A pozzolan is broadly defined as a siliceous material, which when added reacts with the lime (or portlandite) to produce an additional amount of Calcium Silicate Hydrate (C- S-H) by hydration, the main cementing component. Thus the pozzolanic material reduces the amount of calcium hydroxide (Ca(OH)2) and increases the amount of C-S-H. In
addition, a pozzolanic reduces the alkalinity of the pore solution (and thereby fibre corrosion) and greatly reduces or even eliminates CH from the matrix) (Purnell, 1998). Very fine pozzolan particles sized between 0.1 and 10 μm, such as SF, can fill the holes between cement grains and fibres.
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The use of Pozzolans, as partial replacement to Ordinary Portland Cement (OPC) offers many advantages and results in cost savings, particularly when these materials are diverted away from waste sites (Uzal et al., 2010). Their introduction is beneficial by: reducing the amount of cement; improving concrete properties; reducing the energy needed for processing natural materials; conserving natural resources. Another advantage is that there is a reduction in cement production which reduces CO2 emissions and energy
consumption (Suneel, 2004). Lothenbach et al. (2011) and Carrasco et al. (2014) have shown that Pozzolans can improve long-term strength and durability and reduce the porosity. The adverse effects of replacing cement are the increase in setting times and the decrease in workability due to an increase in the surface area of particles (Sahmaran et al., 2006).
Several researchers (Bagel 1998; Khan et al. 2000; Pandey and Sharma 2000) reported that the combination of two or more kinds of pozzolanic materials emerges as a superior choice when improving concrete and mortar properties. A study by Toutanji et al. (2004) agreed with these studies and confirmed that a combination of SF, GGBS and PFA can produce cementitious materials with higher strength and effective resistance to freeze- thaw. Middendorf et al. (2005) characterised the effect of adding an additive to show that the matrix becomes denser. Recently, Zhou et al. (2012) stated that by partially replacing cement with PFA or GGBS the mix’s setting times is delayed. The effect of the three common additives of SF, GGBS and PFA on mechanical properties of cementitious materials is described in the following three sub-sections.
2.8.1 Silica Fume (SF)
SF, known also as micro silica, is a waste product from the silicon industry and contains 95-99% active silica by weight. Since the 1970s the application and interest in SF
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has increased. SF particles are extremely small; more than 95% can have a diameter less than 1 µm. They have a very high water demand, which necessitates the addition of a mix of high range water reducers or superplasticisers (Vitro, 2008). This is in part due to the adverse effect SF has on workability (Purnell, 1998). SF particles are usually grey in colour, darker or lighter grey depends on their carbon and iron content. The specific surface area of SF ranges from 13,000 to 30,000 m2/kg with a bulk density range from 130 to 430 kg/m3 (Siddique, 2011). The high specific surface increases the rate of the pozzolanic reaction and also that of hydration generally, probably due to the small SF particles acting as nucleation sites (El-Hadj and Duval, 2009). It has been stated by Larbi et al. (1990) and Duchesne and Berube (1994) that adding SF reduces the alkalinity in the cement pore solution. SF is generally added as a partial replacement for cement at lower weight fractions than GGBS and PFA, at about 5-25% according to Sengupta and Bhanja (2003). Yogendran et al. (1987), Toutanji et al. (1993), Mobasher and Li (1996), Fu and Chung (1998), Chung (2000), Shannag (2000), Chung (2002), Shihada and Arafa (2010), Ismeik (2010) and Zulkarnain and Ramli (2011) all confirm that the typically replacement level is 15%.
