In order to know the possibility of using MK as a pozzolanic material, pozzolanic activity was studied as described in the methodology. With this reaction, two phases are obtained, liquid and solid, which are then analyzed.
3.3.1. Solid Phase
XRD results corresponding to the study of the kinetics reaction in the pozzolan/lime systems are presented in Figure 3
and Table 5
and Table 6
Using XRD, stratlingite was detected at 12.61 Å, 6.28 Å, 4.15 Å, and 2.87 Å, while LDH-type compounds (phyllosilicate/carbonate) were detected at 7.60 Å, 7.41 Å, and 3.78 Å [29
]. In addition, muscovite reflections appeared at 10.01 Å, 5.03 Å, 4.48 Å, and 3.35 Å, although slightly displaced toward 9.97 Å with respect to 10.01 Å and 3.34 Å with respect to 3.35 Å. These variations suggest that the pozzolanic reaction generates the 2M1
muscovite restructuring toward a 3T muscovite with characteristic spacing at 9.97 Å and 3.34 Å (Figure 4
Furthermore, calcite formation was detected at 3.86 Å, 3.03 Å, 2.49 Å, 2.28 Å, 2.09 Å, 1.91 Å, and 1.87 Å, and quartz reflections were maintained at 4.26 Å, 3.34 Å, and 1.81 Å from the original material.
Calcination at 750 °C/2 h increased MK reactive and amorphous material content (Table 6
). It should be noted, under these conditions, that amorphous phase content increased at long ages (28, 90, and 360 days).
The characterization of solid products from the pozzolanic reaction indicates the minimal solidification of C-S-H gels with and without aluminum in their structure [30
]. Therefore, and since there is little literature available on the pozzolanic reaction of MK that includes specific analyses of aqueous solutions, speciation analysis and geochemical modeling of the resulting aqueous phases were then carried out, after treating the activated natural kaolin with a saturated lime solution at one, seven, 28, 90, and 360 days, to be able to predict the most stable associations.
3.3.2. Liquid Phase
shows the evolution of the lime content fixed in the calcined kaolin/lime systems at different reaction times.
The fixed lime values for the samples calcined at both temperatures show high pozzolanic activity at all reaction times. These results correspond to a logarithmic growth up to 90 days, in which time the maximum is practically reached. This situation is comparable for all the activation conditions and for longer times.
ZnO addition before the thermal activation process produced an increase in the reactivity of MK with the calcium hydroxide of the saturated lime solution. This increase was detected at all times, mainly during the first seven days of pozzolanic reaction (Figure 5
). This fact can be explained by the disaggregation in the microgranular constituents with the addition of the chemical activator [19
The consumption of Ca2 +
(aq.) in the solution is associated with the pozzolanic reaction, where MK reacts with the Ca2 +
(aq.) available in the alkaline medium to form, mainly, gels without aluminum (C-S-H) and with aluminum (C-A-S-H). Gel compounds are frequently found as metastable phases in this type of reaction, decreasing their stability with increasing temperature of reaction [36
]. In some cases, they act as precursors to the formation of other stable phases such as stratlingite or zeolites [39
The pH is the parameter that controls the Ca2+ (aq.) concentration in the solution. As the reaction evolved, the pH decreased, and the system reached a steady state in a time up to 28 days (age included as a limit in the cement standards applicable in this case).
In order to set the experimental conditions, a pilot experiment was initially designed that included natural kaolin calcined at 600 °C/2 h and treated with a saturated lime solution for one, seven, 28, 90, and 360 days. The pH and Ca2+ (aq.) concentrations (determined via EDTA titration and by ICP/MS spectrometry) were measured in the laboratory immediately after each extraction, at the end of the hydration reaction. Differences in both measures were linked to temperature dependence under high-pH conditions.
presents the Ca2+
(aq.) concentration evolution as a function of the experimental pH values obtained for each reaction time using both methods (EDTA titration and ICP/MS) and, in turn, variations in pH with regard to the Ca2+
concentrations measured for each reaction time experimentally and using the PHREEQC program.
No significant differences were observed regarding the Ca2+ concentration measured using both methods, except for the first set of data, carried out at six hours of reaction, which differed by more than three units (mM). Measurements of Ca2+ (aq.) depended on the temperature under conditions of high pH. In addition, it can be seen that the pozzolanic reaction ended after 28 days, since the Ca2+ concentration remained, from that moment, constant in the solution.
