1. Introduction
Calcium aluminate cement (CAC) or high-alumina cement is widely used and usually adopted in engineering applications with special purposes, such as refractory, biomaterial, ability to resist acid exposure, fast-setting, durable, and additive manufacturing concretes [
1]. Several research programs were being determined, especially for the purposes of property improvement, durability, and more eco-friendly production processes. In addition, during the manufacturing process of CAC, depending on the availability of the source, it is reported that there is a significant reduction in carbon dioxide emissions generated in the atmosphere from lower sintering temperatures compared to portland cement, so CAC can be considered an eco-cement material.
One of the primary applications of CAC is in the construction sector. It is widely used in refractory applications due to its ability to withstand extreme temperatures, making it invaluable in the manufacturing of furnaces, kilns, and foundry linings. Moreover, CAC is employed in high-performance concrete formulations, where its rapid setting and exceptional strength gain are critical, particularly in precast concrete elements and underwater construction [
2].
However, the widespread adoption of CAC-based materials is not without challenges. Regulatory constraints governing the use of CAC can vary significantly from one country to another. These constraints may pertain to environmental considerations and quality control measures. Understanding these variations and how they impact the use of CAC-based materials is essential for engineers and researchers.
One of the primary concerns associated with CAC is the hydrate conversion process during hydration, which can impact the material’s performance. The CA phase is the fundamental hydraulic phase in all CAC systems. Regarding the affecting factors, the most significant factor is the curing temperature, since the hydration product directly relies on the curing temperature [
3]. CAH
10 and C
2AH
8 phases are regarded as metastable at ambient temperature [
4] and ultimately change to the more stable C
3AH
6 and AH
3 phases due to thermodynamic stabilization. The conversion from the metastable to stable phases leads to porosity gains, more permeability, and hence strength reduction [
5]. The conversion reactions are presented in Equations (1) and (2). The hydration reaction of CA produced is dependent on the temperature, which is given in
Table 1. The strength progression of CAC concrete at room temperature is shown in
Figure 1. The strength first rises rapidly to a high early strength (corresponding to CAH
10 and C
2AH
8), then gradually drops to a minimum (during the transformation to C
3AH
6 and AH
3) before rising once more from prolonged hydration.
Boris et al. [
6] investigated the conversion process, especially in the presence of nanomaterials. These nanomaterials vary in chemical composition and grain size, and their effects on CAC depend on their type and quantity. Nano-silica was found to accelerate the CAC hydration process, shortening the induction period and intensifying the secondary heat release effect. Despite similar types of hydration products in all CAC pastes, the quantitative composition could vary [
6]. Due to this conversion effect, the implementation of CAC in structural engineering practice is restricted. Although CAC is no longer a novel material, the conversion raises questions about how well it performs under harsh loading and the stability of its microstructure. The processes that transform the metastable phases of CAH
10 and C
2AH
8 into stable, long-lasting, and dense C
3AH
6 hydrates are responsible for this thermodynamic instability [
7,
8]. Many researchers have attempted to identify the chemical reactions and the effects of the conversion on the overall performance of CAC concretes [
8,
9]. Some researchers [
3,
4,
5] have investigated whether replacing some of the CAC with pozzolanic materials such as ground granulated blast furnace slag (GGBFS), metakaolin, silica fume, and rice husk ash is a proper technique to mitigate the hydrate conversion process and strength reduction.
Therefore, utilizing coal fly ash (FA) as pozzolan in CAC is carried out to overcome the conversion process. The incorporation of FA could improve the hydrogarnet due to an aluminate hydrate reacting alternatively to silica in the C-A-S-H phase or stratlingite (C
2ASH
8) [
2,
10] as revealed by the subsequent equations:
Majumdar and Singh [
11] found that replacing CAC with pozzolanic materials such as micro-silica, GGBFS, and metakaolin reduced the hydrate conversion while also increasing its strengths. Its hydrate conversion of hexagonal hydrated structures could be prevented by the pozzolanic reaction in the CAC system, which has been observed in the reaction of CA. Lastly, this reaction could prevent hexagonal (C
2AH
8) formation and C
3AH
6 transformation [
11,
12].
FA is an industrial by-product with very fine particles that is manufactured from the combustion of coal in power plants. FA has a spherical shape that has a high silica and calcium composition [
13]. Although the CAC hydration process may receive some attention for inclusion with pozzolanic materials by many researchers, the mechanical and microstructural performances of such systems have not been systematically investigated in engineering applications. In addition, the effect of pozzolanic materials on the hydration reaction of CAC and its mechanical performance have not been completely investigated. Similarly, a study that examines how FA impacts the mechanical and microstructural characteristics of CAC systems is lacking.
