Predictive Model of Setting Times and Compressive Strengths for Low-Alkali, Ambient-Cured, Fly Ash/Slag-Based Geopolymers
Abstract
:1. Introduction
- Curing Conditions: Curing in air-tight conditions can improve the strengths [46].
2. Materials and Methods
2.1. Materials
2.1.1. SCMs
2.1.2. Alkaline Activator
2.1.3. Fine Aggregate
2.1.4. Mix Design
2.1.5. Mixing, Sample Preparation, and Curing
- The fly ash and blast furnace slag were dry mixed in a 5 L Hobart mixer for 3 min.
- The alkaline activator solution was added to the mix and blended in the Hobart mixer for 5 min in order to activate the fly ash and blast furnace slag.
- Sand was added and the mixing continued for a further 5 min.
- The majority of the blended mix was transferred to a brass mould and spread into sixteen individual mould cavities.
- Mould filling was optimised by vibratory casting for 3 min on a vibrating table.
- Insufficient filling was compensated by supplementation from the remainder in the Hobart mixer bowl; excess filling was removed by guillotine scraping. These procedures were followed by additional vibratory casting for 1 min.
- The mould was placed in a zip-lock plastic bag and sealed without removing excess air.
- The cast samples were cured for 24 h at 23 ± 2 °C.
- The samples were removed from the mould, placed in an air-tight plastic container (7 L volume), which contained a bowl of water (200 mL volume), and retained as such until testing.
2.2. Methods
2.2.1. Setting Time
2.2.2. Compressive Strength
3. Results
3.1. Setting Time
3.2. Compressive Strength Data
- Figure 6A–C show that the compressive strengths fall into regimes of low FA/S ratios (high-compressive strengths) and high FA/S ratios (low-compressive strengths).
- Figure 6A–F show that the kinetics of compressive strength development fall into regimes of rapid reaction (1–7 days) and slower reaction (14–28 days).
- Figure 6G–I show that the Ms has a decreasing effect on the rate of compressive strength development as the age increases.
- Figure 6G shows that the FA/S ratio and Ms exhibit competing effects at early age (1–7 days), where:
- High FA/S ratios: Low Ms decreases the rate of compressive strength development.
- Low FA/S ratios: Low Ms increases the rate of compressive strength development.
- Figure 6G also shows that for high FA/S ratios, the early age rate of compressive strength development is greatest for Ms = 1.7.
- Figure 6E suggests that there is a difference between the middle age compressive strength development between the lowest Ms and the two higher Ms. The elucidation of the specific reaction mechanisms of the geopolymerisation process currently is underway. Concerning the present data, it is likely that the geopolymerisation mechanisms can be differentiated between Ms of 1.5 and that of Ms of 1.7 and 2.0. The rapid increase in the rate of strength development between 30 and 40 wt% slag content for Ms of 1.7 and 2.0 suggests that the greater homogeneity of blast furnace slag relative to fly ash and the stress concentrations associated with the presence of crystalline quartz and mullite in the latter [10,66] equate to a shift from an inhomogeneous to a homogeneous microstructure at the transition. Conversely, the absence of the rapid increase in the rate of strength development for Ms of 1.5 results from greater influence of the Ms over that of the mineralogy. This is discussed in more detail below.
- More generally, these data suggest that a minimal slag content of 40 wt% can be expected to yield the most reproducible behaviour and hence, the greatest similarity in geopolymerisation mechanisms.
- Figure 6A–C,G–I demonstrate the highest compressive strengths and the most rapid compressive strength development results from minimal slag contents of 40 wt%.
3.3. Prediction of Long-Term Compressive Strength
4. Conclusions
4.1. Mix Design
- These mixes fail to meet both the final setting time and compressive strength requirements due to sluggish kinetics.
- These mixes have the most flexible setting times, but they fail to meet the compressive strength requirement.
- These mixes have high compressive strengths but, owing to the high blast furnace slag content, they set too quickly; also, there is no strength increase at slag contents >50 wt%.
- These mixes exhibit the most flexible setting times for those that meet the compressive strength requirement; however, at the slag content of 30 wt%, the compressive strengths depend on the Ms (Figure 6), which makes the process sensitive to the alkalinity of the activator.
- These mixes pass both the setting time and strength requirements, representing reproducible mix designs in terms of both blast furnace slag content and Ms; the mix in region 6 is optimal in that it shows the highest extrapolated ultimate compressive strength (~132 MPa at 60FA/40S at Ms = 1.7).
