Population growth and improvements in living standards are leading to a rapid increase in demand for buildings, contributing to the growing economic importance of construction [1]. The growing importance of the construction sector is also reflected in the energy sector. This is partly due to an increase in demand for renewable energy [2] and partly due to environmental considerations [3]. Nowadays, the area and population density of cities are increasing, with the continuous growth of the built and paved areas [4]. At the same time, passenger transport demand is also increasing [5]. This contributes to the growing economic importance of the construction industry. The construction industry provides more than 100 million jobs worldwide and accounts for 6% of global GDP, demonstrating its significant impact on the world economy. Demand for built infrastructure is expected to remain high over the next two decades, which, in numerical terms, means that global infrastructure investment is expected to reach 3.7×10¹² USD/y by 2040 [6]. Many good examples of initiatives responding to these challenges can be seen in the form of valuable and worthwhile transport- [7] and innovative megaprojects [8]. Based on the experience of the construction case studies, three materials were found to have a significant impact on EF in all cases: cement, steel and ceramics [9]. The problem is that cement production, which is considered one of the most important raw materials for the construction industry, has a negative impact on the environmental performance of the industry [10], as it produces significant carbon emissions [11]. In the construction industry, research has also highlighted the social dimension of the triad of environment, society and governance (ESG) [12]. Several indicators are used to measure the environmental impact, such as the water footprint [13] and the product footprint [14]. The most important indicator is the carbon footprint both in general [15] and in the cement industry [16]. The calculation of the ecological footprint is of great importance both in general [17] as well as the in the energy industry [18] and in the cement industry [19]. Ecological footprint theory attempts to estimate the area needed to support a given human population [20], but the methodology can also be adapted to manufacturing [21] and construction processes [22]. The largest part of the ecological footprint, disaggregated by land use category, is the carbon footprint [23]. However, the use of the ecological footprint is not redundant, as it can be used as an indicator to help determine the upper limit of growth [24]. Cement plants are a source of many negative environmental externalities. Research on the composition of the cement used suggests that the amount of cement used can be significantly reduced and thus the environmental pollution resulting from overproduction [25]. The cement industry is one of the world's largest emitters of CO₂, accounting for 8% of global carbon dioxide emissions [26]. Annual greenhouse gas emissions from the production of Portland cement are approximately 1.5×10⁹ t, which are estimated to account for 6% of total emissions [27]. There are several strategies to reduce carbon emissions from cement production. These include promoting innovative production pathways with near-zero emissions, improving energy efficiency, switching to lower carbon fuels, and promoting material efficiency (by changing the clinker-to-cement ratio to reduce overall demand) [28]. However, it is important to understand the issue as part of a complex system [29].

Hardened concrete is a composite material consisting of two main components: the aggregate and the cement paste. Temperature increases cause changes in both components as the strength properties of the concrete deteriorate. The structure of the concrete collapses and eventually fails. These include internal microcracks that form during heating. The change in strength properties of concrete at high temperatures depends on many parameters [30]. High temperatures change the structure of concrete. This was already investigated in the 1970s: Waubke (transformation of quartz at elevated temperatures) in 1973 [31], Schneider and Weiβ (temperature dependence of concrete strength) in 1977 [32], and in the 1980s: Khoury et al. (changes in the microstructure of cement stone at elevated temperatures) in 1985 [33] and Hinrichsmeyer (changes in the porosity of cement stone at elevated temperatures) in 1987 [34], but it is still an interesting topic today thanks to CT examinations. At elevated temperature the concrete strength decreases but the ductility increases. There are two possible reasons for the detachment of layers in concrete surfaces. From the cracks the water vapour escaping, and the and the forces from thermal expansion increasing and flakes off. The spalling of the concrete surfaces was investigated by Hertz [35].

The critical air temperature for spalling of the concrete surface is 374 °C. They found that if the moisture content is less than 3–4%, the probability that the concrete surface will spall is very low. The spalling of the surface of high-strength concrete is usually caused by stresses due to the increase in temperature. In normal concrete, the surface layers are usually stressed by the water vapour escaping from the concrete. Lower compressive stresses and a smaller dimension is sufficient to prevent the phenomenon of spalling.

The novelty of the research is that it aims to provide a complex answer based on the joint examination of two aspects (ecological and technical) and determines the optimal solution. While the fire protection characteristics have already been examined before, the evaluation from an ecological point of view represents a new aspect. The aim of the research is to highlight that correctly selected components not only have a positive effect on technical parameters, but can also reduce CO₂emissions. A case study is used to illustrate the magnitude of the ecological impact.

