Introduction

The cement industry, for example, consumes enormous amounts of energy and non-renewable natural sources in its manufacturing process. Global production of Portland cement is expected to be up to 8 Gt/year by 2100, depending on Shared Socioeconomic Pathways (Renforth, 2019). As a result, they are responsible for approximately 8-9% of global carbon dioxide (CO₂) emissions, of which the burning of fossil fuels and calcination account for approximately 40% and 50% of these emissions, respectively. The remaining 10% comes from transporting raw materials and electricity (Benhelal; Zahedi; Hashim, 2012; Monteiro; Miller; Horvath, 2017). In Brazil, approximately 866 kg of CO₂ is estimated per ton of clinker and 564 kg of CO₂ per ton of cement is produced (Ribeiro et al., 2022). This last rate is lower due to using renewable alternative fuels, waste co-processing, and clinker replacement with mineral additions in the final composition (SNIC, 2019).

In addition to greenhouse gases (GHG), emissions of heavy metals (Cd, Cr, As, Hg, Ni, Pb, Cu, Zn, etc.), other air-polluting gases (nitrogen oxides, sulfur dioxide and carbon monoxide) carbon) and particulate materials are the other considerable impacts of this sector (Ali; Saidur; Hossain, 2011; Richards; Agranovski, 2015; Van Den Heede; De Belie, 2012). Average emission levels for suspended particles are estimated at 25 mg/Nm³ and 300 mg/Nm³ for nitrogen and sulfur oxides (Kuenen et al., 2019).

It must explore new materials and innovative processing methods to steer the cement industry towards a more sustainable future and enhance its role in a circular economy (CE). This approach aims to minimize the adverse effects of resource extraction, greenhouse gas emissions, and energy consumption in cement production. The European Commission (EC) advocates for waste reduction and resource preservation through novel techniques that not only cut costs but also generate revenue, thereby avoiding resource scarcity and fostering economic growth (Jayakumar et al., 2020; Manninen et al., 2018; Moraga et al., 2019).

CE’s shared values focus on decoupling economic growth from resource consumption and efficiency, waste management, sharing, reduction of GHG emissions, LCA, and closing loops (Ness; Xing, 2017). Practitioners consider CE a practice encompassing regenerative industrial transformations and achieving sustainable production and consumption (Agyabeng-Mensah et al., 2021).

Among the potential residues as alternative raw materials for cement production are ornamental rocks processing sludge (ORPS) and calcium carbonate sludge (CCS). ORPS comes from the cutting and polishing of ornamental rocks, with an estimated generation of 20 to 40% of the volume of the block extracted in the quarry (Alyamaç; Tuğrul, 2014; Mashaly et al., 2016). CCS comes from the cellulose and paper industry through the Kraft process, with a generation of 20 to 30kg, dry basis, per ton of cellulose processed in Brazil (Ribeiro et al., 2022).

To further maximize the environmental bias, the mentioned industrial solid waste can enable the production of belitic cement. Belitic cement is the binder with the highest belite content (40-60% by weight), unlike Portland cement, which has the highest alite content. Furthermore, it has a lower CO₂ footprint, as it consists of a binder with lower demand for high-quality limestone, lower energy demand, lower clinkerization temperature, lower heat of hydration in the first ages of hydration, more excellent production of C-S-H and prolonged durability (Cuesta; Ayuela; Aranda, 2021; Irico et al., 2022), despite its late hydration. Compared to Portland cement, to produce 1 ton of clinker, the production of alite requires 578 kg of CO₂ and 511 kg to produce belite, a CO₂ generation saving of 12% (Gartner, 2004). Finally, around 200 to 350 ºC can be saved in emissions due to fuel consumption to produce this belite cement (Redondo-Soto et al., 2022; Ribeiro et al., 2022).

The significant increase in the consumption of construction materials driven by population growth leads to environmental impacts from extraction to demolition, with the need to apply life cycle thinking to identify and mitigate these effects (Huang et al., 2020). Considering the high domestic demand for cement worldwide, as well as its importance in terms of environmental impacts, the use of environmental management methodologies to track the environmental burdens linked to its production and use is fundamental, not only for the industry but also for the entire civil construction sector, including regulatory bodies. In this context, Life Cycle Assessment (LCA) is a tool that allows analyzing the environmental impacts of products or services throughout their life cycle from a holistic perspective (ABNT, 2014a, 2014b; EC, 2010, 2012; Hellweg; Canals, 2014). Consequently, it is a suitable methodology for identifying critical environmental hotspots and possible mitigation strategies in different civil construction sectors (Ivanica et al., 2022; Lima et al., 2022; Luca et al., 2023; Rigon et al., 2021; Vieira et al., 2023; Yilmaz; Seyis, 2021).

