**1. Introduction**

The impact of concrete, being one of the most commonly used construction materials worldwide, on sustainability can be significant. Concrete, in general, has a relatively low embodied energy compared to other construction materials. Fly ash, a by-product from thermal power stations, has been proven to have a lower embodied energy compared to ordinary Portland cement (OPC) [1]. The use of fly ash as a supplementary cementitious material (SCM) in concrete is well recognised for its economic and performance advantages such as improved workability and durability [2–5]. In fact, fly ash is specified in various Standards for use as a SCM [6] and in General Purpose and Blended Cements [7]. Studies have shown that by using high volumes of fly ash (>50%) it is possible to achieve the desired properties of concrete with a minimized cost [6,7].

The pozzolanic reaction of fly ash is a slow process. Therefore, the early strength of fly ash concrete is much lower than the concrete which does not contain any fly ash [8]. Different approaches have been used to accelerate the pozzolanic reaction of fly ash in concrete [9–12]. One of the approaches studied is the incorporation of very small size pozzolanic materials. In particular, microsilica has been used to improve the early age strength properties of concrete containing fly ash [13–15]. Ultra fine fly ash (UFFA) is a recently developed material. It is produced by a proprietary separation system with a mean particle diameter of 1–5 microns and contains 20% more amorphous silica than typical class F fly ash (particle diameter of 1–300 microns) [16]. Therefore, not only have the benefits of using UFFA in concrete been studied [17–19], but also the effectiveness of UFFA in improving the strength of fly ash concrete at early age has been evaluated [20]. The use of UFFA in concrete also contributes to the sustainability. This is because, compared to cement production, the UFFA production does not require any high energy-intensive process.

It has been recognised that, in general, ordinary Portland cement (OPC) concrete has minimal (almost no) resistance to acid attacks. While some weaker acids can be tolerated if exposed occasionally, OPC is known to be unable to hold up against any solution with a pH of 3 or lower [21]. Sulphuric acid

(H2SO4) is one of the most deleterious acids to act on concrete due to the combination of acid and sulphate attack. The deterioration of concrete sewer pipes due to sulphuric acid attack is a global problem all around the world. Moreover, industrial waste often contains a large amount of sulphuric acid. Therefore, concrete structures in industrial areas are exposed to of sulphuric acid attack. Sulphuric acid reacts with calcium hydroxide (CH), hydration product of cement in concrete and produce gypsum. The creation of gypsum in concrete causes volume increase. The gypsum also reacts with calcium aluminate hydrate (C3A) to produce ettringite. The volume of ettringite is almost seven times more than the initial compounds [22]. Ettringite causes inner pressure in concrete leading to the formation of cracks [23]. Ultimately, the corroded concrete loses its mechanical strength that contributes to more cracking, spalling and finally leads to completely destruction [24].

Nitric acid (HNO3) is another powerful corrosive acid that is immensely aggressive in nature. Nitric acid occurs in chemical plants producing explosives, artificial manure and similar products. Although nitric acid is not as strong as sulphuric acid, its effect on concrete at brief exposure is more destructive. The nitric acid reacts with CH of concrete and produces a highly soluble calcium nitrate salt. This salt weakens the cement paste structure and reduces the strength of concrete.

Different strategies have been used to enhance the resistance of concrete in acidic environment. One of the strategies, found to be very effective, is the use of various supplementary cementitous materials such as fly ash, slag, microsilica and calcite laterites [25–27]. Although extensive research has been carried out on the use of UFFA in concrete either individually or in combination with fly ash, very few studies evaluated its effectiveness on the durability properties of fly ash concrete. This paper reports the results of an investigation on the behaviour of a concrete in sulphuric acid and nitric acid environment where cement was replaced with fly ash and UFFA.
