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CO2 emissions and uptake in rendering mortars: sustainable approach

Emissões e captura de CO2 em argamassas de revestimento: abordagem sustentável

Abstract

This study aimed to estimate CO2 emission and uptake due to the mixed rendering mortars carbonation process, replacing natural aggregate (NA) with recycled aggregate (RA) and lime contents to produce less emissive final material. Mortars were subjected to natural carbonation in three environments. Carbonation depth was evaluated until 119 days of CO2 environmental exposure, and thermogravimetric analysis determined absorbed carbon content. The scenario evaluation considered different depths for the potential CO2 uptake. It was found that the higher the lime content, the higher the carbon emissions and capture. However, the CO2 captured amount (25.41 KgCO2/m³) does not balance its emission from raw materials production. Replacing NA for RA, the mortars reduced emissions associated with production by approximately 8.15%. The scenario simulation proved that constructive control is essential for rendering mortar depths less than 20 mm, the maximum carbon fixation is reached before rendering. Mortars with recycled aggregate are less emissive, which is essential in selecting more sustainable building materials.

Keywords:
CO2 sequestration; Sustainability; Compensatory measures; Construction waste.

Resumo

Este estudo teve como objetivo estimar a emissão e captura de CO2 devido ao processo de carbonatação de argamassas mistas, substituindo agregado natural (AN) por agregado reciclado (AR) e teores de cal para produzir material final menos emissivo. As argamassas foram submetidas à carbonatação natural em três ambientes. A profundidade de carbonatação foi avaliada aos até 119 dias de exposição ao tempo com presença de CO2, e a análise termogravimétrica determinou o teor de carbono absorvido. A avaliação dos cenários considerou diferentes profundidades para o potencial absorção de CO2. Verificou-se que quanto maior o teor de cal, maiores as emissões e captura de carbono. Contudo, a quantidade de CO2 capturada (25,41 KgCO2/m³) não neutraliza a sua emissão proveniente da produção de matérias-primas. Substituindo o AN pelo AR, as argamassas reduziram as emissões associadas à produção, em aproximadamente 8,15%. A simulação dos cenários comprovou que o controle construtivo é essencial, pois em espessuras inferiores a 20 mm a fixação máxima de carbono é alcançada antes da aplicação de revestimento. As argamassas com agregados reciclados são menos emissivas, o que é essencial na seleção de materiais de construção mais sustentáveis.

Palavras-chave:
Sequestro de CO2; Sustentabilidade; Medidas compensatórias; Resíduos de construção.

Introduction

The construction industry (CI) is one of the most significant contributors to anthropogenic emissions. Only the cement industry is globally responsible for about 7% of all carbon dioxide emissions (IEA; WBCSD, 2018INTERNATIONAL ENERGY AGENCY; WORLD BUSINESS COUNCIL FOR SUSTAINABLE DEVELOPMENT. Technology Roadmap: Low-Carbon Transition in the Cement Industry. Paris, 2018.). CO2, a major Greenhouse Gas (GHG), is responsible for increasing the average planet temperature and consequent global warming (IPCC, 2021INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE. Climate change 2021: the physical science basis. contribution of working group I to the sixth assessment report of the IPCC. New York: Cambridge University Press, 2021.). Several carbon capture processes have been studied to reduce the impacts associated with CO2 emissions. One of them is the carbonation reaction method (recarbonation), which in cement-based materials corresponds to CO2 reaction from the atmosphere with alkaline compounds such as calcium hydroxide and portlandite (Ca(OH)2) present in the cementitious material, forming calcium carbonate (CaCO3) and water (H2O (Wijayasundara; Mendis; Ngo, 2017WIJAYASUNDARA, M.; MENDIS, P.; NGO, T. Comparative assessment of the benefits associated with the absorption of CO 2 with the use of RCA in structural concrete. Journal of Cleaner Production, v. 158, p. 285-295, 2017.; Yang; Seo; Tae, 2014YANG, K. H.; SEO, E. A.; TAE, S. H. Carbonation and CO2 uptake of concrete. Environmental Impact Assessment Review, v. 46, p. 43-52, 2014.)). This reaction also occurs in lime-based materials, where calcium and magnesium hydroxides may be present, depending on the rock mineralogical origin used in lime production (Ergenç; Fort, 2018ERGENÇ, D.; FORT, R. Accelerating carbonation in lime-based mortar in high CO2 environments. Construction and Building Materials, v. 188, p. 314-325, 2018.; Kang; Kwon; Moon, 2019KANG, S.-H.; KWON, Y.-H.; MOON, J. Quantitative analysis of CO2 uptake and mechanical properties of air lime-based materials. Energies, v. 12, n. 15, p. 2903, 2019.).

In Brazil, it is common the use lime in mortars, due to its satisfactory properties, such as lower drying shrinkage and greater water retention, workability, and plasticity (Chever; Pavía; Howard, 2010CHEVER, L.; PAVÍA, S.; HOWARD, R. Physical properties of magnesian lime mortars. Materials and Structures, v. 43, n. 1-2, p. 283-296, 2010.). Cement and hydrated lime rendering mortars are very porous, allowing the rapid diffusion of atmospheric CO2 into the matrix when applied in small thicknesses, which favors CO2 fixation by the natural carbonation process. However, in a high porosity network, CO2 capture is also intertwined with the chemical composition (Ergenç; Fort, 2018ERGENÇ, D.; FORT, R. Accelerating carbonation in lime-based mortar in high CO2 environments. Construction and Building Materials, v. 188, p. 314-325, 2018. Kang; Kwon; Moon, 2019KANG, S.-H.; KWON, Y.-H.; MOON, J. Quantitative analysis of CO2 uptake and mechanical properties of air lime-based materials. Energies, v. 12, n. 15, p. 2903, 2019.; Borges et al., 2023BORGES, P. M. et al. Mortars with recycled aggregate of construction and demolition waste: mechanical properties and carbon uptake. Construction and Building Materials, v. 387, 2023.). CO2 may chemically react with (Ca(OH)2) and Mg(OH)2, forming calcium carbonate (CaCO3) and magnesium carbonate (MgCO3), respectively (Delabona; Gava; Rufatto, 2020DELABONA, F. J.; GAVA, G. P.; RUFATTO, M. Avaliação do potencial de captura de CO2 em argamassas de revestimento devido à carbonatação natural. In: ENCONTRO ANUAL DE INICIAÇÃO CIENTÍFICA, 6., Cascavel, 2020. Anais [...] Cascavel, 2020.; Fontolan; Gava; Silva, 2020FONTOLAN, B. L.; GAVA, G. P.; SILVA, T. B. Contribuição ao sequestro de CO2 devido a carbonatação natural de argamassas com agregado de resíduo de construção e demolição. In: ENCONTRO ANUAL DE INICIAÇÃO CIENTÍFICA, 6., Cascavel, 2020. Anais [...] Cascavel, 2020.).

