•Aggregates with the real geometry shape was generated by using clump module.•Flat-Joint Model is adopted as the constitutive law.•The damage evolution process of concrete is analyzed from ...microscopic point of view.•Effects of aggregate surface texture on the mechanical behaviors of concrete are studied.
As the main component of concrete, aggregate has an important influence on the performance of concrete. In this study, the influence of coarse aggregate content and surface characteristics on the compressive mechanical properties and the damage evolution process of high-strength concrete were studied by using a 3D discrete element method (DEM). In order to improve the reliability of simulation results of concrete, a more accurate numerical simulation method was proposed. First, clumped-particle modules from the real aggregate geometry obtained by 3D Scanning were generated and then randomly distributed in the concrete model according to the specified aggregate content. Then the Flat-Joint Model (FJM) which can represent the micro-structure characteristics of concrete more reasonably was used to define the contact mechanical characteristics between different components. Based on this, the compressive strength, number of microcracks and damage evolution process of concrete with different aggregate surface texture factor (STF) and volume ratio were analyzed. The simulation results showed that aggregate surface texture and volume ratio were closely related to the compressive strength of concrete, and both have an exponential correlation with the compressive strength. The STF and volume ratio of aggregate have a significant effect on the proportion of tensile microcracks, and the concrete with large aggregates STF has more uniform stress distribution and wider damage range.
•Physicochemical properties and pozzolanic activity of sludge ashes were investigated.•The sludge ash calcined at 800 ℃ presented high pozzolanic activity.•Sludge ash addition enhanced the formation ...of Al-bearing hydrates.•No leaching risk was detected for the use of sludge ash in building material.
The application of aluminum-based flocculant in wastewater treatment results in a large amount of aluminum-rich sewage sludge. This work investigated the influence of calcination process on physicochemical characteristics and pozzolanic activity of aluminum (Al)-rich sludge ash and studied the effect of sludge ash on cement hydration. The results showed that higher calcination temperature from 600 ℃ to 900 ℃ increased the amorphous content in sludge ash. The pozzolanic activity of sludge ash calcined at 800 ℃ and 900 ℃ was confirmed by Frattini test. In view of strength activity index of blended mortar and energy conservation, the optimal calcination condition of sewage sludge ash was calcined at 800 ℃ with air-cooling. The addition of sludge ash promoted the transformation of ettringite to monosulfate phase in cement paste. However, the high Al concentration dissolved from S6 and S7 ash inhibited significantly the cement hydration and resulted in low compressive strength values of the blended mortars. The pozzolanic reaction of S8 and S9 ash produced more hydration heat and additional Al-bearing products such as katoite and monosulfate which contributed to the strength development of mortars. Furthermore, the heavy metals in sewage sludge can be immobilized in ash structure during calcination process and the structure of hydration products, which ensures the environmental security of sludge ash utilization in construction materials.
Gypsum has been known to regulate cement setting since more than 100 years, whereas the understanding of the physio-chemical mechanism remains incomplete. Here, we investigated the influence of ...gypsum on elasticity evolution of fresh tricalcium silicate (C3S) paste through small amplitude oscillation shear method. Isothermal calorimetry, X-ray diffraction, 1H nuclear magnetic resonance, and zeta potential measurements were performed to explore the corresponding changes in hydration, microstructure, and colloidal interaction. It was found that incorporating gypsum is beneficial for inter-particle agglomeration by significantly weakening electrostatic repulsion force between particles, which accelerates elasticity development in the early period of setting. Although the retardation effect of gypsum on C3S hydration hinders the further development of elasticity in the later period, the more divergent needle-like calcium silicate hydrate (C-S-H) and the stronger cohesion of the C3S particle network in presence of gypsum lead to a higher elasticity at the same hydration degree.
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•The steam-cured cement paste has a higher porosity and pore connectivity.•The pore structure of steam-cured cement paste has a less fractal characteristic.•Steam curing process ...brings a microstructural damage to hydrated cement paste.
