Progress in Green and Low-carbon Technologies development of Building Ceramics Industry

As global and domestic concerns about climate change intensify, the development and adoption of green and low-carbon manufacturing technologies to effectively reduce resource consumption, energy usage, and greenhouse gas emissions have emerged as primary trends in the evolution of the building ceramics industry under the backdrop of the “dual carbon” strategy. This article systematically reviews the commercially applied and research-stage green and low-carbon technologies within the current building ceramics industry. It conducts a carbon emission reduction potential analysis for various technologies, identifies constraints in the technology promotion process, and highlights the substantial carbon reduction potential of clean energy substitution technologies and manufacturing process optimization technologies. Technologies such as raw material substitution, high-quality service, and waste ceramic tile recycling and regeneration contribute significantly to economic and environmental benefits, but are still in developmental stages. The article also discusses the application of methods such as life cycle assessment and carbon footprint analysis in the evaluation of green and low-carbon technologies in the field of building ceramics. It discusses the trends and limiting factors of different green and low-carbon technologies, offering suggestions and support for the building ceramics industry’s transition towards a low-carbon and environmentally friendly direction.


Introduction
Building ceramics refer to ceramic products that are utilized on building surfaces or as structural elements.They are composed of various mineral raw materials and additional additives, which are carefully proportioned, subjected to processes such as pulverization, mixing, shaping, glazing, and firing, resulting in the formation of ceramic materials [1] .In recent years, China has maintained its position as the world's largest producer of building ceramics, with an annual output ranging from 8.1 billion to 8.6 billion square meters, accounting for approximately 64% of global production.The national production levels and growth trends of building ceramics from 2012 to 2022 are depicted in Figure 1.Year-on-year growth rate (%) only considers greenhouse gas emissions but does not account for other pollutants and resource energy consumption.It provides a relatively comprehensive and accurate assessment and analysis of greenhouse gas emissions, making it more targeted in evaluating greenhouse benefits.This method calculates the total emissions of all greenhouse gases and then converts them into CO2 eq.CO2 eq is a calculated mass for comparing the radiative forcing of a greenhouse gas to that of carbon dioxide.Currently, countries worldwide are promoting the construction of carbon quantification and accounting systems to achieve green and low-carbon development.LCA is widely used for quantifying global warming potential.In addition to this indicator, LCA can provide multidimensional environmental impact indicators, including resource depletion, land acidification, human toxicity, ozone depletion, etc.This approach avoids the limitations of single indicators and can comprehensively consider the environmental impacts of the ceramic manufacturing industry.Based on this, it can calculate the environmental benefits of the same product under different production technologies, providing data support for choosing lower-carbon and greener process technologies.It can also analyze the environmental impact contribution rate of different production materials, identify key stages of resource consumption and environmental pollution, and is frequently used to identify environmental impact hotspots.It systematically evaluates the efficiency of raw material and energy use, identifies pollution transfer between stages, and provides a reference for improving subsequent processes.
With the proliferation of ceramic production lines across the nation, the substantial surge in capacity engenders the extensive depletion of mineral resources, the generation of copious ceramic waste, and the accumulation of discarded ceramic tiles surpassing their intended service duration.The process of ceramic preparation and energy utilization predominantly contributes to the emission of greenhouse gases, thereby engendering ecological degradation, exacerbating the depletion of mineral resources and energy reserves.In light of these circumstances, a comprehensive perspective encompassing the entire lifecycle of building ceramics categorizes technologies into five primary domains: raw materials, energy, manufacturing processes, service life, and recycling.This technological framework is visually depicted in Figure 2.This study systematically examines the current landscape of green and low-carbon technologies in the ceramic industry, encompassing both commercially viable applications and those still under research and development.It provides a comprehensive overview of the application of mainstream evaluation methodologies, such as life cycle assessment (LCA) and carbon footprint analysis, in assessing of green and low-carbon technologies within the domain of building ceramics.Drawing on these findings, the study explores the developmental trends and constraining factors associated with different types of green and low-carbon technologies.Ultimately, this research aims to offer informed recommendations and support for the low-carbon and green transition of the building ceramics industry.

Fig. 2.
Green and low-carbon technology system of building ceramics.

