Construction work and industry

Comprehensive research has been dedicated to the subject of fly ash admixture concrete and the properties that it exhibits. Due to the irresistible information that is obtainable on the topic of fly ash addition to concrete, it is outside the scope of this study to do anything more than draw attention to some of the existing research. Research has found that fly ash used as an additive to Portland cement has a number of positive effects on the resulting concrete. These positive effects are detailed below. Ahmaruzzaman (2010) in his work highlighted essentially three applications for fly ash in cement, including (1) replacement of cement in Portland cement concrete (2) pozzolanic material in the production of pozzolanic cements, and (3) set retardant ingredient with cement as a replacement of gypsum. One of the advantages of usage of fly ash in concrete is the reduction of construction cost by partial replacement of cement with fly ash. Other beneficial effects, include lower water demand for similar workability (Halstead, 1986), reduced bleeding, and lower evolution of heat. Fly ash is used particularly in mass concrete applications and large volume placement to control expansion due to heat of hydration and also helps in reducing cracking at early ages. Fly ash concrete provides much strong and stable protective cover to the steel against natural weathering (Ahmaruzzaman, 2010). The utilization of fly ash in concrete produces less permeability because of the spherical particles, and therefore improved packing, i.e. more dense paste and pozzolanic reaction.

The availability of high-lime fly ash containing compounds found in cement has led to high- strength concretes produced by the addition of fly ash and plasticizers. High-strength and high- performance concrete can also be made with Class F fly ash. The utilization of fly ash in concrete produces less permeability because of the spherical particles, and therefore improved packing, i.e. more dense paste and pozzolanic reaction. Class F fly ash produce concrete with lower heat of hydration compared to straight Portland cement concrete. Whereas Class C fly ash may not lower the heat of hydration. Abrasion resistance of concrete made with Class C fly ash was better than both concrete without fly ash and concretes containing Class F fly ash (Tikalsky et al., 1988). Naik et al. (1998) found that blending of Class C fly ash with Class F fly ash showed either comparable or better results than either the reference mixture without fly ash or the unblended Class C fly ash. Blending of fly ash, therefore, leads to comparable or better quality and reduced cost, due to the use of Class F versus Class C fly ash in concrete. Siddique (2004) found that Class F fly ash can be suitably used up to 50 % of cement replacement in concrete for use in precast elements, and reinforced cement concrete construction. The use of high volumes of Class F fly ash as a partial replacement of cement in concrete decreased its 28 days compressive, splitting tensile and flexural strengths, modulus of elasticity, and abrasion resistance of the concrete. The presence of CaO and CaSO4 in some fly ash contributed to their

utilization in soil stabilization and as fillers in road bases than in building materials (Steenari et al., 1999a). Both CaO and CaSO4 form hydration products, such as portlandite (Ca(OH)2) and

ettringite (Ca6Al3(SO4)3(OH)12·26H2O), with significantly larger volumes than those of the

reactants. Calcium oxide or calcium sulfate present in a concrete construction may thus create expansion cracks due to delayed reactions with water (Steenari et al., 1999a).

Fly ash increase resistance to corrosion, and ingress of corrosive liquids by reacting with calcium hydroxide in cement into a stable cementitious compound of calcium silicate hydrate (Scheetz and Earle, 1998; Ahmaruzzaman, 2010). Mehta (1996) also noted in his review paper that the agents responsible for concrete expansion and cracking are alumina-bearing hydrates, such as calcium monosulfo-aluminate and calcium aluminate hydrate, which are attacked by the sulphate ion to form ettringite and calcium trisulfoaluminate. Acidic-type interactions between sulphate ions and calcium hydroxide also lead to strength and mass loss. The original calcium hydroxide was soluble, whereas the calcium silicate hydrate is less soluble in fly ash concrete, thereby

calcium silicate hydrate being less soluble, reaction products tend to the filling of capillary voids in the concrete mixture, thereby reducing permeability of the concrete (Halstead, 1986; Ahmaruzzaman, 2010).

Gao et al. (2007) investigated the utilization of fly ash in the construction of concrete dams. The compressive strengths of dam concrete with 50 % of fly ash in 90 days are higher than those with 30 % of fly ash or without fly ash. Fly ash may decrease the deformation of dam concrete with 50 % of fly ash, and the shrinkage and expansive strain was reduced significantly–about 33 % and 40 % less than the specimens without fly ash, respectively (Ahmaruzzaman, 2010). The low- lime fly ash was used to develop chloride-resistant concrete by improving its physical resistance to the ingress of chlorides and binding capacity of these ions in the cover zone (Dhir and Jones, 1999).