Sulfate-Resistant Clinker Base Cement with New Secondary Main Constituents: A Technical, Economic, and Environmental Analysis

Academic Editor: Jaroslav Pokorný Received: 16 December 2024 Revised: 27 January 2025 Accepted: 28 January 2025 Published: 4 February 2025 Citation: Martínez Infante, M.Á.; Navarrete Rubia, B.; Vilches Arenas, L.F. Sulfate-Resistant Clinker Base Cement with New Secondary Main Constituents: A Technical, Economic, and Environmental Analysis. Buildings 2025,15, 479. https://doi.org/ 10.3390/buildings15030479 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Sulfate-Resistant Clinker Base Cement with New Secondary Main Constituents: A Technical, Economic, and Environmental Analysis Miguel Ángel Martínez Infante 1,*, Benito Navarrete Rubia 2,* and Luis Francisco Vilches Arenas 2 1School of Industrial Engineering, University of Valladolid, 47011 Valladolid, Spain 2Department of Chemical and Environmental Engineering, Technical School of Engineering, University of Seville, 41012 Seville, Spain; [email protected] *Correspondence: [email protected] (M.Á.M.I.); [email protected] (B.N.R.) Abstract: The Spanish cement sector must adapt its production model to a green economy model. This study focuses on the use of new secondary main constituents (SMCs) suitable for a cement plant that specializes in sulfate-resistant (SR) cement production, defining a framework of technical conditions for their usage and their economic and environmental feasibility. Low-calcium-carbonate-content albero, steel slags, and iron silicate were the tested SMCs; however, they are not currently permitted in cement manufacture. CEM I 42.5 R-SR 3 (type I-SR) was mixed with 5%, 20%, and 30% of these new SMCs. XRF, XRD, leaching and other chemical tests, setting, and hardening tests were performed with no significant issues. Albero is the best option, on the whole, because of the following characteristics: availability, >100 Mt; proximity, 3 km; and acceptable compressive strength level. However, black slag cement with 30% SMC after 28 days shows the best performance, with a compressive strength of 41.3 MPa compared to 35.3 MPa for albero cement and 56.5 MPa for the type I-SR reference. Albero and steel slag at 30% content are the best option according to the cost savings of 32% ( − 31.5 EUR/t and − 31.6 EUR/t, respectively) compared to the type I-SR reference. Regarding the carbon footprint, albero and steel slag at 30% content have the least impact, showing a 31% reduction ( − 254.8 kg CO 2 /t and − 255.2 kg CO 2 /t, respectively) compared to the type I-SR reference. The studied SMCs meet the analytical conditions and—with the corresponding regulatory changes—offer potential cost savings for SR cement production, exhibiting a competitive advantage. Keywords: sustainable cements; SR clinker; albero; iron silicate; white slag; black slag; economic viability; carbon footprint 1. Introduction The integrated cement production process is highly demanding in terms of electrical and thermal energy and requires substantial mineral resources. Significant amounts of CO 2 are emitted during production due to the unavoidable decarbonation of raw materials and the combustion needed for clinker calcination [1]. The European Union (EU) is a global leader in the fight against climate change, imposing specific requirements on industrial production models, including the cement industry. The European legislative framework is strategically oriented towards the circular economy, energy efficiency [2], and the reduction in emissions, including CO2[3–5]. The survival of the European, and specifically the Spanish, cement sector hinges on transitioning from a linear production model to a more efficient, socially responsible, Buildings 2025,15, 479 https://doi.org/10.3390/buildings15030479 Buildings 2025,15, 479 2 of 26 and environmentally sustainable model. This new model must also prioritize economic sustainability to prevent the industry from relocating to countries with less stringent environmental regulations, which would not address the underlying problems. To achieve climate neutrality objectives by 2050, the Spanish cement sector has established a roadmap with specific policies aligned with the European Green Deal and the 2030 Agenda. It is based on circularity and the life cycle of products, aiming to position the cement industry as a driver of change towards a more sustainable way of building [6]. Carbon emissions from clinker can be reduced through the use of decarbonized raw materials, biomass fuels, the incorporation of eco-fuels such as green hydrogen, and electrification in the production process, as well as the implementation of technologies to store, capture, and utilize the emitted carbon. This study focuses on minimizing the carbon footprint of the produced cements by reducing their clinker content and optimizing the production process. Regarding product innovation, the cement sector, alongside increasing the use of permitted pozzolanic materials such as calcinated clays and hydraulic materials such as granulated blast furnace slag (granulated slag), is mainly developing new clinkers and alternative SMCs for cement manufacturing, such as by-products and industrial wastes. There are different innovative trends for manufacturing more environmentally friendly cements. Belite cements have been extensively studied and offer significant environmental and energy advantages [ 7 ]; however, they also present deficiencies in terms of reactivity. Geopolymers and the alkaline activation of aluminosilicates show great strength and chemical attack development at early ages, but have workability issues [ 8 ]. There are also environmental concerns about the significant CO 2 emissions of the alkaline solution production process [ 9 ], as well as the formulation of cements incorporating new SMCs that are currently not allowed, and new concentrations of currently allowed SMCs. Regarding this last trend, several research projects have been carried out on waste materials with a remarkable pozzolanic performance, such as recycled activated metakaolin [ 10 , 11 ], organic materials such as sugarcane bagasse and thermally activated rice husks [ 12 ], ceramic waste [ 13 ], clay waste [ 14 ], volcanic lava [ 15 ], construction and demolition waste [ 16 ], thermally activated coal mining waste [ 17 ], and biomass bottom ash [18]. The European and Spanish regulations regarding the hydraulic conglomerate promote the use of SMCs in cements [ 19 – 22 ], but are cautious due to the performance repercussions, mainly on durability, that the new cements may have. This study addresses new types of cement based on the use of SMCs that are not currently permitted by the EU regulations, taking a sulfate-resistant (SR) clinker base cement as a reference. The SR clinker base cement is from a cement plant with 100% sulfate-resistant clinker production because of the very low Al 2 O 3 content in the main raw materials. Due to the low tricalcium aluminate (C 3 A) content (<3%), it is more difficult to produce cements with a high content of SMCs at the same level of tricalcium