Eco-efficient concrete using blast furnace slag: optimized performance, lower costs, reduced CO₂eq emissions

Concreto ecoeficiente com escória de alto-forno: desempenho otimizado, menores custos e redução das emissões de CO₂eq

Abstract

This study evaluated eco-efficient concretes with partial replacement of Portland cement by blast furnace slag (BFS), considering compressive strength, CO₂eq emissions, and production cost. A factorial experimental design was adopted with cement replacement by BFS (%CR) at 0%, 40%, and 80%, fines-to-aggregate ratio (%FA) at 18%, 20%, and 22%, and curing times of 7, 28, 56, and 91 days. Compressive strength was experimentally determined, while the environmental assessment was performed through a cradle-to-gate Life Cycle Assessment (LCA), using 1 m³ of concrete as the functional unit. Costs were estimated based on material consumption and local databases. The results showed that 40% BFS resulted in compressive strength values close to those of the reference mixtures at later ages, while 80% BFS reduced strength by approximately 28%. CO₂eq emissions decreased from 259 kg CO₂eq/m³ in C-18 to 90.3 kg CO₂eq/m³ in 80E-18. Cost reductions of approximately 14% and 29% were obtained for 40% and 80% BFS, respectively. Response surface methodology combined with desirability analysis indicated an estimated optimal cement replacement level of 58.94% by BFS and 18% FA, with global desirability of 79.09%.

Keywords
Blast furnace slag; Eco-efficient concrete; Life cycle assessment; CO₂eq emissions

Resumo

Este estudo avaliou concretos ecoeficientes com substituição parcial do cimento Portland por escória de alto-forno (ES), considerando resistência à compressão, emissões de CO₂eq e custo de produção. Foi adotado planejamento experimental fatorial com substituição do cimento por ES (%CR) nos teores de 0%, 40% e 80%, relação entre finos e agregados (%FA) nos níveis de 18%, 20% e 22%, e cura aos 7, 28, 56 e 91 dias. A resistência à compressão foi determinada experimentalmente, enquanto a avaliação ambiental foi realizada por Avaliação do Ciclo de Vida (ACV), com abordagem do berço ao portão e unidade funcional de 1 m³ de concreto. Os custos foram estimados pelo consumo de materiais e bases locais. Os resultados mostraram que 40% de ES resultou em resistências próximas às das misturas de referência em idades avançadas, enquanto 80% reduziu a resistência em aproximadamente 28%. As emissões diminuíram de 259 kg CO₂eq/m³ em C-18 para 90,3 kg CO₂eq/m³ em 80E-18. As reduções de custo foram de 14% e 29% para 40% e 80% de ES, respectivamente. A superfície de resposta associada à desejabilidade indicou teor ótimo estimado de substituição do cimento por ES de 58,94% e 18% de FA, com desejabilidade global de 79,09%.

Palavras-chave
Escória de alto forno; Concreto ecoeficiente; Avaliação do ciclo de vida; Emissão de CO₂eq

1 Introduction

The construction sector has a significant influence on global resource consumption and greenhouse gas emissions. The Global Status Report for Buildings and Construction 2024/25 indicates that buildings and construction accounted for approximately 34% of global carbon dioxide (CO₂) emissions and 34% of global energy consumption in 2023 (United Nations Environment Programme, 2025). In this context, concrete plays a central role because it is one of the most widely used construction materials and depends directly on Portland cement as its main binder (Guo et al., 2023). In 2022, global cement production reached approximately 4.2 billion tons, with China and India as the main producers (Garside, 2023). According to the International Energy Agency (2023), cement production accounts for approximately 8% of global CO₂ emissions. Therefore, reducing cement consumption and improving the environmental performance of cementitious materials are important strategies to support the transition toward lower-carbon construction.

One of the strategies adopted to reduce the environmental impact of cementitious matrices is the partial replacement of Portland cement by supplementary cementitious materials and industrial residues. Studies by Faldessai et al. (2023), Eugênio et al. (2023), Özkiliç et al. (2023), França et al. (2023), and Kaplan et al. (2023) investigated the use of ceramic waste, iron ore tailings, marble waste, sugarcane bagasse ash, blast furnace slag, and fly ash as alternatives to reduce cement consumption in concretes and mortars. In these studies, replacement levels varied from 2% to 80%, depending on the type of material, its physical and chemical characteristics, and the required mechanical performance.

Among these materials, blast furnace slag has received attention because of its chemical compatibility with cementitious systems and its availability as a coproduct of the steel industry. Blast furnace slag is generated during pig iron production, and global pig iron production through the blast furnace route reached 1,286.54 million tons in 2023 (World Steel Association, 2024). It is estimated that approximately 20% by mass of pig iron production is converted into slag. Due to its contribution to sulfate attack resistance, permeability reduction, and workability improvement, blast furnace slag is widely used by the cement industry (Zeng et al., 2023). Its composition is mainly formed by oxides of silicon, calcium, aluminum, magnesium, and oxygen, which may represent up to 95% of its mass. This chemical similarity with Portland cement enables its use as a supplementary cementitious material (Silva et al., 2023).

The use of blast furnace slag in cementitious materials is already consolidated in research and industrial practice. In Brazil, NBR 16697 (ABNT, 2018a) allows high slag contents in blast-furnace Portland cement, especially in CP III cement, which confirms that the material is not new to cement technology. However, its performance in concrete still depends on several variables, including replacement level, slag fineness, cement composition, water-to-binder ratio, aggregate proportion, curing age, and dosage method. Therefore, the scientific relevance of studies involving blast furnace slag does not lie only in confirming its feasibility, but in defining how different mixture conditions affect mechanical, environmental, and economic responses.

