Feasibility analysis for implementing CO 2 curing in a concrete block industry in the São Paulo Region

: The project feasibility analysis determines whether the project should be carried out from different spheres: strategic, technical, operational, legal, economic-financial, environmental, marketing, political, fiscal, location, among others. These are not excluding analyzes, all aspects must be assessed before implementing a new project or new process. This work will focus on the analysis of the technical feasibility of the innovative project to implement carbonation curing in a concrete block factory in the region of São Paulo. An overview of the CO 2 curing process and changing needs is presented, including identifying local CO 2 sources and delivering cost, gas consumption for chamber saturation, estimation of CO 2 uptake by masonry units during curing, estimation of consumption and monthly cost of CO 2 , curing chamber changes needs, new equipment acquisition, estimation of cost for retrofit and new installations, potential best definitions on optimal temperature, humidity, and CO 2 concentration. International succeeded cases are presented. The study concludes that the technology can be easily implemented in the region, with few changes on a plant production process and on the curing chamber. There would be an increase of 4% to 14% on the block cost depending on the distance to the CO 2 supplier. Considering the Brazilian production of concrete blocks, up to 168,780 tons of CO 2 per year can be sequestered, this value is equivalent to the CO 2 sequestered by 21,100 trees.


INTRODUCTION
Accelerated carbonation curing or CO 2 chemical curing of concrete blocks is an innovation. According to the Oslo Manual [1], innovating means implementing a new or significantly improved product (good or service). However, chemical curing with CO 2 should not be considered just an innovation, but a sustainable innovation. According to 290,000.00 ton of CO 2 if all production had been cured with CO 2 . This value corresponds to the CO 2 sequestration of 36,250,000.00 trees in a year, since, according to the Totum Institute, a tree sequesters approximately eight kilograms of CO 2 in a year [13]. In addition, the carbonated concrete paving pieces showed superior axial compressive strength at 2-days age than the reference pieces (non-carbonated), although the 28-days were equivalent [6].

Production of concrete blocks that sequester CO2
The manufacturing process of concrete blocks involves the stages of mix design, in which the materials are proportioned; mixing, generally carried out in an orbital mixer; production (molding) of dry concrete in molds (forms), compaction and vibration through vibrating-pressing machines; curing and storage before delivery [14]. Concrete blocks that store CO 2 are produced in the same way as conventional concrete blocks, regardless of the industry that will produce them. It is not necessary to change the mix proportion, equipment and cycle time. The differentiation takes place in the curing procedure, which, unlike traditional industries, is not through steam or humidity, but with CO 2 .

Innovative CO2 chemical cure technology
The technology in question is a chemical cure that promotes the mineralization of CO 2 , that is, the carbon dioxide through the cure is chemically transformed into a mineral, calcium carbonate. The permanent absorption of CO 2 in the pores of the concrete is possible due to the accelerated carbonation reaction. The main carbonation reaction occurs between Ca(OH) 2 and CO 2 in the presence of water, as shown in Equation 1.1; and in a second moment, when most of the calcium hydroxide has already been acquired, the occurrence of carbonation of hydrated calcium silicate is reported, as shown in Equation 1.2. These reactions are show below.