Although the use of solid waste as alternative materials in the production of Portland cement has been practiced for some time, more needs to be done regarding the real environmental benefit of such substitutions in alternative cement, considering LCA. Therefore, the objective of this study is to evaluate the environmental impacts, through the LCA tool, associated with the concomitant use of the processing of ornamental rocks sludge and calcium carbonate sludge in the production of belitic clinker on a laboratory scale.

Materials and methods

The raw materials used for the production of belitic clinkers (BC) were: calcitic limestone, kaolinite clay, ornamental rock processing sludge (ORPS) and calcium carbonate mud (CCS). The oxide balance in these materials for preparing the raw clinker flour composition was calculated according to the chemical moduli of C150-19a (ASTM, 2019), with the lime saturation factor (LSF = 77.0), silica modulus (SM = 3.0) and alumina modulus (AM = 2.65), typical belitic clinker patterns. The physicochemical compositions of the investigated materials and the developed formulations can be found in Tables 1 and 2, respectively. The clinker production process was carried out in a 30-ton manual hydraulic press, and the sintering temperature was 1100 ºC for 290 minutes in each cycle, according to an investigation by Ribeiro et al. (2022).

The LCA study was conducted according to Brazilian standards NBR ISO 14040 (ABNT, 2014a) and NBR ISO 14044 (ABNT, 2014b), covering all the steps foreseen in an LCA study (definition of objective and scope; life cycle inventory; life cycle impact assessment; and interpretation of results). To construct the inventory, primary data collected on-site through direct measurement during the process’s execution in the laboratory were used. For the waste used as raw material in the study, attributional modelling based on the cut-off approach was adopted, with no charges for the process.

Objectives and scope

The study aims to identify the potential environmental impacts, particularly the carbon footprint of BC, produced from ornamental rocks processing sludge and calcium carbonate sludge on a laboratory scale and based on estimates of values if produced on an industrial scale Brazil. The scope adopted for the study was cradle-to-gate, in which the “cradle” corresponds to the extraction and processing of raw materials and inputs used in the BC, including transportation, and the “gate” is the manufacturing process. The functional unit considered was 396 grams of wet pellet produced in cycle 1 of each formulation, resulting in a belitic clinker variation of 210 to 261 grams, depending on the formulation. The variation in obtaining belitic clinker occurs due to differences in the compositions of the formulations due to the release of CO₂ from natural and alternative raw materials. Figure 1 illustrates the laboratory process of obtaining belite cement.

Inventory analysis

Primary data was measured in loco, estimated and calculated according to each cycle to construct the inventory. Due to the innovative nature of the construction material, it was not feasible to carry out measurements directly in the industry. As an alternative, a bibliographical review of case studies related to the carbon footprint of Portland clinker was initially carried out due to the similarity of the production processes. It was observed that most of the greenhouse gas emissions in the life cycle of Portland clinker come from the rotary kiln’s calcination processes and energy consumption. Given that many of these studies were conducted abroad, it was considered important to evaluate the typical energy consumption of the Brazilian cement industry, according to Dorileo, Bajay and Gorla (2010) and the production of electricity and fuels, considering the specific characteristics of the Brazilian energy matrix. Thus, the measurements that should be carried out in the laboratory were identified: the CO₂ emission in the calcination process and the energy consumption of the muffle furnace.

Estimation of CO₂ emissions and energy consumption in the clinkerization process

Preliminarily, carbon dioxide emissions in the calcination process and energy consumption in a rotary kiln were estimated as if this clinker production were carried out on an industrial scale, according to Passuello et al. (2014). Both analyses can be measured by thermogravimetry (TG/DTG) through mass variations (loss or gain) of the formulations during the clinkerization process under continuous and uniform heating. The tests were carried out on a PerkinElmer thermobalance, model STA 8000, using alumina crucibles with approximately 20 mg of sample, using a heating rate of 10 ºC per minute in a nitrogen atmosphere (N₂) and a temperature range of 25 to 1000 ºC. The temperature range of the DTG curve adopted to calculate the mass loss related to decarbonation varied from 600 to 850 ºC. For purposes of comparing energy consumption, it was considered that the clinkerization of Portland clinker consumes 3.35 MJ/kg of clinker, according to Dorileo, Bajay and Gorla (2010).

It should be noted that this estimate is quite conservative, as it disregards the fact that BC is produced at lower temperatures than Portland clinker. In this study, 1100 versus 1450 ºC were used, respectively. However, this estimate is a basis for a first approximation of pilot-scale experiments.