4.2. Predictive Models
- Variation of the alkaline activator/SCMs mass ratio;
- Variation of the water/binder mass ratio;
- Interpolative refinement of the Ms <1.50 and in the range 1.70–2.00;
- Optimisation of the sand/SCMs mass ratio <2.00 [63];
- Alteration of the alkaline activator from sodium silicate to potassium silicate;
- Variation in the fly ash type (Class C, Class F), mineralogy, glass chemistry, and particle size/surface area;
- Variation of the slag mineralogy, glass chemistry, and particle size/surface area.
Supplementary Materials
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Aluminosilicate Source 1 | Alkaline Activator Proportions | Mix 3A:L:S | w/b 4 | Specimen | Curing Conditions | Compressive Strength (MPa) | Reference | |||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Fly Ash | Slag | Components | Mix | Shape | Size (mm) | Temperature (°C) | Age (Day) | |||||
Na2SiO3 | NaOH | Silicate Modulus 2 | ||||||||||
70% | 30% | Ms = 2.78 | 14 M | Ms = 1.20 | 1.0:0.4:1.6 | 0.20 | Cube | 50 × 50 × 50 | 23 | 28 | 65.0 | [40] |
80% | 20% | 48.0 | ||||||||||
90% | 10% | 40.0 | ||||||||||
100% | - | 26.0 | ||||||||||
- | 100% | Ms = 2.06 | 15 M | Ms = 0.96 | 1.0:0.75:2.75 | 0.66 | Cube | 50 × 50 × 50 | 20 | 28 | 39.6 | [52] |
10% | 90% | 48.2 | ||||||||||
20% | 80% | 53.3 | ||||||||||
30% | 70% | 47.3 | ||||||||||
40% | 60% | 55.6 | ||||||||||
50% | 50% | 62.5 | ||||||||||
100% | - | Ms = 2.39 | 6 M | Ms = 1.52 | 1.0:0.6:1.0 | - | Cube | 50 × 50 × 50 | 25 | 28 | 38.5 | [45] |
10 M | Ms = 1.25 | 50.5 | ||||||||||
14 M | Ms = 1.06 | 56.0 | ||||||||||
- | 100% | Ms = 2.03 | - | Ms = 1.00 | 1.0:0.16:2.75 | 0.35 | Cube | 50 × 50 × 50 | 25 | 28 | 73.0 | [53] |
50% | 50% | 98.0 | ||||||||||
100% | - | 22.0 | ||||||||||
- | 100% | 0.50 | 64.0 | |||||||||
50% | 50% | 89.0 | ||||||||||
100% | - | 20.0 | ||||||||||
- | 100% | 0.65 | 35.0 | |||||||||
50% | 50% | 30.0 | ||||||||||
100% | - | 19.0 | ||||||||||
40% | 60% | Ms = 3.41 | - | Ms = 1.40 | 1.0:0.57:3.0 | 0.45 | Cube | 40 × 40 × 40 | 20 | 28 | 59.0 | [54] |
50% | 50% | 52.0 | ||||||||||
60% | 40% | 43.0 |
Parameter | Criteria | ASTM C150 Type I | AS 3972 Type GP |
---|---|---|---|
Setting Time | Minimum | 45 min | 45 min |
Maximum | 375 min | 360 min | |
Mortar Compressive Strength | 3 days (min) | 12 MPa | - |
7 days (min) | 19 MPa | 35 MPa | |
28 days (min) | - | 45 MPa |
Oxide | Fly Ash (wt%) | Slag (wt%) |
---|---|---|
SiO2 | 64.63 | 33.84 |
Al2O3 | 24.40 | 13.76 |
Na2O | 0.73 | 0.38 |
K2O | 2.31 | 0.28 |
CaO | 1.68 | 41.75 |
MgO | 0.63 | 5.59 |
Fe2O3 | 2.91 | 0.56 |
SO3 | 0.14 | 2.42 |
Loss on Ignition (LOI) | 1.34 | 0.13 |
Total | 98.77 | 98.71 |
Mix No. 1 | FA/S 2 | Fly Ash | Slag | Na2SiO3 | NaOH | Sand 3 | Free Water |
