3.1. Raw Material Characterization
The chemical composition of the BOF, BFS slags, and gypsum waste is shown in
Table 4. The main oxides found for each raw material are different concerning their origin. BFS has high SiO
2, CaO, and Al
2O
3 content, essential for hydration product formation, such as aluminates and hydrated calcium silicates. In BOF slag, the oxides CaO, Fe
2O
3, and SiO
2 predominate, which help in the chemical activation of BFS and the formation of new hydration products, while the gypsum residue basically presents CaO and SO
3, essential in the formation of ettringite.
The gypsum waste from civil construction corresponds to the hydration residue of calcium sulfate hemihydrate. For this reason, its mineralogical composition corresponds mainly to gypsum;
Table 5.
The mineralogical composition of the BOF slag used (
Table 5) demonstrates that despite the different percentages found, the majority of phases in the three samples correspond to wustite (FeO), calcite (CaCO
3), belite (β-2CaSiO
2), brownmillerite, and magnetite. Portlandite was also identified in all slags. However, portlandite content was significant for BOF2 and BOF3 slags.
BOF slag exerts a relevant influence on the reactivity of the reactional environment by the alkaline activator function. In this sense, BOF3 presented the highest portlandite content and had the highest basicity (3.7). Ca(OH)
2 has high solubility in water (1.6 g/L) [
10], being easily solubilized and quickly raising the pH of the solution. On the other hand, BOF1 showed a high CaCO
3 content. Calcite can also change the pH of the solution by slowly releasing hydroxyl ions due to its low solubility in water (1 mg/L), as shown in the following equations [
11]:
In this way, both phases (CaCO3 and Ca(OH)2) can affect the basicity of the medium, but, depending on solubility, with different speeds.
Another factor relevant to reactivity is the percentage of the amorphous phase, which, due to the slow cooling of BOF slags, usually has a low crystallization energy level retained in its atomic structure. For this reason, BOF slags tend to be difficult to solubilize, given the high stability of the crystalline phases. In this sense, BOF2 and BOF3 slags presented similar contents of around 10%, while BOF1 presented twice the glass phase in its atomic structure, which may indicate greater reactivity and binding capacity [
12,
13].
In terms of hydraulic behavior, the most reactive phase present in BOF, in the initial ages, corresponds to brownmillerite, which appeared prominently in BOF2 (19%), as it begins hydration in the first 24 h to form hydrated calcium ferroaluminates or hydroxy compounds. At the final ages, belite is the phase responsible for the formation of hydrated calcium silicates that corroborate the gain of mechanical resistance in the cement and was identified in a higher percentage in BOF3 (25%) [
13].
3.2. Cement Chemical Composition and Alkalinity
The evolution of the hydration process and the number of products formed in ECO cement are closely dependent on the chemical composition of the cement. Also, the development of mechanical resistance is mainly related to the precipitation of hydration products such as ettringite and hydrated calcium silicates (C-S-H).
Table 6 shows the chemical composition of the cement.
By the time the anhydrous grains of the cement mixture come into contact with water, the dissolution of the blast furnace slag begins, but very slowly, which makes its use unfeasible. This is because the dissolution of the slag occurs by hydroxylic attack, that is, by OH
− ions. Thus, if the solution in contact with the grains is alkaline, dissolution will be faster as the solubility of the glassy structure of the silica that constitutes BFS is increased. In addition, the formation of hydrated silico-aluminous products on the surface of the slag grains is avoided, which would prevent the dissolution from continuing [
14,
15].
It is estimated that the initial pH of cement rich in blast furnace slag is around 11–12.5 and decreases as the hydration process develops [
4,
11]. The increase in pH in the solution is mainly related to the hydrated lime that makes up the BOF slag since calcium sulfate does not change the pH of the solution significantly.
This statement is confirmed by observing the alkalinity of the cements ECO3 802010 and 802015, which differ only in the BOF slag content, for which the alkalinity corresponds to 12.3 and 13.1, respectively. In addition to increasing the pH level, it is expected that lime introduces Ca2+ ions into the solution, causing the equilibrium of the solubility product to shift towards saturation, accelerating the hydration product precipitation. At the beginning of cement hydration, due to the high alkalinity, there are hydrates precipitated, such as ettringite, C-S-H, and C-A-H, ensuring the cement’s high resistance at early ages. On the other hand, excessive alkalinity, as observed in ECO3 802015, can create instability in the products formed and can lead to covering the surface of anhydrous grains with precipitated products, harming the development of resistance at later ages as a consequence of earlier product precipitation.
For calcium sulfate, the dissolved SO42− reacts with the aluminum released from BFS dissolution to form ettringite, one of the first products to be precipitated in this cement, which prevents the formation of low-permeability products on the surface of the slag particles. Therefore, the aluminum content is also a relevant factor for the initial strength gain.
