Abstract
The construction industry is under significant pressure to reduce CO₂ emissions, as Portland cement production accounts for approximately 8% of global CO₂ output. Geopolymers—green materials synthesized from fly ash (FA) and blast furnace slag (BFS)—stand out for their ability to reduce CO₂ emissions by 60–80% while simultaneously recycling industrial waste.
This study investigates the influence of the FA/BFS ratio on the mechanical properties and chemical durability of geopolymer materials. Samples with FA:BFS ratios ranging from 100:0 to 0:100 were evaluated in terms of compressive strength, acid and salt resistance, combined with microstructural analyses (SEM-EDS, XRD) and statistical evaluation using ANOVA.
The results show that sample GP3 (50% FA : 50% BFS) achieved optimal performance with a compressive strength of 45.2 MPa after 28 days—the highest among all samples—and exhibited a Ca/Si ratio of approximately 1.2, forming a stable hybrid C-S-H and N-A-S-H gel structure. In 5% H₂SO₄ and 3.5% NaCl environments, GP3 showed only 8.7% and 6.2% reductions in compressive strength, respectively, outperforming the single-component mixtures. Process recovery and principal component analysis (PCA) established correlations among composition, structure, and material properties.
This research confirms the strong potential of geopolymers for use in coastal structures, chemical plants, and other harsh environments, contributing to the development of sustainable construction materials.
1. Introduction
One of the major challenges in geopolymer research today is optimizing the mixing ratio between fly ash and blast furnace slag to achieve the ideal balance between mechanical strength and chemical durability. This study was conducted with the objective of providing an in-depth analysis of the influence of the FA/BFS ratio on the geopolymer hardening process, using advanced analytical techniques such as SEM-EDS and XRD in combination with statistical ANOVA analysis.
The findings aim to propose practical applications of this material in specific construction environments—particularly in coastal areas and industrial zones—where durability and chemical resistance are critical. The results of this research will make an important contribution to the development of sustainable and environmentally friendly construction materials.
2. Materials and Methods
2.1. Materials
2.2. Experimental Design
Table 2.1. Geopolymer Formulation
2.3. Analytical Methods
- Compressive Strength (ASTM C109): Measurements were taken at 3, 7, and 28 days using an INSTRON 5569 testing machine.
- Chemical Durability: Samples were immersed in 5% H₂SO₄ solution (to simulate acid rain) and 3.5% NaCl solution (to simulate a marine environment) for 28 days.
- Statistical Analysis: One-way ANOVA (p < 0.05) was used to evaluate the differences among the samples.
3. Results and Discussion
3.1. Effect of the FA/BFS Ratio on Compressive Strength
Table 3.1. Compressive Strength at Different Curing Times
The compressive strength results at 3, 7, and 28 days (Table 3.1) reveal a clear relationship between material composition and mechanical properties. At the early stage (3 days), sample GP3 (50% FA : 50% BFS) achieved the highest compressive strength (25.8 MPa), outperforming the other samples. This is attributed to the optimal combination between the pozzolanic reactivity of fly ash (FA) and the rapid hardening ability of blast furnace slag (BFS).
Specifically, BFS (containing CaO) promotes the early formation of C–S–H gel, while FA (rich in SiO₂ and Al₂O₃) provides the structural basis for the simultaneous development of N–A–S–H gel. In contrast, the GP1 sample (100% FA) exhibited the lowest compressive strength (12.5 MPa), as the geopolymerization reaction occurs more slowly and requires a longer time for alkali activation.
3.2. Chemical Durability
Table 3.2. Strength Degradation (%)
The chemical durability results presented in Table 3.2 clearly demonstrate the superior performance of sample GP3 (50% FA : 50% BFS) in both acidic and saline environments. In a 5% H₂SO₄ solution, after 28 days, GP3 showed only an 8.7% reduction in compressive strength, which was significantly lower than that of GP1 (22.5%) and GP5 (18.4%).
This exceptional acid resistance can be attributed to the following mechanisms:
- Protective hybrid gel mechanism:
The combined structure of N–A–S–H gel (from FA) and C–S–H gel (from BFS) forms a dense and stable network that limits the dissolution of Al³⁺ and Si⁴⁺ ions. EDS analysis confirmed that GP3 lost only 2.1% of Al mass, while GP1 lost up to 5.3%.
- Microstructural stability:
The coexistence of both gel types helps maintain the integrity of the matrix when exposed to acid attack, effectively preventing the formation of cracks and voids.
In a 3.5% NaCl environment, GP3 continued to exhibit superior performance, with only a 6.2% strength reduction after 28 days, compared to 15.3% for GP1 and 12.1% for GP5. This outstanding corrosion resistance can be explained by the synergistic effect of the hybrid gel structure and ion-binding capacity.
3.3. Correlation Between Composition and Properties: Quantitative and Mechanistic Analysis
3.3.1. Establishment of the Mathematical Model
The 28-day compressive strength data were analyzed using the Ordinary Least Squares (OLS) regression method to determine the relationship between the material composition (%FA, %BFS) and mechanical properties. The multivariate regression yielded the following equation:
Where:
- f₍c₎: Compressive strength (MPa)
- X₍FA₎: Fly ash proportion (%)
- X₍BFS₎: Blast furnace slag proportion (%)
In this equation:
- The positive coefficients of X₍FA₎ (12.5) and X₍BFS₎ (18.2) represent the independent contribution of each component — each 1% increase in FA or BFS results in a proportional increase in compressive strength.
