Process Development of Fly Ash-Based Geopolymer Mortars in View of the Mechanical Characteristics
Abstract
:1. Introduction
2. Experimental and Numerical Analyses
2.1. Materials
2.2. Mix Proportion and Casting
2.3. Testing Procedure
3. Test Results and Discussion
3.1. Workability of Geopolymers
3.2. Bulk Density, Water Absorption, and Porosity
3.3. Compressive and Flexural Strengths
3.4. SEM, EDX, XRD, and TGA-DTA Investigations
4. Conclusions
- This study showed that geopolymer mortars can be manufactured through activation of locally available class F fly ashes (FA and FB) with different alkaline solution types and contents. The acceptable workability was classified as moderate based on the slump flow diameter (15 ± 2 cm), which was obtained for all geopolymer mixtures without adding superplasticizers.
- The chemical compositions of source materials for the geopolymers should be rich in silicon and aluminum oxides. These oxides in class F fly ash react with liquid alkaline to produce geopolymer mortars that bond the fine aggregates or unreacted particles. Considering the test results, the geopolymer mortars designed with FB had gave results than those of FA due to the ratio of silicon oxide (SiO2) to aluminum oxide (Al2O3) by mass of the raw material, which should preferably be in the range of 2.0 to 3.5 to produce geopolymers with better strength characteristics.
- The highest bulk density was 1985 kg/m3 for 40B-3-70, whereas 40A-3-100 gave the lowest value (1355 kg/m3). Considering FA and FB, the fineness or particle size distribution of F was one of the main parameters in terms of the physical characteristics of the geopolymer mortars, as well as the Na2SiO3/NaOH and L/S ratios. The bulk density was increased by the ratios of L/S and Na2SiO3/NaOH at low curing temperatures due to the geopolymerization reaction not being fully completed at such curing temperatures for the hardened mortars. However, curing at high temperature (100 °C) resulted in a reduction in the bulk density due to the expansion and tough structure of the geopolymer samples.
- The water absorption and porosity values changed between 10.07% and 17.98% and between 16.71% and 27.77%, respectively. Hardened geopolymer mortars produced with an L/S ratio of 0.4 resulted in higher values of water absorption and apparent porosity when compared to the ones produced with a lower L/S ratio based on the increases in the alkaline solution and water. However, regardless of the L/S ratio, the apparent porosity and the water absorption increased with the increasing ratio of Na2SiO3/NaOH for the samples cured at 100 °C due to the geopolymerization.
- The highest compressive and flexural strength values were found to be 60.1 MPa and 9.9 MPa for the mixture with the L/S ratio of 0.2 cured at 100 °C for 28 days. It can be concluded that using excessive activator solutions caused less alkali reaction, which weakened the bonding in the geopolymer matrix. On the other hand, the decrease in mechanical properties of geopolymers after using the optimal Na2SiO3/NaOH ratio was attributed to an increase in the coagulation of the silica.
- The heat curing process substantially affected the chemical reaction that occurs in the geopolymer mortar based on the type of fly ash. Higher curing temperatures resulted in better compressive strength for both fly ashes due to the improvement of kinetic energy, as well as the reaction degree, which caused the geopolymer mortars to have a stronger Al-Si-O network.
- Considering all of the test results, in order to obtain the best strength characteristics, the curing temperature, L/S ratio, and Na2SiO3/NaOH should be 100 °C, 0.2, and 2.5, respectively, for FA, whereas a curing temperature of 100 °C, L/S ratio of 0.2, and Na2SiO3/NaOH ratio of 2 were optimal for FB.
- SEM analysis indicated that the compact composite matrix was enhanced for 20FB-2-100, which had the highest compressive strength among the tested samples. The EDX analysis showed that FB with more soluble silica, increased temperature, and an increased Na2SiO3/NaOH ratio improved the mechanical performance by providing a better geopolymerization reaction. According to the XRD analysis, SiO2 and Al2O3 in F for the 20FA-1-70 mixture did not fully take part in the geopolymerization, which resulted in lower compressive strength. In addition, DTA peaks observed in the range of 30–165 °C supported the mechanical tests by showing that the N-A-S-H peak increased with the increased compressive strength.
- The production of fly-ash-based geopolymers not only provides an opportunity to use F as a sustainable green material via alkaline activation but also avoids the disposal problems associated with F. Additionally, this method contributes to the environment by decreasing CO2 emissions into the atmosphere, since it is a cement-free technology. Thus, geopolymers manufactured in this study with the desired strength characteristics can be used as brick building blocks, tiles, and prefabricated materials, as well as in infrastructure works.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Analysis Report (%) | FA | FB |
---|---|---|
CaO | 2.00 | 1.53 |
SiO2 | 54.08 | 62.28 |
Al2O3 | 26.08 | 21.46 |
Fe2O3 | 6.68 | 7.01 |
MgO | 2.67 | 2.37 |
SO3 | 0.73 | 0.07 |
K2O | 4.53 | 3.81 |
Na2O | 0.79 | 0.26 |
Others | 2.44 | 1.21 |
Loss on ignition (LOI) | 1.52 | 1.78 |
Specific gravity | 2.04 | 2.25 |
BET (m2/g) | 1.11 | 2.26 |
Fly Ash (g) | Sand (g) | Liquid/Solid Ratio | Na2SiO3/NaOH Ratio | NaOH (g) | Na2SiO3 (g) | Total Water/Solid Ratio |
---|---|---|---|---|---|---|
600 | 600 | 20% | 1.0 | 120 | 120 | 0.18 |
1.5 | 95 | 144 | ||||
2.0 | 80 | 160 | ||||
2.5 | 68 | 172 | ||||
3.0 | 60 | 180 | ||||
40% | 1.0 | 240 | 240 | 0.20 | ||
1.5 | 192 | 288 | ||||
2.0 | 160 | 320 | ||||
2.5 | 136 | 344 | ||||
3.0 | 120 | 360 |
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Öz, H.Ö.; Doğan-Sağlamtimur, N.; Bilgil, A.; Tamer, A.; Günaydin, K. Process Development of Fly Ash-Based Geopolymer Mortars in View of the Mechanical Characteristics. Materials 2021, 14, 2935. https://doi.org/10.3390/ma14112935
Öz HÖ, Doğan-Sağlamtimur N, Bilgil A, Tamer A, Günaydin K. Process Development of Fly Ash-Based Geopolymer Mortars in View of the Mechanical Characteristics. Materials. 2021; 14(11):2935. https://doi.org/10.3390/ma14112935
Chicago/Turabian StyleÖz, Hatice Öznur, Neslihan Doğan-Sağlamtimur, Ahmet Bilgil, Aykut Tamer, and Kadir Günaydin. 2021. "Process Development of Fly Ash-Based Geopolymer Mortars in View of the Mechanical Characteristics" Materials 14, no. 11: 2935. https://doi.org/10.3390/ma14112935
APA StyleÖz, H. Ö., Doğan-Sağlamtimur, N., Bilgil, A., Tamer, A., & Günaydin, K. (2021). Process Development of Fly Ash-Based Geopolymer Mortars in View of the Mechanical Characteristics. Materials, 14(11), 2935. https://doi.org/10.3390/ma14112935