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• A novel geopolymer concrete is developed incorporating waste ceramic aggregates. • The designed geopolymer exhibit ultra-high residual strength after 1000 ℃ thermal exposure. • The high temperature resistance and mechanism of designed geopolymer are investigated. A novel geopolymer concrete is developed, incorporating waste ceramic aggregates, with ultra-high residual strength after exposure to elevated temperatures. Different aggregate volume fractions and particle size gradations are designed, and the variations in mass loss, volume shrinkage, compressive strength, mineral composition, and microstructure of geopolymer mortar after exposure to high temperatures ranging from 20 ℃ to 1000 ℃ are investigated by testing mass loss, volume change, strength, XRD and SEM. The results show that the waste ceramic aggregates (WCA) applied in the preparation of geopolymer mortars improve the mechanical properties and high-temperature resistance. By optimizing aggregate particle size gradation, the microstructure of the geopolymer mortar becomes denser and more homogeneous, and the high-temperature resistance is further improved. The residual compressive strength at 1000 °C is 115.2 MPa, 476 % higher than that of reference. These findings demonstrate the great potential of geopolymer mortars using ceramic aggregates with optimized gradation for high temperature resistant applications. [ABSTRACT FROM AUTHOR]

• The functionally graded composite slabs (FGCS) exhibit enhanced penetration and crack resistance. • The FGCS target is fabricated by one-time grouting cast to achieve improved interfacial bond. • A reduced perforation limit is observed for the FGCS target under in-service bullet impact. This study develops functionally graded composite slab (FGCS) by a one-time grouting approach based on the design concepts of ultra-high performance fibre reinforced concrete (UHPFRC) and two-stage concrete (TSC). The anti-penetration performance of the FGCS targets is investigated against 7.62 mm in-service bullets with shooting velocity of 843 m/s. The results show that while the TSC target presents superior bullet penetration resistance, it concurrently poses a significant threat of widespread cracking damage. Upon weighing the factors of bullet penetration and risk of cracking, the FGCS emerges as a more viable candidate for protective structures. The FGCS target is cast through a one-time grouting method, and there is no interface delamination under the projectile impact. The optimized FGCS exhibits enhanced in-service bullet impact resistance and a reduced safety thickness of 80 mm compared to the normal UHPFRC. [ABSTRACT FROM AUTHOR]

This paper investigates the high temperature resistance performance and mechanism of potassium-activated blended precursor geopolymer with silica fume. The failure morphology, volume, and mass loss, compressive strength deterioration, hydration production, and pore structure are measured and analyzed. The results show that introducing slag into fly ash-based geopolymer could greatly improve the 28 d compressive strength but reduce the thermal stability. In contrast, the partial substitution of fly ash by metakaolin contributes to excellent high temperature resistance with slightly enhanced 28 d compressive strength. After being exposed at 800 °C, the residual compressive strength of F7M3 remains at 37 MPa, almost 114% of the initial ambient-temperature strength. An appropriately enlarged silica fume content in geopolymer results in increased compressive strength and enhanced thermal stability. However, an excessive silica fume content is detrimental to the generation of alkali-aluminosilicate gels and ceramic-like phases and thus exacerbates the high temperature damage. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the material synergy and parameter optimization of circular ultra-high performance concrete (UHPC) filled steel tubes towards excellent axial bearing capacity and material utilization efficiency. 12 stub columns of UHPC-filled steel tubes (UCFST) were tested and analysed combined with 293 literature data sets under axial compression. The results show that increasing confinement index ξ and steel ratio α can enhance the ductility of the axial load-displacement curve and the failure mode of the column. Threshold values of ξ = 1.0 and α = 0.25 are proposed to distinguish the failure modes. Appropriately higher steel strength grade, lower D/t , larger α and ξ are recommended to ensure the good axial bearing capacity of the UCFST columns. To produce excellent material synergy and axial bearing capacity, the optimum steel and UHPC strength grades are recommended around C100 ∼ C160 and Q345 ∼ Q690 for the axially-loaded UCFST short columns, respectively. Furthermore, the optimum ranges of diameter-to-thickness ratio D / t , steel ratio α and confinement index ξ are <90·(235/ f y), 0.10–0.30, 0.7–2.0, respectively. The current normal CFST design codes are inappropriate for the UCFST columns due to the parameter values exceeding limit values. Therefore, a reliable analytical model is proposed and well validated to predict the axial bearing capacities of circular UCFST short columns. • 12 circular UCFST stub columns are tested and analysed combined with 293 literature data. • The material synergy and parameter optimization of the UCFST columns are explored. • The optimum ranges of material strength grades, D / t , α and ξ are proposed. • An analytical model is proposed to predict the axial bearing capacities of UCFST stub columns. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the latest advancement in physical and chemical activation strategies for ultra-high performance concrete (UHPC), addressing challenges such as high autogenous shrinkage, insufficient hydration, and inadequate steel fibers utilization, to further enhance its performance and achieve broader application. Firstly, an in-depth discussion is provided on physical activation strategies, encompassing design methodologies, nanomaterials, mixing, casting, and curing regimes. Subsequently, the discourse comprehensively covers chemical activation strategies, including active nanomaterials, multi-cementitious material systems, chemical admixtures and alkali-activator. Throughout this investigation, particular emphasis is placed on identifying research gaps within various activation strategies to foster more profound subsequent research and encourage innovative contributions to civil engineering. Finally, a focused discussion is provided on the influence of synergistic physical and chemical activation on UHPC performance. This comprehensive review serves as a crucial reference for deep research and broader application of UHPC in engineering, offering valuable insights for further exploration and optimization of the UHPC performance. [ABSTRACT FROM AUTHOR]