silicate (C 3 S) because they result in a lower strength at early ages. This could be a competitive disadvantage compared with a higher C 3 A clinker base cement, depending on the cement applications. It is even more challenging to find new SMCs that are available in sufficient quantity and proximity to the cement plant to allow for a reduction in the clinker content of the cement without compromising its properties. Therefore, the choice of new SMCs has to do with the promotion of the principle of the circular economy, waste usage, or elements with a lower carbon footprint to improve the competitiveness of cement plants in Europe. The experimental work focuses on using a type I cement, CEM I 42.5 R-SR 3 (type I-SR), as a reference. This cement is ground at the laboratory level with SMCs in different Buildings 2025,15, 479 3 of 26 proportions, 5%, 20%, and 30%, to formulate a cement suitable for most common applications, from structural purposes to mortar. The results of the test plan will establish a correlation to evaluate the possibility, with the corresponding regulatory changes, of using these new SMCs for the manufacture of type II/A (5% and 20% SMC mixtures) and type II/B (30% SMC mixtures) cements. Regarding compressive strength performance, it would be possible to produce type 42.5 R with 5% SMC mixtures, type 42.5 N with 20% SMC mixtures, and type 32.5 R with 30% SMC mixtures. Regarding the SMCs, three materials were chosen which are permitted according to the EU standards [ 23 ]—limestone, blast furnace steel-granulated slag, and siliceous fly ash—and four materials whose usage is not currently permitted as SMCs in cement: low-calcium-carbonate-content albero [ 24 ], iron silicate, white electric steel furnace slag, and black electric steel furnace slag. All of these alternative materials are available at the cement plant. Once all the SMCs had been characterized, physical and chemical tests were carried out to verify the behavior of the cements manufactured with the alternative SMCs compared with cements produced with the permitted ones. Strength, soundness, leaching, and granulometry are some of the variables that should be considered before making a proposal for the use of these new materials. This study evaluates the performance of cements manufactured with SR clinker and non-regulated SMCs compared to traditional ones. It presents a range of options, enabling the adaptation of cements manufactured with SR clinker in an economically and environmentally optimal way, particularly from a carbon footprint perspective. 2. Materials and Methods Limestone (Cementos Portland Valderrivas, S.A., Gilena, Seville, Spain), granulated slag (ArcelorMittal, S.A., Gijón, Asturias, Spain), and siliceous fly ash (Repsol, S.A., Espiel, Cordoba, Spain), which are permitted cement components according to UNE-EN 197-1:2011 [23], have been studied. In the same way, four non-permitted cement SMCs have also been examined: low-calcium-carbonate albero (Cementos Portland Valderrivas, S.A., Alcalá de Guadaíra, Seville, Spain), iron silicate (Atlantic Copper, S.A., Huelva, Spain), white electric furnace slag, and black electric furnace slag (Siderúrgica Sevillana, S.A., Alcalá de Guadaíra, Seville, Spain). 2.1. Materials 2.1.1. SR Clinker: Primary Cement Component The manufacture of SR cement is regulated by European regulations through the UNE-EN 197-1:2011 standard, with cements subject to CE marking. Additionally, in Spanish territories only, Royal Decree 1313/1988 [ 25 ] approves the manufacture of other sulfate-resistant cements and cements resistant to marine environments according to UNE 80303-1:2017 [ 26 ] and UNE 80303-2:2017 [ 27 ]. In all these cases, except type III cement production according to UNE-EN 197-1, clinker with a restricted C 3 A content is used. In the Spanish regulations, Royal Decree 256/2016 [ 28 ] (an instruction for the reception of cements) and Royal Decree 470/2021 [ 29 ] (a structural code) establish the general technical prescriptions that cements must satisfy, along with regulations for their reception on site and conditions for their use in structural applications. The properties of SR clinker have been widely studied. It has been shown to increase the durability of manufactured concretes and mortars due to its resistance to attack by sulfates and seawater compounds [ 30 ]. On the other hand, its lower reactivity due to the lower C 3 A content vs. ordinary Portland cement clinker means that cements manufactured with SR clinker have a lower initial compression strength while containing equal C 3 S con- Buildings 2025,15, 479 4 of 26 tents [ 31 ]. Moreover, the lower heat generation of SR cement during hydration negatively affects the setting performance in mortar pastes and concrete curing in cold environments. In Spanish integrated cement plants, the production of SR and MR cements represented 12% of the total, 1.8 Mt, in 2022, of which 65% were type I cements, 7% type II, 16% type III, and 12% type IV [ 32 ]. By strength category, types 52.5 and 42.5 accounted for 95% of the SR and MR cements manufactured. Furthermore, some cement plants, due to the characteristics of their main raw materials for manufacture, cannot competitively produce clinkers other than SR. The adaptation of compressive strength performance would necessitate increasing the C 3 S content of the manufacturing clinker, which would then be more difficult to calcine. This would increase its carbon footprint, independently of the repercussions on the cost of raw materials and the production performance of the clinker kiln. To the authors’ knowledge, excluding SR and MR cements, this situation was true for 4% of the cement produced in Spain in 2022, or approximately 0.65 Mt [32]. 2.1.2. Albero Low-calcium-carbonate-content albero, under 75%, is an SMC identified with the calcarenites of Los Alcores, a region east of Seville, Spain. The calcarenite baseline composition, the cement plant control over product availability, the material being located in a quarry owned by the cement production center, and the proximity to the production center are the main reasons to consider this SMC in the study. 2.1.3. Iron Silicate Iron silicate is a by-product obtained via the metallurgical process of copper metal production. Its availability in proximity to the cement plant and its composition and mineralogy are the reasons for it to be included in this study [ 33 ]. Iron silicate has been used as an SMC in the manufacture of cement in Spain under the following standards: UNE 80301-85 Cement—definitions, classification and specifications [ 34 ], and UNE 80302-85 Cement—chemical specifications for cement components [ 35 ]. The revision of standard UNE 80301-96 Cement—definitions, classification and specifications, eliminated the usage of non-ferrous slag in the manufacture of cements [ 36 ]. Concerns were raised about the safety of its use due to the potential leaching of heavy metals or other compounds in applications involving contact with drinking water, although no conclusive studies have been conducted on the matter. 