2.5.4. Scanning Electron Microscopy (SEM)

The surface morphology of both materials (bamboo ash and fly ash) as well as 100% fly ash and 5% bamboo ash+95% fly ash geopolymer concrete after 7 days of curing were conducted by scanning electron microscopy (SEM, Jeol, Tokyo, Japan) operated at 15 kV.

### 2.5.5. Ultrasonic Pulse Velocity (UPV) Test

The UPV value of concrete samples was determined according to BS EN 12504-4:2004 [39] after exposure to high temperature.

#### 2.5.6. Fire Endurance Test

The fire endurance test was carried out in accordance with ASTM E119-12a [40] using automatic electrical furnace (Figure 1) with 100 mm × 100 mm × 100 mm concrete. All samples were cured for 28 days at 25 ± 2 ◦C before being subjected to high temperature. An electrically-heated furnace designed for a maximum 1200 ◦C was used. All samples were heated for the duration of 1 hour at 200 ◦C, 400 ◦C, 600 ◦C and 800 ◦C as targeted temperature. Two different cooling approaches were tested which were air cooling (AC) and water cooling (WC) regimes with curing conditions at 25 ◦C and 60% relative humidity. Different cooling regimes for normal cement composite had a significant influence on the mechanical properties of the concrete after exposure [41,42]. Before the test were conducted, all samples were weighed to determine their initial density and initial ultrasonic pulse velocity (UPV) value. After acquiring the temperature level, a further experiment was then carried out to determine the UPV loss, weight loss, physical appearances, and residual compressive strength.

**Figure 1.** Overview of an automatic electrical furnace.

#### **3. Results and Discussion**

#### *3.1. Characterization of the Binder*

#### 3.1.1. X-ray Fluorescence (XRF)

The chemical composition of bamboo ash and fly ash were determined using XRF. Based on the results shown in Table 3, the main oxides composition of bamboo ash is silica, potassium oxide, calcium oxide, and sulfur trioxide containing 35.2%, 33.1%, 13.5%, and 8.3%, respectively. For fly ash, most of the compounds are silica and alumina. Furthermore, silica/alumina ratio for bamboo ash is unidentified while fly ash is around 2.0. Silica and alumina are very important for geopolymer synthesis. A nil percentage of alumina in bamboo ash is too unrealistic for geopolymerization to occur. The presence of high amount of calcium oxide in bamboo ash reduces the setting time of fly ash containing geopolymer concrete. The fly ash used in the present study was considered as Class F, revealing that the summation of silica, alumina and iron oxide is more than 88%.

**Table 3.** Chemical composition (%) of BA and FA.


#### 3.1.2. X-ray Diffraction (XRD)

XRD of both binder materials is shown in Figure 2. Fly ash shows the presence of quartz (SiO2, JCPDS = 88-2487) and mullite (3Al2O3.2SiO2, JCPDS = 06-0259) in the XRD pattern (Figure 2a), while bamboo ash diffraction patterns show the presence of quartz (JCPDS = 88-2487), potassium oxide (K2O, JCPDS = 26-1327), rosenhahnite i.e., calcium hydroxide silicate (Ca3Si3O8(OH)2, JCPDS = 83-1242), and sulfur trioxide (SO3, JCPDS = 76-0760) (Figure 2b). Potassium oxide, rosenhahnite, and sulfur trioxide are found only in bamboo ash. This result corroborates with XRF data (Table 3). A sharp peak at 28.28◦ indicates a mainly crystalline structure consisting of quartz, as well as potassium oxide and rosenhahnite in bamboo ash.

**Figure 2.** X-ray Diffraction (XRD) of (**a**) fly ash and (**b**) bamboo ash.