Environmental aspects demand solutions for low-carbon materials (Coppola et al., 2019COPPOLA, L. et al. An Empathetic Added Sustainability Index (EASI) for cementitious based construction materials. Journal of Cleaner Production, v. 220, p. 475-482, 2019.). According to Fukui et al. (2013FUKUI, E. et al. CO2 liberado na produção de argamassas. In: SIMPÓSIO BRASILEIRO DE TECNOLOGIA DAS ARGAMASSA, 10., Fortaleza, 2013. Anais [...] Fortaleza, 2013.), the amount of CO2 released from mortar production is directly proportional to the cement and hydrated lime percentage in its composition. The use of these materials ought to be balanced (Forster et al., 2020FORSTER, A. M. et al. Lime binders for the repair of historic buildings: Considerations for CO2 abatement. Journal of Cleaner Production, v. 252, p. 119802, 2020.; Maddalena; Roberts; Hamilton, 2018MADDALENA, R.; ROBERTS, J. J.; HAMILTON, A. Can Portland cement be replaced by low-carbon alternative materials? A study on the thermal properties and carbon emissions of innovative cements. Journal of Cleaner Production, v. 186, p. 933-942, 2018. ). Cement emission in Brazil is around 564 kg per ton of product (IEA; WBCSD, 2018INTERNATIONAL ENERGY AGENCY; WORLD BUSINESS COUNCIL FOR SUSTAINABLE DEVELOPMENT. Technology Roadmap: Low-Carbon Transition in the Cement Industry. Paris, 2018.), and lime emission range from 607 to 2169 kg per ton of product due to the different technologies and combustion materials used and the raw materials' decarbonation. The high lime emission is due to the lack of efficiency and control of the lime kiln used in manufacturing, which varies according to the type and amount of energy consumed (John; Punhagui; Cincotto, 2014JOHN, V. M.; PUNHAGUI, K. R. G.; CINCOTTO, M. A. Produção de cal: economia de baixo carbono, impactos de novos marcos regulatórios e tecnologias sobre a economia brasileira. Ribeirão Preto: Funpec, 2014.).

Approximately 50% of the cement produced in Brazil is used for mortar production, whereas 4% is industrialized and 96% is made on-site (Punhagui et al., 2018PUNHAGUI, K. R. G. et al. Estudo de baixo carbono para a indústria de cimento no estado de São Paulo de 2014 a 2030: sumário executivo. São Paulo: CETESB, 2018. v. 1.) by artisanal and not controlled techniques. According to Sindicato Nacional da Indústria do Cimento (SNIC, 2022SINDICATO NACIONAL DA INDÚSTRIA DO CIMENTO. Dados do setor. São Paulo. 2022. Available: Available: http://snic.org.br/numeros-do-setor.php . Access: 01 jan. 2023.
http://snic.org.br/numeros-do-setor.php...
), the Brazilian construction sector produced 65.80 million tons of cement in 2020. That is 28.08 million tons of cement destined for mortar production. Based on a standard mortar mix (1:1:6), the demand is 167 million tons of sand. Considering that 50% of mortars at the end of their life cycle turn into fine aggregate, 83.56 million tons of RA are generated with potential use in mortar - a non-structural material used in Brazil in large volume - production.

Literature studies (Šavija; Luković, 2016ŠAVIJA, B.; LUKOVIĆ, M. Carbonation of cement paste: understanding, challenges, and opportunities. Construction and Building Materials, v. 117, p. 285-301, 2016.; Shin; Kim, 2022SHIN, B.; KIM, S. CO2 emission and construction cost reduction effect in cases of recycled aggregate utilized for nonstructural building materials in South Korea. Journal of Cleaner Production, v. 360, p. 131962, 2022.) indicate the technical and economic feasibility of using recycled fine aggregates (RA) in rendering and/or laying mortar production. Their use in mortar production minimizes associated waste’s environmental impacts Costa and Ribeiro (2020COSTA, F. N.; RIBEIRO, D. V. Reduction in CO2 emissions during production of cement, with partial replacement of traditional raw materials by civil construction waste (CCW). Journal of Cleaner Production, v. 276, p. 123302, 2020.). It contributes to the CO2 capture process due to the existence of chemical compounds that can undergo carbonation. Construction and demolition waste has calcium hydroxide (Ca(OH)2) and hydrated calcium silicate (C-S-H) in its composition, contributing to carbonation chemical reaction development (Cai; Wang; Xiao, 2018CAI, J.; WANG, S.; XIAO, Z. A study on the CO2 capture and attrition performance of construction and demolition waste. Fuel, v. 222, p. 232-242, 2018.). Using RA in cementitious materials reduces CO2 emissions by increasing waste utilization. When demolished RAs are not fully carbonated, they may increase the potential for CO2 capture when used as aggregates. In this process, new surfaces are exposed, and a new carbonation cycle begins, fixing CO2 from the atmosphere (Kaliyavaradhan; Ling, 2017KALIYAVARADHAN, S. K.; LING, T.-C. Potential of CO2 sequestration through construction and demolition (C&D) waste: an overview. Journal of CO2 Utilization, v. 20, p. 234-242, 2017.).

The CO2 captured by recarbonation can be quantified indirectly based on the carbonated depth or directly by TGA (Possan, 2019POSSAN, E. Captura de CO2 em materiais cimentícios. Concreto & Construção, v. 1, p. 60-66, 2019.). Thermogravimetric analysis (TGA) is a powerful method for determining the amount of carbonate material amount present in concrete and mortars. Calcium carbonate decomposition analysis allows for studying the cementitious materials samples’ carbonation and CO2 absorption (Villain; Thiery; Platret, 2007VILLAIN, G.; THIERY, M.; PLATRET, G. Measurement methods of carbonation profiles in concrete: Thermogravimetry, chemical analysis and gammadensimetry. Cement and Concrete Research, v. 37, n. 8, p. 1182-1192, 2007.). Such a method is feasible for determining the cement-based materials' carbonation degree; it quantifies the calcium carbonate content in a sample extracted from concrete mortar and reduced to powder. By the derived curve (DTG), the potential CO2 capture is obtained by balancing the mass lost between the carbonated and non-carbonated samples (Mazurana et al., 2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.; Neves Junior et al., 2019NEVES JUNIOR, A. et al. Determination of CO2 capture during accelerated carbonation of engineered cementitious composite pastes by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 138, n. 1, p. 97-109, 2019.).

