Understanding the Effect of Waiting for the Dissolution of Sodium Hydroxide in Geopolymer Concrete Mixes
Abstract
:1. Introduction
2. Materials
3. Methodology
3.1. Mix Design
3.1.1. Approach 1: Optimizing the Preparation of the Alkaline Activator
3.1.2. Approach 2: Optimizing the Mixing Order of the Geopolymer Concrete (GPC) Ingredients
3.2. Alkaline Activator Preparation
3.3. Geopolymer Concrete Specimen Preparation and Testing Methods
4. Results and Discussion
4.1. Alkaline Activator Temperature (Approach 1)
4.2. Workability
4.3. Unconfined Compressive Strength
5. Conclusions
- ▪
- The use of an alkaline activator is crucial for the geopolymerization process; its temperature at the time of mixing needs to be controlled to ensure proper reaction kinetics, essential for achieving optimal compressive strength in geopolymer concrete.
- ▪
- The temperature of the SH solution increased significantly after adding SF due to the start of the exothermic chemical rection to form the new bonds of the sodium silicate alternative and water.
- ▪
- All mixes exhibited satisfactory workability properties ranging between 150 mm and 260 mm, except those that did not contain the NaOH solution (SD0HV and 1W-SG).
- ▪
- High segregation was shown by mix (SD0HV), where no alkaline activator preparation was required beforehand; instead, the geopolymer ingredients were added to the mix in various orders.
- ▪
- Mixing the total water content with the NaOH pellets, GGBS, and SF caused mix (1W-SG) to fail at the first stage (during chemical preparation) due to the early start of the hydration process, with the presence of GGBS aided by the heat generated from the SH dissolution in water.
- ▪
- The highest slump value was achieved in mix (SD3H), which contained an alkaline activator with a temperature equivalent to room temperature after cooling for 2 h.
- ▪
- The alkaline activator used to prepare GPC with acceptable fresh and hardened properties does not necessarily need 24 h to fully reach; it can be used when it has cooled and reached room temperature.
- ▪
- An inverse relation was observed between the alkaline activator temperature at mixing time and the compressive strength value—as the alkaline activator temperature decreased, the compressive strength value increased.
- ▪
- The mixes consisting of alkaline activators achieved the highest compressive strength values despite the varying concentrations of NaOH and SF in the mix.
- ▪
- The compressive strength values of all mixes continued to increase with time.
- ▪
- Up to 64% of the compressive strength was achieved at 7 days due to the relatively high content of MgO in the GGBS, which contributed to the formation of the hydrotalcite (Mg6Al2(OH)16CO3), which plays a significant role in the early strength development of slag-based geopolymer concrete (Wang et al., 2020) [32].
- ▪
- Mix G-(0.5W-S) (which was formulated by adding the aggregates + GGBS, then half of the water mixed with NaOH pellets + SF, and then the other half of water) was considered the optimized mix in terms of its mechanical performance and properties.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Oxide | Composition (wt %) | |
---|---|---|
GGBS | SF | |
SiO2 | 35.54 | 97.1 |
Al2O3 | 11.46 | 0.2 |
Fe2O3 | 0.42 | 0.1 |
Na2O | 0.37 | - |
CaO | 37.99 | 0.2 |
MgO | 8.78 | 0.1 |
K2O | 0.43 | 0.2 |
TiO2 | 0.7 | - |
V2O5 | 0.04 | - |
P2O5 | 0.02 | 0.03 |
SO3 | 1.54 | 0.1 |
Mn2O3 | 0.43 | - |
BaO | 0.09 | - |
L.O.I | 2.00 | 0.5 |
Property | GGBS | SF |
---|---|---|
Specific gravity | 2.9 | 3.15 |
Bulk density (kg/m3) | 1200 | 300 |
Insoluble residue | 0.3 | - |
Glass content (%) | 90 | - |
Blaine fineness (m2/kg) | 450 | - |
Alkalinity value (pH) | 10.4 | 7 |
Color | Off-white | Grey |
Physical form | Fine powder | powder |
Physical Properties | Coarse Aggregate | Fine Aggregate (Sand) | |
---|---|---|---|
20 mm | 10 mm | ||
Uniformity coefficient (CU) | 1.3 | 3.3 | 0.11 |
Curvature coefficient (CC) | 7.5 | 1.5 | 1.75 |
Flakiness index (%) | 23 | 30–35 | - |
Elongation index (%) | 12 | 17–22 | - |
Shape index (%) | 7 | 12 | - |
Impact value | 15 | 23 | - |
Fineness modulus (mm) | - | 4 | 1.54 |
Uncompacted bulk density (kg/m3) | 2570 | 1350 | 1500 |
Pre-dried particle density (kg/m3) | - | 2690 | 2600 |
Water absorption (%) | 1.1 | 2 | 21 |
Mix | Description |
---|---|
PD | The alkaline activator was prepared 24 h prior to the mixing day. |
SD0H | Reducing the activator preparation period from the previous day (24 h) to same day mixing, immediately before mixing without cooling. |
