The role of external additive water reducing agent and its use
2015/06/10 Source：China Cement Spare Parts Network
Water reducing agent is an admixture that can reduce the amount of mixing water and increase the strength of concrete under the condition of unchanged concrete compatibility and cement dosage; or save cement dosage under the condition of unchanged compatibility and strength. Compared with ordinary water reducing agent, water reduction and enhancement are stronger.
A, the role of water reducing agent
1) Electrostatic repulsion theory
After hydration of cement, due to the van der Waals force between ions and cement hydration minerals, the main minerals of cement in the hydration process with different charges and cohesion, resulting in a flocculation structure of concrete. Water reducing agent is mostly anionic surfactants, mixed into the concrete, water reducing agent in the negative ions - SO-, - COO- will be adsorbed on the cement particles under the action of the positive charge Ca2+ mineral, the formation of diffusion double electric layer (Zel. a potential) ion distribution, the formation of the surface .
2) Steric potential resistance effect
In the cement slurry mixed with water reducing agent, the long chain of organic molecules of water reducing agent is actually present on the surface of cement particles in various adsorption states. Different adsorption state is due to the different structure of the molecular chain of high efficiency water reducing agent, which directly affects the slump of concrete mixed with this type of water reducing agent changes over time. Some studies have shown that the adsorption state of naphthalene and melamine water reducing agent is rod-like chain, so it is a flat adsorption, electrostatic repulsion is weak. The result is that the Zeta potential decreases quickly and the electrostatic equilibrium is easily destroyed with the development of cement hydration process, so that Van der Waals gravity dominates and the slump changes over time.
3) Lubrication effect
The polar hydrophilic group of water reducing agent is oriented to adsorb on the surface of cement particles, mostly in the form of hydrogen bonding with water molecules, plus the hydrogen bonding between water molecules, constituting a stable water film on the surface of cement particles, preventing direct contact between cement particles, increasing the sliding capacity between cement particles, playing a lubricating role, thus further improving the fluidity of the slurry. Cement slurry towel of tiny bubbles, the same for water reducing agent points of the directional adsorption of polar groups wrapped, so that the bubble and bubble and bubble.
After the concrete is mixed with water reducing agent, along with the hydration reaction, water reducing agent molecules dispersed in the dispersion system, uniform adsorption on the surface of cement particles, destroy the agglomeration of cement particles, so that the cement particles due to the special role of the presence of water reducing agent molecules in a highly dispersed stable state. At low water content, it has high fluidity. For high performance water reducing agent in the cement particle surface adsorption state and dispersion mechanism of research there are many, which are more famous stereo effect theory, vacancy stabilization theory, D-L-V-O theory, etc..
Second, the use of water reducing agent
1. In the case of not changing the ratio of various raw materials (except cement) and the strength of concrete, can reduce the amount of cement, mixed with cement quality 0.2% to 0.5% of the concrete water reducing agent, can save the amount of cement 15 ~ 30% or more.
2. Without changing the ratio of various raw materials (except water) and the slump of concrete, reducing the amount of water can greatly improve the strength of concrete, the early strength and late strength are 60% and more than 20% higher than the concrete without water reducing agent, by reducing water, it can realize the pouring of C100 grade high-strength concrete.
3. Without changing the ratio of various raw materials, the rheology and plasticity of concrete can be greatly improved, so that concrete construction can be carried out by self-flow, pumping, without vibration, etc., improving the construction speed and reducing construction energy consumption.
4. Mixing with concrete high-efficiency water reducing agent can improve the life of concrete by more than double, even if the normal service life of the building is extended by more than double.
5. Reduce the shrinkage rate of concrete solidification and prevent cracks in concrete members; improve frost resistance and facilitate winter construction.