3.3.2. Residual Compressive Strength

The residual compressive strength of Csample and Asample at different temperature and cooling regimes is shown in Table 8. For both cooling types, the residual compressive strength increases at 200 ◦C and achieved a maximum strength for Csample and Asample. At 400 ◦C, the Csample has the least strength loss of 41% and 43% for AC and WC, respectively. For 600 ◦C, the loss trend continues to increase for both types of samples and achieved a maximum loss of compressive strength at 800 ◦C. 78% (AC) and 80% (WC) of strength loss gained by Csample which is the highest loss of compressive strength, compared with Asample with 65% (A/C) and 66% (W/C) loss.

**Table 8.** Residual compressive strength of samples exposed at different temperature.


According to a previous researcher [40], geopolymer concrete strength was enhanced in heat conditions. Changes in chemical structure and the dehydration of free and chemically-bound water was caused from the exposure to high temperature. As the temperature increases, the moisture particles inside the samples tend to escape to the surface.

In this research, it was proved that fly ash geopolymer concrete (Asample and Csample) does not complete a geopolymerization process until 28 days have passed. Within 200 ◦C exposure, both type of samples gain an improvement in terms of strength and also the matrix structure. These are associated with the reported changes in the value of compressive strength after exposure.

It is seen that the water molecule is expelled from the geopolymer concrete during the presence of heat, which improves the strength of the concrete and also leads to the discontinuous nano-pores of the matrix. It is possible that not all water molecules were expelled due to high temperature, especially in a larger sample. Higher surface tensions in larger samples will dissipate moisture at slower rates compared to in smaller samples.

Starting from 400 ◦C, 600 ◦C and 800 ◦C levels of heat exposure, the the compressive strength decreased for both type of samples. But in the presence of bamboo ash, the residual compressive strength of Asample tends to show a positive result at 800 ◦C where the strength is higher compared to Csample.

This clearly shows that at 800 ◦C, the presence of bamboo ash contains potassium oxide, thus producing a significant compressive strength compared to Csample. If alkali metal oxides content is above a certain limit, it exhibits a high coefficient of thermal expansion. With the presence of bamboo ash at high temperature, the tendency of the oxides lead to change its shape to have a higher area and volume. Thus, more solid and packed molecules are formed and provide better structural components for the geopolymer concrete.

#### 3.3.3. Ultrasonic Pulse Velocity (UPV) Value after Exposure

The change in UPV value due to exposure to high temperature is depicted in Table 9. The UPV values at the initial temperature (27 ◦C) were 3854 m/s (AC) and 3850 m/s (WC) for Csample, and 3810 m/s (AC) and 3802 m/s (WC) for Asample. UPV value increases at 200 ◦C yielded values of 4451 m/s (AC) and 4417 m/s (W/C) for Csample, and 4438 m/s (AC) and 4394 m/s (WC) for Asample, which is the highest value among all studied temperature. At 400 ◦C, the trends changed, after which the Csample experienced UPV losses of 13.3% (AC) and 14.0% (WC) compared to Asample, which experienced 12.9% (AC) and 16.1% (WC) UPV losses. At 600 ◦C, the loss trends continued to increase and achieved a maximum loss at 800 ◦C for both types of samples. 67.8% (AC) and 64.7% (WC) losses was achieved by Csample, while 42.8% (WC) and 45.1% (AC) were achieved for Asample. The Asample exhibited a smaller loss of UPV compared to Csample. Generally, the reduction of the velocity in the concrete was due to the deformation of the microstructure in the geopolymer concrete. A rise in the temperature increases the amount of air voids in the concrete samples. Thus, the transmission speed of sound waves decreased with the increase in the traveling time of the ultrasonic pulse transmission.


**Table 9.** UPV value after exposure to different temperatures.

The quality of the concrete can be classified based on Table 10 [47]. Based on Table 10 values, good quality concrete can be produced at 200 ◦C. It was proven that the highest compressive strength

corresponded with the highest UPV value. Apart from that, at 800 ◦C, Asample gained a higher UPV value compared to Csample, which corresponded with the residual compressive strength results.


**Table 10.** Quality of concrete based on UPV.
