Environmental, economic and engineering performances of aqueous carbonated steel slag powders as alternative material in cement pastes: Influence of particle size.

The low reactivity and volume expansion issue of steel slag limits its application as alternative to cement. Studies demonstrated that aqueous carbonation (AC) can enhance the cementitious properties of finely sized steel slag as a cementitious supplementary material (SCM). However, the impact of particle size on the CO2 uptake capacity and its association of performance of carbonated steel slag remains unexplored. This study aims to optimize the grinding levels by examining the fineness of the steel slag used as SCM to reduce the high-energy consumption while maintaining the CO2 sequestration and properties of SCM. The results show that reducing the size of steel slag is favorable for CO2 sequestration (particle size 22.4-112.6 µm corresponds to sequestration of ∼88.5-37.9 kg CO2/t steel slag) and improve the leaching of Mg ions for mineralization. The life cycle assessment shows that the global warming potential of AC of steel slag is ∼96.2-24.9 kg CO2-eq/t steel slag, which can offset the carbon emissions due to further grinding. The 28-day compressive strength of the cement pastes blended with finer carbonated steel slag was also relatively higher due to the formation of mono-carboaluminates and stabilization of ettringite in facilitating the bond strength between the carbonated steel slag particle and the cement paste matrix. According to 3E (engineering, environmental and economic) triangle model, 22.4 µm steel slag powder showed the best comprehensive performance, including an increased revenue of 40.8 CNY/ton steel slag.

Report of RILEM TC 281-CCC: outcomes of a round robin on the resistance to accelerated carbonation of Portland, Portland-fly ash and blast-furnace blended cements.

Many (inter)national standards exist to evaluate the resistance of mortar and concrete to carbonation. When a carbonation coefficient is used for performance comparison of mixtures or service life prediction, the applied boundary conditions during curing, preconditioning and carbonation play a crucial role, specifically when using latent hydraulic or pozzolanic supplementary cementitious materials (SCMs). An extensive interlaboratory test (ILT) with twenty two participating laboratories was set up in the framework of RILEM TC 281-CCC 'Carbonation of Concrete with SCMs'. The carbonation depths and coefficients determined by following several (inter)national standards for three cement types (CEM I, CEM II/B-V, CEM III/B) both on mortar and concrete scale were statistically compared. The outcomes of this study showed that the carbonation rate based on the carbonation depths after 91 days exposure, compared to 56 days or less exposure duration, best approximates the slope of the linear regression and those 91 days carbonation depths can therefore be considered as a good estimate of the potential resistance to carbonation. All standards evaluated in this study ranked the three cement types in the same order of carbonation resistance. Unfortunately, large variations within and between laboratories complicate to draw clear conclusions regarding the effect of sample pre-conditioning and carbonation exposure conditions on the carbonation performance of the specimens tested. Nevertheless, it was identified that fresh and hardened state properties alone cannot be used to infer carbonation resistance of the mortars or concretes tested. It was also found that sealed curing results in larger carbonation depths compared to water curing. However, when water curing was reduced from 28 to 3 or 7 days, higher carbonation depths compared to sealed curing were observed. This increase is more pronounced for CEM I compared to CEM III mixes. The variation between laboratories is larger than the potential effect of raising the CO2 concentration from 1 to 4%. Finally, concrete, for which the aggregate-to-cement factor was increased by 1.79 in comparison with mortar, had a carbonation coefficient 1.18 times the one of mortar. Supplementary Information: The online version contains supplementary material available at 10.1617/s11527-022-01927-7.

Upcycling of wastes for sustainable controlled low-strength material: A review on strength and excavatability.

