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We know that in addition to cement and water in concrete, the remaining materials on the water reducing agent molecules are basically adsorption, so the water reducing agent for the main or cement.
So first of all, let's introduce the hydration process of cement. The hydration process of general-purpose silicate cement is divided into three stages.
① Dissolution stage: When the cement comes into contact with water, the surface of the particles begins to hydrate, generating a small number of hydration products, which are immediately dissolved in the water. The new surface is exposed so that the hydration continues until a saturated solution of hydration products is generated.
② gelation stage: As the solution is saturated, the product of continued hydration can no longer be dissolved, and directly precipitated as colloidal particles. With the increase in hydration products, hydride aggregation, and cement paste gradually lost plasticity producing the phenomenon of coagulation.
③ Crystallization stage: The colloid composed of microscopic crystals is not stable and can gradually recrystallize to generate macroscopic crystals so that the mechanical strength of the cement paste hardening body is continuously increased and finally becomes a cement stone with certain mechanical strength.
In the cement slurry, the dispersing mechanism of polycarboxylate superplasticizer is generally considered to be the main role of space potential resistance, electrostatic repulsion as a secondary role, and the complexation of calcium ions and hydration film lubrication also play a certain important role.
Next, let's talk about the working mechanism of the water reducing agent.
① The role of spatial site resistance: The hydration reaction of cement will make the cement molecules attract each other and appear flocculation structure, flocculation structure formation process of cement molecules will be wrapped part of the water molecules, the water molecules wrapped is about 10%-30% of the total mixing water (this may be the reason for the existence of water reducing agent water reduction rate limit). This part of the water is wrapped by cement molecules, so it cannot participate in the free flow between cement molecules to play the role of lubrication, thus affecting the fluidity of the concrete mix. When the polycarboxylic acid water reducing agent molecules start to contact with cement particles, the negatively charged main chain in the polycarboxylic acid water reducing agent molecules can be adsorbed on the surface of positively charged cement particles, i.e. the phenomenon of "anchoring". The long side chain of polycarboxylic acid water reducing agent molecule is stretched in the liquid phase of cement slurry, forming a certain thickness of adsorption layer, and can be simultaneously stretched with other long side chains of polycarboxylic acid water reducing agent molecule in the liquid phase of the cement slurry to form a three-dimensional, cross conformation. When the cement particles move in opposite directions, the adsorption layer overlaps, and the greater the overlap, the greater the repulsion between cement particles, thus improving the dispersion of cement particles, destroying the flocculation structure between cement particles, and releasing the water trapped by cement flocculation. Number.
② Electrostatic repulsion theory: Polycarboxylic acid water reducing agent molecule contains anionic groups (COO2-). And in the early hydration process of cement, the surface of cement particles contains a positive charge (Ca2+). Therefore, the anionic polycarboxylic acid water reducing agent molecule adsorbs to the surface of cement particles containing a positive charge. It turns the cement particles into a negatively charged "hedgehog". Because of the same negative charge, each cement particle repels the other (electrostatic repulsion), which improves the dispersion between cement particles. At the same time, the negatively charged carboxyl group in the cement paste and the positively charged calcium ion generate unstable complexes, and the calcium ion dissolved in the concrete system is captured, the concentration of calcium ion is significantly reduced, reducing the concentration of calcium ion in the system, which can reduce the formation of gel particles and thus slow down the hydration of cement, inhibit the hydration of cement and improve the dispersion of cement particles to maintain performance. The greater the content of carboxylic acid ions in polycarboxylic acid water reducing agent (the greater the anion charge density), the more it can substantially improve the dispersion performance of cement particles. The reduction of calcium ion concentration has an inhibitory effect on the initial hydration of cement, but as hydration continues, the complex will decompose on its own, so the polycarboxylic acid water reducing agent has a certain slow setting effect, and does not affect the strength of concrete.
③ Lubricating effect: The branched chains of polycarboxylate superplasticizer molecules have hydrophilic groups, and these groups form a water film on the surface of cement particles through the combination with water molecules, so that the surface energy of cement particles is reduced. The water film separates the cement groups and makes them easy to slide with each other. The combined effect of the above makes the cement particles separate from each other. The macroscopic manifestation is a concrete mix with better fluidity.
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