Sustainable concrete: Building a greener future
Graphical abstract
Introduction
Annually, around ten billion metric tons of concrete, mostly ordinary Portland cement (OPC), are produced worldwide with over 500 million tons in the United States alone (Meyer, 1992), representing two tons of conventional concrete per family in the United States. The production of OPC is predicted to reach two billion tons in the United States by 2050, which is four times higher than the level in 1990 (Crow, 2008). For each ton of OPC, approximately one ton of CO2 is produced (Hanein et al., 2018; Hasanbeigi et al., 2010). CO2 emissions are related to the energy consumption of the raw materials and external heat used during production. Given current production rates, OPC factories are responsible for 7% of total worldwide CO2 emissions (Chen et al., 2010). This extraordinary amount of cement and CO2 emissions has elevated global awareness and prompted scientists to develop alternative, sustainable concrete and cement options.
Several studies have been conducted to reduce carbon footprint and energy consumptions due to manufacturing of Portland cement. Wu et al. proposed ultrafine cement and incorporated high volume of fly ash and blast furnace slag; hence, the energy consumption and carbon emissions were reduced by 47% and 41% respectively in comparison with Portland cement (Wu et al., 2018). Some techniques and strategies for reducing carbon footprint and energy consumption were introduced by Ishak and Hashim's review work such as energy efficiency enhancement, using renewable energy, or producing an alternative Portland cement (Ishak and Hashim, 2015). Geopolymer concrete is one of sustainable cementitious materials that reduce carbon emissions by 20–50% (Maddalena et al., 2018).
Geopolymer concrete and cement is a more sustainable alternative to OPC. It is a mixture of aluminate silicate source materials such as fly ash, blast furnace slag or metakaolin, and an activating solution including either sodium silicate, sodium hydroxide (Salami et al., 2014; Li et al., 2013), or silica fume, sodium hydroxide and water (Assi et al., 2016a). Geopolymer concrete has been shown to have properties on par and even superior to OPC, including good resistance against sulfate attack and acid, high early and final compressive strength, and high resistance to fire, in the presence of external heat (Okoye et al., 2017; Zhuang et al., 2016; Wallah, 2011; Diaz et al., 2010). Recently, good compressive strength has been achieved in ambient conditions (Nath and Sarker, 2015), reducing the amount of energy needed during production and as result CO2 emissions. As a result of the reduced CO2 emissions (McLellan et al., 2011) and the use of waste materials such as fly ash, geopolymer concrete is considered a more sustainable alternative to OPC.
While geopolymer concrete is considered more sustainable than OPC, there are concerns about high costs and actual CO2 emissions that have hindered market adoption of the concrete. There is evidence to suggest that the cost of geopolymer concrete may be a significant premium to OPC, 93%–139% of the cost of OPC (McLellan et al., 2011). The major cost driver of geopolymer concrete, approximately 80%, is due to the activating solution used in the mixture (Abdollahnejad et al., 2015). To date, no new mix designs have been proposed to reduce the cost of geopolymer concrete. Additionally, there are mixed results concerning carbon emissions, calling into question the environmental benefit of geopolymer concrete. Some researchers have claimed that there are slightly less carbon emissions, but others have claimed that CO2 emissions are higher in cases where geopolymer concrete was used (Turner and Collins, 2013) (Habert et al., 2011). However, these assumptions are suspicious because the external heat was assumed to be primary, and the CO2 emissions of sodium silicate was not calculated correctly (Davidovits, 2015). Meanwhile, McLellan et al. investigated carbon emissions in Australia for geopolymer paste in comparison with OPC and found that greenhouse gas emissions were 44–64% of OPC (McLellan et al., 2011). Consistent with these results, Yang et al. showed that the reduction in CO2 emissions was between 55 and 75% when geopolymer concrete was compared to OPC (Yang et al., 2013). The research presented herein, not only proposes new geopolymer concrete mixtures that reduces the costs associated with the activating solution, but it also demonstrates significant reductions in CO2 emissions relative to OPC.
