2.4.1. Compressive Strength

The compressive strength testing was performed in accordance with the guidelines given in AS (1012.9-2014 [28]) at the age of 7, 14, 28, 56 and 90 days. At each age, three specimens were tested and the mean value of these measurements is reported.

#### 2.4.2. Strength and Mass Loss

After 28 days of curing, the 100 mm cube samples were immersed in sulphuric acid of 3% concentration (H2SO4, pH ≈ 3) and nitric acid of 1.5% concentration (HNO3, pH ≈ 3) for a period up to 90 days. These concentrations have been taken from existing literatures [29,30]. The solutions of acids were prepared by mixing concentrated acids with a predetermined amount of tap water. The pH level of acid solutions was monitored regularly using a portable digital pH meter (standard error: ±0.05). To maintain the desired pH levels, the concentrated acid was added either weekly or when the pH level went up. It has to be mentioned that the pH value depends on the degree of dissociation of radicals, and it may not be a true indicator of the concentration of acid in the solution [31]. Therefore, in the present study, the concentration was used directly as an indicator of the aggressiveness of the exposure environment.

The samples were removed from the acid solution after the exposure period and brushed carefully to remove the loose particles from the surface. They were then left for drying under room temperature for 1 h before determining the loss in compressive strength and the mass changes. The loss in compressive strength was calculated by determining the strengths at 7, 14, 28, 56 and 90 days. The mass loss was determined at 3, 7, 14, 28, 56 and 90 days.

#### 2.4.3. Scanning Electron Microscope (SEM)

The microstructure was studied using scanning electron microscope, Zeiss EVO-40 (Carl-Zeiss, Germany). The small cut samples were polished using silicon carbide paper and coated with platinum before imaging.

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

#### *3.1. Compressive Strength Development*

The compressive strength development of concrete containing different amounts of fly ash and ultra fine fly ash is shown in Figure 1. The compressive strength values are summarised in Table 3 along with standard deviation. In Figure 1, it can be seen that the strength development for Mix 1 (OPC) was much faster than the other mixes containing FA and UFFA. The compressive strength of Mix 1 (OPC) at seven days was 31.8 MPa, while this was below 30 MPa for other mixes. This trend also continues at the 14 day of curing. Therefore, it can be said that the strength gain of concrete mixes containing high volumes of FA is much slower than the concrete mixes without FA. This is due to the slower pozzolanic reactions of FA, in which the reaction between FA and water creates a slower hydration rate compared to the reaction between cement and water. However, at later ages (28 days or after), it can be seen that the strength for all the FA concrete mixes begins to develop at an accelerating rate, most notably for Mix 2 (20% FA + 10% UFFA). At 90 days, it can be seen that the compressive strength of mix Mix 2 (20% FA + 10% UFFA) far exceeds that of Mix 1 (OPC) and the rest, with Mix 3 (30% FA + 10% UFFA) coming in at the second. This also conforms to the findings of existing literature [32] that FA concrete has a slower strength gain at early age, but the strength exceeds the OPC concrete without any FA at 90 days.

**Figure 1.** Compressive strength development of concrete.


**Table 3.** Compressive strength results.

#### *3.2. Behaviour in Sulphuric Acid Environment*

Figure 2 shows the compressive strength loss for the five mixes when they are immersed in 3% sulphuric acid for a period of up to 90 days. The losses of compressive strength are summarised in Table 4 along with standard deviation. It can be observed from Figure 2 that Mix 1 (OPC) had the highest loss in the compressive strength at 90 days. Although Mix 1 (OPC) possessed the highest compressive strength initially, it was only able to retain 37.1% of its seven-day compressive strength after 90 days. This indicates that Mix 1 (OPC) was affected the most in 3% sulphuric acidic environment. Mix 2 (20% FA + 10% UFFA) showed the second largest variance, with compressive strength of 18.6 MPa compared to 26.0 MPa at seven days. Mix 3 (30% FA + 10% UFFA) also showed a declining compressive strength trend. However, this was not as severe as Mix 1 (OPC) and Mix 2 (20% FA + 10% UFFA). Mix 4 (40% FA + 10% UFFA) and Mix 5 (50% FA + 10% UFFA) showed minimal changes in strength loss, with less than 15% strength loss.

