Briefly describe the main manifestations of the unsuitability of cement admixtures

The adaptability of cement admixtures refers to the characteristics of mixing qualified chemical admixtures into concrete or mortar prepared with cement and mineral admixtures that meet national standards to obtain expected performance. Its application is different from the application of other materials such as cement and aggregates. First, it is necessary to select a reasonable type of admixture, clarify its dosage and mixing method, and it should have good adaptability to cement. Since the performance of concrete and the quality of construction projects are directly affected by the adaptability of admixtures and cement, the adaptability of concrete admixtures and cement has gradually become an urgent problem in our country.

The incompatibility between admixtures and cement is mainly manifested in the following points

1. Abnormal condensation of cement

When cement uses anhydrite as a setting regulator, When the sulfate is exhausted or the concentration of sulfate ions is too low, a large number of lignin sulfonate salt molecules disappear, and the adaptability of cement and water-reducing agents becomes worse.

Excessive retardation of cement is another manifestation of abnormal cement setting caused by superplasticizer.

2. Poor workability of concrete

Specific manifestations include: at the beginning of the induction period, the fluidity drops sharply; concrete bleeds and segregates; carbon adsorption water-reducing agent leads to changes in dispersion performance Poor; insufficient retardation and plastic retention; water reducing agent and retarder are incompatible. The main manifestations of poor concrete work are: at the beginning of the induction period, its fluidity decreases rapidly; the main manifestations of concrete bleeding and segregation are that the concrete sticks to the bottom plate during trial mixing, and the pumping The frictional resistance with the inner wall of the pipeline increases, and the phenomenon of pump clogging is easy to occur; the adsorption of carbon to the water-reducing agent will lead to a reduction in dispersion efficiency; insufficient retardation and plastic retention; the water-reducing agent and retardant do not blend together; ordinary Air-entraining agents and polycarboxylic acid admixtures have poor adaptability, and sometimes even aggravate concrete bleeding and segregation.

Factors affecting the adaptability of cement admixtures and cement

Cement

Since cement clinkers are different, their performance in adsorbing water reducing agents is also different. , its order of action is mainly: C3A>C4AF>C3S>C2S. After adding the water-reducing agent to the cement slurry system, because C3A has the fastest hydration rate, its adsorption capacity for the water-reducing agent is also the largest. A large amount of water-reducing agent will be absorbed first.

Therefore, for cement with a higher content of C3A, under the same conditions of the same amount of water-reducing agent, the more adsorbed water-reducing agent will definitely affect other components in the cement slurry. The amount of water-reducing admixture required for the mineral component has an impact, which is mainly manifested in the low fluidity of the concrete. At this time, it is necessary to appropriately increase the amount of water-reducing agent. The alkali content in the cement will have a great impact on the function of the water-reducing agent. Under normal circumstances, the alkali in the cement has a significant early and setting-accelerating effect, so the fluidity of the concrete is reduced, which in turn leads to the use of water-reducing agents. The effect of the agent is not as expected. Therefore, low-alkali cement should be used as much as possible.

Water reducing agent

(1) Types of water reducing agent

Most carboxylic acid concrete admixtures have better adaptability to cement. , at this time, the carboxylic acid additives are mainly formed through graft copolymerization. However, polycarboxylic acid admixtures sometimes have certain restrictions on the range of certain cement dosages. A little more will easily cause serious bleeding and segregation; sometimes they are not suitable for cement planting. The performance is relatively poor and is particularly sensitive to the amount of water required for concrete. For example, for C30 concrete, when the water consumption is 105kg/m3, the slump of the concrete is only 5cm, but when the water consumption is increased to 5kg/m3, The slump will be higher than 21cm, and serious bleeding and segregation may even occur.

　(2) Compounding of water-reducing agent

Many manufacturers often use naphthalene-based water-reducing agents in combination with sulfamate admixtures, which can improve and reduce concrete slump. The loss of strength and the ease of construction of concrete containing naphthalene-based water-reducing agents ensure that pumped concrete has good construction performance. However, mixing polycarboxylic acid-based admixtures with naphthalene-based water-reducing admixtures will lead to a reduction in concrete strength and poor working performance.

For example, there is such a case-

The production staff mixed the remaining small amount of concrete prepared with naphthalene-based water-reducing agent into polycarboxylic acid-based water-reducing agent. Among the concrete prepared with the agent, it was found that the slump of the concrete when it left the factory was 190mm, and the slump of the concrete after it arrived at the construction site was only 100mm, and its fluidity was almost zero. When vibrating, the vibrator vibrated It will take a long time for the hole to close after pulling it out.

　(3) Adoption of cement admixture adding technology

There are three methods of adding admixtures, including co-mixing, first-mixing, and post-mixing.

Simultaneous mixing method, that is, when mixing concrete, water and admixture solution are mixed into the concrete at the same time.

First mixing method, that is, first mix the dry powder of the admixture with the cement, and then mix it with sand, stone, and water.

Post-mixing method, that is, adding admixtures in batches for mixing after the concrete is mixed. The post-mixing method is divided into water retention method and batch addition. Water retention method means that during the process of mixing concrete, water is added first, and then the admixture is added after 1 to 3 minutes; batch addition means adding admixtures in batches. Restore and compensate for slump values.

Through research, it was found that woodThe plasticizing effect of the first-mixing method of sulfonate is basically the same as that of the stagnant water method and the simultaneous mixing method; while the plasticizing effect of the first-mixing method of naphthalene-based high-efficiency water-reducing agent is slightly worse. Therefore, the above research shows that the use of post-mixing method is more economical and reasonable than other methods. The amount of water-reducing agent used in the post-mixing method is small. The amount of water-reducing agent used in the post-mixing method is only 60% of that in the first-mixing method, but the fluidity is the same.

In addition, the post-mixing method can also save the amount of cement when the strength and fluidity are the same. In some cements, adding high-efficiency water-reducing admixture after adding water will significantly improve the fluidity of fresh concrete and save the amount of water-reducing admixture. Sulfate seriously affects the adaptability of cement varieties and water-reducing agents. Compounding an appropriate amount of sulfate can improve the plasticizing effect of the water-reducing agent. An appropriate amount of sulfate can increase the water-reducing agent in the solution and improve the slump retention and dispersion capabilities of the water-reducing agent.

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