1. Dispersion
After the cement is stirred with water, the cement particles will attract each other to form a flocculation structure of the slurry. This flocculation structure encapsulates 10%-30% of the water between the cement particles, so that these water molecules cannot flow freely and have a lubricating effect. The flocculation of cement will affect the fluidity of the cement slurry. Polycarboxylic acid molecules will make the surface of cement particles have a negative charge by adsorbing on the surface of cement particles, thus forming electrostatic repulsion, promoting the mutual dispersion of cement particles, destroying the flocculation structure, and releasing the encapsulated water molecules to participate in the flow. This is the reason why the polycarboxylate superplasticizer molecule has a water-reducing effect. It can be understood that the polycarboxylate superplasticizer molecule increases the utilization rate of water molecules, and does not increase or decrease the molecular weight of water for no reason. At the same time, the formation of a complex of COO2- and Ca2+ ions reduced the concentration of calcium ions in the cement slurry, delayed the formation of Ca(OH)2 crystallization, and reduced the formation of C-S-H gel, thereby delaying the cement hydration process. Therefore, the polycarboxylic acid molecule also has a certain retarding effect. As an aside, the difficulty in the research and development of early-strength polycarboxylic acid mother liquor may be related to its own retarding effect. At present, most of the early strength admixtures on the market rely on the compound to increase sodium nitrite and triethanolamine to achieve the purpose.
2. Lubrication
The branched chain of the polycarboxylic acid superplasticizer molecule has hydroxyl groups and ethers (-C-O-C-C) with hydrophilicity, and these groups are combined with water molecules in the form of hydrogen bonds to form a layer of water film on the surface of cement particles, which has a good lubricating effect, which can effectively reduce the resistance between cement particles and increase the fluidity of concrete.
3. Steric hindrance
The branched chain in the structure of the polycarboxylate superplasticizer has a spatial structure at the microscopic level, and the closer the cement particles are, the greater the steric hindrance effect. This effect increases the slump retention effect of the concrete.
The particle size of cement particles is at the micron level, and the polycarboxylic acid molecule is at the nanometer level, which is about 3 orders of magnitude worse. One cement particle can adsorb several polycarboxylic acid molecules. The adsorption of cement particles by polycarboxylic acid delays the hydration process, but with the hydration of cement, the polycarboxylic acid molecules will eventually be encapsulated by the hydration products and lose the effect of polycarboxylic acid. In general, the adsorption of polycarboxylic acid molecules does not change the properties of cement, but only changes the rheological process of concrete, and has no effect on the later strength of concrete.
Post time: Mar-10-2025