Ones cement concrete is set and solid, it is as hard as rock? How it happens?Are pebles or cource sand in it helps? Or is it cement powder that becomes hard?
the cement powder is what gets hard. it is a chemical reaction where the point is to make the stone into a pourable item, then as it dries it turns back in to stone. the pebbles and sand are present to reduce cost and to help with expansion and cracking problems.
When you mix Portland Cement, sand, gravel, water, and in some cases, an entraining agent, the mixture undergoes a chemical reaction due to the cement called hydration. This mixture hardens into what is called concrete. Gravel and sand in the concrete are what are called aggregates. The cement is what binds the sand and the gravel together. The gravel and the sand are what give the concrete its compressive strength (sometimes you add prestressed steel to give it better strength). If you simply used Portland Cement, it would fall apart easily. The sand is present to fill the voids, or empty spaces, between the gravel rocks. An entraining agent is often added (a liquid poured from a bottle) to remove the air from the mixture, resulting in a higher quality concrete.
