Life cycle assessment of Hybrid alkali-activated cement production with red mud as an alkali activator

doi:10.21203/rs.3.rs-3536065/v1

The production of Hybrid Alkali Activated Cement (HAAC) has generated considerable interest in environmental issues. In this research, the environmental impacts of utilizing red mud (RM) as a partial activator of alkali-activated ground granulated blast furnace slag (GBFS) in HAAC production have been evaluated. A contribution analysis was carried out using life cycle assessment (LCA) to assess the environmental significance of six important substances in HAAC production. A comparative analysis of the environmental consequences of producing Ordinary Portland Cement (OPC) and two HAACs using various activators in the same plant was conducted. The results showed that the calcination and preparation of alkali-activated cementitious materials are the two processes with the highest environmental impacts. Marine ecotoxicity was identified as the primary impact category, followed by freshwater ecotoxicity and fossil depletion. Compared to OPC, HAAC yields superior benefits in the majority of environmental impact categories. Additionally, the inclusion of RM as a partial alkali excitant to HAAC results in even more pronounced environmental benefits when compared to NaOH alone, particularly in terms of cleaner production areas.

Hybrid alkali-activated cement

Red mud

Alkali activator

Life cycle assessment

Environmental impact

Ordinary Portland Cement (OPC) has become a popular binder for mine backfilling due to its excellent cementing properties. Its demand has significantly increased in recent years, contributing to improving the quality of the Cemented paste backfill (CPB) in underground mining areas (Zhang, Zhang, et al. 2022). However, OPC production is associated with severe environmental impacts and is responsible for 6%-8% of global CO₂ emissions from thermally decomposing limestone and the combustion of fossil fuel (Zheng, Zhang, et al. 2021). Around 0.80 tons of CO₂ are released for each ton of OPC produced, contributing to global environmental problems by causing an imbalance in the ecosystem (Ninan, Radhakrishnan, et al. 2023). In addition, high electricity consumption for grinding raw materials and using coal and clinker for producing fine cement, as well as the high heat consumption in the calcination of limestone and sand for the production of clinker, further aggravates the scenario (Madlool, Saidur, et al. 2011). Cement manufacturing also involves high costs and consumption of natural resources, including limestone, clay and laterite (He, Zhu, et al. 2019), raising sustainability concerns. Thus, it is imperative to effectively decrease cement consumption in raw materials production and CPB processes. Many researchers have used industrial solid wastes or by-products as partial substitutes for raw materials to develop sustainable construction materials for improving eco-friendliness.

Alkali-activated materials are produced by the reaction between aluminosilicate-rich materials and alkali activators to generate a novel hydraulic binder widely used in composites for construction due to its performance and environmental benefits (Almutairi, Tayeh, et al. 2021). The production of alkali-activated materials is associated with lower CO2 emissions than OPC (Luukkonen, Abdollahnejad, et al. 2018). It is worth noting that incorporating alkali activators with solid wastes (such as GBFS) as precursors exhibit comparable or even superior mechanical and durability properties to OPC in the same conditions (Zakira, Zheng, et al. 2023). This is mainly because alkali activators can provide a higher alkaline environment, accelerating the hydration reaction and facilitating better performance(Chen, Zhou, et al. 2023). Hybrid alkali-activated cement (HAAC) consists of a high proportion of supplementary cementitious material (SCM), a low proportion of OPC, and a small quantity of alkali activators (Palomo, Fernández-Jiménez, et al. 2007). It is sustainable and offers engineering advantages, such as low heat of hydration and resistance to chemical attacks (Mohapatra and Pradhan 2022). HAAC normally utilizes sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH) solutions as activators. However, using liquid activators on construction sites is potentially dangerous and uneconomical (Kang 2012). Therefore, discovering sustainable and safer alkali activators is crucial.

More recently, researchers have investigated developing activators from alkaline solid wastes(Yaseri, Jafarinoor, et al. 2023), allowing the waste alkali contained therein to be used as an activator instead of alkalis. This solves a large amount of solid waste disposal and is environmentally friendly (Guo, Zhao, et al. 2022). Živica (2006) used silica powder as a waste alkali activator and found its positive effect on enhancing the production of calcium silicate hydrate (C – S – H) and densifying the pore structure formed by the activated binder. Torres and Puertas (Torres-Carrasco and Puertas 2015) used waste glass as a source of silica to replace the commercial activator sodium silicate hydrate in fly ash geopolymers. (Geraldo, Fernandes, et al. 2017) demonstrated the usefulness of using rice husk ash (RHA) in sodium hydroxide by determining the optimal hydrothermal process of sodium silicate solution.