The proposed geopolymer concrete mixtures extend previous research on the use of sodium hydroxide as part of the activating solution. Several research projects have been conducted to investigate the effect of sodium hydroxide concentration on the mechanical and chemical properties of geopolymer concrete. Chindaprasirt and Chalee studied the effect of sodium hydroxide concentration on chloride penetration and steel corrosion of fly ash based geopolymer concrete. Both chloride penetration and corrosion were decreased when sodium hydroxide increased (Chindaprasirt and Chalee, 2014). The compressive strength and reaction products were found to be strongly related to sodium hydroxide concentration (Bidwe and Hamane, 2015) (Phoo-ngernkham et al., 2015). Other researchers found the setting time, conductivity, porosity, slump, flexural strength, and tensile strength were improved when sodium hydroxide concentration increased. While the extant research investigates the effect of sodium hydroxide on varied chemical and mechanical properties (Onutai et al., 2016; Hanjitsuwan et al., 2014), its effect on the cost and fuel (thermal energy) usage has not been investigated with different sodium hydroxide concentrations. Our research suggests that reduced quantities of sodium hydroxide, with the addition of OPC, not only improves the properties of geopolymer concrete but also reduces production cost and fuel usage associated with the production process, a major driver of carbon emissions.
Although previous research has been dedicated to the omission of external heat in the geopolymer concrete curing and aging periods, external heat still plays a dominant role in geopolymer concrete production. Researchers have investigated geopolymer concrete performance at ambient conditions (Nath and Sarker, 2015; Nematollahi et al., 2015). It has been discovered that early compressive strength, elastic modulus, and flexural strength properties are reduced when the elevated external heat is removed (Xie and Ozbakkaloglu, 2015). The ambient curing conditions accompanied with moisture curing show early compressive strength enhancement compared with external heat-cured specimens (Nematollahi et al., 2015). The early compressive strength and initial setting time were improved when a small proportion of OPC was used in the mixture (Nath and Sarker, 2015). While evidence suggests that geopolymer concrete can be produced without external heat, other research has suggested that the bond strength may suffer at an ambient curing temperature (Yan et al., 2015). Additionally, mechanical and structural properties, fracture behavior, the role of microwave radiation, thermal behavior, compressive strength and transport properties of geopolymer concrete, mortar or paste were investigated. The results showed that geopolymer concrete behaves better when external heat is applied (Aslaner and Osman, 2016) (Kürklü, 2016). However, there is no specific research investigating the effect of elevated heat on the cost of geopolymer concrete and fuel (thermal usage) energy in the United States.
Section snippets
Objectives
The current research addresses the cost concerns of geopolymer concrete revealed in the extant literature and establishes that geopolymer concrete represents a more sustainable alternative to OPC. New fly ash-based geopolymer concrete mixtures with an activating solution that is a combination of silica fume, sodium hydroxide, and water are proposed. The reduction of sodium hydroxide concentration in the activating solution results in significant cost reductions compared to prior mixtures. Then,
Materials and methodologies
The source of energy consumption, which is required to produce geopolymer concrete will be due to sodium hydroxide production, the activating solution preparation, and external heat for curing if it is presented. The required energy for fly ash and silica fume will not be taken into consideration because they are byproduct materials. The required energy for transportation will be considered and evaluated in future work according to the available data of the product source and assumed geopolymer
Calculation of energy requirements and predicted cost for the standard mix and corresponding Portland cement compressive strength
In this section, the energy requirements are calculated. The compressive strength will be based on the experimental results of Assi et al. (Assi et al., 2016a). The seven-day compressive strength as shown in Table 2 is 106 MPa in the presence of external heat for two days. The 90% compressive strength was achieved in less than seven days. Accordingly, a similar compressive strength is chosen for Portland cement concrete based on the Portland Cement Association (PCA) book (Kosmatka et al., 2008
Conclusions and implications for theory, and practice on cleaner production/sustainability
The studies presented herein propose new mixtures for geopolymer concrete that represent a cost-competitive, more environmentally friendly alternative to OPC. Through the use of a sodium hydroxide and silica fume-based activating solution and the addition of OPC to the mixture, the new mixtures have superior properties, lower costs, and lower fuel (thermal energy) usage than both current geopolymer mixtures and OPC. The addition of OPC to the mixture eliminates the need for external heat to
Acknowledgement
This research is based upon work supported partially by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences and Office of Biological and Environmental Research under Award Number DE-SC-00012530.
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