Sustainable assessment and synergism of ceramic powder and steel slag in iron ore tailings-based concrete

Solid waste management is a critical issue worldwide. Effectively utilizing these solid waste resources presents a viable solution. This study focuses on Iron ore tailings (IOTs), a solid waste generated during iron ore processing, which can be used as supplementary cementitious materials (SCMs) but have low reactivity, hindering their large-scale application in concrete production. To address this, ternary SCMs were prepared using ceramic powder (CP) and steel slag (SS) to enhance the performance of concrete incorporating IOTs. The study found that the synergistic effect of CP and SS significantly improved the compressive strength of concrete, with a notable increase of up to 21% compared to concrete with IOTs alone. Mercury intrusion porosimetry (MIP) and backscattering electron (BSE) analyses revealed that the ternary SCMs significantly optimized the characteristics of the interfacial transition zone (ITZ), which in turn enhanced the compressive properties of the concrete. This contributed to maintaining the structural integrity of the concrete, even amidst variations in the pore structure. Importantly, the incorporation of ternary SCMs led to a 23% reduction in carbon emissions, from 400.01 kg CO2/m3 to 307.48 kg CO2/m3, and elevated eco-strength efficiency from 0.1 to 0.14. The study highlights the role of multi-material synergy in developing composite SCMs systems, fostering the sustainable advancement of green building materials.

Effect of fine recycled aggregate on the strength and durability properties of concrete modified through two-stage mixing approach

To overcome the scarcity of river sand and dumping of construction and demolition wastes, the fine recycled aggregate (FRA) collected from C&D wastes is being utilized as a replacement to river sand. Many earlier studies reported that the higher water absorption of fine recycled aggregate and weak interfacial transition zone (ITZ) resulted in the development of the concrete with less strength and durability requirements. This study surmounts the above two factors through pre-saturation and a two-stage mixing approach (TSMA) technique. The concrete mixes prepared at 0.45 w/c ratios with 0%, 25%, 50%, and 100% of FRA were evaluated through hardened properties such as compressive strength, split tensile strength, flexural strength, and durability properties such as water absorption, sorptivity, shrinkage, rapid chloride penetration, and carbonation tests. The results indicate that the optimum level of replacement of fine recycled aggregate was 25% and the increase of fine recycled aggregate decreases the strength and durability properties. However, increased curing of concrete resulted in better strength and durability properties. The strength of the concrete was increased by 12% at 28 days and 17.46% at 90 days by two-stage mixing approach. The water absorption, porosity, shrinkage, chloride penetration, and carbonation of two-stage mixing approach was decreased by 7.45%, 15.38%, 16.57%, 18.18%, and 13.51% compared to normal mixing approach. Microstructural investigations show improvement in the interfacial transition zone with two-stage mixing approach compared to normal mixing approach.

Enhancing the quality of recycled coarse aggregates by different treatment techniques-a review

Generation of solid wastes due to industrialization and urbanization results in dumping of wastes in landfills causing contamination of soil, air, and water. One of the important solid wastes is construction and demolition wastes generated during destruction of engineering structures. These wastes reduce the percolation of rain water that recharges the underground water level table and affects the integrity of the environment. In recent times, concrete fractions recycled from the construction wastes have been used as an alternative to fine and coarse aggregates. However, the adhered mortar on the surface of recycled aggregates possesses micro-cracks resulting in the higher water absorption compared to the natural aggregates. Removal of adhered mortar and densification of micro-cracks on the surface of recycled aggregates are performed through various treatments to enhance the quality of recycled aggregates. This paper reviews various treatments and processing techniques to improve the quality of aggregates recycled from the construction wastes for its efficient utilization in the concrete. The review on various literatures infers that the microbial treatment to recycled aggregates was more effective in improving the properties of the concrete. Microbial treatment precipitates dense CaCO₃ crystals that clog the micro-cracks on the adhered mortar and reduces the porosity of the recycled aggregates. It is also observed that several advanced concrete mixing techniques strengthen the weaker interfacial transition zone (ITZ) in the recycled aggregate concrete without any additional surface treatments to recycled aggregates.

