Polycarboxylate superplasticizer is a kind of high performance superplasticizer, which is a kind of cement dispersant in the application of cement concrete. It is widely used in highway, bridge, dam, tunnel, high-rise building and other projects.
Function in cement
Water reducing agent is an important role of surfactant in the hydration process of cement; Or the workability of concrete can be improved without changing the unit water consumption; At present, the concrete superplasticizers used are all surfactants, which belong to anionic surfactants. After cement and water are mixed, hydration reaction occurs, and some flocculating structures appear. It is wrapped with a lot of mixing water, thus reducing the workability of fresh concrete (also known as workability, It mainly refers to the performance of fresh concrete in construction, that is, in the process of mixing, transportation and pouring, it can keep uniform and dense without delamination and segregation. In order to maintain the required workability in construction, it is necessary to increase the mixing water volume, because the increase of water volume will cause excessive pores in the cement structure, which will seriously affect the physical and mechanical properties of hardened concrete, If the water can be released, the water consumption of concrete can be greatly reduced. In the process of preparing concrete, adding an appropriate amount of water reducing agent can play such a good role
When adding water reducing agent
When water reducing agent is added into concrete, the hydrophobic group of water reducing agent will be adsorbed on the surface of cement particles, while the hydrophilic group will be adsorbed on the water solution, forming a monolayer or multi-molecular layer adsorption film. Due to the directional adsorption of surfactant, the surface of cement particles will have the same sign of charge. Therefore, under the action of homomorphism and repulsion, not only the cement water system will be in a relatively stable suspension state, The addition of water reducing agent can not only improve the workability of fresh concrete, but also reduce the pore volume and make the cement stone more compact due to the decrease of water cement ratio, The addition of superplasticizer also affects the hydration rate and setting time of cement. These properties are very important in practice
After mixing with water, the cement particles will attract each other to form a flocculation structure. This flocculation structure will wrap 10% – 30% of water between cement particles, so that these water molecules can not flow freely and play a lubricating effect. The flocculation of cement will affect the fluidity of cement paste. Polycarboxylic acid molecules can make the surface of cement particles with negative charge by adsorption on the surface of cement particles, thus forming electrostatic repulsion, promoting the dispersion of cement particles, destroying the flocculation structure, releasing the encapsulated water molecules to participate in the flow. This is the reason why polycarboxylate superplasticizer has water reducing effect. It can be understood that polycarboxylate superplasticizer is to improve the utilization rate of water molecules, and does not increase or reduce the molecular weight of water for no reason. At the same time, the formation of COO2- and Ca2+ ions reduced the calcium concentration in cement pastes, delayed the crystallization of Ca (OH) 2, and reduced the formation of C-S-H gel, thus slowing down the hydration process of cement. Therefore, polycarboxylic acid molecules also have a certain retarding effect. In addition, the difficulty of 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 are achieved by adding sodium nitrite and triethanolamine.
The branched chain of polycarboxylate superplasticizer has hydroxyl and ether (- c-o-c-c), which is hydrophilic. These groups combine with water molecules through hydrogen bonds to form a water film on the surface of cement particles. This water film has good lubrication effect, which can effectively reduce the resistance between cement particles and increase the fluidity of concrete.
The branched chain of polycarboxylate superplasticizer structure has a spatial structure at the micro level, just like “xiaopang” in the cartoon above, which prevents the “big fat” cement particles behind by curling hair. The closer the cement particles are, the greater the steric hindrance effect is. This effect increases the slump retaining effect of concrete.
During the preparation of polyacid superplasticizer, some branched chains will be grafted on the superplasticizer molecules. These branched chains can not only provide steric hindrance effect, but also be cut off gradually in the high degree of cement hydration environment, so as to release polycarboxylic acids with dispersing effect, which can improve the dispersing effect of cement grains and control the slump loss.