Studies on the chemical curing process with CO2 in precast concrete materials without steel reinforcement
According to Fortunato et al. [15] several studies involving the use of accelerated carbonation curing in nonreinforced precast cementitious products have been carried out to develop a curing process to use. Table 1 summaries the related research and the carbonation parameters used. In Table 1, the non-reinforced cement precast elements that underwent a compaction process for their production were shaded in gray. It was found that only Shao and Lin [5] and Fortunato et al. [15] produced the sample in the industry using a vibro-pressing process, El-Hassan et al. [19] produced blocks in the laboratory using a manual machine. The other studies used samples produced in laboratory obtained by other compaction process rather than using a manual, pneumatic or hydraulic machines, not appropriate for concrete blocks or pavers production [23]. It was observed that in the studies of Shao et al. [4], El-Hassan et al. [19] and Zhan et al. [20] the concrete blocks did not have the dimensions specified by ABNT NBR 6136:2016 and are probably not hollow. It is known that the production/compaction process and the dimensions/formats of prefabricated cementitious products directly affect the speed of the carbonation reaction and the advance of the carbonation front, promoting important variability in the percentage of CO2 absorbed.
In most studies the concentration of CO 2 inside the carbonation chamber was 100%. The maximum concentration of CO 2 inside the chamber allows the curing of the carbonation to be accelerated by maintaining its objective, which is the maximum production of CaCO3 in the cement matrix.
In the studies of Shao and Lin [5], Fortunato et al. [15], Rostami et al. [17], Boyd et al. [18], El-Hassan et al. [19], El-Hassan and Shao [21] it was possible to verify that the initial curing (open air curing or wet curing) carried out before the carbonation curing, directly interfered in the hydration of the non-reinforced cementitious product, and contributed to strength gain. Nonreinforced cementitious precast not subjected to initial cure absorbed more CO 2 than those submitted to initial wet curing. The hypothesis is that the non-performance of initial wet curing generated less hydration products, reducing the alkaline barrier, providing a greater advance of the carbonation front, allowing more CO 2 to be incorporated into the cement matrix. In all reported studies, the prefabricated cementitious non-reinforced carbonated yielded greater mechanical resistance when compared to the non-carbonated (reference) specimen at the earliest ages. This result is attributed to the fact that carbonation makes the sample premature, that is, its maturation is accelerated, and it gains the expected mechanical properties much faster. Among the unreinforced prefabricated samples reported tests, the concrete blocks absorbed the most CO 2 .This is probably due to the smaller thickness of the block walls, allowing greater CO 2 diffusivity.
Further analyses on the literature review are reported in Fortunato et al.
[ [15]], that concludes the best conditions for carbon cure of concrete blocks are using a chamber at 100% of CO 2 concentration, to perform an initial air cure for at least two hours before carbon cure, the humidity shall be between 50 to 80%, temperature should be up to 60 o C to maximize carbonation.

Based material -CO2 captured
The carbon dioxide to be used in curing is captured by specialized companies as AirLiquide and WhiteMartins from emitting sources with high concentrations (> 90% CO 2 ) and high purity (absence of contaminants such as moisture and/or other gases). After capture, the CO 2 is purified and liquefied and can be transported by pipeline within the industry itself, as in the case of cogeneration of utility systems in which the CO 2 customer itself uses the CO 2 produced, or else, the captured CO 2 is transported in tank trucks that will supply customers in bulk or go to a filling center, where the CO 2 will be discharged into a tank and later fill the cylinders [24], [25]. Figure 1 demonstrates in a simplified way the CO 2 path from capture to the final customer. To store CO 2 on the premises of Brazilian industries, there is no need for a government-issued environmental license. The supplying companies must only provide a safety data sheet. Since the CO 2 is an inert gas and heavier than air, it can occupy the air space. Every care must be taken to avoid cases of asphyxia. It is important that the tanks are in an open and ventilated place and that the chamber where the curing will be carried out has an efficient exhaustion system [24], [25].

Curing chamber facilities and parameter control equipment
In the case of an existing plant, there is the possibility of retrofitting the existing chambers, if they are perfectly sealed with the installation of appropriate doors, thus avoiding the risk of accidents due to CO 2 leakage. There is also the possibility of installing new designed CO 2 curing chamber, that can be make of masonry or appropriate containers can be used [24], [25]. The control of temperature, humidity, pressure, and CO 2 concentration parameters use equipment such as a manometer to check the internal pressure of the chamber a CO 2 sensor, a temperature sensor, a humidity sensor, among others [27], [28].

Research of technological processes and similar products -Success Cases
According to Castro [29], in a technical feasibility study, it is necessary to verify if the technology to be used is applied in practice and if the technical knowledge is available. Most companies choose to invest in already used mature technologies in which the problems and adjustment that certainly arise in the development process are already known and solved. Few companies choose to use the technology present only in the state of the art. Some successful cases of industries that apply CO 2 curing in non-reinforced cement prefabricated units are presented below.