Red mud (RM) is a strong alkaline solid waste produced during alumina extraction (Li, Zhang, et al. 2021). The RM generated per ton of alumina produced is estimated to be between 1.5 and 2.5 tons and is increasing globally to 200 million tons annually (Gao, Zhang, et al. 2021). Ground granulated blast furnace slag (GBFS) is another industrial by-product of the steel industry, with an annual production of about 125 million tons (Manojsuburam, Sakthivel, et al. 2022). (Tian, Wang, et al. 2022) explored the strength development and microstructure of alkali-activated slag/red mud (AASR) mortars with varying RM and GBFS ratios using sodium silicate as the alkali activator. (Zhu,, Ji-xiang,, et al. 2018) found that the compressive strength of AASR slurry was higher than that of AASC or alkali-activated RM. (Choo, Lim, et al. 2016) used RM as a solid alkali activator for developing one-part alkali-activated fly ash geopolymers and investigated their resulting compressive strength. These studies suggest that RM can be used not only as a precursor in alkali-activated binders but also as an alternative activator to strong alkali activators. Most previous studies focused only on the hydration process, microstructure, and mechanical properties of such mortars, and the environmental impact of the whole process was not assessed.

Life cycle assessment (LCA) is a widely used method to evaluate the environmental consequences related to a product (Beylot, Bodénan, et al. 2022). (Galvez-Martos and Schoenberger 2014) demonstrated that LCA results of the cement plant depended on initial assumptions and were determined by a calculation related to global warming. (Meshram and Kumar 2022) conducted a comparative LCA analysis of ordinary silicate cement and geopolymer cement in the context of India and confirmed that geopolymers appear to be more sustainable than conventional cements. The comparative LCA analysis of solid waste cement and conventional cement conducted by (Tao, Lu, et al. 2022) revealed that the impact of solid waste cement on each environmental type was less than that of conventional cement. Even though LCA can describe the environmental consequences of the cement industry, some limitations associated with the previous research findings hinder its appropriate applicability. Previous studies have shown the associated environmental benefits rather than the excellent performance of HAAC. Also, the significance of the primary data was neglected in many of the past research studies, resulting in a degree of uncertainty.

Therefore, this research aims to evaluate the environmental impact of using HAAC as a filling cementitious material, where incorporating GBFS as an SCM and RM as a partial substitute for NaOH as an alkali activator. The environmental impact of HAAC using RM as the main activator was also evaluated while considering its strength. The LCA analysis was based on the primary data obtained from a cement plant in Changsha (Hunan Province), China. Considering the situation of the cement plant, 13 impact categories were considered in this study, i.e., global warming potential (GWP), ozone depletion potential (ODP), ozone formation potential (OFP), particulate matter formation potential (PMFP), terrestrial acidification potential (TAP), freshwater eutrophication potential (FEP), marine eutrophication potential (MEP), terrestrial ecotoxicity potential (TETP), freshwater ecotoxicity potential (FETP), marine ecotoxicity potential (METP), human toxicity potential (HTP), mineral depletion potential (MDP), and fossil depletion potential (FDP). The in-depth contribution analysis on substances was also carried out. Moreover, the LCA results were compared to OPC production at the same cement plant.

LCA is an advanced tool widely used to assess environmental impact and resource consumption throughout the life cycle of a product. Based on ISO 14040/14044 (Pryshlakivsky and Searcy 2013), four steps are followed, i.e., goal and scope definition, life cycle inventory analysis, life cycle impact assessment (LCIA), and life cycle interpretation (Tam, Zhou, et al. 2022). The first three steps are presented in the subsequent sub-sections, whereas the last step is discussed in Section 3.

