Portland silica fume cement

When a high early strength is required, such as for post-tensioned structures or pre-tensioned precast elements, Portland cement with a strength class of 52.5 may be used. Blended cements are usually not suitable because of the slow rate of hydration, with the exception of Portland-silica fume cement and special (fast) slag cements. 

II Portland-silica fume cement 11/A-D 90-94 Silica fume 6-10... 

Portland silica fume cement Addition of silica fume can yield exceptionally high strengths, and cements containing 5-20% silica fume are occasionally produced. However, silica fume is more usually added to Portland cement at the concrete mixer. 

Mehta, P. K., Gjorv, O. E. (1982) Properties of Portland cement concrete containing fly ash and condensed silica fume, Cement and Concrete Research, 12(5) 587-95. 

Richardson, I. G. 2004. Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H applicability to hardened pastes of tricalcium silicate, p-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume . Cement and Concrete Research 34 (9) 1733-1777. 

The addition of silica fume to dry-mix shotcrete in proportions of 10-15% by mass of Portland cement substantially improves the adhesive and cohesive properties of the freshly applied shotcrete. Silica fume creates a very dense and sticky mix with an almost complete lack of bleeding. These characteristics of the material are attributed to the extreme densification achieved by packing of the ultrafine silica fume particles between the cement particles in the plastic (fresh) shotcrete. Practical ramifications of these characteristics include the observations which follow [107, 117-119]. 

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. 

Silica fume. 10%, 20%, 30% and 40% by weight of the Portland cement replaced by waste silica fume... 

Figure 2. Particle size and shape of ground waste glass type I, type II, silica fume, rice husk ash and ordinary Portland cement.

Two types of pulp sand mortar were used as the core plate of sandwich specimens. Mortar of Type M was made by mixing high-early-strength Portland cement, silica fume and pulp sand. Mortar of Type P was made directly with pulp sand. Mix proportions of each type of mortar are shown in Table 1 and Table 2. Pressurized forming was used to produce mortar plates of each type, whose pressure was lOMPa. Then specimens were put in a moist room of 20 C for about one day. After steam curing at 50 C for 24 hours, they were autoclave-cured on condition of 180 C for 3 hours. Finally they were dried out in an electric oven of 110 C for 24 hours. The geometry of the specimen was 50x10x200 (mm). 

Silica has played a key role since the beginning of civilization, first in flint for tools and weapons and in clay and sand for pottery. The high strength and durability of Roman cement 2000 years ago is now known to be due to the use of a special volcanic ash that is an almost pure form of amorphous colloidal silica. Today there is active research on the use of the somewhat similar silica fume from electric furnaces to make a super-strong Portland cement. 

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. 

In non-carbonated and chloride-free concrete, the concentration of hydroxyl ions (OH ) varies from 0.1 M to 0.9 M, due to the presence of both NaOH and KOH (the latter is predominant, especially in Portland cement). Other ions, e. g. Ca and S04 , are present only in very low concentrations. Addition of blast furnace slag or fly ash to Portland cement results in a moderate reduction of ionic concentration, and thus in pH. From hydroxyl ion concentrations in Table 2.1, values of pH of 13.4-13.9 can be calculated for Portland cement, and pH values of 13.0-13.5 for blended cements. Addition of condensed silica fume in higher percentages may lead to a decrease in the pH to values to below 13 [4, 10]. 

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]... 

Table 2.3 Resistivity [Cl m) of concrete made with Portland cement (OPC), blast furnace slag cement (GGBS), and Portland cement with addition of 5% silica fume (SF) iv/c = 0.45. lues were determined after 1.5—2.5 y exposure in a fog room or 20°C 80% R.H. climate [37]...

Tables 11.8-11.12 present the mean values of the DuraCrete parameters introduced in the previous section for Portland cement concrete. In most cases, values are given for slag, fly ash and silica fume concrete as well. The tabulated values were based on literature surveys, experiments on wide ranges of concrete or expert opinion [21,29].

Portland cement and silica fume are the major components. The slump values are in the range 150-200 nun strength after 28 days is in the range 75-100 MPa [21]. 

PC - Portland cement SF - silica fume FA - fly ash GBS - granulated blast fumance slag... 

Portland cement+granulated blast furnace slag+silica fume (Shizawa et al, 1992 ... 

Portland cement+metakaolin+silica fume (Chung, 1996) ... 

Kovler, K. (1998) Setting and hardening of gypsum— Portland cement—silica fume blends. Cement and Concrete Research 28,423-438. 

Nagataki, S., and Wu, C. (1995) study of the properties of Portland cement incorporating silica fume and blastfurnace slag. American Concrete Institute SP-153, pp. 1051-1068. 

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