Sustainability, durability and mechanical characterization of a new recycled textile-reinforced strain-hardening cementitious composite for building applications

Tesi en modalitat de compendi de publicacionsCementitious materials have one of the highest compressive strength-to-weight ratios compared to other construction materials. Nonetheless, both tensile strength capacity and toughness result to be an order of magnitude less respect to the former, which thereby leads to cracking under tensile stresses caused by service loads. This lack of tensile strength capacity of the material leads to cracking and fragile failure in case the material is insufficiently reinforced. Within this context, fibers can be used in cementitious matrices aiming at enhancing the toughness, energy absorption capacity, post-cracking behavior as well as flexural and tensile strength. Although during the past decades, various types of fibers such as asbestos, steel, glass, and polymeric have been tested in brittle matrices, there have been some disadvantages such as detrimental health effects, high cost, and specifically, substantial environmental footprint. Likewise, based on the statistics, the construction sector is responsible for about 40% of the European Union's total final energy consumption, 35% of its total CO2 emissions, and 45% of waste generation. That is why significant efforts should be devoted to applying the ‘3Rs’ concept of reducing, reusing, and recycling in the building sector and material fabrication. On the other hand, the textile leftover is one of the predominant waste resources worldwide while only less than 20% is being recycled. The textile industry produces textile wastes (TW) from the primary stages of garment production (pre-consumer waste such as fiber, yarn, and fabric) to the end of its useful life (post-consumer waste: discarded clothes). Thus, the reuse of this textile waste in construction is becoming interesting and convenient due to the shortage of natural mineral resources and increasing waste disposal costs. Recently, sustainable fibers produced from renewable, biodegradable, waste, recycled, available, and low-cost resources becoming a focal point. In this sense, vegetable and cellulosic fibers have already been used as reinforcement in cementitious materials for low- to medium-performance structural applications. TW fiber could be another sustainable alternative for reinforcement in cementitious composites. In view of the abovementioned, this research comprehensively verifies, by means of physical, mechanical, and durability-based material characterization tests, the possibility of incorporation of short TW fiber as well as the nonwoven TW fabric in the cementitious composites as internal reinforcement to produce a sustainable, ductile, and durable composite to be used in building applications. In this regard, several experimental tests were carried out on different mix design samples to characterize the mechanical, microstructural, durability, thermal, acoustic, fire, and shrinkage properties. The results have shown that the recycled TW fiber, especially in the form of nonwoven fabric, could be a technically feasible, sustainable, and durable reinforcement to be used in the cementitious mortar for low to medium-performance structural applications (e.g., façade panels, roofing, raised floors, and masonry structures). Further, the sustainability of the optimum composite as a façade cladding panel (as an example of one projected application) was assessed through the MIVES, a new comprehensive and integrated Multi-Criteria Decision-Making method that embraces the three pillars of sustainability: economic, environmental, and social. Future works on this kind of fiber-reinforced cementitious mortar could be to develop a numerical model simulation or produce a 3D concrete printing (3DCP) prototype by employing additive manufacturing technology.Los materiales