3.1.4. Scanning Electron Microscopy (SEM) Analysis

Figure 4 shows the surface and shape morphology of the fly ash as well as the bamboo ash used in this study. Most of the fly ash particles have a glassy and spherical structure (Figure 4a), and are also known as cenospheres [43]. This ash contains a series of spherical vitreous particles of different sizes. Meanwhile, Figure 4b shows the SEM micrographs of bamboo ash, which depicted that bamboo ash has a rectangular structure and different particle sizes. It also has a porous structure, as can be seen from Figure 4b.

**Figure 4.** Scanning Electron Microscopy (SEM) micrographs of (**a**) Fly ash (**b**) Bamboo ash.

#### *3.2. Appropriate Mix Proportion*

#### 3.2.1. Workability

Table 4 represents the workability result of geopolymer concrete using different mix proportions. The standard deviation of the different mix proportions is found to be 2–8 mm. It is depicted that the slump value increases with the increase of the activator to binder ratio. The mix was very stiff and no flow was observed when the activator to binder ratio was 0.40 for both types of sample. Based on the visual observation, the mix was not homogenous and difficult to mix and compact. Apparently, the bamboo ash absorbs more water during the mixing, thus reducing the workability of the mix.

Once the activator to binder ratio was reached at 0.45, the slump value increases compared to 0.40 for 100% FA (Csample) and 5% BA+ 95% FA (Asample). Based on the observations, the mix was quite homogenous and the mixing process was easier compared to for the 0.40 ratio. The workable mix with the highest flow value was obtained when the activator to binder ratio was 0.50 for both samples.

In conclusion, the small percentage i.e., 5% addition of bamboo ash at a 0.45 ratio, gave a much smaller slump loss in Asample compared to Csample. However, with 0.40 and 0.50 ratios, the slump loss is 10 mm for Asample. Thus, we decided to perform another experiment in a 0.45 activator to binder ratio.


**Table 4.** Slump test result.

#### 3.2.2. Compressive Strength

Table 5 shows the compressive strength as well as the standard deviation of samples with curing duration. This table shows the effect of bamboo ash to fly ash ratio (%) on the compressive strength of geopolymer concrete at a 0.45 activator to binder ratio. The results show that the compressive strength of geopolymer concrete is increased with the increasing of curing time. This is due to the geopolymerization process between alumina and silica from the binder with the alkaline solution. But our studies failed to get desired ultra-high performance fiber reinforced concrete compressive strength [31–35]. However, as the percentage of bamboo ash increased, the compressive strength of the geopolymer concrete was decreased. This was probably due to a lack of the amount of alumina in bamboo ash, which reduced the geopolymerization product and contributed to the strength of concrete [44]. For 7 days of curing, replacement with 5% bamboo ash shows higher compressive strength at 18.94 MPa compared to 100% of fly ash i.e., 16.15 MPa. The increased early age compressive strength of BA with FA is due to the hydrolysis which imposed to form crystalline phases and reduced the porosity of the samples. Besides, it was also concluded that surface area of the mixture was increased due to the addition of bamboo ash, since the specific gravity of BA (2.05) is less than FA (2.20).

The standard deviation in the compressive strength for different mix proportions was found to be 0.11 to 1.31 MPa. A very stable proportion was needed in order to obtain a very good concrete mix for construction purposes. The addition of 5% bamboo ash was shown to be the most suitable mixture to get a better performance in regards to early compressive strength compared to other mixtures.


**Table 5.** Effect of curing period on compressive strength of the bamboo ash (BA) to fly ash (FA) ratio (%) in the 0.45 activator to binder ratio.

To understand the formation of phases after 7 days of curing, XRD of Csample (100% FA) and Asample (5% BA + 95% FA) was performed and results are shown in Figure 5. Quartz and mullite were obtained in both samples. There is a possibility that bamboo ash in Asample was completely mixed and dissolved in the matrix of fly ash attributed to the high dissolution rate of potassium oxide, rosenhahnite ,and sulfur trioxide in high alkaline condition, as explained by other researchers [45], during 7 days of curing. Thus, only quartz and mullite are found as present in 100% fly ash. This result confirms that owing to the dissolution of oxides present in bamboo ash control, the crack formed and filled out the porosity of geopolymer concrete (SEM results will be shown in Figure 6), leading to higher compressive strength being observed for 5%BA + 95%FA (Table 5) after 7 days of curing. The presence of quartz and mullite in Csample and Asample explained that SiO2 and Al2O3 are not fully utilized for geopolymer formation. Bamboo ash and activator are the materials that are involved in the synthesis of fly ash based geopolymer concrete. The pure geopolymer network actually consists mainly of Si, Al, and O with alkali Na<sup>+</sup> or K+. In this reaction, all minerals did not participate in the geopolymerization process.

**Figure 5.** XRD analysis for Csample (100% FA) and Asample (5% BA + 95% FA) after 7 days of curing.

Figure 6 shows the SEM results of 100% fly (Csample) and 5% bamboo ash + 95% fly ash (Asample) after 7 days of curing at 0.45 activator to binder ratio. 100% FA (Csample) containing sample shows macro and micro cracks in Figure 6a, however, 5% bamboo ash + 95% fly ash (Asample) does not show any defect in Figure 6b which indicated that the potassium oxide, sulfur trioxide, and rosenhahnite has dissolved and filled out the cracks/pores in the concrete. Thus, after 7 days, the 5% bamboo ash + 95% fly ash sample shows higher compressive strength (Table 5). The bamboo ash has the property to hold the paste together, thus controlling the cracks within the concrete owing to the reactive compositions which contain potassium oxide and other oxides. Generally, previous researchers [46] agreed with the addition of fiber into concrete, which increases the strength of the concrete structure. Moreover, Csample exhibits cracks, pores, and defects, which decreases the positive properties of concrete.

**Figure 6.** SEM micrographs of (**a**) 100% fly ash (Csample) and (**b**) 5% bamboo ash + 95% fly ash (Asample) after 7 days of curing.

#### *3.3. Fire Endurance Test*

#### 3.3.1. Cooling Effect on the Physical Appearance of the Concrete

High temperature i.e., heat exposure is one of the most important parameters which affects the surface characteristics, surface outlook, shape, and color of concrete. Although it does not give significant information regarding the distortion suffered by the concrete, it will give an immediate impression of the failure tendency of the concrete. Tables 6 and 7 represent a detailed picture of the concrete after exposure to different temperature and cooling regimes for Csample and Asample, respectively.

It can be observed from the entire cooling regime at 200 ◦C that the color of the sample was grey for both Csample and Asample with a smooth and sharp edges. These characteristics are maintained

up to 400 ◦C. At 600 ◦C, the light grey color is observed for air cooling (AC) with rough surface while water cooling (WC) exhibits yellowish grey with cracks appeared in Csample whereas in Asample there is no cracks. At 800 ◦C, the Csample develops heavy cracks throughout the surface in AC and WC, whereas Asample shows lesser cracks compared to Csample in both AC and WC.

The change in the matrix structure is a consequence of the dehydration in the phases of the binder during exposure to different temperatures. It is manifested as changes in porosity and color, as well as physical defects such as the presence of cracks. It is matched with the results presented in Tables 6 and 7 for Csample and Asample, respectively, which show the physical appearance of the concrete after exposure to high temperature. In general, the color changes experienced in both samples as a result of heating may be linked to the chemical transformations taking place in the heated samples.


**Table 6.** Cooling type, color, and texture of Csample.


