I. INTRODUCTION reducing agent is one of the essential components of modern concrete, concrete and mortar mainly used to disperse the cement particles, reduce water consumption, improve concrete strength and durability. Whether it is a simple structure of lignin sulfonate, melamine-formaldehyde condensates and naphthalene sulfonate formaldehyde condensates, etc., but also
Superplasticizer is one of the essential components of modern concrete, concrete and mortar mainly used to disperse the cement particles, reduce water consumption, improve concrete strength and durability. Whether it is a simple structure of lignin sulfonate, melamine-formaldehyde condensates and naphthalene sulfonate formaldehyde condensates, or structural diversification of polycarboxylates, the mechanism of the relationship between structure and properties of water-reducing agent has been a hot research .
Second, water reducer surfactants
Superplasticizer are some surface-active substances, and their performance water-reducing mechanism are surface active role. Surface-active substances are organic molecules having a hydrophilic group and hydrophobic group, after adding an aqueous solution can reduce water surface tension of the water-air interface) and interfacial tension (water-solid interface).
The hydrophobic groups are typically organic hydrocarbons, while the hydrophilic groups are typically able to dissociate the various ion salts, so that negatively charged hydrophilic group, which is an anionic surfactant. Also cationic surfactants (hydrophilic group which dissociate ions, positively charged hydrophilic group), amphoteric surfactants (can dissociate positive ions and negative ions, having two hydrophilic groups) and nonionic surfactant (hydrophilic group separated solved ions, but with polar groups, polar groups adsorbed water molecules act as a hydrophilic group) hydrophobic group can be adsorbed on the cement particle surface, while pro water-based cement group to improve the hydrophilic particles. Save water dispersion of water-reducing agent is a reducing agent in the role of adsorption particles and the surface of the cement hydration products achieved. Adsorption water reducer changed the nature of the cement water dispersion of solid-liquid interface, so the force between the cement particles changes, thus affecting form of solid particles in a liquid dispersion properties, hydration kinetics and hydration products. Superplasticizer molecules of different structures, different adsorption characteristics, the impact on the solid-liquid interface is also different.
Third, the dispersion mechanism
Superplasticizer dispersion mechanism electrostatic repulsion effect, steric hindrance, film lubrication, entraining isolation "ball" effect. Superplasticizer different mechanism of action of different classes, which have been widely accepted that the electrostatic repulsion and steric hindrance.
1, the electrostatic repulsion
After incorporating fresh concrete water reducer, superplasticizer molecules adsorbed on the cement particle surface orientation, some polar groups pointing to the liquid phase. Since the ionization of hydrophilic polar group, so that the surface of cement particles to bring the same charge, and the charge amount increases with the concentration of reducing agent until saturation, so that the electrostatic repulsion between the particles of cement, cement particles flocculation structure disintegrated particles dispersed each other, releasing wrapped in floc free water, thereby effectively increasing the flowability of the mix. Linear ionic polymer superplasticizer (e.g., naphthalene sulfonate formaldehyde condensates, melamine sulfonate formaldehyde condensate) adsorbed on the surface of cement particles, can significantly reduce the negative potential of cement particles (absolute value increases), and thus the order electrostatic repulsion based dispersing cement particles with ionic sulfonate superplasticizer polymer electrolyte, a strong electrostatic repulsion, a polymer electrolyte superplasticizer with carboxylate ions, followed by electrostatic repulsion, a hydroxyl group and an ether group, a non- ionic surfactant water reducer, electrostatic repulsion is minimized.
2, steric hindrance
Polymer superplasticizer adsorbed on the surface of cement particles, then a layer of a certain thickness of the polymer molecules adsorbed layer is formed on the surface of cement particles. When the cement particles near the adsorption layer begins to overlap, which produces a repulsive force acting between the particles, the more the overlap, the greater the repulsion. Prevent cement particles closest mechanical separation forces which overlaps the polymer adsorption layer generated near, called steric repulsion. Copolymer having a branched superplasticizer (such as cross-chain polyacrylic acid, acrylic acid and acrylate copolymer carboxyl containing polyethylene oxide graft polyacrylic acid copolymers, etc.) cement particles adsorbed on the surface, although its cement Particles smaller negative potential is reduced, but because its main chain is connected to the surface of cement particles, extends into the liquid phase the branched polymer molecules form a thick adsorption layer, so as to have a large steric repulsion. Branched chain amino superplasticizer structure and more, and on the cement particles ring and caudate adsorption was, therefore steric hindrance as well. Therefore, it would be for the cement particles have a significant role in dispersing dosage is small.
3, hydrate film lubrication
Superplasticizer molecules containing a large amount of polar groups, such as a lignin sulfonate containing a sulfonic acid group, a hydroxyl group and an ether group; naphthalene sulfonate formaldehyde condensate and melamine sulfonate formaldehyde condensate containing a sulfonic acid group; sulfamic acid formaldehyde condensates containing a sulfonic acid group and
Amine; polycarboxylate superplasticizer containing a carboxyl group and an ether group. These genes have a strong polar hydrophilic interaction, especially hydroxyl, carboxyl and ether groups, etc. can form hydrogen bonds, so the more hydrophilic. Therefore, water reducer molecules adsorbed in the surface of cement particles, due to the effect of hydrophilic polar group, can form a layer of solvent cement particle surface of the water film has a certain mechanical strength. Hydration film may undermine flocculation of cement particles, release package in which mixing water, cement particles well dispersed, and improves wettability of the surface of cement particles, while the relative motion of cement particles and aggregates of particles of lubricating effect, so the macro performance of fresh concrete increased mobility.