Raw materials substitution technology
At present, the total production of building ceramics in China has surpassed 8 billion square meters, with an annual consumption of natural mineral raw materials ranging from 150 to 200 million tons [4] .The direct industrial process carbon emissions caused by the decomposition of carbonate raw materials account for approximately 5% of the carbon emissions in the production process of building ceramics, while various raw materials contribute to indirect greenhouse gas emissions during the extraction and production processes.To address these challenges, the development of innovative technologies that utilize industrial waste materials, such as fly ash, silica fume, coal gangue, waste stone powder, and ceramic waste, for the production of high-performance ceramic products has gained attention.These approaches not only achieve carbon reduction objectives but also bring about additional environmental benefits, such as resource conservation and land preservation.For instance, Yang Luo [5] has pioneered a novel all-coal-ash building ceramic production technology, which involves blending raw fly ash with activated ash to achieve all-coal-ash ceramic tiles with a fly ash content of up to 100%.The resulting all-coal-ash building ceramics exhibit significantly superior performance compared to the requirements stipulated in the national standard GB/T4100-2015.In a similar vein, Wang S H et al. [6] have explored the utilization of polishing waste as a raw material for the production of porcelain tiles by optimizing the formulation and process conditions.Their research has led to the identification of an optimal scheme with a polishing waste content of 20 wt% and a firing temperature of 1150°C, resulting in the development of high-performance porcelain tiles with a remarkable flexural strength of 60.52 MPa.
Some scholars have conducted research on the potential of raw material substitution technologies to mitigate pollution and reduce carbon emissions.Barbosa et al. [7] employed a life cycle assessment (LCA) approach to investigate the environmental impacts of manufacturing building ceramics using waste decorative stone as a substitute raw material.The findings revealed that this technology achieved a 35.74% reduction in greenhouse gas emissions and a 14.83% reduction in mineral resource consumption compared to traditional ceramic tiles.LópezGarcí a A et al. [8] conducted an LCA-based comparative analysis between traditional ceramic tiles and tiles with 3%, 7%, and 10% olive residue additions.The results demonstrated that the incorporation of olive residue led to a reduction of over 8% in ecological toxicity, human toxicity, and eutrophication potential at the raw material stage compared to traditional ceramic tiles.Morfino et al. [9] compared the environmental impacts of utilizing zircon sand and alumina in different ceramic mixtures.The results indicated that zircon sand ceramic tiles exhibited superior environmental performance across various impact categories compared to alumina ceramic tiles.
The substitution of raw materials offers the potential for carbon reduction and the concurrent environmental benefits of material and land conservation.However, the practical application of these techniques necessitates careful consideration of their potential impact on the quality of building ceramics.For instance, in the case of fly ash, significant variations in composition exist among different sources, complicating the complete phase transformation of fly ash silicates.This complexity in processing routes, coupled with the absence of effective fly ash modification methods, poses challenges to product quality control.As a result, the wide-scale dissemination and extensive utilization of these technologies may be hindered to some extent.

Clean energy technology
The production process of building ceramics contributes to approximately 80% of the total greenhouse gas emissions, predominantly resulting from fuel combustion [10] .The implementation of clean energy substitution technologies, such as biomass, hydrogen, and electricity, has significant potential for mitigating carbon dioxide and exhaust emissions, leading to substantial environmental benefits.Numerous scholarly publications have explored the application of clean energy substitution technologies.Zhang et al. [11] emphasize the role of natural gas as a clean and low-carbon fossil fuel, highlighting its potential to reduce carbon dioxide emissions by nearly 40% compared to coal, assuming equivalent heat generation.Xiao et al. [12] demonstrate enhanced combustion efficiency, achieving a 3% to 5% increase when utilizing ordinary biomass fibers and employing advanced methods for hightemperature combustion control using biomass micron fuel.Additionally, Cheng et al. [13] successfully pioneered the production of zero-carbon ammonia-fueled green ceramic tiles by adopting a 100% ammonia fuel approach and implementing innovative kiln ignition and heating processes.
Some scholars have conducted research on the emission reduction and carbon mitigation potential of clean energy substitution technologies.Wang et al. [14] conducted a comparative analysis of different fuel scenarios and found that ceramic tiles predominantly fueled by natural gas demonstrated lower human health and ecosystem impact indicators, with reductions of 16.92% and 3.15%, respectively, compared to ceramic tiles fueled by coal gas.Almeida et al. [15] employed a life cycle assessment (LCA) approach to study four ceramic tile factories in Portugal and concluded that optimizing energy consumption, electricity use, and raw material transportation distances significantly reduces the overall environmental burden.Ancona et al. [16] discussed the energy and environmental performance of cogeneration systems in the ceramic industry, highlighting their ability to reduce carbon dioxide (CO2) and nitrogen oxide (NOx) emissions to maximum values of 81g/kWh and 173mg/kWh, respectively.Under gas turbine operation, carbon monoxide (CO) emissions were reduced by 185 mg/kWh.Monteiro et al. [17] investigated the environmental and economic life cycle performance of ceramic factories utilizing photovoltaic power generation.Despite the higher initial investment costs, self-production through photovoltaics resulted in a 33% reduction in carbon dioxide equivalent (CO2eq) emissions and yielded cost savings and a reduced payback period.
The ultimate objective of clean energy substitution technologies in the ceramic industry is to achieve near-zero emissions in energy-intensive processes.Currently, natural gas is widely adopted in the industry, although factors such as price fluctuations and supply limitations pose certain constraints on its full-scale implementation.The economic viability, safety considerations, and stability issues surrounding hydrogen energy need to be addressed, alongside the compatibility challenges it presents for kiln furnaces, which limits its widespread adoption within the sector [18] .The maturity of biomass energy technologies and the relatively low calorific value of biomass gas pose challenges in meeting the sintering requirements of ceramic products.Consequently, it is anticipated that fossil fuels, with natural gas as a representative source, will remain the predominant energy source for the next two decades, albeit with a growing emphasis on coexistence and integration of multiple energy sources in the industry.

