With the development of science and technology, people have put forward various new and higher requirements for the performance of concrete. Since the promotion of concrete admixtures in the 1940s, its development has not only changed the internal structure of hardened concrete from the microscopic submicroscopic level, but also changed the structure of fresh concrete in the process.
Water reducing agents, also known as dispersants or plasticizers, are the most commonly used and most important admixtures. When it is used, the water consumption of the fresh concrete can be reduced without affecting the workability of the concrete. Its main component is surfactant, and its effect on fresh concrete is also mainly surface active.
The water reducing agent can reduce the water consumption of the concrete mixture, improve the strength and durability of the concrete, impermeability, improve the workability of the mixed soil, improve the construction speed and construction quality, meet the mechanized construction requirements, and reduce the noise and Labor intensity, saving cement and so on.
Water reducing agent development history
In the 1930s, it was found that the incorporation of sulfite pulp waste in concrete improved the workability of the mixture, and the strength and durability were also improved. In 1935, the E.W.Scripture in the United States was first developed as a water reducing agent containing lignin sulfonate as a main component.
In 1962, Japan first developed a water reducing agent containing sodium salt of B-naphthalenesulfonic acid formaldehyde condensate as a main component, which is referred to as a naphthalene water reducing agent. These water-reducing agents have the characteristics of high water-reducing rate and are suitable for preparing high-strength (compressive strength up to 100 MPa) or slump up to 20 cm.
Subsequently, in 1964, the Federal Republic of Germany successfully researched sulfonated melamine formaldehyde resin water reducing agent. This type of water reducing agent and naphthalene system water agent have the characteristics of high water reduction rate, good early strength and low air entrainment, and steamed concrete. Cement products with high content of products and aluminates (mainly C3A) have good adaptability and can produce high-strength or large-flow concrete.
In the late 1970s, many people improved the lignin-based water reducing agent and developed a modified lignosulfonate superplasticizer.
In the early 1990s, the United States first proposed the concept of high-performance concrete (Hc), which required high-strength, high-flow, high-endurance properties. High-performance concrete puts higher demands on water-reducing agents and requires high performance. The water reducing agent has the characteristics of high honest water rate, large fluidity and small loss of slump. Some new high-efficiency water-repellent agents have been rapidly developed and applied, such as polycarboxylic acid-based and sulfamic acid-based high-efficiency water-repellent agents.
In summary, the water reducing agent has undergone a history of development from lignin sulfonate, naphthalene sulfonate condensate, melamine formaldehyde condensate, sulfamate system, polycarboxylic acid system, etc., and the water reduction rate is also from 8 % increased to around 30%. The application of high-performance water reducer means that concrete with the same performance can save 20%~30 of cement, energy saving from the source, saving money, and clean production.
Type of water reducing agent
The types of water reducing agent are lignosulfonate, naphthalene water reducing agent, melamine water reducing agent, polycarboxylate water reducing agent, casein water reducing agent, sulfamate water reducing agent, acrylic acid reduction Aqueous agent, etc.
Lignosulfonate: It is a common water-reducing agent. Its raw material is lignin, which is generally extracted from coniferous wood. The lignin is polymerized by three lignin monomers such as musk alcohol, coniferyl alcohol and sinapyl alcohol. In the mortar, it can improve the workability, fluidity, and strength, and the water reduction rate is 5%-10%.
Naphthalene sulfonate water reducing agent: It is the first high-efficiency water reducing agent used in China. It is a product of naphthalene sulfonation by sulfuric acid and condensation with awakening. It is an anionic surfactant. The appearance of this kind of water reducing agent can be light yellow to dark brown powder depending on the product. It is easily soluble in water and has good dispersion effect on many powder materials such as cement. The water reduction rate is 25%.
Melamine water reducing agent: a product of melamine sulfonation by sulfuric acid and condensation with formaldehyde, and thus the chemical name is sulfonated melamine formaldehyde resin, which is an anionic surfactant. The water-reducing agent has a white powder appearance, is easily soluble in water, has good dispersion of the powder material, has high water-reducing rate, and has good fluidity and repairability.
Powder polycarboxylate: It is a new type of high performance water reducer developed in recent years. It has excellent water reduction, fluidity and permeability. The strength of the cement mortar is obviously enhanced, but the production process is complicated and the general price is high.
Casein: It is a biopolymer obtained by precipitating milk with acid and drying it through a cylinder.
