics is drastically altered. This result underlines the risk of underestimating the absorption capacity of SAP in concrete. The suggested method and prepared solutions can be used to reasonably and efficiently determine the amount of required extra water for internal curing. Furthermore, the investigated ion-dependent characteristics will enhance our understanding of the absorption kinetics and contribute to the development of hydrogel products for concrete.
•Two experimental methods (3D μ-CT and MIP) were used to analyze a wide range of pores in UHPC.•The pore structure of UHPC was altered by superabsorbent polymer (SAP)-based internal curing and ...heat-treatment.•A linear relationship was found between the compressive strength of UHPC and the total porosity determined by MIP.•The effect of heat-treatment was diminished due to the internal curing effect.
The addition of superabsorbent polymer (SAP) mainly has two opposite effects on the pore structure of ultra-high performance concrete (UHPC): the densification of microstructure due to internal curing (IC) and the increase of large voids by swollen SAP particles. To further elucidate these effects on various properties of UHPC, the pore structure of internally cured UHPC is studied in a wide range of 3 nm–10 mm (in diameter), by two methods of micro X-ray computed tomography (μ-CT) and mercury intrusion porosimetry (MIP). Owing to the different measurable pore scale, each method provides different total porosity and pore size distribution. The SAP voids significantly increase the porosity in the size range of 267 µm–2.7 mm, whereas IC makes the pore structure finer within 3–100 nm. In particular, the microstructural change due to the IC has a more decisive influence on the compressive strength than larger SAP voids. In addition, it is discovered that the positive effect of heat-treatment (HT) for UHPC is diminished due to the applied IC. The IC and HT have to share limited amounts of raw materials and space necessary to promote the hydration reaction. However, the IC consumes parts of them before the HT does, thereby limits conventional positive effect of the HT.
In this study, the interdependent relationships among hydration reaction, internal relative humidity (RH), and strength of internally cured ultra-high performance concrete (UHPC) were investigated, ...to emphasize the importance of drying on superabsorbent polymer-based internal curing (IC). Experiments showed that the self-desiccation of UHPC cannot be prevented by external curing, such as water curing, but can be prevented by IC. Although the desiccation and resulting shrinkage of UHPC were effectively mitigated by the IC, a slow strength development was found when maintaining a high internal RH. Under water-curing conditions, the internally cured UHPCs showed 12–17% lower strength at 28 days compared with the reference sample. However, the results were 0–1% when exposed to dry air (RH 60%) between 7 and 28 days, showing accelerated external drying. The results show that the early-age shrinkage-related problem of UHPC can be fundamentally resolved, without a negative effect on strength, by controlling the drying period.
In this study, the impact of triisopropanolamine (TIPA) on both the crystallographic and surface properties of cubic and orthorhombic tricalcium aluminate (C
3
A) were investigated together with ...their hydration behavior and strength development. When TIPA was added during the grinding process, the pre-hydration and carbonation of C
3
A were effectively prevented, and crystal structural changes of C
3
A were confirmed. It leads to altering the hydration mechanism of C
3
A phases: in the case of cubic C
3
A, it promotes the formation of Al-hydrogarnet phases instead of OH-AFm phases even on the first day of curing. Similar hydration behavior was observed with orthorhombic C
3
A, but the phase transition of OH-AFm phases to Al-hydrogarnet occurs during a specific time period in 1–3 days. The latter was revealed as a very interesting endothermic reaction which can be the only heat absorptive behavior in complex cement hydration as reported so far.
This study aimed at investigating the effects of ozone treatment on the dispersion of carbon nanotubes (CNTs) in aqueous solution and the resulting influence on the material properties of ultra-high ...performance concrete (UHPC). The CNT suspension was fabricated by the ozone treatment and used to produce UHPC/CNT composites. Using spatially-resolved small angle X-ray scattering, the degree of dispersion of CNTs in UHPC matrix was quantitatively evaluated. It was confirmed that the ozone treatment enhanced the dispersion of CNTs in aqueous solution by formulating oxygenic and carboxylic groups on the surfaces of CNTs. Thus, interfacial interaction between the CNTs and UHPC was enhanced, leading to the higher nucleation effect at early ages. Ozone treatment did not significantly modify the hydration mechanism of UHPC. Instead, it provided multiple nucleation sites and double steric repulsion thorough the improved degree of dispersion of CNTs, which resulted in accelerated hydration at early ages and improved compressive strength at later ages.