The aluminum content in cement brings two relevant considerations: (i) the percentage of aluminum presented by the BFS, in which the higher the Al content, the higher the contribution to the increase in its dissolution speed; (ii) the aluminum content available in the mixture for the formation of ettringite.
Therefore, the dissolved aluminum content of the slag must be sufficient to react with the sulfate from the gypsum to appropriately form ettringite before the silicates. A low aluminum content and excess sulfate can impair the hydration of the BFS due to the rapid occurrence and growth of ettringite and monosulfate crystals on the surface of the BFS grain, causing its isolation and preventing hydration.
The inferences drawn from this study, together with the initial investigation for the ECO patent [
7] and the relevant literature [
4,
5,
12,
13,
14,
15], have consistently demonstrated the points made in the previous observations and comments.
3.3. Compressive Strength of Cement Mortar Using ECO Formulations
Compressive strength results for cement mortar specimens with ECO1, ECO2, ECO3, and ECO4 cement are shown in
Figure 1,
Figure 2,
Figure 3 and
Figure 4. Using analysis of variance (ANOVA), the effects of ‘type of cement’ and ‘hydration age’ were analyzed for all formulations. For all cement formulations, these effects are statistically significant variables to explain the different mechanical performances observed. In other words, within the same cement group, such as ECO2, the mechanical strength results of distinct formulations showed significant differences. The same was observed for ECO3 formulations.
Figure 1 presents the results of the compressive strength of ECO1 cement, produced based on the initial formulation of ECO cement [
7]. The cement showed a gain in resistance over the ages. However, the resistance at the initial ages was lower than the Brazilian normative limits [
6]. This behavior is attributed to the BOF slag content used in the formulation, the lowest among all ten proposed cement formulations, and the basicity of the BOF1 slag (2.8). It is understood that both factors may not have favored the beginning of the hydration process. On the other hand, due to the blast furnace slag used (BFS1) and the high aluminum content present in its composition, a resistance gain was observed, even if more slowly.
In the compressive strength results for the ECO2 cement, shown in
Figure 2, it can be seen that the cement formulations showed a significant evolution in compressive strength over the ages.
At the initial ages, ECO2 cement exhibited high compressive strength. At 3 days, the compressive strength varied between 8 and 10 MPa, and at 7 days, from 20 to 40 MPa. At this last age (R7), the values approached the normative limits defined for most types of Portland cement at 28 days, as defined by [
6]. The ECO2 cements were produced using BOF2, which presented high basicity (3.1) and the highest content of the brownmillerite phase, factors that may corroborate the development of initial resistance.
At early hydration ages, the ECO2-802013 formulation performed the best. It is noteworthy that this corresponds to the cement with the highest addition of BOF2 among the three formulations, which generated an environment of greater alkalinity, resulting in increased dissolution of blast furnace slag and accelerating the formation of hydration products.
At the later ages, the ECO2-802010 formulation demonstrated the best performance, with a compressive strength at 180 days greater than 80 MPa, corroborating the consistent strength growth throughout the hydration process. The ECO2-851510 formulation is attributed the worst performance due to the high BFS content and the reduction of gypsum, and consequently, the reduction in the contribution of calcium ions and sulfate ions, which compromised the availability of ions for the formation of hydration products.
For the ECO3 cement formulations, shown in
Figure 3, the results demonstrate that the evolution of compressive strength over the ages did not occur uniformly for all cements. Compressive strength after 3 days of hydration reached values between 14 and 20 MPa, the highest among all the formulations studied. This behavior is understood through the basicity of the slag used, BOF3, which is the highest and equivalent to 3.7, responsible for accelerating the decomposition of blast furnace slag, given the alkalinity of the reaction environment. The best behavior is observed for the ECO3-802010 formulation, which maintains a constant evolution of compressive strength at all ages. It is seen that the ECO3-802015 cement had a compromised performance compared to the others, and this difference is more evident in the final ages. The mentioned formulation is composed of the highest BOF slag content, which allowed a rapid gain in resistance after the 3rd day, but from this age onwards, little evolution was observed (R7: 20.4 MPa to R180: 35.8 MPa).
The compressive strength results for the ECO4 formulation cements are shown in
Figure 4 and demonstrate that they correspond to the lowest values observed among all formulations. Despite the development of initial resistance, there is no significant evolution over the ages, so between 3 and 180 days of curing, the difference was approximately 15 MPa. Among the formulations, the compound with the highest contents of BFS and BOF slag had slightly better results (ECO4-901013).
Figure 5 shows the average results for each group of cements. Observing the comparison of the evolution between them, it is seen that the cement mixtures of the ECO2 formulation, in addition to presenting a continuous gain in mechanical strength, showed less dispersion of the results, which demonstrates a more homogeneous behavior among them.
The ECO2 cement formulations were produced using BOF2 slag, with high basicity, the highest brownmillerite content, and a relevant dicalcium silicate content, corroborating the development at the initial and final ages. In addition, the BFS1 slag was used, which presented the highest aluminum content in its composition, supporting its solubilization and ettringite formation.