Cement production from major global producers increased from approximately 2.63 billion tonnes in 2009 to 3.99 billion tonnes in 2018 (CEMBUREAU, 2019), and produces an estimated 4–8% of global CO2 emissions (Andrew, 2018a; Davidovits, 2015). Cement is a hydraulic binding material that glues aggregates together to form mortar or concrete, constituting 10–15% of the concrete mix by volume (Gagg, 2014).
The two main aspects of cement production that produce CO2 emissions are:
1. The chemical reaction that occurs when heat is applied during clinker production (clinker is the primary constituent of cement). Here, carbonates are decomposed into oxides and CO2, stoichiometrically indicating the amount of CO2 emitted for a given amount of produced CaO as shown in Equation 1.1 (Andrew, 2018a; Gagg, 2014).
CaCO3 + HEAT → CaO + CO2 (1.1)
2. Fuel combustion during heating of raw materials at temperatures over 1000 °C (Andrew, 2018a).
Besides CO2 emissions, other emissions from cement production include dust, NOx, and SO2 (CEMBUREAU, 2019). The most common cement is CEM I Portland cement (PC) (95% to 100% clinker). As clinker is the primary constituent of cement, the higher the clinker share, the higher the CO2 emissions from the chemical reaction to form clinker.
Thus, other cement types “blended cements” were introduced in addition to PC substituting some percentages of clinker with industrial by-products and/or waste such as blast furnace slag and coal fly ash (Davidovits, 2015) to improve the environmental performance of PC (CEMBUREAU, 2015a, 2015b, 2015c). These blended cements are CEM II Portland composite cement (65% to 94% of clinker), CEM III blast furnace cement (5% to 64% of clinker), CEM IV pozzolanic cement (45% to 89% of clinker), and CEM V composite cement (20% to 64% of clinker) (BS-EN197-1, 2011).
Furthermore, the need for binders with even better environmental performance than PC, which could potentially further reduce CO2 emissions, led to the development of geopolymer binders. Geopolymers result from the “polycondensation of polymeric aluminosilicates and alkali-silicates, yielding three-dimensional polymeric frameworks”
They do not necessitate extreme high-temperature kilns with high fuel expenditure as in PC production, thereby reducing energy consumption (Davidovits, 2015, 1994).
Geopolymeric raw materials include naturally occurring aluminosilicates such as metakaolin, or industrial by-products or wastes with high silica-to-alumina ratios, such as coal fly ash (CFA) and granulated blast furnace slag (GBFS) (Davidovits, 2015).
Geopolymers can be applied for environmental use in the containment of hazardous and toxic wastes, as well as in construction (Davidovits, 1994). The efficient recycling and use of silica and alumina-rich industrial wastes in geopolymers also reduces the potential environmental impacts of final disposal.
Alkali-activation is a common term employed in the reaction of aluminosilicates and alkali-silicates or alkali activators to produce a binder (Provis, 2018). Geopolymers are regarded as a sub-group of alkali-activated materials and are used interchangeably to some degree in the literature. However, the terminology of geopolymer and alkali-activated materials is under debate (Luukkonen et al., 2018). In some of the published articles in which this dissertation is based on, alkali-activated material terminology was used; however, geopolymer terminology is adopted in this dissertation as most of the cited references use this term.
Geopolymer concretes are produced by mixing geopolymer binders with fine and coarse aggregates and water, while geopolymer mortar is produced by mixing geopolymer binders with fine aggregates and water, as in cement concrete and mortar, respectively.
On the other hand, geopolymer composites contain hardened binders with a blend of inorganic, metallic, or polymeric materials. Two or three of these mixtures comprise a composite (Wu and Zhang, 2018). Geopolymer binders can be produced in two ways:
one- and two-part mixes. Two-part mix is the more conventional method of geopolymer binder production, and it occurs when a solid aluminosilicate powder reacts with an aqueous alkali activator (e.g. sodium silicate and sodium hydroxide). However, managing significant volumes of corrosive and hazardous alkali solutions in the two-part mix is impractical, resulting in one-part mix geopolymer binder which is produced when a solid aluminosilicate powder is reacted with a solid alkali activator and water. A one-part mix can be more scalable in the future as the binder is used in the same way as cement—by simply adding water (Luukkonen et al., 2018; Provis, 2018). The one-part and two-part geopolymer binder mixes are presented in Figure 1.1 and Figure 1.2.
Figure 1.1: Two-part mix geopolymer binder Solid alumino-silicate precursor e.g., coal fly ash, granulated blast furnace
slag, metakaolin etc.
Aqueous alkali-activator e.g., sodium silicate solution and
sodium hydroxide
Two-part geopolymer binder Water
Figure 1.2: One-part mix geopolymer binder
As stated earlier, geopolymers are produced from aluminosilicate precursors, thus providing possibilities for different geopolymeric precursor options. These different options have varying availability, cost, and reactivity globally; hence, they are not standardised materials, in contrast to PC. However, this also makes geopolymers locally adaptable and versatile (Provis, 2018). Furthermore, some precursors such as CFA and GBFS are in demand for blended cements, thereby creating competition for raw materials in the supply chain. In addition, factors such as the long-term availability of precursors, such as in CFA (due to the implementation of renewable energy use), incorporation of sustainability measures in environmental regulations and industries, and varying composition of raw materials (for example, type of coal combustion and source of coal) (Wescott et al., 2010), hinder the future development of geopolymers. Nevertheless, this also leads to the exploration of locally suitable precursors and predisposes the development of geopolymers as a locally adaptable binder than a globally standardised material. In contrast to these concerns, the advantages of geopolymers over PC include acid and temperature resistance, high strength and durability, cold and quick setting, stable bonding of heavy metals and harmful substances, and simple manufacturing techniques (Weil et al., 2005). Compared to PC, geopolymers are considered more environmentally sustainable, as they lead to 70–80% CO2 reduction (Davidovits, 2015).
However, due to differences in precursors, alkali activators, system boundaries, methodologies, transportation, allocation, among others, environmental assessment studies of geopolymers vary and make comparisons challenging.
The most common methodology used in conducting environmental assessments of geopolymers is life cycle assessment (LCA). LCA “addresses the environmental aspects and potential environmental impacts throughout a product’s life cycle from raw material acquisition through production, use, end-of-life treatment, recycling and final disposal”
(ISO 14040, 2006). Geopolymer production includes all or some of these life cycle phases which need to be considered when assessing environmental performance.
Solid alumino-silicate precursor e.g., coal fly ash, granulated blast furnace
slag, metakaolin etc.
Solid alkali-activator e.g., sodium silicate powder
One-part mix binder Water