Project: research plan and methods
There is an ongoing worldwide effort for more sustainable alternatives to cement. The production of this material accounts for a high percentage of the World’s carbon dioxide emissions and thus more effective mitigation measures must be taken. Since climate change and global warming are key factors for sustainable development, relevant enterprises must be capable of understanding and managing the risks of greenhouse gas emissions, to guarantee their success in a competitive business environment.
Another identified societal issue is the quantity of generated waste from households and some industrial activities, despite a significant effort towards proper management of these wastes. The incineration of municipal solid waste (MSW) is one of the preferred treatment techniques since it allows the recovery of energy apart from MSW mass and volume reduction by 70% and 90%, respectively. The ash resulting from this process is a hard grey/black amorphous material, with extensive particle size distribution, that contains high levels of several toxic metals, which may leach out and accumulate over time in sensitive recipients.
In Portugal, despite other entities that co-incinerate waste with fuel, the only facilities that thermally treat the majority of MSW from Oporto and Lisbon are Lipor and Valorsul, respectively. However, despite their success in recovering energy and effectively reducing the volume of MSW, thousands of tonnes of ashes are produced yearly, which need to be further valorised into new construction materials.
A consensus has been reached that the use of alkali-activated materials (AAM) (also known as inorganic polymers or geopolymers), as the binder in the manufacture of alkali-activated concrete, is the most cost-effective, eco-friendly and technically viable candidate to replace cement. Existing structural codes for conventional concrete have been assembled and modified throughout several decades, progressively embedding the most up-to-date developments on the material’s technology and its structural behaviour. However, significant research on the use of AAM as building materials is still needed before it can be safely used in the manufacture of structural elements and included in those codes, so that stakeholders and professionals in the construction industry, knowing that there is sufficient evidence, will feel greater confidence on its use as a cement substitute.
Concerning the project’s innovation , aside from the fact that AAM, based on activated fly ash (FA) or blast furnace slag (BFS), are inherently innovative since their existence is widely unheard of within the industry, the combined use of activated municipal solid waste ashes (MSWA) and recycled concrete aggregates (RCA) from construction and demolition waste (CDW) is yet to be investigated. The alkali activation of MSWA, apart from being a technically viable alternative to cement and demonstrating a lower carbon footprint, will also answer part of the aforementioned societal issues by valorising a deeply unappreciated resource while simultaneously immobilizing its hazardous elements within the new construction material. The proposed project will:
- Evaluate the solidification/stabilization of hazardous elements in MSWA, sourced from Valorsul’s MSW incineration unit, by alkali activation;
- Further minimize the environmental impact of AAM by alkali activation of MSWA with RCA;
- Facilitate the adaptation of existing standards and structural codes to the properties of AAM, enabling their marketing and safe use.
Therefore, this project’s mission is to reduce the environmental impacts of cement-bound materials (i.e. concrete and mortar) as a response to three societal issues:
- Cement’s excessive environmental impact, by reducing its content;
- Huge production of MSWA and CDW, by incorporation in the manufacture of new construction materials;
- Heavy metal contamination of soils from leachable MSWA, via their solidification/ stabilization into a value-added product.
To accomplish this mission, three strategic objectives were defined:
- Study the viability of totally replacing cement with alkali-activated MSWA;
- Establish the influence of RCA, as a natural aggregate replacement, in the production of alkali-activated concrete and mortar specimens;
- Facilitate the introduction and commercialization of AAM as an alternative to cement.
For each of the strategic objectives, operational ones were defined.
1
For strategic objective no. 1, the research team will:
- Characterize and select the aluminosilicate precursors (i.e. MSWA, FA, BFS), with adequate reactivity for alkali activation;
- Evaluate the influence of varying contents of the alkaline activator on the strength development of different combinations of the solid precursor;
- Establish ideal curing regimens for the maximum long term mechanical and durability performance of AAM.
2
For strategic objective no. 2, the research team will:
- Analyse the role of RCA, in terms of its reactivity with the alkaline activator and ascertain the formation of new products of hydration capable of enhancing the interfacial transition zone microstructure;
- Compare and correlate the properties of RCA containing AAM with those of ordinary Portland cement recycled aggregate concrete.
3
For strategic objective no. 3, the research team will:
- Demonstrate the reproducibility of AAM by correlating their properties with those of conventional concrete;
- Strive for compatibility of the manufacturing process and properties of AAM with existing standards and specifications for conventional concrete (i.e. EN 206 and Eurocode 2);
- Demonstrate the reduced carbon footprint and economic viability of AAM by means of a life cycle assessment and life cycle cost.