Previous studies have shown that blast furnace slag can influence hydration, mechanical behavior, and the microstructure of cementitious materials. Barabanshchikov et al. (2020) reported that the addition of blast furnace slag to Portland cement can control the hydration rate and reduce heat release during hydration by up to 13% compared with pure cement. Kaya et al. (2025) investigated the partial replacement of ordinary Portland cement (OPC) with blast furnace slag (BFS) at proportions of 0%, 20%, 40%, and 60%, with a water-to-binder ratio of 0.3 and 41% aggregate volume. The authors evaluated fresh-state strength, buildability stress, and extrusion workability, the latter being relevant for 3D printing applications. The results showed that 40% BFS increased fresh-state strength by up to 163.41% and improved static yield stress, resulting in greater cohesion and structural performance at early ages.

Other studies also indicate that the performance of slag depends on its characteristics and on the cementitious system analyzed. Ghasemalizadeh and Khoshnazar (2024) verified that ultrafine ground granulated blast furnace slag, at proportions of 30% and 40% by weight, significantly increased compressive strength at all ages, outperforming mixtures containing conventional slag and even pure cement. The performance improvement was attributed to the higher reactivity of ultrafine slag, which promoted calcium hydroxide consumption and accelerated clinker phase hydration, resulting in a denser and more homogeneous microstructure. Montoya et al. (2023) observed that replacing up to 50% of OPC with blast furnace slag in alkali-activated systems favored the formation of additional hydration products, such as C-A-S-H gel, which are responsible for increased strength and reduced porosity of the cementitious matrix.

Although these studies demonstrate the technical potential of blast furnace slag, mechanical and environmental performance are often evaluated separately. Practical mixture design requires simultaneous consideration of compressive strength, CO₂eq emissions, production cost, and eco-efficiency, since lower emissions may not coincide with higher strength, and higher strength may not ensure lower cost or environmental impact. Therefore, optimal replacement requires joint technical, environmental, and economic analysis.

In this context, the contribution of the present study is not the use of blast furnace slag itself, since this material is already widely studied and applied in the cement industry. The contribution lies in the integrated evaluation of concretes produced under the same experimental dosage method, combining compressive strength, cradle-to-gate Life Cycle Assessment, production cost, and eco-efficiency indicators. In addition, the study applies factorial design, response surface methodology, and desirability analysis to estimate an optimal replacement level based on the simultaneous optimization of these variables. This approach provides a clearer understanding of the balance between mechanical performance, CO₂eq emissions, and cost in concretes containing blast furnace slag.

Thus, this study aimed to evaluate eco-efficient concretes with partial replacement of Portland cement by blast furnace slag, considering compressive strength, CO₂eq emissions obtained through cradle-to-gate Life Cycle Assessment, and production cost. The objective was to determine, through factorial design, response surface methodology, and desirability analysis, an estimated optimal replacement level capable of reducing environmental and economic impacts while maintaining adequate mechanical performance under the adopted dosage conditions.

2 Materials and methods

2.2 Materials

The following materials were used in this study:

1. high early strength and sulfate-resistant cement (CP), CPV ARI RS, manufactured by Liz Cimentos;

2. natural sand (NS) obtained from the municipality of Sem Peixe, Minas Gerais, Brazil;

3. artificial sand (AS) supplied by Pedreira MBC;

4. coarse aggregates (G0 and G1), also supplied by Pedreira MBC, located in São Geraldo, Minas Gerais, Brazil;

5. blast furnace slag (BFS), type G2A-4K, provided by Gerdau Graphene;

6. groundwater obtained from an artesian well; and

7. a water-reducing admixture manufactured by GCP, category MIRA SET 504.

Table 1 presents the physical characteristics of the materials, and Table 2 shows the chemical characterization of Portland cement and blast furnace slag. Both materials are mainly composed of calcium oxide (CaO), silica (SiO₂), and alumina (Al₂O₃), which supports the use of blast furnace slag as a supplementary cementitious material. The physical characterization of Portland cement and blast furnace slag was presented through specific density and fineness/specific surface area values. The particle size distribution curves of cement and slag were not experimentally determined in this study; therefore, this limitation was considered in the interpretation of the fines-to-aggregate ratio. Figure 1 presents the particle size distribution curves of the aggregates used in the mixtures, which exhibited fineness modulus values of 2.79 for AS, 2.47 for NS, 5.69 for G0, and 6.92 for G1.

Table 1
Physical characteristics of the materials

Material	Specific density (g/cm³)	Fineness (g/cm³)	Specific surface area (m²/kg)	pH
CP	3.05	-	483	-
BFS	2.96	4000 ± 300	-	-
NS	2.60	-	-	-
AS	2.88	-	-	-
G0	2.81	-	-	-
G1	2.87	-	-	-
Water	1.00	-	-	6.8
Admixture	1.055 - 1.095	-	-	-

Table 2
Chemical characteristics of Portland cement and blast furnace slag

Components		CaO	SiO2	Al2O3	MgO	Fe2O3	SO3	MnO	TiO2	K2O	Na2O	Others
Contents (%)	BFS	44.00	33.10	11.00	5.20	1.90	1.80	0.60	0.50	0.50	0.20	1.20
	CP	60.30	20.16	5.23	1.74	2.55	2.81	-	-	0.60	0.06	6.62

Figure 1
Particle size distribution curves of the aggregates

The mineralogical characterization of blast furnace slag was evaluated by X-ray diffraction (XRD), as shown in Figure 2. The XRD pattern exhibited a broad diffuse halo within the intermediate 2θ range, supporting the predominantly amorphous nature of the material. Low-intensity reflections superimposed on this halo were tentatively associated with calcium-, magnesium-, and aluminum-containing silicate phases. Therefore, the BFS should be considered predominantly, but not entirely, amorphous. Because of the broad halo, peak overlap, and signal noise, the crystalline phase assignments were interpreted qualitatively. The predominantly amorphous structure is consistent with the latent hydraulic behavior commonly associated with blast furnace slag in cementitious systems

Figure 2
X-ray diffraction pattern of blast furnace slag

2.2 Experimental design

The proportions of the concrete mixtures containing blast furnace slag were defined through a full factorial experimental design, allowing the evaluation of the individual effects of the factors, their interaction effects, and possible curvature in the responses. Two controlled factors were investigated: the percentage of Portland cement replacement by blast furnace slag (%CR), at nominal levels of 0%, 40%, and 80%, and the fines-to-aggregate ratio (%FA), at nominal levels of 18%, 20%, and 22%. Both parameters were defined on a mass basis.