Solidia Technologies ▪ Company:
The American start-up Solidia was founded in 2008, with an investment of around US$ 80 million, through investors such as LafargeHolcim, Total, Air Liquide, Oil & Gas Climate Initiative, BASF Venture Capital, BP Ventures, Kleiner Perkins Caufield &Byers, Bright Capital, among others [30]. ▪ Products sold: The company commercializes concrete masonry units and concrete paving pieces that store CO 2 , as well as the CO 2 curing technology [31].
▪ Manufacture and molding of prefabricated parts that store CO2: The same equipment, trace, cycle time used in the production of traditional prefabricated products are used [32]. ▪ Accelerated carbonation cure: -Based-material: according to Meyer et al. [30] the CO 2 used by the company is supplied by AirLiquide and is usually stored in tanks like in Figure 2. -Installations: according to Meyer et al. [30] as for the structure of the chamber, the company initially developed a prototype applied in several places of the world with more than 50 tests in companies interested in the technology. Figure 3 shows the test-chamber used by the company to carry out the cure with CO 2 on a small scale. By proving the CO 2 absorption and mechanical strength improvements, the technology matured, and permanent carbonation chamber structures were developed across the world. Figure 4 shows carbonation chambers located in the United States, Canada, and the United Kingdom [30]. -Equipment and accessories: according to Hall [34] the gas flow in the chamber occurs to ensure uniform evaporation on the surface of all products. This allows batch processing time to be minimized as the slowest curingtime product in the chamber defines the duration of the entire process.
Solidia has designed and built chambers 3.00 m wide x 6.00 m high x 23.00 m long [34]. The devices used for control and ventilation are shown in Figure 5. ▪ Advantages of CO2 curing: Jang et al. [31], DeCristofaro et al. [32] and Meyer et al. [30] indicate the following advantages: -It does not consume water, but CO 2 ; -It takes advantage of existing factory facilities, production process, equipment, and raw materials; -It is adaptable to existing production lines; -Smart and fast curing, as the compressive strength results of non-reinforced precast cured with CO 2 obtained in 24 hours are equivalent to the compressive strength results of conventional unreinforced precast at 28 days; -Reduced inventory, due to faster curing time, enabling just-in-time production and delivery; -Better performance and greater durability than conventional concrete; -No occurrence of primary efflorescence.

Carboclave
▪ Company: The company is a Canadian start-up founded in 2016 after demonstrating a series of successes in the development, validation and expansion of the CO 2 curing processing, not only with autoclaving but with all other conventional concrete curing methods.

▪ Products sold:
The company commercializes concrete masonry units that store CO 2 , as well as CO 2 curing technology that can be applied to masonry units and pavers [35].
▪ Manufacture and molding of prefabricated parts that store CO2: According to Hargest and Al-Ghouleh [35] the process for producing precast products in an airtight enclosure, which comprises the steps of a carbonation of pre-dried concrete precast units by feeding CO 2 , gas into a closed airtight enclosure under near ambient atmospheric pressure (psig between 0 and 2) and/or low pressure ( between 2 and 15 psig ) conditions, wherein said pre -dried concrete units have lost between 25 to 60% of their initial mix water content. The manufacturing and curing process is shown in Figure 6 below. ▪ Accelerated carbonation cure: -Based-material: the CO 2 used by the company Carboclave is supplied by the partner Praxair.
-Installations: for the chamber structure , the company makes the technology available in two possibilities, retrofit and new installations. When the curing chamber already exist, it is used with adaptations, working with atmospheric pressure. The new chamber is specially designed and built for curing with CO 2 in an autoclave at high pressures. Figure 7 demonstrates a type of curing chamber used by Carboclave. -Equipment and accessories: according to Hargest and Al-Ghouleh [35] before curing with CO 2 , vacuum step is used to exhaust 50 to 90% of the volume of air initially present in the enclosure, then, after the CO 2 injection, applying a pressure greater than atmospheric, which is controlled by a manometer. ▪ Advantages of CO2 curing: Hargest and Al-Ghouleh [35] verified: -CarboClave concrete masonry units are 25% more ecological than concrete masonry units, as they absorb up to 250g of CO 2 per 19x19x39cm concrete masonry units; -Non-reinforced CO 2 -cured precast have greater resistance to axial compression than regular ones, greater resistance to freezing/thawing; greater resistance to sulfate attack; greater resistance to drying and atmospheric shrinkage; reduced sensitivity and permeability; and reduced efflorescence effect.

CarbonBuilt ▪ Company:
CarbonBuilt was created in 2014 at the Institute for Carbon Management at the University of California, Los Angeles (UCLA). In 2020, with support from the US Department of Energy and NRG COSIA Carbon XPRIZE, they took the prototype off paper and developed CO 2 curing in practice [36].