Cement pastes subjected to initial steam-curing cycle have a poor later strength and durability. This adverse effect is originated from the steam-curing process. In order to investigate the microstructure changes of cement paste during steam-curing process, the XCTs with different spatial resolutions were combined used to detect the pore structure and the hydration development of cement pastes subjected to different curing regimes. The hydration heterogeneity and spatial distribution of pore network of cement pastes at various steam-curing ages were obtained on both mesoscale and microscale. The results indicate that the XCT is competent for figuring out the hydration development and of which the steam-cured cement paste is much higher than the standard-cured one at early age but has slight difference at later age. Besides, the steam-cured cement paste has a higher porosity and pore connectivity in the studied scales than the standard-cured one with the same hydration volume. Moreover, the pore structure of steam-cured cement paste has a less fractal characteristic, resulting in a microstructural damage and thus a lower fracture toughness and higher brittleness under elevated temperature steam-curing condition.
•An innovative design approach of binder system of eco-efficient ultra high performance concrete is proposed.•The design principles of binder system are based on physical packing and chemical effect ...optimization.•The chemical effect is ensured by optimizing the oxide composition and reaction degree index.•The designed eco-efficient ultra high performance concrete shows a better performance and environmental impact.
This research presents an innovative design approach of binder system for eco-efficient UHPC (EUHPC) to achieve a densely compacted hardened paste by optimizing the physical packing and chemical effect of binder system. The influences of binder system composition on the compressive and environment impact of UHPC under different curing regime were studied. The hydration and microstructure of the designed EUHPC was investigated. The results show that the initial packing density and CaO/SiO2 ratio of the binder system have remarkable impact on the environmental impact and mechanical performance of UHPC. The optimal range of CaO/SiO2 ratio and packing density of binder system for eco-efficient UHPC is 0.9 to 1.2 and 0.7 to 0.73, respectively. According to this design method, the EUHPC with a very low carbon dioxide index (about 3.4 kg/MPa‧m3) and compressive strength of 150 to 200 MPa is successfully prepared.
The tensile strength is one of the main parameters in the design of concrete structures and has an important influence on the cracking performance and durability of concrete materials. As the weak ...region in concrete, the strength of the interface transition zone (ITZ) directly affects the dynamic mechanical response of concrete. Meanwhile, considering that the tensile performance of concrete is far weaker than its compressive performance, the study of the strength of ITZ is of great significance to the dynamic tensile fracture characteristics and mechanical properties of concrete. In this paper, a method combining 3D scanning technology and Clump-Cluster was proposed to generate a real geometry shape model of crushable aggregate. On this basis, the 3D discrete element model (DEM) of high-strength concrete was constructed using the Flat-Joint Model (FJM). The effects of the ITZ strength on the failure mode, number of microcracks, tensile strength and strain rate effect of high-strength concrete under strain rates from 0.01/s to 100/s were studied by splitting tensile test. The simulation results indicate that the number of microcracks generated decreases with the increase of ITZ strength, and the proportion of ITZ cracks decreases obviously. When the ITZ strength exceeds the critical value of 0.8 times that of mortar, the tensile strength is almost not affected by ITZ strength. In addition, based on the numerical simulation results, a modified formula that can reflect the functional relationship between the ITZ strength and the dynamic increasing factor of the concrete tensile strength (TDIF) is proposed, and the rationality of the TDIF modified formula is verified.
•Crushable aggregates with the real geometry shape are generated by using clump-cluster method.•Flat-Joint Model is adopted as the constitutive law.•The tensile strength is almost not affected by ITZ strength when its strength exceeds 0.8 times that of mortar.•A modified formula that can reflect the functional relationship between the ITZ strength and the TDIF is proposed.
One-part geopolymeric binders which are considered as cleaner non-Portland cementitious materials are prepared by first mixing the aluminosilicate materials with the solid alkaline activators before ...adding water, which makes the mixing method of the geopolymer being the same as Portland cement. In this paper, sinking spherical beads (SSB) is introduced into the raw materials containing fly ash (CFA) and ground granulated blast furnace slag (GGBS) to synthesize a ternary one-part geopolymer. A systematic investigation of the effect of SSB on the geopolymer pastes and mortars is performed through measuring the fluidity, final setting time, compressive strength and flexural strength. It is found that SSB can significantly improve the fluidity and mechanical properties of geopolymer due to its high reactive effect and remarkable morphological effect. The action mechanism of SSB for improving the performance of the geopolymer was also dissected. It is interesting to note that SSB has little influence on the initial heat flow and increases the cumulative heat relating to the strength development. The changes in the formation of C-A-SH and N-A-SH gels after the introduction of SSB are well captured by the XRD and EDX analysis. It clearly appears that the N-A-SH gel with the optimal Na/Si ratio of 0.5 can be generated by optimizing the rational proportion of raw materials and alkaline activators. This study not only demonstrates the superiority of SSB in synthesizing a ternary one-part geopolymer, but also develops a method to design the novel non-Portland cement only by using industrial wastes.
