Do you have a comprehensive knowledge of concrete admixtures? Five technical knowledge questions and answers to help you fully analyze the key points of knowledge.
1. What is the origin of the development of concrete admixtures?
Before concrete was produced, the great Chinese people had already begun to add chemical substances to building materials to improve building materials.
When the Qin Dynasty built the Great Wall in 221 BC, clay, lime, etc. were used as gel materials, and glutinous rice juice, pig blood, tofu juice, etc. were used to increase its viscosity. This can also be regarded as the earliest admixture.
In 258 AD, Cao Cao added vegetable oil to the dust to build the Tongque Tower. Glutinous rice juice was added to the lime used in the Hezhou City Wall in the Song Dynasty.
In the Ming Dynasty, "Tiangong Kaiwu" recorded that 1 part lime, 2 parts river sand, glutinous rice and Yang peach vine juice were added and mixed evenly to make a storage tank.
2. How does the water-reducing agent increase the fluidity and strength of concrete?
After the cement is mixed with water, due to the molecular attraction between cement particles, , and produce many flocs, forming a flocculation structure. In this structure, the cement particles are surrounded by a lot of mixing water, thereby reducing the workability of the concrete mixture. At this time, if an appropriate amount of water-reducing agent is added, due to its surface activity, the hydrophobic end will be directionally adsorbed on the surface of the cement particles, and the hydrophilic end will point to the aqueous solution, so that the surfaces of the cement particles will be charged with the same charge. The electrostatic repulsion between cement particles is increased, causing the cement particles to disperse with each other. The flocculation structure disintegrates and the trapped free water is released, thereby effectively increasing the fluidity of the concrete mixture. On the other hand, due to the dispersion effect of water-reducing agent on cement, the surface of cement particles in contact with water increases and the hydration is relatively sufficient, thereby improving the strength of concrete.
3. What is the technical and economic effect of high-performance concrete water-reducing agent?
① When the water consumption remains unchanged, the slump of concrete can increase by 10 to 20cm;
② Keeping the workability of concrete unchanged, water can be reduced by 10% to 50%, and concrete strength can be increased by 15% to 20%, especially the early strength improvement is more significant;
③ When the strength of concrete is kept constant, the cement consumption can be saved by 10% to 15%;
④ The heat release rate of cement hydration slows down and the heat peak is delayed;
⑤ Concrete The phenomenon of bleeding and segregation has been greatly improved;
⑥The water permeability of concrete can be reduced by 40% to 80%, and the ability to resist permeability, frost resistance, chemical corrosion and rust is improved;
⑦Special concrete can be prepared, which will be more economical, simpler and more flexible than using special cement.
4. What items should be inspected for the quality of admixtures?
1. Quality inspection of admixture products, measuring the composition, performance and quality stability of a certain admixture to determine whether it is Reach the specified physical and chemical performance indicators. Such as appearance, active substance content, solubility, specific gravity, pH value of aqueous solution, chloride ion content, sulfate content, sugar content, foaming property, surface tension and whether there is rust on steel bars, etc.
2. Performance testing of admixtures mixed into cement slurry (also known as clean slurry) is used to check the performance indicators that admixtures should meet when adapting to different types of cement. Its measurement items include slurry fluidity, setting time, exothermic changes in hydration heat, stability, settlement test, etc.
3. The admixture is mixed into the concrete for adaptability test, that is, the concrete mixed with the admixture is compared with the benchmark concrete to test the water reduction rate, bleeding rate, and content of the new concrete. Air volume, condensation time, slump loss, and test the compressive strength, impermeability, frost resistance, shrinkage, creep, and corrosion of steel bars after hardening of the concrete.
5. How to choose pumping concrete admixtures?
General admixtures have the functions of retarding, reducing water, and strengthening. Composite powder admixtures also increase the micro-expansion and anti-permeability effect. , for pumped concrete construction, the impact of admixtures on the pumping effect is mainly considered. Therefore, there are several main aspects to consider when selecting admixtures.
1. Most air-containing admixtures have an air-entraining effect after being added to concrete, producing a certain proportion of particulate bubbles. If the air content is too low, it will affect the fluidity and pumpability of concrete, so it should be controlled within a reasonable range. In addition, adjusting the gas generating agent in the admixture will also increase or decrease the strength level of concrete.
2. Water reduction rate An important indicator of admixtures is the water reduction rate. Reducing water consumption and lowering the water-cement ratio are important means to improve the strength of concrete. Generally, water consumption is reduced by about 15%. For example, for C30 concrete, if the cement dosage is 400kg and the water-cement ratio is 0.5, the water consumption should be 200kg, the water reduction rate is 15%, and the water consumption is reduced by 30kg. The actual water use is 170kg, the water-cement ratio becomes 0.42, and the strength level increases. More than 10%.
3. Slump After using water-reducing agent, when the concrete is transported to the construction site, the concrete pump pipe is blocked due to the sharp loss of slump.
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