**3.1 Micro silica (silica fume)**

Micro silica, also known as silica fume, is a byproduct of producing silicon metal or ferrosilicon alloys. Micro silica, as a mineral pozzolanic admixture, has a very fine particle size that averages 0.5 micro-meter in diameter. The average size of silica fume particle is 100 times finer than Portland cement size. Silica fume is commercially available in a densified and un-densified form, with similar chemical composition, and different densities. The chemical composition of silica fume is shown in **Table 1**.

*Application of Supplementary Cementitious Materials in Precast Concrete Industry DOI: http://dx.doi.org/10.5772/intechopen.100249*


#### **Table 1.**

*Chemical composition of micro-silica.*

#### **Figure 3.**

*Excessive surface salt due to efflorescence phenomena.*

Micro-silica improves concrete mechanical properties through two different mechanisms by contributing to the Portland cement hydration. When mixing water is added to cement, without incorporating micro-silica, the following chemical reaction takes place during hydration:

$$\begin{aligned} \text{Cement} + \text{Water} & \longrightarrow \text{Calcium Science Hydroxate} + \text{Calcium Hydroxide} \\ (\text{H2O}) & \qquad (\text{C}-\text{S}-\text{H}) & \qquad \text{Ca(OH)}\_2 \end{aligned} \tag{1}$$

The main outcome of the hydration process includes the calcium silicate hydrate that acts as a binder and is directly responsible for the compressive strength of hardened concrete. The compressive strength depends on the amount of produced binder. Thus, the quantity of cement, cement fineness, water-cement ratio, and sufficient mixing energy and time are crucial to attain the required compressive strength of concrete upon hardening. The secondary outcome of the hydration process is calcium hydroxide, Ca (OH)2, which does not act as a binder, and has no contribution to the strength of the mix. In addition, excessive amounts of Ca(OH)2 may react with carbon dioxide and form a soluble salt that could leach within the hardened concrete causing efflorescence, as shown in **Figure 3**, and reduce the long-term performance of the structure due to its susceptibility to sulfate attacks, chemical attacks, and accelerating alkali-silica reactivity (ASR).

When micro silica is included in the mix, the added pozzolan reacts with the formed calcium hydroxide to produce an additional binder, which increases the hardened concrete strength and eliminates the salt formation by halting the efflorescence. The following equation describes the chemical contribution of micro-silica:

$$\begin{aligned} \text{Calcium Hydroxide} + \text{Microsifica} + \text{Water} & \xrightarrow{\text{Calcium Science}} \text{Calcium Science} \\ \text{Ca(OH)}\_2 \text{ (SiO2)} & \quad (\text{H20}) & \quad (\text{C}-\text{S}-\text{H}) \end{aligned} \tag{2}$$

In addition to its contribution to the concrete mechanical properties through the aforementioned reaction, micro-silica results in an improved packing order of the mixed granular material. The improved packing order results in reduced porosity

and increased resistivity to chloride attacks and de-icing salts, and reduces the rate of steel reinforcement corrosion. Improved packing order of micro-silica concrete is shown in **Figure 4**.
