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Synthesis And Performance Of Workability Polycarboxylate Superplasticizer
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Ordinary concrete is generally composed of water, cement, sand, and stone. To improve concrete performance, appropriate additives are often added. The effects of additives on the application need to be comprehensively analyzed using effective testing methods. In the actual testing process, there are some objective problems that urgently need to be solved. It is necessary to clarify the scope of application of different types of additives and provide scientific guidance for practical construction.
Concrete admixtures are materials added during concrete mixing that can improve concrete performance. According to their different functions, they can be divided into four main types:
One admixtures that improves the rheological properties of concrete mixtures, such as air entraining agents, alkaline agents, and pumping agents;
Secondly, admixtures that improve the hardening performance and setting time of concrete, such as early strength agents, retarders, and accelerators;
Thirdly, admixtures that improve the durability of concrete, such as waterproofing agents, air entraining agents, rust inhibitors, and antifreeze agents;
Fourthly, admixtures that improve other properties of concrete, such as expansion agents, air entraining agents, and coloring agents.
The use of concrete admixtures can generate significant economic benefits, reduce labor intensity, facilitate mechanized operations, and is of great significance for ensuring the construction quality, safety, schedule, cost, and other goals of concrete engineering.
According to relevant standards, specialized benchmark cement should be used to test the performance of concrete admixtures. Some companies believe that using benchmark cement to test qualified admixtures is difficult to implement in actual construction, so they tend to use engineering cement for testing.
Research has found that the compressive strength ratio for engineering cement is greater than that for benchmark cement. The results of engineering cement testing are not significantly different across different additive amounts, whereas benchmark cement can reflect the quality of the additives. Therefore, national testing standards should be strictly followed, and benchmark cement should be used for additive quality testing.
Air content is an important performance indicator of concrete. A certain air content can improve the durability and workability of concrete. After mixing, transportation, pumping, and vibration, the air content of concrete differs significantly from that determined in laboratory tests. Moreover, the air content measured with benchmark cement is higher than that with engineering cement, mainly because engineering cement contains more mixed materials, which can absorb some of the gas.
The existing mechanism sand has a high mud content and poor grading, and the vast majority of it cannot meet the Class II sand standard. When preparing admixtures to meet concrete workability requirements, increasing the amount of air-entraining agent can easily lead to excessive air content. Therefore, within a certain range of air-entraining agent dosage, if it has little effect on concrete strength, the air content index can be appropriately relaxed.
According to relevant regulations, the dosage of concrete admixtures needs to be in accordance with the manufacturer’s specified dosage. However, the manufacturer’s specified dosage is often too high, leading to bleeding and bottom scratching. The detected water-reduction rate is relatively high, and the compressive strength ratio is similarly high. So, in the inspection, the dosage can be determined by solidification. High-performance water-reducing agents have uniform solidification contents of 16% and 1%, while others have uniform solidification contents of 8% and 1.6%. At the same time, the uniformity index clearly specifies the requirement to increase the solid content. When the solid content meets the requirements, the actual dosage of the admixture can correspond to the solid content, making it more reliable to use the solid content uniformly.
Incompatibility issues with cement admixtures can affect concrete performance. Many cement compositions are complex, and differences can occur between manufacturers and batches. The quality of admixtures varies, which can affect the compatibility between different substances. In the relevant technical regulations, the compatibility testing method is clearly stated, and clear requirements are set for aspects such as mortar mix ratio, water-cement ratio, and total mortar volume. Experiments have shown that compatibility testing is reliable and can roughly reflect the actual condition of concrete.
A concrete water-reducing agent is an admixture that provides both water-reducing and strengthening effects, offering significant technical and economic benefits.
Using water reducing agents while maintaining a constant slump can reduce the water consumption per unit of concrete, lower the water cement ratio, thereby improving concrete strength, compactness, and durability; Using water reducing agents can enhance the fluidity of concrete while maintaining a constant water consumption; Using water reducing agents can reduce the amount of cement used while maintaining the same strength of concrete, achieving the effect of saving materials and reducing energy consumption.
In the actual construction process, commonly used types of alkali water reducers include ordinary and high-efficiency, which should be selected reasonably, depending on the specific situation, based on the specific.
The main components of ordinary water-reducing agents are lignosulfonates, such as calcium lignosulfonate, magnesium lignosulfonate, and sodium lignosulfonate, which can achieve air entrainment, water reduction, and slow setting. They are usually used in climatic conditions with a daily minimum temperature of 5 ℃ or higher, requiring concrete with a strength grade below C40. At the same time, specific requirements are set for concrete curing, and ordinary alkali water agents should not be used alone for steam-cured concrete.
A high-performance water reducer is a new type of admixture, mainly composed of polycarboxylate superplasticizer and can be divided into various types, such as standard, retarding and early-strength types. It has a higher water-reduction rate, better slump retention, smaller drying shrinkage, and certain air-entraining performance.