2.1. Material and strength testing

GBFS and RM were from a steel smelter and alumina plant in China, respectively. OPC was supplied by a Hunan cement company. A reagent-grade sodium hydroxide (SH; 99.5%) was used as the activator. Tailings used as cement reinforcement come from a mineral processing plant. The SH beads were dissolved in distilled water to produce an activator solution with the desired concentration (10M). Table 1 shows the chemical composition of RM, GBFS, and OPC. From the table, the silica/alumina molar ratio of GBFS is 1.95, while the chemical composition is mainly SiO2, CaO and Al2O3. The silica/alumina molar ratio of RM is 0.83, and the chemical composition is mainly Fe2O3, Al2O3, SiO2 and Na2O. OPC mainly comprises CaO and SiO2.

Table 1

Chemical composition of materials
Constituents		Chemical analysis (% by mass)
Constituents	Fe₂O₃	SiO₂	CaO	Al₂O₃	MgO	SO₃	K₂O	Mn₂O₃	Na₂O	TiO₂	CuO	P₂O₅
GBFS	1.03	34.5	34	17.7	6.01	1.64	1.2	1.5	0.41	0.5	-	0.03
RM	34.5	19	2.57	23	0.2	0.33	0.08	-	12.05	1.72	-	0.15
OPC	2.77	20.9	63.2	5.45	2.7	2.54
Tailings	15.04	35.63	17.06	5.99	7.02	15.66	1.20	0.21	0.50	0.24	0.14	0.09

The ground and dried HAAC were mixed in proportion to their composition, as given in Table 2. The HAAC is subsequently combined with tailings and water in the ratios illustrated in Fig. 1. The amount of each material was calculated based on the number of test groups and the volume of the specimen mold. The mixture of HAAC, tailings and water was stirred well in a mixer according to the mix proportions. The mixed pastes were then cast into standard molds of 50 mm in diameter and 100 mm in height to prepare the sample. The specimens were demolded after 24 h and placed in a curing chamber at a curing temperature of 21°C and relative humidity of 90%. The specimens were cured until desired testing ages of 3, 7, and 28 days. Two samples per group were prepared and tested. The average value was taken as the representative of the respective mix. The binder containing OPC was used as a reference (control) group. Finally, the compressive strength of backfill slurry was determined.

Table 2

Mix proportion of HAAC (wt%)
Components	GBFS	OPC	Alkali activator
Components	GBFS	OPC	Red mud	NaOH
Content	52.5%	25%	15%	7.5%

2.2. Goal and scope definition

The main objective of this paper is to assess the environmental impact of HAAC using RM as the main alkali activator and GBFS as an SCM. The functional unit (FU) is 1 kg of HAAC. The alkali-activated GBFS was used as SCM and RM with NaOH as an alkali activator. The SCM and basic activating agent were taken as 52.5% and 22.5% by weight, respectively. A 5M aqueous NaOH solution with a concentration was prepared in this study. NaOH was produced at the chlor-alkali chemical plant near the studied cement plant. The chlor-alkali process primarily consists of mercury, diaphragm, and membrane cells. The membrane method has been preferred in recent years due to its higher energy efficiency and lower environmental damage. Thus, it was used in this research. The impact associated with RM and GBFS was not considered except for the impact incurred during grinding and alkali activation. “From cradle to gate” was employed as a boundary condition in this paper, including extraction of raw materials, preparation of raw meal, alkali-activated SCM preparation, calcination, and production of HAAC. Detailed procedures associated with cement production are described in the literature (Her, Park, et al. 2022). The system boundary is shown in Fig. 2.

2.3. Life cycle inventory

Life cycle inventory (LCI) analysis quantifies the inputs and outputs of all production processes in the studied system boundary (Petek Gursel, Masanet, et al. 2014). In this research, the inventory data of the foreground system were gained in a typical cement plant located in Changsha, Hunan Province, China. Other limited data regarding the background system were collected from the Ecoinvent database (Frischknecht, Jungbluth, et al. 2004). The LCI, covering the inputs and outputs in the whole life span, is presented in Table 3. All processes, such as the extraction of raw materials, energy depletion, and environmental emissions, are based on the FU in this paper.

However, errors existing in primary data and inevitable data gaps could result in uncertainties in the data, which are expressed as standard deviations. In order to quantify uncertainties, it was assumed that measurement data is lognormally distributed in LCA analysis. It should be stated that the square of the geometric standard deviation (GSD2) covers a 95% confidence interval, which is a typical feature of the lognormal distribution. It can be described as Eq. (1).