Manufacturing process optimization technology
The primary stages contributing to CO2 emissions in the production of ceramic tiles are the drying and firing processes, in which high-energy-consuming equipment such as spray dryers and kilns are involved.These two stages account for approximately 95% of the total energy consumption in the overall production process.Moreover, the carbon emissions from the product manufacturing process constitute more than 80% of the complete life cycle of ceramic tiles in the field of building ceramics [19] .To achieve carbon emission reductions, manufacturing process optimization techniques employ measures such as equipment improvements and technological innovations to mitigate the thermal energy (coal) and electricity consumption in ceramic tile production.These techniques encompass various aspects, including low-carbonization technologies for powder preparation, slurry formulation, drying procedures, kiln operations, and efficient fuel utilization.Notable technologies that have been implemented include dry powder processing, microwave drying, high-efficiency large-scale coal-water slurry gasification, in-depth recovery of water vapor in cooling towers, and utilization of exhaust heat from flue gases, among others.Xu Xuefeng [20] suggests that the moisture content in the slurry of continuous ball milling can be reduced to 32%, which is slightly lower than the energy-intensive intermittent ball milling with a moisture content of 33%-34%.This is beneficial for reducing water consumption and energy-saving in spray drying.The comprehensive energy-saving and coal-saving effect of this technology is about 30% and 40%, respectively.Wu Yupeng et al. [21] evaluated the impact of dry powder processing technology on the synergistic reduction of atmospheric pollutants and CO2.Their findings demonstrated that dry powder processing saves approximately 30% of energy in the drying process, resulting in a 51% reduction in CO2 emissions, as well as reductions of 42%, 45%, and 42% in particulate matter, nitrogen oxides (NOx), and sulfur dioxide (SO2) emissions, respectively.Liu Shuai et al. [22] developed a waste heat utilization scheme and engineering steps in a project case study, achieving an annual standard coal saving of 1960 tons, a comprehensive waste heat utilization rate of 90.8%, and a significant annual reduction of 4815.1 tons in CO2 emissions.
Some scholars have conducted research on the carbon emission reduction, economic viability, and environmental benefits of manufacturing process optimization techniques.Wang et al. [23] quantitatively compared the carbon emissions of dry powder processing, low-temperature fast firing, and multi-layer drying kiln technologies, analyzing their respective carbon reduction potentials.The findings revealed that dry powder processing exhibited a substantial carbon reduction potential of 50.76%, with energy production and product manufacturing stages contributing 21.13% and 29.63% to the overall reduction potential, respectively.The low-temperature fast firing technique demonstrated a carbon emission reduction of 13.98%.Similarly, employing a five-layer drying kiln instead of a one-layer drying kiln yielded a carbon reduction potential of 1.44%.Wang et al. [14] conducted a comparative analysis of different powder processing methods and found that dry powder processing exhibited lower human health, ecosystem, and resource impact indicators compared to wet powder processing, with reductions of 22.89%, 22.01%, and 39.04%, respectively.Monteiro et al. [17] evaluated the environmental and economic life cycle performance of waste heat recovery in ceramic factories.The results demonstrated that by utilizing waste heat from flue gases to preheat combustion air, a significant reduction of 13% in CO2eq emissions was achieved, accompanied by substantial economic savings and a presumed payback period within the investment lifespan.
The optimization of manufacturing processes holds great potential for energy efficiency and emissions reduction in the peak period of the ceramics industry.However, certain constraints still exist.The lack of standardization in domestic ceramic raw materials leads to significant variations in particle size and material characteristics among different ceramic factories.Consequently, raw materials need to undergo pre-processing to meet specific standards before entering the ball milling stage [24] , thereby limiting the development of powder processing technologies.Moreover, the utilization of nonstandardized mineral materials often results in elevated temperatures during the pre-firing and mid-firing stages of kiln operations, leading to increased gas consumption during ceramic firing and posing challenges to low-carbon drying technologies.The widespread implementation of efficient large-scale coal-water slurry gasification technologies and other fuel-efficient utilization techniques is hindered by issues such as low slurry concentration and poor rheological properties, which reduce gasification furnace efficiency, impede cost reduction and productivity improvement, and limit stable production.
Although high-temperature waste heat recovery technologies have been applied in the ceramics industry, their adoption remains limited, and the utilization of low-temperature waste heat is still in its nascent stage due to experimental and developmental constraints.As technology maturity advances, the adoption and dissemination of manufacturing process optimization techniques will likely increase.

