Recycling coral waste into eco-friendly UHPC: Mechanical strength, microstructure, and environmental benefits

Mechanical and Microstructural Performance of UHPC with Recycled Aggregates Modified by Basalt Fiber and Nanoalumina at High Temperatures

This study investigates the mechanical properties and microstructure of basalt fiber (BF) and nanoalumina (NA)-modified ultra-high-performance concrete with recycled aggregates (UHPC-RA) under high-temperature conditions. The effects of different replacement rates of recycled aggregates (RAs), BF content, and NA content on the compressive strength, splitting tensile strength, and elastic modulus were evaluated at ambient temperatures and after exposure to 200 °C, 400 °C, 600 °C, and 800 °C. The results show that mechanical properties decrease with temperature rise, but specimens containing BF exhibited improved crack resistance and better high-temperature integrity. The incorporation of NA enhanced the thermal stability and heat resistance of the concrete. Digital image correlation (DIC) was used to monitor real-time surface deformation, and scanning electron microscopy (SEM) analysis revealed improved microstructure with reduced porosity and cracks. This study demonstrates that the combination of BF and NA significantly enhances the high-temperature performance of UHPC-RA, which holds promising potential for applications in environments subjected to elevated temperatures.

Enhancing Radiation Shielding Capabilities with Epoxy-Resin Composites Reinforced with Coral-Derived Calcium Carbonate Fillers

This study investigates the development of epoxy-resin composites reinforced with coral-derived calcium carbonate (CaCO3) fillers for enhanced radiation shielding and mechanical properties. Leveraging the high calcium content and density of coral, composites were prepared with filler weight fractions of 0%, 25%, and 50%. SEM and EDS analyses revealed that higher filler concentrations (50%) increased particle agglomeration, affecting matrix uniformity. Mechanical testing showed that while the tensile and flexural strengths decreased with the increased filler content, the compressive strength significantly improved, reaching 135 MPa at a 50% coral content. Radiation shielding evaluations demonstrated enhanced attenuation with a higher filler content, achieving 39.63% absorption at 60 kVp for the 50% coral composite. However, the shielding efficiency was notably lower compared to lead, which achieves over 99% absorption at similar energy levels. These quantitative comparisons highlight the material's limitations in high-radiation environments but emphasize its suitability for moderate shielding applications. Despite their lower shielding efficiency, the composites provide an environmentally friendly and non-toxic alternative to lead, aligning with sustainability goals. Future work should focus on optimizing filler dispersion, mitigating agglomeration, and exploring hybrid systems to enhance the shielding efficiency and mechanical properties. The further quantitative evaluation of parameters such as Zeff and cross-sections is recommended to comprehensively assess the material's performance.

Efficient utilization of coral waste for internal curing material to prepare eco-friendly marine geopolymer concrete

To address shortages in construction materials for island engineering, tackle the accumulation of solid waste, and inhibit the shrinkage of geopolymers, coral waste was utilized as the internal curing material to prepare high-performance marine geopolymer concrete (MGC) with seawater, sea-sand, and normal limestone aggregate (LsA). The coral coarse aggregate (CorA) used in this investigation has a total porosity ranging from 50% to 58.3% with internal pore diameters spanning 50-400 μm. The water desorption of CorA followed a two-stage pattern within a relative humidity (RH) range of 75%-85%, becoming nonlinear above 90% RH, which released about 85% of its moisture within 200 h at 97% RH, demonstrating potential for internal curing. Adding a small amount of CorA to MGC increased slump and setting time by providing internal curing water. However, as CorA content exceeded 30%, the slump significantly decreased due to reduced mixing water and elevated activator concentration, while the initial setting time slightly decreased. Furthermore, the inclusion of saturated CorA in MGC significantly reduced autogenous shrinkage, with higher CorA contents (exceeding 30%) leading to slight expansion in the early stages and nearly eliminating shrinkage at contents above 40%. The greater drying shrinkage in geopolymer systems compared to ordinary Portland cement is due to capillary pressure compressing the product framework, converting larger gel pores into smaller ones. Additionally, the layered calcium aluminosilicate hydrate (C-A-S-H) gel exhibits more pronounced creep characteristics under low internal humidity conditions. The higher CorA content in MGC promoted the formation of hybrid C, N-A-S-H gel and hydrotalcite-like phases, and reduced carbonation issues. The interfacial transition zone (ITZ) between CorA and the geopolymer matrix formed a robust mechanical interlock, enhancing tensile strength and minimizing shrinkage-induced cracks. Based on overall performance and marine material utilization, an optimal substitution rate of CorA between 40% and 50% is recommended.