▪ Products sold:
The company sells reverse technologies for industries which seek to make a beneficial use of the CO 2 . Accelerated carbonation curing is one of them. They are producing concrete masonry units that absorb CO 2 and soon they intend to expand the range of ecological non-reinforced prefabricated elements. They develop projects in the United States, Europe and India [37].
▪ Manufacture and molding of prefabricated parts that store CO2: The same equipment, mix proportion, cycle time used in the production of traditional prefabricated products are used [36].
▪ Accelerated carbonation cure: -Based-material: the CO 2 used comes from the plant that emits it [37]. Figure 8 shows the CO 2 piping coming from a thermoelectric plant, the gas will be sent to the carbonation chamber installed on the thermoelectric plant infrastructure. -Installations: for the structure of the chamber, an adapted container was used for the loading of concrete masonry units through forklifts. Figure 9 shows the container-chamber [38]. -Equipment and accessories: the chamber is controlled by CO 2 , humidity, and temperature sensors [39]. Figure 10 demonstrates their inspection. ▪ Advantages of CO2 curing verified by the company: The main advantage emphasized by the company is the definitive incorporation of CO 2 in the concrete and the removal of this environmental liability from the atmosphere [39].

Carbicrete ▪ Company:
CarbiCrete is a Canadian company that commercializes CO 2 absorption technology in concrete masonry units and pieces for paving through accelerated carbonation curing. The company's patented technology was developed at McGill University and includes the production of concrete by replacing cement with steel slag [40]. ▪ Products sold: The company licenses the technology to concrete masonry units (CMUs) and precast panels; and oversees the retrofit for implementation of the process in the industry [40]. ▪ Manufacture and molding of prefabricated parts that store CO2: According to Hahn [40] the same equipment and cycle time used in the production of traditional prefabricated products are used. The composition has replacement of cement by steel slag. ▪ Accelerated carbonation cure: -Raw-material: the CO 2 used by the company Carbicrete is supplied by the partner Praxair and is stored in tanks [41].
-Installations: the possibility of retrofitting existing chambers and new installations using an adapted container was reported. Figure 11 demonstrates the container-chamber [41]. -Equipment and accessories: the chamber is controlled by CO 2 , pressure, humidity, and temperature sensors. It was reported that the curing procedure with CO 2 lasts from 5 hours to 6 hours [41]. ▪ Advantages of CO2 curing: according to [41]: -High strength gain before 24 hours after curing with CO 2 ; -Compressive strength of CO 2 cured concrete masonry units is 30% than conventionally cured concrete masonry units; -According to the company, to produce a conventional concrete masonry unit, 2kg of CO 2 is generated, while the concrete masonry units cured with CO 2 is negative (-1kg), since the CO 2 is absorbed.

Location
According to Corrêa and Corrêa [42] decisions related to the location of the implementation of an enterprise are expensive and difficult to reverse, given that the location of an operation affects both its ability to compete and other internal and external aspects. The location directly affects the transport costs of inputs and final product, labor costs (since different regions may have different salary levels) and cost and availability of energy, in addition the choice of location influences directly on expenses, investments and even revenues. The location study has as main objective to determine the best place for the enterprise. This location, also known as the optimal location, can be understood as the one that gives the project the best cost/benefit ratio in an adequate period.

Location Variables
Location variables are factors that must be considered when choosing the ideal location for the enterprise. In general, the availability of raw materials, proximity to the consumer market and/or factors related to the production process must be considered. The analyzes must weigh between the main expenses and gains with these choices, and will be better described below [43]: • As for the availability of raw materials: when this requires large volumes and transport is difficult or distant, it is recommended that the location of the enterprise is close to the sources of inputs; • Regarding the consumer market: If the focus of the enterprise is the relationship with customers, the location must be close to the consumer market; • Regarding the production process: some production processes may require certain conditions to take place, for example, they may need a source of water nearby for cooling or being close to a power substation for their perfect functioning, among others. However, it is desirable that the location of the unit is aligned with the needs of the production process.