High-quality serviceability technology
The development of high-quality service technologies for building ceramics, aimed at enhancing strength, prolonging lifespan, or promoting lightweight applications, plays a crucial role in reducing the lifecycle environmental burden of ceramic products.Among these technologies, the thinning of building ceramics has been successfully employed.By reducing the thickness of ceramic tiles to 6-10mm, while maintaining their functional performance, the consumption of resources and energy during manufacturing is significantly reduced, transportation costs are lowered, and the structural load on buildings is reduced, thus achieving environmental friendliness.For example, the application of Mona Lisa 5mm-thick large-format ultra-thin porcelain panels results in savings of over 60% in raw materials, a 58.8% energy saving, and reductions of 59.5% in sulfur dioxide emissions and 58.8% in carbon dioxide emissions, compared to traditional ceramic tiles.In addition to ultra-thin tiles, there is considerable potential for the thinning of conventional ceramic products [25] .Furthermore, longevity technologies aimed at extending the lifespan of building ceramics are currently being explored in laboratory settings.The mechanical activation technique involves pre-processing ceramic powders to reduce particle size, enhance sintering activity, and improve the densification process during the formation of green bodies, thereby achieving densification reinforcement in ceramics.Ion exchange technologies rely on the diffusion of alkali ions (typically Li + or Na + ions) from materials containing alkali ions into molten salts with different ionic radii (such as KNO3 or CsNO3).The resulting "jamming effect" or differential coefficients of thermal expansion generate pre-stressing, leading to ceramic strengthening.
Xie Adi et al. [26] conducted an evaluation of the environmental impacts of ceramic wall and floor tiles and thin panels using a life cycle assessment (LCA) method.The results indicated that the environmental impacts in the life cycle of ceramic wall and floor tiles primarily stem from heavy oil production, ceramic tile manufacturing, electricity production, and clay production.In comparison, thin panels exhibited significantly better performance in terms of non-renewable resource consumption, primary energy consumption, and eutrophication indicators when compared to wall and floor tiles.Pini et al. [27] conducted a comparative analysis of carbon emissions throughout the life cycle of large-sized thin ceramic panels (1000mm×3000mm×3.5mm)and traditional ceramic tiles, including raw material extraction, manufacturing, transportation, use, and disposal.The results showed that thin ceramic panels had lower carbon emissions than ceramic wall and floor tiles.
High-quality service technologies have the potential to generate significant environmental benefits in the field of building ceramics.However, the current maturity level of longevity technologies for building ceramics is relatively low, and they are still in the stage of laboratory research and development.While thinning technology for building ceramics has been partially implemented, it is not fully aligned with the existing ceramic tile construction standards.High demands are placed on the installation base, adhesives, and construction processes.Furthermore, the advancement of production technology requirements also presents a significant constraint on the widespread adoption of thin ceramic tiles.Currently, ceramic thin panels are priced higher in the market compared to regular ceramic tiles.Nonetheless, the value-added benefits of ceramic thin panels far outweigh those of ordinary ceramic tiles, leading to overall cost reduction.Despite these advantages, ceramic thin panels have yet to gain mainstream acceptance in the industry.

