Water reducing agent (also known as superplasticizer, superfluidizer, high performance AE water reducing agent) is a key material for the preparation of modern concrete and is an indispensable component. The excellence achieved in the field of modern concrete comes mainly from the technological developments in the field of water reducing agents. The role of water reducing agents is to reduce the amount of mixing water while maintaining the same concrete slump, so that the water-cement ratio is at a minimum or to be able to significantly improve the fluidity of concrete or fresh mortar to maximize fluidity while maintaining the same amount of mixing water.

　　At present, there are many types of water reducing agent, according to its different water reducing capacity can be divided into ordinary water reducing agent, high efficiency water reducing agent, high performance water reducing agent; according to its appearance of the state can be divided into powder and water agent; according to the composition of its chemical composition can be divided into lignin sulfonate, naphthalene system, melamine system, amino sulfonate system, fatty acid system, polycarboxylate system, etc.. Water reducing agent has a dispersing effect on cement particles, the use of water reducing agent can not only adjust the fluidity of concrete slurry on a macroscopic level, but also from the microscopic, sub-microscopic change the structure of concrete slurry, so by adding the appropriate amount of water reducing agent can be modified from macroscopic and microscopic and other multi-angle concrete slurry. This paper will do a summary of the mechanism of water reducing agent adsorption, water reducing agent adsorption behavior at the cement-water interface and its interaction with cement components and hydration products.

　　1 water reducing agent mechanism of action

　　In recent years, the water reducing agent dispersion mechanism of action of a wide range of opinions, there is no completely unified concept. Most scientific and technological researchers believe that the water reducing agent is mainly in two aspects, one is the role of dispersion (electrostatic repulsion, spatial resistance), the second is the role of lubrication. Dispersion role refers to the concrete after the addition of mixing water, there will be flocculation structure, and after the addition of water reducing agent, water reducing agent molecules can quickly adsorbed on the surface of cement particles, so that cement particles with the same charge, electrostatic repulsion, so that the flocculation structure of cement is destroyed. The lubricating effect is because the water reducing agent is a kind of surfactant, and the polarity of hydrophilic group is extremely strong, so the water reducing agent molecules adsorbed on the surface of cement particles will produce a solventized water film with water molecules, and the water film will play a very good lubricating effect. Qu Jindong et al. believe that the naphthalene system and sulfonated melamine system and other traditional high-efficiency water reducing agent, the molecular skeleton is only a rigid straight chain, straight chain with many amine groups and sulfonic acid groups and other functional groups, in the form of plane rigid straight rod adsorption on the cement particles, after the water reducing agent is added to the cement cementitious system, the water reducing agent quickly adsorbed on the surface of the cement particles, so that the surface of the cement particles to produce a double electric layer structure, improve the concrete slurry The fluidity of the concrete slurry. Yan Bingshan et al. believe that the molecular structure of high performance water reducing agent such as polycarboxylic acid system is special, it is a kind of comb-type structure of copolymer, the form of adsorption is curly spatial three-dimensional adsorption, and the degree of curl increases with the increase of water reducing agent admixture. Jiang Nan et al. believe that the molecular skeleton of polycarboxylic acid water reducing agent consists of the main chain and side chain, the main chain, side chain contains a large number of functional groups such as hydroxyl, ether, ester, carboxyl, polyoxyalkyl, sulfonic acid groups. The mechanism of action of polycarboxylic acid system water reducing agent is that the main chain is adsorbed on the active site of cement particles, generating electrostatic repulsion and forming a double electric layer structure; the side chain with hydrophilic properties will be free in the liquid phase, generating spatial site resistance, further improving the stability of dispersion between cement particles.