In recent decades, the use of controlled low-strength material (CLSM) in densely populated cities has increased. CLSM is designed for future excavation with great fluidity, appropriate early strength, and low final strength. CLSM mixtures exhibit variable strength properties and performance due to the distinctive features of wastes (i.e., combustion residues, industry slags, and construction and other solid wastes) produced from various sources. CLSM should increase early strength quickly enough to allow traffic to resume within a few hours while maintaining a low strength for future re-excavation. It is suggested that the initial mixture design for each waste reported in the literature be changed until the combination meets the application standards defined in ACI 229R-13. The effects of adjusting other ingredients (i.e., cement, water, and admixtures) in the wastes incorporated into CLSM mixtures on the strength and re-excavatability properties are also detailed and discussed in this review. From practical and economic perspectives, the supply of materials in the waste streams, transport distance, and material properties and cost are important aspects to consider before their introduction to the construction industry.

CO2 pretreatment of municipal solid waste incineration fly ash and its feasible use as supplementary cementitious material.

In this study, municipal solid waste incineration fly ash (MSWIFA) was pretreated with CO2 via slurry carbonation (SC) and dry carbonation coupled with subsequent water washing (DCW). Both the treated MSWIFAs were then used as cement replacement in cement pastes by weight of 10%, 20% and 30% to investigate the influence on hydration mechanisms, physico-mechanical characteristics and leaching properties. The results showed that carbonates formed on the surface of SC-MSWIFA particles were finer (primarily 20-50 nm calcite) than those from the corresponding DCW-MSWIFA (mostly 130-200 nm vaterite). Hence, SC-MSWIFA blended cement pastes led to shorter setting time and higher early compressive strength than the DCW-MSWIFA pastes. In contrast, the presence of vaterite-rich DCW-MSWIFA in the blended cement pastes could accelerate the cement hydration after 24 h. Both the CO2-pretreated MSWIFA can replace cement up to 30% without sacrificing the long-term strength and mechanical properties of cement pastes, demonstrating excellent performance as a supplementary cementitious material. Moreover, volume stability in terms of expansion and lead leaching of CO2-pretreated MSWIFA cement pastes were far below the regulatory limits.

Environmental benefit assessment of steel slag utilization and carbonation: A systematic review.

The rapid increase in steel slag generation globally highlights the urgent need to manage the disposal or utilization processes. In addition to conventional landfill disposal, researchers have successfully reused steel slag in the construction, chemical, and agricultural fields. With the large portions of alkaline silicate mineral content, steel slag can also be used as a suitable material for carbon capture to mitigate global warming. This article comprehensively reviews the environmental performance of steel slag utilization, especially emphasizing quantitative evaluation using life cycle assessment. This paper first illustrates the production processes, properties, and applications of steel slag, and then summarizes the key findings of the environmental benefits for steel slag utilization using life cycle assessment from the reviewed literature. This paper also identifies the limitations of quantifying the environmental benefits using life cycle assessment. The results indicate steel slag is largely utilized in pavement concrete and/or block as a substitution for natural aggregates. The associated environmental benefits are mostly attributed to the avoidance of the large amount of cement utilized. The environmental benefits for the substitution of traditional energy-intensive material and carbonation treatment are further discussed in detail. Due to the presence of heavy metals, the potential risks to human and ecological health caused by the manufacturing process and usage stage are examined. Finally, the current challenges and global social implications for steel slag valorization are summarized.

Roles of chlorine and sulphate in MSWIFA in GGBFS binder: Hydration, mechanical properties and stabilization considerations.

In this study, municipal solid waste incineration fly ash (MSWIFA) was first washed (pretreatment) with pure water with liquid to solid (L/S) ratio of 2, 3, 6, 10, to understand the removal efficiency of chlorine and sulphate, as well as its consequent ability as alkaline activator for granulated blast furnace slag (GGBFS). Washed MSWIFA was blended with GGBFS at a fixed ratio of 3:7 to examine their impact on mechanical properties, reaction mechanism, microstructure and leaching behavior. The results showed that chlorine in MSWIFA (>70%) can be washed out easily, while the removal of sulphate was largely depended on the L/S. GGBFS can be better activated by a low L/S (e.g. 2) washed-MSWIFA with attaining the compressive strength of 45.2MPa at 28 days. The higher chlorine and sulphate contents retained in the washed-MSWIFA, the higher the total heat release in the activated GGBFS system. Calcium silicate hydrate (C-S-H), ettringite (AFt) and Friedel's salt were the main hydration products of the activated binders. The rapid formation of AFt was mainly responsible for the 1-day strength development. Large amounts of Friedel's salts were formed from 1 day to 3 days associated to the inhibition of sulphate, and the presence of C-S-H played the key role in long-term strength development. The leaching test of heavy metals and soluble ions also demonstrated that washed MSWIFA activated GGBFS binders were harmless to the environment.

