USING SILICA FUME TO IMPROVE CONCRETE SULFATE RESISTANCE

Prior to develop the advantages of using silica fume to improve the concrete sulfate resistance, it may be interesting to highlight the basics form of sulfate attacks and the properties of sulfate resisting cement.
The use and deterioration of concrete in environments containing sulfates has led to the development of special sulfate resisting cements. It has also spawned significant research into the use of supplementary cementitious materials to improve sulfate resistance.  Sulfate resisting cements (type V) have a low C3A content to minimize the risk off sulfate attack. However, this does not necessarily provide immunity:
  • as certain sulfate react with hydrated lime and the calcium silicate hydrate the sulfate resisting cement provide less protection than expected.
  • Low C3A cements are more susceptible to reinforcement corrosion attack.

THE BASIC FORMS OF SULFATE ATTACK ARE THE FOLLOWING:-

The reactive aluminates in the cement will react with the gypsum in cement during hydration. This process is harmless as the ettringite does not produce expansive forces and is stable in sulfate solutions.
If the quantity of reactive aluminates in the cement is too high, then their hydrate form will be available to react with sulfates after the cement has hardened. This will produce expansive ettringite and cracking of the concrete.
Aluminate Hydrate + Calcium Hydroxide + sulfate + water => Ettringite
The second principle cause attack is the acid interaction of sulfates ions and calcium hydroxide, causing gypsum formation.
Calcium Hydroxide + sulfate + water => Gypsum
It has been shown that the cation (calcium, Magnesium, Aluminium, Ammonium) of the sulfate salt affects the type and the severity of the attack.

AN ALTERNATIVE SOLUTION FOR SULFATE RESISTANCE: PORTLAND TYPE 1 + SILICA FUME:-

The utility of Silica Fume for enhancing the resistance of concrete to attack in sulfate environment has been examined for many years. Cement Type 1+Silica Fume have been chosen for their ability to produce a durable concrete.
Silica Fume serves to reduce the intensity of sulfate attack for the following reasons:
  • By reducing the cement content (replace by silica fume), the mount of lime and C3A present in the concrete is reduced, so the amount of gypsum and ettringite then can be formed is also reduced.
  • Some of the lime that is formed during hydration reacts with Silica Fume to form CSH. This further reduces the amount of lime that is available for gypsum formation.
  • Reduction of concrete permeability. The higher CSH levels and the physical blocking of pores by the Silica Fume particles reduce the permeability of the concrete. Consequently, the rate of diffusion of sulfates into the cement paste, and hence the rate of sulfate attack, is reduced.

Comments

Post a Comment

Popular posts from this blog

Lane’s Weighted Creep Theory

Image
Bligh, in his theory, had calculated the length of the creep, by simply adding the horizontal creep length and the vertical creep length, thereby making no distinction between the two creeps. However, Lane, on the basis of his analysis carried out on about 200 dams all over the world, stipulated that the horizontal creep is less effective in reducing uplift (or in causing loss of head) than the vertical creep. He, therefore, suggested a weightage factor of 1/3 for the horizontal creep, as against 1.0 for the vertical creep. Thus in Fig–1,  the total Lane’s creep length (L l ) is given by L l = (d 1 + d 1 ) + (1/3) L 1 + (d 2 + d 2 ) + (1/3) L 2 + (d 3 + d 3 ) = (1/3) (L 1 + L 2 ) + 2(d 1 + d 2 + d 3 ) = (1/3) b + 2(d 1 + d 2 + d 3 ) To ensure safety against piping, according to this theory, the creep length Ll must no be less than C1H L , where H L is the head causing flow, and C 1 is Lane’s creep coefficient given in table –2 Table – 2: V
Read more

Khosla’s Method of independent variables for determination of pressures and exit gradient for seepage below a weir or a barrage

Image
In order to know as to how the seepage below the foundation of a hydraulic structure is taking place, it is necessary to plot the flow net. In other words, we must solve the Laplacian equations. This can be accomplished either by mathematical solution of the Laplacian equations, or by Electrical analogy method, or by graphical sketching by adjusting the streamlines and equipotential lines with respect to the boundary conditions. These are complicated methods and are time consuming. Therefore, for designing hydraulic structures such as weirs or barrage or pervious foundations, Khosla has evolved a simple, quick and an accurate approach, called Method of Independent Variables. In this method, a complex profile like that of a weir is broken into a number of simple profiles; each of which can be solved mathematically. Mathematical solutions of flow nets for these simple standard profiles have been presented in the form of equations given in Figure  and curves given in Plate, wh
Read more

What is Soil in Geotechnical Engineering.

Image
Definition of Soil The word ‘soil’ is derived from the Latin word solium which, according to Webster’s dictionary, means the upper layer of the earth that may be dug or plowed; specifically, the loose surface material of earth in which the plants grow. The above definition of soil is used in the field of agronomy (The application of soil and plant sciences to land management and crop production) where the main concern is in the use of soil for raising crops. In geology, earth’s crust is assumed to consist of unconsolidated sediments, called mantle or regolith, overlying rocks. The top soil contains a large quantity of organic matter and is not suitable as a construction material or as a foundation for structure. The top soil is removed from the earth’s surface before the construction of structure. The term ‘ soil ’ in the soil engineering is defined as an unconsolidated material , composed of solid particles , produced by the disintegration of rocks . The void space
Read more