　　2 Water reducing agent adsorption behavior

　　After the water reducing agent is added to the concrete cementitious system, because there are many active sites on the surface of the cement particles, the water reducing agent will be rapidly adsorbed on the surface of the cement particles, adsorption is the basis for other physical and chemical changes. The water reducing effect of water reducing agent is caused by the adsorption of water reducing agent molecules on the surface of cement particles. The research results of Qu Jindong and other scholars show that there are many active sites on the surface of cement particles and their hydration products, and the water reducing agent can be selectively adsorbed on the surface of cement particles and cement hydration products. Initially, the surface activity of cement particles and their hydration products is high, and there are more adsorption sites, so the adsorption of water reducing agent is relatively fast, but as the adsorption continues, there are fewer and fewer adsorption sites, and the adsorption rate gradually slows down, and finally reaches equilibrium, even if the amount of water reducing agent is increased, the adsorption amount no longer changes. Wang Qian et al. selected four representative water reducing agents, starting from the adsorption of naphthalene high efficiency water reducing agent on the surface of cement particles, to study the limit adsorption of naphthalene water reducing agent and its influence on the key factors of cement slurry settling and fluidity. The results show that for naphthalene-based high-efficiency water reducing agent, the water reducing agent with large polymerization degree has large spatial site resistance, small adsorption amount and low adsorption rate. Wu Yonghua et al. studied the characteristics of the adsorption behavior of naphthalene water reducing agent on the surface of cement particles and its effect on the fluidity of cement slurry, and proposed that there are three adsorption forms of naphthalene high efficiency water reducing agent: ring type, horizontal type and tail type, and established different adsorption models according to these three different adsorption forms, which will change accordingly with the change of adsorption amount and time, and finally affect the cement slurry flowability. Zheng Dafeng et al. studied the adsorption pattern of three different water reducing agents on the surface of cement particles doped with fly ash. The experiments were characterized using UV-Vis spectrophotometer, and the results showed that the adsorption behavior of water reducing agent on the surface of cement particles satisfied Langermuir isothermal adsorption, and the adsorption layer was single molecule.

　　3 The effect of concrete components on the adsorption of water reducing agent

　　The raw materials used in concrete mixing are coarse aggregate, fine aggregate, cement, water and admixture. Among them, coarse aggregate and fine aggregate account for more than 75% of the total content of concrete, coarse aggregate mainly refers to crushed stone, fine aggregate mainly refers to sand. Sand and gravel are mostly collected from mountains and rivers and contain more mud. Wang Ziming et al. believe that the clay content in sand and gravel affects the adsorption amount of polycarboxylic acid water reducing agent and the fluidity of concrete slurry. The amount of water reducing agent adsorbed in kaolin is 5 to 10 times higher than in cement, and in bentonite is 50 times higher than in cement. Qian Jue Shi et al. found that many components of modern concrete contain sulfate, such as stone, sand, admixtures, admixtures, cement, etc.. The main component of concrete on the impact of water reducing agent adsorption is sulfate, sulfate and water reducing agent there is a competitive adsorption relationship between sulfate, sulfate will affect the water reducing agent to reach adsorption equilibrium time. Sulfate ion will destroy the double electric layer structure of concrete slurry, reduce the absolute value of zeta potential, reduce the thickness of water reducing agent adsorption layer, and affect the water reducing agent dispersion effect on cement slurry. yamada et al. selected polycarboxylic acid water reducing agent containing graft chain, added sulfate ion in liquid phase, and studied the effect of the concentration of added sulfate ion on water reducing agent adsorption effect. The results showed that the participation of sulfate ions would reduce the adsorption of water reducing agent on the surface of cement particles and their hydration products and decrease the thickness of the adsorption layer. The reason is mainly twofold, on the one hand, it is the competitive adsorption effect of sulfate ions and carboxyl groups; on the other hand, the high ion concentration will shorten the polyoxyethylene branched chain, thus reducing the spatial site resistance. jiang et al. studied the effect of sodium sulfate on the adsorption effect of naphthalene water reducing agent on the surface of cement particles and their hydration products, and the results showed that: the addition of appropriate amount of sodium sulfate can enhance the adsorption effect of water reducing agent, when The slump of the slurry has a small loss through time, but when the amount of sodium sulfate is too much, the high ionic strength of sulfate will compress the double electric layer structure of the concrete cementing system, weakening the electrostatic repulsion effect and leading to an increase in the viscosity of the concrete cementing system. Wang Zhi et al. studied the effect of sulfate admixture on the adsorption equilibrium time of polycarboxylic acid water reducing agent. The results show that: compared with no sulfate, when the amount of sodium sulfate is 0.2%, the time for water reducing agent to reach adsorption equilibrium increases, and the adsorption rate constant increases; when the amount of sodium sulfate is 1%, the time for water reducing agent to reach adsorption equilibrium also increases, but the adsorption rate constant decreases.