---|---|---|---|---|---|---|---|
Ms = 2.0 | |||||||
1 | 90/10 | 641.4 | 71.3 | 356.3 | 0.0 | 1069.0 | 62.0 |
2 | 80/20 | 570.1 | 142.5 | 356.3 | 0.0 | 1069.0 | 62.0 |
3 | 70/30 | 498.9 | 213.8 | 356.3 | 0.0 | 1069.0 | 62.0 |
4 | 60/40 | 427.6 | 285.1 | 356.3 | 0.0 | 1069.0 | 62.0 |
5 | 50/50 | 356.3 | 356.3 | 356.3 | 0.0 | 1069.0 | 62.0 |
6 | 40/60 | 285.1 | 427.6 | 356.3 | 0.0 | 1069.0 | 62.0 |
Ms = 1.7 | |||||||
7 | 90/10 | 638.4 | 70.9 | 340.9 | 13.8 | 1064.0 | 72.0 |
8 | 80/20 | 567.5 | 141.9 | 340.9 | 13.8 | 1064.0 | 72.0 |
9 | 70/30 | 496.5 | 212.8 | 340.9 | 13.8 | 1064.0 | 72.0 |
10 | 60/40 | 425.6 | 283.7 | 340.9 | 13.8 | 1064.0 | 72.0 |
11 | 50/50 | 354.7 | 354.7 | 340.9 | 13.8 | 1064.0 | 72.0 |
12 | 40/60 | 283.7 | 425.6 | 340.9 | 13.8 | 1064.0 | 72.0 |
Ms = 1.5 | |||||||
13 | 90/10 | 636 | 70.7 | 330.2 | 23.1 | 1060.0 | 80.0 |
14 | 80/20 | 565.3 | 141.3 | 330.2 | 23.1 | 1060.0 | 80.0 |
15 | 70/30 | 494.7 | 212.0 | 330.2 | 23.1 | 1060.0 | 80.0 |
16 | 60/40 | 424.0 | 282.7 | 330.2 | 23.1 | 1060.0 | 80.0 |
17 | 50/50 | 353.3 | 353.3 | 330.2 | 23.1 | 1060.0 | 80.0 |
18 | 40/60 | 282.7 | 424.0 | 330.2 | 23.1 | 1060.0 | 80.0 |
Cube Specimen (mm3) | Compressive Strength (MPa) | Conversion Factor | Nature of Data | ||
---|---|---|---|---|---|
7 Days | 28 Days | 7 Days | 28 Days | ||
150 × 150 × 150 | 11.40 | 15.80 | 0.70 | 0.68 | Determined |
100 × 100 × 100 | 14.20 | 19.00 | 0.87 | 0.81 | Determined |
50 × 50 × 50 | 15.00 | 21.80 | 0.92 | 0.93 | Determined |
25 × 25 × 25 | 16.23 | 23.37 | 1.00 | 1.00 | Extrapolated |
Setting Time (min) | Low Slag Content (<40 wt% Slag) | High Slag Content (≥40 wt% Slag) |
---|---|---|
Initial | (−309.37Ms + 833.9) × e(Slag × [−0.0100Ms − 0.0280]) | (−244.12Ms + 553.7) × e(Slag × [+0.0142Ms − 0.0466]) |
Final | (−619.12Ms + 1734.4) × e(Slag × [−0.0061Ms − 0.0358]) | (−607.53Ms + 1413.1) × e(Slag × [+0.0224Ms – 0.0691]) |
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Ukritnukun, S.; Koshy, P.; Rawal, A.; Castel, A.; Sorrell, C.C. Predictive Model of Setting Times and Compressive Strengths for Low-Alkali, Ambient-Cured, Fly Ash/Slag-Based Geopolymers. Minerals 2020, 10, 920. https://doi.org/10.3390/min10100920
Ukritnukun S, Koshy P, Rawal A, Castel A, Sorrell CC. Predictive Model of Setting Times and Compressive Strengths for Low-Alkali, Ambient-Cured, Fly Ash/Slag-Based Geopolymers. Minerals. 2020; 10(10):920. https://doi.org/10.3390/min10100920
Chicago/Turabian StyleUkritnukun, Supphatuch, Pramod Koshy, Aditya Rawal, Arnaud Castel, and Charles Christopher Sorrell. 2020. "Predictive Model of Setting Times and Compressive Strengths for Low-Alkali, Ambient-Cured, Fly Ash/Slag-Based Geopolymers" Minerals 10, no. 10: 920. https://doi.org/10.3390/min10100920