1. Influence of cement mineral composition

As we all know, there are four main constituent minerals of Portland cement. Their hydration properties are different and their proportion in cement affects the overall properties of cement. Although C3a contributes greatly to the early strength, the hydration heat is several times of other minerals. Therefore, early strength cement with high C3a content is easy to crack due to early temperature shrinkage, self shrinkage and drying shrinkage.

At present, superplasticizers and other admixtures are widely used in China's concrete, especially in concrete with medium or above strength grade. Because C3a has the fastest hydration speed and the largest adsorption capacity of superplasticizer, it firstly adsorbs a large amount of superplasticizer. Therefore, the cement with high C3a content generally has poor adaptability to admixtures.

2. Influence of cement fineness on concrete

At present, under most cement grinding conditions in China, the finer the cement is ground, the more fine particles are. Increasing the specific surface area of cement can improve the hydration rate and early strength of cement. But the particle size is below 1 um, the particles are completely hydrated in less than a day and almost have no contribution to the later strength. It contributes to the early hydration heat, self shrinkage and drying shrinkage of concrete - the hydration heat of cement particles with fast hydration is released early; Due to the rapid hydration, the water in the concrete is consumed quickly, resulting in the self drying shrinkage of the concrete; Fine particles are easy to hydrate and produce more easily dried and shrinking gels and other hydrates. The reduction of coarse particles reduces the stable volume of unhydrated particles, which affects the long-term performance of concrete. Our existing concrete structure. The general design life is 60 years, and some experts predict that due to the excessive content of ultra-fine cement particles, after 50 years, our concrete strength may only reach 40% of the design strength.

With the increase of cement specific surface area, the adaptability to the same superplasticizer is poor. In order to reduce the fluidity loss, more superplasticizers need to be added. It not only increases the construction cost, but also increases the amount of cement in concrete and affects the durability of concrete. The fineness of cement will also affect the frost resistance and crack resistance of concrete.

3.Relationship between alkali content in cement and concrete cracking

High alkali content is easy to induce alkali aggregate reaction. The premise of the reaction must be that there is sufficient alkali content, sufficient amount of active aggregate and sufficient water supply in the concrete, and the three conditions exist at the same time. Generally, it will not occur in the dry environment, but alkali can promote the shrinkage cracking of cement and cause the deterioration of concrete structures.

4. Effect of mixture on workability of concrete

Due to the different varieties, properties and dosage of mixtures, the workability of concrete from batching plant and the adaptability to admixtures are also different. Therefore, the mixture with easy bleeding and large fluidity loss and the mixture with good water retention and small fluidity loss can make up for each other and prevent bleeding and segregation. Improve the workability of concrete and adaptability to admixtures. For example, slag, steel slag and copper slag are used together with fly ash and coal gangue. Good results can be obtained. Using limestone as admixture can stimulate the early strength of cement. Using fly ash and slag as admixture can not only reduce the hydration heat of concrete. If superfine powder is added, the cement strength can be greatly improved and the clinker ratio can be reduced.