The principle of the water reducing agent
The water reducing agent is usually a surfactant and is an anionic surfactant. It adsorbs on the surface of the cement particles to make the particles exhibit electrical properties. The particles repel each other due to the same charge, so that the cement particles are dispersed to release excess water between the particles to produce water reduction. On the other hand, due to the addition of the water reducing agent, the surface of the cement particles forms an adsorption film, which affects the hydration speed of the cement, makes the growth of the cement stone crystals more perfect, reduces the capillary gap of water evaporation, and the network structure is more compact and improved. The hardness and structural compactness of cement mortar. It is divided into the following steps: Dispersion: After cement and water mixing, due to the molecular gravitational effect of cement particles, the cement slurry forms a floc structure, so that 10% ~ 30 of the mixing water is wrapped in the cement particles, not Participates in free flow and lubrication, which affects the fluidity of the concrete mix. When the water reducing agent is added, since the water reducing agent molecules can be adsorbed on the surface of the cement particles, the surface of the cement particles carries the same electric charge (usually a negative charge), forming an electrostatic repulsion, causing the cement particles to disperse and disintegrate the flocculation structure. , releasing part of the water that is wrapped and participating in the flow, thereby effectively increasing the fluidity of the concrete mixture. Lubrication: The hydrophilic group in the alkaline water agent is very polar, so the water-reducing agent adsorption film on the surface of the cement particles can form a stable solvated water film with water molecules. This water film has good lubrication effect. It can effectively reduce the sliding resistance between the cement particles, thereby further improving the fluidity of the mixed soil. The steric hindrance effect: a polyether side chain having hydrophilicity in the structure of the water reducing agent, extending in the aqueous solution, thereby forming a hydrophilic stereoscopic adsorption layer having a certain thickness on the surface of the adsorbed cement particles. When the cement particles are close, the adsorption layers begin to overlap, that is, steric hindrance occurs between the cement particles. The more overlap, the greater the steric hindrance force, and the greater the hindrance to the cohesion between the cement particles, causing the concrete to fall. The degree is kept good. Slow release of graft copolymerization branch: a new type of water reducing agent such as polycarboxylic acid watering agent is grafted with some branches on the molecule of the water agent during the preparation process, and the branch can provide not only space The resistance effect, and in the high alkali content environment of cement hydration, the branch can be slowly cut off, thereby releasing the polycarboxylic acid having a dispersing action, thereby improving the dispersion effect of the cement particles and controlling the enthalpy. Loss of the drop.
Effect of water reducing agent on the performance of concrete
1. Effect of water reducing agent on rheological properties of fresh concrete
In order to prepare fresh concrete with good flow properties, it is necessary to dismantle the viscous structure that reduces the flow between the cement particles, so that the cement particles are well dispersed in the aqueous medium. There are many properties affecting cement gelation, such as the mineral composition of cement, the shape and size of cement particles, the completeness of mineral crystallization, and the operation of parent and environmental factors. The various factors described above directly or indirectly control the stability of the cement particles in the slurry. Different media conditions may change the value of the charge of the cement particles in the slurry, that is, change the electrostatic repulsion between the particles.
When a proper amount of water reducing agent is added to the fresh mixed bauxite, the potential of the cement particles increases, and the electrical repulsion between the cement particles increases greatly, resulting in a decrease in the viscosity of the fresh mixed soil, thus promoting the entire dispersion system. The stability is improved and the fluidity is improved. In addition, there is an intermediate state between the cement slurry from the diluted to the condensed state, that is, the thixotropic state. This is caused by the re-dispersion of the agglomerated structure in the cement paste at an increased shear rate. Specifically, when the shear rate increases, the resistance decreases and the viscosity decreases. That is, the slurry is not in a state of segregation at the same time, and the fluidity is regained if the agglomerated slurry is stirred or shaken. Generally, the addition of an appropriate amount of water reducing agent to the cement slurry can promote the strong thixotropy of the fresh concrete. This is due to the formation of the solvation film layer of the water-reducing agent on the surface of the cement particles and the increase of the potential, and if it is slightly vibrated, it will exhibit better fluidity. The thixotropy of fresh concrete without water reducer is much weaker.
2. Effect of water reducing agent on the workability of fresh concrete
There are many factors affecting the workability of fresh concrete, mainly cement, aggregate, water consumption, the nature and amount of admixture, temperature and other factors. When the other conditions are the same, the workability is related to the type and amount of the water reducing agent. The workability of fresh concrete is usually measured by the slump value. After mixing from concrete to watering, it is necessary to have a period of transportation waiting for parking time, which often makes the concrete workability worse and causes construction difficulties. Experiments show that the use of water reducing agent can improve the initial workability of concrete, but often its slump loss is larger than the reference concrete without water reducer, the reasons are: (1) mineral adsorption and water reduction in cement The ability of the agent is strong or weak. The order of the main mineral adsorption water reducing agent in cement is C3AQAF>C3S>CzS. When the water is stirred, more dispersing agent will accumulate on the surface of the cement particles, and the concentration of the water reducing agent will decrease in the whole liquid phase. The dewatering dose of the dispersion is gradually insufficient, and the slump gradually decreases with time. (2) Bubble overflow and evaporation of water. Even if the non-aerobically water-reducing agent is incorporated into the concrete, there is a certain gas introduction, and during the transportation and the like, the bubbles continuously overflow and dissipate, and accompanied by evaporation of the water, the performance of the superplasticizer is particularly remarkable. (3) After the water-reducing agent is added, due to the action of dispersing and wetting, the initial hydration speed of the cement is too fast, the hydration product increases, the solid content increases, and the viscosity of the whole system increases, causing the slump value to decrease rapidly, and the high temperature condition More even worse.
