The construction industry faces dual challenges of developing sustainable materials while advancing innovative manufacturing technologies. With increasing scarcity of fly ash due to coal power plant closures, alternative precursors are needed. Municipal solid waste incineration bottom ash (MSWI BA), currently landfilled in large quantities, presents an opportunity to address both material scarcity and waste management challenges. However, 3D concrete printing technology requires materials with precisely controlled rheological properties balancing conflicting requirements: sufficient fluidity for pumping and extrusion, yet adequate stiffness for shape retention. Developing mixtures meeting these stringent printability requirements while incorporating waste materials represents a significant challenge.

This study aims to investigate the impact of municipal solid waste incineration bottom ash (MSWI BA) as a sustainable alternative to fly ash (FA) on the printability and early-age behaviour of slag-based alkali-activated materials for 3D printing applications. The research followed a systematic three-phase approach: mix development and optimization, investigation of MSWI BA effects on rheology and early-age reaction kinetics, and environmental impact assessment through life cycle analysis.

The first phase involved development of two 3D printable slag-based AAM mortars through systematic optimization for printability. The optimization process evaluated buildability through slump tests, flowability through slump flow measurements, extrudability through mini-extrusion tests, setting time through Vicat testing, and mechanical strength development through compressive and flexural tests. The optimized reference mix composition comprised 80% slag and 20% FA with water-to-binder ratio of 0.38, alkali content (Na₂O/b) of 5%, activator modulus (SiO₂/Na₂O) of 0.5, and sand-to-binder ratio of 1.5. The target mix comprised 80% slag and 20% MSWI BA, maintaining identical parameters except water-to-binder ratio increased to 0.40 to achieve comparable workability, necessitated by BA's angular morphology and finer particle size. Both mixes met all printability requirements and exceeded mechanical strength targets.

The second phase revealed fundamental mechanisms governing the observed behavioural differences. Yield stress evolution via slugs test showed the FA-based mix exhibited rapid structuration with brittle discontinuity at 80 minutes, while the BA-based mix maintained slug formation throughout 140 minutes, confirming extended printable window. Pore solution analysis through revealed the BA system consistently showed lower elemental concentrations and slower consumption rates of key elements (Na, Si, Ca, Mg), indicating reduced precursor dissolution. This was attributed to dilution effects from higher water content and, more significantly, heavy metals (Cr, Zn, Pb) from MSWI BA forming hydroxide precipitates on slag surfaces, hindering slag dissolution. Fourier transform infrared spectroscopy confirmed slower reaction kinetics, showing persistent low-polymerized silicate oligomers in the BA system that sustained electrostatic repulsion between particles, delaying percolated network formation and extending printability window.

Environmental assessment demonstrated both the developed slag-based AAM mixes achieved approximately 68% reduction in shadow costs compared to 3D printable ordinary Portland cement mortar, with MSWI BA contributing less than 5% to total environmental impact. This research demonstrates that MSWI BA can effectively replace fly ash at 20% binder content in 3D printable slag-based AAMs, providing extended printable window without compromising performance. ... Read more

The development of an efficient and sustainable transportation system is crucial for the effective functioning of economies. Concrete, a widely used material for road construction, poses significant environmental challenges. A sustainable alternative is to use alkali-activated concrete (AAC) for road construction. However, the aggregates used in AAC are generally mined, contributing to the negative environmental impact. To address this concern, municipal solid waste incineration (MSWI) ashes, specifically MSWI bottom ash (BA), can be utilised to replace natural aggregates. Thus the construction of pavements using sustainable concrete facilitates its growing demand without burdening the environment.

This research focuses on the utilisation of municipal solid waste incineration bottom ash (MSWI BA) aggregates in AAC for potential application in pavements. The study was divided into five phases: aggregate characterisation, mechanical performance evaluation, long-term performance study, microstructure analysis, and life cycle assessment (LCA) analysis. The physical properties of MSWI BA aggregates indicated that the aggregates are porous and weak. Nonetheless, these aggregates showed comparable properties to natural aggregates and can still be used for pavement application. However, the metallic aluminium in the MSWI BA aggregates releases hydrogen gas leading to concrete cracking and swelling, thus, hindering its use in various applications. To address this concern, alkaline pre-treatment using sodium hydroxide solution was employed in this research. An optimal replacement level of 30% was chosen based on the effectiveness of the pre-treatment in removing metallic aluminium and the compressive strength of AAC containing MSWI BA aggregates.

