This paper investigated the possibility of red mud as a raw material for producing cementless blended binder for strong brick production using a Ca(OH)2-Na2CO3-activated fly ash binder system. This study also presented several ways of enhancing the flowability of the fresh mixture and compressive strength development in the system. Overall, although the replacement of red mud up to 30% largely reduced early strengths and flowabilities compared to those of the original binder without red mud, the 7- and 28-day strengths were still high enough to be used as a structural binder for brick production. This study also demonstrated that the use of NaOH was very effective in increasing flowability and early strength in the red mud-CNF blended system despite the red mud inclusion because NaOH greatly improved the reaction degree of the system, resulting in more C-S-H formation as well as significant pore size refinement, at early days. The detailed conclusions made were the following:
1. The red mud in this study mainly consisted of Fe2O3, Al2O3, and SiO2 (SiO2 + Al2O3 + Fe2O3 > 75.7 wt. %). Compared to other sources of red muds in literature, the red mud in this study had relatively small Fe2O3 and CaO contents, but rather large contents of Al2O3, SiO2, and Na2O. Main mineral phases in this red mud were hematite, quartz, natrodavyne, sodalite, anatase, and boehmite. Overall particle size of red mud was much smaller than that of fly ash in this study.
2. As contents of red mud increased, flowability (or mixability) showed a tendency to considerably decreased and the compressive strength disployed a tendency to decrease in this study; however, as curing days progressed, the decreased strength was recovered to almost the same strength as that of the sample without red mud.
3. The higher water content improved flowability (or mixability), but it largely lowered 28-day strength; however, it enhanced the early compressive strength at 3 days mainly due to the improved mixability.
4. The addition of NaOH largely increased the flowability and it improved early strengths of the samples with red mud before 7 days, but the strength improvement was nearly lost at 28 days. The addition of 5% NaOH increased the strength more than 3 times, and even 1% NaOH addition
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showed approximately 1.5 times higher early strength at 3 days than that of the sample with 0%
NaOH addition.
5. The XRD and TGA results showed that C-S-H was the main reaction product and no geopolymeric product was found in all the samples. As more red mud was incorporated, the less Ca(OH)2 was consumed, resulting in less C-S-H formation. However, by adding NaOH, the reaction degree was greatly improved at early days despite the inclusion of red mud; Ca(OH)2 was rapidly consumed and more C-S-H quickly formed.
6. In MIP, after red mud was included, fine red mud particles instantly removed pores with a size range of 0.5-3µm by the filling effect; however, the fraction of smaller sizes of pores below 0.5 µm and total porosity were somewhat increased.
7. The increased w/b temporarily produced a decreasing effect on the total porosity at 3 days, probably due to the increased paste mixability, resulting in slightly improved 3-day strength;
however, the total porosity turned was the largest among samples at 28 days.
8. The addition of NaOH had a great impact on the 3-day pore size distribution as it generated remarkable pore-size refinement, which, however, disappeared at 28 days.
9. The BSE images on the polished samples showed that all the samples similarly possessed heterogeneous microstructures with numerous particles of red mud and unreacted fly ash. The line elemental scanning results of all the tested samples confirmed that major reaction product was C- S-H with Al substitution.
10. In the elemental mapping results on raw red mud and red mud treated with NaOH, there were significant changes in the elemental distributions of Ca, Ti, and Fe after adding NaOH; all of these elements were widely dispersed and it would be more advantageous to have a rapid reaction. This suggests that adding NaOH into red mud slurry helped the elements of the red mud to be well dispersed; as a result, strength development at early age could be enhanced through improved reaction degree.
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11. Trial bricks were prepared using a representative mixture, RM2840N5, to examine a possible toxicity of produced brick product using the TCLP test. All targeted toxic elemental concentrations were significantly lower than the required concentrations.
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