Display omitted
•0.1 ppm level of ozone treatment enhanced the dispersion of carbon nanotubes in suspension.•The ozone treatment helped to attach oxygenic and carboxylic chemical groups on the surface of carbon nanotubes.•This chemical modification increased the flowability of ultra-high performance concrete due to double steric repulsion effect.•Spatially-resolved small angle X-ray scattering analysis confirms the uniform dispersion of carbon nanotubes in solid matrix.•The well dispersed carbon nanotubes accelerated hydration and improved materials properties due to multiple nucleation effect.
This study explored the hydration reaction of ultra-high-performance concrete (UHPC) by using X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA) as ...analysis methods. The partial- or no-known crystal structure (PONKCS) method was adopted to quantify the two main amorphous phases of silica fume and C-S-H; such quantification is critical for understanding the hydration reaction of UHPC. The measured compressive strength was explained well by the degree of hydration found by the PONKCS method, particularly the amount of amorphous C-S-H. During heat treatment, the pozzolanic reaction was more intensified by efficiently consuming silica fume. After heat treatment, weak but continuous hydration was observed, in which the cement hydration reaction was dominant. Furthermore, the study discussed some limitations of using the PONKCS method for studying the complicated hydration assemblage of UHPC based on the results of TGA and NMR. Generally, the PONKCS method underestimated the content of silica fume in the early age of heat treatment. Furthermore, the structural evolution of C-S-H, confirmed by NMR, should be considered for more accurate quantification of C-S-H formed in UHPC. Nevertheless, PONKCS-based XRD could be useful for understanding and optimizing the material properties of UHPC undergoing heat treatment.
The effects of various ions in cement-based solutions on the water retention capacity and ion absorbency of superabsorbent polymers (SAPs) are studied herein. It is verified that the retention ...capacity of the SAP is reduced due to the absorbed Ca2+ from solution, and the SAP releases monovalent cations such as Na+ into the solution upon absorption of Ca2+. Importantly, it was determined that non-multivalent cations in solutions play a critical role in the retention capacity of SAPs. As the total ion concentration (TIC) of the solution increases, the retention capacity improves. Higher TIC yields a weaker osmotic pressure, which reduces the driving force for the initial absorption of SAP. Therefore, the amount of the Ca2+ absorbed in the SAP decreases and the retention capacity improves. Based on the accurately measured ionic characteristics during absorption and desorption, the complicated retention phenomenon and its ionic dependence in cement-based solutions are clearly understood.
This study investigated the feasibility of using calcined layered double hydroxides (CLDHs) to prevent chloride-induced deterioration in reinforced concrete. CLDHs not only adsorbed chloride ions in ...aqueous solution with a memory effect but also had a much higher binding capacity than the original layered double hydroxides (LDHs) in the cement matrix. We investigated this adsorption in hardened cement paste in batch cultures to determine adsorption isotherms. The measured and theoretical binding capacities (153 mg g−1 and 257 mg g−1, respectively) of the CLDHs were comparable to the theoretical capacity of Friedel's salt (2 mol mol−1 or 121 mg g−1), which belongs to the LDH family among cementitious phases. We simulated chloride adsorption by CLDHs through the cement matrix using the Fickian model and compared the simulation result to the X-ray fluorescence (XRF) chlorine map. Based on our results, it is proposed that the adsorption process is governed by the chloride transport through the cement matrix; this process differs from that in an aqueous solution. X-ray diffraction (XRD) analysis showed that the CLDH rebuilds the layered structure in a cementitious environment, thereby demonstrating the feasibility of applying CLDHs to the cement and concrete industries.
•We examine the adsorption equilibrium and kinetics of CLDH in the hydrated cement.•CLDH capacity to bind chloride ions in the hydrated cement paste is determined.•We model chloride adsorption by CLDH through the cement matrix.•CLDH reforms the layered structure with ion adsorption in the cement matrix.
PDF