In the laboratory tests, a heating curve close to the ISO 834 curve. The maximum temperatures (between 50 °C and 800 °C in different steps) were kept for two hours. After the heat exposure, the specimens were kept at room temperature and allowed to cool. After the cooling the specimens were tested at room temperature. The compressive strength test was performed on the concrete cubes. The length of the edges of the cubes was 150 mm. The concrete cubes were tested for fracture using a crusher of the type ALPHA 3 3000S. The flexural tensile strength of the concrete was measured on slabs with an edge length of 70 × 70 × 250 mm, which were cast in a horizontal position. The flexural tensile strength was measured using a crusher of the type WPM ZDM 10/91.

The tests carried out were partly visual and partly mechanical strength tests, the results of which are described in detail in the following section. Based on the results of the tests carried out, the next subchapter analyses the ecological economics of the etalon mixture and the optimum fire resistance mixture M17.

Several aspects must be considered when choosing the right concrete formulation. First, the variation of the strength parameters as a function of temperature is important. It is important that the selected product meets at least the same requirements as the reference product at room temperature. In addition, the selection should consider that the concretes should be more resistant at high temperatures. In this case, they should not completely lose their structural integrity, and the expected deterioration should be gradual rather than rapid. In addition, it must meet the production and economic requirements. Figure 2 illustrates the connections between the laboratory tests.

Figure 2.

Connections between the laboratory tests

The technical analysis showed that the M17 formulation is the best in terms of fire resistance. In the ecological analysis, only the M17 concrete was investigated in addition to the standard M11 concrete. The composition of the two formulations is summarised in Table 1. and Table 2. The weight of the tested constituents per 1 m³ of concrete forms the basis for the analysis.

In the calculations, the environmental impact determined according to the two formulations depends only on the quantity of the materials and their specific emissions. Therefore, in the second step of the analysis, we determined the specific CO₂e values for each component based on [37]. The specific values are summarised in Table 3.

Note: In the case of andesite, since no exact value was found, it was assumed to be the same since they are rocks near the surface and easy to interpret. This value is higher than the average value in the case of stones! Since air is introduced into the material during the mixing process, we do not include a characteristic value for limestone in our calculations.

The Gotthard Base Tunnel (GBT) with its scale and duration, is a complex megaproject [38]. The 57 km long GBT is the largest technical structure of the AlpTransit project in Switzerland and was opened to rail traffic in June 2016 [39]. The amount of concrete used for the tunnel was 170,000 m³. [40] Multiplying this by the density of concrete (2.5 t/m³) gives the mass of concrete used. This value is 425,000 t. It follows from the type of investment that the fulfilment of fire protection requirements is of fundamental importance. The amount of concrete used highlights that the right mixture can lead to a significant reduction in CO₂ emissions.

The tests carried out were partly visual and partly mechanical strength tests, the results of which are described in detail in the following points.

• The cracks that developed on the specimens after thermal stress were examined by visual inspection. The 70 × 70 × 250 mm plates and the 150 × 150 × 150 mm cubes showed no visible cracks up to 500 °C. The cracks in the cubes heated at 500 °C and 800 °C. Based on a visual inspection of the cube specimens, at 500 °C, all specimens maintained their structural integrity, and at 800 °C, the structure of the concrete collapsed, and we heard a dull sound when tapping on the surface.

• For the evaluation, the average measurement results of 3 - 3 tests were. Based on the values of the flexural tensile strength of the columns, M14 samples showed the highest initial strength, and M10 formulations behaved most unfavourably in the flexural tensile test. At 300 °C, a local peak in flexural and tensile strength is observed in the results of the M11 and M10 formulations. At temperatures above 400 °C, a rapid decrease in flexural tensile strength values is observed.

• For the evaluation, the average measurement results of the 3 - 3 tests were as follows: Based on the compressive strength values of the cubes for the specimens loaded at 150 °C, we observed a local minimum point. At 300 °C or 400 °C, a local maximum point of compressive strength was observed. Above 400 °C, an increase in the rate of degradation is observed. The best values for compressive strength are obtained with formulation M17.

• In Figure 3 were compiled the average values of flexural and tensile strengths as a function of temperature for all concrete formulations. Figure 4 shows the relative values of the flexural tensile strength compared to the reference mix design. The figures only show curves characteristic of the M11 and M17 mixtures considered relevant! Figure 3.

  Average flexural-tensile strength values at the column (calculated from the average of 3 specimens)

  Figure 4.