Environmental impact assessment

The assessment of potential environmental impacts was carried out using the midpoint scope, with an attributional approach, in which the 16 impact categories of the EF 3.0 method were considered. These categories are: climate change, depletion of the ozone layer, ionizing radiation, formation of photochemical ozone, particulate matter, human toxicity (cancer and non-cancer), acidification, eutrophication (terrestrial freshwater and marine), freshwater ecotoxicity and use of land, water and resources (fossils, minerals and metals). The modelling was carried out using the SimaPro Expert 9.1.1.1 software with the ecoinvent 3.6 database.

Results and discussion

Estimation of CO₂ emissions and energy consumption in the clinkerization process

Considering that carbon dioxide emissions in cement kilns are directly related to the decomposition of limestone (CaCO₃ → CaO + CO₂), Figure 2 presents the thermal analyses by thermogravimetry (DTG) of the in-nature formulations, and Table 3 shows the estimates of CO₂ emissions per ton of clinker produced.

It is possible to verify (Table 3) that the REF mixture obtained the highest decarbonation, measured at 30.68%. This impact was expected due to the higher limestone content used. The other clinkers showed lower CO₂ generations than REF, reaching a reduction of up to 16.62 kg/tonne of raw mix with F1. Given this, ORPS and CCS can directly impact minimizing emissions and extracting natural resources. It is essential to highlight that these theoretical results did not consider emissions arising from the thermal and electrical energy of the process.

According to Belizario-Silva, Oliveira and John (2022), one ton of Portland clinker generates 564 kg of CO₂ in Brazil. This makes it possible to verify that the belitic clinkers in this investigation are more environmentally sustainable, mainly with the F1 mixture (347.98 kg/ton of clinker), reducing the carbon footprint by 23.92% of clinker in the clinkerization process. This result could be exciting and significant for the cement industry, using lower CaCO₃ levels.

From this same perspective, other studies have investigated the production of binders with less environmental impact in the laboratory. The reductions achieved in emissions ranged from 330 to 525 kg CO₂/ton. clinker (Costa; Ribeiro, 2020; Gao et al., 2021; Iacobescu et al., 2016; Malacarne et al., 2021; Mariani et al., 2019; Passuello et al., 2014; Santos; Cilla; Ribeiro, 2022).

Still, according to Figure 2 and Table 4, the results indicate lower mass losses in the BC with industrial solid waste than in the BC with natural materials. Table 3 presents the results obtained for the energy demand of belitic clinkers compared with the data for Portland clinker used by Passuello et al. (2014).

Table 4 shows that producing BC exclusively with ORPS and CCS (F1) requires up to 15.22% less energy than Portland cement clinker and 6.88% less energy than the REF formulation produced exclusively with natural raw materials. Other energy uses in industry, such as lighting and process heat, were considered constant during the analysis. It is important to highlight that the decrease in energy consumption is remarkably significant since direct heating represents 79.30% of the total energy consumed in the cement industry in Brazil (Dorileo; Bajay; Gorla, 2010).

Comparative LCA of the belitic clinkers

Among the 16 impact categories considered in the study, climate change, freshwater ecotoxicity, land use, water use, and use of fossil resources proved to be the most significant, presenting potential environmental impacts, according to Figure 3.

The results in Figure 3 show that BC formulations produced with industrial solid waste are better regarding climate change by up to 11.17% and natural resource use by 10.71%, using the F1 formulation compared to the REF formulation. This is because the limestone released more CO₂ at the levels set in the REF composition, even at 1100 ºC.

However, BC produced with waste in the other categories performed less than REF. In the particulate matter category, emissions of up to 38.02% can come from the extraction, crushing, grinding and transportation of alternative raw materials. For the freshwater ecotoxicity category, the impact contribution to the process was up to 17.71%, affected by emissions of toxic substances in freshwater and marine ecosystems during the extraction and processing of materials. Subsequently, BC damage in acidification reaches up to 14.31%, resulting from emissions of nitrogen and sulfur oxides (NO_x and SO₂).

Despite the production of different more sustainable cements, the results of the impacts of each category of this investigation are comparable to the study by Malacarne et al. (2021), mainly on the potential to reduce global warming from less use of ordinary Portland clinker (OPC). In the categories evaluated, the type of fuel used in the furnace and the energy consumption of the cement factory have the potential to influence the magnitude of these impacts (Chen et al., 2010).