cementosos tienen una de las más altas resistencias a la compresión comparada con la de otros materiales de construcción. No obstante, tanto la capacidad de resistencia a la tracción como la tenacidad resultan ser un orden de magnitud menor que la primera, lo que conduce a la fisuración bajo los esfuerzos de tracción provocados por las bajas cargas de servicio. Esta falta de capacidad de resistencia a la tracción puede provocar grietas y roturas frágiles.. Por esta razón, las fibras se han utilizado predominantemente en matrices cementosas con el objetivo de mejorar la tenacidad, la capacidad de absorción de energía, el comportamiento posterior al agrietamiento y la resistencia a la flexión y a la tracción. Aunque durante las últimas décadas, se han probado varios tipos de fibras como asbesto, acero, vidrio y polímeros en matrices frágiles. Algunas de ellas presentan algunas desventajas como efectos perjudiciales para la salud, alto costo y, específicamente, una huella ambiental sustancial. Asimismo, según las estadísticas, el sector de la construcción es responsable de cerca del 40% del consumo total de energía final de la Unión Europea, el 35% de sus emisiones totales de CO2 y el 45% de la generación de residuos. Es por eso que se deben dedicar esfuerzos significativos a aplicar el concepto de las '3R' de reducción, reutilización y reciclaje en el sector de la construcción y la fabricación de materiales. Por otro lado, el sobrante textil es uno de los residuos predominantes a nivel mundial del cual se recicla menos del 20%. La industria textil produce residuos textiles (TW) desde las etapas primarias de la producción de prendas de vestir (restos previos al consumo, como fibras, hilados y telas) hasta el final de su vida útil (excedentes posteriores al consumo: ropa desechada). Así, la reutilización de estos residuos textiles en la construcción se está volviendo interesante y conveniente debido a la escasez de recursos minerales naturales y al aumento de los costes de eliminación de residuos. Recientemente, las fibras sostenibles producidas a partir de recursos renovables, biodegradables, de desecho, reciclados, disponibles y de bajo costo estan adquiriendo protagonismo. En este sentido, las fibras vegetales y celulósicas ya se han utilizado como refuerzo en materiales cementosos para aplicaciones de baja y media resistencia. La fibra TW podría ser otra alternativa sostenible para el refuerzo en compuestos cementosos. En vista de lo anterior, esta investigación verifica exhaustivamente la posibilidad de incorporar fibra corta de TW, así como fieltros de TW en los compuestos cementosos como refuerzo interno para producir un compuesto sostenible, dúctil y duradero para ser utilizado en aplicaciones de construcción. Se realizaron varios ensayos experimentales sobre diferentes muestras para caracterizar las propiedades mecánicas, microestructurales, de durabilidad, térmicas, acústicas, al fuego y de contracción. Los resultados han demostrado que la fibra de TW reciclada, especialmente en forma de fieltro, podría ser un refuerzo técnicamente factible, sostenible y duradero para ser utilizado en el mortero cementoso para aplicaciones estructurales de rendimiento bajo a medio (por ejemplo, paneles de fachada, techos, pisos elevados y estructuras de mampostería). Además, la sostenibilidad del compuesto óptimo destinado a panel de revestimiento para fachada ventilada (como ejemplo de aplicación) se evaluó a través de MIVES, un nuevo método integral e integrado de toma de decisiones de criterios múltiples que abarca los tres pilares de la sostenibilidad: económico, ambiental y social. La investigación futura en este tipo de mortero cementoso reforzado con fibra podría ser el desarrollo de modelos de simulación numérica o la formulación específica de material para impresión de hormigón 3D mediante el empleo de tecnología de fabricación aditiva.Enginyeria de la construcci

