11, Apr 2022
As widely known, high strength cement paste, mortar and concrete, can be obtained by the incorporation of a well–designed combination of cement, filler, and superplasticizer (SP) . Quite often, SNF is applied as good filler, due to its high surface area and is a very reactive pozzolana because of its highly amorphous silica and its ability to react with free calcium hydroxide (CH), to the form hydrosilicates.
Cement containing SF exhibits higher strength than non–blended materials, therefore it is used to manufacture high-strength concrete . Moreover, SF acts not only as filler or pozzolanic material, but also to improve the cement paste–aggregate interface, which contributes to further improving concrete strength. Incorporation of SF, also affects fresh cement flowability, however, substantially raising the water demand, and its high early age hydration reactivity increases the heat of hydration and consequently intensifies total shrinkage.
Superplasticizers are frequently used in concrete technology for three different purposes or a combination of these: – to increase the workability without changing the mix composition to enhance placing characteristics of concrete, – to reduce the required water of standard consistency in order to increase the strength of concrete , – to reduce both water and cement at a given workability in order to save cement and reduce creep, shrinkage and thermal strains caused by heat of cement hydration . The fresh characteristics of concrete made with Portland cement (PC) can be modifified and controlled with superplasticizers. Achieving the steepest decline in the water–cement ratio, greatest workability and decreasing the viscosity is, however, contingent upon the compatibility between the admixture chosen and the cement. The presence of flfly ash (FA) and SF may positively affect the interaction between SP and cement components
Superplasticizers improve the rheological performance of cement paste, due to the dispersion of agglomerated cement and hydrating particles.Such agglomerates formed during the initial mixing of OPC with water; result from relatively large Van der Waals attractive forces, which operate at inter–particle distances in the range of 5 to 7 nm. Loosely entrapped quantities of water are associated with the agglomerated network of particles.
Anionic polymers, such as SNF or SMF have the ability to overcome the Van der Waals forces by imparting a stronger repulsive force at the surface–liquid interface, freeing the entrapped water. The dispersing effect of superplasticizer was ascribed to the development of the electrostatic (negative) charge on the cement particles .