Where m represents the measurement data of inputs and outputs, and GSD2 is defined according to the pedigree matrix originally researched by (Weidema 1996). It is given as Eq. (2).

U1, U2, U3, U4, and U5 represent uncertainty factors of reliability, completeness, temporal correlation, regional correlation, and other technological relationship, respectively. Ub is the basic uncertainty factor. Corresponding scores are acquired from the pedigree matrix in line with the data source and type. According to the calculation method above, detailed information about uncertainties is given in Table 3.

Table 3

Life cycle inventory of HAAC based on the defined FU.
	Substance	Unit	Amount	GSD²
Raw materials	Explosives	kg	9.75E-06	1.26
	Limestone	kg	3.65E-01	1.30
	Clay	kg	4.65E-02	1.28
	Laterite	kg	1.57E-02	1.06
	Gypsum	kg	1.69E-02	1.29
SCM	Granulated Blast Furnace Slag	kg	5.250E-01	1.34
	Red Mud	kg	1.50E-01	1.59
	NaOH	kg	7.65E-02	1.13
Energy consumption	Crude oil^a	L	4.03E-04	1.25
	Electricity^b	kWh	8.44E-02	1.26
	Raw coal	MJ	1.31E + 00	1.30
Emission to air	Carbon dioxide	g	5.92E-01	1.65
	Carbon monoxide	g	3.26E-04	1.64
	Sulfur dioxide	g	2.16E-04	1.65
	Nitrogen oxides	g	3.46E-03	1.68
	Particulates	g	2.84E + 00	1.64
	Lead	kg	1.39E-07	1.62
	Chromium	kg	2.49E-08	1.58
	Arsenic	kg	1.65E-07	1.32
	cadmium	kg	1.39E-08	1.65
	Mercury	kg	7.15E-09	1.64
Emission to water	Industrial wastewater	kg	1.38E-01	1.52
	COD	kg	3.06E-05	1.53
	Liquid waste	kg	6.54E-04	1.52
Emission to soil	Solid waste	kg	7.06E-03	1.29

^a The calorific value of crude oil is 35.96 kJ/L.

^b About 30% of electric energy is generated from thermal power, wherein raw coal consumed is considered in the system boundary.

Table 3 shows that the main inputs during OPC production are raw materials (i.e., limestone, sandstone, iron ore, gypsum) and energy (i.e., raw coal, crude oil, and electricity). As previously mentioned, the OPC percentage in HAAC with RM as the main activator is 25 wt.%. The involved energy consists of thermal (raw coal and crude oil) and electricity. Raw coal and crude oil are mainly depleted during calcination and transportation, respectively, whereas electricity is consumed during the crushing, conveying, and grinding of raw materials, as well as the preparation of NaOH.

Besides, the main outputs cover all environmental emissions, including air, water, and soil. Most of the emissions to the atmosphere come from the calcination process, such as carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter (PM) (Josa, Aguado, et al. 2007). Other pollutants are also released, such as different substances toxic to land, freshwater and seawater organisms. The transportation of materials and fossil fuels depends on the distances between the suppliers and the cement plant (Stafford, Dias, et al. 2016).

2.4. Life cycle impact assessment

Life cycle impact assessment (LCIA) comprises quantification of the environmental effect of a product. In this research, LCA software SimaPro 9.0.0 was used to analyze environmental consequences. SimaPro is advanced and powerful software that can be associated with many databases, such as the Ecoinvent database (Herrmann and Moltesen 2015). The Ecoinvent database provides various background data sources in the cement and concrete industries. The ReCiPe 2016 Midpoint (H), a damage-oriented method, was chosen for quantitative analysis, indicating the direct environmental impact of the whole life span. As previously illustrated, the LCA model quantified results into 13 impact categories, i.e., GWP, ODP, OFP, PMFP, TAP, FEP, MEP, TETP, FETP, METP, HTP, MDP, and FDP. Ecosystems, human health, and resources are the impact categories covered.

3.1. Comparison of HAAC and OPC cement strength

The mortar strength test results of HAAC incorporating RM as the main activator are presented in Table 4. It is observed that the strength of HAAC is higher than that of OPC at 3d, 7d, and 28d, and the compressive strength increased with the curing time.

