3.1. Hydrothermal Treatment of Biomass Fly Ash
The hydrothermal treatment was applied to three samples, the abbreviations of which are presented in
Table 2. The isothermal holding time at 200 °C for all samples was chosen at 2, 4, 8, and 24 h.
It was determined that in all the biomass fly ash samples, after 2 h of hydrothermal treatment, tobermorite (
d-spacing: 1.130, 0.548, 0.308, 0.298, 0.282, 0.184) was obtained (
Figure 3). By prolonging the duration of synthesis to 4, 8, and 24 h, the intensities of the tobermorite diffraction peaks increase only slightly (
Figure 3, curves 4–24 h). Meanwhile, the intensities of the diffraction maxima characteristic of quartz also slightly decreased. It should be noted that only one other primary crystalline compound, calcium carbonate, remained in the synthesis products under all experiment conditions. This finding shows that the hydrothermal treatment of biomass fly ash results in a product consisting only of quartz, calcite, and calcium silicate hydrates (tobermorite).
This affirmation was confirmed by the results of the simultaneous thermal analysis (
Figure 4,
Table 3). Because the curves of the DSC analysis of all the samples are closely similar as in the case of the XRD analysis, only the graphs after 2 and 24 h of synthesis are presented.
In all the DSC curves of the samples treated for 2 h under hydrothermal conditions (
Figure 4a), four endothermal effects are visible. Three effects describe the thermal changes in materials identified by the XRD method. The endothermal effect with a maximum at 132 °C is attributed to the removal of absorptive/structural water from the synthesis products, which is also characteristic of the partial decomposition of tobermorite [
26]; a peak at 573 °C describes the recrystallization of quartz from alpha to beta modification; and a peak at 730 °C is attributed to the decomposition of CaCO
3. In addition to these described peaks, another non-intense endothermic effect is visible in the temperature range of 350–390 °C in the DSC curves of all samples. This peak is typical for the decomposition of hydrogarnets with varying degrees of silicate substitution [
27], which, due to its amorphous form, was not identified by the XRD method.
The main difference between the profiles of the DSC curves is the exothermic effect at ~845 °C, which appears in the curve of sample AR1.5. This peak is indicative of the transformation of semi-crystalline or amorphous calcium silicate hydrates with a lower CaO/SiO
2 ratio (C-S-H(I)) into wollastonite [
28]. This shows that not only crystalline but also amorphous calcium silicate hydrates can form in the synthesis products, which are not identified by the XRD method. After 24 h of synthesis, the peak of recrystallization of these amorphous compounds into wollastonite is already visible in the DSC curves of all samples (
Figure 4b). The earlier formation of amorphous calcium silicate hydrates in sample AR 1.5 is apparently related to the nature of the starting materials. This sample contains the largest amount of R-type fly ash, in whose XRD curve (
Figure 1) amorphous compounds were identified.
Since the target synthesis product was calcium silicate hydrates,
Table 3 presents the TG analysis data for the mass loss of all hydrothermally treated samples at temperatures between 50 and 200 °C.
As can be seen from the provided data, calcium silicate hydrates already formed after 2 h of hydrothermal treatment. By extending the synthesis time to 24 h, the amount of new calcium silicate hydrates formed increased slightly. The highest mass losses during the decomposition of calcium silicate hydrates were recorded in the AR1 sample and the lowest were recorded in the AR 1.5 sample.
On the other hand, 2 h of hydrothermal treatment at 200 °C was a sufficient amount of time for the formation of calcium silicate hydrates. The slightly higher amount of these compounds, formed by extending the duration of synthesis, does not compensate for the energy costs required for synthesis.
In addition to the study of the composition of the synthesis products, a study of the chemical composition of the liquid medium remaining after the filtering of the synthesis products was carried out. The data of the ICP-OES analysis results after 2 h of hydrothermal treatment are presented in
Table 4.
As can be seen from the data in
Table 4, the most soluble components—SO
3, K
2O, Na
2O, and CaO—were mostly transferred to the liquid medium during hydrothermal synthesis. The concentration of all other components, including heavy metals, in the liquid medium was extremely low. The total amount of leached materials from ash during the hydrothermal treatment was the highest in sample S1, which contained the most K
2O.
It should be noted that due to the small amount of components transferred to the liquid medium, this medium can be reused for synthesis. After several stages of synthesis, the liquid medium can be easily purified with anionic and cationic sorbents. Therefore, it can be concluded that the hydrothermal biomass fly ash processing method is a sustainable method that allows the prevention of environmental pollution.
3.2. The Influence of Synthesized Additive on Portland Cement Hydration and Hardening
The cement paste samples were formed by substituting 5–15 wt.% of ordinary Portland cement with synthesized additives (S1, AR1, and AR1.5).
Table 5 displays the mixing ratios and principal properties of the cement paste samples.
As depicted in
Table 5, the water requirement to achieve normal consistency in cement pastes containing additives exceeds that of pure cement pastes. This phenomenon is ascribed to the sorption properties of the synthesized additives, which prompt a heightened water demand. All samples with additives exhibited slightly longer setting times (except the initial setting time of sample 10S1) compared to the pure Portland cement paste. The longer setting times in samples with additives could be related to the elevated sulphate content (
Table 1) in biomass fly ash.
For an estimate of the influence of synthesized additive on the early hydration process of Portland cement, an isothermal calorimetry test was performed. The test was carried out with a pure Portland cement sample and samples in which 10% by weight of Portland cement was replaced by additives S1, AR1, and AR1.5. The test results are presented in
Figure 5.
