Materials and Structures最新文献

Kenaf geopolymer for 3D printing is a promising representative for comprehensive and intelligent utilization of industrial and agricultural wastes. Aiming at exploring the effect of raw materials’ proportion on printing performances, alkali activator dosage (10 wt.%, 15 wt.%, 20 wt.%) and mass ratio of ground granulated blast furnace slag (GGBFS) to fly ash (FA) (15:85, 20:80, 25:75) were adopted as main variables in this paper. Results have shown that increasing the activator dosage and decreasing the mass ratio of GGBFS to FA can improve the flowability, and adjusting these two parameters can tailor the fluidity to a suitable range. Moreover, dry density of kenaf geopolymer was more significantly affected by alkali activator dosage compared with mineral powder ratio, and lightweight characteristic due to kenaf participation effectively improved shape retention ability of printed specimens. Besides, microstructure analysis further confirmed that with appropriate alkali activator dosage and mineral powder ratio, high level of geopolymerization reaction can be achieved to produce enough gel product for a satisfactory internal structure, which externally manifested as excellent printability and mechanical strength. Finally, formula with alkali activator dosage of 15 wt.% and GGBFS to FA mass ratio of 25:75 was recommended for satisfactory printing performance and mechanical properties.

In order to overcome the brittle behavior of conventional geopolymers, of late, a paradigm shift towards development of hybrid geopolymers has commenced. This study describes hybrids synthesized by co-milling metakaolin and solid organics (epoxy resin: diglycidyl ether of bisphenol A and hardener: dicyandiamide) followed by alkali activation. The developed hybrid geopolymers exhibit enhanced mechanical and physical properties. Physical and mechanical properties of such hybrids depend on the extent of molecular-level interactions and microstructural evolution during geopolymerisation. Evolution of molecular structure from precursor stage (co-milled samples) to hybrid geopolymers is studied using transmission electron microscopy (TEM) and ²⁷Al, ¹³C, ²⁹Si solid-state nuclear magnetic resonance (NMR) spectroscopy. NMR and TEM analyses of the hybrid geopolymers illustrate the formation of Si–O–C bonds and uniform C distribution (with no phase separation); this confirms inorganic–organic chemical interactions during geopolymerisation. Detailed assessment of pore characteristics using TEM, mercury intrusion porosimeter, and Brunauer–Emmett–Teller reveal formation of a dense gel (with reduced pore size and pore volume) in hybrid geopolymer vis-à-vis MK-based inorganic geopolymer. The implication of such microstructural features on mechanical and physical properties is discussed. Lastly, the suitability of developed hybrids as fire-retardant materials used in mass transit applications is highlighted.

The decarbonization of the concrete industry is an ongoing pursuit. One solution towards this goal is the use of limestone powder in portland cement. Waste eggshell has tremendous potential as an alternative calcite filler in cement due to its similarities with limestone. In this research, the feasibility of adding 15% and 35% ground eggshell in portland cement to make cement mortars was investigated. The hydration mechanism of eggshell and limestone blended cements was compared through the heat of hydration, phase assemblage, electrical resistivity, compressive strength, and shrinkage measurements. The experimental results showed that cement mortars with ground eggshell attained similar compressive strength as that with limestone. However, eggshell mixtures demand more mixing water to compensate the hydrophobicity of the eggshell membrane. The high calcite content in both eggshell and limestone accelerates the hydration of cement at 15% replacement, but ground eggshell retards cement hydration at 35% replacement due to the dominant influence of the membrane. Overall, eggshell waste is a feasible sustainable alternative to limestone powder at up to 15% portland cement replacement levels. Lifecycle assessment and cost analysis showed that adding 15% ground eggshell in cement concrete further reduces its embodied carbon and energy and cost compared to cement concrete containing limestone powder.

