X-ray fluorescence (XRF) was performed on raw materials using beads fused with lithium boride (B₄Li₂O₇). The equipment used was a Thermo ARL 9800 XP spectrometer.

A BLF 17/3 Carbolite furnace was used to produce the clinker, with a maximum temperature of 1700 °C and 220 V. The heating rate for all tests was the same (10 °C/min); only the maximum clinkering temperature changed. The residence time for each sample at higher temperatures was always 40 minutes. Clinkering processes were carried out in an artisanal crucible made from refractory brick, reference ALMAG AF of Refratechnik. When the heating process for each test was completed, the crucible with the red hot sample was removed from the furnace. All clinkers were cooled with air from a compressor at 5 psi for 10 minutes to reach a final temperature of approximately 40 °C.

The XRD patterns were collected in a PANalytical X'Pert PRO MPD, with a Cu source, Bragg-Brentano configuration, a range from 15° to 70° of 2theta, an X Celerator detector step of 0.016° and an accumulation time of more than 10000 counts in the main peak. The mineralogical analysis was performed by Rietveld methodology using High Score Plus software and the Inorganic Crystal Structure Database (ICSD).

This was performed using SDT Q600 equipment with an inert atmosphere of N₂, an alumina melting pot of 90 mL, a scan rate of 20 °/min and a temperature ramp from 25 °C to 1500 °C.

An OLYMPUS MMQC-005 petrographic microscope, model BX41, was used to qualitatively analyze the clinker obtained, in order to identify the phases formed. The petrographic analysis was performed with reflected light on polished specimens etched with nital (nitric acid at 1% in isopropyl alcohol).

This test was performed to provide an indicator of the degree of clinkering in the material. Free lime was measured by dissolving CaO in a methanol:ethylene glycol (1:2) mixture at a temperature of 80 °C with constant agitation. After the solution was filtered, it was titrated with 0.1 N hydrochloric acid using bromocresol green - Methyl red as a mixed indicator.

A Box-Behnken statistical design was used in this study. The factors considered were fluorite content, fluorite/gypsum ratio and clinkering temperature. The response variable was free lime. The levels of each factor were taken from previous studies on mineralization (26, 35). This study explored the effects of fluorite and gypsum at low and high levels. Table 1 shows the matrix design for randomized trials. The central points are additional experimental runs located at a midpoint between the lower and higher level of all factors. This is done to estimate experimental error and adjust the model.

The results obtained were grouped by clinkering temperature, as follows (Figures 1, 2, and 3):

At 1300 °C and 1250 °C (Figures 2 and 3) all the formulations with mineralization had less free CaO than the reference sample, as a result of more efficient clinkering (the reaction of the raw materials with the mineralizers increased alite production). The reference sample at 1350 °C (Figure 1) had lower free CaO than the mixtures with fluorite and gypsum at the same temperature. One hypothesis to explain this behavior is that when the samples reached 1350 °C, fluorine volatilization occurred and this became a limiting factor for mineralization. It is well known that fluorine evaporates easily at high temperatures, like alkalies and sulfates (28).

The tests that showed the lowest free CaO (tests 6, 2, and 5) are highlighted in Figures 1, 2, and 3. These formulations had a CaF₂/CaSO₄.2H₂O ratio of 2/15. The high fluorite content (0.75 and 0.5%) facilitated the reaction of C₂S with CaO to form C₃S.

In Figure 2 and Table 3, when comparing the reference at 1300 °C (without mineralization) with test 2 (CaF₂/CaSO₄.2H₂O=2/15, temperature=1300 °C, and fluorite=0.75) an 81% decrease in free CaO can be observed in the mineralized clinker. Moreover, in Figure 3 and Table 3, when comparing the reference at 1250 °C with test 5 (CaF₂/CaSO₄.2H₂O=2/15, temperature=1250 °C, and fluorite=0.50) a 56% decrease in free CaO can be observed. Thus, the CaF₂/CaSO₄.2H₂O=2/15 ratio plays an important role in reducing clinkering temperature, i.e., in the formation of calcium silicates (C₂S and C₃S). The effect of the 2/15 fluorite/gypsum ratio on the clinker was more noticeable at 1300 °C.

Table 4 and Figures 4, 5, and 6 show the identification and quantification of clinker phases with and without mineralizers. In order to ensure a good fit of the calculated pattern to the observed data, we looked for a weighted-profile R-value (Rwp) below 10% for all samples.

Figure 4 (1350 °C) and Table 4 show that alite content decreased and belite content increased when mineralizers were present. This is consistent with the higher free lime content found in these samples by titration, which means that the clinkering process for the mineralized clinker was incomplete at this temperature.

Figures 5 and 6 (1300 °C and 1250 °C) and Table 4 show that alite content increased and belite content decreased when mineralizers were present. This is consistent with the lower free lime content found in these samples by titration, which means that the mineralized clinker was more sintered at these temperatures. This result is consistent with those of researchers including Akin (21), who found that C₃S formation increased with the addition of fluorine.

The free CaO of the mineralized clinker was lower than that of the clinker without fluorite and gypsum (Figures 5 and 6). This indicates that the mineralizers improved the clinkering process.

Figures 7, 8, and 9 show that the raw material with mineralizers presented clinkering (C₃S and C₂S formation) at lower temperatures than the raw material without mineralizers.

The DSC technique also showed that the mineralizers had the greatest effect at temperatures between 1250 °C and 1300 °C.

Figures 10, 11, and 12 show better-defined particles of calcium silicates and calcium aluminates in the mineralized clinker than in the clinker without mineralizers at the same temperature. This indicates that the mineralizers had a positive effect on the clinkering process. The clinkering process at 1250 °C (Figure 12) requires a longer time or higher sintering temperature in order to achieve the complete formation of the clinker's phases.

An idiomorphic, equidimensional alite shape was obtained at a CaF₂/CaSO₄.2H₂O ratio of 2/15 and a temperature greater than or equal to 1300 °C (tests 2 and 6). The clinker also had improved reactivity in these conditions because it obtained a reactive phase of belite as α’ and α at lower temperatures. In contrast, xenomorphic crystals with irregular shapes were observed in the clinker without mineralization, indicating a lack of clinkering. This result is consistent with those of researchers including García-Díaz et al. (19), who found that CaF₂ affected clinker phase content and polymorphism.

In regard to the assessment of alite size, the samples with mineralizers always presented larger alite sizes (Table 5). Test 2 reached a regular size (28 micrometers) with evidence of a complete reaction and crystallization. Based on this, it can be concluded that the fluorite/gypsum ratio of 2/15 works efficiently at 1300 °C, allowing crystalline phases to form at a lower temperature.

Free CaO measurement in the clinker, XRD, DSC and optical microscopy were used to show that fluorite and gypsum have a significant effect at temperatures of 1300 °C and 1250 °C. It can be concluded that the fluorite/gypsum ratio of 2/15 works efficiently at 1300 °C, since the best results corresponded to that sample (the greatest amount of alite, the least belite, the smallest amount of free lime and regularly sized alite).

The R-programming language was used in the analysis and modeling of the Box-Behnken experimental design, because it has statistical packages to optimize the process variables. For this boundary, curves were generated to identify the trend of the variables studied. The results obtained in the simulation showed the same behavior as the experimental results. The behavior obtained (Figure 13) showed that increases in fluorite and temperature led to decreased free lime in the clinker, while the lowest fluorite/gypsum ratio (2/15) improved clinkering (green areas).