Investigation of the bond strength and microstructure of the interfacial transition zone between cement paste and aggregate modified by Bayer red mud

In this study, a method is proposed for modifying aggregate with Bayer red mud (RM), and the bond strength and microstructure of the interfacial transition zone (ITZ) in the concrete prepared using the modified aggregate is determined. Compared to concrete prepared using natural basalt aggregate, concrete prepared with RM-modified basalt aggregate aged for 7 and 28 days had a 25.08 % and 21.75 %, respectively, higher compressive strength and a 39.53 % and 15.30 %, respectively, flexural strength. Compared to concrete prepared using natural limestone aggregate, the compressive and flexural strengths of concrete prepared with RM-modified limestone aggregate increased by over 10.00 % and 20 % respectively, after aging of both 7 and 28 days. The RM had a higher wettability to cement paste than basalt and limestone, implying that cement paste on the surface of RM-modified aggregate had a correspondingly stronger microflow and filling capacity. In addition, concrete prepared with the RM-modified aggregate had a low voidage, a compact ITZ structure and strong interfacial adhesion, resulting in considerably enhanced mechanical properties. This study provides novel applications for RM that can be widely used in building materials and waste reduction and a new method for improving the mechanical properties of concrete.

Evaluation of morphology and size of cracks of the Interfacial Transition Zone (ITZ) in concrete containing fly ash (FA)

Interfacial Transition Zone (ITZ) of coarse aggregate cement matrix is commonly regarded as the weakest element of concrete. In this phase - the first cracks in the material are initiated, and the process of destruction of the composite begins. An improvement of the ITZ properties are positively influenced by the mineral additives used for the composite. One of such a substitute for a binder is, potentially hazardous industrial waste, siliceous fly ash (FA). In this paper the ITZ between aggregate and cement paste in concretes containing FA is considered. The paper presents the results of tests on the effect of the addition of FA in the amount of: 0, 20 and 30% by weight of cement on morphology and size of cracks of the ITZ in composites. In matured concretes the smallest cracks occur in composite with the 20% FA additive. It can be concluded that composites with 20% addition of FA are characterized by low permeability and therefore high durability. The results of tests carried out can be helpful in obtaining concrete with the highest possible: strength, durability and reliability of operation. Moreover, such procedures also cause a restriction storage of hazardous materials, i.e. FA - by 160 million tons per year.

Microstructure of ultra high performance concrete containing lithium slag

Lithium slag (LS) is discharged as a byproduct in the process of the lithium carbonate, and it is very urgent to explore an efficient way to recycle LS in order to protect the environments and save resources. Many available supplementary cementitious materials for partial replacement of cement and/or silica fume (SF) can be used to prepare ultra high performance concrete (UHPC). The effect of LS to replace SF partially by weight used as a supplementary cementitious material (0%, 5%, 10% and 15% of binder) on the compressive strengths and microstructure evolution of UHPC has experimentally been studied by multi-techniques including mercury intrusion porosimetry, scanning electron microscope and nanoindentation technique. The results show that the use of LS degrades the microstructure of UHPC at early ages, and however, the use of LS with the appropriate content improves microstructure of UHPC at later ages. The hydration products of UHPC are mainly dominated by ultra-high density calcium-silicate-hydrate (UHD C-S-H) and interfacial transition zone (ITZ) in UHPC has similar compact microstructure with the matrix. The use of LS improves the hydration degree of UHPC and increases the elastic modulus of ITZ in UHPC. LS is a promising substitute for SF for preparation UHPC.

Effects of maximum aggregate size on UPV of brick aggregate concrete

Investigation was carried out to study the effects of maximum aggregate size (MAS) (12.5mm, 19.0mm, 25.0mm, 37.5mm, and 50.0mm) on ultrasonic pulse velocity (UPV) of concrete. For investigation, first class bricks were collected and broken to make coarse aggregate. The aggregates were tested for specific gravity, absorption capacity, unit weight, and abrasion resistance. Cylindrical concrete specimens were made with different sand to aggregate volume ratio (s/a) (0.40 and 0.45), W/C ratio (0.45, 0.50, and 0.55), and cement content (375kg/m(3) and 400kg/m(3)). The specimens were tested for compressive strength and Young's modulus. UPV through wet specimen was measured using Portable Ultrasonic Non-destructive Digital Indicating Tester (PUNDIT). Results indicate that the pulse velocity through concrete increases with an increase in MAS. Relationships between UPV and compressive strength; and UPV and Young's modulus of concrete are proposed for different maximum sizes of brick aggregate.