Summary of Concrete Admixture

Under normal circumstances, people will be in the mixing process and before the mixing of concrete to improve the performance of concrete substances are collectively referred to as admixtures. The quality of admixtures mixed into concrete shall generally not be greater than 5% of the cement quality. Retarder, accelerator and early strength agent can adjust the hardening performance and setting time of concrete. The main function of admixtures such as air-entraining agent, water-reducing agent and pumping agent is to improve the rheological properties of the concrete mixture, making it easier to be pumped to the designated location. Admixtures such as waterproofing agent, air-entraining agent and rust inhibitor are used to improve the durability of concrete. In order to improve other properties of concrete, it is necessary to incorporate admixtures such as colorants, air entraining agents, anti-freezing agents, and expansion agents into concrete.

Effect of water reducing agent on concrete

(1) the impact of concrete slump

In general, practitioners will add a certain amount of water reducing agent to concrete to achieve the purpose of increasing the slump of concrete, so as to effectively improve the workability of fresh mixed concrete.

Experience shows that the concrete without water reducing agent loses less slump within 1 hour than the concrete with water reducing agent. The main reason for this "strange" phenomenon is that the concrete needs to go through mixing, transportation and pouring processes after the water reducing agent is mixed. Therefore, in the concrete construction site need to use water recovery has been mixed with water reducing agent concrete slump method, however, this method will significantly reduce the coagulation such as strength and other properties, which may lead to concrete cracking, hardening anomalies and other phenomena.

In order to effectively solve the problem of concrete slump loss mixed with water reducing agent, it is suggested that employees adopt batch component mixing water reducing agent and post mixing method. Among the above methods, batch component mixing method can not only effectively avoid the problem of concrete slump loss, but also control the production cost to a certain extent and realize the benefit of concrete construction.

(2) The influence of excessive dosage of water reducing agent on concrete

According to the relevant construction standards, the amount of water reducing agent in concrete should be 0.25~0.75, but due to the influence of various adverse factors, the quality of water reducing agent mixed into concrete in the actual construction process often exceeds the standard, and the consequences of the amount of water reducing agent exceeding the standard, especially the serious exceeding the standard, are very serious. The test data show that when the quality of the water reducing agent in the concrete exceeds the recommended quality by more than 1 times, the hardening and setting time of the concrete will be greatly extended, and the early strength of the concrete will not meet the design requirements, and in some cases the concrete will not be solidified.

In the actual operation process, employees must strictly follow the relevant national regulations on the dosage of concrete admixture. Before formally carrying out the admixture addition work, the construction unit must arrange professional personnel to carry out concrete trial mixing work, so as to determine the dosage of zui good admixture and ensure the reliable quality of concrete.

(3) Gas introduction

After adding water reducing agent to concrete, the internal air content of concrete will increase significantly. Under normal circumstances, if the concrete contains a certain amount of uniform bubbles, the water bleeding of concrete will be greatly reduced, and the workability can be effectively improved, and the impermeability and freeze-thaw resistance of concrete can be improved to a certain extent. Therefore, the concrete water reducing agent mixing method often appears in underground waterproof engineering and other projects with high requirements on concrete impermeability.

However, in recent years, some practitioners have found that in some cases, the addition of water reducing agent to concrete can not only improve the performance of concrete, but also significantly reduce the flexural strength and compressive strength of concrete.

Through a large number of controlled variable experiments, technicians found that the influence of concrete air content on concrete performance is very subtle. The experimental conclusion points out that under the premise of the same water-cement ratio of concrete, the flexural strength of concrete will decrease by 2% ~ 3% and the compressive strength by 4% ~ 5% when the air content in concrete increases by one percentage point. In addition, when the concrete induced air volume exceeds the critical point of 6%, the durability and strength of concrete will be significantly reduced, and the concrete at this time can no longer meet the requirements of the project.

Therefore, the air-entraining water reducing agent has a strong instability, of course, this does not mean that you can not use the air-entraining water reducing agent, as long as the practitioners in the process of using the air-entraining water reducing agent to pay attention to the air content of the concrete can be controlled within a reasonable range.

(4) Adaptability of cement and water reducing agent

In a concrete mixing work, it was found that after a small amount of water reducing agent was mixed into cement, the hardening state of cement changed abnormally. After a few minutes, the setting speed of cement suddenly became faster, resulting in the so-called quick setting phenomenon. Through analysis, zui finally determined the cause of the abnormal phenomenon: gypsum, mixed materials and mineral raw materials in cement are not suitable for water reducing agent. In view of the above situation, employees can not adopt the construction method of forced water addition when they find that the fluidity of concrete is poor or the phenomenon of quick setting occurs after being mixed with qualified water reducing agent, but should start from the adaptability of cement and water reducing agent to find solutions, so as to ensure the quality of finished concrete products to a large extent. In the composite use of different varieties of admixtures, it is necessary to pay great attention to its compatibility, to prevent the performance of concrete from being adversely affected, such as concrete over-setting, pseudo-setting or rapid setting phenomenon, can not meet the requirements of the slump and lead to poor cohesion, water retention, mobility and so on.

Therefore, before the formal use, the corresponding admixture compatibility test must be done to ensure that it meets the corresponding requirements before it can be officially put into use.

The role of antifreeze and early strength agent

Antifreeze can significantly reduce the freezing point of concrete at the specified temperature, so that the concrete liquid phase is not frozen or only partially frozen, so as to ensure the hydration of cement and obtain the expected strength within a certain period of time. Antifreeze containing nitrite and carbonate is strictly prohibited for prestressed concrete structures; antifreeze containing hexavalent chromium salt, nitrite and other harmful components is strictly prohibited for drinking water projects and projects in contact with food, and is strictly prohibited for consumption; antifreeze containing ammonium nitrate, urea and other irritating gases is strictly prohibited for office, residential and other construction projects.

(1) Effect of chloride ion on steel bar

Excessive chloride ions and alkali-active substances in concrete will affect the performance of concrete, so the industry defines chloride ions and alkali-active substances as harmful substances. In order to effectively improve the strength of concrete, people will add a large number of steel bars in the concrete, and the chloride ions in the concrete will chemically react with the steel bars, thereby accelerating the corrosion of the steel bars and releasing gas. Zui eventually promotes the expansion of the concrete and cracks appear, which seriously affects the appearance and strength of the concrete.

Part of the concrete early strength agent used at this stage contains chloride ions, so in order to eliminate or reduce the adverse effects of chlorine-containing additives on concrete, it is recommended to use chlorine-containing additives in a timely manner to the concrete mixed with the right amount of rust inhibitor. According to the chemical principle, the chemical reaction of chloride ion with iron is more intense in the environment of oxygen and water, so the admixture containing chloride ion should be avoided in the open concrete, so that the quality of concrete can be effectively guaranteed. (2) Dosage control problem

Excessive admixture may cause serious adverse effects on concrete. Experience shows that if the early strength agent and antifreeze agent are excessive, the performance of concrete may be seriously reduced. Triethanolamine is a more common application of concrete admixture, a large number of experimental data show that, if the content of triethanolamine is greater than 0.05 of the quality of cement, it will inevitably reduce the strength of the finished concrete, the more serious the content exceeds the standard, the greater the adverse effects on concrete. In addition, if sulfate admixtures are added to concrete without calculating the content of sulfate ions in cement, the content of sulfate ions in concrete may be seriously exceeded, thus reducing the durability and strength of concrete products.