Cement-silica fume pastes

Figure 8 shows the amount of calcium hydroxide formed in cement-silica fume pastes. At all curing periods, lime content is less in portland cement pastes incorporating silica fume. Extrapolation of the curve shows that, at about 24% silica, no lime would be left in the pastes. This figure may vary depending on the materials and method of estimation. 

Figure 8. Amount of calcium hydroxide formed in cement-silica fume pastes hydrated for different times.

Silica fume. Silica fume (SF) is a waste product of manufacturing ferro-sihcon alloys. It consists of an extremely fine powder of amorphous silica. Average particle diameter is about 100 times smaller than that of Portland cement and the specific surface area is enormous 13000-30000 m /kg compared to 300-400 m /kg for common Portland cements. Silica fume shows an elevated pozzolanic activity and is also a very effective filler. For these reasons, addition of silica fume to Portland cement may lead to a very low porosity of the cement paste, increasing the strength and lowering the permeability. It is usually added in the proportion of 5 to 10 % and it is combined with the use of a superplasticizer in order to maintain adequate workability of the fresh concrete. 

The air-bubble generating and stabilizing process requires a minimum paste consistency. Silica fume particles are smaller than those of Portland cement and addition of silica fume therefore increases the fine fraction of the particles. The higher fraction of smaller particles then increases the surface area causing a greater binding of the water in the mix. This removes the water required for the bubble-generating process. 

Table 2.1 Ionic concentration (in mmol/L) measured in the pore solution extracted from cement pastes, mortars and concrete made with ordinary Portland cement (OPC) and with additions of blast furnace slag (GGBS), fly ash (PFA) and silica fume (SF). n.a. = concentration not available [15]... 

C. L. Page, 0. Vennesland, Pore solution composition and chloride binding capacity of silica fume cement pastes . Materials and Structures, 1983, 16, 91. 

The similar reduction of water permeability was found in the case of a paste from cement with silica fume, rice hush ash and slags [138, 141], Also concretes produced of cement with 35 % fly ash addition show 2-5 times lower permeability, as compared to concrete from cement with no mineral additions [139]. 

Fig. 6.15 Effect of silica fume addition on the construction of paste-aggregate transition zone eg—cement grains,. sf silica fume, (a, b) c, d hydration progress. (According to [23])...

These results are important as the ability of chloride ions binding in the pastes from cements with mineral additions is discussed. For example Diamond [208], as well as Page and Vennesland [209] found that the silica fume addition is significantly increasing the free chloride ions concentration in the paste pore solution. In the... 

Special cements with silica fume, giving condensed, compact pastes (DSP) with high content of ultra-fine particles, have very good freeze-thaw resistance [80], It is caused by extremely low capillary porosity and hence very low content of freezable water in these pastes. The same rerrrark can be related to the reactive powder concrete (RPC), for example the Ductal type composite [335], This question will be discussed in Chap. 10. 

Coleman, N.J., and Page, C.L. (1997) Aspects of pore solution chemistry of hydrated cement pastes containing metakaolin. Cement and Concrete Research 27,147-154. Cong, X. et al. (1992) Role of silica fume in compressive strength of cement paste, mortar and concrete. ACl Materials Journal 89,375-379. 

Durekovic, A. (1986) Hydration of ahte and CjA and chances of some stractural characteristics of cement pastes by addition of silica fume, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 4, pp. 279-284. 

Durekovic, A., and Popovic, K. (1987) The influence of silica fume on the mono/disilicate anion ratio dirring the hydration of CSF containing cement paste. Cement and Concrete Research 17,108-114. 

Gantefall, O. (1986) Effect of condensed silica fume on the diffusion of chlorides through hardened cement paste. ACI SP-91, pp. 991-998. 

Mitchell, Hinczak, 1., and Day, R.A. (1998) Interaction of silica fume with calcium hydroxide solutions and hydrated cement pastes. Cement and Concrete Research 28, 1571-1584. 

Brouwers, 1998). In the presence of these additives the dissolved calcium hydroxide tends to react with the residual non-reacted fly ash, yielding additional amounts of C-S-H, and thus reducing the permeability of the paste. The optimum amounts of these additives to be interblended with Portland cement were found to be around 35 mass% of fly ash and about 8 mass% of silica fume (van Eijk and Brouwers, 1998). However, cement combined with 70 mass% of granulated blast furnace slag behaves not too differently from a cement that contains just Portland clinker alone (Faucon et al., 1996). 

Lobo, C., and Cohen, M.D. (1993) Hydration of type K expansive cement paste and the effect of silica fume. Cement and Concrete Research 23,104-115. 

The use of silane treated carbon fibers and silane treated silica fume led to increases in tensile strength of cement paste by 56% and modulus and ductility by 39% [30], as compared to the values obtained for cement paste with either as-received carbon fibers or as-received silica fume. Silane treatment of fibers and silica fume contributed about equally to the strengthening. Silane treatment of fibers and silica fume also decreased the air void content. The strengthening and reduction in air void content were less when the fiber treatment involved the use of K2Cr20v instead of silane, and even less when the treatment involved the use of O3. The effectiveness of silane is due to its hydrophilic nature. 

---