Waste ceramic tile recycling and regeneration technology
In the process of ceramic production and application, a considerable amount of ceramic waste, approximately 20 million tons annually, is generated, presenting significant opportunities for recycling and utilization [28] .The recycling and reuse of ceramic waste, particularly in the form of building ceramic waste, have been explored through various technologies for integration into other product systems such as concrete admixtures, asphalt mixtures, and stone materials.Ceramic waste can be classified into two categories: the former includes waste from raw materials, glazes, fired ceramics, as well as waste generated during post-processing operations, including grinding and polishing waste [29] .The latter refers to waste materials, such as damaged ceramic tiles during transportation or tiles that have exceeded their expected service life.In recent years, several studies have investigated ceramic waste recycling technologies, providing a theoretical basis for their application in the production line of building ceramics.For instance, Yue Aijun et al. [30] employed ceramic waste as aggregates in concrete by crushing and blending them with cement, sand, and other raw materials, achieving optimal ceramic content ranging from 50% to 70% in recycled ceramic concrete for applications in construction and road sectors.Ren Yongli et al. [31] utilized ceramic waste as coarse aggregates in asphalt mixtures, leading to an asphalt mixture with improved thermal insulation properties and enhanced resistance to rutting, thereby effectively lowering the temperature of the road surface.These approaches not only address the issue of ceramic waste disposal but also contribute to resource conservation and reduction in production costs, representing promising avenues for sustainable recycling and reuse.
Several researchers have conducted studies on the environmental impact and economic benefits of waste ceramic tile recycling and regeneration technology.Meena et al. [32] evaluated the performance of self-compacting concrete under sulfuric acid exposure by introducing ceramic waste bricks as natural fine aggregates at different replacement levels.The 60% replacement level demonstrated the highest optimal performance in terms of compressive strength and sulfuric acid resistance.The self-compacting concrete mixture produced from waste ceramic tiles was cost-effective and had lower CO2 emissions.Chang et al. [33] investigated the impact of concrete with 10% ceramic waste content on natural resources, climate change, ecosystem quality, and human health.The results showed reductions of 6.78%, 8.68%, 7.18%, and 7.19% in the respective impacts.Furthermore, the impacts on non-renewable energy, ozone depletion, and global warming were also reduced by up to 7%, 6%, and 9% respectively.Namra et al. [34] conducted a life cycle assessment of production from recycled brick waste and recycled ceramic tilebased geopolymer.The results indicated reduced environmental impacts, with global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), and ozone depletion potential (ODP) of 1352 kg CO2eq, 3.01 kg SO2eq, 0.19 kg Neq, and 6.56E-9 kg CFCeq respectively.
After years of research and practical application, the recycling and utilization of unfired waste materials, such as greenware and clay waste, have been largely achieved, leading to certain economic and social benefits.However, the utilization of solid waste from hard ceramics, primarily composed of waste ceramics and polishing waste, poses significant challenges.These waste materials undergo hightemperature sintering, resulting in altered physicochemical properties and rendering them difficult to grind due to their complex composition.While large-scale ceramic enterprises with advanced technologies and equipment have achieved nearly zero solid waste discharge, smaller and less technologically advanced enterprises may face higher costs in waste collection and pretreatment, which can surpass the costs of raw material procurement.Moreover, the recycling of polishing waste lacks effective solutions.At the industry level, the overall utilization of ceramic waste remains relatively low, primarily due to the absence of corresponding regulations and technical standards.

Conclusion
Driven by the imperatives of achieving carbon peak and carbon neutrality, the imperatives of green, low-carbon, and environmentally friendly practices have emerged as pivotal trends in the development of the building ceramics industry.By strategically charting the trajectory of the building ceramics sector, capitalizing on advancements within production chains, and embracing innovations in green and lowcarbon technologies, the industry is poised for a transformative evolution.This paper elucidates the current state of research in green and low-carbon technologies within the domain of building ceramics.It systematically categorizes and assesses the potential of such technologies in the realm of building ceramics manufacturing.The paper also provides an exposition of prominent assessment methodologies for evaluating low-carbon technologies, including building ceramics' low-carbon product certification standards, carbon footprint analysis, and life cycle assessment (LCA).It discusses the advantages and limitations of these assessment methodologies while outlining their respective applicability domains.
The five categories of green and low-carbon technologies in building ceramics introduced in this article are still in a developmental stage overall.While there have been some achievements in carbon reduction, economic benefits, and environmental benefits within the industry, there are still significant challenges for these current green and low-carbon technologies when it comes to applicability, cost, safety, and other aspects, especially in light of the long-term goal of carbon neutrality.There is a lack of corresponding regulations and technical standards, and issues such as the imbalance in regional green development within the industry still persist.With the government's increasing focus on energy consumption and environmental protection, the costs associated with energy and environmental pollution are expected to rise.Consequently, green and low-carbon building ceramics will possess a competitive edge.The continued research and development, as well as the synergistic application of diverse technologies, hold the promise of significantly reducing greenhouse gas emissions from China's building ceramics products in the foreseeable future.