As expected across all categories, hotspots came from electricity and transport. This was due to the number of clinker production cycles used to obtain better warlike clinkers, which were linked to the time and temperature of the muffle furnace, as well as the difficulty in obtaining local limestone and clay to produce the REF formulation. It is essential to highlight that grinding and cooling BC may require more incredible energy because they are less friable and require a thermal shock to improve their reactivity, respectively (Akono; Reis; Ulm, 2011; Schneider, 2019). According to Valderrama et al. (2012), grinding is the industry process that demands the most electricity. Although Stafford et al. (2016) also found that transporting raw materials for OPC production in Brazil is the main contributor to global warming potential, the authors believe this large contribution was due to clinker imported from an external source, which is not common in Brazilian cement plants.

Although oil predominates and the use of natural gas is growing, renewable sources maintain a significant presence in the Brazilian energy matrix, representing 47%, a proportion notably higher than the world average of 14%. The Brazilian electrical matrix is based on the participation of renewable sources, distributed in hydraulic energy (58%), wind (12.6%), biomass (8.8%), solar photovoltaic (3.9%), nonrenewable (15.7%) and nuclear (1.1%). Specifically, the southern region is the leader in per capita electricity consumption, with 3,084 kWh per inhabitant, due to the high concentration of the electro-intensive industry (EPE, 2023).

On the other hand, Brazil has a territorial area of more than 8515759 km², widths of 4319 km from east to west and 4394 km from north to south of the country (IBGE, 2022). Road freight transport impacts all production chains since, at some point in its life cycle, every product must use road transport to deliver raw materials and inputs to customers or specific logistical services (Caldas; Sposto, 2017). According to CNT (CNT, 2022), 64.7% of all cargo was transported on Brazilian highways. Despite this, it is known that cement industries are located close to mines to facilitate and maximize their manufacturing process, minimizing the impact related to transportation.

According to Passuello et al. (2014) and Ige et al. (2021), there are two significant limitations in the environmental assessment of innovative construction materials. Firstly, the need for full-scale production of the material represents a challenge. In this context, LCA offers a basis for comparing laboratory-scale materials based on insights into the possible impacts of industrial-scale production. However, it is essential to fully understand all impacts and their components in a configuration spanning raw material extraction to product delivery. Its effectiveness largely depends on the accuracy and integrity of the inventory that gathers mass and energy input and output data throughout the various stages of the life cycle. With a comprehensive and reliable inventory, the usefulness of the LCA may be protected due to the uncertainty associated with the subsequent Life Cycle Impact Assessment (LCIA) or the possible incompleteness of the environmental impact categories considered.

The second limitation concerns the assessment of the material’s durability in relation to its environmental performance. The durability of products plays a crucial role in determining the period during which the building will provide services and the number of resources required for maintenance. Therefore, it is considered a fundamental aspect in evaluating the building’s environmental, social, and economic impact. However, many researchers need to pay more attention to this issue, possibly because LCA is typically applied to evaluate products with shorter life cycles compared to buildings that may have a projected lifespan of more than 100 years. In the case of materials such as clinker and cement, durability will depend not only on the product’s characteristics but also on design criteria. With access to this information and the product’s environmental profile, designers will have relevant data to make more conscious decisions, considering social, economic and environmental aspects.

Conclusions

The feasibility of reusing processing of ornamental rocks sludge and calcium carbonate sludge in the production of belitic clinkers from a life cycle assessment perspective was investigated in this study. Based on the results, it can be concluded that:

1. the use of industrial solid waste meets the concept of cleaner production;

2. industrial solid waste reduced the use of limestone and clay by up to 100% to produce of belitic clinker;

3. formulation F1 provided the lowest carbon footprint, being 23.92% lower compared to REF;

4. belitic clinkers require up to 17.71% less energy when compared to Portland cement; and

5. belitic clinkers with industrial solid waste are better in the impact categories of climate change and use of natural resources.

The environmental assessment highlighted that significant reductions could be achieved in the Brazilian cement industry using alternative materials, specifically ornamental rock processing sludge and calcium carbonate sludge. This research provides a promising route for the industry, demonstrating the potential for significant reductions in the consumption of natural resources and GHG emissions during production. These findings inspire and motivate us to explore more sustainable ways of production, such as using these waste materials to prepare more sustainable binders.

Acknowledgements

The authors acknowledge the Brazilian governmental research agencies CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico). The participation by R.C.E.M. was sponsored by CNPq through the research fellowship PQ2 310369/2021-5. The participation of C.A.M.M. was sponsored by CNPq through the research fellowship DT 306585/2021-9. The participation of F.A.B. was sponsored by CNPq through the Research fellowships DT 304755/2022-2.

References

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