In the next phase of the research, the mechanical and long-term performance of AAC with optimum replacement level was evaluated. The results demonstrated that the concrete satisfied the mechanical performance requirements for pavements. However, the freeze-thaw resistance of the AAC was below the norm requirement due to the air voids and associated cracking, which was confirmed through scanning electron microscopy (SEM) and X-ray computed tomography analysis.

SEM analysis revealed reactive phases in the MSWI BA aggregates and poor aggregate-matrix bonding for the coarser fraction compared to the finer aggregate fraction, leading to decreased mechanical performance. Despite these findings, the AAC containing MSWI BA aggregates satisfied the majority of the norm requirements, indicating its potential for road pavement application. However, evaluating the environmental impact of adding MSWI BA aggregates in concrete is essential. The life cycle assessment analysis demonstrated that the optimal MSWI BA sample exhibited better environmental effects, indicated by a lower environmental cost indicator value compared to AAC and ordinary Portland cement concrete samples with similar performance.

In conclusion, the pre-treatment method utilised in this research is optimal for a replacement level of 30% of gravel with MSWI BA aggregates. The AAC with 30% replacement level meets all the mechanical property requirements stipulated by the norm for pavement application, exhibits good air void distribution, and has limited environmental impact owing to the lower ECI value. This study evaluated the feasibility of applying MSWI BA aggregates in AAC for pavement application and showed promising results.
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It has been reported that due to the rapid urbanization and economic growth the municipal solid waste (MSW) would double in volume from 1.3 billion tons per year (in 2012) annually by the end of 2025, challenging environmental and public health management worldwide. Given that, most of the MSW incineration (MSWI) bottom ash (BA) are disposed in landfill currently, and technically and economically viable techniques for the reuse and recycling of MSWI BA is still at a premium. This issue would seriously challenge the environmental and public health management worldwide.
In some European countries and the US, MSWI BA has been utilized as aggregate in pavement construction or as aggregate in concrete. Previous studies also proved the feasibility of using MSWI BA in concrete, either as aggregates or binder substitute materials. However, it is worth noticing that there are several significant drawbacks of using MSWI BA in concrete, including the potential risk of leaching due to the existence of heavy metals and harmful salts, the low reactivity due to high content of quartz and unburned organic matters, and the metallic aluminum-induced expansion.
Therefore, in this study, a characterization of as-received MSWI BA was conducted at the beginning to find out the potential problems when used in concrete, namely the metallic aluminum content, low reactivity and unburned organics. A comprehensive pretreatment was performed subsequently to solve the problems. Specifically, both physical and chemical treatments were carried out to get rid of the metallic aluminum in BA. Afterwards, thermal treatment was conducted to enhance the reactivity of BA and remove the unburned organics. Pre-treated BA samples were characterized again to reveal the effectiveness of pretreatment. The results showed that both chemical and physical treatment were highly effective in removing metallic aluminum. Meanwhile, thermal treatment was proved to be a proper activation method which also removed the remaining organic matters through the high-temperature process.
Subsequently, the investigations of the effects of pre-treated BA addition on compressive strength, reaction products and hydration heat development were conducted on cement paste level by varying the replacement material (BA with different treatment methods) and ratio. A proper method of pretreatment was proposed as well as an optimization of a maximum replacement level of BA in cement paste without detrimentally influence the performance of concrete paste was studied. Compared with nonreactive micronized sand (only works as filler) and pure cement, the addition of physically treated BA has a certain amount of contribution to the hydration process from the viewpoint of heat release. Results show that BA do have pozzolanic activity but is much lower than cement, and physically treated BA is suitable to be used as filler in concrete. Additionally, physically treated BA was further activated through thermal treatment according to the result of compressive strength test, which delivered the highest strength among all the treated BA under the same replacement ratio.
Finally, to extend the application of MSWI BA in concrete, the mix design was made by blending treated BA with the highest compressive strength into concrete and make it suitable for structural application. The effects of treated BA addition on the workability and compressive strength of concrete were investigated. The addition of treated BA brought slight negative impact both in workability and strength due to the existence of nonreactive phases in BA (quartz and organics).
Accordingly, this study proved the potential of BA with proper treatment to be used as a cement substitute material in concrete as well as promoted the understanding of the influence of BA on the hydration process, which also brings the possibility that BA could be widely reused in concrete system in future industry.
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