Methodology
In order to verify the technical feasibility and location for the implementation of CO 2 curing in concrete block factories located in the São Paulo region, several rounds of brainstorming were carried out with concrete block manufacturers, which were recruited by Associação Bloco Brasil, as well as meetings with Brazilian CO 2 suppliers. From the discussion, the equipment, material and other needs were assessed and listed, both for the case of a new or retrofit chamber. The CO 2 suppliers and block producers plant location were identified with the state. The materials, equipment, CO 2 and other supplies costs were assessed, as well as the estimated initial cost for a new or retrofit chamber. From the literature review the efficiency of the carbon cure was estimated. From this data it was possible to assess the technical and location feasibility for concrete block carbon cure implementation in the São Paulo region.

Based-material -captured CO2
The supply of captured CO 2 is possible and viable to be carried out in the state of São Paulo, with more than one company supplying this gas. The carbon dioxide must be transported from the captured CO 2 producer to the concrete block plant using tank trucks. Then, it will be stored in tanks located in the concrete block plant premises and will later be transported through pipes provided and installed by the CO 2 supplier to the accelerated carbonation curing chambers.
For the installation and operation of the CO 2 tank, there is no need for a government environmental license. The gas is inert, does not explode and is not inflammable, but it must be stored in a well-ventilated place, as it can cause suffocation.

Installations
The CO 2 chamber can be new or can be adapted from the existing infraestructure. In both cases, airtightness must be guaranteed. Figure 12a shows a model of a new installation, like a cold chamber with temperature, humidity, and airtightness control, made in Brazil and which can be used, with the necessary adjustments, in the concrete masonry units manufacturing industry with the purpose of carrying out curing with CO 2 .  Figure 12b shows a steam curing chamber used in an large concrete block plant located in the Great São Paulo region. To use this chamber there is the need of installing doors, control equipments, a exhaust system and the CO 2 supplier pipeline.
The analysis will be carried out considering a curing chamber with concrete masonry walls and reinforced concrete structure slab. For the installation of the CO 2 curing chamber, it must be ensured: The airtightness of the chamber must be guaranteed by installing a guillotine door or sliding door in which carbon dioxide leakage is guaranteed no occurrence and the walls and ceiling must also be waterproofed to ensure that CO 2 does not escape through cracks or crevices. The indicated wall treatment is to apply a semi-flexible two-component waterproofing coating that shall also provide airtightness.

• Thermal insulation
Thermal insulation is important to standardize the curing procedure and prevent temperature changes from interfering with the process. Therefore, it is recommended that the walls and ceiling of the chamber be coated with insulating material such as isopanels composed of profiled steel sheets interspersed with polyisocyanurate.

• CO2 input
The entry of CO 2 should occur in the lower region of the chamber. The pipeline from the tank to the chamber is designed by the CO 2 supplier. It is made of stainless steel and must have a pressure regulator before entering the chamber, since the pipeline will work at atmospheric pressure and the gas is pressurized to approximately 15 bar in the tank. The CO 2 supplier will design the chamber filling time according to the possible flow rate according to the pipeline diameter.
• CO2 and air output (exhaust) An exhaust system must be provided so that air is removed from the chamber when filling it with CO 2 , as well as the CO 2 being expelled at the end of the curing procedure. It is recommended to use axial exhaust fans equipped with butterfly valves activated by hydraulic or pneumatic mechanisms, designer according to the dimensions of the chamber. A duct located in the lower region of the chamber must also be provided for the exhaustion of CO 2 after curing.

• Ventilation
Ventilation inside the chamber is essential so that the CO 2 is distributed evenly. The fans (blowers) must be positioned in the sides of the walls, in the upper region at one side and in the lower region on the other side, creating an air flow to promote greater contact between the CO 2 and the blocks.

• Air conditioning
The air in chamber must be conditioned to specific temperature and humidity. The system must contain an automation panel to make it possible to set the desired parameters. The air flow must be carried out through a buster interconnected in flexible and shielded pipe made with high performance material to prevent CO 2 from escaping. The humidification system must be coupled to the refrigeration equipment.

• Safety
An automated security panel equipped with red and green lights must be installed outside of the chamber, which will secure the opening of the chamber door. The red light will be directly related to the high concentration of CO 2 inside the chamber and the green light will indicate the low concentration of CO 2 in the chamber, communicating to the operator the prohibition or permission to open the door of the carbonation chamber.

Equipment and sensors for parameter control
The temperature, humidity, pressure, and CO 2 concentration parameters must be controlled using the equipment described below.

