Ceramisation of hazardous elements: Benefits and pitfalls of the inertisation through silicate ceramics

Classification and predictive leaching risk assessment of construction and demolition waste using multivariate statistical and machine learning analyses

Managing construction and demolition waste (CDW) poses serious concerns regarding landfilling and recycling because of the potential release of hazardous elements after leaching. Ceramic materials such as bricks, tiles, and porcelain account for more than 70% of CDW. Fourteen samples of different CDW products from Ferrara (Northeast Italy) were subjected to geochemical analyses, including leaching tests, in accordance with UNI EN 12457-2. The interaction between ceramics and concrete was examined, highlighting the influence of mixed environments on the leaching behavior. Results were compared with an extensive database of more than 150 samples collected from the literature on different CDW types worldwide. Multivariate statistical analysis and machine learning were used to classify the CDW compositions based on the bulk chemical data. Various metrics-contaminant factors (Cf and Cd) and hazardous quotients (HQ and HQm)-were introduced to quantify the key environmental hazards of leachates. The results of this study underscore the potential of the proposed approaches in automating CDW classification and predicting Cf and HQ using only the starting bulk chemical composition. The findings enhance CDW management practices and support sustainability efforts in the construction industry.

Target immobilized phases of heavy metals in hazardous waste based lightweight aggregate

The potential leaching risk poses a concern for the large-scale recycling of hazardous waste as lightweight aggregates (LWAs). This paper investigated the combination state of heavy metals in target immobilized phases of LWA through both theoretical calculations and experimental verification. Results reveal that Pb can enter the feldspar crystal cell to form stable interstitial solid solutions, while Cu, Cr, and Ni can replace specific ions in spinel to form replacement solid solutions. The addition of target immobilized phases generally weakened the physical performance of LWAs, while reducing the leaching risk. The appropriate amount of the spinel phase favored the immobilization of Cu, Cr, and Ni, whereas albite contributed to the immobilization of Pb with low leaching values. Due to the lower melting temperature, albite could facilitate the introduction of a high-temperature liquid phase, enhancing the migration of Pb²⁺ for better immobilization in glassy phase. In contrast, anorthite exhibited a higher viscosity at 1100 °C, leading to ineffective physical encapsulation of heavy metal ions by the liquid phase. Heavy metal ions react with additional spinel phase at high temperatures to form stable solid solution phases. This study provides a novel method for regulating heavy metal leaching in hazardous waste-based LWA.

Hazardous element inertisation in vitrified silicate ceramics: Effect of different matrices

The ceramic industry is a production sector that can efficiently recycle its own processing residues, achieving a reuse index of almost 100%. Recently, the range of waste from other industrial sectors that can be used as secondary raw materials in ceramic bodies has expanded. However, such an expansion potentially introduces hazardous components. This study aimed to quantitatively assess the efficiency of inertising hazardous elements (HEs) through ceramisation. The ceramics were characterised through XRPD, SEM-EDS and leaching tests to determine their leaching behaviour and the mechanisms of element immobilisation in neoformation phases during the ceramisation process. The results indicate high immobilisation efficiency for Ba, Co, Cr, Cu, Pb, Sb, Sn and Zn. However, Mo is the main element of concern owing to its poor retention in ceramic bodies. This is likely due to the formation of oxyanionic complexes that are difficult to immobilise in silicate matrices. In addition, the ceramic bodies exhibit substantial differences that appear to be associated with variations in pseudo-structural components and the degree of polymerisation of their vitreous phase.

Mitigating heavy metal volatilization during thermal treatment of MSWI fly ash by using iron(III) sulfate as a chlorine depleting agent

In the thermal treatment of municipal solid waste incineration fly ash (FA), the presence of chlorides leads to the pronounced volatilization of heavy metals at high temperature, making heavy metals stabilization challenging. Conventional washing processes struggle to remove chlorides completely, and even minor residual chlorides can lead to significant heavy metal volatilization. This study innovatively applied iron(III) sulfate as a chlorine depleting agent, which can form FeCl3 (boiling point 316 °C) and volatilize to remove the residual chlorides at below 500 °C, thus preventing the chlorination and volatilization of heavy metals at 600-1000 °C. Using water-washed FA to produce lightweight aggregate (LWA) preparation, after adding iron(III) sulfate, the volatilization rates of Pb and Cd at 1140 °C decreased to 5.4% and 9.3%, respectively, a reduction of 82.8% and 84.1% compared to before its addition. The LWA met standard requirements in both performance and heavy metal leaching toxicity. The mechanism was further studied through thermodynamic equilibrium calculations and heating experiments of pure chemicals. This study presents novel approaches and insights for suppressing the volatilization of heavy metals in FA at high temperature, thereby promoting the advancement of thermal treatment techniques and the safe, resourceful disposal of FA.

