1. Introduction
Durability of concrete is a major concern when exposed to aggressive environments, especially chloride and carbon dioxide (CO
2) causing chlorination and carbonation, respectively. These ions induce the corrosion of embedded steel rebars [
1]. Once the steel rebar starts to corrode, the corrosion products induce internal expansion, resulting in cracks and spalling, which leads to the failure of concrete structures [
2].
Chlorination can also affect the durability of concrete more often than carbonation [
3]. However, the increase of urban population density and industrialization as well as more technologies have led to higher emission of carbon, which dramatically increases the concentration of CO
2 in the atmosphere.
The theory of carbonation is a complex process. Carbonation occurs once calcium carbonate (CaCO
3) forms [
4]. Generally, the formation of CaCO
3 occurs when calcium hydroxide (Ca(OH)
2) encounters atmospheric CO
2 in the presence of water [
5]. Ca(OH)
2 is an alkaline substance, which is consumed during carbonation process; thereafter the concrete becomes more acidic, causing a reduction in the pH of the pore solution and successfully breaking down all other hydrate phases [
6]. Thus, the final products become a mixture of carbonates with silicate, ferrite and aluminum-hydroxide phases. In other words, the chemistry of carbonation is always the same; however, the penetration difficulties into the concrete vary in different concrete mixtures.
The carbonation process begins when CO
2 from the atmosphere diffuses into concrete. The gaseous CO
2 cannot directly react with the hydrates of cement paste. However, it dissolves in water (H
2O) and forms bicarbonate (HCO
3−) ions (Equation (1)) and thereafter reacts with calcium ions (Ca
2+) of the pore water [
6]. However, since the pH value inside the concrete is high, the HCO
3− will dissociate and form carbonate (CO
32−) ions (Equation (2)). Lastly, the carbonate ions will react with the Ca
2+ ions in the pore solution and form CaCO
3 in Equation (3) [
7]. The carbonation process is described by the following chemical equations [
8]:
There are three different factors affecting the carbonation process, namely the temperature, relative humidity, and concentration of CO
2 present in the surroundings [
7]. When the temperature increases, the diffusivity of CO
2 into concrete is amplified due to the increase of molecular activity [
9]. Secondly, the relative humidity acts as the major role in determining the diffusivity of CO
2. The highest rate of carbonation happens at the relative humidity of 50–70% [
10]. The carbonation process generally occurs in the presence of water. However, if it is too wet, water acts as the obstruction for penetration of CO
2, which in turn decreases the rate of the carbonation process. Lastly, the CO
2 concentration in the environment is certainly the main factor controlling the carbonation process. If the atmosphere has high concentration of CO
2, this can induce the carbonation process [
11]. In the diffusion process of carbonation, CO
2 flows from higher concentration to lower concentration. Therefore, the rate of diffusivity depends on the level of concentration. However, different surface-repairing materials are becoming prominent to reduce the carbonation effect on concrete [
12,
13]; but there are some issues related to post-treatment regarding cost, durability, and practical application.
Blended cement has now become very popular, owing to better performance in terms of mechanical properties as well as carbonation resistance when compared with ordinary Portland cement (OPC) [
14,
15]. As a result, the trend of using pozzolanic materials as partial replacement of cement is expanding [
16].
The three largest producers of palm oil fuel ash (POFA) are Indonesia, Malaysia, and Thailand. In 1960, palm oil cultivation was limited to 54,000 hectares in Malaysia; however, it has substantially increased to 4.85 million hectares in 2010 and 5.39 million hectares in 2014. Moreover, in Indonesia, palm oil cultivation was 6.5 million hectares in 2012. Palm oil cultivation produces large portions of biomass waste such as fronds, effluent, leaves, trunks, kernel shell, mesocarp fiber, and empty fruit branches after the harvesting of fruits, and processing and re-plantation of palm trees [
16]. These huge portions of biomass are burnt at 800–1000 °C in an industry to generate electricity [
16]. The end product of this burning process is known as POFA, which is normally thrown into the landfill causing an environmental impact. Moreover, when wind is blown through the landfill, it could cause health hazards, as it is toxic to breathe [
17]. Due to the high production of POFA, it is a severe risk to health conditions and environmental damage. The situation will be worse as it is forecast that the generation of POFA will tend to increase in the future. Therefore, reusing POFA from landfill is good opportunity for reducing environmental and health effects and at the same time achieving better performance of concrete.
Raw POFA has zero cost and the treating process requires only a few devices such as an oven, grinder and ball mill, and thus it is not expensive. This is helping the environment, because all unused raw POFA will be thrown into landfill, causing pollution. As a result, reusing POFA can save the environment. The cost can also be reduced by using POFA instead of cement. Cement is more expensive than POFA. Therefore, replacing cement with POFA can make concrete production cost-effective in addition to reducing the environmental load.
POFA has very good potential for improving concrete’s properties due to its chemical composition, such as calcium and silicon oxides present inside [
18]. Thus, POFA is proven to be the emerging waste material for improving the durability of concrete, owing to the formation of secondary calcium silicate hydrate (C-S-H) gel through a pozzolanic reaction, which is caused by silica [
19]. A certain amount of micro POFA (mPOFA) enhances the durability of concrete, which is 10–30% in replacement of OPC [
20,
21,
22,
23]. POFA shows a lower strength activity index at an early stage of curing, but it exhibits improved values at a later stage [
24]. It is used in the production of high-strength concrete by reducing the average size to around 10 µm with a 0–30% replacement of OPC [
22,
25,
26]. At the age of 28 days, the highest compressive strength was found with 20% replacement of OPC by POFA [
22,
25]. The inclusion of ultrafine POFA reduces the early-stage compressive strength up to 7 days and it was particularly observed for a higher amount of POFA [
27]. It was also seen that if mPOFA is converted into nano-POFA (nPOFA) and included together to produce concrete, the result had a positive influence in filling out the porosity of concrete and enhancing the compressive strength [
28].
According to Thomas et al. [
16], POFA has been proven to bring benefits to carbonation resistance by using micro sizes of POFA. There is a high possibility of using nano-size POFA to achieve higher carbonation resistance of concrete, compared to micro POFA [
29]. Therefore, this study tried to use POFA as a supplementary cementitious material for concrete. Islam et al. found that using 30% mPOFA exhibited better performance, but once the amount increased, it showed detrimental effects to the concrete [
18]. Furthermore, it was found that using a little amount of nPOFA can replace a huge amount of mPOFA [
30]. Therefore, in the present study, the concrete mixtures containing a low amount of nPOFA along with mPOFA were prepared and their synergistic effects on carbonation resistance under accelerated conditions were measured.
In the present study, the amount of POFA has been optimized for high resistance to carbonation. This study emphasizes the assessment of the carbonation resistance properties of concrete by incorporating micro and nano-POFA. The accelerated carbonation experiment was performed according to BS 1881-2010:2013 in 4.0% (±0.5%) CO
2 at 20 °C (±5 °C) temperature and 55% (±5%) relative humidity [
31]. In this standard method, there is a tolerance value for both temperature and relative humidity of ±5 °C and ±5%, respectively. This would not have any effect on the acceleration of carbonation if the temperature is around room temperature and relative humidity is within 50–70% [
7]. Under these circumstances, a maximized effect would be gained in this research.