Table 4

Comparison of HAAC and OPC strength results
Specimen	Flexural strength (MPa)
Specimen	3d	7d	28d
OPC	3.25	4.04	5.21
HAAC	3.28	4.39	5.56

3.2. LCIA results

The environmental effects of each input and output can be expressed in the LCIA. It is well-known,\ that the production of HAAC incorporating GBFS and RM would affect ecosystems, human health, and resources (Sevgili, Dilmaç, et al. 2021). Therefore, the environmental consequences are divided into 13 impact categories in this study, i.e., GWP, ODP, OFP, PMFP, TAP, FEP, MEP, TETP, FETP, METP, HTP, MDP, and FDP, based on the ReCiPe 2016 Midpoint (H) approach. The data corresponding to the extraction of raw materials, energy depletion, and environmental emissions were assigned to these categories in SimaPro 9.0.0 to obtain LCIA results.

3.2.1. Characterization results

Characterization results for each impact category were scaled to 100% to evaluate the impact of each process on the individual categories. Identifying which process or category accounts for more significant environmental effects is difficult. The total processes were divided into five stages: (i) Extraction of raw materials, (ii) preparation of raw meal, (iii) preparation of alkali-activated SCM, (iv) calcination, and (v) final production of HAAC. Characterization results of each phase of HAAC are shown in Fig. 3. Table 5 presents the values of characterized results that are calculated using the ReCiPe 2016 Midpoint (H) method. The uncertainty analysis was also carried out – the results are expressed by GSD2 values in Table 5.

Table 5

LCIA results based on per FU (Characterization).
Impact categories	Unit	Amount	GSD²
GWP	kg CO₂ eq	4.18E-01	1.27
ODP	kg CFC11 eq	8.97E-08	1.36
OFP	kg NO_x eq	5.54E-04	1.29
PMFP	kg PM2.5 eq	2.27E-04	1.18
TAP	kg SO₂ eq	5.24E-04	1.29
FEP	kg P eq	8.04E-05	1.21
MEP	kg N eq	7.63E-06	1.30
TETP	kg 1,4-DCB	1.25E + 00	1.33
FETP	kg 1,4-DCB	1.36E-02	1.42
METP	kg 1,4-DCB	1.81E-02	1.40
HTP	kg 1,4-DCB	8.20E-02	1.28
MDP	kg Cu eq	1.34E-03	1.56
FDP	kg oil eq	1.12E-01	1.18

Figure 3 shows that the calcination process has predominant contributions to the five impact categories of TETP, FETP, METP, MDP and FDP – especially for category FDP, which is responsible for 85.43% of the total FDP. This is attributed to the large consumption of raw coal during this phase. Alkali-activated SCM preparation significantly affects GWP, ODP, TAP, FEP, and MEP. It has a contribution rate of 71.57% to the impact category ODP, followed by FEP (51.35%), MEP (49.17%), TAP (48.71%), and GWP (44.23%). SCM preparation involves grinding of SCMs (GBFS and RM) and NaOH preparation using the chlor-alkali approach, which results in electricity consumption and emissions of chlorinated hydrocarbons and other pollutants.

Compared to the above-mentioned two phases, raw meal preparation has little effect on most impact categories, expected for OFP and MDP at 20.59% and 14.50%, respectively. Further, the environmental impact due to HAAC production is responsible for 13.60% of impact category MDP, which produces a lower impact on other categories. It also can be observed that the effect involved in extracting raw materials is negligible.

3.2.2. Normalization results

In order to comprehensively compare different impact categories, normalized analysis was carried out by quantifying the contribution of each category to the total environmental consequences (Fig. 4). The largest environmental load was induced by the category METP. Moreover, FETP and FDP accounted for significant impacts, while the effects of other impact indicators are relatively lower. These findings suggest that greater attention should be paid to water pollution, including freshwater and marine water. Meanwhile, limiting and controlling massive fossil fuel consumption is also imperative. The development of alternative fossil fuels can be a good option to reduce emissions associated with fuel combustion.

