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Round rock, such as river rock, is considered the best type of aggregate for improving the flow of concrete when being pumped due to its smooth, rounded shape. The rounded edges reduce friction between particles, facilitating easier movement through the pump and minimizing the likelihood of blockages. This shape allows for better interlocking and placement within the concrete mix, enhancing the overall workability of the mixture.

Furthermore, when aggregates have a rounded shape, they require less water to achieve a workable consistency, which is beneficial in pumping scenarios where maintaining the correct water-cement ratio is crucial for strength and durability. In contrast, angular aggregates, like crushed stone or granite gravel, may create more friction and resistance when being pumped, resulting in a lower flowability compared to the smoother, rounded aggregates.

Sand can also contribute to the mix, but it is primarily used for filling voids and improving the paste content in the concrete rather than significantly enhancing the flow characteristics by itself when pumped. Thus, round rock is specifically advantageous in enhancing concrete flow during pumping applications.