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The Adaptability of Polycarboxylate Admixtures to Other Concrete Raw Materials

The development and successful production of polycarboxylate superplasticizers at the end of the 20th century ushered in a new era of concrete admixtures. The engineering industry has highly praised this type of water-reducing agent due to its low dosage, high water reduction rate, and small loss of concrete slump, which has also driven the rapid development of concrete technology.

In recent years, the process of national infrastructure construction has accelerated, and the number of construction enterprises has also increased. The supply of raw materials has become increasingly tight, and the quality of raw materials is uneven. Lack of banned mining of river sand resources, high fineness modulus of mechanism sand, excessive mud content, and poor grading; The proportion of cement composite admixture increases, etc.

This seriously affects the quality of concrete, and the adaptability of additives to other raw materials has become the biggest issue.

The Influence of Cement and Admixture Adaptability on Concrete Quality

Cement is one of the most important components in concrete and the most important factor determining concrete performance. Currently, cement plants focus only on strength and excessively grind cement, which seriously affects concrete quality.

Previously, the cement admixture was generally between 10% and 15%, and the compressive strength of 42.5 cement at 28 days could be maintained at around 50MPa. Now, to reduce costs, cement plants have increased the admixture to 20% to 30%. It is difficult for the compressive strength of 42.5 cement at 28 days to reach 48MPa.

The fineness of cement is not necessarily better the larger it is. If the fineness is too large, the water demand of concrete will also increase while achieving the same workability, and the loss of concrete slump will also increase. If the tricalcium aluminate content in cement is high, or the amounts of hemihydrate gypsum and sodium sulfate in cement are also high, the slump loss of concrete is particularly large, and it will lose fluidity in about half an hour, seriously affecting construction.

When the alkali content in cement is high, concrete flowability decreases, and slump loss increases over time, especially when using low-sulfate-content water reducers. However, water reducers with higher sulfate content can significantly improve this situation.

This is mainly because the calcium sulfate contained in low-concentration water-reducing agents is generated during synthesis and neutralization, and has excellent water solubility. So, when using high-alkali cement, adding a certain amount of sodium sulfate and hydroxy acid salt retarder when compounding the water reducer can improve the fluidity and slump of the concrete.

When the alkali content of cement is high and the pH value of polycarboxylate superplasticizer is low, concrete will first undergo an acid-base neutralization reaction. The temperature of concrete will not only rise, but also accelerate the hydration of cement. The flowability and slump of concrete will experience significant losses in a short period of time. So when encountering cement, it is important to avoid using citric acid retarders and instead use alkaline retarders such as sodium hexametaphosphate and sodium polyphosphate, which have better effects.

When the alkali content in cement is low, the fluidity of concrete is also relatively poor, and increasing the dosage appropriately does not have a significant effect. Moreover, concrete is prone to bleeding. The main reason for this phenomenon is the insufficient content of sulfate ions in cement, which reduces the hydration effect of tricalcium aluminate in inhibiting cement. At this point, a certain amount of sulfates, such as sodium thiosulfate, should be added during the compounding process to supplement the soluble alkali in the cement.

When there is yellow paste, needle holes, and many bubbles in the concrete, it can be concluded that this is caused by difficulty adapting the mother liquor and cement. At this time, various mother liquors such as ether and ester, and fat can be compounded. At the same time, it is necessary to consider reducing the amount of pure water, reducing the mother liquor, adding melamine and sodium hexametaphosphate, and then using an appropriate amount of defoamer.

Do not use products such as thickeners. The use of thickeners can cause air bubbles not to come out, resulting in excessive air content, a decrease in concrete bulk density, and a significant decrease in strength. If necessary, tannic acid or yellow lead can be added.

When the foaming component of the grinding aid in cement is high, the concrete is also prone to yellow paste and has a very poor static state for about 10 seconds. Sometimes, it may be mistakenly thought that the water reduction rate is high or the air intake is increased when the water reducing agent is compounded, but in fact, it is a problem with the cement grinding aid. When encountering this problem, defoamers must be used according to the foaming amount of the grinding aid, and air entraining agents cannot be used in compounding.

The Influence of Mechanism Sand Quality and Admixture Adaptability on Concrete Quality

There are significant differences in the mother rock and production technology of mechanism sand in different regions. The water absorption rate of mechanism sand affects the slump loss of concrete to a certain extent. In contrast, a high content of mechanism sand and mud powder not only affects the strength, especially the rebound strength and durability of concrete, but also causes surface powdering of concrete, which is unfavorable for cost control of mixing plants.

The fineness modulus of the currently produced machine-made sand is basically between 3.5 and 3.8, even 4.0, and the grading is severely broken and unreasonable. The proportion between 1.18 and 0.03mm is very small, which is a challenge for pumping concrete.

During the production of machine-made sand, it is necessary to strictly control the stone powder content to around 6% and the mud content to within 3%. The stone powder content is a good supplement for intermittent machine-made sand.

When preparing concrete, try to achieve reasonable gradation while maintaining a certain amount of stone powder content, especially when controlling the amount of 2.36mm or more.

On the premise of ensuring the strength of concrete, the sand ratio should be controlled well, and the proportion of small and large crushed stones should be reasonable. The amount of small crushed stones can be appropriately increased.

Most of the sand used in washing machines is precipitated with coagulants to remove mud, and a considerable amount of coagulants will remain in the finished sand. High-molecular-weight flocculants have a particularly significant impact on water-reducing agents, doubling the dosage of admixtures while causing significant losses in concrete flowability and slump.

The influence of adaptability of admixtures and additives on concrete quality

Power plant fly ash is already a scarce commodity, and ground fly ash is produced accordingly. Conscientious enterprises will add a certain proportion of raw ash, while unscrupulous enterprises are all made of stone powder. The burning loss of fly ash seriously exceeds the standard, and the water demand ratio is too high. The activity is basically between 50% and 60%. The amount of limestone powder added to fly ash not only affects its combustion loss but also its activity.

Strengthen the detection of ground fly ash, grasp the changes in its loss on ignition, and closely monitor the water demand ratio.

A certain amount of clinker can be added appropriately during fly ash grinding to increase its activity.

It is strictly prohibited to use materials with high water absorption such as coal gangue or shale to grind fly ash.

A certain amount of products with water-reducing components can be added appropriately during fly ash grinding, which can help control the water demand ratio. The quality of different materials has a particularly significant impact on the state of concrete, and solving adaptability problems requires a detailed analysis process.