Concrete is the most widely used building material. Currently, in the production of modern concrete, chemical admixture of concrete has become an indispensable component. Looking back at the history of concrete development, from traditional concrete to the emergence of modern concrete, chemical admixture of concrete plays a significant role. Practice has proven that chemical admixture of concrete has an important role in improving the performance of fresh and hardened concrete. With the research and exploration of chemical admixtures, the construction process and new varieties of concrete have made great progress.
Chemical admixture of concrete is classified into four types based on their main functions: admixtures that improve the rheological properties of concrete mixtures; admixtures that adjust the setting time and hardening properties of concrete; admixtures that improve the durability of concrete; and admixtures that improve other properties of concrete.
Chemical admixture of concrete is classified based on their effects into water-reducing agents (normal water-reducing agents, high-efficiency water-reducing agents, polycarboxylate-based high-performance water-reducing agents, air-entraining water-reducing agents); setting agents (retarders, early strength agents, quick-setting agents); air-entraining agents, aerating agents; waterproofing agents; rust inhibitors; antifreeze agents; pumping agents; expanding agents; etc. Below is a detailed introduction to several chemical admixtures of concrete.
Water-reducing agents are the most widely used type of chemical admixture of concrete, primarily playing adsorption dispersion, lubrication, and wetting roles in concrete. Normal water-reducing agents are suitable for concrete with a minimum daily temperature above 5°C and a strength grade below C40, and are not suitable for independently used in steam-cured concrete. When using normal water-reducing agents containing lignosulfonates, a cement adaptability test should be conducted first and used only after passing. High-efficiency water-reducing agents can be used for plain concrete, reinforced concrete, prestressed concrete, and for preparing high-strength concrete. The standard type high-efficiency water-reducing agents are suitable for concrete with a minimum daily temperature above 0°C and can also be used for steam-cured concrete.
Setting agents mainly achieve the required concrete effects by altering the cement hydration process.
Retarders are additives that reduce the speed of cement hydration and heat of hydration, extending the setting time. They enable fresh concrete to retain good plasticity for a longer time, greatly facilitating pouring operations and improving construction efficiency. The mechanism of retarders is through reacting with chemicals on the surface of cement particles to form a thin film that obstructs the normal hydration process of cement. This obstruction is not permanent but gradually diminishes over time, allowing the concrete to start setting and hardening at the appropriate time.
Hence, the use of retarders in appropriate amounts does not adversely affect the long-term performance of the concrete. Retarders can be classified into inorganic and organic types based on their chemical composition. Inorganic retarders include phosphates, zinc salts, ferrous sulfate, copper sulfate, borates, fluorosilicates, etc.; organic retarders include lignosulfonates, hydroxycarboxylic acids and their salts, polyols and their derivatives, sugars, and carbohydrates, etc.
Early Strength Agents are additives that accelerate the hydration speed of cement mortar and enhance the early strength of concrete. Different early strength agents or the same early strength agent mixed with different types of cement concrete have varying effects. Here, we analyze the principle of a few types of early strength agents.
Sulfate-based early strength agents
Examples include anhydrous sodium sulfate, which dissolves in water and reacts with calcium hydroxide produced during cement hydration to form sodium oxide and calcium sulfate. These newly formed calcium sulfate particles are extremely fine and have much higher activity than added calcium sulfate, rapidly reacting with C3A to generate ettringite, speeding up the cement setting and early strength. However, sulfate early strength agents can corrode steel in concrete and reduce the later strength of cement mortar. Hence, the use of chloride and sulfate early strength agents is decreasing.
Formate-based early strength agents
Calcium formate can influence concrete strength by changing the concentration of tricalcium silicate in the concrete system. It lowers the pH value of the system, accelerates the hydration of C3S, and can increase the concentration of Ca2+ in the liquid phase, speeding up the dissolution of calcium silicate and enhancing the early strength of cement.
Nano C-S-H crystallization seed agents
These are new types of early strength agents for concrete. They provide uniform nucleation sites for the crystallization of C-S-H gel components dissolved in the mixing water during cement hydration. As the gel grows, the solution repeatedly transitions from saturation to unsaturation, accelerating cement particle hydration and increasing early strength. The addition of seed agents makes the hydration products more uniform and optimizes the microstructure of hydration products, enhancing long-term strength and durability of concrete.
Flash setting admixture, which enable concrete to set and harden quickly, are essential components of shotcrete. They can make the cement slurry set initially within a few minutes and reach final setting within ten minutes. Based on the product form, they are classified into powder flash setting admixture (FSA-P), liquid flash setting admixture (FSA-L). Flash setting admixture are further classified based on the dispersion state of solid substances: solution-type liquid flash setting admixture (FSA-LR) and suspension-type liquid flash setting admixture (FSA-LX). Based on alkali content, they are classified into alkali-free flash setting admixture (FSA-AF) and alkaline flash setting admixture (FSA-A).
Waterproofing agents are additives that can reduce the permeability of mortar and concrete under static water pressure, improving the impermeability and hydrophobicity of concrete. The types of waterproofing agents mainly include inorganic waterproofing agents, organosilicon-based waterproofing agents, expansion-type waterproofing agents, and fatty acid-based waterproofing agents.
In the early stages, inorganic waterproofing agents used were primarily chloride-based waterproofing agents. During the hardening process, these waterproofing agents react with cement to form complex salts, filling the voids in concrete and mortar, increasing the density and impermeability of concrete, and thereby achieving waterproofing and anti-seepage effects. Examples include ferric chloride and calcium chloride. These waterproofing agents are effective but contain chloride ions that can corrode steel reinforcement and embedded metals.
Organosilicon-based waterproofing agents are small molecular weight water-soluble polymers that decompose easily in weak acid to form water-insoluble waterproof films. These films surround the constituent particles of concrete, providing hydrophobic properties. However, in poor curing conditions, organosilicon waterproofing agents can crack, and the hydrophobic film formed on the hardened concrete surface tends to peel off.
Expansion-type waterproofing agents consist mainly of materials capable of micro-expansion, compensating for shrinkage in concrete to prevent cracks and achieve rigid self-waterproofing of concrete. They are effective in humid conditions, but poor early curing conditions can cause later cracking, affecting waterproofing results.
Fatty acid-based waterproofing agents have a different waterproofing mechanism compared to other waterproofing agents. Fatty acids are hydrophobic materials, reducing the permeability of hardened concrete and blocking pores to prevent water penetration. When these soluble soaps react with calcium ions in the cement gel, they form insoluble calcium salts. These insolubles deposit on the walls of capillaries, blocking pores and turning capillary walls hydrophobic, thus providing waterproofing effects. However, in prolonged immersion, effective components can leach out, reducing waterproofing effectiveness.
Air-entraining agents are additives that introduce a large number of uniformly distributed, closed, and stable tiny air bubbles into the concrete mix during mixing, enhancing the workability, cohesiveness, and water retention of the mix and the durability of the concrete.
Air-entraining agents primarily introduce air bubbles, followed by dispersion and wetting actions. Air-entraining agents were initially used to improve the impermeability, frost resistance, and salt frost erosion resistance of concrete. The primary mechanism is that air-entraining agents introduce tiny air bubbles that cut off capillary channels, reducing capillarity and thus improving impermeability. These micro air voids can absorb and release the expansion pressure of ice crystals in capillaries during freezing, preventing destructive pressures and reducing and preventing freeze-thaw damage, thereby enhancing concrete's frost resistance.
As a professional concrete admixture factory, ARIT provides various types and uses of chemical admixtures to meet any of your concrete admixture needs.
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