Sustainability, durability and mechanical characterization of a new recycled textile-reinforced strain-hardening cementitious composite for building applications

Tesi en modalitat de compendi de publicacionsCementitious materials have one of the highest compressive strength-to-weight ratios compared to other construction materials. Nonetheless, both tensile strength capacity and toughness result to be an order of magnitude less respect to the former, which thereby leads to cracking under tensile stresses caused by service loads. This lack of tensile strength capacity of the material leads to cracking and fragile failure in case the material is insufficiently reinforced. Within this context, fibers can be used in cementitious matrices aiming at enhancing the toughness, energy absorption capacity, post-cracking behavior as well as flexural and tensile strength. Although during the past decades, various types of fibers such as asbestos, steel, glass, and polymeric have been tested in brittle matrices, there have been some disadvantages such as detrimental health effects, high cost, and specifically, substantial environmental footprint. Likewise, based on the statistics, the construction sector is responsible for about 40% of the European Union's total final energy consumption, 35% of its total CO2 emissions, and 45% of waste generation. That is why significant efforts should be devoted to applying the ‘3Rs’ concept of reducing, reusing, and recycling in the building sector and material fabrication. On the other hand, the textile leftover is one of the predominant waste resources worldwide while only less than 20% is being recycled. The textile industry produces textile wastes (TW) from the primary stages of garment production (pre-consumer waste such as fiber, yarn, and fabric) to the end of its useful life (post-consumer waste: discarded clothes). Thus, the reuse of this textile waste in construction is becoming interesting and convenient due to the shortage of natural mineral resources and increasing waste disposal costs. Recently, sustainable fibers produced from renewable, biodegradable, waste, recycled, available, and low-cost resources becoming a focal point. In this sense, vegetable and cellulosic fibers have already been used as reinforcement in cementitious materials for low- to medium-performance structural applications. TW fiber could be another sustainable alternative for reinforcement in cementitious composites. In view of the abovementioned, this research comprehensively verifies, by means of physical, mechanical, and durability-based material characterization tests, the possibility of incorporation of short TW fiber as well as the nonwoven TW fabric in the cementitious composites as internal reinforcement to produce a sustainable, ductile, and durable composite to be used in building applications. In this regard, several experimental tests were carried out on different mix design samples to characterize the mechanical, microstructural, durability, thermal, acoustic, fire, and shrinkage properties. The results have shown that the recycled TW fiber, especially in the form of nonwoven fabric, could be a technically feasible, sustainable, and durable reinforcement to be used in the cementitious mortar for low to medium-performance structural applications (e.g., façade panels, roofing, raised floors, and masonry structures). Further, the sustainability of the optimum composite as a façade cladding panel (as an example of one projected application) was assessed through the MIVES, a new comprehensive and integrated Multi-Criteria Decision-Making method that embraces the three pillars of sustainability: economic, environmental, and social. Future works on this kind of fiber-reinforced cementitious mortar could be to develop a numerical model simulation or produce a 3D concrete printing (3DCP) prototype by employing additive manufacturing technology.Los materiales cementosos tienen una de las más altas resistencias a la compresión comparada con la de otros materiales de construcción. No obstante, tanto la capacidad de resistencia a la tracción como la tenacidad resultan ser un orden de magnitud menor que la primera, lo que conduce a la fisuración bajo los esfuerzos de tracción provocados por las bajas cargas de servicio. Esta falta de capacidad de resistencia a la tracción puede provocar grietas y roturas frágiles.. Por esta razón, las fibras se han utilizado predominantemente en matrices cementosas con el objetivo de mejorar la tenacidad, la capacidad de absorción de energía, el comportamiento posterior al agrietamiento y la resistencia a la flexión y a la tracción. Aunque durante las últimas décadas, se han probado varios tipos de fibras como asbesto, acero, vidrio y polímeros en matrices frágiles. Algunas de ellas presentan algunas desventajas como efectos perjudiciales para la salud, alto costo y, específicamente, una huella ambiental sustancial. Asimismo, según las estadísticas, el sector de la construcción es responsable de cerca del 40% del consumo total de energía final de la Unión Europea, el 35% de sus emisiones totales de CO2 y el 45% de la generación de residuos. Es por eso que se deben dedicar esfuerzos significativos a aplicar el concepto de las '3R' de reducción, reutilización y reciclaje en el sector de la construcción y la fabricación de materiales. Por otro lado, el sobrante textil es uno de los residuos predominantes a nivel mundial del cual se recicla menos del 20%. La industria textil produce residuos textiles (TW) desde las etapas primarias de la producción de prendas de vestir (restos previos al consumo, como fibras, hilados y telas) hasta el final de su vida útil (excedentes posteriores al consumo: ropa desechada). Así, la reutilización de estos residuos textiles en la construcción se está volviendo interesante y conveniente debido a la escasez de recursos minerales naturales y al aumento de los costes de eliminación de residuos. Recientemente, las fibras sostenibles producidas a partir de recursos renovables, biodegradables, de desecho, reciclados, disponibles y de bajo costo estan adquiriendo protagonismo. En este sentido, las fibras vegetales y celulósicas ya se han utilizado como refuerzo en materiales cementosos para aplicaciones de baja y media resistencia. La fibra TW podría ser otra alternativa sostenible para el refuerzo en compuestos cementosos. En vista de lo anterior, esta investigación verifica exhaustivamente la posibilidad de incorporar fibra corta de TW, así como fieltros de TW en los compuestos cementosos como refuerzo interno para producir un compuesto sostenible, dúctil y duradero para ser utilizado en aplicaciones de construcción. Se realizaron varios ensayos experimentales sobre diferentes muestras para caracterizar las propiedades mecánicas, microestructurales, de durabilidad, térmicas, acústicas, al fuego y de contracción. Los resultados han demostrado que la fibra de TW reciclada, especialmente en forma de fieltro, podría ser un refuerzo técnicamente factible, sostenible y duradero para ser utilizado en el mortero cementoso para aplicaciones estructurales de rendimiento bajo a medio (por ejemplo, paneles de fachada, techos, pisos elevados y estructuras de mampostería). Además, la sostenibilidad del compuesto óptimo destinado a panel de revestimiento para fachada ventilada (como ejemplo de aplicación) se evaluó a través de MIVES, un nuevo método integral e integrado de toma de decisiones de criterios múltiples que abarca los tres pilares de la sostenibilidad: económico, ambiental y social. La investigación futura en este tipo de mortero cementoso reforzado con fibra podría ser el desarrollo de modelos de simulación numérica o la formulación específica de material para impresión de hormigón 3D mediante el empleo de tecnología de fabricación aditiva.Postprint (published version

