**3. Life Cycle Assessment (LCA)**

A life-cycle assessment (LCA) is a method for assessing all the potential environmental impacts of a product, process, or activity over its entire life cycle [104]. Several LCA studies have focused on the sustainability of concrete [105,106]. It is important to integrate recycled EOL products at the beginning of concrete's life cycle, and re-valorise it at the end of concrete's life cycle in another production process or even maybe, for the concrete production industry (see Figure 13) [107–110].

**Figure 13.** Cradle-to-gate life cycle assessment (LCA): studied system boundaries of the concretes (plain line—included processes; dashed line—non-included processes) [111].

Sustainability through re-use of accessible economic and social resources is a way to attain equilibrium with the environment, while ensuring long-term development and endurance [26]. A beneficial difference for the environment could result from substituting cement and sand with by-products or EOL products from intersectoral industrial activities. By this way, we may be able to reduce the adverse environmental impacts stemming from cement (74%–93%) and sand (0.3%–2%) consumption in the total LCA of EOL material-based concretes. The minimum contribution of sand to the entire environmental assessment of concrete makes this issue important to concrete design [111–113].

With regard to the LCA of concrete, 4 aspects should be considered: (1) design, (2) production/execution, (3) usage and (4) end-of-life disposal. Structural concrete has life expectancy differentiated by application, such as in pillars, beams and walls. While the durability over time for foundation or load-bearing structural elements is 50–300 years, the corresponding lifetime for cover walls is only 20–50 years [111]. Therefore, sufficient data is not presently available for the EOL stage of structural concrete and its disposal conditions [111,114–116].

The necessity for further LCA studies on the treatment and re-use of construction waste is clear. Instead of being released into the environment, it can be re-valorised in the life cycle of new designs of concrete. Use of waste like fly ash, blast furnace slag and other mineral admixtures as a binder for the production of concrete is becoming common in the construction industry. Replacing the principal factor responsible for the negative environmental effects of concrete is the key to generating an ecologically beneficial life cycle for concrete [107,117,118].

With regards to economic impact, regenerating alternative EOL materials for binder components in concrete would decrease the cost of construction without sacrificing performance. Other costs can also be considered, such as those concerning the source and transport of the alternative SCM materials, controlled combustion process and also savings as a result of diversion, such as disposal management. Consequently, the environmental advantages will reduce the enormous demand for Portland cement per unit volume of concrete, in addition to a concurrent and meaningful reduction in Greenhouse Gas (GHG) emissions [108,119,120].

#### **4. Conclusions**

Concrete represents one of the most widely used construction materials worldwide by volume. Portland cement production is highly energy intensive, and emits significant amounts of CO2 through the calcination process, which contributes substantial adverse impact on global warming. Efforts are needed to design and develop more ecologically friendly concrete with improved performance in strength and durability.

SCMs are frequently applied in concrete mixtures as a substitute for clinker in cement or cement in concrete. This approach yields concrete with reduced cost, decreased environmental impact, higher long-term strength and better long-term durability. Presently the two most common SCMs are silica fume and fly ash. As a by-product of the silicon and ferrosilicon alloy fabrication, SF contains more than 90% SiO2 and is present as spheres with average diameters about 100 times smaller than cement particles. The large specific surface of SF, ca. 20,000 m2/kg, is 10 to 20 times greater than that of other pozzolanic materials, imbuing it with high pozzolanic activity and fluidization properties.

FA has also been widely used in concrete mix design, due to its growing availability and substantial environmental complications appeared by release of fly ash. Physical characteristics of FA can differ on the nature of coal, rank, mineral matter chemistry and mineralogy, furnace design, furnace operation and method of particulate control, while chemical characteristics are relatively insensitive those factors. FA has been used in the manufacturing of bricks, concrete and cement-composites like columns, beams, slabs, columns, sheets, pipes, wall panels, etc. Typically, about 25% of FA is used as a substitute for cement to achieve effective final products. FA increases durability, workability, density and workability of concrete, while simultaneously decreasing water demand, porosity and permeability of concrete.

In the present, the use of industrial EOL materials in concrete has been demonstrated. However, it is clear that more research is needed to assess the feasibility of long-term performance, develop more

ecologically sound production, in addition to quality assessment of these materials. When ashes of high quality can be regularly obtained with reduced financial and most importantly environmental costs, their use in the engineering domain will become more widespread. Technical and economic performances of alternative SCM is evident, proving that along with the studying of the material's mechanical and physico-chemical properties, the review of its life cycle is as well important and will mention whether it will be environmentally feasible to apply the SCM admixture at the total scale of the life cycle of concrete.

**Author Contributions:** Conceptualization, A.I.N., B.S.V.; investigation, A.E.S.; data curation, B.S.V., A.E.S., A.I.N.; writing—review and editing, A.E.S., B.S.V., M.V., N.Š.R., S.S., C.O.-Y., M.A.G., Z.B.B., I.C.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was possible with the support of European Commission, Horizon 2020, ERA-MIN2 Research & Innovation Programme on Raw Materials to Foster Circular Economy – "RECEMENT – Re-generating (raw) materials and-of-life products for re-use in Cement/Concrete" funded project.

**Conflicts of Interest:** The authors declare no conflict of interest.
