LCA based recommendations for the selection of eco-concretes using blast furnace slag and fly ash

. The development of circular economy models for the construction sector brings important challenges and opportunities to research and particularly to make knowledge transfer, for example through public policies. Santiago de Cali, third major city of Colombia (South America), is developing a circular economy model policy, which is mainly based on waste valorisation. Among others, the model includes a family of eco-concrete products that replaces Portland cement by blast furnace slag and fly ash coming from local industries. Therefore, the aim of this research is to develop scientific based recommendations to stakeholders for the selection of different concrete products (i.e. beams, columns, slabs) using blast furnace slag and fly ash (eco-concretes). Considering that life cycle assessment (LCA) is the most employed tool for consolidating, comparing, and assessing sustainability impacts [1], the LCA of these concrete products for the design of a sustainable residential house was performed. The LCA software Building for Environmental and Economic Sustainability (BEES) developed by the National Institute of Standards and Technology (NIST) was selected for the study. Results include a discussion on the environmental impacts. Furthermore, a methodology for the selection of sustainable building materials is presented.


Introduction
Through a sustainability construction handbook and its certification, Santiago de Cali (Colombia) is formulating a circular economy model for the construction sector, which approaches the circular economy as a production and consumption system that promotes efficiency in the use of materials, water and energy, taking into account the resilience of ecosystems and the circular use of material flows through the implementation of technological innovations, alliances and collaborations between stakeholders for the promotion of business models that respond to the fundamentals of sustainable development [2]. Among others, the model includes products such as recycled aggregates or eco-aggregates, eco-concretes and ecomortars, eco-prefabricated products and modules, and smart construction materials, for example using titanium dioxide (TiO2). So, considering the wide spectrum of technological solutions, making decisions about the best sustainable materials for a building or infrastructure might be a complex process.
On the other hand, taking into account that using environmental, economic, and social indicators, Life Cycle Assessment (LCA) allows consolidating, comparing, and assessing sustainability impacts, this article presents LCA based recommendations to stakeholders for the selection of eco-concretes using * Corresponding authors: anibal.mauryramirez@ugent.be ; nele.debelie@ugent.be blast furnace slag and fly ash, local industrial wastes that have the potential of reducing significantly the current consumption of cement, which is projected to keep increasing in the remaining global south region until 2050, as indicated by the recent Global Consensus on Sustainability in the Built Environment [3].

LCA methodology
This study used the conceptual framework of LCA developed by the International Organization for Standardization (ISO), which includes four steps: objective and scope definition, inventory analysis, impact assessment, and results interpretation ( Figure 1). On the other hand, the online software Building for Environmental and Economic Sustainability (BEES) version 2.1, which was developed by the National Institute of Standards and Technology (NIST) from the U.S. Department of Commerce, was selected for the LCA implementation. This computational tool which was launched in 1997 as executable with less than 65 data base products, has now more than 248 data base products and it runs with free access on-line (Table 1). Similarly, BEES uses a Microsoft SQL Server relational database management system, which includes two phases. First, LCA software (SimaPro v8.0) and database (Ecoinvent 2017) are used to generate the Life Cycle Impact Assessment (LCIA) results for each building product. Second, the product characteristics, costs, and LCIA results are manually compiled into source data tables using an excel format. These source data tables are pulled into the BEES application in CSV format. Finally, the database is reviewed and validated by LCA experts and NIST research team. Additionally, a sample review of the database was performed by a third party in 2020 [4].

Objective and scope
Although the goal of BEES is to calculate the environmental impacts and economic performance for building products from North America, the lack of systematized information and the existing similarities in the standardization and production processes from the building materials industry from Colombia (South America), allow to use BEES as reference software to estimate the environmental impacts of eco-concretes from the product portfolio of the circular economy model of Santiago de Cali (Colombia). The goal and scope definitions include the system boundaries to produce Portland cement concrete products (Figure 2), cut-off criteria (cradle-to-grave, 60 years' durability), the functional unit of concrete (1 yd 3 or 0.76 m 3 for columns and beams), and the data collection strategy.