SD3H | Reducing the activator preparation period from the previous day (24 h) to same day mixing, up to 3 h prior to mixing. |
SD3HF | Reducing the activator preparation period from the previous day (24 h) to same day mixing, aided by cooling in the freezer for up to 3 h. |
SD0HV | Reducing the activator preparation period from the previous day (24 h) to adding all activator ingredients in various orders during mixing. |
Mix Design | Alkaline Activator | Precursor (kg) | Sand (kg) | Aggregate (kg) | Water (W3) (L) | |||||
---|---|---|---|---|---|---|---|---|---|---|
SSA | SH Sol. (kg) | |||||||||
SH Sol. (mL) | SF (kg) | |||||||||
Water (W1) (L) | SH Pellets (kg) | Water (W2) (L) | SH Pellets (kg) | 10 mm | 20 mm | |||||
PD, SD0H, SD3H, SD3HF | 0.455 | 0.182 | 0.281 | 0.455 | 0.182 | 2.67 | 8 | 4 | 8 | 1.29 |
SD0HV | 0 | 0 | 0.281 | 0 | 0.364 | 2.67 | 8 | 4 | 8 | 2.2 |
Mix | Description |
---|---|
PD | The control mix, the same control mix prepared in Approach 1, consisted of an alkaline activator prepared 24 h prior to the mixing day. |
0.5W-SG | Investigating the effect of changing the addition order of the GPC ingredients, starting by adding the aggregates, then the NaOH solution (half water amount + NaOH pellets), and then the mixture of SF and GGBS with the other half of water. |
1W-SG | Investigating the effect of changing the addition order of the GPC ingredients, starting by adding the aggregates and then a solution of the total amount of water + NaOH pellets + GGBS + SF. |
G-(0.5W-S) | Investigating the effect of changing the addition order of the GPC ingredients, starting by adding the aggregates + GGBS, then half of the water mixed with NaOH pellets + SF, and then the other half of water. |
G-1W-S | Investigating the effect of changing the addition order of the GPC ingredients, starting by adding the aggregates + GGBS to the NaOH solution (total amount of water + NaOH pellets) and then adding SF at the end. |
G-(1W-S) | Investigating the effect of changing the addition order of the GPC ingredients, starting by adding the aggregates + GGBS, then the slurry (total amount of water + NaOH pellets + SF). |
Mix Design | Alkaline Activator | Precursor (kg) | Sand (kg) | Aggregate (kg) | Water (W3) (L) | |||||
---|---|---|---|---|---|---|---|---|---|---|
SSA | SH Sol. (kg) | |||||||||
SH Sol. (mL) | SF (kg) | |||||||||
Water (W1) (L) | SH Pellets (kg) | Water (W2) (L) | SH Pellets (kg) | 10 mm | 20 mm | |||||
PD | 0.455 | 0.182 | 0.281 | 0.455 | 0.182 | 2.67 | 8 | 4 | 8 | 1.29 |
0.5W-SG, G-(0.5W-S) | 1.1 | 0.364 | 0.281 | 0 | 0 | 2.67 | 8 | 4 | 8 | 1.1 |
1W-SG, G-1W-S, G-(1W-S) | 0 | 0.364 | 0.281 | 0 | 0 | 2.67 | 8 | 4 | 8 | 2.2 |
Time | Temp. °C (PD) | Temp. °C (SD0H) | Temp. °C (SD3H) | Temp. °C (SD3HF) | Temp. °C (SD0HV) | |||||
---|---|---|---|---|---|---|---|---|---|---|
NaOH | SSA | NaOH | SSA | NaOH | SSA | NaOH | SSA | NaOH | SSA | |
Prep. time (0 min.) | 92 | 103 | 91 | 103.4 | 93 | 104.7 | 93 | 105 | No alkaline activator | |
30 min. after prep. | N/A | N/A | N/A | N/A | 62 | 77 | 54 | 61 | No alkaline activator | |
60 min. after prep. | N/A | N/A | N/A | N/A | 44.6 | 47.6 | 21.4 | 25.7 | No alkaline activator | |
90 min. after prep. | N/A | N/A | N/A | N/A | 38 | 39.9 | 14.2 | 12.5 | No alkaline activator | |
Mixing time | room temp. (25 ± 2) (24 h) | room temp. (25 ± 2) (24 h) | 73.2 (15 min) | 88.6 (15 min) | 31 (2 h) | 32 (2 h) | 15.5 (1 h and 45 min) | 13 (1 h and 45 min) | No alkaline activator |
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Altameemi, S.; Adeleke, B.O.; Kinuthia, J.M.; Oti, J. Understanding the Effect of Waiting for the Dissolution of Sodium Hydroxide in Geopolymer Concrete Mixes. Materials 2025, 18, 849. https://doi.org/10.3390/ma18040849
Altameemi S, Adeleke BO, Kinuthia JM, Oti J. Understanding the Effect of Waiting for the Dissolution of Sodium Hydroxide in Geopolymer Concrete Mixes. Materials. 2025; 18(4):849. https://doi.org/10.3390/ma18040849
Chicago/Turabian StyleAltameemi, Samara, Blessing O. Adeleke, John M. Kinuthia, and Jonathan Oti. 2025. "Understanding the Effect of Waiting for the Dissolution of Sodium Hydroxide in Geopolymer Concrete Mixes" Materials 18, no. 4: 849. https://doi.org/10.3390/ma18040849
APA StyleAltameemi, S., Adeleke, B. O., Kinuthia, J. M., & Oti, J. (2025). Understanding the Effect of Waiting for the Dissolution of Sodium Hydroxide in Geopolymer Concrete Mixes. Materials, 18(4), 849. https://doi.org/10.3390/ma18040849