Synthesis of structural binder based on red mud and fly ash activated with Ca(OH) 2 and Na 2 CO 3. Synthesis of structural binder for brick production based on red mud and fly ash activated with Ca(OH)2 and Na2CO3. Test bricks were also prepared to examine potential toxicity due to the inclusion of red mud and fly ash using the Toxic Characteristic Leaching Procedure (TCLP) test.
INTRODUCTION
Research Backgrounds
Consequently, the red mud technologies developed in one nation may not be suitable for use in others. Therefore, it has been difficult to determine a universal way to consume large volumes of red mud worldwide.
Research Objectives
MATERIALS AND METHODS
Red mud and fly ash
Preparation of samples
After four weeks, the samples were dried in a vacuum desiccator for two days to eliminate any residual solvent inside the samples. The cut samples were impregnated with EPO-TEK epoxy resin under vacuum at room temperature for one day. To investigate any possible influence of the use of NaOH on morphology or material characteristics of raw red mud particles during the mixing process, two additional samples were prepared for SEM BSE analysis: (1) raw red mud (denoted RM) and (2) red mud treated with NaOH (denoted RM+NaOH).
Testing of samples
A leaching test was performed on the prepared brick samples according to the toxicity characteristic leaching procedure (TCLP) of the US Environmental Protection Agency (EPA) [44]. Inductively coupled plasma optical emission spectroscopy (ICP-OES) (Varian 700-ES, Walnut Creek, USA) was used to determine the concentrations of As, Ba, Cd, Cr, and Pb. To determine the water absorption of the produced bricks, which is an important property of brick product, the 7-day cured bricks were immersed in water for 24 hours at 22°C.
RESULTS AND DISCUSSION
Characterization of raw red mud and fly ash
As the distribution curves show, the mean particle size of red mud was noticeably smaller than that of fly ash.
Flowability and compressive strength development
Whereas, the addition of NaOH produced more of an improvement in fluidity than increasing the w/b ratio. For example, the RM2840N5's paste overflowed the measurement table in the mini-drop test (see RM2840N5 in Figure 4). All the compressive strength test results of the hardened paste samples are summarized in Figure 6 and Table 3.
As the data show, the compressive strength developed a tendency to decrease when more red mud was incorporated; however, its reducing effect decreased as healing days increased. It is well known that the higher water content usually causes the lower compressive strength [53]. However, when the fluidity of fresh pasta during mixing is very low, the addition of water can be beneficial for the compressive strength, possibly due to the increased miscibility.
As shown in Figure 6(b), after increasing the water content from w/b = 0.40 to 0.44 and 0.48, not only the miscibility of the mixtures was improved, but also the early strengths after 3 days increased slightly compared to that of w/b = 0.40. However, the compressive strengths after 28 days were influenced more by the negative effect of an increased w/b ratio than by the degree of improved miscibility. The addition of NaOH largely improved strength development after 3 and 7 days, but the improvement virtually disappeared after 28 days.
XRD
Mineralogical phases of these samples were very similar to those of RM0040N0; C-S-H and calcite were observed and residual calcium hydroxide was identified. Although, similar to class F fly ash, red mud did not have the Ca content to produce C-S-H, the C-S-H formation was observed in these samples due to the significant use of Ca(OH)2. Overall, in the XRD patterns of all the samples with red mud, the phases at 3 days were also found at 28 days; however, one clear difference between 3- and 28-day cured samples was the peak reduction of Ca(OH)2 in XRD (Figure 8).
The reflection intensities of Ca(OH)2 in these samples at 28 days were significantly smaller than those at 3 days; this implies that Ca(OH)2 has been consumed in the curing process. In this study, C-S-H and calcite were possible candidates that could be generated by the consumption of Ca(OH)2 in the curing process [37]. Thus, the different rate of Ca(OH)2 reduction implies the different extent of C-S-H formation after 3 days.
The degrees of reaction could be compared between samples by comparing the intensities of Ca(OH)2 reflections in XRD on a specific curing day. In fact, RM2840N5 had a lower content of Ca(OH)2 than that of the other samples in its original mixing ratio because NaOH was further added to RM2840N0 (Table 1); thus, the smallest peaks of Ca(OH)2 in RM2840N5 after 3 days may not be abnormal; even considering the initial content of Ca(OH)2, the reflection intensities of Ca(OH)2 of RM2840N5 were much smaller than those of the other samples after 3 days, and therefore more C-S-H was probably formed in RM2840N5 after 3 days. This interpretation was also consistent with the strength test results, which showed a 2.6 times higher strength of RM2840N5 compared to the strength of RM2840N0 after 3 days.
From this observation, we can conclude that the addition of NaOH promoted the early formation of C-S-H by rapidly consuming Ca(OH)2.