3.3. Contribution analysis

In order to determine the specific pollutants, a contribution analysis of substances was conducted. The contributions of primary substances to several important impact categories are shown in Fig. 5. As well-known, GWP is widely analyzed and researched by scholars and decision-makers in cement production (Benhelal, Zahedi, et al. 2013). The measurement to GWP is CO2 equivalent. It has been reported that nearly 0.80-1.00 kg of CO2 is released per kg of produced OPC (Feiz, Ammenberg, et al. 2015). Due to the enormous CO2 emissions from cement clinker production, these emissions are the predominant contributions to the GWP category. Huntzinger and Eatmon showed that emissions from the cement kiln are the major contributors to GWP, even half (Huntzinger and Eatmon 2009). Nevertheless, in this study, emissions were derived from these production steps that could exacerbate the effect of GWP. Figure 5 shows that the emissions from the kiln (i.e., clinker unit) accounted for merely 34.78% of the total GWP (4.18E-01 kg CO2 eq) in this study, followed by raw materials (25.22%) and NaOH (16.52%). This is attributed to the lower raw material amounts used to produce clinker, resulting in less CO2 emissions. Other units, i.e., GBFS and RM, have small impacts on GWP since only their grinding process is involved.

ODP is characterized by increased ozone layer destruction due to unnatural emissions of stubborn chemicals containing chlorine or bromine atoms (Goedkoop 2012). ODP is scaled using CFC 11 equivalent (kg CFC 11 eq), which is called “Trichlorofluoromethane” or “Freon 11”. It is primarily determined by chlorinated or brominated hydrocarbons released from the fossil fuels combustion (clinker unit) and preparation of NaOH. Because of this, these two units are the main contributors to the category ODP. It is worth mentioning that this value of ODP was usually less considered in the previous studies. The discrepancy can be explained by considering the impacts of NaOH preparation by the chlor-alkali method.

Freshwater and marine eutrophication are usually measured by phosphorous and nitrogen equivalent (kg P eq and kg N eq), respectively. However, these two categories (FEP and MEP) demonstrated different results. Figure 5 shows that FE is mainly affected by raw material acquisition, whereas ME is primarily determined by emissions from clinker production. This is because FE relies on phosphorus-containing compounds (Li, Geng, et al. 2022). In contrast, MEP depends on nitrogen compounds generated from the oxidation process of nitrogen molecules in combustion air and the oxidation process of nitrogenous compounds in fossil fuel (NOx). It should be noted that clinker production comprises the depletion of a mass of fossil fuels during calcination, and thus clinker unit becomes the major contributor to MEP. In addition, raw materials obtaining occupies about 17.96% of MEP.

FDP measurement is based on the oil equivalent (kg oil eq). In this study, it reached 1.12E-01 kg of oil eq. Fossil fuel, especially raw coal, is mainly used in the calcination stage to provide intense thermal energy for sintering raw materials in the cement kiln. Figure 5 shows that the clinker unit is the greatest contributor to FDP, accounting for 70.43%. In addition, the impacts caused by GBFS and RM occupy lower proportions, only around 12.00%, due to lower energy consumption in their grinding process than calcination in the kiln.

The toxicity-related environmental effects are measured in terms of 1, 4 dichlorobenzene equivalent (kg 1,4-DCB), as indicated by (Dorca-Preda, Fantke, et al. 2022). (Chen, Habert, et al. 2010) determined 7.60E-02 1,4-DCB equivalent for each kg of produced OPC for HTP, compared to 8.20E-02 for this study. Most of these impacts are induced by emissions from the cement kiln, i.e., during calcination, followed by NaOH preparation. However, the 8% higher impact on HTP in this study is attributed to more energy consumption due to the poor grindability of GBFS and RM during milling. Also, some heavy metals emissions in coal-fired power generation plants can increase the impact of HTP. On the other side, preparing NaOH produces chlorine and hydrogen chloride emissions, contributing to severe toxicity potential for humans. In addition, the raw materials acquisition affects this impact category (13.97%). Regarding freshwater and marine ecotoxicity, Fig. 5 shows similar results – the raw materials unit has a comparable contribution (about 25%). The preparation of NaOH exerts a lower impact, merely accounting for nearly 10% respectively.

