The admixture that can significantly reduce the amount of mixing water under the condition that the concrete slump is basically the same is called a high-efficiency water-reducing admixture. High-efficiency water reducing agent has a strong dispersing effect on cement, which can greatly improve the fluidity of cement mixture and concrete slump, while greatly reducing water consumption and significantly improving the workability of concrete. This article mainly discusses the effect of high-efficiency water-reducing admixture on the structure of hardened cement stone and the mechanism of water-reducing admixture.
High-efficiency water reducing agent is suitable for precast and cast-in-place reinforced concrete in various industrial and civil construction, water conservancy, transportation, ports, municipal and other projects; suitable for high-strength, ultra-high-strength and medium-strength concrete, and Concrete requiring early strength, moderate frost resistance, and high fluidity; suitable for precast concrete components in the steam curing process; suitable for water-reducing reinforcement components (i.e., masterbatch) of various composite admixtures.
1. The effect of high-efficiency water-reducing admixture on fresh concrete
1.1 Water-reducing effect.
(1) After adding high-efficiency water-reducing admixture to concrete, the water-cement ratio can be significantly reduced while maintaining fluidity. The water reduction rate of high-efficiency water-reducing agents can reach 10% to 25%, while the water-reducing rate of ordinary water-reducing agents is 5% to 15%. This is why high-efficiency water-reducing agents are named. The reason for the water-reducing effect is mainly due to the adsorption and dispersion of water-reducing agent by concrete.
(2) During the process of mixing with water and setting and hardening, cement will produce some flocculation-like structure. There are many reasons for the formation of flocculated structure. It may be due to the different charges carried by cement minerals (C3A, C4AF, C3S, C2S) during the hydration process, resulting in flocculation due to the attraction of opposite charges; it may also be due to the cement particles in the solution. The thermal movement of particles collides, adsorbs, and attracts each other at certain edges and corners to form a flocculated structure; van der Waals attraction between particles and the initial stage of hydrolysis and hydration reactions can also cause flocculation. In these flocculated structures, a lot of mixing water is wrapped, thereby reducing the amount of water required for cement hydration and reducing the workability of fresh concrete. In order to maintain the required workability of fresh concrete during construction, the amount of water must be increased accordingly during mixing, which will promote the formation of excessive pores in the cement stone structure, thus seriously affecting a series of physical properties of hardened concrete. Mechanical properties, if this excess water can be released, the water consumption of concrete mixing can be greatly reduced. Adding an appropriate amount of water reducing agent during the preparation of concrete can play this role well.
(3) Studies have shown that after adding a water-reducing agent, the hydrophobic groups of the water-reducing agent are directionally adsorbed on the surface of the cement particles, and the hydrophilic groups are directed toward the aqueous solution, forming a single-molecule or multi-molecule adsorption membrane. .
(4) Due to the directional adsorption of surfactant molecules, the surface of cement particles has charges of the same sign. Therefore, under the action of electrical repulsion, not only the cement water system is in a relatively stable suspension state (the electric double layer potential increases), and disperses and disintegrates the flocculated structure formed in the initial stage of adding water to the cement, releasing the free water in the flocculated aggregates, thereby achieving the purpose of water reduction.
1.2 Plasticization.
(1) After adding water-reducing agent to concrete, the fluidity can be increased while keeping the water-cement ratio unchanged. General water-reducing agents can increase the slump of fresh concrete by more than 10cm while keeping the cement dosage unchanged. High-efficiency water-reducing agents can prepare concrete with a slump of 25cm.
(2) In addition to the effects caused by adsorption and dispersion mentioned above, the plasticizing effect also has wetting and lubricating effects.
Wetting effect: After the cement is mixed with water, the surface of the particles is moistened by water. Its moistening status has a great impact on the performance of fresh concrete. When this type of diffuse wetting occurs naturally, it can be calculated by the equation proposed by Glbbs the amount by which surface free energy is reduced.
(4) Lubricating effect: The polar hydrophilic groups in the water-reducing agent are directionally adsorbed on the surface of cement particles and can easily associate with water molecules in the form of hydrogen bonds. The force of this hydrogen bond association is much greater than the molecular attraction between water molecules and cement particles. When the cement particles absorb enough water-reducing agent, with the help of the hydrogen bond association between R-SO3θ and water molecules, coupled with the hydrogen bond association between water molecules, a stable layer of solvation layer is formed on the surface of the cement particles. Water film, this film plays a three-dimensional protective role, prevents direct contact between cement particles, and acts as a lubrication between particles.