Some studies (Mazurana et al., 2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.; Borges et al., 2023BORGES, P. M. et al. Mortars with recycled aggregate of construction and demolition waste: mechanical properties and carbon uptake. Construction and Building Materials, v. 387, 2023.) have evaluated CO2 capture in rendering mortars with different RA contents. Mazurana et al. (2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.) evaluated CO2 capture in mortars with RA and hydrated lime using thermogravimetry and reported that rendering mortar samples exposed to the action of atmospheric CO2 resulted in carbonate depths ranging from 3 mm to 20 mm at 28 days of natural carbonation, depending on the mortars' ratio. Borges et al. (2023BORGES, P. M. et al. Mortars with recycled aggregate of construction and demolition waste: mechanical properties and carbon uptake. Construction and Building Materials, v. 387, 2023.) evaluated carbon capture by carbonation in cementitious mortars with three granulometric distributions of recycled aggregates and found that CO2 capture was higher in matrices with 100% recycled aggregate.

The studies did not approach the influence of hydrated lime content on CO2 capture and did not evaluate projections to later ages based on the thermogravimetric method. Hydrated lime is widely used in Brazil and several countries where masonry construction system is coated with mortar. Lime production contributes to significant CO2 due to the decarbonization and the use of fossil fuels and/or wood burning. Understanding how this material influences CO2 emissions and capture due to mineralization in the life cycle is relevant for the selection of the material that leads to less emissive mortar production. This study can also help to fit materials to ease carbon capture throughout the building life cycle.

Andrade et al. (2018ANDRADE, J. J. O. et al. Evaluation of mechanical properties and carbonation of mortars produced with construction and demolition waste. Construction and Building Materials, v. 161, 2018.) studied rendering mortars with RA without hydrated lime using analytical equations based on the samples' carbonation depth and reported that rendering mortar samples exposed to the action of atmospheric CO2 resulted in carbonate depths ranging from 3 mm to 20 mm at 250 days of natural carbonation.

Further studies are needed to determine potential CO2 uptake in cement and lime-based rendering and laying mortars to assess its influence and the exposure environment. By thermogravimetric analysis, this research aims to estimate the amount of CO2 captured, in rendering mortars produced with different lime contents and with natural aggregate replaced by different fine aggregate contents from construction demolition waste, exposed to natural carbonation. By evaluating the CO2 emissions and potential capture of rendering mortars with construction and demolition waste over time, this study converges with the Paris Agreement’s goal, signed at the 21st United Nations Framework Convention on Climate Change (UNFCCC). The agreement intends to limit global warming to 1.5 °C above pre-industrial levels by reducing GHGs, especially anthropogenic carbon dioxide emissions. The Sustainable Development Goals (SDGs) 12 - Responsible Consumption and Production and 13 - Action United Nations are also within the research's scope. Studying the CO2 capture in cement-based mortars and the influence of lime and recycled aggregate on it, it’s important in the current context. In addition, promoting the utilization of waste materials in the construction industry can help offset a portion of the emissions generated during the manufacturing of construction materials.

Material and methods

Materials and mortars' production and characterization

To evaluate the CO2 uptake in rendering mortars, 40 x 40 x 160 mm prismatic specimens were produced as per Table 1. Each specimen is classified based on its lime content and whether it consists solely of natural sand (NA) or includes recycled aggregate (RA). Pozzolanic Portland cement (CP II - Z - 32, similar to ASTM C-595 I P), dolomitic lime (CH-III), quartz-origin natural sand, and RA (Figure 1) were used as per the chemical and physical characteristics shown in Table 2.

Table 1 -
Materials consumption for 1m³ of rendering mortar production

Figure 1 -
Materials used in mortar production

Table 2 -
Materials' Physico-chemical composition

The mortars (Table 1) were produced in the laboratory using a mechanical mixer and characterized in a fresh and hardened state, 7 samples of each mixture. The consistency index was set at 270mm±20mm according to NBR 13276 (ABNT, 2016ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 13276: mortars applied on walls and ceilings: determination of the consistence index. Rio de Janeiro, 2016.). Flexural tensile strength was performed in triplicate on air-cured specimens measuring 40x40x160 mm at 28 days, according to NBR 13279 (ABNT, 2005aASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 13279: argamassa para assentamento e revestimento de paredes e tetos: determinação da resistência à tração na flexão e à compressão. Rio de Janeiro, 2005a.). Compressive strength was determined at 28 days on six specimen fragments obtained from the flexural tensile test. The mechanical tests were performed on an Emic Mue 100 model universal machine using a 5-ton cell. The capillarity coefficient was determined according to NBR 15259 (ABNT, 2005bASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 15259: argamassa para assentamento e revestimento de paredes e tetos: determinação da absorção de água por capilaridade e do coeficiente de capilaridade. Rio de Janeiro. 2005b. ), using 3 specimens for each mixture. The elastic modulus was assessed using the Impulse excitation technique (IET) in three specimens, using the longitudinal method according to E-1876 (ASTM, 2021AMERICAN SOCIETY FOR TESTING AND MATERIALS. E-1876: standard test method for dynamic young’s modulus, shear modulus, and poisson’s ratio by impulse excitation of vibration. Philadelphia, 2021.) and Thomaz, Miyaji and Possan (2021THOMAZ, W. de A.; MIYAJI, D. Y.; POSSAN, E. Comparative study of dynamic and static Young’s modulus of concrete containing basaltic aggregates. Case Studies in Construction Materials, v. 15, p. e00645, 2021.) using an ATCP Physical Engineering nonelastic equipment.

The water retention test was carried out following NBR 13277 (ABNT, 2012ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 13277: argamassa para assentamento e revestimento de paredes e tetos: determinação da retenção de água. Rio de Janeiro, 2012.). Three specimens were tested for each mixture with the Buncher funnel. Using a vacuum pump, the mortar was subjected to a consistent pressure of 51 mmHg for a duration of 15 minutes. The result was obtained from the ratio between the mass difference of the set before and after suction.

Mortars carbonation

40x40x160mm rendering mortar specimens were exposed to CO2 natural action for 119 days in three different exposure environments (indoor, outdoor without rain protection, and outdoor with rain protection). Three specimens were submitted to the carbonation test for each studied combination, fractured over time. CO2 concentrations were monitored automatically during the specimens’ exposure time (Figure 2) from an Arduino-based electronic system developed for the study. CO2, relative humidity, and temperature sensors were attached to the system.

Figure 1 showed that CO2 concentration was about 492 ppm in the exposure environment, higher than the global annual average (417 ppm) (Stein, 2023STEIN, T. NOAA index tracks how greenhouse gas pollution amplified global warming in 2022. 2023. Available: Available: https://research.noaa.gov/2023/05/23/noaa-index-tracks-how-greenhouse-gas-pollution-amplified-global-warming-in-2022/. Access: 26 mar. 2024.
https://research.noaa.gov/2023/05/23/noa...
). The indoor environment had the highest average concentration as it is a closed laboratory environment without any ventilation. It was noted that CO2 concentration was higher in the protected environment than in the unprotected one. These environments were near a parking lot, implying high CO2 exposure throughout the day.