  Relative flexural-tensile strength values at the column (calculated from the average of 3 specimens)

  

• Based on the values of the flexural tensile strength of the columns, M14 samples showed the highest initial strength, and M10 formulations behaved most unfavourably in the flexural tensile test. At 300 °C, a local peak in flexural and tensile strength is observed in the results of the M11 and M10 formulations. At temperatures above 400 °C, a rapid decrease in flexural tensile strength values is observed.

• For the evaluation, the average of the measurement results of the 3 - 3 tests were. Figure 5 compiles the average values of the compressive strengths measured on the cube as a function of temperature according to the concrete formulation. Figure 6 shows the relative compressive strength values compared to the reference formulation. The figures only show curves characteristic of the M11 and M17 mixtures considered relevant! Figure 5.

  Average compressive strength values at the cube (calculated from the average of 3 specimens)

  Figure 6.

  Relative compressive strength values at the cube (calculated from the average of 3 specimens

  

• From the point of view of fire resistance, the M17 formulation has proven to be the most suitable. This concrete recipe contains andesite CEM II A-S 42.5 N cement and 30 kg/m³ blast-furnace slag admixture. As a result, they lose less strength under thermal load, the rate of degradation is moderate, and the cracks are narrower and more widely distributed over the surface. On the basis of the results of the tests carried out, the next chapter analyses the ecological economics of the etalon mixture and the optimum fire resistance mixture M17.

The results of the calculations are described in detail in the following points.

• Emissions per 1 m³ are 394.76 kgCO₂e/m³ (Table 4.) for the M11 formulation and 351.54 kgCO₂e/m³ (Table 5.) for the M17 formulation. These values can also be specified to mass. So, the specific emissions are 0.162 kgCO₂e/kg for the M11 formulation and 0.143 kgCO₂e/kg for the M17 formulation. Table 4.

  The investigated concrete mixture-M11

  Component	Volume	Specific CO2e emission [kgCO2e/kg]	Total CO2e emission [kgCO2e
  cement volume(kg∕m3) cement type	370 (CEM II/A-V 42.5 R)	0.8150	337.44
  water (kg/m3)	148	0.0010	0.05
  aggregate 0–4 mm (kg/m3)	879	0.0048	6.77
  aggregate 4–8 mm (kg/m3)	496
  andesite NZ 4/11 (kg/m3)	537	0.0900	48.33
  super plasticizer (kg/m3)	1.3	1.6700	2.17
  blast furnace slag (kg/m3)	0	0.0416	0.00
  Σ	–	–	394.76

  Table 5.

  The investigated concrete mixture-M17

  Component	Volume	Specific CO2e emission [kg CO2e/kg]	Total CO2e emission [kg CO2e]
  cement (kg/m3) cement type	360 (CEM II/A-V 42.5 R)	0.8150	289.02
  water (kg/m3)	148	0.0010	0.05
  aggregate 0–4 mm (kg/m3)	845	0.0048	6.61
  aggregate 4–8 mm (kg/m3)	498
  andesite NZ 4/11 (kg/m3)	577	0.0900	61.93
  super plasticizer (kg/m3)	1.6	1.6700	2.67
  blast furnace slag (kg/m3)	30	0.0416	1.25
  Σ	–	–	351.54

• The change in formulation resulted in a saving of 43.22 kgCO_2e emissions. This represents a saving of ~10% compared to the reference value.

• In order to calculate the ecological footprint, we would need kgCO₂e values instead of kgCO₂. Typically, in the case of building materials, there is no significant difference between the two values, but the first is always higher than the second one. So, using our calculated values, we can also give a conservative estimate of the ecological footprint in terms of built-in materials. This requires multiplying the calculated emissions in tonnes by 0.338 gha/t. Thus, we get 0.134 gha/m³ in the case of the etalon, while 0.119 gha/m³ for the M17 formulation. This value implies that 1 m³ of concrete for the etalon requires 1340 gm² of hypothetical land area, while the M17 formulation requires 1190 gm². The „gha” or „gm²” means the world's average productive land area [41]. The difference between the two values is ~10% as well.

Multiplying the amount of 425,000 t of concrete used by the difference between the emissions of the M11 and M17 mixtures (43.22 tCO2e/t) gives the value of the emission savings. Comparing the two materials, the CO2e saving is 8075 t, which translates into a saving of 2729.35 gha of CO_2e when converted to world average land productivity. According to the Global Footprint Network, the organisation responsible for calculating the ecological footprint, the ecological footprint of a Swiss inhabitant is 4 gha/person (2023) [42]. This means that this saving is equivalent to an annual ecological footprint of 682.35 Swiss inhabitants.