Analysis of Different Early Strength Agents on the Performance of Prefabricated UHPC

Precast ultra-high-performance concrete (UHPC) has emerged as indispensable in the engineering sector due to its cost-effectiveness and superior performance. Currently, precast UHPC grapples with challenges pertaining to slow setting times and insufficient early strength, largely attributed to its high water-reducing agent content. Effective utilization of early strength agents to augment UHPC's early strength is pivotal in addressing this issue. This study investigates the efficacy of two distinct concrete early strength agents, namely calcium formate (Ca(HCO2)2) and aluminum sulfate (Al2(SO4)3). A UHPC system with a water/cement ratio of 0.17 was used; both single and compound doping experiments were conducted using varied dosages of the aforementioned early strength agents. Our results show that both early strength agents significantly reduce setting time and enhance early strength at appropriate dosages. Specifically, the addition of 0.3% Ca(HCO2)2 led to a 33.07% decrease in setting time for UHPC. Moreover, the incorporation of 0.3% Ca(HCO2)2 and 0.5% Al2(SO4)3 resulted in a strength of 81.9 MPa at 1.5 days, representing a remarkable increase of 118.4%. It is noteworthy that excessive use of Ca(HCO2)2 inhibits the hydration process, whereas an abundance of Al2(SO4)3 diminishes the early strength effect. Simultaneously, this article provides recommendations regarding the dosage of two distinct early strength agents, offering a novel solution for expediting the production of prefabricated UHPC with a low water/cement ratio and high water-reducing agent content.

Mechanical properties, durability and environmental assessment of low-carbon cementitious composite with natural fibrous wollastonite

Cementitious composite is one of the most widely used construction materials around the world, but cement production is accompanied by energy waste and CO2 emission. Wollastonite, a natural fibrous silicate mineral, can be a potential supplementary cementing materials (SCMs) for cementitious composite owing to its similar calcium silicate system, and it has an eco-friendly and convenient production process. Furthermore, its unique fibrous structure can possibly act as reinforcement for the cement matrix. In view of this, this study aimed to investigate the effects of different sizes (the median diameter of 3 μm, 6 μm, 9 μm and 12 μm) and contents (0%, 5%, 10%, 15% and 20% by mass) of wollastonite combined with 5% silica fume on the mechanical strength, durability and microstructure of cementitious composite by compressive test, flexural test, shear test, rapid chloride migration test, sulfate corrosion test, X-ray diffraction(XRD), scanning electron microscope (SEM) and fourier transform infrared spectrometer (FTIR). The results showed that the strength and durability of samples increased and then decreased as wollastonite content increased. An addition of 10% wollastonite into the cement matrix increased compressive strength, flexural strength and shear strength by 6.22%, 29.3% and 18.4%, respectively. However, an addition of 5% wollastonite was found to be more beneficial for improving resistance to chloride and sulfate corrosion. Additionally, samples prepared with 3 μm wollastonite performed better, which can be attributed to the fact that the small size of wollastonite contributed to both the filling effect and the skeletal support and bridging effect of microfibers. The CO2 emissions of cementitious composites decreased as the wollastonite percentage increased. The findings confirm that natural wollastonite as SCMs for cementitious composite has performance enhancement and environmental benefits, however, it is recommended that the wollastonite content should not exceed 15%.