▪ CO2 sensor
The sensor for evaluating carbon dioxide concentration must have an evaluation range between 0% and 100%. The sensor must be installed in the upper region of the chamber to verify that it is filled with CO 2 . Since it is heavier than the air it tends to stay below reaching 100% at the top ensures the chamber is fully filled. Another CO 2 sensor must be installed in the lower region, close exhaustion duct . This will indicate when the CO 2 concentration inside the chamber is low, communicating the security system that will automatically turn on the green indicator light, allowing the door to be safely open.
• Relative humidity sensor The Relative Humidity Sensor monitors the relative humidity in the range of 0 to 95% (± 5%). This sensor should be positioned in the center region of the chamber, as this location represents the average humidity of the chamber.
• Temperature sensor The temperature sensor, in general must range between -40°C to 135°C. This sensor should also be positioned in the center region of the chamber.

• Manometer
The Bourdon type manometer, stainless steel case and brass alloy internals can be used, which controls the pressure from 0 to 20kgf/cm 2 and has an accuracy of 1.6%. Figure 13 demonstrates the installations of the chamber and the supply of CO 2 , as well as the equipment and sensors necessary to carry out the curing with CO 2 in the industry, both for chambers made for this purpose and for retrofit chambers.

Location
This study assesses industries consolidated in the market, within the state of São Paulo. The locational variable is related to the availability of raw material, CO 2 . The industries that are closer located to the sources that generate CO 2 will have lower transport costs, not to mention the environmental issue related to lower gas emission due to transport. Figure 14 demonstrates the CO 2 gas sources sources and the concrete block plants in the state of São Paulo. In Figure 14 it is possible to see that most of the main cities in the state of São Paulo have concrete block plants. CO 2 suppliers are more concentrated close to São Paulo city, with some suppliers in the inland part od the state. In this figure a radius of 100 km of the CO 2 suppliers was delimited. It is possible to observer that several concrete block plants that are within these perimeters.

Estimated CO2 consumption for curing chamber saturation
To calculate the consumption of CO 2 for saturation of the curing chamber, the dimensions and storage capacity of concrete masonry units of an existing steam curing chamber in a concrete block plant, as shown by the Figure 15. -Net volume occupied by the concrete blocks in the chamber: 7.49 m 3 ; -Volume occupied by the trays in the chamber: 2.57 m 3 -Volume occupied by the shelves in the chamber: 1.00 m 3 The calculation of the volume of CO 2 to be injected into the chamber for its saturation is demonstrated through One kilogram of CO 2 occupies 0.534m 3 , then considering a 10%-waste, 109 kg of CO 2 will be needed to saturate the curing chamber.
Considering a production of three cycles per minute of the concrete block machine and eight hours worked per day, 5,760 concrete block units are produced per day. This production will fill six curing chambers/day, each cycle consuming 654 kg CO 2 /day. Considering 22 workdays per month at total of and 14,388 kg CO 2 /month for saturation of the curing chamber. A total of 126,720 blocks are produced per month.

Estimation of CO2 absorption during curing by concrete masonry units
To calculate the monthly estimate of CO 2 absorption by the concrete masonry units, the mix proportion is considered as in Table 2. With this mix proportion, 64 concrete masonry units are produced, so block consumes 0.625 kg of cement. Considering the monthly production of 126,720 concrete masonry units, 79,200 kg of cement/month is consumed.
Considering the literature review, it was found that the lowest CO 2 absorption rate in concrete masonry units produced with natural aggregates, cured with 100% CO 2 concentration, is 8.62% for 2 hours of cure with CO 2 as (MacMaster and Tavares [22]). The highest reported absorption is 36.03% but with 24 hours of curing with CO 2 (Zhan et al. [20]). Considering the 2-hours cure and the smallest reported absorption, it is possible to sequester from 6,827 kg CO 2 /month. Since six 2-hours cycles are necessary per day, two chambers will be necessary for the 8-hour workday.