3. Effect of water reducing agent on concrete setting time
Concrete setting time is an important parameter in construction, especially for mass concrete construction. Appropriate amount of retarder can delay the setting time of concrete and solve the problems in construction.
4. Effect of water reducer on concrete compressive strength
Compressive strength is one of the most important mechanical properties of mixed soil. Under certain conditions, engineering requires that other properties of concrete tend to be closely related to the strength of the mixed soil. The basic starting point for studying the theory of concrete strength for a long time is to consider the compressive strength of cement stone as the main influencing factor, and to establish a series of relationships between cement void ratio and compactness and strength:
R=ARC(CW-B) where: R-concrete compressive strength,
A B empirical constant: actual strength of RC-cement; c/w-grey water ratio.
It can be seen from the above formula that the important factor of concrete strength is the water-cement ratio of the cement installation and the degree of hydration. After adding the water reducing agent, the water-cement ratio in the concrete is greatly reduced, the void volume inside the cement stone is obviously reduced, and the cement stone is more dense, so that the compressive strength of the concrete is significantly improved.
5. Effect of water reducing agent on durability of concrete
5.1 Impact on freezing and thawing
The freeze-thaw resistance of concrete is subject to two important factors, the water-cement ratio and the combined gas volume, under the same conditions. The experiment found that the water-cement ratio of concrete is reduced, its freeze-thaw resistance is better, and the water-reducing agent with certain air-inducing effect is added, and its freeze-thaw resistance is more improved. At present, the commonly used water reducing agents in China have different degrees of water reduction and air entrainment, so it will also be beneficial to improve the freeze-thaw resistance of concrete.
It should also be proved that the porous multiphase aggregates of concrete contain pores of various sizes, and the A properties of water in the pores vary greatly depending on the pore size. From the research results of the relationship between concrete bubble structure and freeze-thaw resistance, it is found that even if the same amount of air is introduced into the concrete, the structure of the bubble in the concrete, that is, the diameter and distribution shape of the bubble, is different due to the different types of the admixture. The effects of freeze-thaw performance are also significantly different. Generally, the introduction of 2% gas into the concrete can improve the durability of the mixed soil. If the air volume exceeds 6%, the strength of the concrete will be significantly reduced, and the durability will also decrease. Therefore, the suitable gas content control range is generally 2% 6-5%.
5.2 Influence of impermeability Generally, the water-cement ratio is relatively large when the water-cement ratio is greater than 0.55. Since the water used for mixing and mixing the soil far exceeds the water required for cement hydration, it is in concrete. The existence of priming residual water, early evaporating water and bleeding channels will lead to a sharp increase in the permeability of concrete, but if the water-cement ratio is too low, it cannot be made due to the poor workability of concrete. Fully dense concrete structure, its impermeability can not be improved. The use of water reducing agent or air-entraining water reducing agent can greatly reduce the mixing water consumption under the same conditions of workability. If a water reducing agent that introduces an appropriate amount of microbubbles is incorporated, it will have a good effect on improving the impermeability and antifreeze performance by reducing the bleeding channel, and if a certain amount of expansion agent is compounded in the water reducing agent, in limited conditions It is also an effective way to make dense concrete with good impermeability.
5.3 Effect on carbonation and steel corrosion
The carbonation of concrete is closely related to the durability of reinforced concrete structures. The concrete structure is subjected to CO2 from the surface, and the hydration product Ca(OH) in the concrete slowly becomes CaCO and loses alkalinity. When the carbonization penetrates into the steel part, the 'passivation film' that originally protects the steel bar is destroyed, so that the steel bar is electrochemically corroded. When the admixture is added and the admixture contains a large amount of chloride ions, the electrochemical corrosion effect on the steel bars will be significantly increased. For this reason, the chloride ion content in reinforced concrete should be strictly controlled. The test results show that the concrete carbonization rate of the water-reducing agent is obviously slower than that of the concrete without water-reducing agent, which can overcome the defects of low carbonation resistance of the slag cement, so that it can reach the level of anti-carbonization of ordinary cement, and the total steel reinforcement of concrete The risk of rust is significantly reduced. If the rust inhibitor (such as sodium nitrite) is compounded in the water reducing agent, the rust resistance of the steel bar will be further improved, and the overall durability of the concrete will be significantly improved.
6. Conclusions and prospects
(1) The effect of the water reducing agent greatly improves the physical properties of the fresh concrete, improves the strength grade and durability of the concrete after hardening, and saves the amount of cement.
(2) The effect of water reducing agent on cement performance has multiple effects. When evaluating the effect of water reducing agent on concrete performance, it should be fully considered. According to different purposes, different water reducing agents are selected, which will help the influencing factors.
(3) The compatibility of water reducer with cement is also one of the indicators for evaluating the performance of water reducer. If incompatibility occurs, the performance of concrete will not be improved, and even concrete members will be more susceptible to cracks.