Effect of water-to-cement ratio induced hydration on the accelerated carbonation of cement pastes.

Recently, the use of accelerated carbonation curing has attracted wide attention as a promising method to reduce carbon dioxide (CO2) emission and improve the mechanical properties of cement-based materials. However, the diffusion mechanism of CO2 in the matrix and the content of hydration products are the key factors that restrict the carbonation reaction rate. To understand the combined behavior of hydration and carbonation reactions, this paper investigates the influence of cement hydration induced by water-to-cement ratio (w/c) (ranging from 0.25 to 0.45) on microstructure and microhardness properties of cement paste. The experimental results demonstrated that carbonation only occurred at the surface layer of cement paste samples and carbonation efficiency was significantly influenced by greater hydration due to higher w/c. The carbonation depth of the sample with 0.45 w/c was about 6 times higher than that of sample with 0.25 w/c after 28 days of CO2 curing. XRD results revealed that calcite-type calcium carbonate is the main carbonation product and consumption of clinker phases (C2S and C3S) during the hydration enhanced the calcite precipitation in the pores of the surface layer. According to FTIR, with increasing w/c, the position of Si-O-Si stretching bond of the carbonated surface changed from Q2 to Q3, confirming the formation of amorphous silica-rich gel, along with the appearance of CO32- bonds related to calcite. In overall, the micro-mechanical analysis in this study showed that the carbonation significantly improved the surface microhardness of cement paste samples, while the refinement of capillary pores due to carbonation also decreased the negative impact of large pores formed in the matrix of cement paste prepared with high w/c.

Highly-efficient green photocatalytic cementitious materials with robust weathering resistance: From laboratory to application.

The combined use of nano-TiO2 with cementitious materials offers an environmentally-friendly way to combat the air pollution problem. However, a trade-off between a high efficiency and a robust weathering resistance has often to be made for most of the attempted nano-TiO2 incorporation methods. This paper developed a simple and effective "spraying" method to coat nano-TiO2 particles on the surface of concrete surface layers (CSL). The results showed that the NOx removal rate of the samples increased with an increase in both the concentrations of nano-TiO2 solutions and the number of times of the spraying action. And the conditions for preparation of the Spray AB (the CSL were first sprayed with the 30 g L-1 TiO2-solution 20 times, followed by mechanical compaction, and for another 20 times after the compaction) were found to be optimal in terms of NOx removal performance and weathering resistance. The Spray AB was superior to the 5% TiO2-intermixed samples with respect to photocatalytic NOx removal ability. Compared with TiO2-dip-coated samples, the Spray AB samples had better and robust weathering resistance. A case study on the factory-fabricated green Eco-blocks (produced by the laboratory-developed spray method and the conventional intermix method) was performed. Examination and comparison on their respective photocatalytic NOx removal further verified the advantages of the spray method over the intermix method.

A critical review of waste glass powder - Multiple roles of utilization in cement-based materials and construction products.

In light of concerns relating to improper waste disposal and resources preservation, reclamation of the discarded glass in construction materials had been extensively carried out since 1963. In the past decade, although more than 100 papers associated with the use of glass powder (GP) in the micron level scale were published, comprehensive review of all practical applications in cement-based materials and construction products is not available. This paper therefore provides a summary of the body of knowledge on the interaction and effects of using GP in cement-based and extended construction materials. This review concludes that GP is an innovative and promising eco-supplementary cementitious material. Beyond that, use of GP is demonstrated to be potentially beneficial as a precursor in geopolymer and suitable for manufacturing eco-cement, artificial lightweight aggregate and composite phase change material. The multiple applications of GP are seen as an important step towards waste glass recycling as a sustainable construction material and for the overall betterment of the industry.

