*3.4. Visual Inspection*

Figure 6 shows the various stages of concrete deterioration in 3% sulphuric acid environment. It can be seen that Mix 1 (OPC) suffered the greatest signs of deterioration at the end of 90 days in compared to the other mixes. Mix 1 (OPC) also showed the signs of peeling and full exposure of the aggregate surface at 28 days. At 90 days, the initial layer was found to be completely disintegrated with some of the initial surface aggregates already falling off. This would also link to the reduction of mass and compressive strength for this mix. Mix 2 (20% FA + 10% UFFA) showed the signs of deterioration, with the formation of gypsum at the surface at 28 days and becoming more porous. It was also observed that the initial layer of the surface started spalling off and exposed aggregates at 90 days. Mix 3 (30% FA + 10% UFFA) and Mix 4 (40% FA + 10% UFFA) showed similar behaviour. However, the deterioration signs were less as the volumes of FA increased. The deterioration was much slower, with the aggregates being slightly exposed at 90 days. The volumes of these two mixes also appeared to have increased at 28 days, which could be as a result of the formation of gypsum. Mix 5 (50% FA + 10% UFFA) appeared to be the most aesthetically resistant, with no major structural changes at the end of 90 days.

**Figure 6.** Concrete deterioration in 3% sulphuric acid solution.

Figure 7 shows the various stages of concrete deterioration in 1.5% nitric acid environment. Similar to 3% sulphuric acid environment, Mix 1 (OPC) showed the most serious damage in 1.5% nitric acid, with spalling of the surface beginning already at 28 days. At 90 days, larger surfaces of the aggregates can be observed with more severe spalling of the surface. Mix 2 (20% FA + 10% UFFA) showed the higher resistance compared to Mix 1 (OPC). However, Mix 2 (20% FA + 10% UFFA) showed

severe spalling and exposed aggregates at 90 days. Other mixes behaved in a similar fashion with structural changes not as severe as Mix 1 (OPC) or Mix 2 (20% FA + 10% UFFA). All mixes showed a browning of colour at 28 days, turning lighter again at 90 days after disintegration and spalling of the initial layer.

**Figure 7.** Concrete deterioration in 1.5% nitric acid solution.

#### *3.5. Microstructural Observation from SEM*

The SEM image of a sample from Mix 1 (OPC) at 28 days is shown in Figure 8. The hexagonal plate-shaped crystals of CH and C-S-H gels are clearly visible in the image. The presence of excess hydrous calcium-sulpho-aluminate hydrate (also known as ettringite) characterised by needle-like structures is also evident. Large number of pores and voids can also be seen in the image. The SEM image of a sample from Mix 3 (30% FA + 10% UFFA) at 28 days is shown in Figure 9. The SEM image shows a denser matrix with much lower trace of the CH crystals. It is considered that the majority of CH content might have reacted with the amorphous silica of FA and UFFA to produce secondary C-S-H gel by the pozzolanic reactions. The denser microstructure is likely to be associated with micro-filing effects of UFFA. The UFFA might have filled the pores and voids between the unreacted particles in the hydrated matrix, effectively densifying the pore structure.

Figure 10 shows the SEM image of a sample from Mix 3 (30% FA + 10% UFFA) exposed to 3% sulphuric acid for 28 days. The surface appears to be highly porous in the image. A large scale of possible micro-cracks and voids can also be observed. A noticeable amount of C-S-H gel appears to have been decomposed into finer particles. Remains of calcium hydroxide crystals and unreacted FA and UFFA also appear to be present. Furthermore, the signs of gypsum can be seen to cover the surface area including particles of FA. The extensive formation of gypsum in the surface regions may have caused the disintegration resulting the spalling of the surface. Figure 11 shows the SEM image of sample from Mix 3 (30% FA + 10% UFFA) immersed in 1.5% nitric acid for a period of 28 days. The surface also appears to be very porous, with the salt by-products on the surface caused by the reaction of the acid with the CH. Small round particles appear are the unreacted FA and UFFA. The broken surface pieces are likely to be the traces of calcium nitrate salt and calcium nitro-aluminate hydrate. It also appears that the ions from the nitric acid have completely disintegrated the C-S-H gel on the outer surface of the sample leading to dissolution and deterioration of the surface layer.

**Figure 8.** SEM image of Mix 1 (OPC) (28 days of water curing).

**Figure 9.** SEM image of Mix 3 (30% FA + 10% UFFA) (28 days of water curing).

**Figure 10.** SEM image of Mix 3 (30% FA + 10% UFFA) in 3% sulphuric acid (exposure period 28 days).

**Figure 11.** SEM image of Mix 3 (30% FA + 10% UFFA) in 1.5% nitric acid (exposure period 28 days).

Total porosity and the presence of microcracks have significant influence on the permeability of concrete. In general, permeability decreases with an increase in porosity up to a certain level, and then the influence of porosity on permeability is negligible. The presence of microcracks also increases the permeability of concrete, and thus encourages more rapid deterioration.