Recycling solid waste to produce eco-friendly ultra-high performance concrete: A review of durability, microstructure and environment characteristics

Recycling waste materials (WMs) is a cost-effective method for saving natural resources, protecting the environment, and reducing the use of high-carbon raw materials. This review aims to illustrate the impact of solid waste on the durability and microstructure of ultra-high performance concrete (UHPC) and to provide guidance for the research of eco-friendly UHPC. The results show that the proper use of solid waste to replace part of the binder or aggregate has a positive effect on the performance development of UHPC, but further enhancement techniques should be developed. When solid waste is prepared as a binder, the durability of waste based UHPC can be effectively improved by grinding and activation. When solid waste is used as an aggregate, its rough surface, potential reactivity and internal curing effect are also beneficial to the improvement of UHPC performance. Since UHPC has a dense microstructure, it can effectively prevent the leaching of harmful elements (heavy metal ions) in solid waste. However, the effect of waste modification on the reaction products of UHPC needs to be further studied, and design methods and testing standards suitable for eco-friendly UHPCs should be developed. The use of solid waste in UHPC effectively reduces the carbon footprint of the mixture, which is beneficial to the development of cleaner production technologies.

Enhancing performance and sustainability of ultra-high-performance concrete through solid calcium carbonate precipitation

Ultra-high-performance concrete (UHPC) exhibits high compressive strength and good durability. However, owing to the dense microstructure of UHPC, carbonation curing cannot be performed to capture and sequester carbon dioxide (CO₂). In this study, CO₂ was added to UHPC indirectly. Gaseous CO₂ was first converted into solid calcium carbonate (CaCO₃) using calcium hydroxide, and the converted CaCO₃ was then added to UHPC at 2, 4, and 6 wt% based on the cementitious material. The performance and sustainability of UHPC with indirect CO₂ addition were investigated through macroscopic and microscopic experiments. The experimental results showed that the method used did not negatively affect the performance of UHPC. Compared with the control group, the early strength, ultrasonic velocity, and resistivity of UHPC containing solid CO₂ improved to varying degrees. Microscopic experiments, such as heat of hydration and thermogravimetric analysis (TGA), demonstrated that adding captured CO₂ accelerated the hydration rate of the paste. Finally, the CO₂ emissions were normalized according to the compressive strength and resistivity at 28 days. The results indicated that the CO₂ emissions per unit compressive strength and unit resistivity of UHPC with CO₂ were lower than those of the control group.

A two-step approach for calculating chloride diffusion coefficient in concrete with both natural and recycled concrete aggregates

This paper presents an analytical approach to calculate the effective diffusion coefficient of chlorides in concrete with both natural and recycled concrete aggregates. In the approach the concrete is treated as a composite consisting of three phases, namely mortar, natural aggregate plus interfacial transition zone, and recycled concrete aggregate plus interfacial transition zone. The effective diffusion coefficient of chlorides in the composite is calculated through two steps. The first step is to calculate the effective diffusion coefficients of chlorides in the natural aggregate plus interfacial transition zone and in the recycled concrete aggregate plus interfacial transition zone by using multilayer spherical approximation, the results of which provide the information about the quality of recycled concrete aggregate in terms of chloride penetration resistance. The second step is to calculate the effective diffusion coefficient of chlorides in the three-phase concrete composite by using effective medium approximation, the results of which provide the information about the influence of recycled concrete aggregate on the diffusivity of recycled aggregate concrete. The analytical expression of the effective diffusion coefficient is derived and carefully compared with the results obtained from both the experiments and numerical simulations, which demonstrates that the present analytical model is rational and reliable. The analytical expression presented can be used to predict the service life of recycled aggregate concrete exposed to chloride environment.

Utilization of sewage sludge ash in ultra-high performance concrete (UHPC): Microstructure and life-cycle assessment

In this research, an economical and eco-friendly ultra-high performance concrete (UHPC) with compressive strength of more than 120 MPa was prepared with the dosage of sewage sludge ash (SSA) at 8 wt%. The results indicate that the addition of SSA has an adverse influence on the workability of UHPC samples due to its special morphology. Furthermore, the microstructure and phase assemblage of SSA-based UHPC were determined and the results show that SSA inhibits the early hydration of cement clinker, while promotes the precipitation of additional hydration products at later curing ages due to its pozzolanic reaction. The pore structure analysis of SSA-based UHPC determined by mercury intrusion porosimetry indicates that the addition of SSA increases the cumulative pore volume, while decreases the large pore volume of UHPC. Economic and environmental analysis indicates that using SSA-based UHPC greatly reduces the unit cost and the impacts on the environment.