Portland cement replacement by BFS was calculated as the ratio between the mass of BFS and the total binder mass, composed of Portland cement and BFS, according to Equation 1:

Eq. 1 $\%CR=\frac{m_{BFS}}{m_{CP}+m_{BFS}}x100$

The fines-to-aggregate ratio was calculated as the ratio between the total binder mass, composed of Portland cement and/or BFS, and the total mass of aggregates, composed of natural sand, manufactured sand, gravel 0, and gravel 1, according to Equation 2:

Eq. 2 $\%FA=\frac{m_{CP}+m_{BFS}}{m_{NS}+m_{AS}+m_{G0}+m_{G1}}x100$

Where:

$m_{CP}$ and $m_{BFS}$ are the masses of Portland cement and blast furnace slag, respectively; and

$m_{NS}$, $m_{AS}$, $m_{G0}$, and $m_{G1}$are the masses of natural sand, manufactured sand, gravel 0, and gravel 1, respectively.

Accordingly, the %FA parameter was calculated exclusively on a mass basis and was not determined from the absolute or apparent volumes of the constituent materials.

As an example, for mixture C-18, the fines-to-aggregate ratio was calculated as 345/(423+454+206+821)×100=18.12%For mixture 40E-18, the BFS replacement level was calculated as 138/(207+138)×100=40%. These calculations demonstrate that both parameters were determined on a mass basis using the mixture quantities presented in Table 6.

Table 6
Unit cost of materials

Material	Unit	Cost (R$)
Portland cement CP-V	Kg	R$ 0,83
Slag	kg	R$ 0,35
Manufactured sand	m³	R$ 143,52
Natural sand	m³	R$ 138,88
Gravel 0	m³	R$ 154,41
Gravel 1	m³	R$ 147,11
Admixture	Kg	R$ 3,80
Water	l	R$ 0,02

Based on the final mixture quantities, the calculated mass-based %FA values were 18.12%, 20.18%, and 22.30%. For the organization of the experimental design and sample nomenclature, these values were represented by the nominal levels of 18%, 20%, and 22%, respectively.

For statistical modeling, the nominal levels of %CR and %FA were coded in Minitab® 19 as −1, 0, and +1. Thus, for %CR, the coded levels corresponded to 0%, 40%, and 80%, respectively, while for %FA they corresponded to the nominal levels of 18%, 20%, and 22%, respectively. This coding procedure was adopted exclusively for the organization and statistical analysis of the experimental design.

Additionally, curing time was included as a controllable factor exclusively for the evaluation of compressive strength, varying at four levels: 7, 28, 56, and 91 days. The water-to-binder ratio was fixed at 0.60, the ratio between fine and coarse aggregates was kept constant, and a water-reducing admixture was added to maintain the slump within the range of 140 ± 20 mm. Table 3 presents the combinations of the experimental levels of %CR and %FA included in the factorial design, whereas the corresponding quantities of materials required to produce 1 m³ of concrete are presented in Table 4. Each mixture was produced in duplicate for each evaluated curing age.All statistical analyses were performed using Minitab^® 19 software, considering a significance level of p-value < 0.05. Analysis of variance (ANOVA) was applied to identify which main effects, interaction effects, and second-order effects significantly influenced the response variables. Regression models were then fitted to describe the behavior of the responses as a function of the controlled factors. For compressive strength, the statistical model considered %CR, %FA, and curing time, whereas the environmental and economic responses were evaluated based on the mixture proportions.

Table 3
Factorial design generated in Minitab^® 19

Treatment	Nominal experimental levels		Coded levels
	% CR	% FA	A	B
1	00	18	-1	-1
2	0	20	-1	0
3	0	22	-1	1
4	40	18	0	-1
5	40	20	0	0
6	40	22	0	1
7	80	18	1	-1
8	80	20	1	0
9	80	22	1	1

• Note: the %FA values shown in the experimental design correspond to nominal levels. Based on the final mixture quantities, the calculated mass-based ratios were 18.12%, 20.18%, and 22.30%.

Table 4
Composition of the concrete samples

Sample	Cement (kg/m³)	Slag (kg/m³)	Natural sand (kg/m³)	Manufactured sand (kg/m³)	Gravel 0 (kg/m³)	Gravel 1 (kg/m³)	Water (L/m³)	Admixture (kg/m³)
C-18	345,0	0,0	423	454	206	821	207,0	2,733
C-20	384,2	0,0	423	454	206	821	230,5	1,333
C-22	424,6	0,0	423	454	206	821	254,8	0,000
40E-18	207,0	138,0	423	454	206	821	207,0	2,733
40E-20	230,5	153,7	423	454	206	821	230,5	1,333
40E-22	254,8	169.9	423	454	206	821	254,8	0,000
80E-18	69,0	276,0	423	454	206	821	207,0	2,733
80E-12	76,9	307,3	423	454	206	821	230,5	1,333
80E-22	84,9	339,7	423	454	206	821	254,8	0,000

• Note: Portland cement replacement by BFS was performed on a mass basis. The %FA parameter was calculated as the mass ratio between the total binder content and the total aggregate content. The values 18%, 20%, and 22% represent the nominal experimental levels, corresponding to calculated mass-based ratios of 18.12%, 20.18%, and 22.30%, respectively.