The highest intensity of heat flow was identified in the Portland cement without additives. Meanwhile, in samples with additives, the induction period came earlier than in the OPC sample and was significantly shorter, especially in samples 10S1 and 10AR1 (
Figure 5a). In these samples, the second peak of heat emission, related to the hydration of calcium silicates, also appeared in a shorter time period from the start of hydration. Meanwhile, the second peak of heat evolution in the AR1.5 sample was reached at the same time as in the OPC sample. The shifting of heat flow curves with additives to the side of shorter durations indicates the acceleration of C
3S hydration in the early period. However, this acceleration does not compensate the reduction in the degree of the C
3S hydration reaction, because in the samples with additives, less C
3S reacts, and more heat of hydration is released from the sample cement without additives (
Figure 5b).
The XRD analysis curves of the samples hydrated for 2 and 28 days are presented in
Figure 6. In all samples, regular cement hydrates were formed, ettringite (PDF 41-1451) and portlandite (Ca(OH)
2) (PDF 84-1271). In addition, unhydrated calcium silicates (C
3S, C
2S) (PDF 42-551) and calcite (CaCO
3) (PDF 5-586) were detected in the samples. Although the XRD curves of the samples after 2 days of hydration (
Figure 6a) appear similar, a disparity can be observed in the intensities of the portlandite peaks and unhydrated calcium silicates. The samples containing additives exhibit higher intensities of portlandite and lower intensities of unhydrated calcium silicates compared to the pure cement sample. The same trend persists after 28 days of hydration (
Figure 6b); however, the differences in the peak intensities mentioned in these XRD curves are not so clear.
The outcome of the simultaneous thermal analysis is presented in
Figure 7 and
Table 6. After 2 days of hydration, as well as after 28 days of hydration, the profiles of the DSC curves of all samples are very similar; only the intensities of the peaks are slightly different. Three endothermic peaks are observed in the temperature range from 30 to 950 °C (
Figure 7). The peak at 50–220 °C is associated with the split of the main cement hydrates (C-S-H, ettringite, calcium aluminate hydrate, hydrogarnets), the peak at ~450 °C identifies the decay of portlandite, and the peak at 650–750 °C indicates the decomposition of calcite [
29]. After 28 days of hydration, the character of the DSC curves remains the same (
Figure 7b), except for one notable difference. Another weak endothermic effect appears in the temperature range of 350–390 °C in the DSC curves of all samples with additives. This effect can also be observed in the DSC curves of samples after hydrothermal treatment (
Figure 4a). As mentioned above, this peak is typical of the decomposition of hydrogarnets, and this fact shows that the investigated additives also promote the formation of hydrogarnets in cement.
In
Table 6, the total mass loss of the samples at 50–220 °C is presented, excluding the mass loss of the additive itself because part of the additive loses its mass at this temperature due to the dehydration of C-S-H. After 2 days of hydration, the mass loss of the sample without additive and the samples with 10% by weight of the additives S1 and AR1.5 was the same at this temperature, while the mass loss of the sample with the AR1 additive was higher. This indicates that, even with a lower amount of cement in the reaction, the samples with all additives produced the same or a higher amount of main cement hydrates. The same trend could be seen after 2 days of hydration in the amount of mass loss of the samples during portlandite decomposition at 450 °C. The mass loss of the samples with additives S1 and AR1.5 was greater than the pure cement sample, while the mass loss of the sample with the AR1 additive was slightly smaller. In summary, it can be said that all the additives promote the hydration of calcium silicates, but in the samples with additives S1 and AR1.5, more portlandite is formed, and in the AR1 sample, more calcium silicate hydrates are formed.
After 28 days of hydration, this trend is no longer observed. The highest mass losses at temperatures of 50–220 °C were determined in the sample without additives and in sample 10S1; however, the highest mass losses at ~450 °C remained in sample 10AR1.5. Therefore, it can be said that during the initial hydration period (up to 2 days), the investigated additives promote the hydration of Portland cement, but as the duration of hydration increases to 28 days, this influence becomes less noticeable.
Figure 8 summarizes the data of the compressive strength of the Portland cement samples for different durations of hydration and different additives.
Following 2 days of hydration, the compressive strength of the samples incorporated with 5 wt.% of various additives was similar to that of pure cement (24.60 MPa), ranging from 24.55 to 24.60 MPa. However, with increasing additive content up to 15 wt.%, the compressive strength of all samples with additives decreased compared to that of the pure cement sample, regardless of the specific additive used. However, it should be noted that the compressive strengths of all samples after 2 days of hardening meet the requirements of the EN 197-1:2011 standard [
30] and are greater than 10 MPa. After 28 days of curing, all samples with 5 wt.% of different additives showed slightly higher compressive strengths (44.2–44.8 MPa) than the Portland cement sample (44.1 MPa) and met the requirements of the mentioned standard for cement 42.5 N. The requirements of this standard are also met by the compressive strength of sample with 10 wt.% of AR1.5 additive (42.9 MPa). The compressive strengths of all other samples with additives decreased in direct proportion to the amount of additive added and were lower than 42.5 MPa.
In summary of the findings of this section, it can be concluded that knowing the volume of cement production, substituting 5% to 10% of the weight of cement with biomass fly ash after hydrothermal treatment allows the sustainable utilization of a significant portion of this waste.