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As we know, cement is not only water-soluble in concrete; the other materials have a significant impact on superplasticizer molecules. Water reducing agents are therefore mainly directed towards cement. First, we will discuss the cement hydration process.
Three stages are involved in the general portland cement hydration process:
(1) Dissolution stage. When cement is placed in direct contact with water it causes the particles to begin to hydrate. A small quantity of hydration product are produced, which can be immediately dissolved in the water. Hydration can continue on the exposed surface until there is a saturated solution.
(2) Gelation stage. Because of the saturation of the solution the product can’t be dissolved and becomes colloidal particles. As the amount of hydration products increases, the cement slurry loses its plasticity and forms a coagulation phenomenon.
(3) Crystallization stage. A colloid made of microscopic crystallines is unstable. It can slowly recrystallize and form macro-crystals. This increases the mechanical strength of cement slurry and makes it a cement rock with an increased mechanical strength.
The main role for steric hindrance is played by the superplasticizers made of polycarboxylic alcohol based superplasticizers in the cement slurry. Complexation of calcium ions, lubrication for hydration film play important roles. important role.
1. Steric hindrance:
Cement molecules are attracted to each other during cement’s hydration, which results in flocculation.
The flocculation structure will form when the cement molecules encapsulate some of the water molecules. It accounts for approximately 10%-30% (this could explain why the water reducer has a maximum water reducing rate). Since it is enclosed by cement molecules it can’t participate in free flow or lubrication of cement molecules. It will affect concrete mix fluidity. Once the concrete particles come in contact with the polycarboxylate concrete cement admixture molecules, the main chain is negatively charged. concrete admixture Positively charged cement particles can attract molecules to their surfaces, causing an “anchoring” phenomenon. The long side chain of the molecule can be extended in cement slurry in order to form an adhesive layer. A three-dimensional cross chain can then form with the other polycarboxylic acids superplasticizer molecules. Conformation. The cement particles moving towards each other will cause the adsorption layers to overlap. A larger overlapping area means that there is more repulsion among the cement particles. This improves the dispersibility. The cement particles must be destroyed from their flocculation. You can understand how cement flocculation causes water to escape. Water reducing molecules are able to increase the lubricating efficiency of water molecules and reduce water molecules.
2. Electrostatic repulsion theory:
The superplasticizer polycarboxylic acids COO2- is an anionic group in molecules. The cement particle’s surface contains positive charges (Ca2+) during early hydration. Anionic Polycarboxylic Acid superplasticizer molecules can adsorb these positively charged particles. You can make the cement particles into a hedgehog with a negative charge. It is possible to improve the dispersibility among cement particles by having them have the same negative charge. In the cement paste, both the negatively charged carboxyl group and the positively charged calcium are responsible for the formation of unstable complexes. Once the calcium is dissolved in cement, it becomes more concentrated, which reduces calcium ions. The formation of gel particles can be slowed down, the cement hydration process is inhibited, and the cement particle dispersion performances improved. A higher level of Polycarboxylic acids superplasticizers with carboxylate and ion ions, e.g., a greater anion charge density, will lead to a better dispersion performance (acid-ether) for cement particles. Inhibiting the cement’s initial hydrolysis can be caused by a decrease in calcium ions. As the cement hydrates, it becomes more complex. The superplasticizer Polycarboxylic Acid has an effect that slows down but does not affect the strength.
The hydrophilic group is found in the branched chains of polycarboxylic acid supraplasticizer molecule. They form a water film over the cement particles, by reacting with water molecules. This decreases their surface energy. The cement particles slip easily. Combining the two causes separation of cement particles. This concrete mixture is more fluid.
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