1 Introduction
2 Experimental Procedures
2.1 Materials
2.2 Sample Preparation and Test Plan
NaOH concentration (mol L−1) | Solution volume (mL) | Fly ash samples (g) | Soaking time (h) | |||||
---|---|---|---|---|---|---|---|---|
3 | 6 | 12 | 24 | 48 | 72 | |||
0 | 400 | 100 | A1 | A2 | A3 | A4 | A5 | A6 |
0.25 | 400 | 100 | B1 | B2 | B3 | B4 | B5 | B6 |
0.75 | 400 | 100 | C1 | C2 | C3 | C4 | C5 | C6 |
1.25 | 400 | 100 | D1 | D2 | D3 | D4 | D5 | D6 |
2 | 400 | 100 | E1 | E2 | E3 | E4 | E5 | E6 |
2.3 Ion Leaching Test
2.4 Test of the pH Value of the Leaching Solution
2.5 XRD Measurement
2.6 FTIR Measurement
2.7 SEM Measurement
3 Results and Analysis
3.1 Ion Leaching Behavior
3.1.1 Leaching Behavior of Si4+
3.1.2 Leaching Behavior of Al3+
3.1.3 Leaching Behavior of Ca2+
3.1.4 Analysis of the Differences in the Leaching Behaviors of Si4+, Al3+ and Ca2+
Active ions | Time (h) | 0.25 mol L−1 | 0.75 mol L−1 | 1.25 mol L−1 | 2 mol L−1 |
---|---|---|---|---|---|
Si4+ | 3 | 4.21 | 7.37 | 12.42 | 13.68 |
6 | 8.33 | 12.50 | 22.08 | 29.17 | |
12 | 12.50 | 16.67 | 29.17 | 37.50 | |
24 | 19.79 | 26.04 | 45.63 | 58.33 | |
48 | 23.96 | 35.42 | 64.58 | 89.06 | |
72 | 26.04 | 39.58 | 76.04 | 107.29 | |
Al3+ | 3 | 26.67 | 48.33 | 95.00 | 106.67 |
6 | 50.00 | 84.76 | 139.52 | 170.48 | |
12 | 64.89 | 91.11 | 184.44 | 215.56 | |
24 | 85.22 | 117.39 | 202.61 | 252.17 | |
48 | 111.30 | 164.35 | 241.74 | 310.87 | |
72 | 120.87 | 170.43 | 258.26 | 334.78 | |
Ca2+ | 3 | − 94.9 | − 94.9 | − 94.9 | − 94.9 |
6 | − 94.4 | − 95.6 | − 96.3 | − 95.0 | |
12 | − 94.5 | − 95.1 | − 95.1 | − 95.1 | |
24 | − 92.4 | − 92.9 | − 92.9 | − 92.4 | |
48 | − 91.8 | − 93.0 | − 93.6 | − 92.4 | |
72 | − 92.4 | − 92.9 | − 92.9 | − 93.5 |
3.2 Changes in the pH of the Leaching Solution
3.3 Changes in the Mineral Composition of the Fly Ash Residues
3.4 Changes in the Functional Groups of the Fly Ash Residues
3.5 Changes in the Surface Morphology of the Fly Ash Residues
4 Reaction Mechanism During Alkali Activation of Low-Calcium Fly Ash
4.1 Dissolution Stage
Oxides | Chemical reaction equations | \( \Delta_{r} G_{m}^{\varTheta } (298.16 \rm {K}) \) (kJ mol−1) | |
---|---|---|---|
Alkaline oxides |
\( {\text{CaO}}(s) + {\text{H}}_{ 2} {\text{O}} \to {\text{Ca}}^{{ 2 { + }}} (aq) + 2{\text{OH}}^{ - } (aq)\)
| (3) | − 26.76 |
\( {\text{Ca}}^{2 + } (aq) + 2{\text{OH}}^{ - } (aq) \to {\text{Ca(OH)}}_{2} (s)\)
| (4) | − 30.00 | |
\( {\text{MgO}}(s) + {\text{H}}_{ 2} {\text{O}} \to {\text{Mg}}^{2 + } (aq) + 2{\text{OH}}^{ - } (aq) \)
| (5) | − 37.79 | |
\( {\text{Mg}}^{2 + } (aq) + 2{\text{OH}}^{ - } (aq) \to {\text{Mg(OH)}}_{2} (s) \)
| (6) | − 76.45 | |
Acid oxides |
\( {\text{SiO}}_{2} (s) + {\text{H}}_{ 2} {\text{O}} + {\text{OH}}^{ - } (aq) \to {\text{SiO(OH)}}_{3}^{ - } (aq)\)
| (7) | − 26.25 |
\( {\text{Al}}_{ 2} {\text{O}}_{ 3} (s) + 3{\text{H}}_{ 2} {\text{O}} + 2{\text{OH}}^{ - } (aq) \to 2{\text{Al(OH)}}_{4}^{ - } (aq) \)
| (8) | − 41.86 |