Effect of accelerated aging and silica fume addition on the mechanical and microstructural properties of hybrid textile waste-flax fabric-reinforced cement composites

Incorporating eco-friendly substances obtained from recycled resources and industrial by-products is gaining increased acceptance among building materials. In this context, a cementitious matrix containing supplementary cementitious materials (SCMs) reinforced by recycled fibers may be a promising solution from both a durability and sustainability perspective. This study presents an extensive experimental program carried out on a cement- based composite with Silica Fume (SF), reinforced with recycled textile waste (TW) nonwoven fabric. Initially, the mechanical strength (compression and flexure) of the Portland cement paste substituted with variable SF content (0%–30%) was characterized. Based on the results, laminate plates having six TW fabric layers impregnated with three different cement pastesPostprint (published version

Mechanical performance of aged cement-based matrices reinforced with recycled aramid textile nonwoven fabric: Comparison with other FRCMs

Utilizing recycled fibers as reinforcement in cement-based matrices is an effective means of promoting waste recycling and adopting a circular economy approach in the construction industry. Within this framework, the recycling and potential reutilization of textile residues can improve the pre- and post-cracking performance of cement-based matrices intended for building components with up to intermediate structural responsibilities (i.e., panels and cladding elements for buildings). This research is focused on the mechanical and durability -through forced aging of dry-wet and freeze-thaw cycles- experimental characterization of laminated fabric-reinforced cementitious matrices (FRCMs) containing 4 and 6 nonwoven fabric layers obtained from end-of-life fire-protecting t-shirts. For this purpose, both direct and flexural tensile tests were conducted to characterize the mechanical performance of the composite. The tests on the 6-fabric layers produced panels with Portland Cement (PC) matrix, after 28-day of curing, led to average values of the maximum tensile strength of 3.7 MPa with associated toughness index superior to 25 kJ/m2, and mean modulus of rupture of 11.6 MPa with a fracture energy index of 4.3 kJ/m2. After dry-wet accelerated aging, the post-cracking performance of the developed composites decreased (on average, 40% in toughness and 11% in strength) due to fiber embrittlement. To better understand the performance of aged composites, shredded fibers recovered from protective clothing (mainly consisting of meta-aramid fibers) were immersed in the binary matrix. Accordingly, the mechanical properties of the fibers after 5 and 10 cycles of dry-wet aging were studied. Based on the results, replacing partially PC by silica fume (between 30% and 50%) was seen as a sustainable alternative to improve the performance of the aged fibers by more than 10%

Serviceability parameters and social sustainability assessment of flax fabric reinforced lime-based drywall interior panels

In the search of more environmentally-friendly construction materials, the use of natural-based fibers has gained much attention as reinforcement in the inorganic-based matrix. In this paper, the nonwoven flax fabric reinforced lime composites are created using a dewatering technique, and the serviceability parameters –thermal conductivity, sound absorption coefficient, and residual flexural resistance after exposure to elevated temperature– are determined experimentally. The tests are carried out on two different lime composites prepared under two distinct curing regimens, i.e., accelerated carbonation in a CO2 chamber and natural carbonation in laboratory conditions, to evaluate the effect of forced carbonation. In addition, the experimental results of the serviceability parameters are included in the MIVES model (Integrated Value Model for Sustainability Assessment) to evaluate the social sustainability of the developed material as an interior drywall panel. MIVES, a type of multi-criteria decision-making method, is based on the value function concept and seminars with experts. According to the results of experimental tests, the accelerated cured sample has higher thermal conductivity (~4 times) and lower sound absorption coefficients (~20%) than the naturally cured one. Nonetheless, the flexural performance of the former is 50% (at room temperature) and 100% (at elevated temperature) better. As for the social sustainability index assessed by the MIVES-based multi-objective approach, it ranges between 0.65 and 0.75 (out of 1.0) for both lime composite panels, at least 20% higher than the control lime panel with no reinforcement. The sustainability model designed for this research can be used for assessing the social sustainability performance of other materials although the weights assigned by the experts could be adapted to reflect the perceptions and local preferences.This work was supported through the project grants PID2019-108067RB-I00/AEI/10.13039/501100011033 and PID2020-117530RB-I00/MCIN/AEI/10.13039/501100011033 by the Ministerio de Ciencia e Innovación (MCIN)/Agencia Estatal de Investigación (AEI) of the Spanish Government. The author Payam Sadrolodabaee acknowledges the Banco Santander for the Research Scholarships (Postdoc-UPC 2022 Grant).Peer ReviewedObjectius de Desenvolupament Sostenible::12 - Producció i Consum ResponsablesPostprint (published version