Inventory analysis
The inventory analysis requires the estimation of the inputs and outputs from the unit process within a product system. One of the primary tasks is the data collection that ensures the product system evaluated is representative and appropriately addresses the cut-off criteria, data (including quality requirements), and other scoping elements. For example, to produce a specific product or intermediate product, inputs collected include energy, fuels, water use, complementary (e.g. plywood forms) and raw materials. Similarly, outputs include direct emissions and waste to air, water, and soil ( Figure 3). On the other hand, although several approaches are used to collect inventory data to develop LCA's for the generic BEES products, generic product data are primary collected using the unit process and facility specific approach. NIST works to obtain product data that represent the closest approximations available of the impacts and features from each product. Some BEES products are built using detailed LCA surveys completed by industry experts, while others are built using published LCA reports, for example, coming from Environmental Product Declarations (EPD's). In most cases, experts from the corresponding industries verify the assumptions related to the unit processes to make sure that the data have been appropriately represented. Nowadays, many general industries and specialized ones have already published their EPD's, which are based on externally validated LCA's. For BEES products that use existing EPD as data source, the information normally comes with the approval of the copyright owner.

Products description
Normally, concrete is associated to different building elements by its compressive strength at 28 days after casting. So, the compressive strengths and products considered by BEES range from 20.7 MPa (3000 psi) to 55.2 MPa (8000 psi) as indicated in Table 2. Additionally, Table 2 describes the amount of cementitious materials replacements (ranges and designations).

Raw materials for concrete products
The analysed concrete products are a mixture of Portland cement, water, fine aggregate (i.e. sand or finely crushed rock), and coarse aggregate (i.e. gravel or crushed rock), which are normally designed for a specific mechanical (i.e. compressive strength 28 MPa after 28 days casting) and durability performance (i.e. 60 years). Ground granulated blast furnace slag and fly ash, coming from local industries, may be substituting a portion of the Portland cement in the concrete mix, which in this study varies from 20 to 40 % for fly ash, and from 30% to 50% for slag. Tables 3 and 4 present the amounts of concrete components for each mix design included in BEES for this case study.  For BEES, production processes characteristics regarding Portland cement and blast furnace slag are obtained from the EPD developed by the Portland Cement Association [5] and the Slag Cement Association [6], respectively. On the other hand, for fly ash, which is considered a waste material from the electricity generation using coal, no input materials and processing are considered due to minimal processing into a usable raw material. However, fly ash transport is considered

Concrete products manufacturing
Although BEES Online version 2.1 runs using the concrete products production data from the EPD developed in 2016 (version 2.0) by the Athena Sustainable Materials Institute for the National Ready Mixed Concrete Association [7], there is already a more recent EPD (version 3.1), which includes new plants and industries. However, the EPD conceptualization is the same as version 2.0, as for example can be seen through the system boundary (cradle-to-gate) in Figure 4.

Transportation
Due to the extended use of fossil fuels (i.e. diesel) for concrete mixers trucks, transportation is an important variable, particularly regarding to the global warming potential. So, BEES gives the possibility to personalize the concrete delivery distance. In this case, although the average distance for delivery among the NRMCA members in North America is 80 km (50 mi), a larger distance of 120 km (74.6 mi) was chosen. This is based on the average delivery distance from Latin America reported in the Overview of Cement and Concrete Production in Latin America and the Caribbean with a focus on the goals of reaching carbon neutrality [8]. In addition, it is important to indicate that the transport included in the construction process stage (Figure 4) refers to the energy portion used for the concrete mixing process in the trucks.

Concrete casting
Although concrete casting has a wide spectrum of requirements regarding steel reinforcement and plywood forms, BEES considers four requirement types for Beams, Columns, Slabs and Basement Walls, respectively (Table 5). Regarding the energy consumption at this stage, a small fraction was considered for placing the concrete in their forms or slab [4].

Use
Considering the high durability of concrete products (beams, columns, and slabs) which are not exposed to aggressive environments (residential or commercial buildings), the possible maintenance activities were considered insignificant during the 60 years of the LCA. However, further research will be performed based on the different degradation mechanisms.

End-of-life (EOL)
At the end of life of the building when concrete is removed, the material can be crushed and reused as fill and road base material. The decision to send crushed concrete to a landfill is a future project decision. It is most representative of current practice to assume that removed concrete is crushed and then reused or recycled to avoid landfilling [4].

Impact assessment
In the current version, BEES includes twenty-one impact categories, ten coming from the Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI) developed by the Environmental Protection Agency from U.S. (EPA), three coming from BEES itself, and eight other categories specified in the Product Category Rules (PCR) developed by the International EPD system ( Table 6). For this study, BEES and TRACI impact categories were considered.