4, entraining isolation "ball effect"
Lignosulfonates, polycarboxylic acid and amino sulfonate superplasticizer, etc., can be reduced due to the liquid-gas interfacial tension, it has a certain air entraining action. The incorporation of superplasticizer concrete mixture, not only can be adsorbed on the solid-liquid interface, and can be adsorbed on the liquid-gas interface, easy to make concrete mix in the formation of many tiny bubbles. Superplasticizer molecular orientation in the liquid-gas interface on the bubble, the bubble surface hydration layer film is formed, and at the same time bring the same charge of cement particles. Between bubbles and blisters, bubbles are generated between the cement particles and the electrostatic repulsion of cement particles produce isolation, thereby preventing agglomeration of cement particles. And the ball and uplift the bubble, but also help the fresh concrete cement particles relative sliding aggregate particles. Therefore, air entraining water reducing agent isolation has the "ball" effect can improve the workability of concrete mixture.
Fourth, the concrete performance
Save water reducing agent on concrete action, citing gas, setting time, slump and slump loss, strength, durability and other properties.
1, setting time
Influence of superplasticizer on the concrete setting time depends on the chemical structure of superplasticizer, naphthalene-based and melamine-based no retarding effect on concrete, and even slightly ahead of the setting time, but sulfamate and polycarboxylates Gao is the retarding effect of the plasticizer. The study found that initial and final set and superplasticizer content linearly related. Start superplasticizer affect condensation, hydration start after reducing agent is no longer affected. Superplasticizer in cement hydroxyl groups form unstable complexes, thereby inhibiting hydration initial concentration in the liquid phase, resulting in a retarding effect. As the hydration process, this instability complexes gradually decompose, so hydration continues normally, weakening the impact of the final set.
Join in concrete superplasticizer will significantly improve the strength of concrete. In the early hydration, due to the high dispersion of cement and water reducer increases the water contact area and improve the hydration level, increase hydration product unit volume, so early hydration accelerate early from a macro point of view intensity increased. On the other hand water reducing agent and a slight delay in the rate of hydration hydration process, this role in the early dispersion due to the impact is not yet clear, but in late, but played a good role, it suppresses gel The shift of the crystal, which is conducive to crystal growth, so long whiskers increased, the crystalline calcium silicate hydrate such long fibers, which surface energy, the contact points, intertwined with each other and form a dense network structure , cement inside the pores are filled with split becomes small, which greatly improve the strength of cement stone. Compared with naphthalene superplasticizer, polycarboxylate water reducer can effectively inhibit the early tricalcium aluminate cement, tricalcium silicate hydration, but it can give full play to the late hydration of cement, conducive to the development of late strength of concrete .
3, slump loss
Incorporation of superplasticizer cement particles surface wetting strengthen, et increase hydration products containing water of crystallization, solid-liquid phase ratio of the entire system is increased, the free water content is relatively smaller, condensed trend accelerated. As hydration proceeds, superplasticizer adsorbed on the surface of cement particles generated hydration products colloid layer covering the surface of the slurry although part superplasticizer molecules will be adsorbed on cement hydration products, but the slurry The remaining amount of reducing agent significantly reduced dispersion of cement particles weaken, cement particles began to appear flocculation, the slurry viscosity, concrete slump when the loss occurred. In addition, air entraining water reducing agent will introduce a certain amount of bubbles, the concrete transport and standing process will overflow from the gas, the role played ball microbubbles reduce concrete slump when the loss occurred. Amino superplasticizer and Polycarboxylate superplasticizer branched Gao performance more, there is a low potential and high spatial repulsion, thus dispersing the adsorption properties of the cement well, and can maintain a stable dispersion system sex. Yamada et al studied the similar molecular weight, branched polymers of different chain length on adsorption isotherms noted that after the cement, may cause long-chain branched entangled between the surface of the particles dispersed branched chain, instead of an aggregated particles, the particle dispersion stabilizer of the poor.
Dispersion superplasticizer concrete mixing water and water-cement ratio drastically reduced. To reduce mixing water so hardened concrete to reduce the porosity, pore size distribution moves to the direction of the hole, increasing the density of concrete. Experimental data show that adding an aliphatic superplasticizer concrete aperture by a decrease to below. On the other hand to reduce mixing water reduces the grout after excess water evaporates and leaves the pores Road bleeding, thus reducing the transmission speed in the concrete, and help improve concrete resistance to chloride ion penetration.
Concrete water reducer mechanism is a rather complex issue, involving the functional group, structure, surface and other aspects of physical and chemical properties. Micro, surfactants superplasticizer changed the nature of the cement water dispersion of solid-liquid interface, so the force between the cement particles changes, thus affecting the solid particles in a liquid dispersion properties, hydration kinetics and hydration form product. At the macro performance of the effect of water reduction effect of concrete, setting time, slump and slump loss, strength, durability and other properties. Therefore, deepen microscopic mechanism in order to study the structure of deeper relationships and properties, synthesis and application guidance superplasticizer.