The optimized mixture was determined using the desirability function, which converts each response into a value from 0, representing an unacceptable result, to 1, representing the most desirable result. Overall desirability was calculated as the geometric mean of the individual values. Optimization maximized 28-day compressive strength and minimized CO₂eq emissions and production cost, identifying the mixture with the best combined mechanical, environmental, and economic performance.

2.3 Production and mechanical performance of concrete specimens

The concrete specimens were produced based on the combinations defined in the experimental design presented in Table 3. Portland cement replacement by BFS was performed on a mass basis. For each nominal %FA level, the total binder mass, corresponding to the sum of Portland cement and BFS, was kept constant, and the required mass of Portland cement was replaced by an equivalent mass of BFS. Thus, BFS represented 0%, 40%, or 80% of the total binder mass, according to the corresponding experimental treatment. Table 6 presents the quantities of materials required to produce 1 m³ of concrete. The mixing sequence followed the procedure established by NBR 12821 (ABNT, 2009) and is illustrated in Figure 3.

Figure 3
Mixing procedure of the concrete samples

To determine the mechanical performance of the concrete, cylindrical specimens with 100 mm diameter and 200 mm height were molded. After 24 hours, the specimens were demolded and subjected to moist curing under controlled temperature of 25 °C. The compressive strength test was carried out in accordance with NBR 5739 (ABNT, 2018b). The specimens were tested at ages of 7, 28, 56, and 91 days using a Solotest hydraulic/electrical testing machine.

2.4 Environmental assessment of concrete

The environmental assessment followed the Life Cycle Assessment (LCA) methodology, according to NBR ISO 14040 (ABNT, 2025a) and NBR ISO 14044 (ABNT, 2025b), which establish its principles, framework, requirements, and guidelines.

The objective of this study was to perform the LCA of the nine concrete mixtures presented in Table 5, considering a cradle-to-gate approach and using 1 m³ of concrete as the functional unit. The aim was to evaluate the influence of the %CR and %FA factors on CO₂eq emissions during concrete production. Modeling was performed using SimaPro software version 9.5 and the Ecoinvent database version 3.9.1 to determine the contribution of the materials to CO₂eq emissions within the global warming impact category.

Table 5
Concrete components

Input	Component
Known inputs from nature (resources)	Water, unspecified natural origin, BR-MG
Known inputs from the technological sphere (materials/fuels)	Cement, CP V RS {BR}| cement production, CP V RS | Cut-off, U
	Chemical, organic {GLO}| chemical production, organic | Cut-off, U
	Concrete mixing factory {RoW}| concrete mixing factory construction | Cut-off, U
	Gravel, crushed {BR}| gravel production, crushed | Cut-off, U
	Lubricating oil {RoW}| lubricating oil production | Cut-off, U
	Sand {BR}| sand quarry operation, extraction from river bad | Cut-off, U
	Sand {BR}| sand quarry operation, open pit mine | Cut-off, U
	Steel, low-alloyed, hot rolled {RoW}| steel production, low-alloyed, hot rolled | Cut-off, U
	Ground granulated blast furnace slag {RoW}| ground granulated blast furnace slag production | Cut-off, U
	Synthetic rubber {RoW}| synthetic rubber production | Cut-off, U
	Tap water {BR}| tap water production, underground water with disinfection | Cut-off, U
Known inputs from the technological sphere (electricity/heat)	Diesel, burned in building machine {GLO}| diesel, burned in building machine | Cut-off, U
	Electricity, medium voltage {BR-South-eastern/Mid-western grid}| electricity voltage transformation from high to medium voltage. Cut-off, U

The dataset “Concrete, 25MPa {BR} | concrete production, 25MPa, for building construction, with cement, CP-II-E | Cut-off, U” was used as the reference for modeling the new concretes, whose component specifications are presented in Table 5. Mass quantities were used to quantify each material in the concrete mixtures, while the quantities of the remaining components were kept unchanged. The selected components were those that best represent local conditions. The results do not include transportation processes or the infrastructure of the concrete production plant.

The system boundary adopted for the cradle-to-gate LCA is summarized in Figure 4. The assessment included the production of the main input materials used in the concrete mixtures, namely Portland cement, blast furnace slag, aggregates, water, and admixture, as well as the modeling of 1 m³ of concrete as the functional unit. The environmental output was expressed as global warming potential, in kg CO₂eq/m³. The figure also summarizes the main CO₂eq emission values obtained for the nine mixtures and the contribution of Portland cement and blast furnace slag to the total emissions. Transportation processes, construction activities, use stage, maintenance, end-of-life stage, and the infrastructure of the concrete production plant were not included in the system boundary.

Figure 4
LCA system boundary adopted for the cradle-to-gate assessment of the concrete mixtures

2.5 Economic assessment of concrete

The economic assessment was conducted using Life Cycle Costing (LCC), a method that structures cost analysis across the defined system stages using updated databases (Piccini et al., 2023).

The objective was to analyze the costs of the concretes corresponding to the nine mixtures presented in Table 3. The LCC analysis was limited to the acquisition costs of the materials composing the final product and adopted 1 m³ of concrete as the reference unit. The costs of cement, slag, and admixture were obtained through surveys conducted in the local market. For aggregates (NS, AS, G0, and G1), data from the Minas Gerais Reference Cost System for Infrastructure Works (SICRO/DNIT, 2023) were used. For water, data were collected from the Autonomous Water and Sewage Service (SAAE, 2023) of the city of Viçosa, considering the average cost for a commercial establishment. All collected data are presented in Table 6, and their unit values were represented in accordance with Table 4.

