Concrete admixture refers to the substance mixed in concrete according to different requirements to improve the performance of concrete. The mixing amount is generally no more than 5% of the cement mass (except special cases). Based on the main functions, admixtures mainly contain: •Water-reducing Agent It refers to the admixture used for reducing water consumption and strengthening functions when the slump degrees of mixtures are basically the same.
By raw materials and chemical components, water-reducing admixtures can be divided into: lignosulfonate, alkyl aryl sulfonates (commonly known as coal tar water-reducing admixture), sulfonated melamine-formaldehyde resin sulfonate (commonly known as melamine water-reducer), molasses and humic aid water-reducer and others. Based on performances and functions, water-reducing admixtures can be divided into: ordinary water-reducer, effective water-reducer, hardening water-reducer, retarder water-reducer, and air entraining water-reducer. Air Entraining Admixture Air entraining admixture refers to the admixture that entrains a large number of uniform, stable and closed tiny bubbles in the process of mixing concrete to reduce the segregation of concrete mixture, improve the workability, and also enhance anti-freeze ability and durability of concrete. It is a kind of surfactant, too. It has influences on concrete as follows: •It can improve the workability of concrete mixtures. The closed bubbles are like balls that can reduce the friction among cement particles to improve the mobility.
Meanwhile, the bubble film can play a role of water conservation. •It can enhance impermeability and frost resistance. The closed stomata entrained by air entraining admixture can effectively cut off the capillary porosity ducts and reduce pores caused by segregation to enhance impermeability. Meanwhile, the closed pores entrained can be an effective buffer for the expansion caused by water freeze to improve frost resistance. •Hardening accelerator Hardening accelerator refers to the admixture that can accelerate the development of early strength of concrete.
Generally, hardening accelerator can be divided into inorganic (chloride, sulfate, etc. ), organic (triethanolamine, tri-isopropanolamine, and sodium acetate, etc. ) and inorganic-organic compound, the three categories. It can accelerate the hydration and the hardening of cement, improve early strength, and shorten conservation cycle so as to enhance the turnover rate of templates and sites and speed up the construction process. It is especially used in winter construction (whose minimum temperature is not less than -5C) and emergency repair works.
•Set Retarder. Set retarder refers to the admixture that can delay the setting time of concrete mixing materials, and have no bad impact on the development of concrete’s latter strength. It often contains lignosulfonate, carbohydrate, inorganic salts and organic acids. The most common ones are calcium lignosulfonate and molasses. And the retardant effect of molasses is better. •Flash Setting Admixture Flash setting admixture refers to the admixture that can promote the rapid hardening of concrete. The concrete added by flash setting admixture can let the gypsum mixed in cement lose its retardant function to make concrete harden quickly.