The specimens were fractured longitudinally at each test age to obtain a slice to measure carbonation depth over time. The top of the specimens was not sealed, considering that the CO2 diffusion rate was not enough to overcome the size of the sample fraction at the test times determined in the experimental design. Carbonation depth measurement was carried out from 14 to 119 days, defined by image analysis using the ImageJ software with the area approximation method for greater accuracy with less operator interference.

CO2 capture determination by thermogravimetric analysis

For thermogravimetric analysis, specimens exposed for 28 days to the CO2 natural action outdoors in a rain protection environment were transversely fractured. Phenolphthalein chemical indicator was sprayed on one side of the specimen, as shown in Figure 3. The material was then collected in the mirror specimen (without chemical indicator spray) from the carbonated (C) and non-carbonated (NC) areas. A small aliquot (approximately 5 g) from each area was extracted (lecron spatula) from a fractured surface of the prismatic specimen after the CO2 exposure. The collected dry samples were inserted in identified small sample holders, closed, and kept in the laboratory until analyses perform. The sample collection and storage were done carefully to avoid contamination. At first, the collected aliquots were milled, posteriorly sifted on 0.15mm sieves, then stored in hermetically sealed and identified specimen holders and kept in the laboratory until the analysis. This process was carried out quickly to avoid the material´s atmospheric carbonation.

Figure 2 -
CO2 concentration over time in different exposure environments collected by Arduino system

Figure 3 -
Sample collection process representation

Thermogravimetric analysis (TGA) was performed using a STA8000 PerkinElmerTM simultaneous thermal analyzer with a reading ranging from 15 to 1600 ºC. Alumina crucible, 50±1 mg mass for each specimen, 20 mL.min-1 nitrogen flow, 20 °C.min-1 equipment heating rate and a heating range from 35 °C to 900 °C were used as test parameters. In the first hour of testing, the specimens were preheated inside the equipment at 35 °C for humidity removal. TG and DTG curves were analyzed using the OriginPro® software to determine the CO2 capture. The compounds' volatilization points from non-carbonated (NC) and carbonated (C) materials and cementitious materials were analyzed to determine the masses corresponding to each characteristic peak's initial and final temperatures. These values were input on tabulation software to calculate the mass loss differences between specimens, as shown in Equations 1 and 2.

% C a C O 3 = ( M L C - M L N C ) / ( M 1 ) Eq. 1

Where:

  • %CaCO3:  Present calcium carbonate percentage;
  • MLC:  Calcium carbonate peak’s mass loss from the carbonated sample;
  • MLNC:  Calcium carbonate peak’s mass loss from the non-carbonated sample; e
  • M1:  Calcium carbonate chemical mole fraction.

% M g C O 3 = ( M L C ) x ( M 2 ) Eq. 2

Where:

  • %MgCO3:   Present magnesium carbonate percentage;
  • MLC:   Magnesium carbonate peaks from the carbonated sample; e
  • M2:   Magnesium carbonate chemical mole fraction.

Using the tangent method described by Scrivener, Snellings and Lothenbach (2016SCRIVENER, K.; SNELLINGS, R.; LOTHENBACH, B. A practical guide to microstructural analysis of cementitious materials. New York: CRC Press, 2016.), it was possible to determine the amount of CO2 captured between the calcium carbonate, magnesium carbonate molar masses, and carbon dioxide, as seen in Equations 3 and 4.

% C a p C O 2 = ( % C a C O 3 x M 1 ) + ( % M g C O 3 x M 2 ) Eq. 3

Where:

  • %CapCO2:   Carbon dioxide captured percentage;
  • %CaCO3:   Present calcium carbonate percentage;
  • %MgCO3:   Percentage of existing magnesium carbonate; e
  • M1 e M2:   Calcium and magnesium carbonate chemical mole fraction, respectively.

C a p C O 2 = d 1 x % C a C O 3 + % M g C O 3 m i x 0,1 Eq. 4

Where:

  • CapCO2:   Amount of carbon dioxide captured (kg.CO2/m³);
  • d1:   Dry weight (kg/m³);
  • %CaCO3:   Present calcium carbonate percentage;
  • %MgCO3:   Present magnesium carbonate percentage; and
  • mi:   initial sample mass (mg).

Emissions determination

The estimative of CO2 emissions for mortar production was obtained by the product of the emission adding linked to each material (Table 3) multiplied by consumption (Table 1). The emissions calculation for cement production is mainly due to clinker production by decarbonation and heating of kilns, as well as the use of energy (Equation 5).

E = E c l i n k e r x % C l i n j e r c e m + E e n e r g y x E F e n e r g y + E f i l l e r Eq. 5

Where:

  • Ecem =   Emission for cement production, in kgCO2/t;
  • Eclinker =   Emission for clinker production, in kgCO2/t;
  • %Clinkercem =   clinker content in cement;
  • Eenergy =   Emission by electrical energy, in kWh/t;
  • EFenergy =   Emission factor of electrical energy, in KgCO2/kWh; and
  • Efiller =   Emission for filler production, in kgCO2/t.

The clinker content value of cement was obtained by deducting the loss of ignition value (8.05% as per Table 2 and the insoluble residue percentage (12.15%), according to the Cement Test Report, considering a 5% sulfate addition. The cement has a 75% clinker content.

Transport emissions were not included in this study since they depend on the materials manufacturing and the mortar production places, which may vary.

Emission balance

The difference between the total CO2 emitted (Etotal) during mortar production and the CO2 captured by mineralization (CapCO2) is the emission balance (BCO2), representing the CO2 captured percentage relative to that emitted (Equation 6).

B C O 2 % = ( C a p C O 2 / E t o t a l ) x 100 Eq. 6

Scenarios analysis

As Kaliyavaradhan and Ling (2017KALIYAVARADHAN, S. K.; LING, T.-C. Potential of CO2 sequestration through construction and demolition (C&D) waste: an overview. Journal of CO2 Utilization, v. 20, p. 234-242, 2017.) and Possan (2019POSSAN, E. Captura de CO2 em materiais cimentícios. Concreto & Construção, v. 1, p. 60-66, 2019.) proposed, it is possible to determine the potential and effective CO2 capture due to the mineralization of cement-based materials. Usually, the potential capture does not consider the different material applications expressed in kg. CO2/m³. The effective or real capture capacity, on the other hand, considers the material during its life cycle, the depth of application, exposure environment, and surface protection, among other factors that affect the CO2 diffusion in the cement-based material Possan (2019). It is possible to measure CO2 capture in different scenarios as CO2 advances over time, which is given by measuring the carbonation depth reaching different ages. With effective capture, it is possible to analyze the behavior of rendering mortar over time by different exposure scenarios analysis, thus forecasting CO2 capture. In this way, the carbonation depths in the mortar specimens were measured at 14, 28, 42, 63, 91, and 119 days, based on the procedures described by Rilem CPC 18 (1988RILEM. CPC-18: measurement of hardened concrete carbonation depth. Materials and Structures, v. 21, n. 6, p. 453-455, 1988.). Table 4 presents the line equation for each mortar and the mathematical model's respective adjustment coefficient (R²). The line equation was obtained with de values from the 119 fist days carbonation depth.