2.6 Eco-efficiency analysis of concrete

The eco-efficiency assessment followed NBR ISO 14045 (ABNT, 2014), integrating life-cycle environmental impacts with the value of the product system. LCA was used to quantify environmental impacts (ABNT, 2014).

The eco-efficiency evaluation consists of five stages, as shown in Figure 5: goal and scope definition, environmental assessment, product system assessment, eco-efficiency quantification, and interpretation. The objective was to evaluate the eco-efficiency of the nine concrete mixtures presented in Table 5, adopting 1 m³ of concrete as the reference unit. Regarding the value of the product system, choices may reflect efficiency in use, resources, production, delivery, utilization, or a combination of these aspects, and may be expressed in monetary terms or based on other criteria (ABNT, 2014).

Figure 5
Phases of an eco-efficiency assessment (Associação Brasileira de Normas Técnicas, 2014)

Two eco-efficiency indicators were used. For variables with directly proportional importance, the indicator (IEDP) presented in Equation 3 was applied, whereas for variables with inversely proportional importance, the eco-efficiency indicator (IEIP) proposed by WBCSD (2006) and exemplified by NBR ISO 14045 (ABNT, 2014) was adopted, as shown in Equation 4. The product system value indicators (IVSP) were mechanical performance and LCC, while the environmental impact indicator (IIA) corresponded to the LCA of the concrete.

Eq. 3 $IEDP=IVSP*IIA$

Eq. 4 $IEIP=\frac{IVSP}{IIA}$

3 Results and discussion

3.1 Influence of %CR and %FA on concrete compressive strength

Figure 6 presents the compressive strength (CS) results at 7, 28, 56, and 91 days. At all ages, CS decreased as the fines-to-aggregate ratio (%FA) increased. This effect should be interpreted with caution because the particle size distributions of Portland cement and blast furnace slag were not experimentally determined. Therefore, the observed variations should not be directly attributed to packing density or porosity, as these properties were not measured in this study. In the adopted dosage method, changes in %FA also modified the total binder content, paste volume, total water content, and admixture demand, while the water-to-fines ratio was kept constant. Thus, the influence of %FA on CS must be analyzed together with the cement replacement level, curing age, and mixture composition. Although previous studies indicate that the filler/cement ratio can affect the behavior of cementitious matrices (Souza et al., 2023), the present study does not allow this effect to be isolated from the other dosage variables.

Figure 6
Compressive strength results at 7, 28, 56, and 91 days

For the same %FA level, the increase in %CR did not produce a single linear behavior for all mixtures and curing ages. The mixtures with 40% BFS presented compressive strength values close to those of the reference mixtures at later ages, whereas the mixtures with 80% BFS showed lower compressive strength, especially at early ages. In general, CS increased with curing time, indicating the contribution of continued hydration and the later-age reactions associated with BFS-containing mixtures. Figure 7 presents the Pareto chart, in which the dashed line corresponds to the reference used to indicate whether the effects are statistically significant, and the value 2.04 represents the t-value associated with the null hypothesis for α = 0.05. The following factors significantly influenced CS: %CR (A), the second-order effect of %CR (AA), %FA (B), time (C), and the second-order effect of time (CC).

Figure 7
Pareto chart for compressive strength

Figure 8 presents the effects of %CR and %FA on compressive strength (CS) for each analyzed age. It can be observed that at 7 days the CS results decrease as the BFS content increases. According to Cabrera-Madrid, Escalante-García and Castro-Borges (2016), this behavior may be attributed to the initial inertness of BFS, since its chemical reactions do not occur at the same rate as cement hydration reactions. Thus, in mixtures containing BFS, strength compensation may occur at later ages, which corroborates the significant effect of the curing time factor. For this reason, with strength development at 28, 56, and 91 days, no difference in mean CS is observed between the reference samples (100% cement) and those containing 40% BFS, suggesting that this replacement level may be suitable for the production of eco-efficient concretes.

Figure 8
Main effects plot for compressive strength at (a) 7; (b) 28; (c) 56; and (d) 91 days

The lower compressive strength observed in mixtures containing 80% BFS should not be attributed exclusively to the slower early-age reaction of blast furnace slag. At this replacement level, the substantial reduction in Portland cement content also decreased the amount of clinker available to generate the alkaline environment and calcium-bearing hydration products required for effective slag activation. BFS is a latent hydraulic material, and the dissolution of its predominantly amorphous phase depends on sufficiently alkaline conditions, which are provided mainly by Portland cement hydration. Therefore, in mixtures containing 80% BFS, the reduced availability of alkaline species and calcium hydroxide may have limited slag dissolution and the subsequent formation of additional C-S-H and C-A-S-H gels, particularly at early ages. Thus, the lower mechanical performance resulted from the combined effects of Portland cement dilution and insufficient chemical conditions for slag activation, rather than solely from an intrinsically low reactivity of BFS (Cabrera-Madrid, Escalante-García and Castro-Borges, 2016)

At later ages, the continued hydration of Portland cement may progressively improve the chemical conditions required for slag reaction, favoring the formation of additional hydration products and contributing to the densification of the cementitious matrix. This effect was more evident in mixtures containing 40% BFS, which maintained a better balance between the amount of Portland cement required to provide adequate activation conditions and the amount of slag available for later-age reactions. In contrast, at 80% replacement, the limited Portland cement content may have restricted the degree of slag activation throughout the evaluated curing period. The predominantly amorphous structure identified by XRD, together with the low-intensity crystalline reflections, is consistent with the latent hydraulic character of BFS; however, its effective reaction depends not only on its mineralogical characteristics, but also on fineness and on the chemical conditions provided by the cementitious system (Zhai; Kurumisawa; Moon, 2023; Sun et al., 2022; Langaro et al., 2017). Particle size distribution and specific surface area may also influence slag reactivity (Pereira et al., 2010). However, since the particle size distribution curves of Portland cement and BFS were not experimentally determined, their influence on particle packing and compressive strength cannot be directly confirmed in this study.

