At present, the more widely used water-reducing agents are lignin-basedwater-reducing agents naphthalene-based superplasticizers, melamine-based superplasticizers and polycarboxylate-based superplasticizers. And their characteristics are as follows:

• Lignin-basedWater-reducing Agents

Lignin-basedwater-reducing agents have lignin sulfonates as the major components, including calcium lignosulphonate, sodium lignosulphonate and magnesiumlignosulphonate, and are ordinary water-reducing agents. They do not have high water-reducing rate, and will retard coagulation and entrain air. Therefore, during use, the dosage should be controlled within a suitable amount. Otherwise, the excessive dosage will lead to decrease of strength and waste of money, and even will not coagulate in a very long time, resulting in construction accidents.

Generally, the suitable amount is 0.2%-0.3% that of cement.

• Naphthalene-basedSuperplasticizers

Naphthalene, methyl naphthalene, anthracene, cumarone, and coal tar mixture water-reducing agents are collectively referred to as coal tar water-reducing agents, because the raw materials for production are all from different distillates of coal tar. Such types of water-reducing agents are all polymer electrolytescontaining monocyclic, polycyclic or heterocyclic aromatic hydrocarbons and polar sulfonic acid groups, with the relative molecular mass in the range of 1500 to 10000. Since sulfonic acid groups have good dispersion to cement, i.e., high water-reducing rate, coal tar water-reducing agents are all belong to the category of superplasticizers, and will not retard coagulation or entrain air within an appropriate molecular weight range. Naphthalene-based superplasticizers are mature in the production technology, stable in the supply of raw materials, as well as large in the output and wide in the application, and gradually predominated, so usually coal tar water-reducing agents mainly refer to naphthalene-based superplasticizers. Naphthalene-based superplasticizers can be used after spray drying in the grouting material as leveling agents

Generally, the suitable amount is 0.2%-1.0% that of cement.

• Melamine-basedSuperplasticizers

Melamine-basedsuperplasticizers are approximate to naphthalene-based superplasticizers in the performance. Both are the non-air-entraining type and have no retarding effects. The water-reducing and strength-enhancing effects are slightly superior to naphthalene-based superplasticizers. The dosage and price, however, are also slightly higher than naphthalene-based superplasticizers. Melamine-basedsuperplasticizers have been widely used in grouting materials, self-leveling mortar and other products after spray drying.

Generally, the suitable amount is 0.5%-2.0% that of cement.

• Polycarboxylate-basedSuperplasticizers

The polycarboxylate-basedsuperplasticizer is a new type of high-performance concrete water-reducing agent, developed with the development and application of high-performance concrete. It has such excellent properties as high strength, good heat resistance, durability, and weather resistance. Its advantages are small dosage, high water-reducing rate, favorable fluidity, good slump resistance (the slump has almost no loss within 90min), no use of formaldehyde in the synthesis, and no pollution to the environment. The application of polycarboxylate-based superplasticizer in the dry-mixed mortar is still in its infancy.

Generally, the suitable amount is 0.05%-1.0% that of cement.