2.1.4. Steel Slags Black slag and white slag are wastes generated in the steelmaking process in electric furnace steel plants. The reasons for considering these SMCs in the study include their availability (their proximity to the production center), complying with the principles of the circular economy (instead of sending it to landfill), and avoiding the usage of other SMCs whose production has a greater environmental impact. Finally, there are already specific regulations about cements made from steel slag in other countries, such as GB/T 13590-2022 [37] Portland cements from steel slag, in China. 2.2. Methodology: SMC Test Plan The procedure forrequestingand preparing SMCsamples wasas in UNE-EN 932-1:1997 [38]. Tests were conducted to determine the general properties of aggregates. Part 1: Sampling methods. The SMC tests represent the average results of five analyses. The heavy metal tests for the specific samples of this study were outsourced, but the reference cement plant already had several heavy metal reports based on different samples of base cement and Buildings 2025,15, 479 5 of 26 SMCs. These were used as raw meal components for clinker production. The results from the heavy metal study samples corroborate the previous ones. Table 1presents the specific test plan carried out on the materials used as SMCs in the formulated cements, reflecting the regulations that describe the methodology of each test and its justification. Table 1. Test plan for the studied SMCs, regulations, and methodology applicable to each test and performance justification. Test Plan Regulation/Test Methodology Comments Chemical composition UNE-EN 196-2:2014 [39] Test methods for cements. Part 2: Chemical analysis of cements. Methodology: X-ray fluorescence spectroscopy. ARL 9800 XP TCA (Thermo Fisher Scientific Inc., Waltham, MA, USA) X-ray fluorescence spectroscopy of the chemical elements, in the form of oxides present in the samples, explains certain behaviors of the cements and concretes manufactured with these SMCs. Soluble chloride determination Ion-selective electrode (Mettler-Toledo International Inc., Columbus, OH, USA) Chlorine is important due to its capacity to affect metallic internal components in concrete structures through corrosion phenomena. Free lime content UNE-EN 1744-1:2010+A1:2013 [ 40 ] Tests to determine the chemical properties of aggregates. Part 1: Chemical analysis. X-ray diffraction methodology. CubiX3 Cement (Malvern Panalytical Ltd., Worcestershire, UK) It may influence volume stability behaviors in the formulated cements depending on the SMC composition. Alkalis (sodium equivalent) UNE-EN 196-2:2014 Test methods for cements. Part 2: Chemical analysis of cements. Methodology: X-ray fluorescence spectroscopy. K2O and Na2O contents may affect the volume stability property due to the reaction of aggregates with the alkalis present in cements used in the manufacture of mortars and concretes. Insoluble residue in hydrochloric acid and sodium carbonate UNE-EN 196-2:2014 Test methods for cements. Part 2: Chemical analysis of cements. Uncombined silica index provides information on the typology of the constituent elements of the SMCs. Humus content UNE-EN 1744-1:2010+A1:2013 Tests to determine the chemical properties of aggregates. Part 1: Chemical analysis. The humus content of an aggregate is determined by the color that appears when a test portion is stirred in a sodium hydroxide solution. It may affect the setting time and hardening of cement. Total organic carbon content DIN EN 13137 [41] Characterization of waste—determination of total organic carbon in waste, sludge and sediments. UNE-EN 13639 [42] Determination of total organic carbon in limestone. Test for the used SMCs. Has the possibility of affecting the setting time and hardening of the cement. Heavy metals Standard referred to in the table of results. Varies depending on the element to be determined. Heavy metal analysis of each SMC and of the cement CEM I 42.5 R-SR 3 used as a base in the formulation. This test is used as a reference for leaching tests. Buildings 2025,15, 479 6 of 26 2.3. Methodology: Cement Test Plan The next step is the preparation of the samples of the formulated cements, using the raw materials analyzed in the previous section as SMCs. The procedure is based on cement type CEM I 42.5 R-SR 3, a sample of ground industrial cement from a Cementos Portland Valderrivas, S.A. plant located in Seville, Spain, to which SMCs were added in proportions of 5%, 20%, and 30% of mass. All the SMCs to be introduced into the lab mill (Retsch GmbH, Haan, Germany) were dried at 105 ◦ C until a constant mass was achieved. The fineness specification for all the SMCs of 33% maximum was retained on the 32 µ m sieve, with 20 min in the lab mill after 80 min lab grinding of coarser SMCs: limestone, albero, iron silicate, and steel slags. No preparation was required for the granulated slag and fly ashes as they were as fine as the cement. The cement resulting from this type I-SR preparation was called “base cement”, with 20 min in the lab mill. Compressive strength represents the average value of two tests for the age of breakage for each cement. For the rest of the tests, an analysis was carried out for each type of cement. Additionally, leaching tests were conducted at a hazardous waste landfill facility, with three analyses of each sample performed to determine the average values. Table 2lists all the formulated cements tested in this study with their detailed composition and the sample code used to simplify the full designation in further tables and figures. The purpose of the test plan, given in Table 3, was to determine the physical and chemical behavior of cements manufactured with the alternative SMCs by comparing the test results obtained with those SMCs with the cements manufactured with the SMCs currently permitted according to UNE-EN 197-1. Buildings 2025,15, 479 7 of 26 Table 2. Cement mixtures’ composition (% mass). Cement Composition (% Mass) Totals 100% Sample Code Clinker Gypsum Ferrous Sulfate Limestone Albero White Steel Slag Black Steel Slag Iron Silicate Fly Ash Granulated Slag Base cement BC 92.00 3.50 0.50 4.00 Base cement + 5% Limestone BC5Limestone 87.40 3.33 0.48 8.80 Base cement + 5% Albero BC5Albero 87.40 3.33 0.48 3.80 5 Base cement + 5% White slag BC5White slag 87.40 3.33 0.48 3.80 5 Base cement + 5% Black slag BC5Black slag 87.40 3.33 0.48 3.80 5 Base cement + 5% Iron silicate BC5Iron silicate 87.40 3.33 0.48 3.80 5 Base cement + 5% Fly ash BC5Fly ash 87.40 3.33 0.48 3.80 5 Base cement + 5% Granulated slag BC5Granulated slag 87.40 3.33 0.48 3.80 5 Base cement + 20% Limestone BC20Limestone 73.60 2.80 0.40 23.20 Base cement + 20% Albero BC20Albero 73.60 2.80 0.40 3.20 20 Base cement + 20% White slag BC20White slag 73.60 2.80 0.40 3.20 20 Base cement + 20% Black slag BC20Black slag 73.60 2.80 0.40 3.20 20 Base cement + 20% Iron silicate BC20Iron silicate 73.60 2.80 0.40 3.20 20 Base cement + 20% Fly ash BC20Fly ash 73.60 2.80 0.40 3.20 20 Base cement + 20% Granulated slag BC20Granulated slag 73.60 2.80 0.40 3.20 20 Base cement + 30% Limestone