Results interpretation
This phase follows to the environmental impact's classification and characterization which results in metrics that are unproportioned or inadequate to make a real comparative analysis (e.g. Global warming potential in kg of CO2 equivalent). So, the results are rectified and normalized to place them in comparable scales. BEES uses an extended version of the EPA normalization references which quantify U.S. economy annual contributions to each impact category (Table 7). Then, normalization is completed by dividing BEES product-level impact assessment results by the fixed U.S. scale normalization references, expressed in the same units, yielding an impact category score for a building that has been placed in the context of annual U.S. contributions to that impact. By placing each product-level impact result in the context of its associated U.S. impact result, the measures are all reduced to the same scale, allowing comparison across impacts because they are now fractions of a percentage from U.S. impact yardstick, which keep the same trend for Colombia. Afterwards, BEES synthesis impact scores by weighting impact categories by its relative importance to overall environmental performance. Although the weight for each environmental impact category can be selected by the user in BEES, the Science Advisory Board from EPA and the BEES stakeholder Panel suggest weights that can be seen in Tables 8 and 9, respectively.

Case study
First, a sustainable house must be designed in Santiago de Cali (Colombia) for a family consisting of five persons. In order to make this, there is available a flat land of which the area is 150 m 2 and a budget of 100,000 USD. Second, a group of students of the course Sustainability in Construction from the Master in Civil Engineering (Pontificia Universidad Javeriana Cali, Colombia) proposed a one-floor house with four bedrooms, two bathrooms, visitor´s toilet, living room, dining room and terrace ( Figure 5). In addition, considering the high seismic risk and looking for a flexible architectural design, a 28 MPa concrete frame structure formed by aerial beams (0.30m×0.35m), columns (0.30m×0.30m) and foundation beams (0.40m×0.3m) was suggested. Third, for the selection process of the best eco-concrete, a matrix with their environmental impact categories, concrete products and associated scores that range from 1 for the best option (with lower environmental impact values), 2 for the second best option and so on, was elaborated.

Results
The on-line software BEES was set using the conditions previously described in the LCA methodology. Then, concrete products were categorized for each environmental category from 1 to 8 based on the environmental impact, being 1 the smallest and 8 the biggest impact, respectively. For example, the results for environmental impact on Global Warming Potential for the house beams can be seen in Figure 6. In this case, numbers 1 and 8 were assigned to 50% Slag Concrete and 100% OPC Concrete (reference), respectively. This result was the same for all environmental impact factors evaluated, except for Acidification, where the behaviour was different with the number 1 assigned to 40% Fly Ash Concrete (Figure 7). The same trend as described above was obtained with concrete products for columns.  So, following the proposed methodology to select the best eco-concrete for the sustainable house a matrix was elaborated (Table 10). It is noticeable that the 1 st option is the 50% Slag Concrete, followed by the 20 -30% Fly Ash -Slag Concrete (2 nd option) and the 40% Slag Concrete (3 rd option). This result does only not apply for acidification, where the lowest environmental impact is obtained with the 40% Fly Ash Concrete (1 st option) followed by the 30% Fly Ash Concrete (2 nd option) and the 20 -30% Fly Ash -Slag Concrete (3 rd option). In order to take a final decision about the eco-concrete for the sustainable house, it was considered that Santiago de Cali is implementing a net zero carbon buildings program [11] and that acidification is only one environmental impact from the 13 analysed ones, so the best option for the beams and columns' project is the 50% Slag Concrete.
On the other hand, based on the magnitude of the environmental impacts, particularly on global warming, the amount of CO2 equivalent produced by the structure made with 50% Slag Concrete is 379.3 kg equivalent CO2 (Table 10), which is approx. 27% smaller than the equivalent CO2 that might be released by traditional concrete (100%OPC). This percentage is slightly higher than the average obtained in Colombia (3 to 25%) using other waste valorisation strategies in the cement industry [12]. Thus, considering the current use of fly ash and blast furnace slag, which are 3.5 and 72.3 kg per ton of cement [12], more efforts should be done to significantly reduce CO2 emissions, while satisfying the huge construction demands coming from a developing country.

Conclusion
The developed life cycle assessment using the BEES software and the proposed methodology allows estimating the environmental impacts at local and regional levels for supporting decision makers about sustainable materials for buildings and infrastructure. For example, based on acidification and global warming potential, two different eco-concretes would be recommended for a sustainable house located in Santiago de Cali (Colombia). In this case study, 40% Fly Ash Concrete or 50% Blast Furnace Slag Concrete would be recommended for reducing Acidification or Global Warming Potential, respectively. However, in order to make a final decision, knowing the sustainable development plan from the city, region or country is fundamental. In this case study, the net zero carbon program to which the city is subscribed supports the decision towards the eco-concrete with less global warming potential (i.e. lower kg CO2/functional unit), which is 50% Blast Furnace Slag Concrete. This decision is in agreement with the current needs of reducing CO2 emissions associated with the cement production, which is estimated to continue increasing in the remaining global south region such as Latin America, particularly in Colombia, one of the fastest growing economies [2].