Table 3 -
CO2 emissions associated with the mortars materials production

Table 4 -
Linear adjustment of mortar carbonation depth (y) as a function of natural exposure time to CO2 (x)

For advanced ages, an extrapolation was performed using the Tuutti (1982TUUTTI, K. Corrosion of steel in concrete. Stockholm: Swedish cement and concrete research intitute, 1982.) model (Equation 7) simulating the carbonated depth that would exceed the 20 mm specimen limit. Equation 7 describes the diffusion of CO2 in concrete. This was used in this study as a CO2 diffusion model for rendering mortars was not found in the literature. Because it is not a material with a structural function, the study of CO2 diffusion in rendering mortars is recent and derived from the current demands for carbon capture due to the carbonation of cement-based materials, which justifies the lack of models. The model was used only in the scenarios study.

d c = k x t Eq. 7

Where:

  • dc =   carbonated depth (cm);
  • k =   carbonation coefficient; and
  • t =   time in days.

The CO2 capture estimative was calculated for the outdoor with rain protection environment, considering the period of six months of natural exposure without surface protection. As shown by Barbosa (2020BARBOSA, M. C. Efeito da proteção superficial do tipo pintura na captura de CO2 por carbonatação em argamassas de revestimento. Curitiba, 2020. Dissertação (Mestrado em Engenharia Civil) - Universidade Tecnológica Federal do Paraná, Curitiba, 2020.) surface protection significantly reduces CO2 capture (up to 38.11% in 164 days).

The determination of scenarios to calculate the masonry amount (m²) that may be coated with 1 m³ of mortar was needed to estimate the capture based on the carbonation depth. The first scenario was determined with an average thickness of 20 mm. From this, it was possible to determine the masonry area that could be coated with 1 m³ of mortar, as shown in Figure 4, resulting in a 50 m²/m³ of mortar ratio. The other scenarios (Table 5) followed the same logic for different depths since different render thicknesses are used due to a lack of control in construction (due to masonry plump correction).

From the thermogravimetric analysis results, it was possible to measure how much CO2 each mortar captured for each age in kg.CO2/m³. The surface was considered without any paint or other type of rendering. The CO2 concentration, relative humidity, and precipitation were not evaluated. For different materials and conditions, other results should be found.

Figure 4 -
Schematic representation of the m²/m³ ratio

Table 5 -
Scenarios description for an outdoor with rain protection environment

Results and discussion

Mortars' characterization

Table 6 shows that the mortars used in this study had a low compressive strength for the mortars. However, they are within limits specified by the ABNT 13281 standard (ABNT, 2005cASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS. NBR 13281: argamassa para assentamento e revestimento de paredes e tetos: requisitos. Rio de Janeiro, 2005c.) for rendering mortars, meeting the density, workability, capillarity, resistance, and compatibility standards for the application surface.

A similar consistency index of the mortars was maintained, between 270±20, varying the water consumption (Table 1). The higher the lime content, the greater the water retention and the greater the amount of water needed to achieve the desired consistency Chever, Pavía and Howard (2010CHEVER, L.; PAVÍA, S.; HOWARD, R. Physical properties of magnesian lime mortars. Materials and Structures, v. 43, n. 1-2, p. 283-296, 2010.). The mortars' performance in the fresh and hardened state must be observed since adding RA at certain ratios may reduce mortars' flexural tensile strength and compressive strength (Mazurana et al., 2021). Andrade et al. (2018ANDRADE, J. J. O. et al. Evaluation of mechanical properties and carbonation of mortars produced with construction and demolition waste. Construction and Building Materials, v. 161, 2018.) observed that the higher the RA contents in the mortar composition, the greater the porosity and water absorption (Table 2), and the lower the density and mechanical resistance. According to the literature, such a decrease is directly related to the increase in the amount of water required to achieve workability, leading to a greater material porosity in the hardened state. Porosity is linked to an increase in water consumption and a decrease in cement content. It is observed that the 1.0L-RA mortar presented similar behavior to 2.0L-NA for such characteristics. Replacing NA for RA by up to 30% gives a satisfactory performance in the fresh and hardened state to produce rendering mortars (Andrade; Sanjuán, 2018ANDRADE, C.; SANJUÁN, M. Updating carbon storage capacity of spanish cements. Sustainability, v. 10, n. 12, p. 4806, 2018.; Muñoz-Ruiperez et al., 2016MUÑOZ-RUIPEREZ, C. et al. Lightweight masonry mortars made with expanded clay and recycled aggregates. Construction and Building Materials, v. 118, p. 139-145, 2016.). The higher the lime content, the lower the flexural tensile strength and the lower the stiffness. The increase in lime content has a negative influence on the compressive strength due to the lower cement content and the greater water retention since cement is the main responsible for the strength development. No correlation was found in this study between the lime and RA content in these properties. For the elasticity modulus, it was observed that the higher the lime content, the lower the stiffness. The replacement of 25% of NA by RA increase only 3% on this property.

Mortar carbonation

According to Pauletti, Dal Molin and Possan (2007PAULETTI, C.; DAL MOLIN, D. C. C. D.; POSSAN, E. Accelerated carbonation: state-of-the art of the research in Brazil. Ambiente Construido, Porto Alegre, v. 7, n. 4, p. 7-20, out./dez. 2007.), the diffusion of CO2 is hindered in unprotected outdoor environments that are subject to rain, whereas indoor environments promote faster carbonation due to better control and facilitation of CO2 diffusion. The environments that most contributed to the increase of the carbonation degree were the outdoor with rain protection environment, followed by the outdoor without rain protection environment, mainly in the early stages of exposure, as seen in Figures 1 and 5. Such an occurrence may be due to the specific characteristics of each environment.