BC30Limestone 64.40 2.45 0.35 32.80 Base cement + 30% Albero BC30Albero 64.40 2.45 0.35 2.80 30 Base cement + 30% White slag BC30White slag 64.40 2.45 0.35 2.80 30 Base cement + 30% Black slag BC30Black slag 64.40 2.45 0.35 2.80 30 Base cement + 30% Iron silicate BC30Iron silicate 64.40 2.45 0.35 2.80 30 Base cement + 30% Fly ash BC30Fly ash 64.40 2.45 0.35 2.80 30 Base cement + 30% Granulated slag BC30Granulated slag 64.40 2.45 0.35 2.80 30 Buildings 2025,15, 479 8 of 26 Table 3. Test plan for the cements formulated in this study with regulations, applicable methodology, and performance justification. Test Plan Regulation/Methodology/(Specimen State) Comments Particle size distribution Laser granulometry: Malvern Mastersizer 2000 (Malvern Panalytical Ltd., Worcestershire, UK)/(dust) Fineness affects cement behavior mainly in the development of initial strengths, hydration reactions, and grinding performance. Blaine UNE-EN 196-6:2019 [43] Test methods for cements. Part 6: Determination of fineness. (dust) Blaine gives a measure of the particle specific surface area. It affects the initial strength, hydration heat, hydration reactions, and grinding performance. Chemical composition—main elements UNE-EN 196-2:2014 Test methods for cements. Part 2: Chemical analysis of cements. Methodology: X-ray fluorescence spectroscopy. ARL 9800 XP TCA (Thermo Fisher Scientific Inc., Waltham, MA, USA). (dust) It may allow us to explain certain behaviors of the cements and concretes that are analyzed in the different tests. Due to the methodology used, they are represented in the form of oxides. Elemental mineralogical composition X-ray diffraction with Rietveld method, CubiX3 Cement (Malvern Panalytical Ltd., Worcestershire, UK). (dust) The characterization of the crystalline phases contained in the tested cements helps to explain certain behaviors seen in the other tests carried out. Setting time UNE-EN 196-3:2017 [44] Test methods for cements. Part 3: Determination of setting time and volume stability. (paste) It explains workability conditions, identifying the stages in which the cement paste loses plasticity by increasing its viscosity (initial setting time), and ceases its deformability by acquiring more rigidity (final setting time). Soundness UNE-EN 196-3:2017 (paste) Evaluation of expansive behavior in cements. Compressive strength UNE-EN 196-1:2018 [45] Test methods for cements. Part 1: Determination of strengths. Mixer, IB32-40E compacting unit, IB32-045E and breaking press, Autotest 250/10 W (IBERTEST S.A.E, Madrid, Spain), climatic chamber, CCK NG (Dycometal S.L., Banyoles, Girona, Spain). (mortar) Evaluation of compressive strength development of cement mortar at 1, 2, 7, 28, 60, and 90 days. Loss on ignition (975 ◦C) UNE-EN 196-2:2014 Test methods for cements. Part 2: Chemical analysis of cements. (dust)Evaluation of calcination weight loss in formulated cements. Insoluble residue in hydrochloric acid and sodium carbonate UNE-EN 196-2:2014 Test methods for cements. Part 2: Chemical analysis of cements. (dust) Evaluation of the calcination loss in cement. It may allow us to explain some behaviors of the tested cements. Free lime UNE 80243:2014 [46] Test methods for cements. Chemical analysis. Determination of free calcium oxide. Ethylene glycol method. (dust)It may influence volume stability behaviors. Pozzolanicity UNE-EN 196-5:2011 [47] Test methods for cements. Part 5: Pozzolanicity test for pozzolanic cements. (dust) Indicator of pozzolanicity of the SMC due to its capacity to react with the portlandite released during cement hydration, creating hydraulically active compounds. Heavy metals The applied regulation is explained in the table of results. (dust)This test is used as a reference for leaching tests. Leaching UNE-EN 12457-4:2003 [48]; UNE-EN 15216:2008 [49] US EPA 9056-1 [50]; Analytical techniques: inductively coupled plasma optical emission spectroscopy, ICP-OAS, Avio 200 (PerkinElmer, Inc., Shelton, CT, USA), and ion chromatography, Metrohm IC Net 2.3.SR3 (Metrohm AG, Herisau, Switzerland) and infrared spectroscopy, LECO CS 230, (LECO Corporation, St. Joseph, MI, USA). (mortar) Characterization of water leaching of the heavy metals present in the cement. 3. Results 3.1. Results of SMC Tests 3.1.1. XRF Analysis SMCs The magnesium content of the slags is expressed as MgO due to its potential expansive origin in the form of periclase, and the sulfur present in the steel slags is expressed in Table 4 as SO3due to its possible effect on the setting of the cements. Buildings 2025,15, 479 9 of 26 Table 4. Elementary chemical composition. Secondary Main Component SiO2Al2O3Fe2O3CaO MgO SO3Na2O K2O Limestone 0.19% 0.10% 0.12% 54.93% 0.48% 0.12% 0.04% 0.18% Albero 23.76% 0.70% 2.53% 39.91% 0.19% 0.11% 0.04% 0.15% White slag 19.17% 13.39% 4.81% 45.63% 13.63% 3.77% 0.13% 0.05% Black slag 18.00% 12.58% 17.60% 32.63% 6.84% 2.08% 0.12% 0.04% Iron silicate 24.00% 2.44% 60.97% 1.95% 0.73% 1.15% 0.54% 0.87% Fly ash 51.93% 25.52% 9.58% 0.69% 0.91% 0.43% 0.50% 2.57% Granulated slag 32.89% 11.04% 0.36% 42.93% 7.41% 0.13% 0.27% 0.47% 3.1.2. Other Tests on the Studied SMCs As indicated in Table 5, the level of soluble chlorides in all the SMCs studied was not a limiting factor for the maximum chloride content in common cements ( ≤ 0.1%). However, the presence of chloride in slags and iron silicate with a content over 50% the chloride allowed in the manufacture of cements (0.1%), according to UNE-EN 197-1: 2011, should be monitored more exhaustively depending on the proportion of use. Table 5. Results of other tests on the studied SMCs. Secondary Main Component (SMC) Insoluble Residue Humus Free Lime Alkalis Na Eq. TOC Soluble Chlorides Na2Oeq Limestone 0.09% Negative 0.16% 0.16% 0.09% 0.02% 0.16% Albero 21.57% Negative 0.05% 0.14% 0.36% 0.03% 0.14% White slag 4.22% Negative 0.42% 0.16% 0.25% 0.07% 0.16% Black slag 6.23% Negative 0.22% 0.15% 0.30% 0.08% 0.15% Iron silicate 5.79% Negative 0.17% 1.11% 0.00% 0.06% 1.11% Fly ash 86.82% Negative 0.28% 2.19% 0.10% 0.04% 2.19% Granulated slag 0.30% Negative 0.39% 0.58% 0.35% 0.05% 0.58% The analysis of K 2 O and Na 2 O content makes it possible to determine expansive behavior by the reaction of aggregates with the alkalis present in cements used in the manufacture of mortars and concretes. Using the ratio of molecular weights, the total content of these two elements is expressed as equivalent sodium: Na 2 Oeq = Na 2 0 + 0.658 × K 2 0. Fly ash and iron silicate contribute the most alkali to the mixture—2% and 1%, respectively. The free lime oxide content of the SMCs apparently does not present any threat to the volume stability of the cements manufactured with these SMCs. In the determination of the insoluble residue, it is worth noting that the value of the albero is due to its quartz content and the value of the fly ash is due to the presence of silicoaluminate compounds and the low lime content. Humus is a substance formed in the soil by the decomposition of animal or vegetable residues. Its presence is evaluated due to its possible effect on the setting and hardening of