Based on the alumina and calcium contents, various CAC systems are remarkably different, with low silica contents. The mechanical properties of this CAC concrete can be changed by variations in the alumina content [
14]. Although it has been researched how the water-to-cement ratio (w/c) affects the fracture performance of CAC concrete, the effect of adopting different silica contents in the CAC system has not been reviewed yet [
15]. According to the review of the literature, the novelty of this work includes:
The experiment explores various aspects of CAC composites incorporating FA, entailing initial setting time, evolution of FA reactivity, workability, densities at 7, 28, and 56 days, compressive strengths at 7, 28, and 56 days, and porosities at 7, 28, and 56 days. To examine the pore structures and chemical composition of CAC, including FA, nitrogen adsorption and thermogravimetric analysis (TGA) have been performed. Furthermore, microstructural analysis using Scanning Electron Microscopy (SEM) with Energy Dispersive Spectrometer (EDS) has been employed to gain a deeper understanding of the transformation of CAC containing FA.
Table 1.
Summary of the temperature history of conversion reaction of CAC from literature.
Table 1.
Summary of the temperature history of conversion reaction of CAC from literature.
Reference | Temperature Range (°C) |
---|
CAH10 | C2AH8 | C3AH6 |
---|
Scrivener et al. [16] | <15 | 15–70 | >70 |
Adams et al. [17] | <15 | 15–27 | >27 |
Khaliq and Khan [18] | <15 | 15–27 | >27 |
Zapata et al. [19] | <15 | 15–35 | >35 |
Son et al. [20] | <20 | 20–40 | 40–60 |
Antonovič et al. [7] | 5 | 20 | 40 |
Ukrainczyk and Matusinović [21] | 20 | 30 | >55 |
Vafaei and Allahverdi [14] | 15–25 | 25–40 | 40–60 |
Zapata et al. [9] | ≥20 | ~30 | >55 |
Mean | <16 | 28 | >45 |
Median | <15 | 15–27 | >40 |
4. Discussion
As aforementioned, the performance of FA in CAC systems is strongly influenced by its physical and chemical properties. The ternary diagram of CAC systems containing different FAs is depicted in
Figure 12. Regarding the early-age properties, different activation energies of CAC systems with FA contribute to calcium reacting faster than silica at a very young age. Different FAs can offer different [
52] activities. These reactions thereafter change hydration reactions. When CAC mortar is converted, it undergoes a process of mineralogical development, and the porosity has increased. Therefore, the conversion method could avoid the reaction that could be formed in the subsequent way [
2,
3,
10,
11,
44]. The mineral admixtures containing silica would react first with the CA, avoiding the formation of C
2AH
8 and, subsequently, the conversion into C
3AH
6. Consequently, instead of this C
3AH
6 phase, the C
2ASH
8 phase was proposed to be formed. This implies that samples containing FA exhibit reduced levels of stable phases and encounter comparatively lesser degrees of strength loss (see
Table 5).
Based on SEM-EDS images as shown in
Figure 13 and
Table 6 of CAC specimens prepared from fractured surfaces after 56-day water curing, The main elements involve Al, Si, and Ca, and other elements C and O. By using FA, a pozzolan with a high specific surface area and strong reactivity, in relation to the CACP sample, the concentration of the remaining Ca(OH)
2 and silicon dioxide (SiO
2) among the compounds was reduced, according to data from EDS elemental compositions for each system. This showed that these components had a higher likelihood of forming C-(A)-S-H gel when FA was present in the specimens, which consequently improved the strength and decreased porosity. It is believed that, compared to the control sample, CAC specimens containing FA exhibit enhanced mechanical characteristics. In addition, it is likely that the specimen lacking FA will perform poorly in terms of mechanical properties given the quantity and size of pores among the samples in this work.
The partial utilization of FA in CAC systems, especially for class-C FA, improved the quality of resultant products from very early to later ages. It can be mainly adopted for increasing sustainability in our manufacturing industries such as construction, refractory, and rapid repair applications, as well as reducing material costs since FA is an industrial by-product that is less expensive, and using it can increase reactivity so that the CAC can be used in smaller fractions.
In summary, the incorporation of FA into CAC offers numerous advantages, including improved environmental sustainability and enhanced material performance. By introducing FA into CAC, we contribute to the cement industry’s efforts to combat climate change. Our study has shown that FA addition leads to a range of beneficial effects on CAC properties. Firstly, it results in a reduced setting time, enhancing construction efficiency and reducing construction time. Additionally, FA improves workability and positively influences microstructural characteristics. This includes the increased formation of C-(A)-S-H phases and the reduction of inter-particle space, indicating the pozzolanic action of FA. These changes lead to improved microstructure, density, and compressive strength, which are highly desirable attributes in the construction industry. Furthermore, the presence of FA in CAC alters the kinetics of hydration reactions, lowering the activation energy required for the conversion process. This has significant implications for the material’s setting time, strength development, and overall performance. While our study highlights the positive impacts of FA on CAC, further research is needed to fully understand the mechanisms underlying FAs’ influence on the conversion process of CAC. This will enable us to optimize the use of FA for specific applications in the construction industry, ultimately advancing both environmental sustainability and construction practices.