TGA
Thus, the relative weights of C-S-H, Ca(OH)2, and CaCO3 can be approximately compared using their DTG peak areas. In all samples, the DTG peaks of C-S-H largely increased from 3 to 28 days, while the Ca(OH)2 peaks significantly decreased. This observation supports the previous conclusion that Ca(OH)2 was mainly consumed during the reaction (i.e. C-S-H formation) after 3 days [57].
This indicates that CaCO3 can be formed immediately by the reaction of Na2CO3 with Ca(OH)2 before 3 days, but, after 3 days, CaCO3 was consumed by the chemical reaction (probably the formation of C-S-H) by 28 days. Note that RM0040N0 exhibited the largest DTG peak of Ca(OH)2 remaining at 28 days, despite its greatest strength development (most likely due to large C-S-H formation); however, this is not anomalous because the RM0040N0 mixture contained the highest initial Ca(OH)2 content in the mixture proportion (Table 1). In particular, when the 3-day DTG peaks were compared, the significant difference of C-S-H content was easily discernible between these two samples.
In addition, the DTG peak size of Ca(OH)2 was also noticeably smaller in RM2840N5 compared to that of RM2840N0. These observations are consistent with the XRD results that the addition of NaOH increases the degree of reaction [i.e. greater extent of C-S-H formation and Ca(OH)2 consumption] at an early age, and consequently a higher early compressive strength was achieved in RM2840N5. .
MIP
However, RM2848N0 was the only sample whose total porosity increased from 3 to 28 days, and this increased porosity was related to the lowest strength of RM2848N0 because it was the highest value among the porosities measured in this study. The comparison of 28-day pore size distributions between RM2840N0 and RM2840N5 in Figure 10 revealed that the use of NaOH only insignificantly affected the pore size distribution and total porosity after 28 days. However, the NaOH addition had a large impact on the 3-day pore size distribution because it significantly reduced the total pore sizes; this pore size refinement should be related to the highest 3-day strength of RM2840N5 (~26.4 MPa) among the samples.
As mentioned earlier, there was little change in the pore size distribution of RM2840N5 between 3 and 28 days. Despite the small temporal change in pore sizes in RM2840N5, the strength increased ~33% from 3 to 28 days. Thus, the influential degree of pore size refinement was not solely decisive for strength development, although it was significant.
SEM
Although the samples with red mud had high fractions of the heavy element Fe due to the high Fe2O3 content of raw red mud (~31.2 wt% Fe2O3 in Table 2), these samples were not brighter than the sample without red mud. Sodium, magnesium, potassium, calcium, aluminium, silicon, iron and titanium were selected for line scanning based on the XRF results of raw red mud and raw fly ash. In RM0040N0 [Figure 12(a)], the concentrations of the main elements (Ca, Al, and Si) were proportionally related to each other in the line scan result on matrix.
On the other hand, although Ca, Al, and Si in RM2840N0 [Figure 12(b) ] were not as strongly proportional in the matrix as those in RM0040N0, they still showed proportionality. In the line scan result of RM2848N0 [Figure 12(c)], there was an area of red mud at the left end of the scanned line, and therefore the other area apart from red mud (i.e., matrix) should be used for interpretation. In the matrix region of RM2840N5 in Figure 12(d) , Ca, Al and Si were also very proportional to each other similar to the other samples, implying C-S-H formation, but the concentration of Na was much higher than in the other samples, because further NaOH was incorporated into the samples.
Thus, the use of NaOH during mixing was unlikely to deform the red mud particle shapes. However, there were significant changes in the elemental distributions of Ca, Ti and Fe after NaOH addition. This visible dispersion of the elements is probably due to the increased fluidity of the fresh RM+NaOH paste, because its precondition is an increased fluidity that allows good mixing of these elements.
Thus, we can conclude that the addition of NaOH to the red mud helped the elements to be well distributed, and as a result,
Trial brick production and tests
Conclusion
As more red mud was incorporated, the less Ca(OH)2 was consumed, resulting in less C-S-H formation. The BSE images of the polished samples showed that all the samples similarly had heterogeneous microstructures with numerous particles of red mud and unreacted fly ash. Recovery of alumina and iron oxide from iron-rich Bayer red mud by reduction sintering.
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![Table 2 shows the oxide chemical compositions of raw red mud and fly ash. Usually, lists of chemical elements in red muds are generally similar regardless of source locations, but their proportions differ widely from source to source [36, 46]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10529050.0/20.892.222.670.594.823/chemical-compositions-usually-chemical-generally-regardless-locations-proportions.webp)
![Figure 3: TGA and derivative thermogravimetry (DTG) curves of raw red mud. Each label indicates possible weight loss due to decomposition of boehmite (BM) [48], natrodavyne (ND) [49, 50], calcite (Cal) [37, 51], and gibbsite (GB) [52]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10529050.0/23.892.170.721.130.482/derivative-thermogravimetry-indicates-possible-decomposition-boehmite-natrodavyne-gibbsite.webp)