Electricity does not contribute significantly to all impact categories due to the efficient usage of electrical energy and advanced technology in the studied cement plant. The world average electricity consumption is 1.07E-01 kWh per kg of OPC (Sousa and Bogas 2021), and this value in the studied plant is around 8.44E-02 kWh, signifying lower electricity usage following this production procedure in this study. 70% of electricity (consumed by the cement plant under consideration) is sourced from hydroelectric power, which is a cleaner and low-carbon approach compared to thermal power.

Incorporating RM as a partial activator for GBFS activation is a sustainable option to reduce cement usage with lower environmental emissions. As discussed earlier, GBFS is an industrial by-product produced and accumulated in large quantities annually. After activation by the alkaline activator (NaOH and RM), its pozzolanic activity was significantly improved, as reflected by better compressive strength, durability, and corrosion resistance (Gomes, Nguyen, et al. 2023). In this case, benefits, including lower costs incurred in cement manufacturing, better resource conservation, and effective disposal of GBFS and RM, were also attained, signifying a triple win from a sustainability standpoint.

3.4. Environmental impact assessment of OPC and two HAACs

3.4.1. Comparison of resource and energy depletion

In this section, a comparison of the resource and energy consumption of the OPC and two HAACs (otherwise identical, but one using NaOH exclusively as an activator and the other using RM as a partial substitute for NaOH) was carried out based on the FU to assess the environmental benefits. As shown in Table 1, 0.36 kg of limestone was used in the HAAC. However, 1.43 kg of limestone was used in OPC, as per the cement manufacturer. Clay was heavily replaced by GBFS and RM in HAAC, and laterite consumption also decreased. It can be inferred that HAAC production reduced limestone consumption and other mineral materials. This can limit energy inputs and environmental emissions during mining, milling, grinding, and calcination.

The energy consumption value of HAAC in this study is 4.74 x 103 kJ, whereas that of OPC is 5.21 x 103 kJ (Qinli Zhang 2022). This is due to the poor grindability of GBFS and RM compared to OPC, causing reduced energy consumption during grinding. At the same time, GBFS and RM incorporated as substitutes for raw materials significantly reduced fossil fuel (i.e., raw coal) consumption for the calcination stage (Qaidi, Tayeh, et al. 2022). Based on mentioned-above two reasons, nearly 16% of energy saving was attained in HAAC production.

3.4.2. Comparison of LCIA results

Figure 5 compares the LCIA results for HAAC with the two selected activators, and OPC produced in the same cement plant. The results elucidate that using RM as a partial activator of NaOH for GBFS activation instead of cement for producing HAAC effectively reduces its environmental impact, especially for categories FDP and TAP. For FDP, it can be described that using alkali-activated SCM in cement production requires reduced raw materials for clinker manufacturing, causing less thermal energy for calcination. Thus, reduced coal consumption is attained, contributing to the reduced impact category FDP. Regarding TAP, as previously reported, SO2 is derived mainly from the oxidation of sulfide and sulfur elements in the raw materials or the raw coal under abundant oxygen and temperature (Zhang, Peiffer, et al. 2022). Terrestrial acidification is also related to the emitted nitrogen compounds (mainly NOx) during raw coal combustion. By and large, decreasing the quantity of raw materials can produce fewer emissions from the cement kiln, causing a lower value of TAP.

Comparing the environmental benefits of the two HAACs using NaOH as the sole activator and using RM as a partial NaOH activator reveals that the HAACs using RM as a partial replacement for NaOH show reductions in most categories, mainly in the categories of TAP, FDP, ODP, and GWP. This is because RM only requires grinding, whereas NaOH production involves the chlor-alkali preparation process. The processed wastewater from the chlor-alkali process is more difficult to dispose of completely, resulting in a certain degree of environmental pollution and the deposition of some harmful gases emitted, which may lead to the reduced pH of the soil, resulting in soil acidification (Roy, Azevedo, et al. 2014). RM addition as a partial replacement for NaOH reduces energy consumption and demand, thereby lowering the FDP value. Reducing NaOH production also reduces harmful emissions, and ODS (Ozone-Depleting Substances) emissions will be reduced, which will help reduce GWP emissions, as ODS emissions (chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, carbon tetrachloride, hydrobromofluorocarbons, chlorobromomethane, and methyl chloroform) are also potent greenhouse gases (Wu, Ding, et al. 2023).