Figure 5 shows that the carbonation depth increased over time in all environments studied. This was due to the CO2 diffusion into the mortar pores, allowing the carbonation reaction to occur. It was noted that the ambient with the highest carbonation depth values was primarily the protected outdoor ambient, followed by the unprotected indoor ambient and the indoor ambient, indicating the relative humidity and CO2 concentration influence on the carbonation front. For mortars with NA at the indoor ambient, the lower the lime content, the greater the carbonation depth, as shown in Figure 5. This may be attributed to mortars with higher lime content and thus higher alkaline reserve. Since they have greater amounts of calcium hydroxide and magnesium hydroxide available for chemical reaction with CO2, it slows down the carbonation front Mazurana et al., (2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.). The carbonation depth increased by replacing NA for RA, corroborating the results of Andrade et al. (2018ANDRADE, J. J. O. et al. Evaluation of mechanical properties and carbonation of mortars produced with construction and demolition waste. Construction and Building Materials, v. 161, 2018.). The RA can lead to an increase in carbonation coefficient K (Leemann; Loser, 2019LEEMANN, A.; LOSER, R. Carbonation resistance of recycled aggregate concrete. Construction and Building Materials, v. 204, p. 335-341, 2019.). This was directly related to RA’s porosity, which facilitated CO2 diffusion and, consequently, the carbonation reactions (Han; Jun; Kim, 2023HAN, S. H.; JUN, Y.; KIM, J. H. The use of monoethanolamine CO2 solvent for the CO2 curing of cement-based materials. Sustainable Materials and Technologies, v. 35, p. e00559, 2023.; Leemann; Loser, 2019; Mazurana et al., 2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.).

CO2 capture

Figures 6 and 7 show the TGA curves of the carbonated (C) and non-carbonated (NC) areas for the mortars studied. It was noticed that the mass loss occurred in three distinct stages, and the residual mass varied according to each sample. A lower residual mass was observed for the carbonated samples than for the non-carbonated ones. Carbonated mortars region (C) showed a greater mass loss at temperatures ranging between 650 °C and 800 °C (Peak 3) than the non-carbonated mortars region (NC). This happened due to the increased calcium carbonate concentration through the mortar carbonation process, where calcium hydroxide reacted with CO2 and precipitated as calcium carbonate (Xuan; Zhan; Poon, 2016XUAN, D.; ZHAN, B.; POON, C. S. Assessment of mechanical properties of concrete incorporating carbonated recycled concrete aggregates. Cement and Concrete Composites, v. 65, p. 67-74, 2016.).

It can be noticed a small carbonation depth from the 2.0L-NA due to the greater alkaline reserve, which diminishes the CO2 diffusion. The TGA curves (Figure 6) indicate a high carbon fixation result (mineralization) which will be higher with an increase in the alkaline reserve, indicating a more conversion of hydroxides into carbonates. In matrices with higher alkaline reserves, the carbonation depth is small, but the carbon fixation is high.

For the non-carbonated mortars, peak 1 was attributed to water loss from the magnesium hydroxide (Mg(OH2)) decomposition in lime at temperatures between 360 °C and 450 °C. For the carbonated specimens, magnesium carbonate decomposition (peak 4) occurred in the same temperature range (Mazurana et al., 2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.). Peak 2 (between 400 °C and 500 °C) was attributed to the calcium hydroxide decomposition, which is less in the carbonated mortars since it reacted with CO2 during mineralization to form calcium carbonate (CaCO3). Peak 2 intensity was higher for non-carbonated samples due to the higher amount of free hydroxides in the matrix (calcium and magnesium) since the carbonation reaction did not occur. The last and highest peak (peak 3) occurred above 650 °C. It was attributed to the calcium carbonate decomposition formed by the chemical reaction of carbonation where calcium hydroxide reacted with CO2 or is from the limestone filler added to the Portland cement production used in the mortar (Li et al., 2018LI, D. et al. Evaluating the effect of external and internal factors on carbonation of existing concrete building structures. Construction and Building Materials, v. 167, p. 73-81, 2018.; Neves Junior et al., 2019NEVES JUNIOR, A. et al. Determination of CO2 capture during accelerated carbonation of engineered cementitious composite pastes by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 138, n. 1, p. 97-109, 2019.).

Table 6 -
Mortars in fresh and hardened state characterization

Figure 5 -
Depth carbonation for different environments

Figure 6 -
carbonated (c) and non-carbonated (nc) mortars’ TGs Curves

Figure 7 -
carbonated (c) and non-carbonated (nc) mortars’ DTGs

Figure 6 shows that the 1.0L-RA specimen had a higher mass loss and a higher peak from 650 °C to 800 °C. Peak 4 increased in the carbonated specimens with RA content, showing a higher amount of calcium carbonate to be decomposed beyond magnesium carbonate; the higher the RA addition, the higher the amount of the carbonates (Cai; Wang; Xiao, 2018CAI, J.; WANG, S.; XIAO, Z. A study on the CO2 capture and attrition performance of construction and demolition waste. Fuel, v. 222, p. 232-242, 2018.; Kaliyavaradhan; Ling, 2017KALIYAVARADHAN, S. K.; LING, T.-C. Potential of CO2 sequestration through construction and demolition (C&D) waste: an overview. Journal of CO2 Utilization, v. 20, p. 234-242, 2017.). This occurred as the RA was already carbonated, as shown in Figure 8, and due to the higher porosity when recycled aggregates are used. While the natural aggregate presented 1.20% absorption, the recycled sand presented 10.48%, facilitating CO2 diffusion. The RA was partially carbonated, with the release of adsorbed CO2 at 250 °C and the release of combined CO2 at 750 °C. The pre-carbonation of recycled aggregates comes from their use as a building material. It can come from the carbonation reactions of the recycled aggregate with the ambient CO2 from its production to its use (El-Dieb; Kanaan, 2018EL-DIEB, A. S.; KANAAN, D. M. Ceramic waste powder an alternative cement replacement: characterization and evaluation. Sustainable Materials and Technologies, v. 17, p. e00063, 2018.; Kaliyavaradhan; Ling, 2017; Ruviaro et al., 2022RUVIARO, A. S. et al. Long-term effect of recycled aggregate on microstructure, mechanical properties, and CO2 sequestration of rendering mortars. Construction and Building Materials, v. 321, p. 126357, 2022.).

When analyzing the non-carbonated specimens produced with RA (Figure 7), minor peaks in the derivative curves at temperatures ranging from 250 °C to 750 °C were noted, evidencing this conclusion. In addition, prominent peaks were noticed for hydrated lime, which, as it was a magnesium lime, released magnesium and calcium hydroxide, resulting in a mass loss (30.94%) higher than cement (7.67%) and RA (10.42%).

Using the tangent method, the CO2 captured values (Table 7) by the samples under study were calculated due to natural exposure to CO2 (Equations 3 and 4). It was noted (Table 7) that the higher the hydrated lime content, the higher the potential of CO2 capture. This was due to the amount of carbonatable materials available in the matrix. In cement-lime mortars, calcium hydroxide comes from Portland cement’s hydration process and calcium and magnesium hydroxides are present in hydrated lime. These products chemically react with carbon dioxide forming stable calcium and magnesium carbonates (Mo; Panesar, 2013MO, L.; PANESAR, D. K. Accelerated carbonation: a potential approach to sequester CO2 in cement paste containing slag and reactive MgO. Cement and Concrete Composites, v. 43, p. 69-77, 2013.). For CO2 diffuses into the mortar, it must react with Ca(OH)2 and Mg(OH)2, so the amount of CO2 captured inside the lime-based mortars may be higher by increasing lime content (Mazurana et al., 2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.).