As shown in this review there is a considerable volume research papers on the use of SF. Larson et al. (1991) reported that SF fills the voids between the particles of cement hence, improving packing between fibre and cement particles to develop the fibre-matrix bond. Bentur and Diamond (1985), Zhu and Bartos (1993) and Bartos and Zhu (1996) experimented with adding SF directly to the fibre bundles by immersion in a SF slurries, prior to manufacturing the FRC. The rationale behind this procedure is that the SF will penetrate into the inter-filament spaces. Previous studies by Yogendran et al. (1987), Toutanji et al. (1998), Urban (2003), Chung (2005) and Yazdanbakhsh et al. (2009)
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concluded that SF is effective in improving the dispersion of short fibres, this is has been introduced in Section 2.7. Chung (2000) reported that the dispersion of CFs of 15 μm diameter and 5 mm length at 0.2% Vf is enhanced by having SF at 15% by weight of cement. Badanoiu and Holmgren (2003) studied the bond for continuous CFs where the tows have 12k (12000) filaments at 7 μm diameter. Results from their study showed a 20% improvement in the bond properties when the matrices contained SF at 10%. Nili and Afroughsabet (2010) found that the addition of SF led to an increase in flexural strength of up to 38% (tested at 28 days). This work also agreed with previous studies that the presence of SF improves the uniformity of fibre dispersion. It is for this reason that the author involves SF in the series of FRC experiments to be reported and discussed in Chapters 5 to 7.
2.8.2
Pulverised Fuel Ash (PFA)
Pulverised fuel ash (also known as fly ash) is a by-product from the burning of pulverised coal in power stations. The major constituents of PFA are silica, alumina, and oxides of iron and calcium. The physical and chemical properties of PFA can vary considerably from one power plant to other due to the differences in the sources of coal. Generally, PFA is made up of glassy, spherical particles which range in size from 2 to 160 μm. PFA has been used in concrete production for over 50 years, and is used to replace up to more than 50% of cement by weight (Carette and Malhotra 1983; Rashad 2014).
The use of PFA in cementitious materials offers many advantages and results in saving costs, particularly when these materials are diverted away from waste sites. PFA is also used to reduce hydration heat and thermal cracking at early ages, to improve the mechanical and durability properties especially at later ages (Siddique and Khan, 2011) and to decrease the shrinkage of the hardened concrete (Atis, 2003). Yuan et al. (1982)
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showed that the water demand increased when the PFA content is more than 20%. Carette and Malhotra (1987) concluded that by adding 20% there is increase in concrete strength. Yuan et al. (1982) confirmed that a mix having 20% PFA exhibited less shrinkage on curing than either the control matrix or a matrix containing PFA at 30 to 50%. Saraswathy
et al. (2003) confirmed that a critical level of 20-30% replacement activated the PFA addition, to improve both the corrosion-resistance and strength. A study by Snelson and Kinuthia (2010) disagrees with the studies by Yuan et al. (1982), Carette and Malhotra (1987) and Saraswathy et al. (2003). They all concluded that the strength decreases as cement is replaced with PFA. The author involves PFA in the series of experiments reported in Chapters 5 to 7.
2.8.3
Ground Granulated Blast Furnace Slag (GGBS)
GGBS is produced as a by-product during the manufacture of iron. This cement replacement is rapidly cooled to form granules, and then ground (crushed) to a fine white powder. This cement material has many similar characteristics to OPC. GGBS contains around 30 to 40% active silica by weight. GGBS is angular in shape with a surface texture much smoother than OPC. The size of GGBS particles varies from 5 to 70 μm, whereas the surface area is between 450 and 685 m2/kg. Replacement levels for GGBS vary from 30% to up to 85% by weight of the cement (Elahi 2009; Siddique and Khan 2011).
It has been demonstrated that GGBS improves the general performance of cementitious materials by decreasing chloride diffusion and chloride ion permeability (Luo
et al. 2003; Yun and Kyum 2005), reducing creep and drying shrinkage, increasing sulphate resistance, enhancing the ultimate compressive strength (Barnett et al., 2006), reducing the heat of hydration and bleeding and reducing the alkalinity of cement pore solution (Duchesne and Berube 1994; Pavia and Condren 2008). Zhou et al. (2012)
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concluded that a matrix with 30% GGBS by weight of cement exhibited higher splitting tensile strength than plain concrete.