With the aid of Minitab^® 19 software, an adjusted regression model was obtained based on the experimental results, with an adjusted R² of 92.81%, indicating adequate agreement between the model and the experimental data. Equation 5 presents the regression model for compressive strength in coded levels. Parameters considered non-significant according to ANOVA were removed from the regression equation. The model indicates that cement replacement by BFS, represented by %CR (A), and curing time (C) exerted the greatest influence on compressive strength, whereas %FA (B) presented a lower effect. These results indicate that the mechanical behavior of the mixtures was governed mainly by the cement replacement level and strength development over time. Therefore, the effect of %FA should be interpreted as secondary and dependent on the adopted mixture-design method, rather than as an isolated factor controlling compressive strength.

Eq. 5 $CS\left(MPa\right)=10.93+13.97A-1.704B+12.74C-4.745A²-1.723C²$

3.2 Influence of %CR and %FA on CO₂eq emissions from concrete production

Figure 9 presents the CO₂eq emission results for the production of 1 m³ of concrete. The emissions decreased as the cement replacement level by BFS (%CR) increased, which was expected due to the reduction in Portland cement consumption. A similar trend was reported by Yang et al. (2014), who observed that the environmental impact index decreases as the blast furnace slag replacement rate increases. In contrast, the increase in the fines-to-aggregate ratio (%FA) resulted in higher CO₂eq emissions under the adopted dosage method because it increased the total binder content and, consequently, the amount of cement in the mixtures. For example, in mixtures without BFS, cement consumption increased from 345.0 kg/m³ in C-18 to 424.6 kg/m³ in C-22. Therefore, the direct relationship between %FA and CO₂eq emissions should not be generalized as an unfavorable effect of fines, but interpreted as a consequence of the mixture design adopted in this study. With the increase of %FA for the same %CR level, emissions increased on average by 9.4% for concretes without cement replacement, 8.7% for concretes with 40% BFS, and 6.5% for concretes with 80% BFS.

Figure 9
CO₂eq emission results for the production of 1 m³ of concrete

The contribution analysis showed that Portland cement was the main source of CO₂eq emissions in all mixtures. In the reference mixtures, cement accounted for 93.8% of total emissions in C-18, 95.1% in C-20, and 96.5% in C-22. Total emissions increased from 259 kg CO₂eq/m³ in C-18 to 310 kg CO₂eq/m³ in C-22, mainly due to the higher cement consumption associated with the increase in %FA under the adopted dosage method. In the mixtures with 40% BFS, total emissions decreased to 175, 190, and 206 kg CO₂eq/m³ for 40E-18, 40E-20, and 40E-22, respectively. In these mixtures, cement contributed between 83.4% and 86.9% of total emissions, while BFS contributed between 7.3% and 7.6%. The lowest impacts were observed for the mixtures with 80% BFS, with emissions of 90.3, 96.0, and 102 kg CO₂eq/m³ for 80E-18, 80E-20, and 80E-22, respectively. In this group, cement contribution decreased to 53.8–58.6%, while BFS contribution increased to 28.1–30.6%. These results confirm that the reduction in Portland cement consumption was the main factor responsible for lowering the global warming potential of the mixtures.

As shown in the Pareto chart (Figure 10), the second-order effect of the %CR factor (AS) was not statistically significant. The regression model for emissions is described in Equation 6 and presented an adjusted R² of 100%, indicating that the proposed second-order model adequately represents a high percentage of the experimentally obtained results. The coefficient with the highest value is associated with the %CR, indicating that cement replacement by slag is the factor that most strongly influences CO₂eq emissions.

Figure 10
Pareto chart for CO₂eq emissions (α = 0.05)

Eq. 6 $CO₂eqEmissions=189.997-93.9883A+15.9267B+0.7767B*B-9.4475A*B$

3.3 Influence of blast furnace slag on concrete cost

Table 7 presents the cost results for producing 1 m³ of concrete. As shown in the interaction plot for cost (Figure 11a), as the amount of slag increases, a cost reduction occurs, corresponding on average to 14% for 40% replacement and 29% for 80% replacement. This result was expected, considering that slag is currently used by the cement and concrete industries to obtain significant financial and environmental benefits (Kaplan et al., 2023). From the Pareto chart presented in Figure 11b, it is observed that the second-order effect of the %CR factor (AS) did not significantly influence the response.

Table 7
Cost to produce 1 m³ of concrete

Sample	Total Cost (R$)
C-18	R$ 485.71
C-20	R$ 513.29
C-22	R$ 542.14
40E-18	R$ 419.43
40E-20	R$ 439.49
40E-22	R$ 460.61
80E-18	R$ 353.16
80E-20	R$ 365.70
80E-22	R$ 379.01

Figure 11
(a) Interaction plot and (b) Pareto chart for cost (α = 0.05)

The regression model for the cost of the samples is described in Equation 7 and presented an adjusted R² of 100%. The controllable factor A, corresponding to %CR, exerted the greatest influence on cost reduction, mainly because blast furnace slag presented a lower unit cost than Portland cement. In contrast, the %FA factor (B) contributed to cost increase under the adopted dosage method because higher %FA levels increased the total binder content and, consequently, the amount of cement in the mixtures. For example, in the mixtures without BFS, cement consumption increased from 345.0 kg/m³ in C-18 to 424.6 kg/m³ in C-22, which explains the higher cost of mixtures with greater %FA. Therefore, the effect of %FA on cost should not be interpreted as an isolated disadvantage of fines, but as a consequence of the mixture design and material consumption adopted in this study.