　　4 water reducing agent on the impact of concrete hydration products microscopic morphology

　　At present, the water reducing agent on the early hydration products of cement microscopic morphology has become a research hot spot. In addition to adsorption on the surface of cement particles, water reducing agent will also be adsorbed on the new cement hydration products, thus affecting the hydration process of cement and the microscopic morphology of hydration products. Chen Huicheng et al. investigated the effect of water reducing agents on the early hydration products of cement using differential scanning calorimetry, and the results showed that the addition of water reducing agents helps to accelerate the early hydration process of cement, reduce the size of hydrated calcium silicate, increase the degree of polymerization, increase the amount of calcium hydroxide and hydrated calcium silicate (C-S-H) gel generation, form a dense structure, and make the concrete have higher strength. The experimental results show that the water reducing agent will slow down the hydration of C3A in addition to the effect on the morphology of calcium alumina. The adsorption of water reducing agent on cement particles is selective and uneven, water reducing agent usually has negative charge, C3A usually has positive charge, due to the electrostatic gravitational effect, water reducing agent will first adsorb on C3A, and the hydration of C3A is the most important content of cement early hydration, so the adsorption amount of water reducing agent is closely related to the content of C3A, which affects the microscopic morphology of cement early hydration products, and there are many reports on this aspect. There are many reports on this. The results showed that the AFt of cement doped with formaldehyde condensate of naphthalene sulfonic acid was growing in a long rod column, the AFt of cement doped with calcium lignosulfonate was growing in a short column with more crystals, the AFt of cement doped with melamine urea-formaldehyde resin was growing in a regular hexagonal column, and the AFt of cement doped with modified calcium lignosulfonate was growing in a short column and a long column. The results of a series of experimental investigations using different methods by YILMAZ et al. showed that the addition of sulfonated melamine-based water reducing agent (SMF) affected the early hydration of gypsum and C3A, as evidenced by the adsorption of the sulfonated melamine-based water reducing agent on calcium alumite, which slows down the growth and nucleation of calcium alumite and hinders the conversion of calcium alumite to AFm. When water reducing agent is not added, calcium alumina has good crystallinity and the size of the crystal is relatively large, elongated needles; after adding, the crystal shape will change significantly, changing to a small size of the cubic shape.

　　5 water reducing agent on the impact of concrete performance

　　With the promotion of water reducing agent and the use of a large number of concrete technology has been more and more rapid development, the strength of concrete continues to improve, but at the same time the water reducing agent will make the concrete dry shrinkage cracking drawbacks have also emerged, the study of the problem of dry shrinkage cracking of concrete has become the focus of research in the field of concrete at home and abroad. Shrinkage will reduce the volume stability of concrete, volume stability is not easy to lead to concrete cracks, cracks as a direct consequence of the main cause of damage to the concrete structure, affecting the service life of the structure. Ma Baoguo and other selected three water reducing agents for experimental investigation. It was found that the incorporation of high-efficiency water reducing agents increased the initial cracking time and reduced the cracking sensitivity of concrete slurry. The effect of reducing the cracking susceptibility of concrete slurry from strong to weak is polycarboxylic acid water reducing agent, high concentration type naphthalene water reducing agent, ordinary naphthalene water reducing agent. All three types of water reducing agents are incorporated to make the free shrinkage value of concrete slurry larger, which is proportional to the amount of water reducing agent, the larger the amount of water reducing agent, the larger the free shrinkage value of concrete. Control the effect of concrete slurry volume stability: polycarboxylic acid system water reducing agent > ordinary naphthalene water reducing agent > highly concentrated polycarboxylic acid system water reducing agent. Fei Zhihua et al. studied the effect of different kinds of water reducing agents on the drying shrinkage performance of concrete slurry. The results show that the factors affecting the drying shrinkage performance of concrete net slurry are: the type of water reducing agent, the amount of water reducing agent, water-cement ratio, expansion degree, etc.

　　6 Existing problems

　　With the rapid development of the construction industry, water reducing agent is used in large quantities, the study of water reducing agent has become the focus of modern concrete admixture research, but at the same time many problems have continued to emerge:.

　　(1) the study of water reducing agent mechanism of action, the conclusion is not quite uniform, so far there is no complete set of concrete workers for reference theory.

　　(2) on the impact of water reducing agents on the microscopic morphology of cement early hydration products is less comprehensive, researchers mostly favor the microscopic morphology of C3A and calcium alumina, and less research on C3S, C2S and other major cement mineral components.

　　(3) The presence of sulfate in the concrete system will make the compatibility problem occur when the water reducing agent and cement particles interact with each other, which will have a bad effect on the concrete slurry flowability