Influence of Concrete Raw Materials on Admixtures

What factors in concrete raw materials significantly affect the adaptability of admixtures? Based on years of experimental summaries, the main factors are as follows:

1. If the content of tricalcium aluminate in the cement is too high, it will cause excessive slump loss over time in the concrete mixture. It is advisable to consult with the cement manufacturer to control the tricalcium aluminate content within a reasonable range.

2. When the standard consistency water requirement of the cement is too high (greater than 145g of water), it will require admixtures to increase the water-reducing rate in order to meet the discharge requirements of the concrete mixture.

3. If the temperature of the cement upon leaving the factory is too high (greater than 70°C), it will cause excessive slump loss in the concrete mixture and accelerate setting. During summer, it is necessary to require the cement manufacturer to keep the temperature below 70°C.。

1. The finer the fly ash, the better the fluidity of the concrete mixture; conversely, coarser fly ash results in poorer fluidity. It is recommended to use Class I or II fly ash with finer fineness as a supplementary material.

2. The higher the carbon content in fly ash (i.e., the greater the loss on ignition), the larger the amount of admixture needed to achieve the same discharge effect, because carbon adsorbs the admixture. It is best to select fly ash with a lower loss on ignition.

3. The smaller the water requirement ratio of fly ash, the better, as this reduces the amount of admixture needed to achieve the same discharge effect; conversely, a larger amount of admixture will be required.

1. If the mud content in sand is too high (greater than 3%), it will adsorb some of the admixtures, causing the concrete mixture to have insufficient slump. The admixture formula must be adjusted to meet the material requirements. It is preferable to choose sand with a lower mud content, or to use manufactured sand or partially manufactured sand.

2. If the mud content in gravel is too high (greater than 2%), it will also adsorb some of the admixtures, resulting in insufficient slump in the concrete mixture. The admixture formula must be adjusted accordingly. It is advisable to select gravel with a lower mud content.

3. If the shale content in gravel is too high (greater than 10%), it will adsorb some of the admixtures and lead to insufficient slump in the concrete mixture, primarily due to shale's high water absorption rate (greater than 3%). It is best to use gravel free from shale as coarse aggregate.

What concrete issues are related to the performance of raw materials? The following problems are associated with the performance of raw materials:

1. Excessive slump loss over time in the concrete mixture is primarily caused by a high tricalcium aluminate content in the cement or excessive temperature of the binding materials. It may also be due to a lack of retarding components in the admixture.

2. Poor workability of the concrete mixture at discharge can result from coarse fly ash, coarse sand, a low sand ratio, insufficient binding materials, or a lack of thickening components in the admixture.

3. Severe segregation of the concrete mixture after discharge is mainly due to excessive amounts of admixture and a high water-reducing rate.

4. Low standard curing strength of the concrete is mainly caused by poor quality of raw materials, a low water-reducing rate of the admixture, and high water usage in concrete production. It may also be due to low temperatures in the curing room.

5. Early cracking of concrete (under good on-site curing conditions) is mainly due to excessive use of binding materials, exceeding 500 kg/m³; or poor quality of supplementary materials in the cement, such as coal gangue or slag. Using admixtures outside the polycarboxylic acid series can also increase concrete shrinkage. Additionally, excessive water usage in production is a contributing factor to early cracking of concrete.