Publications II, III, and IV were executed as part of the Urban Infra Revolution project (project number: UIA02-155). The project lasted three years from 2017 to 2020 and was co-financed by the European Regional Development Fund through the Urban Innovative Actions initiative. The primary objective of the project was to find solutions for reducing CO2 emissions in urban construction development by incorporating sustainability and circular economy in future construction schemes. This was accomplished by developing novel FRGCs by recycling and reusing local industrial wastes for cement substitutes.
Environmental performance calculations were used to guide the design and development of geopolymer composites to ensure better environmental performance compared to conventional PC-based products.
As an article-based dissertation, this dissertation summarises and outlines the main features and results of the four publications presented in Section 1.2. By comparing and linking the findings of these publications, novel findings and conclusions were obtained.
This dissertation comprises five sections:
Section 1. Introduction – This section provides background and an overview of the topic, objectives, scope, and limitations of the current research. The research process and dissertation structure are described.
Section 2. State of the art – This section identifies and reviews the academic literature that is most relevant to the environmental performance of geopolymers and outlines the key differences between the LCA studies performed on geopolymers. This section also provides an overview of the precursors, alkali activator, fibre reinforcement, curing conditions, and carbonation, respectively, within the scope of this dissertation.
Section 3. Materials and Methods – The principle of LCA methodology is discussed in this section, and the LCA studies performed in Publications I, II, III, and IV are described.
The goals of the different publications that make up this dissertation, functional units, system boundaries, life cycle inventories, and life cycle impact assessments are provided.
Section 4. Results and discussions – In this section, the key results of the research performed based on the research objectives of this dissertation are highlighted and discussed in a wider context.
Section 5. Conclusions – Recap of the results of this dissertation and conclusions of the research.
2 State of the art
The development of geopolymer binder/concrete has evolved over time. The first geopolymer binder composed of metakaolin-750, slag, and potassium silicate in a ratio of 1:1:2. This mix design was not considered a worthy competitor to PC because it was costly and not environmentally friendly owing to high amount of potassium silicate. Thus, it was promoted for specific niche applications. Subsequently, a second category geopolymer binder, known as a rock-based geopolymer binder, was developed comprising metakaolin-750, slag, volcanic tuff, and alkali-silicate in the ratio 1:1:2:1. The alkali-silicate in the second mix design was reduced to 20% by weight from 50% by weight in the first geopolymer binder. Furthermore, another rock-based geopolymer mix design was developed with slag, weathered granite, and alkali-silicate in the ratio 1.5:3.5:1, reducing alkali-silicate to 17% by weight. This second rock-based cement mix design has high mechanical strength (100–125 MPa on day 28) and becomes a more competitive option with 80% lower CO2 emissions if slag is considered waste with no allocation, and 70% lower CO2 emissions if slag is allocated environmental burden from previous processes (Davidovits, 2015). Geopolymer development evolved to a third category, based on low-calcium CFA. This third category is of two types: alkali-activated fly ash material and slag/fly ash-based geopolymer binder. The former requires reacting CFA with NaOH and heat curing at 60–80 °C, while the latter involves obtaining geopolymer binder from CFA, GBFS, and silicate solution at room temperature at a ratio of 5:1:1 with the amount of alkali-silicate reduced to 15–20% by weight from 50% by weight of the first geopolymer binder. This slag/fly ash-based geopolymer binder can produce a compressive strength of 100 MPa at 28 days; however, for a lower strength of approximately 40 MPa, the alkali-silicate can be reduced to 10–15% by weight with a ratio of 8:1:1. Davidovits (2015) contended that alkali-activated fly ash should not be qualified as a geopolymer because of its causticity. However, this is still debated, as many studies have labelled it a geopolymer.
The following subsections present an overview of specific precursors, activators, fibre reinforcement, curing, and carbonation, as included in the scope of this dissertation. In addition, this chapter provides an overview of research on the LCA of geopolymers.
2.1
Aluminosilicate precursors2.1.1 Coal fly ash (CFA)
CFA is the most common solid aluminosilicate precursor and is usually used with GBFS in a geopolymer mix design (Luukkonen et al., 2018). CFA is a coal combustion residue with an estimated production of 92 million tonnes in 2016 in the EU and an estimated 50% utilisation rate in the construction industry (ECOBA, 2016). CFA is commonly captured from flue gases by electrostatic precipitators or other particle filtration equipment before flue gases enter chimneys (Zhuang et al., 2016). CFA is of two classes:
class C, high calcium content, and class F, low-calcium content. Class C CFA is mostly
produced from the combustion of lignite coal and has between 50–70% by weight content of SiO2, Al2O3, and Fe2O3, and more than 20% by weight content of CaO; Class F CFA is mainly produced from the combustion of anthracite or bituminous coal and has over 70% by weight content of SiO2, Al2O3, and Fe2O3, and less than 10% by weight of CaO (Zhuang et al., 2016). The alumina and silica contents in CFA make it suitable for geopolymer production which typically demonstrates mechanical properties comparable to those of hydrated PC (Zhuang et al., 2016). However, Class C CFA is rarely used in geopolymers because of its low availability and rapid setting (Luukkonen et al., 2018).
2.1.2 Granulated blast furnace slag (GBFS)
GBFS is a by-product of pig iron production in blast furnaces and is generally utilised as a calcium-rich aluminosilicate precursor. Approximately 17 million tonnes of GBFS were produced in Europe in 2018, with approximately 13 million tonnes used in cement and construction, and 3 million tonnes used in road construction (Euroslag, 2019). GBFS has approximately 42% silica and alumina content (Adesanya et al., 2018). Geopolymers made from GBFS have a rapid setting time which is largely unfavourable, but it can be used to enhance the reactivity of low-calcium CFA (Hasnaoui et al., 2019). GBFS is relatively consistent in physical and chemical properties; however, compositions still vary between ores and furnaces (Duxson and Provis, 2008).
2.1.3 Metakaolin
Metakaolin has 97.5% silica and alumina content (Niu et al., 2020) which makes it a great precursor for geopolymers. Metakaolin is produced by heating kaolinite in a regulated manner at temperatures between 500 and 800 °C to expel hydration water. This heating process is referred to as calcination and typically occurs in rotary kilns or fluidised bed processes, which reduce the calcination duration from hours to minutes (NLK, 2002).
Kaolinite is a hydrated aluminium disilicate and virgin material formed by the weathering of aluminosilicate minerals (NLK, 2002). Kaolinite production in Europe is estimated to be 35 million tonnes (Euromines, 2019) and has been used to produce ceramics for centuries (NLK, 2002).
2.1.4 Mine tailings
Mine tailings are waste generated from mining and quarrying, accounting for 26% of the total waste generated by economic activities and households in the EU (Eurostat, 2020a), and are currently underutilised. Mine tailings are collected in large sediments, which can lead to several environmental issues including environmental contamination and pollution (Sedira et al., 2017). In addition to the environmental risk involved in storing them, storage and maintenance are also resource intensive. Mine tailings vary in type and composition and are generally not highly reactive precursors; thus, they are largely used in combination with reactive precursors (Niu et al., 2020). Mine tailings often require pre-treatment before use which can include mechanochemical activation, such as grinding,
which can be more energy-efficient and less time consuming than high-temperature pre-treatments (Niu et al., 2020).