The sinking spherical beads can improve the fluidity and compressive strength of one-part geopolymer to some extent due to the high reactive effect and remarkable morphological effect and has little influence on the final setting time. This study gives a perspective on the synthesis of one-part geopolymeric products with the generation of the desired C-A-SH and N-A-SH gels by designing the rational proportion of starting materials and alkaline activators. Display omitted
•In this study, active sinking spherical beads is firstly introduced into one-part geopolymer.•The mechanisms of the sinking spherical beads in geopolymer have been studied by multiple methods.•The optimal gelatinous phases can be designed by the rational contents of precursor materials and activators.
Electrolytic manganese residue (EMR) stockpiles contain significant amounts of Mn2+ and NH4+-N which pose a risk of environmental pollution. For EMR safe disposal, an innovative approach is proposed ...that involves direct sodium silicate-sodium hydroxide (Na2SiO3–NaOH) collaborative technology. This approach utilises Na2SiO3 and NaOH as the solidifying agent and activator, respectively, to treat EMR without hazardous effects. The study also provides insights into the kinetics of Mn2+ leaching under the effect of Na2SiO3–NaOH. Leaching efficiency was determined by varying parameters such as stirring rate, reaction temperature, pH of the initial solution, Na2SiO3 concentration, and reaction time to investigate the efficacy of this method. The study indicates that the co-treatment technology of Na2SiO3–NaOH can achieve maximum solidification efficiencies of 99.7% and 98.2% for Mn2+ and NH4+-N, respectively. The process can successfully solidify Mn2+ by synthesising Mn(OH)2 and MnSiO3 in an alkaline environment under optimal conditions including stirring rate of 450 rpm, initial solution pH of 8, test temperature of 40 °C, test time of 420 min, and Na2SiO3 content of 5%. The findings of this study have confirmed that surface chemistry plays a vital role in regulating the test rate and the proposed equation accurately describes Mn2+ leaching kinetics. Overall, the co-treatment technology involving Na2SiO3–NaOH is a viable solution for EMR resource utilisation without compromising environmental safety. This method has the potential to be implemented for other waste streams with comparable compositions, ultimately promoting the sustainable management of waste.
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•A direct Na2SiO3–NaOH collaborative treatment technology for Mn residue.•Synergistic impact greatly enhances the Mn2+ leaching efficiency.•Treatment procedure is well described by Mn2+ kinetic equation.•The maximum solidification efficiency of NH4+-N and Mn2+ are 98.2% and 99.7%.•The ideal strength of Mn residue-based paste has the potential as binder material.
•The effects of nanoparticles on cement hydration at elevated temperatures were investigated by isothermal calorimeter.•The nucleation and growth process of hydrates in nanoparticles modified cement ...were analyzed by a kinetics model.•The mechanism of nanoparticles on the hydration kinetics of cement was discussed.
The nanometer materials and technology are becoming new ways for cementitious composite innovation due to the significant improvement of microstructure and mechanical performance of cement-based materials. In this study, the isothermal calorimetry was employed to measure the heat release rate and total heat release of multi-scale cement system incorporated with different nanoparticles at elevated temperatures. The nucleation and growth processes of hydration products were simulated through a kinetics model. It is discovered that the effects of nanoparticles on cement hydration depend on itself chemical reactivity and physical properties as well as ambient temperature. Both nano-SiO2 and nano-C-S-H can obviously shorten the induction period of cement hydration and have acceleration effects obviously. Addition of 1% nano-CaCO3 seems to have no obvious effect on cement hydration process at elevated temperatures. The acceleration effects of nano-particles mainly refer to the improvement of the nucleation rate of hydrates. Generally, the effect of nanoparticles on nucleation process is more significant than that on growth process. Nano-SiO2 and nano-CaCO3 have slight influence on the growth rate, but the nano-C-S-H can increase the growth rates to some extent at different temperatures.