Estimate of initial cost
For the new installation of the curing chamber, budgets were made considering a cold chamber with dimensions of 4.00 × 4.00 × 4.00 m (length × height × depth), equipped with a temperature, humidity, thermal insulation and tightness. The average value found was R$ 37,500.00. The devices for inlet and outlet of the pressure and CO 2 system, equipped with a pressure regulator and axial exhausters, cost around R$5,525.00. The CO 2 , humidity, temperature, pressure gauge and interface sensors averaged R$ 8,760.00. Totaling R$ 51,786.00.
For retrofitting of an existing curing chamber with the same dimensions of 4.00 × 4.00 × 4.00 m (length × height × depth), it was verified through the SINAPI Price Bulletin (base date August/2022) that the waterproofing service walls and ceiling costs R$ 3,594.00. For thermal insulation, after budgeting with companies in the sector, the average value was R$18,697.00. The air conditioning system, including humidity and temperature control, has an average value of R$ 20,500.00. The devices required for the inlet and outlet of air and CO2, equipped with a pressure regulator and axial exhaust fans with shutters to maintain the system's sealing, amounted to R$ 5,525.00. The CO 2 , humidity, temperature, pressure gauge and interface sensors averaged R$ 8,760.00. Totaling R$57,076.00.
Considering that the chamber is a durable good, diluting the investment value for a minimum period of ten years. Considering that the production of concrete blocks for the same period is 15,206,400; the increase in cost per block will be R$0.0034 for a new installation and R$0.0037 for a retrofit, without impacting the final cost of the product.

Estimate of monthly CO2 consumption and cost
To analyze the CO 2 cost to produce 126,720.00 concrete masonry units two situations were considered. The first scenario considers that concrete block plant is located 30 km from the CO 2 supplier. In the second case the plant is 100 km away from the CO 2 supplier. In the first situation, the cost of CO 2 will be R$ 14,000/month or R$ 0.11/ block. In the second case, the CO 2 cost will be R$ 46,673/month or R$ 0.37/block. According to a survey with the industries in May 2022, the conventionally cured 14×19×39 cm concrete masonry unit cost R$2.65 (not considering the delivering freight cost) each. Therefore, the concrete masonry units cured with CO 2 will present an extra cost that can vary from 4% to 14%, depending on the distance from the factory to the CO 2 supplier and the CO 2 absorption rate applied. The closer the industry is to the CO 2 supplier; the more viable the CO 2 -cured concrete masonry units will be.
In any case, considering the environmental benefits this cost does not seem to be a barrier to the technology implementation. Also, it has already been discussed to monetary compensate the CO 2 sequestration. Although no practical regulation is in practice, this good environmental-friendly process may soon turn into a monetary profit.
The CO 2 emission related to cement production in 2020 in Brazil was approximately 34,907,896 tons [ [44]]. In that year, 61,052,000 tons of cement were produced, of which 3,952,000 were representative of the non-reinforced prefabricated industry and 1,958,000 were for the manufacture of concrete masonry units [ [45]]. This industry mainly uses CP-V cement which emits about 858 kg of CO 2 per cement tonne [ [46]]. With the CO 2 absorption rate considered in this study, of 8.62%, it would have been possible to sequester approximately 168,780 ton of CO 2 per year if the entire production of concrete masonry units had been subjected to carbonation cure. This would correspond to ~ 0.5% of the total CO 2 emitted by the Brazilian cement production in 2020, or ~ 10% of the concrete masonry units industry, and equivalent to the sequestration of 21,100 trees in one year.

CONCLUSION
Carbonic curing is a sustainable innovation that helps to reduce global warming. According to the International Energy Agency [47], CO 2 capture and storage technology, by the year 2050, will be responsible for a reduction of up to 56% of CO 2 emissions in the cement sector. Therefore, accelerated carbonation curing technology becomes indispensable to achieve this objective.
This innovation is already successfully applied in countries such as Canada, the United States, and the United Kingdom. Cases of success are detailed in this paper.
According to the technical feasibility here reported, is possible to implement the technology in the state of São Paulo, with all of the innovative process need available in the region. A map showing CO2 suppliers and concrete block plants located in the state is presented; the greater the distance between the supplier and the plant, the higher will be the cost.
An increase of R$ 0.11 and R$ 0.37 was estimated after implementing the carbonation cure if the plant is located 30km and 100 km from the CO 2 supplier, respectively. Those costs represent 4 to 14% of the concrete block production cost (not considering the delivering freight cost).
If all of the Brazilian concrete blocks industry implements the carbonation cure, considering the smallest absorption rate, it would have been possible to sequester approximately 168,780 ton of CO 2 /year. This would correspond to ~ 0.5% of the annual total CO 2 emitted by the Brazilian cement production, or ~ 10% concrete block industry, and the sequestration equivalent to 21,100 trees in one year.