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Superplasticizer in concrete

Superplasticizer (also called superplasticizer) refers to an admixture that can greatly reduce the amount of water used for mixing while keeping the concrete slump basically the same.

The first generation of superplasticizers—naphthalene-based superplasticizers and melamine resin-based superplasticizers were developed in the early 1960s. Because of their higher performance than ordinary water-reducing agents, they were developed in the late 1930s. Lignosulfonate is a representative-it has a significant improvement, so it is also called a superplasticizer. The second-generation superplasticizer is sulfamate, although in chronological order it is after the third-generation superplasticizer—polycarboxylic acid. The graft copolymer with both sulfonic acid group and carboxylic acid group is the most important third-generation superplasticizer, and its performance is also the most excellent high-performance superplasticizer.

The water reduction rate of the superplasticizer can reach more than 20%. Mainly naphthalene series, melamine series and water reducers made from them, of which naphthalene series are the main ones, accounting for 67%. Especially in my country, most of the superplasticizers are naphthalene series superplasticizers with naphthalene as the main raw material. Naphthalene-based superplasticizers can be divided into high-concentration products (Na2SO4 content <3%), medium-concentration products (Na2SO4 content 3%-10%) and low-concentration products (Na2SO4) according to the level of Na2SO4 content in their products. Content>10%). Most naphthalene-based superplasticizer synthesis plants have the ability to control the content of Na2SO4 below 3%, and some advanced companies can even control it below 0.4%.

Naphthalene-based water reducing agent is the most widely produced and most widely used superplasticizer in China (accounting for more than 70% of the amount of water reducing agent). It is characterized by a high water reducing rate (15% to 25%) and no air entrainment. , Has little effect on setting time, has relatively good adaptability to cement, can be used in combination with various other additives, and is relatively cheap. Naphthalene-based water-reducing agents are often used to formulate high-fluidity, high-strength, and high-performance concrete. The slump loss of concrete mixed with naphthalene-based water reducing agent is faster. In addition, the adaptability of naphthalene-based water reducers to certain cements needs to be improved.

Surfactant is a substance that significantly changes (usually reduces) the surface tension of a liquid or the interfacial tension between two phases. Its molecule consists of two parts: a hydrophilic group and a hydrophobic group. After the surfactant is added to the aqueous solution, the hydrophilic group in the molecule points to the solution, and the hydrophobic group points to the air, solid or non-polar liquid and aligns to form a directional adsorption film to reduce the surface tension of water and the two-phase interphase The interfacial tension. It shows surface activity in liquid. When the water reducing agent is added to the cement slurry, the hydrophobic groups in the water reducing agent molecules are oriented to adsorb on the surface of the cement particles, and the hydrophilic groups point to the aqueous solution, forming a single or multi-molecular adsorption film on the surface of the cement particles. Under the action of repulsion, the flocculation structure formed by various factors such as the molecular cohesion between cement particles after the original cement is added with water (Figure 1 below) is opened, and the free water trapped in the flocculation structure is released. This is called water reduction. Dispersion caused by the adsorption of agent molecules. After the cement is added with water, the cement particles will be wetted by water. The better the wetness, the less mixing water is required under the same working performance, and the speed of cement hydration will increase. When the surfactant is present, it reduces the surface tension of water and the interfacial tension between water and cement particles, which makes the cement particles easy to wet and facilitate hydration. At the same time, the water-reducing agent molecules are oriented and adsorbed on the surface of cement particles, and the hydrophilic groups point to the aqueous solution, which thickens the solvation layer on the surface of cement particles, increases the sliding ability between cement particles, and plays a role of lubrication [Figure 2 below (a), (b)]. If it is an air-entraining water reducing agent, the lubricating effect is more obvious.

The main performance of superplasticizer in concrete:

(1) The effect of alkaline water

After mixing concrete with superplasticizer, the water-cement ratio can be significantly reduced while maintaining fluidity. The water reduction rate of superplasticizer can reach 10%-20%, while that of ordinary water reducer is 5%-15%. The reason for the effect of alkaline water is mainly due to the adsorption and dispersion of water reducing agent by concrete. Due to the directional adsorption of surfactant molecules, the surface of the cement particles has the same sign of charge, but it is under the action of electrical repulsion. Not only makes the cement water system in the same stable and suspendable state, but also disperses and disintegrates the flocculated structure formed by the cement at the initial stage of adding water, and releases the free water in the flocculated agglomerates, thereby achieving the purpose of alkaline water.

2) Plasticization

When the concrete is mixed with water reducing agent, the fluidity can be increased while keeping the water-cement ratio unchanged. The general water-reducing agent can increase the slump of fresh concrete by more than 10cm while keeping the amount of cement unchanged. High-efficiency water agent can formulate concrete with a slump of 25cm.

(3) Wetting effect

After the cement is mixed with water, who is wetted by the surface of the particles, its wetness has a great influence on the performance of the freshly mixed concrete.

(4) Lubrication

The polar hydrophilic groups in the water reducer are directionally adsorbed on the surface of the cement particles and are easily combined with water molecules. When the cement particles absorb enough water reducing agent, a stable solvated water film is formed on the surface of the cement particles. This film plays a three-dimensional protective effect, prevents the direct contact between the cement particles, and provides lubrication between the particles.

The superplasticizer has a strong dispersing effect on the cement, which can greatly improve the fluidity of the cement mixture and the slump of the concrete, while greatly reducing the water consumption and significantly improving the workability of the concrete. However, some superplasticizers will accelerate the loss of concrete slump, and excessive mixing will cause bleeding. The superplasticizer basically does not change the setting time of concrete. When the content is large (overdose incorporation), it has a slight retardation effect, but it does not delay the growth of the early strength of hardened concrete.

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