Alkali-activated binders based on municipal solid waste incineration bottom ash

Abstract

Municipal solid waste incineration (MSWI) is the most widely used solution in those countries where landfilling areas are limited. Incineration allows reducing the total volume of waste (up to 90%) and generating energy resulting from combustion. The main by-product generated in waste-to-energy plants is known as incineration bottom ash (IBA), which is a heterogeneous mixture of ferrous and non-ferrous metals, ceramics, and glass. IBA is classified as a non- hazardous material due to its composition rich in calcium oxide, silica, and iron. IBA composition and morphology are very similar to natural siliceous aggregates after an ageing treatment where the weathered bottom ash (WBA) is obtained. This maturation process makes feasible the WBA valorisation as a secondary aggregate in the field of construction and civil engineering. Moreover, the high percentage of glass and aluminium found in the WBA would allow its valorisation as a precursor in the alkali-activated binders (AABs) formulation. The main goal of this PhD thesis was the scientific and technological development of new AABs based on the alkali activation of WBA (AA-WBA binders), to reduce the use of ordinary Portland cement (OPC) in building and civil engineering fields. In this sense, this aim is related to the use of more sustainable cement-based materials, which promote the circular economy and zero-waste principle through the valorisation of WBA. The potential of WBA as a precursor in the AA-WBA binders’ formulation was evaluated along with the PhD thesis through different studies that can be classified into four blocks. The first block was based on the evaluation of the WBA potential as a precursor in AABs based on its particle size. This study demonstrated the variability in the reactive SiO2 and Al2O3 availability as a function of the particle size. The potential of the entire fraction (EF) and the 8-30-mm fraction highlighted the possible use of them as precursors in the AABs formulation. The second block of this thesis was focused on the study of AA-WBA binders using the WBA as a sole precursor. Mixtures of sodium silicate (WG) and NaOH (2M, 4M, 6M, and 8M) were used as alkaline activator solutions to assess the effect of the NaOH concentration on the final properties. It was demonstrated the possibility of developing AA-WBA. The influence of alkaline activator solution concentration on the final properties of the AA-WBA was evidenced, obtaining better mechanical performance with the use of the WG/NaOH 6M solution. The results revealed the enhancement in the mechanical properties when the 8-30-mm fraction was used. However, the environmental results revealed

arsenic and antimony leaching values that require further research to validate the environmental feasibility of AA-WBA. In the third block, the 8-30-mm fraction was mixed with other precursors with greater availability of Al2O3 (metakaolin and PAVAL®). The main purpose was to improve the mechanical properties and the heavy metal stabilisation effect of the AA- WBA obtained in the second block. In both cases, mechanical performance was improved due to the inclusion of Al2O3. However, the environmental properties continued to show leaching values that did not ensure the environmental viability of the AA-WBA binders. Finally, the fourth block of the thesis was focused on carrying out an environmental and ecotoxicological assessment to validate the use of AA-WBA binders as construction material. The results showed a medium-low level of ecotoxicity in the AA-WBA formulated with the 8-30-mm fraction, similar to the binders activated with MK (AA-MK).

El principal subproducte generat durant la incineració de residus sòlids urbans es coneix com a cendra de fons. La seva composició és molt similars als agregats silícics naturals després d’un tractament d’envelliment on s’obté la cendra de fons madurada (weathered bottom ash; WBA segons les sigles angleses). El seu alt contingut en vidre i alumini el converteixen en un potencial candidat com a precursor en la fabricació d’aglutinants activats alcalinament (alkali-activated binders, AABs segons les sigles angleses). L’objectiu principal d’aquesta tesi doctoral va consistir en el desenvolupament de AABs mitjançant l’activació alcalina de WBA (aglutinants AA-WBA). El potencial de la WBA i els aglutinants AA-WBA es va avaluar mitjançant diferents estudis que es poden classificar en quatre blocs. Al primer bloc es va avaluar el potencial de WBA com a precursor en funció de la seva mida de partícula. Aquest estudi va demostrar el potencial de la fracció sencera i de la fracció 8-30 mm. El segon bloc es va centrar en l’estudi d’aglutinants AA-WBA que utilitzaven el WBA com a únic precursor. Es va evidenciar la influència de la concentració de la solució activadora alcalina en les propietats finals dels aglutinants AA-WBA. Els resultats van revelar la millora de les propietats mecàniques quan es va utilitzar la fracció 8-30 mm. No obstant, els resultats ambientals van revelar valors de lixiviació d'arsènic i antimoni que requerien la validació a nivell ambiental dels aglutinants. Al tercer bloc, la fracció 8-30 mm es va barrejar amb altres precursors rics en d’Al2O3 (metakaolin i PAVAL®) per millorar les propietats mecàniques i l’estabilització de metalls pesants dels aglutinants obtinguts al segon bloc. En ambdós casos, es va millorar el rendiment mecànic, tot i que les propietats ambientals van continuar mostrant valors de lixiviació que no asseguraven la viabilitat ambiental dels aglutinants AA-WBA. Finalment, al quart bloc es va realitzar una avaluació ambiental i ecotoxicològica per validar l’ús d’aglutinants AA-WBA com a material de construcció. Els resultats van mostrar un nivell mitjà-baix d’ecotoxicitat a l’AA-WBA formulat amb la fracció de 8 a 30 mm, similar als aglutinants activats amb MK (AA-MK).