"Water reduction" mechanism of polycarboxylic ether based superplasticizers
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Update time : 2022-10-21 09:49:53
Today's concrete superplasticizersare basically prepared by compounding various materials such as polycarboxylate water-reducing mother liquor and polycarboxylate slump-preserving mother liquor plus retarders, air-entraining agents, and defoaming agents. Among them, the role of water-reducing mother solution in superplasticizer is irreplaceable. Maybe the amount of mother solution will change due to the change of concrete material, but I have never heard that water-reducing mother solution is not added to superplasticizer because the sand is too good.
The water-reducing mother liquid is used as the absolute main material in the water-reducing agent, and its function is to reduce the concrete water-cement ratio (indirectly increase the strength of the concrete), and at the same time ensure the fluidity and workability of the concrete mixture, and ensure the strength of the concrete. At the same time, it also facilitates the work of the construction unit.
We know that in addition to cement and water in concrete, the effect of the remaining materials on the superplasticizer admixture molecules is basically adsorption (waste), so the water reducing agent is mainly aimed at cement. So first, let’s learn about the hydration process of cement.
The hydration process of general portland cement is divided into three stages:
(1) Dissolution stage: When the cement is in contact with water, the surface of the particles begins to hydrate, and a small amount of hydration products are generated, which are immediately dissolved in water. The exposed new surface allows hydration to continue until a saturated solution of hydration products is formed.
(2) Gelation stage: Due to the saturation of the solution, the product that continues to be hydrated can no longer be dissolved, but is directly precipitated as colloidal particles. With the increase of hydration products, the hydration aggregates, and the cement slurry gradually loses its plasticity, resulting in a coagulation phenomenon.
(3) Crystallization stage: The colloid composed of microscopic crystals is not stable, and can gradually recrystallize to form macroscopic crystals, so that the mechanical strength of the hardened body of the cement slurry is continuously improved, and finally it becomes a cement stone with a certain mechanical strength.
In the cement slurry, the dispersion mechanism of polycarboxylic ether based superplasticizers is generally considered to be the main role of steric hindrance and the secondary role of electrostatic repulsion. In addition, the complexation of calcium ions and the lubrication of hydration film also play a role. important role.
1. Steric hindrance:
The hydration process of cement will make the cement molecules attract each other, resulting in a flocculation structure.
During the formation of the flocculation structure, the cement molecules will encapsulate a part of the water molecules, and the encapsulated water accounts for about 10%-30% of the total mixing water (this may be the reason why the water reducing agent has a water reducing rate limit). Because it is wrapped by cement molecules, it cannot participate in the free flow and lubrication between cement molecules, thus affecting the fluidity of concrete mixtures. When the polycarboxylate concrete admixture molecules first come into contact with the cement particles, the negatively charged main chain in the polycarboxylate concrete admixture molecules can be adsorbed on the surface of the positively charged cement particles, that is, an "anchoring" phenomenon occurs, while the polycarboxylate superplasticizer The long side chain in the molecule is extended in the liquid phase of the cement slurry to form an adsorption layer with a certain thickness, and can form a three-dimensional and crossed long side chain with other polycarboxylic acid superplasticizer molecules extending in the liquid phase of the cement slurry at the same time. Conformation. When the cement particles move toward each other, the adsorption layer overlaps, and the larger the overlapping range, the greater the repulsion between the cement particles, thereby improving the dispersibility of the cement particles. Destroy the flocculation structure between cement molecules. The release of the water trapped by cement flocculation can also be understood as the water reducing agent molecules improve the lubricating utilization rate of the water molecules to the cement molecules, and do not increase the water or reduce the number of water molecules out of thin air.
2. The theory of electrostatic repulsion: Polycarboxylic acid superplasticizer molecules contain anionic groups (COO2-), and in the early hydration process of cement, the surface of cement particles contains positive charges (Ca2+), therefore, anionic Polycarboxylic acid superplasticizer molecules adsorb to positively charged cement particles surface. Make the cement particles into a "hedgehog" with negative charge. Because the cement particles with the same negative charge repel each other (electrostatic repulsion), the dispersibility between the cement particles is improved. At the same time, the negatively charged carboxyl groups and positively charged calcium ions in the cement paste form unstable complexes. After the calcium ions dissolved in the concrete system are captured, the calcium ion concentration decreases significantly, reducing the calcium ions in the system. It can reduce the formation of gel particles and thus slow down the hydration rate of cement, inhibit the hydration of cement, and improve the dispersion performance of cement particles. The greater the content of carboxylate ions in the Polycarboxylic acid superplasticizer, that is, the greater the anion charge density, the greater the dispersion performance of cement particles (acid-ether ratio). The decrease of calcium ion concentration can inhibit the initial hydration of cement. With the continuous progress of hydration, the complex will decompose by itself, so Polycarboxylic acid superplasticizer has a certain retarding effect without affecting the strength of concrete.
3. Lubrication
The branched chain of the polycarboxylic acid superplasticizer molecule has hydrophilic groups. These groups form a water film on the surface of the cement particles by combining with water molecules, which reduces the surface energy of the cement particles. Slips easily. The combined effect of the above results in separation between the cement particles. The macroscopic performance is that the concrete mixture has better fluidity.
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