Eq.7 $Cost=439,493-73,8783A+20,5767B+0,5167B*B-7,6450A*B$

3.4 Eco-efficiency indicators

Table 8 presents the indicators CS/E, C*R, and C/CS, which respectively correlate average compressive strength with CO₂eq emissions, production cost with CO₂eq emissions, and production cost with average compressive strength. These indicators allow the comparison between concretes containing BFS and the reference mixtures without cement replacement, supporting the selection of mixtures with a better balance between mechanical, environmental, and economic performance.

The CS/E index indicates the compressive strength obtained per kilogram of CO₂eq emitted; therefore, higher values are more favorable. Conversely, lower C*R and C/CS values are more favorable, since they indicate lower combined cost and emissions, and lower cost per unit of compressive strength, respectively. Thus, these indicators should be interpreted together, as the mixture with the lowest emissions is not necessarily the one with the best mechanical or economic performance.

Table 8
Eco-efficiency indicators of concrete related to CO₂eq emissions, compressive strength, and cost

Sample	CS/E (MPa/kg CO₂eq)				C*R (R$/kg CO₂eq)	C/CS (R$/MPa)
	7 days	28 days	56 days	91 days		7 days	28 days	56 days	91 days
C-18	0,114	0,145	0,153	0,160	126002,32	16,49	12,87	12,25	11,73
C-20	0,098	0,125	0,137	0,141	145723,68	18,37	14,52	13,24	12,79
C-22	0,085	0,113	0,121	0,124	168230,70	20,63	15,46	14,47	14,08
40E-18	0,155	0,215	0,241	0,255	73325,33	15,46	11,15	9,94	9,39
40E-20	0,130	0,189	0,201	0,212	83543,39	17,72	12,25	11,51	10,89
40E-22	0,114	0,168	0,177	0,189	95066,21	19,50	13,29	12,60	11,78
80E-18	0,232	0,310	0,328	0,341	31890,24	16,87	12,62	11,92	11,48
80E-20	0,198	0,267	0,288	0,302	35106,76	19,27	14,26	13,21	12,62
80E-22	0,172	0,234	0,264	0,280	38810,89	21,55	15,82	14,04	13,24

The results should first be compared between mixtures with the same fines-to-aggregate ratio, since this allows a clearer interpretation of the effect of cement replacement by BFS. For the 18% FA group, the comparison between C-18, 40E-18, and 80E-18 shows that increasing BFS replacement improved the CS/E and C*R indicators, mainly due to the reduction in Portland cement consumption and, consequently, in CO₂eq emissions and production cost. Similar behavior was observed for the 20% FA group, when comparing C-20, 40E-20, and 80E-20, and for the 22% FA group, when comparing C-22, 40E-22, and 80E-22. Therefore, within each %FA level, the improvement in environmental and economic indicators was primarily associated with cement replacement by BFS.

However, the C/CS indicator showed a different behavior because it also depends directly on compressive strength. In this case, sample 40E-18 presented the best performance, indicating a better balance between cost reduction and mechanical performance. This result suggests that 40% BFS was sufficient to reduce cement consumption and cost while maintaining compressive strength values close to those of the reference mixtures at later ages. Although 80% BFS strongly reduced CO₂eq emissions and cost, this replacement level also reduced compressive strength, especially at early ages, which limited its performance in the C/CS indicator.

The effect of %FA should be interpreted separately from the effect of BFS replacement. Under the adopted dosage method, increasing %FA also increased total binder consumption and, in the reference mixtures, increased cement consumption from C-18 to C-22. Therefore, the higher cost and CO₂eq emissions observed in mixtures with 20% and 22% FA are not a general consequence of using more fines, but a consequence of the mixture proportions adopted in this study. In this sense, sample 80E-18 showed the best performance for the CS/E and C*R indicators because it combined high cement replacement by BFS with the lowest total binder consumption. In contrast, sample C-22 presented the least favorable performance for these indicators because it contained no BFS and had the highest cement consumption among the reference mixtures.

3.5 Optimization of cement replacement by blast furnace slag

Table 9 presents the global desirability values (DSJ) for each mixture, considering equal weighting for the response variables: average compressive strength at 28 days, CO₂eq emissions, and cost. It is observed that the mixture presenting the highest desirability corresponds to 40% BFS replacement of cement and 18% FA.

Table 9
Global desirability of the mixtures

Sample	DSJ
C-18	0,40
C-20	0,25
C-22	0,00
40E-18	0,72
40E-20	0,63
40E-22	0,53
80E-18	0,70
80E-20	0,61
80E-22	0,51

According to the optimization results presented in Figure 12, the optimal proportion corresponds to 58.94% slag replacing cement and 18% FA, with a desirability of 79.09%. Figure 13 presents the contour and surface plots for compressive strength at 28 days, emissions, and cost. These plots indicate that it is not possible to define an overlapping region that simultaneously maximizes compressive strength and minimizes both cost and emissions; therefore, the obtained desirability was lower than 100%.