Experimental characterization of comfort performance parameters and multi-criteria sustainability assessment of recycled textile-reinforced cement facade cladding

Within the building construction sector, fiber cement boards have attracted interest as facade cladding materials in the last ten years, especially those that incorporate –for reinforcing purposes– natural and/or recycled synthetic fibers (i.e, from the textile industry). So far, the design-governing parameters of facade cladding panels have been mechanical strength, durability, constructability, aesthetics, insulation capacity, and fire resistance. From the sustainability perspective, the impact of the facade on the economic and energy efficiency performance is most often the parameter that leads the decision-making process. Within this context, the quantification of the sustainability performance of the facade –accounting for economic, environmental, and social indicators– is unfrequently carried out in design and project phases, this being attributed to the lack of methodologies that allow considering and quantifying some relevant indicators representative of the facade sustainability performance. As consequence, decisions made based on solely economic and on some of the environmental indicators might lead to solutions with lower sustainability performance than that required (or expected). Recycled textile waste fabric-reinforced cement board as a facade-cladding material for building envelopes is the focus of this research. In order to characterize the fire resistance, and thermal and acoustic insulation –as relevant serviceability parameters– of this material, an experimental program was carried out. Likewise, the sustainability performance of this facade-cladding is assessed through a method based on the Integrated Value Model for Sustainability Assessment (MIVES). This multi-criteria decision making (MCDM) model relies on the value function concept and the multi-disciplinary participation of experts to identify and quantify the relevant indicators of the facade sustainability performance and the relative importance of indicators and requirements. The MIVES-based model generated for this research can be straightforwardly used for assessing the sustainability performance of facade-cladding techniques made of any material and for any type of building (and location). The application of the MIVES model led to the sustainability index of this new material for facade-cladding ranging from 0.68 to 0.71 (/1.00) for different weighting scenarios.The authors express their gratitude to the Spanish Ministry of Economy, Industry, and Competitiveness for the financial support received under the scope of the projects CREEF (PID2019-108978RB-C32) and RECYBUILDMAT (PID2019-108067RB-I00).Peer ReviewedPostprint (published version

A textile waste fiber-reinforced cement composite: comparison between short random fiber and textile reinforcement

Currently, millions of tons of textile waste from the garment and textile industries are generated worldwide each year. As a promising option in terms of sustainability, textile waste fibers could be used as internal reinforcement of cement-based composites by enhancing ductility and decreasing crack propagation. To this end, two extensive experimental programs were carried out, involving the use of either fractions of short random fibers at 6–10% by weight or nonwoven fabrics in 3–7 laminate layers in the textile waste-reinforcement of cement, and the mechanical and durability properties of the resulting composites were characterized. Flexural resistance in pre- and post-crack, toughness, and stiffness of the resulting composites were assessed in addition to unrestrained drying shrinkage testing. The results obtained from those programs were analyzed and compared to identify the optimal composite and potential applications. Based on the results of experimental analysis, the feasibility of using this textile waste composite as a potential construction material in nonstructural concrete structures such as facade cladding, raised floors, and pavements was confirmed. The optimal composite was proven to be the one reinforced with six layers of nonwoven fabric, with a flexural strength of 15.5 MPa and a toughness of 9.7 kJ/m2.This research was funded by the Spanish Ministry of Economy, Industry, and Competitiveness, RECYBUILDMAT (PID2019-108067RB-I00) and CREEF (PID2019-108978RB-C32).Peer ReviewedObjectius de Desenvolupament Sostenible::12 - Producció i Consum ResponsablesPostprint (published version

