1concrete mixing plantAt present, the major environmental issue of slurry water recycling in concrete mixing plants is the wastewater and waste discharged during the cleaning of concrete in mixing trucks and mixers, including cementitious materials (cement, mineral powder, fly ash, etc.), sand, stone, and additives. If these slurry and waste materials are not utilized and recycled, they will cause damage to the surrounding environment, be a waste of resources for enterprises, and also be detrimental to environmental protection. Concrete slurry is alkaline, with a pH value of around 12. Improper discharge can pollute the environment and lose social benefits.

2. The recycling process of slurry water. By using sand and gravel separation equipment, the aggregates are separated, and the discharged slurry water flows into the mixing tank through the sewage ditch. The slurry water is intermittently and periodically uniformly mixed by the mixer in the mixing tank, and then transported to the mixing building for proportional metering and use. The recycling process is as follows: mud → dilution (below 4%) to clean the residual sewage on the mixer truck → dilution (below 4%) → cleaning the car surface with mixing water → precipitation recovery.

3. The technical requirements for concrete mixing water in our Ministry of Construction JGJ63-2006 "Concrete Mixing Water" standard is to classify concrete mixing water into groundwater (including well water) that meets national standards, washing water for concrete production plants and commercial concrete plant equipment, industrial wastewater that has been inspected and qualified, surface water (including water from rivers, streams, and freshwater lakes), and domestic drinking water. The slurry water from concrete component factories, washing mixers and transport vehicles, and pre mixed concrete mixing plants specified in the standards can be used as mixing water for concrete production, but attention should be paid to reducing the impact of cement and admixture types in the water on the mixed concrete. The setting time experiment of cement slurry should be conducted by comparing distilled water or drinking water with slurry water. The difference between the initial and final setting times of the two should not exceed 30 minutes, and their initial and final setting times should also comply with the national standard for cement. The 28 day compressive strength ratio of the prepared cement mortar or concrete shall not be less than 90%. The content of each substance in the mixing water should be within the limit, as shown in Table 1. (4. Analysis of the substances contained in the slurry and their impact on the performance of concrete. Concrete is made by mixing cement, sand, stone, mineral admixtures, and various chemical additives with water in proportion, and the components of the slurry also come from it.).

4.1 Analysis of Substances in Slurry Water Due to cement hydration, slurry water solution mainly contains Ca2+, Na+, K+, OH -, and SO42-. As the early strength agents and antifreeze agents we currently use do not contain chloride salts, the ions brought in by concrete admixtures in slurry water include Na+and K+. Sand and gravel often contain harmful impurities, mainly mud, sulfides, and sulfates. Coarse particles contain harmful impurities such as clay, silt, some sulfates, sulfides, etc. Due to the fact that the mud content of sand and stone in concrete should comply with the provisions of JGJ52-2006, the mud content dissolved in the slurry water is very low.

4.2 Analysis of the Effects of Substances in Slurry on Concrete The harmful ions in slurry that affect concrete performance are mainly Na+, K+, S2-, and SO42-. Sulfate can affect the durability of concrete; Sulfides can cause brittle fracture of steel bars. For Na+and K+in slurry water, they come from admixtures, additives, cement, and so on. According to the 1% concrete residue in each transport vehicle, cement accounts for 50% of it, and the calculated alkali content is 0.015%, which is extremely small. Therefore, slurry water will not increase the alkali aggregate reaction and cause harm. There is a portion of mud in the slurry, which makes the water contain insoluble substances. The residual concrete content of concrete transport vehicles is generally 0.5%, and the mud content in the slurry can be calculated to be between 2% and 3%. The actual measured residual amount is around 1%. The influence of inactive fine powder on the performance of concrete in this part has been shown by many domestic experimental results, with concentrations ranging from 4% to 6%, which will not affect the performance of concrete.

4.3 Determination of Setting Time Difference and Compressive Strength Ratio The determination of setting time difference is based on the standard GB1346-2001 "Cement Standard Consistency, Setting Time, Inspection Method". Cement setting time experiments are conducted using drinking water and slurry water respectively, and the water consumption is based on the standard consistency of drinking water. The difference between initial and final setting time is calculated. The experimental results indicate that; The use of slurry water has no harmful effects on the setting time and compressive strength of cement. The setting time difference is within 30 minutes, and both the initial and final setting times meet national standards. The ratio of compressive strength between 3d and 28d is over 90%.

4.4 The impact of slurry on concrete performance. Through our application of recycled water in mixing plants, we have two experiences. One reason is that excessive concentration of slurry water will reduce the slump of freshly mixed concrete, deteriorate workability, and make construction inconvenient. When the slurry water concentration is between 2% and 4%, it has little effect on the workability of concrete. If it exceeds this range, increasing the water consumption for slump will reduce the strength of concrete. In order to maintain the same water cement ratio, it is necessary to increase the amount of cement used, and increasing the flowability of additives also increases costs. Secondly, after comparing the same ratio of different slurry water and normal mixing water, within the range of slurry water concentration, the early strength of concrete is basically the same, and the later strength will not decrease.

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