The pH calculated theoretically by the PHREEQC method progressively decreased up to 28 days of reaction in an order of magnitude from 12.4 to 11.4, and then remained in the range of 11.4/11.3. The observation was in accordance with the determinations made using Ca2+
(aq.) and confirmed that the ion availability controlled the reaction. However, the experimentally determined pH in the laboratory did not indicate the end of the hydration reaction nor did it justify values for long times (28, 90, and 360 days) whose pH differed from that calculated in the measurements performed after seven days (Figure 6
ICP/MS analysis was the method used to measure the ion concentrations considered in this work at a constant ambient temperature, and the chemical speciation determined by this method was the one introduced in the PHREEQC program in order to optimize pH measurements at 25 °C.
(aq.) concentration evolution with the reaction time for the thermally activated kaolin is shown in Figure 7
. A Ca2+
concentration decrease in the solution was observed for 600 °C /2 h and 750 °C /2 h, with a rapid decrease up to 28 days and another, slower decrease on longer time scales. The pozzolanic reaction stabilized after 90 days, at which point the Ca2+
concentration remained almost constant in solution. In 750 °C /2 h conditions, Ca2+
content in the solution was lower for each reaction time.
At both temperatures, the rapid drop in Ca2+ concentration could be explained by the incorporation of the ions in these metastable structure phases, such as C-S-H gels and LDH-type compounds (phyllosilicate/carbonate), at short reaction times.
At longer time scales, i.e., 28, 90, and 360 days, metastable structures reorganized, evolving into stable phases. In these mechanisms, there was less calcium ion incorporation into the stable phases, which would explain the slower decrease in Ca2+ concentration in the solution. The activation at 750 °C/2 h enhanced the pozzolanic reaction, as reflected in the small Ca2+ content in the solution for all times.
When ZnO was added to the 600 °C/2 h and 750 °C/2 h systems, there was a slight decrease in Ca2+
(aq.) concentration in the solution, although the same trend was maintained regarding the evolution over time. The addition showed a similar behavior to Ca2+
(aq.) for Zn2+
(aq.), but with a lower content of Zn2+
in the solution (Figure 8
These results agree with those obtained using SEM, where the presence of ZnO favored the incorporation of calcium and zinc in the pozzolanic reaction products. This behavior may explain the decrease in Ca2+
(aq.), compared to the samples without Zn2+
, where Zn2+
had a similar behavior, but with a lower content of ions in the solution [40
]. The increment in temperature and reaction time increased these effects, since the reactivity of the activated phyllosilicates was major.
Aqueous silica and alumina provided additional information on the phase reaction with aluminosilicates. At both activation temperatures, the aqueous silica increased in solution up to 28 days and then decreased in all samples. However, the concentration range was still less than 0.1 mM, which is a very low value compared to the supposed content of reactive silica. These data suggest a rapid nucleation process of C-S-H gels on the reactive surface of MK, with little release of silica in the solution. The 750 °C/2 h conditions and the chemical activator presence did not seem to influence the behavior of aqueous silica (Figure 9
and Figure 10
At 600 °C /2 h and 750 °C /2 h, the aqueous alumina increased in solution during the first 28 days and remained almost constant in the 1.3/1.5 mM range on longer time scales. The presence of 1% ZnO did not influence the behavior of the alumina in solution. The evolution of the aqueous alumina over time and the high concentration in solution suggest, in addition to a strong degradation of the primary minerals, the difficulty of the species of aqueous alumina (AlO2− in the alkaline medium) to be incorporated into the secondary minerals formed.
Aqueous K+ concentration increased over time, suggesting its replacement by Ca2+ in the interlaminar region of the 2:1 phyllosilicates from natural kaolin and in the calcined materials. The K+ concentration evolution in solution suggests that the process was favored by temperature and reaction time. The K+ (aq.) content confirmed that it did not precipitate in any secondary form. Any soluble salt that may have been present in the initial solid sample was excluded, due to a correction made with the solutions through the reference solution. The presence of zinc helped the substitution of interlaminar potassium for calcium and zinc, which was incorporated into the 2:1 phyllosilicate structure. Aqueous Mg2+ and Na+ did not show relevant information since their concentration was very small.
Calcite dissolution/precipitation reactions determine the carbonate and bicarbonate concentrations in solution and, therefore, the contribution of these species to alkalinity. It is assumed that the content of calcite present in the sample can have an influence on the chemistry of the solution, basically buffering the pH and, as a Ca2+
source, partially balancing the positive charge in the solution. Moreover, it should be considered that the exposure of the solution to the atmosphere, produced during the manipulation of the samples, leads to the partial carbonation of the solution [28