This paper proposes a novel concept of 'central angle' to extend cross-section design of round-ended rectangular concrete-filled steel tube (RRCFST). Axial compressive behaviour and confinement mechanism of RRCFST stub columns are experimentally and numerically researched, including effects of central angle θ of round ends, aspect ratio κ of middle rectangular part, cross-sectional type, steel wall-thickness t. The results show that local buckling and shear failure of core concrete usually occur. The θ and κ can clearly describe the cross-sectional geometrical characteristics of RRCFST sections, as well as unifying circular and rectangular CFST. When the θ gradually changes from 0 to 180°, the round-ended confinement increases but the corner confinement reduces continuously, which results in overall confinement effect and material efficiency followed by parabolic trends. A larger κ leads to a weaker overall confinement and material efficiency, due to relatively wider unconstrained or weak constrained zones. The optimized cross-sections with θ around 60° and relatively smaller κ are recommended to design RRCFST subjected to axial compression, which contribute to good overall confinement effect, high material efficiency, high axial bearing load and ductility. A new reliable predicting analytical model for axial ultimate bearing capacity of RRCFST is proposed and validated. [ABSTRACT FROM AUTHOR]

• An ambient CO 2 pre-treatment is conducted to improve the reactivity of steel slag. • Carbonated steel slag as SCM contributes to a higher strength of UHPC. • CO 2 can be sequestrated in eco-friendly UHPC by incorporation of carbonated steel slag. • 45% of cement can be replaced by carbonated steel slag in UHPC (>150 MPa). The converter steel slag is a by-product during the steel-making process, which usually acts as an inert ingredient in construction due to the relatively low reactivity. However, its mineral composition results in a high reactivity in a CO 2 rich environment. This study reveals the modification of converter steel slag by an ambient CO 2 pretreatment, and the application of modified converter steel slag as the supplementary cementitious material (SCM) in the design of eco-friendly ultra-high performance concrete (UHPC) by using a particle packing model. The results show that converter steel slag after ambient CO 2 pretreatment is feasible to be used in eco-friendly UHPC design, which achieves a relative higher compressive strength over 150 MPa with the cement substitution from 15% to 45%, compared to non-carbonated steel slag. The ambient CO 2 pretreatment can modify the physical and chemical properties of converter steel slag particles, which produces a rough and porous surface of slag particles because of the precipitation of calcium carbonate and amorphous silica gel as carbonation products. Consequently, the incorporation of carbonated steel slag improves the cement hydration, enhance the formation of ettringite and C-S-H, while densify the microstructure compared to non-carbonated slag in eco-friendly UHPCs. The leaching of potentially hazardous elements, e.g. Cr and V from carbonated steel slag can be efficiently reduced below legal limits in UHPC mixtures. [ABSTRACT FROM AUTHOR]

This study investigates the effects of various mineral admixtures, such as fly ash, metakaolin, and limestone powder, on the mechanical, chloride, and sulfate resistance properties of alkali-activated slag (AAS). The resistance of alkali-activated concrete (AAC) to chloride and sulfate is evaluated using the rapid chloride migration (RCM), wetting-drying cycles, SEM and XRD, TG/DTG. The results reveal that introducing fly ash improves workability but degrades the strength and durability of AAS. The 15% slag substitution by metakaolin is recommended to improve durability. Incorporating 15% limestone powder increases the compressive strength and reduces porosity due to filling and nucleation effects. The replacement ratio of 30% for each mineral admixture induces a considerable Ca2+ dilution, resulting in reducing the strength and durability of AAC. Under short-term wetting-drying cycles, the continued hydration of raw materials under MgSO 4 erosion and the subsequent production of gypsum, hydrotalcite, and brucite from the matrix initially increase the AAC strength. However, the continued substitution of calcium in the C-(A)-S-H gel by magnesium and precipitation of gypsum under excessive cycles adversely deteriorates the compressive strength. The chloride migration degree in AAC is closely related to the porosity. Although the porous gel N-A-S-H facilitates chlorine migration in accelerated tests, it prevents further chlorine penetration by adsorbing chloride ions in large quantities, exhibiting the dominant effect on chloride immobilization. The formation of Friedel's salt, stemming from the chemical bonding of chloride, is present only in the high-calcium specimen G100 and the highly reactive aluminum specimen G85M15. Improvements in the gel chemistry and porosity are key factors in bolstering the durability of AAC. The findings of this study contribute to advancing the application of eco-friendly AAC in marine environments. • Alkali-activated slag-based concretes (AACs) with different mineral admixtures are developed and evaluated. • The mechanisms on sulfate erosion and binding, chloride migration and binding of the designed AACs are explored. • Optimized alkali-activated system for marine concrete is proposed considering strength, pore structure and durability. [ABSTRACT FROM AUTHOR]