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Cement Concrete Additives and Its Rational Application

Cement concrete has become the basic material in modern social architecture, which is directly or indirectly involved in almost every aspect of daily life. With the development of new structure and new technology, concrete is required to have such properties as adjustable setting, early strength, high strength, light weight and high durability. Now more and more people realize that admixture is the fifth indispensable material in concrete except cement, sand, stone and water. The application of admixture is one of the most significant actions in the history of the concrete technological change, which promoted the development of concrete technology, the progress of new design theory on concrete structure and the development of construction technology. In addition, the application of admixture can significantly improve the performance of concrete, reduce the usage amount of cement, reduce the cost and save energy. Therefore, the application of admixtures in concrete has very important technical and economic significance. However, it needs to be emphasized that the admixture must be used reasonably. If not used properly, the admixture will not only fail to achieve the expected effect, but will even bring terrible influence on the concrete quality.


1. Classification of concrete additives

According to the current national standard GB8075-87 of Classification, Designation and Definition of Concrete Admixtures, concrete admixtures are added in the process of mixing concrete to improve the performance of concrete, and the mixing amount is not more than 5% of the cement quality (except special circumstances). In terms of functions, concrete admixtures are divided into five categories:


(1) Improve the rheological property of fresh concrete. Water reducing admixture, air entraining agent, water retaining agent, etc.;


(2) Adjust the concrete setting and hardening performance. Retarder, early strength agent, setting accelerator, etc.;


(3) Regulate the gas content in concrete. Air entraining agent, gas forming agent, foaming agent, defoaming agent, etc.;


(4) Improve the concrete durability. Air entraining agent, corrosion inhibitor, antifreeze agent, anti-permeable agent, anti-corrosion agent, etc.;


(5) Provide special properties for concrete. Gas forming agent, foaming agent, colorant, expanding agent, alkali-aggregate reaction inhibitor, etc.;


2. The applications of concrete additives


Water reducing admixture

Water reducing admixture is a kind of surface active material. When the water reducing admixture is added to concrete, it diffuses the cement particles and releases the free water in the cement aggregate, which keeps the working performance of concrete unchanged to significantly reduce the water consumption, reduce the water-cement ratio, improve workability, increase fluidity and save cement. The water reducing mixture varieties include lignosulphonate reducer, naphthalene sulfonate reducer, water-soluble melamine resin reducer, water reducer for molas, etc.


Air entraining agent

Air entraining agent can produce a large number of small evenly distributed closed bubbles in mortar and concrete, which can improve the workability, anti-permeability, frost resistance and durability of concrete. The varieties of air entraining agent include alkyl sodium sulfonate, rosin soap, OP emulsifier, 8012 and so on.


Thickening time control agent

Thickening time control agent is an admixture to adjust the setting time of cement, and it has a significant influence on the development of concrete setting time and strength. Thickening time control agent include retarder, early strength agent and setting accelerator. Retarder is an admixture that delays the reaction between cement and water, thus prolonging the setting time of concrete. The varieties of retarder include boric acid, phosphoric acid, NNO, lignin sulfonic acid calcium, sodium, etc.. Early strength agent can improve the early strength of concrete, accelerate the turnover of formwork, accelerate the project progress, and save the construction cost in winter. The varieties of early strength agent are chloride, sulfate, triethanolamine, calcium formate and so on. Setting accelerator is generally used in spraying or urgent repair works, the accelerator can make the concrete coagulate and harden within a few minutes, and has the characteristics of high adhesive property; In general, the initial setting time is 1min to 5min, and the final setting time is 2min to 10min. The varieties of thickening time control agent include Red Star type 1, type 711, type 782, etc.


Expanding agent

Expanding agent is the admixture that causes concrete or mortar to expand due to the chemical reaction. The main function is to compensate the shrinkage of concrete. Expanding agent products include calcium sulphoaluminate, gypsum, iron powder, lime, alunite and so on.


Air entraining agent

The air entraining agent added in concrete can react with the alkali in cement, then it produces gas and forms countless independent bubbles of micro-particles to form aerated concrete. The products of air entraining agent include flaky aluminite powder, sodium abietate, rosin pyrolytic polymer and so on.


Foaming agent

Foaming agent can reduce the surface tension of solution, produce uniform and stable foam, and produce foamed concrete. Foaming agent also has a strong effect of slowing coagulation. Foaming agent products include rosin soap, animal hair, resin saponin lipid, petroleum aluminium sulfonate, etc.


Corrosion inhibitor

Corrosion inhibitor is an admixture to prevent or reduce the corrosion of steel or metal embedded parts in concrete. The varieties of corrosion inhibitor include benzoate, phosphate, fluorosilicate, chloro aluminate, etc.


Antifreeze agent

Antifreeze agent is a kind of admixture to reduce the freezing point of water in concrete. The varieties of antifreeze agent are calcium chloride, sodium chloride, sodium nitrite, ammonia, carbamide and so on.

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