This paper presents the results of a laboratory test program designed to investigate the adhesive effects of large-scale (bulk) ice on concrete. Medium-strength concrete cylinders were sawn into discs, and attached to a sample table. Freshwater ice samples, frozen using smaller, standard-sized concrete cylinders, were adhered to the concrete with both varying bond times and added weight during bonding. Shear strength tests were conducted at a set displacement rate, under a number of temperatures. The effect of these variables on the adhesive strength of ice to concrete was examined, as well as whether there was any noticeable removal of concrete cement paste or aggregate during testing. The tests indicate that the adhesive strength is negligible when the method of adhesion is “dry” (no liquid layer at the onset of adhesion). Tests with “wet” adhesion indicated a significantly higher strength. The nominal versus the apparent contact area had significant implications for the determination of the adhesive strength of the bond between the ice and the concrete. Removal of cement paste was evident in a number of tests, however the amount was not significant. The results have relevance for design of structures in a marine environment, such as revetement dams or rubblemound breakwaters, as well as for the standardization of adhesion tests with ice and concrete.

Autogenous shrinkage and shrinkage-induced cracking present significant challenges in ultra-high-performance concrete (UHPC). To address this issue, this study explores the feasibility of using soda residue (SR), an industrial waste product, as a sustainable internal curing agent for UHPC. Experimental results demonstrate that the inclusion of SR substantially mitigates shrinkage and cracking in UHPC, while also enhancing compressive strength. The use of SR with additional water showed comparable or superior performance in reducing shrinkage and cracking compared to higher dosages of SR. Through internal humidity measurements, thermogravimetric analysis, and scanning electron microscopy, two primary mechanisms for the improvement were identified: (1) SR increases internal humidity by releasing additional water, thus preventing shrinkage; and (2) the ettringite formation induced by SR expands the solid phase volume, compensating for shrinkage. Furthermore, utilizing SR as a recycled material not only improves the early-age properties of UHPC but also contributes to sustainable construction practices.

The oil well cement undergoes various physical and chemical changes during the hydration process, leading to the formation of pores of different sizes within the cement stone. These pores can affect the mechanical properties of the cement stone. In the civil engineering field, extensive attempts have been made to predict the mechanical properties of concrete based on pore parameters, yielding good results. This paper explores in detail the methods for predicting the strength of oil well cement based on porosity and pore size distribution. Through referencing the strength prediction methods for concrete in civil engineering, porosity and pore size distribution are used as prediction parameters. The accuracy of predictions made by empirical models and deep learning models is compared, and it is concluded that neither empirical formulas nor ordinary deep learning models can provide accurate fitting results. However, due to the optimization of its algorithm and structure, the KAN model can give more accurate predictions of the pore-size-strength relationship of cement stone. Additionally, the quantitative relationship between pore size and strength of cement stone is explored. The application of the KAN model in strength prediction provides strong guidance for monitoring and optimizing cementing quality during the construction process.

Hollow slabs have been widely used for medium- and small-span bridges, but hollow slab bridges occasionally encounter connecting problems. The key to solve the problems is to ascertain the contact mechanism and subsequently find an approach to simulate them. This paper focuses on developing a cohesion–friction model for concrete interfaces and implementing nonlinear analysis of hollow-slab bridges through a general finite element software. First, the positions and moving tendencies of interfaces in hinge joints are investigated; the stress states on these interfaces can be divided into compression‒shear, tension–shear and pure shear states. Second, a simplified cohesion–friction model for concrete interfaces is proposed in which the shear resistance under different stress states is deduced based on Mohr’s strength theory. Third, the accuracy of the proposed model is verified by a conventional cohesion–friction model. Finally, the proposed model is applied to predict the behaviours of hollow-slab bridges in finite element analysis. Research results indicate that the shear resistance of a concrete interface can be predicted by the proposed model and the behaviours of hollow-slab bridges can be represented by the finite element model. The slab deflection is seriously affected by the friction coefficient and shear strength of the interfaces; hence, it is necessary to calibrate these indexes in advance of numerical simulation analysis.