Nevertheless, the category HTP of HAAC increases by 7.60% compared with that of OPC (Fig. 6). This can be explained by the poor grindability of GBFS and RM, causing more electricity consumption during grinding and milling. It is worth stating that human toxicity is primarily affected by heavy metal elements produced by the power generation plant. Further, preparing NaOH using chlor-alkali processes produces some elemental emissions that may harm humans. Hence, the absolute value of HTP slightly increases. The total environmental result of HAAC reduces by 13.67%, showing that HAAC production not only enhances the benefit of materials conservation but reduces the environmental effect of cement manufacturing (Boesch and Hellweg 2010).

It is emphasized that each environmental consequence heavily depends on its source. Resource and energy depletion or toxicity-related impacts are related to regional or local impacts, and these results would be lightened with increasing source distance. Also, inappropriate waste treatment may disturb the surrounding area. Global warming and other environmental impacts considered holistically are also important to the receiving environment (Finnveden, Hauschild, et al. 2009).

In this study, Life Cycle Assessment (LCA) analysis was used to evaluate the environmental impact of a cement plant producing HAAC incorporating RM as a new alkali activator for slag by replacing part of the NaOH in the blend. The LCIA results with uncertainty information were characterized and normalized. Contribution analysis was also carried out to determine the impact of specific substances on each category. A comparative analysis of the environmental impact of OPC and two HAACs, otherwise identical but using NaOH as the sole activator in one case and RM as a new activator in the other, was then carried out.

The main conclusions drawn from the obtained results are as follows.

Calcination and alkali-activation of SCM phases significantly contribute to most impact categories. The extraction of raw materials has very little impact.
METP causes the largest environmental burden. FETP and FDP also have significant impacts on the total environmental consequences.
Clinker results in huge global warming, marine eutrophication, fossil depletion, and human toxicity, with a contribution proportion of 34.77%, 30.96%, 70.43%, and 37.91% of the total category, respectively. NaOH contributes nearly 40.43% to ozone depletion. Obtaining raw significantly affects freshwater eutrophication as well as freshwater and marine ecotoxicity.
Compared to OPC production, environmental benefits were added for most categories. Comparing the LCA results for two HAACs using different excitants, the environmental results using RM as a new excitant instead of partial NaOH were significantly better than those using NaOH as the sole activator, the total environmental result of HAAC reduces by 13.67%. OPC replacement by HAAC incorporated with RM as a new activator instead of NaOH effectively reduces the overall environmental impact.

The HAAC in this paper is incorporated with RM as a partial activator of NaOH for slag activation to improve waste disposal in the cement industry and reduce the environmental impact. Future research should be into the feasibility of completely replacing NaOH as an activator with solid industrial waste. In addition, it is necessary to find innovative low-carbon cementitious materials to mitigate environmental load.

Funding

This work was financially supported by the National Science Foundation of China (No.52204166, No.52104156, and No.52074351). Supported by the State Key Laboratory of Safety and Health for Metal Mines （2021-JSKSSYS-05）.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contributions

Conceptualisation was conducted by Qinli Zhang, Yan Feng and Qiusong Chen. Methodology was carried out by Qinli Zhang and Cunyu Zhao. The first draft of the manuscript was written by Cunyu Zhao and Qinli Zhang. All authors participated in manuscript reviewing and editing. All authors read and approved the final manuscript.

Ethical Approval

Not applicable.

Consent to Participate

Not applicable.

Consent to Publish

Not applicable.

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Life cycle assessment of Hybrid alkali-activated cement production with red mud as an alkali activator

Status:

Journal Publication

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Abstract

Figures

1. Introduction

2. Methodology

2.1. Material and strength testing

2.2. Goal and scope definition

2.3. Life cycle inventory

2.4. Life cycle impact assessment

3. Results and Discussion

3.1. Comparison of HAAC and OPC cement strength

3.2. LCIA results

3.2.1. Characterization results

3.2.2. Normalization results

3.3. Contribution analysis

3.4. Environmental impact assessment of OPC and two HAACs

3.4.1. Comparison of resource and energy depletion

3.4.2. Comparison of LCIA results

4. Conclusions

Declarations

Funding

Declaration of competing interest

Author Contributions

Ethical Approval

Consent to Participate

Consent to Publish

References

Status:

Journal Publication

Version 1