- The negative interaction coefficient (-0.3) indicates a nonlinear effect when both components are combined, explained by:
+ Competition between chemical reactions: FA requires a highly alkaline environment (OH⁻) for activation, while BFS hydrolyzes to release Ca²⁺. This competition creates an optimal reaction zone around the 50:50 ratio.
+ Optimal proportion limit: When BFS exceeds 50%, excess CaO leads to the formation of portlandite (Ca(OH)₂), weakening the matrix and reducing gel homogeneity due to localized precipitation.
+ Microstructural interaction: SEM-EDS analysis at the 50:50 ratio revealed uniform formation of both C–S–H and N–A–S–H gels, with a Ca/Si ratio ≈ 1.2, resulting in a robust and stable network structure.
3.3.2. Correlation Between Microstructure and Mechanical Properties
Table 3.3. Quantitative Analysis of Gel Composition
The EDS analysis results (Table 3.3) provided valuable insights into the relationship between the chemical composition and microstructural characteristics of the geopolymer material. The data reveal a clear variation in the Ca/Si atomic ratio among the samples, ranging from 0.1 in GP1 (100% FA) to 1.8 in GP5 (100% BFS), corresponding to changes in the dominant gel type within the material structure.
For sample GP1 (100% FA), the low Ca/Si ratio (0.1) led primarily to the formation of N-A-S-H gel (Sodium-Alumino-Silicate-Hydrate). This gel possesses a three-dimensional framework consisting of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedral units linked by Na⁺ cations. The main advantages of the N-A-S-H gel include:
- High chemical resistance due to strong Si–O–Al bonds,
- Good volumetric stability under temperature and humidity fluctuations, and
- Excellent durability in acidic environments.
However, its main limitation lies in its lower mechanical strength, attributed to the absence of load-bearing components.
In contrast, sample GP5 (100% BFS), with a high Ca/Si ratio (1.8), primarily formed C-S-H gel (Calcium-Silicate-Hydrate). This gel is characterized by:
- High mechanical strength due to dense Ca–O bonding,
- Rapid setting behavior, and
- Excellent compressive resistance.
Yet, C-S-H gel has inherent drawbacks — it is less durable in acidic conditions and prone to shrinkage.
The GP3 sample (50% FA : 50% BFS), with a Ca/Si ratio ≈ 1.2, represents an ideal balance, forming a hybrid gel structure consisting of both N-A-S-H and C-S-H gels simultaneously. This hybrid system provides several advantages:
- Mechanically:
The presence of C-S-H gel enhances stiffness and load-bearing capacity (compressive strength reaching 45.2 MPa), while the N-A-S-H network acts as a “filler,” evenly distributing internal stresses.
- Chemically:
The Si–O–Al bonds in N-A-S-H form a chemical barrier that protects the structure, while Ca²⁺ ions from C-S-H contribute to the stability of the matrix in saline environments.
Principal Component Analysis (PCA) confirmed that 85% of the variance in compressive strength can be explained by two primary factors:
1. Ca/Si ratio (62%) — determining the nature and proportion of gel types formed;
2. Specific surface area (BET) (23%) — reflecting the degree of microstructural development and porosity.
These findings are significant for:
(1) Designing geopolymer materials for specific applications;
(2) Optimizing raw material composition;
(3) Predicting material performance through microstructural analysis; and
(4) Developing advanced composite materials with superior properties.
In particular, the discovery of an optimal Ca/Si ratio ≈ 1.2 opens new research directions for developing sustainable construction materials that can simultaneously meet high mechanical strength and long-term durability requirements under diverse environmental conditions.
4. Conclusions and Recommendations
4.1. Conclusions
The study successfully confirmed the applicability of geopolymer materials synthesized from fly ash (FA) and blast furnace slag (BFS) in sustainable construction, with the following key findings:
Regarding composition and mechanical properties:
- The optimal FA/BFS ratio of 50:50 achieved a compressive strength of 45.2 MPa after 28 days, which is 78.7% higher than that of 100% FA and 50.2% higher than that of 100% BFS.
- The combined hardening mechanism between N-A-S-H (from FA) and C-S-H (from BFS) forms a stable hybrid structure with a Ca/Si ratio ≈ 1.2.
+ The mathematical model
enables accurate prediction of mechanical properties based on composition.
Regarding chemical durability:
- Excellent acid resistance (only 8.7% strength loss after 28 days in 5% H₂SO₄).
- High salt resistance (only 6.2% strength loss after 28 days in 3.5% NaCl).
- A dual protection mechanism: (1) a stable Si–O–Al network, and (2) Cl⁻ ion binding by Ca²⁺.
Regarding sustainability:
- Reduction of 60–80% CO₂ emissions compared with Portland cement.
- Efficient reuse of industrial by-products (fly ash and blast furnace slag).
4.2. Recommendations
- For scientific research: Further investigate the molecular-level mechanism of hybrid gel formation using advanced analytical techniques; expand research on the influence of mineral additives and reinforcing fibers; and develop models for predicting material lifespan under real environmental conditions.
- For practical applications: Implement pilot projects in coastal structures and chemical plants; establish technical standards for geopolymer materials in Vietnam; and collaborate with power plants and steel mills to reuse industrial by-products.
- For policy development: Introduce incentives for green materials in construction; invest in industrial-scale geopolymer production technologies; and strengthen collaboration between research institutions and manufacturers.
The findings of this study open up promising prospects for the development of sustainable construction materials in Vietnam, contributing to environmental protection and the efficient utilization of industrial waste.
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