Strong concrete needs evenly distributed cement.
Producing a strong concrete requires the appropriate blending of aggregates with Portland cement and water. With the correct recipe and a proper mixing process these material will conjoin into a “stew” that can harden into the most durable material on earth.
This concrete stew looses its effectiveness when the “glue” created by cement is not properly distributed. The glue is created by a chemical reaction that starts when water touches cement powder in a process called hydration. The resulting cement and water paste must then completely coat, surround and fill the spaces between each grain of aggregate in the concrete mixture. The more completely the conjoining of the glue with the sand and the stone, the better the concrete. Insuring a complete and even hydration of cement and water is a critical component of the concrete mixing cycle. A short blending period will result low hydration and aggregates that don’t receive adequate coatings. The stew must spend some time tumbling around together in order to receive a coating of this glue. Time and contact are needed to make this process work consistently and evenly.
The best way to visualize this hydration process is to consider how you would blend a colored concrete mixture using a powdered black pigment. If the pigment is dropped into a continuous style mixer that moves materials quickly through the process, the result will be a concrete with splotches or stripes (see examples above). The final mix would range from rich blacks in some areas to various shades of gray in others. If you took that same splotchy concrete and put it back into a regular drum mixer for a longer period of time, it would eventually blend together into a consistent shade of black. Like the black pigment illustration, cement powder needs adequate time to hydrate and to evenly coat all of the aggregate within the mix and not be splotchy. This is the reason why most concrete producers mix materials for several minutes in a drum mixer so that the cement powder will blend into the entire concrete stew. Inadequate mixing of this cement glue can create a concrete slab with splotchy areas of weakness. On the other hand, proper hydration of the glue will result in a consistently strong structure that will match the specification of the concrete recipe. Proper blending is the key to creating good concrete. A good concrete recipe deserves the best cement hydration possible, and the best hydration requires proper mixing times.