(5) The addition of water-reducing agent is accompanied by the introduction of a certain amount of bubbles (even non-air-entraining water-reducing agents will introduce a small amount of bubbles).
These tiny bubbles are surrounded by a molecular film of directional adsorption of the water-reducing agent, and have the same sign of charge as the cement particle adsorption film. Therefore, the electrical repulsion between the bubbles and the cement particles causes the cement particles to Dispersion, thereby increasing the sliding ability between cement particles (such as ball bearing action). This effect is more obvious for concrete mixed with air-entraining water-reducing admixtures.
(6) Due to theWith the dispersion, wetting and lubrication effects, concrete can be easily mixed evenly with only a small amount of water, thus improving the workability of fresh concrete.
2. Effect of high-efficiency water-reducing agent on the structure of hardened cement stone
(1) Due to the dispersion effect of water-reducing agent, more cement particles remain isolated and hydrated In the early stage, the reaction area of cement particles is increased. The better the dispersion of the water-reducing agent is, the more obvious this effect will be. At this stage, the cement hydration reaction proceeds in the form of a dissolution-hydration-crystallization process. In addition to the surface area, there are also salt effects and the formation of unstable complexes, which increase the solubility and accelerate the dissolution process of cement, thereby increasing hydrates. Although the film formation of the water-reducing agent will hinder the reaction, its impact is small. Its overall effect is to increase the initial hydration reaction speed.
(2) The cement has a certain geometric shape after final setting. What is formed in the initial stage is mainly insufficiently developed microcrystalline agglomerates, or hydrate gels. These very small crystallites are deposited randomly on the surface of cement clinker particles. Continuing hydration causes these microcrystals to grow outward in a radial pattern, forming fibrous crystals with sharp and forked ends. These fibrous crystals grow around the cement particles to form many pores of varying sizes, enclosing water in the middle. Continued hydration causes the fibrous crystals to elongate outward again, causing the cement particles to overlap each other to form a three-dimensional network structure. Further hydration makes the network structure gradually denser and increases its strength.
(3) The water-reducing agent somewhat delays the transformation process from gel to crystal. This is because the water-reducing agent increases the solubility and accelerates the dissolution rate. After the initial addition of water-reducing agent, there will be more microcrystalline agglomerates in a metastable state, and a layer of water-reducing agent film on the surface hinders and delays the transformation process of microcrystals to crystalline state. From the perspective of thermodynamic stability, small particles have a large surface area, high surface free energy value, and the thermodynamic state is unstable. The microcrystalline agglomerates will automatically dissolve and redeposit on the crystal surface to make them enter the water-reducing agent, which reduces the solid-liquid interface. grow up. The added interface energy makes the free energy change of the transformation process smaller, which weakens the tendency of the gel transformation process.
(4) As the reaction proceeds, hydration products gradually accumulate on the surface of cement particles. The faster the speed of the previous stage, the more products there are and the more hydrates covering the cement particles, but it has not yet constituted the main control factor on the reaction speed. At this stage, the hydration reaction is still dominated by the dissolution reaction process, and the solution has basically reached saturation. The ion diffusion rate that controls the reaction rate is close to a constant. The combined effect of several effects keeps the hydration reaction rate unchanged. Adding a water-reducing agent to form a complex will affect the ability of the reactants to participate in the reaction; the film formed will also hinder the hydration reaction; the association of the water-reducing agent with water molecules will affect the movement of water molecules, and the hydration products in the previous stage will The amount also affects the speed of this stage. Therefore, adding water reducing agent will slow down the cement hydration reaction at this stage.
(5) In the middle and late stages of the hydration reaction, after the hydration product reaches a certain thickness, the diffusion rate of water molecules through the hydration product layer will become the main factor controlling the hydration reaction. The hydration reaction begins with Solid phase reaction. Add water-reducing agent to make the water in the capillary pores become a solution with a certain concentration of water-reducing agent. Due to the reaction of osmotic pressure on diffusion, it will hinder the diffusion of water molecules into the hydration product layer, making the capillary pore diameter smaller, which will increase the The cohesive force of water in the macropores binds water molecules. Coupled with complexing, film-forming and other functions, the water-reducing agent slows down the hydration reaction of cement in the middle and late stages.
(6) From the perspective of pore structure, one of the main purposes of water reducing agents is to reduce water consumption, which makes the capillary pores in hardened cement stone smaller and the pore volume reduced. When water consumption is not reduced, although the total pore volume does not change much due to dispersion and crystallization inhibition, the capillary pore size becomes smaller, which will have a significant impact on the improvement of cement stone strength. </p
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