## 4 Results and Discussion
### Durability against sulphuric Acid  
Long-term chemical resistance of concrete structures is important for sustainable economies; cementitious materials with high acid resistance are essential to build or maintain critical infrastructures [73], considering that the adverse effects of acid attack is accompanied by socioeconomic losses that cost a huge amount of money worldwide [74]. In particular, to build and maintain sewer structures in which biogenic sulphuric acid attack is the main mechanism of destruction, concrete and mortar with high sulphuric acid resistance are required [75; 76; 77]. Due to the distinct nature of the existing anion in sulphuric acid (sulphate ion), the damage mechanism for this type of acid is different from that of other acids. sulphuric acid reaction with hydrated cement paste components is presented in Equations (1) and (2). Following the sulphuric acid reaction with hydrated cement paste components, calcium sulphate (gypsum or plaster), the salt reacts with hydrated calcium alumino-sulphates or not hydrated aluminate components that are available in the hardened paste, producing ettringite. Equations (3)-(5) describe the sulphate attack. An increase in volume accompanies ettringite formation and consequently internal stress, which may cause micro-cracks or even large cracks and intensify sulphuric acid attack [78]. \[\mathrm{H_{2}SO_{4}+Ca(OH)_{2}\to CaSO_{4}+2H_{2}O} \tag{1}\]  
\[\mathrm{H_{2}SO_{4}+CaO.SiO_{2}.2H_{2}O\to CaSO_{4}+Si(OH)_{4}+H_{2}O} \tag{2}\]  
\[\mathrm{3 CaO.Al_{2}O_{3}.CaSO_{4}.12H_{2}O+2 CaSO_{4}.2H_{2}O+16H_{2}O\to 3 CaO.Al_{2}O_{3}.3 CaSO_{4}.32H_{2}O} \tag{3}\]  
\[\mathrm{3 CaO.Al_{2}O_{3}.Ca(OH)_{2}.12H_{2}O+3 CaSO_{4}.2H_{2}O+13H_{2}O\to 3 CaO.Al_{2}O_{3}.3 CaSO_{4}.32H_{2}O+Ca(OH)_{2}} \tag{4}\]  
\[\mathrm{3 CaO.Al_{2}O_{3}+3 CaSO_{4}+32H_{2}O\to 3 CaO.Al_{2}O_{3}.3 CaSO_{4}.32H_{2}O} \tag{5}\]  
Figure 16: Compressive strength at 120 days (tap water and 20% sodium sulphate attack). Figure 17: Compressive strength reduction percentage at 120 days (20% sodium sulphate). Extensive investigations have been conducted in order to improve concrete durability in harsh acidic environments. Previous research recommended less use of cementitious materials due to the vulnerability of cement hydration products, such as lime crystals and C-S-H gel, against sulphuric acid. Due to the nature of pozzolanic reactions, using cement replacement materials leads to a reduction in lime content and improves concrete permeability. Thus, using cement replacement materials such as natural pozzolans and GGBFS can improve concrete durability in environments subjected to sulphuric acid. However, some researchers suggested that cement replacement materials can also negatively affect concrete durability. Physical properties, chemical compounds, fineness, and pozzolanic reactivity of SCMs, along with test conditions, can be the affecting factors for such a contradiction in research results. Compressive strength results of specimens in 25% sulphuric acid solution and tap water bath are compared in Figure 18.  
The compressive strength of specimens with up to 20% cement replacement was similar to that of the control samples, while for more than 20% replacement ratios compressive strength decreased, and 50% replacement led to the lowest strength. Therefore, it can be noted that partial cement replacement with waste can either positively or negatively impact durability of concrete against sulphuric acid based on content, shape, and texture of waste particles [16]. As seen in Figure 18, the control mix had maximum strength against sulphuric acid with around 44% reduction in compressive strength, while specimen with 50% cement replacement had the lowest resistance against sulphuric acid, with about 57% reduction in compressive strength. It is considered that a reduction in permeability helps decrease acid penetration into the mortar mix, and as a result, resistance against acid attack will be improved [79; 80].