cement. The humus content of an aggregate is determined by the color that appears when a test portion is shaken in a sodium hydroxide solution; the results for the presence of humus were all negative. Total organic carbon is another element that may affect the cement setting time. It is determined according to UNE-EN 13639 Determination of Total Organic Carbon for limestone and for the rest of the SMCs according to DIN EN 13137 Characterization of waste and Determination of total organic carbon in waste, sludge and sediments. As a reference, the limestone used as an SMC in common cements, according to UNE-EN 197-1, must not exceed 0.5% of the total organic carbon content by mass according to UNE-EN 13639. All the SMCs have a total organic carbon (TOC) content below this reference value. According to UNE-EN 13639, the TOC content of the SMCs used is irrelevant. Buildings 2025,15, 479 16 of 26 3.3.2. Mechanical Performance of Formulated Cements Based on the results of compressive strength and the equivalent normative requirements, all cements formulated with the new SMCs meet the normative limits at 2 and 28 days of UNE-EN 197-1. The lab grinding procedure was effective in that there was no significant correlation between the particle size distribution and the compressive strength. The strength results of all the formulated cements, with 5%, 20%, and 30% SMC in the base cement, were acceptable and homogeneous. The differences in compressive strength of the formulated cements were more appreciable with 30% SMC (Figure 4). According to the breaks at 2 and 28 days, they all complied with a compressive strength category of 32.5 R cement. In the case of cement with granulated slag, it showed a strength category of 42.5 N. The compressive strength at 2 days showed no relevant variations between SMCs. At 28 days, the granulated slag had the highest strength. Buildings 2025, 15, x FOR PEER REVIEW 16 of 26 Figure 4. Results of compressive strength 30% SMC and base cement [23]. As to the heavy metal components (the fluoride, chloride, and sulfate content of the SMCs), no notable reduction in compressive strength was associated with the zinc, copper, cobalt, or arsenic content present in the cements with iron silicate or in the cements with black slag; the concentration of chromium and manganese is another parameter to be monitored. According to the standard deviation in Figure 5, the results are in an acceptable range, <2 MPa. Note again that the addition of black slag cement produces a slightly higher standard deviation, above 1 MPa, at 7 days, which is a normal value. Figure 5. Standard deviation (SD) results of compressive strength 30% SMC and base cement. Figure 4. Results of compressive strength 30% SMC and base cement [23]. Regarding strength recovery after rupture, with this proportion of supplementary cementitious materials (SCMs), the hydraulic properties of granulated slag, the pozzolanicity of fly ash, and, to a lesser extent, iron silicate, exhibited the best performance between 28 and 90 days. Black slag was the fourth addition with the best strength recovery from 28 days to 90 days. As to the heavy metal components (the fluoride, chloride, and sulfate content of the SMCs), no notable reduction in compressive strength was associated with the zinc, copper, cobalt, or arsenic content present in the cements with iron silicate or in the cements with black slag; the concentration of chromium and manganese is another parameter to be monitored. Buildings 2025,15, 479 17 of 26 According to the standard deviation in Figure 5, the results are in an acceptable range, <2 MPa. Note again that the addition of black slag cement produces a slightly higher standard deviation, above 1 MPa, at 7 days, which is a normal value. Buildings 2025, 15, x FOR PEER REVIEW 16 of 26 Figure 4. Results of compressive strength 30% SMC and base cement [23]. As to the heavy metal components (the fluoride, chloride, and sulfate content of the SMCs), no notable reduction in compressive strength was associated with the zinc, copper, cobalt, or arsenic content present in the cements with iron silicate or in the cements with black slag; the concentration of chromium and manganese is another parameter to be monitored. According to the standard deviation in Figure 5, the results are in an acceptable range, <2 MPa. Note again that the addition of black slag cement produces a slightly higher standard deviation, above 1 MPa, at 7 days, which is a normal value. Figure 5. Standard deviation (SD) results of compressive strength 30% SMC and base cement. Figure 5. Standard deviation (SD) results of compressive strength 30% SMC and base cement. Similar conclusions can be drawn for the cements with 5% SMC: they are all characterized as cements in the 42.5 R strength category, while cements with 20% SMC are all characterized as cements in the 42.5 N strength category. The compressive strength data at 2 days clearly show the slower evolution of cement with granulated slag. The strength of this, however, increases at 28 days and higher because of its hydraulic properties. This behavior is also observed with fly ash and iron silicate but there is less of an increase in strength. According to the standard deviation, all the results are in an acceptable range (<2 MPa). 3.4. Leaching Test Given the composition of the new SMCs, leaching phenomena were evaluated to identify any element in the cement with the capacity to be released in an aqueous medium and cause some risks, mainly environmental. According to the results in Table 10, all the heavy metals, chlorides, fluorides, sulfates, dissolved organic carbon, DOC, and total dissolved solids (TDS) values were below the leaching limits for nonhazardous wastes according to Decision 2003/33/EC of 19 December 2002, which established criteria and procedures for the acceptance of waste in landfills [ 55 ]. Compared to inert waste regulation limits, the specific values of certain pollutants exceeded the limits, e.g., chromium [ 56 ] limits were exceeded in the base cement, cements with ashes, and 5% granulated slag. In the case of molybdenum, cements with the presence of iron silicate had the highest values. As for the DOC test, there was a wide distribution in terms of cements that exceeded the limit value of inert residues, including the cement with 5% black slag. In conclusion, the leaching tests conducted produced no significant results when comparing the cements with the new SCMs, the base cement, and with the cements manufactured with the regulated SMCs. Buildings 2025,15, 479 18 of 26 Table 10. Cement leaching test. TDS: total dissolved solids; DOC: dissolved organic carbon. Note: Cement with SMCs that contain higher heavy metal concentrations marked with an asterisk (*). The highest values of the cement leaching tests for each element are in bold red. Release to Ratio Liquid/Solid = 10 L/kg mg/kg Dry Material As Ba Cd Cr Total Cu Hg Mo Ni Pb Sb Se Zn Chloride Fluoride Sulfate DOC TDS mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR mg/kg DR ppm ppm ppm ppm ppm BASE CEMENT <0.2 5.9 <0.1 0.59 