Comprehensive study of recycling municipal solid waste incineration fly ash in lightweight aggregate with bloating agent: Effects of water washing and bloating mechanism

Recycling into lightweight aggregate (LWA) by sintering is a promising technology for disposal of municipal solid waste incineration fly ash (FA). In this study, FA and washed FA (WFA) were combined with bentonite and SiC (bloating agent) to make LWA. The performance was comprehensively studied by hot-stage microscopy and laboratory preparation experiments. Water washing and increased FA/WFA improved LWA bloating extent, while shorten the bloating temperature range. Water washing also increased the 1 h-water absorption rate of LWA, making it harder to meet the standard. Excessive FA /WFA usage (70 wt%) will prevent LWA from bloating. For the goal of recycling more FA, mixture with 50 wt% WFA could prepare LWA that meet standard GB/T 17431 at 1140-1160 °C. After water washing, the ratio of Pb, Cd, Zn, and Cu stabilized in LWA increased by 279 %, 410 %, 458 %, and 109 % for 30 wt% FA/WFA addition, and 364 %, 554 %, 717 %, and 697 % for 50 wt% FA/WFA addition, respectively. The change of liquid phase content and viscosity at high temperature were determined using the thermodynamic calculations and chemical compositions. The bloating mechanism was further investigated by integrating these two properties. To obtain accurate results of the bloat viscosity range (2.75-4.44 log Pa·s) for high CaO systems, the composition of the liquid phase should be taken into account. The liquid phase viscosity required for bloating start was proportional to the liquid phase content. With temperature increasing, bloating would end when viscosity drops to 2.75 log Pa·s or liquid phase content reach 95 %. These findings provided further understanding of the heavy metal stabilization during LWA production and the bloating mechanism of high CaO content systems, and could contribute to the feasibility and sustainability of recycling FA and other CaO-rich solid wastes into LWA.

Recycling municipal solid waste incineration fly ash in super-lightweight aggregates by sintering with clay and using SiC as bloating agent

Municipal solid waste incineration (MSWI) fly ash is classified as hazardous waste and requires proper treatments. Sintering of MSWI fly ash for the production of lightweight aggregate (LWA) is a promising treatment technology, while the dependence on natural bloating clay to produce high quality LWA has limited its wide application. In this study, by using SiC as a bloating agent, normal clay could be used to produce super-lightweight aggregate (bulk density <500 kg/m³) with MSWI fly ash. Effects of SiC addition amount, sintering temperature and duration on LWA performance were studied. The results showed that LWA with SiC addition of 0.1-0.5 wt% had significant expansion at sintering temperature of 1120 °C-1160 °C. The optimal conditions were 0.3 wt% SiC addition and sintering at 1120 °C for 30 min, and the bulk density could reach 212 kg/m³ with other properties meeting the LWA standard (GB/T 17431.1-2010). Further, the heavy metal leaching toxicity was significantly decreased after sintering and met the MSWI fly ash utilization standard (HJ 1134-2020). The X-ray diffraction results revealed the formation of a complex diopside-based phase after sintering. This study provides a new approach for recycling MSWI fly ash in LWA without dependence on specific clay resources, and makes this technology wider applicability.

Treatment of Soil Contaminated by Mining Activities to Prevent Contamination by Encapsulation in Ceramic Construction Materials

Mining is an essential activity for obtaining materials necessary for the well-being and development of society. However, this activity produces important environmental impacts that must be controlled. More specifically, there are different soils near new or abandoned mining productions that have been contaminated with potentially toxic elements, and currently represent an important environmental problem. In this research, a contaminated soil from the mining district of Linares was studied for its use as a raw material for the conforming of ceramic materials, bricks, dedicated to construction. Firstly, the contaminated soil was chemically and physically characterized in order to evaluate its suitability. Subsequently, different families of samples were conformed with different percentages of clay and contaminated soil. Finally, the conformed ceramics were physically and mechanically characterized to examine the variation produced in the ceramic material by the incorporation of the contaminated soil. In addition, in this research, leachate tests were performed according to the TCLP method determining whether encapsulation of potentially toxic elements in the soil occurs. The results showed that all families of ceramic materials have acceptable physical properties, with a soil percentage of less than 80% being acceptable to obtain adequate mechanical properties and a maximum of 70% of contaminated soil to obtain acceptable leachate according to EPA regulations. Therefore, the maximum percentage of contaminated soil that can be incorporated into the ceramic material is 70% in order to comply with all standards. Consequently, this research not only avoids the contamination that contaminated soil can produce, but also valorizes this element as a raw material for new materials, avoiding the extraction of clay and reducing the environmental impact.