Figure 12
Optimization as a function of 28-day compressive strength, cost, and CO₂eq emissions

Figure 13
Contour and surface plots: (a) 28-day compressive strength; (b) CO₂eq emissions; and (c) cost

In order to provide additional optimization scenarios, Figure 14 presents the relationship between %CR and the relative importance assigned to cost and CO₂eq emissions. For the development of these scenarios, the importance of compressive strength (CS) was evaluated at three levels: 1, 5, and 10. For all scenarios, no substantial variation in the estimated %CR was observed as a result of changes in the relative importance assigned to cost and CO₂eq emissions. This behavior indicates that cement replacement by BFS affected both parameters in a similar direction, since increasing %CR reduced cement consumption and, consequently, reduced both production cost and CO₂eq emissions. However, when greater importance was assigned to compressive strength, the optimization indicated lower %CR values, reflecting the need to balance environmental and economic gains with mechanical performance.

Figure 14
Optimal %CR content as a function of the variation in relative importance between cost and emissions for different levels of compressive strength importance

Regarding %FA, the optimization indicated 18% for all evaluated scenarios. However, this result should be interpreted with caution and should not be generalized to all cementitious systems containing BFS. In the experimental design adopted in this study, increasing %FA also increased total binder consumption, paste volume, total water demand, and, consequently, cost and CO₂eq emissions. Therefore, the selection of 18% FA represents the most favorable condition only under the dosage method and material proportions adopted in this study.

According to the different scenarios presented in Figure 15, convergence toward cement replacement levels by blast furnace slag greater than 40% can be observed. Thus, even for applications in which maximizing compressive strength is ten times more important than reducing cost and CO₂eq emissions, this replacement level is still considered optimal.

Figure 15
Optimal %CR content as a function of the variation in compressive strength importance for different importance levels of the cost × CO₂ emission relationship

4 Conclusion

This study evaluated the partial replacement of Portland cement by blast furnace slag in eco-efficient concrete mixtures, considering compressive strength, production cost, and CO₂eq emissions. Based on the experimental results and on the statistical and optimization analyses, the following conclusions can be drawn:

1. the effect of the fines-to-aggregate ratio (%FA) on compressive strength should be interpreted with caution. In the adopted dosage method, increasing %FA also changed the total binder content, paste volume, total water content, and admixture demand. Therefore, the observed variations in compressive strength cannot be directly attributed to porosity or particle packing, since these properties were not experimentally measured;

2. cement replacement by BFS (%CR) and curing time were the main factors controlling compressive strength. Mixtures with 40% BFS presented compressive strength values close to those of the reference mixtures at later ages, indicating that this replacement level provided a better balance between cement hydration and the later contribution of slag reactions;

3. mixtures with 80% BFS showed lower compressive strength, especially at early ages. This behavior is mainly associated with the dilution of Portland cement and the slower early-age reactivity of BFS. The XRD characterization of BFS supported the discussion of its mineralogical composition, but the mechanical behavior should also be interpreted considering the amorphous fraction, fineness, alkaline activation, and interaction with calcium hydroxide released during cement hydration;

4. the increase in %CR reduced CO₂eq emissions because it decreased Portland cement consumption, which was the main contributor to the environmental impact of the mixtures. In contrast, the increase in %FA led to higher CO₂eq emissions under the adopted dosage method because it increased total binder consumption and, consequently, cement consumption. This result should not be generalized as an unfavorable effect of fines in cementitious systems;

5. the incorporation of BFS contributed to production cost reduction, reaching approximately 29% for mixtures with 80% replacement. The cost increase associated with higher %FA levels was mainly related to the higher consumption of cement and total binder, not to the processing requirements of fine materials;

6. the eco-efficiency indicators showed different optimal responses depending on the criterion analyzed. Sample 80E-18 presented the best performance for the CS/E and C*R indicators because it combined high cement replacement with lower binder consumption. However, sample 40E-18 presented the best C/CS indicator, indicating a better balance between cost reduction and compressive strength; and

7. the response surface methodology combined with desirability analysis indicated an estimated optimal cement replacement level of 58.94% by BFS and 18% FA, with global desirability of 79.09%, considering equal importance for compressive strength, CO₂eq emissions, and cost.

The results indicate that BFS can reduce the environmental and economic impacts of concrete production while maintaining adequate mechanical performance when the replacement level is properly defined. Future studies should include additional durability analyses, such as permeability, sulfate attack resistance, and chloride ion diffusion, as well as complementary physical and morphological characterization of cement and BFS, especially particle size distribution and microstructural analysis.

Declaration of Generative AI and AI-Assisted Technologies in the Writing Process

During the preparation of this manuscript, the authors used ChatGPT as an AI-assisted tool to improve syntax, cohesion, and clarity, as well as to support translation and language refinement in English. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the final version of the manuscript.

Financiamento

The authors gratefully acknowledge the financial support provided by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001, and by the Research Support Foundation of the State of Minas Gerais (FAPEMIG)

SILVA, G. L. M. da; MOURA, H. R. de C. V.; MENDES, Marcus V. de F.; SANTOS, J. A. dos; BOUSADO, J. T.; XAVIER, L. M.; CARVALHO, J. M. F. de; FERRIERA, F. A. Eco-efficient concrete using blast furnace slag: optimized performance, lower costs, reduced CO₂eq emissions. Ambiente Construído, Porto Alegre, v. 26, e153987, jan./dez. 2026. ISSN 1678-8621 Associação Nacional de Tecnologia do Ambiente Construído. http://dx.doi.org/10.1590/s1678-86212026000101024

Data Availability Statement

The data that support the findings of this study are available from the corresponding author, Marcus Vinícius de Freitas Mendes, upon reasonable request.

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Correspondence

Corresponding author: [e-mail]

Editor:

Marcelo Henrique Farias de Medeiros

Conflito de interesse

Not applicable.

Publication Dates

• Publication in this collection
  21 Sept 2026
• Date of issue
  Jan-Dec 2026

History

• Received
  03 Mar 2026
• Reviewed
  12 June 2026
• Accepted
  15 Aug 2026