Exposure period (Days)

**Figure 2.** Compressive strength loss of concrete in sulphuric acid (3%).

**Table 4.** Compressive strength loss of concrete in sulphuric acid (3%).


Figure 3 shows the percentage mass loss of concrete cubes immersed in 3% sulphuric acid for a period of up to 90 days. The mass losses are summarised in Table 5 along with standard deviation. It can be observed that the maximum mass loss occurred in Mix 1 (OPC). The per cent mass reduction increases as the exposure period prolongs, showing an almost linear rate of mass loss. Mix 1 (OPC) showed a mass loss of 1.2% at three days, increasing to 10.3% at 28 days up to 22.7% at 90 days. While not as significant as Mix 1 (OPC), Mix 2 (20% FA + 10% UFFA) presented a mass loss of 2.7% at 28 days up to 8.9% at 90 days. Other mixes showed minimal mass loss, with less than 1% change at the end of 90 days. It can also be seen that the percentage of mass loss decreases as the volume of FA increases in each mix. The minimal mass loss per cent change in Mix 3 (30% FA + 10% UFFA), Mix 4 (40% FA + 10% UFFA) and Mix 5 (50% FA + 10% UFFA) is associated with the greater volume of FA to cement replacement, which provides a higher resistance to sulphuric acid attack. The could also be due to the accumulation of gypsum at the surface, effectively blocking or reducing further reactions from occurring, whilst already possessing a denser matrix.

**Figure 3.** Mass loss of concrete in sulphuric acid (3%).


**Table 5.** Mass loss of concrete in sulphuric acid (3%).

#### *3.3. Behaviour in Nitric Acid Environment*

Figure 4 shows the compressive strength loss of concrete immersed in 1.5% nitric acid for a period of up to 90 days. The compressive strength losses are summarised in Table 6 along with standard deviation. In Figure 4, it can be seen that Mix 1 (OPC) had the greatest decline in compressive strength at 90 days, with a compressive strength of 21.3 MPa. Mix 1 (OPC) was only able to retain 72% of its seven-day compressive strength after 90 days. Mix 2 (20% FA + 10% UFFA) showed the second largest variance, with compressive strength of 22.5 MPa compared to 27.4 MPa at seven days. The other mixes showed minimal changes in strength loss, with less than 10% strength loss. Comparing Figures 2 and 4, it can also be observed that the strength loss of concrete in 3% sulphuric acid is much greater than in 1.5% nitric acid.

**Figure 4.** Compressive strength loss of concrete in nitric acid (1.5%).



Figure 5 shows the percentage mass loss of concrete cubes immersed in 1.5% nitric acid for a period of up to 90 days. The mass losses are summarised in Table 7 along with standard deviation. It can be observed in Figure 5 that Mix 1 (OPC) had the most significant loss in mass, from 1.5% at three days, 4% at 28 days and 5% at 90 days. Rest of the mixes showed a much lower rate of loss with less than 0.6% at three days and less than 1% at 28 days. At 90 days, Mix 2 (20% FA + 10% UFFA) reached a mass loss of 2.7% while Mix 3 (30% FA + 10% UFFA) reached 2.2%. Both Mix 4 (40% FA + 10% UFFA) and Mix 5 (50% FA + 10% UFFA) showed a mass loss of 2% at 90 days, indicating the highest resistance. All mixes showed a consistent trend with the mass loss per cent increasing as the exposure period increased. This indicates that the resistance improves as the FA replacement level increases. The reduction of mass loss in mixes containing FA and UFFA can be attributed to the lower traces of CH due to pozzolanic reactions, minimising further reactions from the nitric acid.

**Figure 5.** Mass loss of concrete in nitric acid (1.5%).


**Table 7.** Mass loss of concrete in nitric acid (1.5%).