High-performance water-reducing agents are suitable for both plain concrete and reinforced concrete, as well as prestressed concrete, especially for high-demand concrete such as high-strength concrete, self-compacting concrete, plain concrete, pumped concrete, steel pipe concrete, prefabricated component concrete, etc. A slow-setting, high-performance water reducer is suitable for large-volume concrete and should not be used when the daily minimum temperature is below 5 ℃. An early-strength, high-performance water-reducing admixture can be used for construction during low-temperature seasons, but the daily minimum temperature cannot be lower than -5 ℃, which meets the requirements of early-strength concrete engineering but is not suitable for large-volume concrete.
A concrete early-strength agent is an admixture that effectively improves concrete’s early strength. If it has both early-strength and water-reducing effects, it should be called an early-strength alkali water agent. Early-strength agents are more suitable for winter construction and emergency repair projects and should not be used in hot conditions or when the ambient temperature is below -5 ℃. Large-volume concrete generates significant hydration heat, and early-strength agents are also not suitable for use.
At present, the commonly used types of early strength agents include chloride early strength agents, sulfate early strength agents, triethanolamine early strength agents, and some composite early strength agents based on these three types.
Common chloride early strength agents include sodium chloride, calcium chloride, etc. Calcium chloride can react with relevant components in cement, increase the proportion of the solid phase in cement stone, promote the formation of the cement stone structure, reduce free water in concrete, lower porosity, shorten setting time, and improve strength, compactness, and frost resistance. However, chloride early-strength agents can corrode steel reinforcement, so they cannot be used in prestressed concrete construction.
Sodium sulfate is a common sulfate early-strength agent that is readily soluble in water. After being added to concrete, it can undergo a series of chemical reactions with the components of cement, ultimately producing hydrated calcium sulfoaluminate, which can accelerate cement hardening.
Chloride and sulfate early-strength agents are inorganic salt agents and are not suitable for use in environments with high relative humidity. They are also not suitable for structures that come into direct contact with corrosive media or for concrete with decorative requirements.
Triethanolamine, an early-strength agent, is an organic chemical that does not corrode steel reinforcement and can be used in prestressed concrete construction. It is not suitable for steam-cured concrete. In practical applications, its effect alone is not satisfactory, and it is usually combined with other salts to form a composite early-strength agent that produces a better effect.
Air entraining agent is a concrete admixture that can introduce a large number of evenly distributed, stable and small bubbles. It can reduce bleeding and segregation of the mixture, improve concrete workability and significantly enhance frost resistance and durability. If it has both air-entraining and water-reducing functions, it should be called an air-entraining and water-reducing agent.
Commonly used air-entraining agents include rosin resins, alkylbenzene sulfonates, fatty alcohol sulfonates, etc. The use of rosin thermal polymers in rosin resins is more effective and has a wider range of applications. The air-entraining and water-reducing agent can not only achieve air entrainment but also improve concrete strength through its water-reducing effect, effectively addressing concrete durability issues in harsh environments. Its application effect is significant in projects such as ports, bridges, roads, and dams.
Air-entraining agents and air-entraining water-reducing agents can be used for anti-seepage, frost resistance, decorative concrete, lean concrete, severely bleeding concrete, and lightweight aggregate concrete. They are not suitable for prestressed concrete and steam-cured concrete.
A retarder is an additive that can delay concrete’s setting time but does not significantly reduce its final strength. If it has both retarder and water-reducing effects, it should be called a retarder water-reducing agent. Common types of retarders include sugars and lignosulfonates. Among them, molasses in sugars has the best effect, and calcium lignosulfonate in lignosulfonates is also widely used. Given their characteristics, retarders are better suited to construction in hot conditions and perform well in concrete projects with specific time requirements, such as large-volume concrete and long-distance transportation concrete. However, they should not be used alone for steam-cured concrete. It is an additive that can increase concrete’s volume. The expansion agent reacts with the moisture in the concrete to form expansive hydrates, which can counteract shrinkage during concrete setting.
Based on their chemical composition, expansion agents include sulfoaluminate-, iron powder-, lime-, magnesium aluminate-, and composite-based expansion agents. Given their characteristics, expansion agents are commonly used for concrete crack prevention, reinforcement, and waterproofing. When used in self-stressing concrete, the expansion rate is much greater than the shrinkage rate, resulting in a prestressed effect. Therefore, they are often used in self-stressing reinforced concrete oil, gas, and water pressure pipes, as well as other self-stressing reinforced concrete components. A nitrogen agent cannot be used in working environments above 80 ℃. For concrete using lime or calcium sulfoaluminate expansion agent, it is not suitable to add chloride salt additives.
In summary, the use of admixtures in concrete construction has become standardized, effectively improving concrete performance, meeting various construction conditions and requirements, and achieving twice the result with half the effort. This article elaborates on the types and functions of concrete admixtures, analyzes testing issues for admixtures and focuses on several commonly used admixtures, aiming to have practical significance, ensure standardized application and improve the quality of concrete engineering implementation.
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