Enhancement of high temperature performance of cement blocks via CO2 curing.

Carbonation for the curing of cement-based materials has been gaining increased attention in recent years, especially in light of emerging initiatives to reduce carbon dioxide (CO2) emissions. Carbonation method or CO2 curing is founded on the basis of the reaction between CO2 and cement products to form thermally stable and denser carbonate, which not only improves the physical and mechanical properties of cement-based materials, but also has the ability to utilize and store CO2 safely and permanently. This study aims to assess the effect of CO2 curing technology on the high-temperatures performance of cement blocks. Upon molding, dry-mix cement blocks were cured under statically accelerated carbonation condition (20% CO2 concentration with 70% relative humidity) for 28 days, followed by exposure to elevated temperatures of 300 °C to 800 °C in order to comprehensively study the principal phase changes and decompositions of cement hydrates. The results indicated that CO2 curing improved the performance of cement blocks, such as enhancement in the residual compressive strength and reducing the sorptivity. At 600 °C, the scanning electron microscopy (SEM) revealed a denser microstructure while thermal analisis and X-ray diffraction (XRD) analysis also clearly demonstrated that higher amounts of calcium carbonate were present in the cement blocks after CO2 curing, suggesting better high-temperature performance compared to natural cured cement blocks. In general, an improved high-temperature performance, specifically at 600 °C of the dry-mixed cement blocks was demonstrated by adopting the CO2 curing technology. This confirms the potential of utilizing CO2 curing technology in not only improving quality of cement blocks, new avenue for storing of CO2 in construction material can be realized at the same time.

Waste resources recycling in controlled low-strength material (CLSM): A critical review on plastic properties.

The exponential growth of waste generation is posing serious environmental issues and thus requires urgent management and recycling action to achieve green sustainable development. Controlled low-strength material (CLSM) is a highly flowable cementitious backfill material with self-consolidating properties. The CLSM efficiency during construction and final performance at the site depends on its plastic properties. Plastic properties are responsible for workability, pumpability, stability, and lateral pressure on adjacent soils. This paper presents a critical review to date on the use of waste materials and/or by-products and their impacts on the plastic properties of the CLSM. Extensive previous studies demonstrated that the basic properties and content of waste materials as well as the amount of water in the mix design, play a dominant role in determining the plastic properties of CLSM. The discussed plastic properties of CLSM include flowability, bleeding, segregation, and hardening time, which are found to be inter-related. Proper mix design adjustment to accommodate the use of waste materials is possible to produce sustainable CLSM with acceptable plastic properties. Additionally, the discussion and analysis presented in this paper could provide a basis for future research advances and the development of sustainable CLSM prepared with waste materials.

Properties of Foamed Mortar Prepared with Granulated Blast-Furnace Slag.

Foamed mortar with a density of 1300 kg/m³ was prepared. In the initial laboratory trials, water-to-cement (w/c) ratios ranging from 0.54 to 0.64 were tested to determine the optimal value for foamed mortar corresponding to the highest compressive strength without compromising its fresh state properties. With the obtained optimal w/c ratio of 0.56, two types of foamed mortar were prepared, namely cement-foamed mortar (CFM) and slag-foamed mortar (SFM, 50% cement was replaced by slag weight). Four different curing conditions were adopted for both types of foamed mortar to assess their compressive strength, ultrasonic pulse velocity (UPV) and thermal insulation performance. The test results indicated that utilizing 50% of slag as cement replacement in the production of foamed mortar improved the compressive strength, UPV and thermal insulation properties. Additionally, the initial water curing of seven days gained higher compressive strength and increased UPV values as compared to the air cured and natural weather curing samples. However, this positive effect was more pronounced in the case of compressive strength than in the UPV and thermal conductivity of foamed mortar.

Development of a method for recycling of CRT funnel glass.

