With the rapid development of high-performance concrete, the issue of high-performance concrete mix ratio has attracted widespread attention. In cement production and concrete construction, the adaptability of cement and admixtures has always been a technical problem that troubles cement manufacturers, concrete construction units and admixture manufacturers. The dialectical relationship between cement and concrete admixtures has naturally become a top priority. What factors affect the adaptability of cement admixtures? There are many factors that affect the adaptability of cement admixtures. The main factors are as follows:
(1) Cement clinker composition
Different clinker ingredients The adsorption of water-reducing agent by material minerals is very different. Cement has two different views on the adsorption form and adsorption products of water-reducing agents. One view is that water-reducing agents selectively adsorb clinker minerals, and the order of adsorption amounts from large to small is C3A > C4AF > C3 S > C2 S. Another view is that cement clinker minerals with a fast hydration rate, a large specific surface area of hydration products, and a large adsorption capacity for water-reducing agents will have poor adaptability to water-reducing agents. As we all know, the hydration rate of cement clinker minerals from fast to slow is: C3A>C4AF>C3S>C2S. Studies by many scholars have also shown that the adsorption capacity of C3A is much larger than that of other minerals. The adsorption capacity of C2 S is very small, and some researchers also believe that high C2 S cement has good adaptability to high-efficiency water reducing agents.
(2) Types and dosage of gypsum in cement
When anhydrite is used in cement production, and calcium lignin and calcium sugar are used as retarding water reducing agents, the concrete The slump loss of the mixture will increase significantly over time, and even “false coagulation” will occur.
When the cement grinding temperature is too high, the dihydrate gypsum mixed in will be partially dehydrated and transformed. It is semi-hydrated gypsum, which will also cause the cement paste to set quickly and affect the adaptability of cement and concrete admixtures.
Industrial by-products such as phosphogypsum and fluorogypsum contain various impurities and active ingredients. Large fluctuations in content will also affect the adaptability of cement and admixtures.
(3) Alkali content in cement
It is generally believed that as the soluble alkali content in cement increases, The adaptability of the water-reducing agent to cement becomes worse, the plasticizing effect of the water-reducing agent is reduced, and the concrete slump loss over time increases.
However, for cement containing Na2SO4 (or Na2SO4 is formed by admixtures (brought in), since alkali exists in the form of sulfate, the solubility and dissolution rate of Na2SO4 are much greater than that of gypsum in cement. The dissolved SO42- reacts with C3A to form ettringite to inhibit cement hydration, which can partially offset the The increase in alkali content will accelerate the setting of cement and deteriorate the adaptability of concrete admixtures and cement.
Therefore, some people have proposed the concept of cement plasticity (SD), that is, SD=SO3/(1.29 Na2O+0.85K2O), it is believed that the impact of the alkali content in cement on the adaptability of admixtures and cement is related to the SO3 content, that is, the SO42- content. When SO3 remains unchanged, as the alkali content increases, the SD decreases and the adaptability changes. Poor; when the alkali content is constant, as SO3 decreases, the SD decreases and the adaptability becomes worse. It is believed that the plasticity degree of cement should generally be controlled between 2.5 and 3.5.
(4) Types and amounts of mixed materials in cement
Different types of mixed materials have different effects on the adsorption of water-reducing agents. The adsorption capacity of slag to naphthalene-based water-reducing agents is less than that of coal gangue, so slag is generally added The adaptability of cement and water-reducing agent is better than that of cement mixed with coal gangue. Generally speaking, because the pozzolanic mixed material has a larger internal specific surface area, its adsorption capacity for water-reducing agent is also larger, so it is better to mix pozzolans. The adaptability of cement and water-reducing agent is poor, which is mainly manifested in poor fluidity of concrete and large loss of slump over time. However, the adaptability of cement and water-reducing agent mixed with different types of fly ash is quite different. Generally, high-quality cement is used. When fly ash (carbon content ≤5%) is used, the plasticizing effect of the water-reducing agent is good; while when coarse fly ash is used, the plasticizing effect of the water-reducing agent is poor when the carbon content is >5%.
Various mineral admixtures added when mixing concrete have the same impact on the effect of concrete admixtures as cement admixtures. When the admixture dosage is the same, the mineral admixtures should be added Compared with concrete without the addition of fly ash, its fluidity is reduced due to viscosity, so at this time the amount of admixture should be appropriately increased or the admixture formula should be adjusted. However, the role of the water-reducing agent in concrete with fly ash The effect may be better than concrete without fly ash.
(5) Fineness and particle composition of cement
Cement plants generally determine the fineness index based on strength. Especially when the strength of clinker is low and the amount of mixed materials is high, the grinding fineness is often increased to ensure the strength of the cement. The greater the fineness of the cement, the greater the fine particle content, the greater the water demand, which is suitable for concrete admixtures. The concrete slump loss is poor and the concrete slump loss is fast.
Depending on the mills used in the final cement grinding system (ball mill, roller mill, vibration mill), the shape of the cement particles obtained will be different.
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Under the same fineness and particle composition, the greater the sphericity of the cement particles, the smaller the water demand and the better the adaptability with admixtures.
(6) Cement production Formation time and temperature
The “fresh cement” that has just been ground has a strong ability to adsorb and agglomerate between particles due to the charge generated during grinding, so it has poor adaptability to water-reducing agents.
Cement made for a short time sometimes has a higher temperature, so the cement hydrates faster, its adaptability to the water-reducing agent will be reduced, and the slump loss of concrete will increase.
Generally speaking, when the cement temperature is less than 50°C, the plasticizing effect of the water-reducing agent is affected.� is not large, but when the cement temperature exceeds 75°C, the plasticizing effect of the water-reducing agent is significantly reduced. When the cement temperature is higher, it may cause the dihydrate gypsum to dehydrate into semi-hydrated gypsum or anhydrous gypsum, causing the water-reducing The adaptability of the agent and cement has become significantly worse.
The above six points are the factors related to cement that affect the adaptability of concrete admixtures
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