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 3.20 0.12 4.78 364 600 BASE CEMENT + 5% SMC Limestone <0.2 4.8 <0.1 0.45 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 2.01 0.09 3.05 257 500 Alberto <0.2 5.1 <0.1 0.39 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 3.01 0.08 4.02 296 700 White slag <0.2 6.3 * <0.1 0.31 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 3.02 0.42 * 7.08 * 299 800 Black slag <0.2 7.8 * <0.1 0.17 * <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 * 0.24 2.58 * 0.15 * 3.99 * 558 700 Iron silicate <0.2 * 5.0 <0.1 * 0.18 <0.2 * <0.2 0.5 <0.2 * <0.2 * <0.2 * <0.4 <0.2 * 2.62 0.27 3.47 * 371 800 Fly ash <0.2 6.7 <0.1 4.6 <0.2 <0.2 * <0.1 * <0.2 <0.2 <0.2 <0.4 <0.2 2.34 0.31 5.68 351 * 600 Granulated slag <0.2 7.0 * <0.1 0.89 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 3.51 0.19 6.82 389 900 BASE CEMENT + 20% SMC Limestone <0.2 6.2 <0.1 <0.1 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 1.29 0.11 4.82 361 600 Alberto <0.2 5.7 <0.1 0.24 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 2.11 0.13 5.11 204 700 White slag <0.2 9.0 <0.1 <0.1 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 2.87 0.31 * 5.33 * 357 900 Black slag <0.2 15.3 <0.1 0.1 * <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 * <0.2 2.05 * 0.34 * 5.99 * 443 700 Iron silicate <0.2 * 5.8 <0.1 * 0.12 <0.2 * <0.2 3.1 <0.2 * <0.2 * <0.2 * <0.4 <0.2 * 1.48 0.41 8.15 * 422 500 Fly ash <0.2 3.8 <0.1 3.7 <0.2 <0.2 * <0.1 * <0.2 <0.2 <0.2 <0.4 0.8 2.48 0.27 5.52 227 * 1000 Granulated slag <0.2 1.8 * <0.1 0.23 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 2.71 0.24 2.25 209 800 BASE CEMENT + 30% SMC Limestone <0.2 4.9 <0.1 0.28 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 1.59 0.11 3.51 309 600 Alberto <0.2 5.2 <0.1 <0.1 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 1.96 0.09 3.76 251 600 White slag <0.2 11.1 * <0.1 <0.1 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 1.89 0.22 * 4.25 * 284 600 Black slag <0.2 17.1 * <0.1 <0.1 * <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 * <0.2 2.99 * 0.18 * 4.01 * 425 1000 Iron silicate <0.2 * 1.8 <0.1 * 0.16 <0.2 * <0.2 5.6 <0.2 * <0.2 * <0.2 * <0.4 <0.2 * 2.31 0.31 8.97 * 230 700 Fly ash <0.2 1.2 <0.1 3.5 <0.2 <0.2 * <0.1 * <0.2 <0.2 <0.2 <0.4 0.39 3.18 0.26 5.45 284 * 700 Granulated slag <0.2 8.3 * <0.1 0.2 <0.2 <0.2 <0.1 <0.2 <0.2 <0.2 <0.4 <0.2 2.14 0.29 2.67 165 600 Limit value Non-hazardous waste [55]2 100 1 10 50 0.2 10 10 10 0.7 0.5 50 15,000 150 20,000 800 60,000 Limit value Inert waste [55] 0.5 20 0.04 0.5 2 0.01 0.5 0.4 0.5 0.06 0.1 4 800 10 1000 500 4000 Buildings 2025,15, 479 19 of 26 The purpose of this analysis was to determine if there is a connection between the heavy metal content of the SMCs and the leaching analysis of the cements. We concluded that, with the concentrations of heavy metals present in the main raw materials of the formulated cements, there is no clear transfer of the tested compounds from the SMCs to the leached solution of the cements, Table 10. In compounds such as barium and molybdenum, a more direct eluate/SMC relationship can be seen; in others such as zinc and chromium, we found an inverse relationship. After carrying out the cement test plan with the new SMCs, we concluded that none of the applied tests have produced parameters that advise against their usage. Key aspects should be taken into account for future legislative proposals, such as greater control of volume stability for formulations with white and black slag due to the presence of elements with the potential for expansion. 3.5. Environmental Analysis: Carbon Footprint Evaluation The environmental impact of the cement manufacturing process is quantified by evaluating CO 2 emissions, Table 11, which are the primary contribution of cement production to global warming [ 57 , 58 ]. To simplify the calculations, we developed a simple methodology for differential evaluation of carbon footprints, considering the direct CO 2 emissions associated with the clinker used in the manufacture of each cement, the CO 2 associated with the logistical activity of the main SMCs used in the cements evaluated, and the CO 2 linked to the extractive and stockpile preparation activities, where applicable. Table 11. Methodology for calculating the carbon footprint of formulated cements [59–61]. kg CO2/t Cement (% Clinker + SMCs Transport + Extractive Activities) CEM I 42.5 R-SR 3 +5% SMC CEM I 42.5 R-SR 3 +20% SMC CEM I 42.5 R-SR 3 +30% SMC kg CO 2 /t Cement Var. vs. CEM I kg CO 2 /t Cement Var. vs. CEM I kg CO 2 /t Cement Var. vs. CEM I Limestone 748.1 −37.8 634.7 −151.2 559.0 −226.8 Fly ash 752.7 −33.2 653.3 −132.6 586.9 −198.9 Granulated slag 787.7 1.8 793.3 7.4 797.0 11.1 Albero 743.4 −42.4 616.0 −169.9 531.0 −254.8 White slag 743.3 −42.5 615.7 −170.1 530.7 −255.2 Black slag 743.3 −42.5 615.7 −170.1 530.7 −255.2 Iron silicate 749.8 −36.1 641.5 −144.4 569.3 −216.6 The environmental impact of the cement manufacturing process, in terms of CO 2 emissions, was quantified to compare the environmental impact of the cements studied against that of a reference, base cement. This methodology takes into account some rules from the UNE-EN 15804:2012+A2:2020 [ 59 ] about environmental product declarations for construction products “from cradle to gate” using equivalent CO 2 emissions from modules A1 and A2, raw materials supply, and transport to the plant gate. A CO 2 emission factor was determined for each cement depending on the SMC and proportion used, which was then compared with the equivalent ratio of the reference cement, CEM I 42.5 R-SR 3, considering only the emissions associated with its clinker content and the transport of the limestone. Other assumptions that informed the methodology included: • Only the CO 2 emissions associated with the clinker production process and the diesel consumption during the transport stage of the SMCs used in the formulation of the cements studied have been used, incorporating the CO 2 associated with the extractive activities of limestone and albero. • CO 2 emissions associated with the production of recoverable materials are not considered. •CO2emissions associated with electricity consumption are not considered. Buildings 2025,15, 479 20 of 26 • The stages of grinding, dispatch, distribution, use, and end of life of the cement, as well as indirect emissions, are not evaluated because they are considered to be differentially neutral. •A ratio of 850 kg of CO2per ton of clinker has been considered. •The clinker content of the reference cement is 92%, as mentioned in Table 2. • The CO 2 emissions associated with the consumption of gypsum and ferrous sulfate are not accounted for. We concluded that, in all cases, except cement manufactured with blast furnace slag, the CO 2 emissions associated with cements that contain SMCs are reduced compared to the reference cement with a higher clinker content. The usage of granulated slag is penalized by the distance to the point of consumption. On the other hand, CO 2 emissions associated with extractive activities are not relevant. 