Finding better solutions to manage and recycle cathode-ray tube (CRT) glass is crucial for reducing the environmental threats due to the disposal of the glass. In this paper, the results of a laboratory study on developing a method for removing lead from crushed funnel glass surface and re-utilizing the treated glass in cement mortar are presented. The results demonstrate that nitric acid at 3-5% concentration levels can be used to remove most of the lead from the crushed funnel glass surface and render it as non-hazardous waste based on toxicity characteristics leaching procedure (TCLP) testing. It is noted that the particle size of glass and number of treatment cycles are significant factors affecting lead extraction. The study further demonstrated that it is feasible to utilize up to 100% of treated funnel glass as a replacement for natural sand for producing cement mortar.

Utilization of recycled cathode ray tubes glass in cement mortar for X-ray radiation-shielding applications.

Recycled glass derived from cathode ray tubes (CRT) glass with a specific gravity of approximately 3.0 g/cm(3) can be potentially suitable to be used as fine aggregate for preparing cement mortars for X-ray radiation-shielding applications. In this work, the effects of using crushed glass derived from crushed CRT funnel glass (both acid washed and unwashed) and crushed ordinary beverage container glass at different replacement levels (0%, 25%, 50%, 75% and 100% by volume) of sand on the mechanical properties (strength and density) and radiation-shielding performance of the cement-sand mortars were studied. The results show that all the prepared mortars had compressive strength values greater than 30 MPa which are suitable for most building applications based on ASTM C 270. The density and shielding performance of the mortar prepared with ordinary crushed (lead-free) glass was similar to the control mortar. However, a significant enhancement of radiation-shielding was achieved when the CRT glasses were used due to the presence of lead in the glass. In addition, the radiation shielding contribution of CRT glasses was more pronounced when the mortar was subject to a higher level of X-ray energy.

Utilization of recycled glass derived from cathode ray tube glass as fine aggregate in cement mortar.

Rapid advances in the electronic industry led to an excessive amount of early disposal of older electronic devices such as computer monitors and old televisions (TV) before the end of their useful life. The management of cathode ray tubes (CRT), which have been a key component in computer monitors and TV sets, has become a major environmental problem worldwide. Therefore, there is a pressing need to develop sustainable alternative methods to manage hazardous CRT glass waste. This study assesses the feasibility of utilizing CRT glass as a substitute for natural aggregates in cement mortar. The CRT glass investigated was an acid-washed funnel glass of dismantled CRT from computer monitors and old TV sets. The mechanical properties of mortar mixes containing 0%, 25%, 50%, 75% and 100% of CRT glass were investigated. The potential of the alkali-silica reaction (ASR) and leachability of lead were also evaluated. The results confirmed that the properties of the mortar mixes prepared with CRT glass was similar to that of the control mortar using sand as fine aggregate, and displayed innocuous behaviour in the ASR expansion test. Incorporating CRT glass in cement mortar successfully prevented the leaching of lead. We conclude that it is feasible to utilize CRT glass in cement mortar production.

Use of wastes derived from earthquakes for the production of concrete masonry partition wall blocks.

Utilization of construction and demolition (C&D) wastes as recycled aggregates in the production of concrete and concrete products have attracted much attention in recent years. However, the presence of large quantities of crushed clay brick in some the C&D waste streams (e.g. waste derived collapsed masonry buildings after an earthquake) renders the recycled aggregates unsuitable for high grade use. One possibility is to make use of the low grade recycled aggregates for concrete block production. In this paper, we report the results of a comprehensive study to assess the feasibility of using crushed clay brick as coarse and fine aggregates in concrete masonry block production. The effects of the content of crushed coarse and fine clay brick aggregates (CBA) on the mechanical properties of non-structural concrete block were quantified. From the experimental test results, it was observed that incorporating the crushed clay brick aggregates had a significant influence on the properties of blocks. The hardened density and drying shrinkage of the block specimens decreased with an increase in CBA content. The use of CBA increased the water absorption of block specimens. The results suggested that the amount of crushed clay brick to be used in concrete masonry blocks should be controlled at less than 25% (coarse aggregate) and within 50-75% for fine aggregates.