This study introduces a microbial-induced calcium precipitation technique into cement-based 3D printing by incorporating Bacillus pasteurii into 3D printing (3DP) mortar. The printability, physical–mechanical properties, and microstructure are analyzed to compare the differences between control concrete and bacterial concrete. Experimental results demonstrated that mixing bacteria in 3DP mortars can enhance printability and increase the uniaxial compressive strength (UCS) and Brazilian splitting tensile strength of printed specimens. Particularly, this method significantly improved the interlayer strength of 3DP concrete. With a bacterial concentration of 1 × 10^7 cells/ml, the UCS improved by 35.8% and 57.3% in the YZ and XY directions, respectively, compared to the control concrete UCS. The tensile strength in the YZ direction improved by 23.65% compared to control concrete at the same bacterial concentration. Moreover, the tensile strength in the XY direction continued to improve with increasing bacterial concentration, while it decreased in the YZ direction, indicating that incorporating bacteria is an effective method for enhancing interlayer tensile strength. Additionally, nitrogen adsorption results revealed that mixing bacteria reduced pore volume and surface area of printed specimens, leading to denser microstructure by filling granular calcium carbonate precipitates at internal pores of 3D-printed concrete, as observed by SEM and XRD. These findings offer a new approach for modifying cement-based 3D-printing mortars and provide valuable insights for enhancing the mechanical performance of architectural 3DP concrete, thereby promoting the advancement of cement-based 3DP technology.

This paper was conducted to upcycle lime-neutralize etching wastewater (LNW), produced from the etching of cenospheres for producing perforated cenospheres, into alkali-activated slag/fly ash, considering the high contents of Ca²⁺ (Calcium ion), Cl⁻ (Chloride ion), and OH⁻ (Hydroxyl). The aim of this study is to limit the discharge of calcium-rich wastewater and minimize the pollution of water and soil resources. To this end, the effects of different LNW content on the fresh properties, hydration products, compressive strength, microstructure and nanomechanical properties of AASF were investigated. Results revealed that the addition of LNW extended the setting times and inhibited the early hydration of AASF due to the existence of Ca²⁺ and NH⁴⁺ (Ammonium) in LNW. In addition, the existence of Ca²⁺ in LNW reacts with NaOH to form Ca(OH)₂, which can work as an auxiliary activator for AASF. Consequently, the LNW-added AASF mortars exhibited denser pore structures and higher compressive strengths. By using 25–100% LNW, the compressive strengths of AASF mortars can be improved by 2.7–18.9% (3d), 4.8–19.5% (7d) and 5.6–19.7% (28d), respectively.

Taking inspiration from the recent rise in interest on calcined clays, the RILEM technical committee RILEM TC 282-CCL on Calcined Clays as Supplementary Cementitious Materials has been summarising knowledge on a wide variety of topics related to the use of calcined clays in cement and concrete. In this article, the working group 2 of this committee summarises recent global efforts on the industrialisation of calcined clay cements, to bring the work of the committee into context. Clays have been a key construction material since Roman times but are now designated as crucial for enhancing cement industry sustainability in the short to mid-term. Several industrial and semi-industrial trials that have recently produced calcined clays through various techniques, such as static, rotary, and suspension calcination technologies are covered in this paper, while worldwide cement production trials with calcined clays and limestone are also discussed. Major projects considering local clays as construction materials are presented as examples of global interest in the subject. Due to interest in achieving climate goals, calcined clays are being rapidly reintroduced into the cement industry, and academic research has played an important role in this process. The examples discussed in this article demonstrate the importance of greater and swifter knowledge transfer from academia to industry. The work also demonstrates the need to upgrade industrial equipment and design new efficient equipment to eliminate the use of fossil fuels for clay calcination, a process that requires relatively lower temperature than clinker production. The challenges in achieving net-zero carbon emissions in clay calcination technologies are also discussed. Overall, this paper presents the context in which the RILEM TC 282-CCL operated.