Evaluation of municipal solid waste incineration filter cake as supplementary cementitious material

https://doi.org/10.1016/j.conbuildmat.2020.118833Get rights and content

Highlights

  • The presence of LDH in early hydration is observed with increasing FC content.

  • The formation of LDH causes a delay in the C3S reaction.

  • The reaction degree of PC slightly rises increasing the replacement rate.

  • Within 5% replacement, the FC does not negatively influence the binder properties.

Abstract

Worldwide, the incineration of household waste generates high amounts of Municipal Solid Waste Incineration (MWSI) bottom ashes (BA). In some European countries, the Filter Cake (FC) is also collected (below 0.25 mm) by the wet sieving treatment of coarse BA (4–32 mm). By using centrifugation, the presence of readily soluble contaminants in FC is decreased. In addition, for reducing the leaching of heavy metals the use of FC in combination with cement represents a valuable option, due to cement immobilization capacity and the FC appropriate particle size and chemical composition. However, to the best of the authors’ knowledge, FC has not often investigated as binder replacement. Therefore, this paper addresses the suitability of FC as substitution for CEM I 52.5 R, between 1 and 20% replacement, by mass. Due to its porous structure, the FC lowers the bulk and particle density of the mixtures and increases the water demand, and widens the particle size distribution. The XRD analysis evidences the formation of layered double hydroxides (LDH) in the early stage of the reaction (1, 2, 7 days) incorporating contaminants as Cl. Kinetically, the hydration of C3S is delayed according to the FC replacement without affecting the long-term flexural strength. Despite this, the presence of FC minimally contributes to the increase of cement reaction degree. Tested accordingly to the EN 124-57, the 28 days mortars fulfill the Dutch legislation for contaminants leaching as unshaped material, independently on the substitution rate.

Introduction

An effective recycling method for municipal solid waste (MSW) is becoming necessary in the European Union, where only in 2014, 2530 million tons of waste have been produced by households and other economic activities [10]. The use of MSW as fuel for waste-to-energy plants has grown by 24% between 2010 and 2014, a preferential option compared to incineration without energy recovery. However, despite the risk of air, water, and soil pollution, 47% of the incinerated waste is still landfilled, in the form of incineration bottom ash (BA) [10]. To reduce the landfilling, recycling of the MSWI BA is supported in the Netherlands through the “Green Deal” [5], which restricts the use of by products as MSWI bottom ash to specific applications, by 2020. Additionally, a new standard is established named the Soil Quality Decree [31], which regulates the leaching of contaminants into the environment of those applications and by-products. Innovative wet separation treatments applied to the coarser BA fraction ensure low contamination and a more straightforward application of the by-product [2]. However, an additional waste stream is produced called MSWI filter cake (FC), by accumulating the finest and most contaminated particles removed from the bigger fractions. Due to the wet sieving and centrifugation applied during its collection, chlorides and sulfates concentrations in FC are lower than in dry sieved BA fines, whereas the presence of heavy metals is hardly reduced, restraining its applicability. To limit the release of those contaminants into the environment, the application of BA in combination with cement (PC) has been proposed [6], [14], [17], [19]. The presence of cement favors the immobilization of ions in the cement matrix, limiting the leaching of the contaminants into the environment [11], [24].

Commonly, dry sieved BA fines (BA-S, Fig. 1) have a small particle size, wide availability and often present hydraulic or pozzolanic phases affecting the cement hydration [28] and thus facilitating their application as supplementary cementitious material (SCM). However, despite the similar chemical and physical properties of FC and BA-S [4], the wet separation of FC limits its reactivity, which is mainly exhausted in contact with the washing water or during weathering [4]. Due to the limited application of this wet sieving treatment, not much knowledge is available about the behavior of FC in combination with cement. Moreover, despite its low reactivity, FC can be used, as cement substitution within a rate of 5% wt., defined as Minor Additional Constituent (MAC) [9]. The fineness of the FC and the presence of small amounts of calcium carbonate have been reported by other studies as beneficial in the cement hydration, providing the surface for nucleation and growth of reaction products [21]. Despite this, the use of FC as SCM or as MAC has so far not been evaluated in any study. Its influence on the physical properties and reaction products of the cement as well as its impact on the binder reaction degree are unclear.

Therefore, this study presents an accurate evaluation of the potential application of FC as SCM and MAC in CEM I 52.5 R, using replacements between 1% and 20% wt. The effects of FC on the early stage of hydration (reaction kinetics and phase formation) are studied by isothermal calorimetry and X-ray Powder Diffraction (XRD). The binder reaction degree and the mechanical performances are measured until the age of 91 days, by thermogravimetric analysis and according to EN 196-1, respectively. The leaching assessment of the final products is also presented according to the Dutch Soil Quality Decree [31], performing one batch leaching tests on the granular material (EN 124-57) [13].

Section snippets

Materials

Fig. 1 displays the production process of the filter cake (FC) used in this study. Initially, the bottom ash below 32 mm was divided into a fine and a coarse stream, called FBA (below 4 mm) and CBA (between 4 mm and 32 mm) respectively, by dry sieving. Due to the adhesion of fine and highly contaminated particles to the core of coarser particles, a wet separation treatment was applied to the coarser BA fraction (4 mm–32 mm) for removing those residues. From the collection of those fines (side

Materials characterization

In Table 2 a comparison of the physical properties of PC and FC is provided. FC shows a lower bulk and particle density compared to PC, in agreement with the high porous structure detected. The nitrogen adsorption measurements display that FC has a pore volume 32 times and a surface area 23 times higher than PC. These properties are mainly related to the presence of ultrafine ashes located on the core of the FC particles, denominated as “fragile zone” [40]. The poor consolidation of those

Conclusions

This study evaluates the application of MSWI filter cake (FC) as supplementary cementitious material (SCM) and Minor Additional Constituent (MAC) for CEM I 52.5 R (PC). The following conclusions can be drawn:

  • The use of FC within the allowed MAC substitution rate (5% wt.) does not modify the physical and mechanical properties of the mixtures. The differences measured are within 5% of the reference, which is well within the experimental error. Therefore the FC can be a suitable candidate for the

credit authorship contribution statement

V. Caprai: Conceptualization, Software, Formal analysis, Investigation, Resources, Writing - original draft. K. Schollbach: Conceptualization, Software, Validation, Resources, Writing - original draft. M.V.A. Florea: Validation, Supervision, Project administration. H.J.H. Brouwers: Conceptualization, Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors would like to acknowledge the financial support provided by NWO domain Applied and Engineering Science, formally NWO (Nederlandse Organisatie voor Wetenschappelijk Onderzoek), the Netherlands, under the project number 13933: “Environmental concrete based on treated MSWI bottom ash.” Moreover, for equipment support, the authors would like to acknowledge the group Chemical Engineering and Chemistry, Chemical Reactor Engineering, Eindhoven University of Technology.

References (39)

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