There are several issues with concrete raw materials! Do you all know? (1)

After decades of development, both the mix proportion and production process of concrete are relatively mature. However, the uneven quality and frequent fluctuations of raw materials, as well as the frequent occurrence of concrete quality problems caused by untimely or inadequate adjustment of concrete mix proportion, are the current difficulties in concrete quality control. For example, the phenomenon of pipe blockage caused by poor aggregate wrapping, segregation, bleeding, or significant loss of slump over time in concrete mixtures is rampant. The problem of unqualified engineering quality caused by fluctuations in raw materials, such as insufficient strength of concrete test blocks to meet design requirements or large strength dispersion, is not uncommon. With the increasing efforts in environmental protection, unstable raw material quality has become the norm. Understanding the current status of concrete raw materials, adapting concrete production to fluctuations in raw material properties, and improving concrete quality are of great significance.  
（1） Cement
Nowadays, the problems reflected by concrete enterprises regarding the cement used in the production of concrete mainly focus on the fine fineness of cement (large specific surface area), high content of C3A and C3S in clinker, high early strength, high alkali content, chaotic addition of admixtures, over mixing, and unknown varieties. (1) The most commonly used cement in concrete production enterprises is ordinary Portland cement (P · O42.5). In order to limit the excessive addition of admixtures in cement, the "General Portland Cement" (GB175-2023) stipulates that the dosage limit of admixtures in ordinary Portland cement should not exceed 20%, and specifies the types of admixtures. However, due to economic considerations, cement companies may use additives such as early strength components or alkali activated slag components to increase the amount of mixed materials beyond the current regulatory range. The variety of mixed materials is also diverse, and the composition and mineral composition of concrete companies are unknown. If cement production enterprises can inform the technical personnel of concrete enterprises of the variety and dosage of cement admixtures, the concrete technical personnel can fully prepare concrete that meets the engineering requirements by adjusting the mix proportion, without causing adverse effects on the engineering quality. The current situation is that the technical personnel of cement enterprises do not disclose the variety and dosage of the admixture in the cement, and the technical personnel of concrete enterprises are unable to take targeted measures. This is one of the important reasons why concrete enterprises are not satisfied with the quality of cement. (2) Another reason why concrete technicians are dissatisfied with cement companies is that the early strength of cement is high, but the growth rate of later strength is not high, stagnant, and even exhibits strength shrinkage. In fact, the strength only theory of cement and concrete is a misconception that construction companies pursue high early strength in order to shorten the demoulding cycle and improve the turnover rate of formwork, which is transmitted from the construction unit to the concrete enterprise and then to the cement production enterprise. Cement companies use methods such as adding early strength agents, alkali activation, and grinding to improve early strength in order to meet the requirements of engineering. Cement with high early strength has the following commonalities: ① high early hydration rate (strength) of clinker; ② Cement clinker contains a high content of finer (<3 μ m) particles; ③ The form and quantity of gypsum in cement do not match well with the mineral composition of clinker; ④ The alkali content in cement is high. The early strength of cement is high, resulting in rapid hydration of cement, concentrated release of hydration heat, increased early shrinkage, increased water reducing agent dosage, and increased slump loss. Undoubtedly, the deterioration of concrete durability over the past half century is largely due to the increase in cement strength, especially early strength, and finer fineness. (3) The early hydration rate of cement is fast, and the development of cement technology significantly improves the calcination strength of clinker, resulting in a C3S content of over 60% in clinker and an increase in the hydration reaction rate of clinker. All of these significantly accelerate the early hydration rate of clinker. The advancement of grinding technology and the use of grinding aids have increased the specific surface area of clinker in cement, making it difficult to reduce the hydration rate. (4) Under the environmental pressure of high alkali content in cement, the dust emissions from the kiln system are significantly reduced, making it impossible to discharge the alkali brought in by the fuel during the cement firing process, and almost all of it remains in the clinker. (5) Due to the shortage of natural gypsum, many cement manufacturers use desulfurization gypsum instead of natural gypsum, resulting in poor compatibility between cement and additives. Cement supply is tight, and many enterprises produce cement with short aging time, high surface temperature, high activity, increased water consumption during use, large dosage of additives, and sometimes increased concrete slump loss. Concrete enterprise technicians should try to choose cement brands from large factories and avoid grinding stations. Strengthen batch testing of cement, conduct statistical analysis on cement strength stability, and adopt the principle of "low rather than high" when designing mix proportions to reduce concrete quality risks.  
（2） Fly ash
Fly ash is a widely used mineral admixture, but currently there are not many high-quality and stable Class I intact ash on the market. Moreover, quality issues such as fake fly ash, desulfurization ash, denitrification ash, and floating black ash often occur in Class II ash. (1) With the widespread application of fly ash and the impact of tight supply, the price of fake fly ash has repeatedly risen, often encountering "fake fly ash". There are two types of so-called 'fake fly ash', one is quality fraud, using inferior materials as good ones; Another type is ingredient fraud, where some suppliers grind limestone, coal gangue, slag and other materials into fly ash or directly sell them as fly ash. Due to its complex composition, it is difficult to evaluate the impact of this' fake fly ash 'on the quality of concrete. It is sometimes difficult to distinguish "fake fly ash" based on standards for testing. Experienced concrete companies have summarized some methods based on practical situations, such as using a microscope to observe particle shape and color, combined with loss on ignition, water demand ratio, and activity for judgment. (2) Under the environmental pressure of denitration ash and desulfurization ash, power plants adopt desulfurization and denitration methods to reduce exhaust emissions. The residues of desulfurization and denitration processes are left on the surface of fly ash or mixed in fly ash, which brings some problems to the use of fly ash. If the concrete mixture contains a stimulating ammonia odor, an increase in bubbles, an increase in air content, volume expansion, and a series of problems such as delayed concrete setting. There is a lack of corresponding guidance on the standard specifications for the use of desulfurization ash and denitrification ash, as well as the methods of use and their impact on the durability of concrete. The residues in desulfurization ash mostly exist in the form of sulfates and sulfites, and sometimes the content of free calcium oxide is high. After adding water and stirring, adding phenolphthalein reagent dropwise turns red. When using, attention should be paid to the influence of sulfates and sulfites in desulfurization ash on external additives, and at the same time, observe whether there is a slow setting phenomenon in the setting time of concrete. Excessive free calcium oxide content in desulfurization ash can easily cause poor stability, leading to concrete expansion and cracking. Stability testing should be taken into account. The residues in denitrification ash should be noted to be ammonium salts such as NH4HCO3 and (NH4) 2SO4, which dissolve in water and produce a pungent odor (NH3) in alkaline environments. This can be used for detection. When the ammonium salt content in desulfurization ash is low, it can be completely released during the concrete construction process and has little impact on the concrete. However, when there is a large residual ammonium salt, the generated NH3 cannot be completely released, and there are yellow spots and blisters on the surface of the hardened concrete, which may affect the later strength. (3) Floating black ash: Some power plants add combustion supporting oils and fats to improve the efficiency of coal combustion. If the added oils and fats are not completely burned, they will remain in the fly ash, and in severe cases, the fly ash will turn black in color and even emit an odor. When using this type of fly ash to produce concrete, black oily substances will float on the surface of the mixture, and concrete technicians refer to this type of fly ash as floating black ash. In practice, an appropriate amount of fly ash can be quickly mixed in water to determine if there is any black oily substance floating on the water surface. If the concrete company mistakenly introduces "floating black ash" and is forced to use it, it should try to prevent bleeding and reduce the floating of black substances as much as possible.