Characterization of eco-friendly lightweight aggregate concretes incorporating industrial wastes

Towards the sustainable development goals in the built environment, the use of waste and recycled sources has been attaining great interest among researchers and policy-makers, especially in concrete as the most used construction material. Excess use of natural aggregates, as one of the main components of concrete, causes the depletion of natural resources and the associated environmental problems, thus, the use of artificial and recycled aggregates is of great importance. In this regard, the production of lightweight artificial aggregates from industrial and hazardous wastes may be a promising solution that not only mitigates the depletion of natural resources but also stabilize those kinds of wastes. This study aimed to investigate the production of concrete with recycled aggregates from industrial wastes, mainly municipal solid waste incineration fly ash (MSWI-FA). To this end, different kinds of mix designs to manufacture the aggregates were developed based on MSWI-FA, ground granulated blast furnace slag (GGBFS), marble sludge (MS), and cement. The concrete samples containing different artificial aggregates, as well as recycled polyethylene terephthalate (PET) in the sand form, were produced and the properties, including compressive strength and thermal insulation, were evaluated. The obtained results of the lightweight concrete demonstrated enhanced thermal property (up to 30%), but at least 30% lower resistance with respect to the normal concrete produced from the natural aggregate.Postprint (author's final draft

Cement composite panels reinforced with textile waste and flax nonwoven fabrics: flexural and accelerated aging performance

Towards more eco-friendly building materials, nonwoven fabrics from natural-based (flax) and recycled (textile waste) fibers were produced and incorporated into the cementitious composite panels. The textile waste fibers were recovered from two different clothing sources including fashion and fire-proofing garments. The flexural and durability (through forced wet-dry aging cycles) properties of the laminated sandwich-like cement panels reinforced with four different nonwoven fabrics (flax, treated flax, hybrid fashion textile waste/flax, and fire-proofing textile waste) were analyzed. Based on the results, all four different composites showed promising flexural-hardening behaviors in unaged conditions, especially those reinforced with the treated flax or hybrid fabrics which reached flexural strength of more than 16 MPa and energy ab-sorption index of more than 10 kJ/m2. After wet-dry aging, the post-cracking performance of all composites degraded due to fiber embrittlement, this was more evident for untreated flax composite (reduced by ~65%). Nonetheless, the treated flax fabric composite and hybrid fabric composite containing silica fume could show a better performance after aging.The authors express their gratitude to the Agencia Estatal de Investigación, Spanish Ministry of Economy, Industry, and Competitiveness (Government of Spain) for the financial support received under the scope of the projects RECYBUILDMAT (PID2019-108067RB-I00/AEI/10.13039/501100011033); 3D-Crete (2019PROD00066/IU68-017262-Codi project: J-02696); and CREEF (PID2019-108978RB-C32/AEI/10.13039/501100011033). The first author acknowledges the Banco Santander for the Research Scholarships (Postdoc-UPC 2022 Grant).Postprint (published version

Compressive and Thermal Properties of Non-Structural Lightweight Concrete Containing Industrial Byproduct Aggregates

This study aimed to investigate the recycling opportunities for industrial byproducts and their contribution to innovative concrete manufacturing processes. The attention was mainly focused on municipal solid waste incineration fly ash (MSWI-FA) and its employment, after a washing pre-treatment, as the main component in artificially manufactured aggregates containing cement and ground granulated blast furnace slag (GGBFS) in different percentages. The produced aggregates were used to produce lightweight concrete (LWC) containing both artificial aggregates only and artificial aggregates mixed with a relatively small percentage of recycled polyethylene terephthalate (PET) in the sand form. Thereby, the possibility of producing concrete with good mechanical properties and enhanced thermal properties was investigated through effective PET reuse with beneficial impacts on the thermal insulation of structures. Based on the obtained results, the samples containing artificial aggregates had lower compressive strength (up to 30%) but better thermal performance (up to 25%) with respect to the reference sample made from natural aggregates. Moreover, substituting 10% of recycled aggregates with PET led to a greater reduction in resistance while improving the thermal conductivity. This type of concrete could improve the economic and environmental aspects by incorporating industrial wastes—mainly fly ash—thereby lowering the use of cement, which would lead to a reduction in CO2 emissions