For specimens with and without RA and the same 25% hydrated lime content, using RA increased the potential of CO2 capture by 6%. The increase of CO2 capture using RA content may be observed when comparing the mortar of the same rate with 100% natural aggregate (1.0L-RA and 1.0L-NA). Replacing NA for RA makes more cementitious materials available and, consequently, calcium hydroxide for carbonation, which favors CO2 sequestration from the environment, as verified by Andrade et al. 2018ANDRADE, J. J. O. et al. Evaluation of mechanical properties and carbonation of mortars produced with construction and demolition waste. Construction and Building Materials, v. 161, 2018.). For TGA analysis, the maximum amount of CO2 captured was observed in the mortar with the highest lime content (2.0L-NA), 25.41 Kg CO2/m³, and the minimum capture was 15.97 Kg CO2/m³ for mortar with the lowest lime content (0.5L-NA).

Figure 9 shows the specimen characterization divided into two groups. The first one compared specimens of NA with different lime contents (0.5L-NA, 1.0L-NA, and 2.0L-NA) (Figures 9a and 9c), and the second one compared the same lime content but varying the RA content (1.0 L-RA and 1.0L-NA) (Figure 9b).

Figure 9c shows that increasing the amount of lime in the mortar's ratio increased the water retention and thus CO2 capture. However, capillarity increased only up to the binders (cement: lime) 1:1 ratio; when the lime content doubled from 1 to 2 in the mixture, capillarity decreased by 18.35%. This was an interesting result as it reduced the possibility of water entering lime- based render (Carasek, 2010CARASEK, H. Argamassas. In: ISAIA, G. C. (org.). Materiais de construção civil e princípios de ciência e engenharia de materiais. São Paulo: IBRACON, 2010.).

Figure 8 -
(a)thermogram and (b) DTG curve for mortar materials production

Figure 9 -
Mortars characterization according to (a) the lime content, (b) the RA content for the same lime content, and (c) properties

Replacing NA for RA in the mixture increased carbon dioxide capture capacity, as the second one has a non-hydrated binder in its composition, which enhances carbonation. Also, increasing the amount of lime added to the mixture increased CO2 capture since the higher the amount of binder, the higher the potential for mortar mineralization.

CO2 emissions

For CO2 emissions associated with rendering mortar production, it was noted that they were directly proportional to the hydrated lime content used (Figure 9). Hydrated lime was the material with the highest influence on emissions, followed by Portland cement. John, Punhagui and Cincotto (2014JOHN, V. M.; PUNHAGUI, K. R. G.; CINCOTTO, M. A. Produção de cal: economia de baixo carbono, impactos de novos marcos regulatórios e tecnologias sobre a economia brasileira. Ribeirão Preto: Funpec, 2014.) stated that this is due to the lime production process in Brazil. As a heterogeneous sector, there are variations in the size and quality of companies and product technology. The CO2 emissions of lime production come from limestone decarbonation inherent to the process and from burning fuels in the manufacturing process. The lack of efficiency and control of kilns and the applied technologies influence the type and amount of fuel consumed, which are directly related to the energy consumed by the sector and CO2 emissions.

Figure 10 shows that the mortar with the highest emission (2.0L-NA) had the highest binder consumption as it was primarily responsible for the CO2 emitted during the mortar production process. The less emissive mortar was the 0.5L-NA, with 211.15 KgCO2/m³, 36.55% lower than the higher emissive one (2.0L-NA, with 332.78 KgCO2/m³). Hydrated lime is an essential component in rendering mortars as it improves plasticity and water retention, as seen in Figure 9. It has a lower cost and easier production than Portland cement and is widely available in Brazil.

A global analysis that considers the properties in the fresh and hardened state, emissions, and CO2 capture is important to establish the optimal content of this material for mortar production. Natural aggregate replaced by recycled aggregate reduced emissions by 7.28% compared to the mortar with NA and the same lime ratio. Using RA in the 1.0L-RA mixture reduced up to 29.1% of the emissions compared to the other ratios, as the emissions for recycled aggregate production were about 83% lower than those of the natural aggregate. The RA carbon footprint can differ depending on the recycling process adopted. The RA used in this study received only mechanical processing, resulting in lower emissions when compared to NA. With other associated RA treatment processes, such as Jigging (Malysz et al., 2022MALYSZ, G. N. et al. Natural and accelerated carbonation in concrete associated with recycled coarse aggregate treated by air jigging technology. Journal of Materials in Civil Engineering, v. 34, n. 7, 2022.) coating with pozzolan slurry (Al-Waked et al., 2022AL-WAKED, Q. et al. Enhancing the aggregate impact value and water absorption of demolition waste coarse aggregates with various treatment methods. Case Studies in Construction Materials, v. 17, p. e01267, 2022.), mechanical (Oliveira; Dezen; Possan, 2020OLIVEIRA, T. C. F.; DEZEN, B. G. S.; POSSAN, E. Use of concrete fine fraction waste as a replacement of Portland cement. Journal of Cleaner Production, v. 273, p. 123126, 2020.; Wang; Mu; Liu, 2018WANG, J.; MU, M.; LIU, Y. Recycled cement. Construction and Building Materials, v. 190, p. 1124-1132, 2018.), thermal (El-Dieb; Kanaan, 2018EL-DIEB, A. S.; KANAAN, D. M. Ceramic waste powder an alternative cement replacement: characterization and evaluation. Sustainable Materials and Technologies, v. 17, p. e00063, 2018.) and chemical treatments such as tannic acid (Fang et al., 2022FANG, Y. et al. A renewable admixture to enhance the performance of cement mortars through a pre-hydration method. Journal of Cleaner Production, v. 332, p. 130095, 2022.; Wang et al., 2022WANG, L. et al. Eco-friendly treatment of recycled concrete fines as supplementary cementitious materials. Construction and Building Materials, v. 322, p. 126491, 2022.), the carbon footprint may be higher. Knowing the RA production process for emission studies is central (Infante Gomes et al., 2021INFANTE GOMES, R. et al. CO2 sequestration by construction and demolition waste aggregates and effect on mortars and concrete performance: an overview. Renewable and Sustainable Energy Reviews, v. 152, p. 111668, 2021.).