3.6. Economic Analysis The cements were also analyzed from an economic point of view, as in Table 12; their production cost was determined and compared with that of the base cement, CEM I 42.5 R-SR 3, under the following assumptions: • Material prices are considered at the cement production plant; the cement CO 2 price also has to be taken into account, considering an emissions ratio of 850 kg CO2/t clinker. • Bulk cement production cost is calculated without considering the cost of the cement grinding stage by multiplying the price of the materials used in their manufacture. •No variation in cement grinding cost between the different cements studied is considered. • The effect on the production cost of differences in the performance of the cements studied is not considered. • Variations in the SMC of gypsum and ferrous sulfate among the cements studied are not considered. •CO2is considered a raw material from the production cost point of view. • Only the clinker production CO 2 emissions factor is considered. No indirect CO 2 emissions have been taken into account. •Costs and prices have been included according to the authors’ criteria. In conclusion, without taking into account the differences in cement properties performance, all the new formulations reduce the production cost compared with the base cement. Additionally, the higher the percentage of SMC, the greater the difference in cost from the reference cement due to the impact of the price of CO 2 on cements with higher clinker content. According to the mentioned criteria, the most profitable SMCs for the studied production center are: 1—white or black slag, 2—albero, 3—limestone, 4—iron silicate, 5—fly ash, 6—granulated blast furnace slag. Moreover, a sensitivity analysis is presented, which was only applied for the comparison of cements formulated with SMCs of 5%, 20%, and 30% and the base cement, based on CO 2 price, the clinker price/cost, and both effects jointly—the worst and best case scenarios. The conclusion of the sensitivity analysis was similar to the base scenario: the greatest savings were achieved with the white and black slag cements ( − 31.6 EUR/t vs. the base cement with 30% SMC at base scenario), obtaining the second best savings with albero (−31.45 EUR/t vs. the base cement with 30% SMC at base scenario). Buildings 2025,15, 479 21 of 26 Table 12. Sensitivity analysis vs. base scenario. Best case scenario: increase of 40 EUR/t of CO 2 and 10 EUR/t clinker. Worst case scenario: decrease of 40 EUR/t CO 2 and 10 EUR/t of clinker. Production cost of cements formulated with SMCs, 5%, 20%, and 30%, on base cement. K—clinker; G—gypsum; L—limestone; FS—ferrous sulfate. Base Scenario Best Case Scenario Worst Case Scenario Clinker 30 EUR/t 40 EUR/t 20 EUR/t Gypsum 11 EUR/t 11 EUR/t 11 EUR/t Ferrous sulfate 60 EUR/t 60 EUR/t 60 EUR/t Limestone 6 EUR/t 6 EUR/t 6 EUR/t Fly ash 12 EUR/t 12 EUR/t 12 EUR/t Granulated slag 40 EUR/t 40 EUR/t 40 EUR/t Albero 2.0 EUR/t 2.0 EUR/t 2.0 EUR/t White slag 1.5 EUR/t 1.5 EUR/t 1.5 EUR/t Black slag 1.5 EUR/t 1.5 EUR/t 1.5 EUR/t Iron silicate 8 EUR/t 8 EUR/t 8 EUR/t CO290 EUR/t 130 EUR/t 50 EUR/t Production cost CEM I 42.5 R-SR 3 K92% G3.5% L4% FS0.5% 99.0 EUR/t 139.4 EUR/t 58.4 EUR/t Production Cost Variation vs. CEM I 42.5 R-SR 3 CEM I 42.5 R-SR 3 +5% SMC CEM I 42.5 R-SR 3 +20% SMC CEM I 42.5 R-SR 3 +30% SMC CEM I 42.5 R-SR 3 +5% SMC CEM I 42.5 R-SR 3 +20% SMC CEM I 42.5 R-SR 3 +30% SMC CEM I 42.5 R-SR 3 +5% SMC CEM I 42.5 R-SR 3 +20% SMC CEM I 42.5 R-SR 3 +30% SMC Limestone −5.13 −20.20 −30.25 −7.23 −28.90 −43.35 −2.83 −11.30 −16.95 Fly ash −4.83 −19.00 −28.45 −6.93 −27.70 −41.55 −2.53 −10.10 −15.15 Granulated slag −3.43 −13.40 −20.05 −5.53 −22.10 −33.15 −1.13 −4.50 −6.75 Albero −5.33 −21.00 −31.45 −7.43 −29.70 −44.55 −3.03 −12.10 −18.15 White slag −5.35 −21.10 −31.60 −7.45 −29.80 −44.70 −3.05 −12.20 −18.30 Black slag −5.35 −21.10 −31.60 −7.45 −29.80 −44.70 −3.05 −12.20 −18.30 Iron silicate −5.03 −19.80 −29.65 −7.13 −28.50 −42.75 −2.73 −10.90 −16.35 4. Discussion The new SMCs were evaluated according to a series of factors to which equal weight is given, as shown in Table 13. We concluded that, for this specific cement plant, the lowcalcium-carbonate-content albero present in the quarry close to the reference production center had the highest score. For the SMCs coming from industrial processes, the scores were very close between iron silicate and electric furnace steel slags, with the steel slags having a slightly better score. Considering only the availability, cost, and circular economy indicators, steel slags are the best options among the new studied SMCs. Table 13. Evaluation of the suitability of the new cement SMCs studied. Scale: 1 (most suitable) to 4 (least suitable). Secondary Main Component Suitability Secondary Main Component Evaluation Heavy Metals Na 2 Oeq. Free Lime TOC Pozzolanic Activity Soundness Availability Cost Carbon Footprint Circular Economy Global Evaluation Albero 1 1 1 4 4 1 1 2 2 4 21 Black slag 3 4 3 3 2 3 2 1 1 1 23 White slag 2 2 4 2 3 4 3 1 1 1 23 Iron silicate 4 3 2 1 1 2 4 3 3 1 24 Considering the test plan of the proposed SMCs, no determining parameters have been found that would rule out the use of the new SMCs in the manufacture of cements. However, certain requirements have to be taken into account for future legislative proposals, such as greater vigilance within production control to ensure the volume stability of formulations with white and black slags due to the presence of magnesium-based expansive compounds. The soundness test of the steel slags’ magnesium content, expressed as MgO, produced a 13.6% highest value in white slag. The sulfur percentage in the steel slags had 3.7% as the highest value in SO 3 . Due to its possible effect on the cement setting time, this should be a major concern. In this case, it would be necessary to adjust the gypsum content in the cement. The chlorides or organic compounds of the new SMCs did not cause any issues in the characteristics of cement. The heavy metals analysis highlighted the unlikely impact of Buildings 2025,15, 479 22 of 26 the copper content in the iron silicate (0.1%) on the cement setting time, the manganese in the black slag (0.4%) on strength development, and the sulfate in the three slags (>0.1%) on the cement setting time and volume stability. From the anhydrous cement test results, it can be seen that the laboratory milling procedure is adequate to achieve an equivalent particle size distribution guaranteeing a minimal differential effect on the performance of the cements to be tested. It was verified that all the analyzed cements meet the regulatory requirements in terms of the SO 3 and Cl content, UNE-EN 197-1. The XRF and XRD analyses did not exclude any cement. The main cement phases were within the normal range. The free lime [0.6–1.2%] and periclase [0–1.2%] contents were in an acceptable range to avoid expansive phenomena, though the presence of periclase was clearly associated with the SMCs based on white slag, black slag, and iron silicate. No effects were expected from the evaluated alkaline sulfates’ phases. Arcanite (K 2 SO 4 ), with a range of [1–3.4%] obtained in 30% SMCs, was highest in black slag cement, with no delay in the setting time. None of the new SMCs, under the parameters of this test, can be considered as new SMCs for the formulation of pozzolanic cements. Regarding the setting and hardening properties of the formulated cements, an increase in setting times without any significant water demand variation was observed as the proportion of SMC increased. Additionally, from the new SMCs, black slag and iron silicate had slightly longer setting times than the albero and white slag. The heavy metal content of these SMCs is therefore correlated with their setting time performance. Compressive strengths at 30% SMC in the cement highlight black slag cement’s performance at 28 days (41.3 MPa vs. 35.3 MPa for albero cement and 56.5 MPa for the type I-SR reference). As there was not much pozzolanicity activity in the new SMCs, there was no strength increase after 28 days. However, the iron silicate cements exhibited a change of +7.8 MPa, and a change of +2.8 MPa was observed in the black slag cements compared to fly ash cement (+20 MPa) and granulated slag cements (+17.4 MPa). The differences in compressive strength results with 5% and 20% SMC cements at 2 and 28 days were not that significant. There does not appear to be any strong influence from heavy metals on the results. The standard deviation was within an acceptable range (<2 MPa). As for the leaching test, there are no significant results compared with the base cement. All the evaluated parameters were below the leaching limits in the nonhazardous waste regulations. The purpose of this analysis was to determine if there is a connection between the heavy metal content of the SMCs and the leachate analysis of the cements based on the concentrations of heavy metals present in the main raw materials of the formulated cements. We concluded there was no clear transfer of the tested compounds from the SMCs to the leached solution of the cements. A more direct eluate/SMC relationship was observed with compounds such as barium and molybdenum, while in others such as zinc and chromium, we found an inverse relationship. In terms of the carbon footprint, scope clinker content, and SMCs transport and extractive activities, for all cases except cement manufactured with blast furnace slag, because of the distance to the cement plant, the CO 2 emissions associated with cements that use SMCs were reduced compared to the reference cement with higher clinker content. The CO 2 emissions associated with extractive activities for albero are not relevant. Albero and steel slags are the best options according to CO 2 emissions ( − 31% with 30% SMC vs. type I-SR reference). In the economic analysis, we observed that all the new formulations reduced production costs compared with the base cement. No free allocations were considered, so the CO 2 price is a relevant factor. Transport cost is another relevant variable. Additionally, the higher the percentage of SMC, the greater the difference in cost to the reference cement due Buildings 2025,15, 479 23 of 26 to the impact of the price of CO 2 in cements with higher clinker content. Albero and steel slags are, therefore, also the best options according to the cost savings ( − 32% with 30% SMC vs. type I-SR reference, 30% iron silicate SMC; −28% vs. type I-SR reference). 5. Conclusions Taking SR cement CEM I 42.5 R-SR 3 as a reference, we proved that it is possible to produce, in accordance with EU regulatory changes, high-performance, low-carbonfootprint and cost-efficient cements using SMCs that are not currently authorized: lowcalcium-carbonate-content albero, iron silicate, white electric steel furnace slag, and black electric steel slag. This study offers information about innovative, alternative, nonregulated SMCs, along with SR clinker base cement production. The main findings of this study are as follows: • There are no concerns arising from the anhydrous, setting, and leaching analyses performed on the studied cements. •No remarkable pozzolanic activity was observed in the nonregulated SMC cements. • Based on compressive strength tests and the equivalent normative requirements, all cements formulated with the new secondary main components meet the regulatory limits at 2 and 28 days of UNE-EN 197-1. Cement with 5% SMC complies with the 42.5 R strength category, 20% SMC cement complies with the 42.5 N strength category, and 30% SMC cement complies with the 32.5 R strength category. Black slag with the highest proportion of SMC, 30%, shows the best behavior after 28 days. • In the carbon footprint evaluation, steel slags were shown to produce the maximum reduction compared to type I-SR: − 42.5 kg CO 2 /t (5%SMC), − 170.1 kg CO 2 /t (20%SMC), and − 255.2 kg CO 2 /t (30%SMC). They were closely followed by cements with albero, at − 42.4 kg CO 2 /t (5%SMC), − 169.9 kg CO 2 /t (20%SMC), and − 254.8 kg CO 2 /t (30%SMC). • Finally, the economic analysis identified steel slag as the best option, generating savings of 5.4 EUR/t (5%SMC), 21.1 EUR/t (20%SMC), and 31.6 EUR/t (30%SMC) vs. type I-SR. Cements with albero also produced remarkable savings of 5.3 EUR/t (5%SMC), 21 EUR/t. (20%SMC), and 31.5 EUR/t. (30%SMC) vs. type I-SR. We have concluded that the low-calcium-carbonate-content albero present in quarries close to the reference production center is the best new SMC option, but there were not significant differences among the various SMCs. Of the SMCs derived from industrial processes, steel slags are the best. Complementary strength and durability tests of the concretes made with this new cement should be performed [ 62 , 63 ] to ensure these cements can be applied in the most important practices, such as in structural concrete. These new high-SMC cements may find their application niche in the short term and, ultimately, improve performance based on the granulometry curves of the cements, through grinding materials independently or new finish mill technologies [ 64 ], and the usage of grinding aid additives [ 65 ] are two working trends that the cement sector will have to address to ensure an improved performance from these new cements. Author Contributions: Conceptualization, M.Á.M.I. and B.N.R.; methodology, M.Á.M.I.; validation, L.F.V.A.; formal analysis, M.Á.M.I.; investigation, M.Á.M.I.; resources, B.N.R.; data curation, B.N.R.; writing—original draft preparation, M.Á.M.I.; writing—review and editing, B.N.R.; visualization, M.Á.M.I., B.N.R. and L.F.V.A.; supervision, B.N.R.; project administration, M.Á.M.I. and B.N.R. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Buildings 2025,15, 479 24 of 26 Data Availability Statement: The data presented in this study were obtained through the research and lab work of the authors. They are available from the authors upon request. Acknowledgments: The authors express their gratitude to Seville University. Conflicts of Interest: The authors declare no conflicts of interest. References 1. Al-Zu’bi, M.; Fan, M.; Al Rjoub, Y.; Ashteyat, A.; Al-Kheetan, M.J.; Anguilano, L. 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