CO2 emissions balance

Table 8 showed that the mortar with the best performance in reducing CO2 emissions was the one at the 1:1:6 ratio with 25% RA replacement, capturing approximately 8.15% of the CO2 emitted during mortar production. This was followed by the 1:2:6 ratio mortar with 0% RA, which captured up to 7.64%. It is essential to evaluate the emissions balance to understand the behavior between emission and capture. As the materials used in mortar production usually have different carbon footprints, these may not be offset only by capture due to mineralization. Lime-based mortars had higher CO2 emissions. However, its CO2 capture was also higher so the decrease in the use of the binder decreased the final balance. The use of RA collaborated positively with the final emissions balance, as this aggregate emitted less CO2 during the production process when compared to natural aggregate (Mazurana et al., 2021MAZURANA, L. et al. Determination of CO2 capture in rendering mortars produced with recycled construction and demolition waste by thermogravimetry. Journal of Thermal Analysis and Calorimetry, v. 147, p. 1071-1080, jan. 2021.) and part of it gets in the mixture already pre-carbonated mixture. The amount of CO2 uptake increased with the increasing replacement of natural sand with recycled sand and with increasing lime. As well as reducing emissions, the replacement of natural sand with RA aggregate also increased CO2 capture, making RA mortar production clearly beneficial.

Scenarios analysis

Figure 11 shows the CO2 capture results, obtained by thermal analysis (Table 7), over time in the various scenarios studied. The TGA results were multiplied by the advance of CO2 over time (Equations 4 and 7), which is given by the projection of the carbonation front.

Figure 10 -
Emissions from 1m³ of mortar production per material

Table 8 -
CO2 balance emissions

Figure 11 -
Effective CO2 capture over time considering the mortar type and depth

Table 9 -
Best time needed for total carbonation in days

Figure 11 shows that the maximum CO2 captured was achieved in short periods at lower render thicknesses, with carbon capture occurring until the carbonated depth is equal to the render thickness. Higher surface area results in greater carbonation depths because it promotes CO2 diffusion. The potential of CO2 capturing was achieved by carbonation completion. 10 mm-thick renderings in direct contact with CO2 for short periods before receiving a finishing layer (paint, ceramic, etc.) will certainly reach their maximum potential of CO2 capture. Figure 5 shows that the maximum CO2 penetration depth into the material was 10.7 mm after 28 days of exposure. Considering the rendering mortar application thickness without a surface layer and its CO2 diffusion capacity over time, later exposure ages generate a higher amount of CO2 capture.

Table 9 shows that thicknesses higher than 20 mm require time exposure to CO2 for more than 3 months for complete carbonation and maximum carbon fixation. The time required for complete material carbonation increases exponentially as the thickness increases, not considering the surface protection effect. Barbosa (2020BARBOSA, M. C. Efeito da proteção superficial do tipo pintura na captura de CO2 por carbonatação em argamassas de revestimento. Curitiba, 2020. Dissertação (Mestrado em Engenharia Civil) - Universidade Tecnológica Federal do Paraná, Curitiba, 2020.) stated that CO2 diffusion in mortars is 3 times smaller when wall painting, affecting carbon capture capacity.

For render thicknesses higher than 30 mm, the time for CO2 diffusion and complete carbonation of the material used in construction is 1 to 4 years, depending on the mortar composition. Thicknesses more than 50 mm, take 4 to 20 times longer to reach maximum carbonation when compared to smaller ones (20 mm).

A six-month exposure period was insufficient for all mortars thicker than 20 mm to reach their maximum carbonation depth and, consequently, their potential CO2 capture. A longer period in direct contact with CO2 before receiving a layer was required for complete carbonation. However, mortar may reach maximum potential CO2 capture in buildings exposed to the wall for long periods.

Renders thicker than 20 mm, resulting from poor quality control and construction execution, increase CO2 emissions, and the construction cost per m² should be avoided.

Although RA cost is lower than natural aggregate, there is still resistance to using this waste due to limited knowledge of cement-based applications. In this way, it is deemed necessary to motivate the use of RA through public policies, which act not only to encourage its use but also in investment and research dissemination.

It highlights the importance of studying CO2 capture in cement-based mortars and the influence of lime and RA, as they may balance part of the emissions generated by the building materials production process.

Conclusions

This work presents the results of CO2 capture from different scenarios of mortar applications, obtained by natural carbonation. In the circular economy and the cleaner production context, the natural aggregate (NA) was replaced by recycled aggregate (RA) and three different hydrated lime contents were evaluated.

  1. the results indicated that the carbonation process in rendering mortars was significant in CO2 uptake, and the thermogravimetric analysis method proved to be effective in calculating carbon fixation;

  2. the mortars studied could neutralize between 7.18 and 8.15% of CO2 emitted during their production, depending on the presence or absence of hydrated lime and the amount of RA used in their production;

  3. mortars presented a high potential for CO2 capture with increasing lime content. The maximum amount of CO2 captured occurred in the 2.0L-NA mortar, 25.41 Kg CO2/m³ at 28 days. For specimens added with RA or not and the same hydrated lime content, using 25% of RA increased the potential for CO2 capture by 6%;

  4. the mortar with the highest emissions was 2.0L-NA (336.78 Kg CO2/m³) due to the high lime content and natural aggregate. High binder levels and the exclusive use of natural aggregates were indeed responsible for increasing the final emissions value;

  5. the production of mortars with RA benefits the CO2 balance and reduces the use of natural resources. For the same lime content, natural sand replaced by RA reduced emissions (254.49 to 237.63 Kg CO2/m³) and increased CO2 capture (18.27 to 19.37 Kg CO2/m³);

  6. the rendering mortar thickness, the type of mortar, and the exposure time to CO2 before applying the finish surface layer affect the carbon capture result. The higher the lime content, the lower the CO2 diffusion speed and the higher the carbon fixation in the rendering mortar;

  7. the m²/m³ ratio is directly proportional to the carbon fixation in the lime-cement-based matrix. The higher this ratio, the shorter the time for the material to be fully carbonated and, consequently, to end the CO2 capture potential. Thicknesses higher than 20 mm required exposure to CO2 for more than 3 months for complete carbonation and maximum carbon fixation. Thicknesses higher than 50 mm took 4 to 20 times longer to reach maximum carbonation when compared to the 20 mm; and

  8. it appears that the rendering mortar thickness application control ensured a lower amount of material used per m², avoiding incorporated losses and an impact on emissions and CO2 capture.

Acknowledgments

The authors would like to thank Itaipu, the Itaipu Concrete Technology Laboratory and Unioeste, for their support in the experimental project, and the Unioeste Foundation and PRPPG | Unila for its research support.

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Editor:

Marcelo Henrique Farias de Medeiros

Publication Dates

  • Publication in this collection
    14 June 2024
  • Date of issue
    2024

History

  • Received
    25 Mar 2023
  • Accepted
    13 Aug 2023
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