The hardness of concrete is determined basically by two things--the mixture of the ingredients and the time you allow for the concrete to set, or cure. The important elements in the mixing of concrete are the amount of concrete itself, the amount of aggregate (small rocks and sand) and the amount of water. Curing is extremely important to the hardness of concrete. In order to make your concrete rock hard you want it to dry out and cure as slowly as possible. This can be achieved by keeping the concrete wet and covered for the first 28 days.
Mix 100 kg of cement (two typical bags of Portland cement) with 40 liters (approximately 3 1/2 standard wheelbarrows) of gravel plus 3 1/2 wheelbarrows of sand. Add water until the mixture is a thick paste. Fill a plastic cup with your cement mixture, pack it and then turn the cup upside down on a flat surface. Carefully remove the cup (the same way you make sand castles at the beach). If the resulting cup-shaped mound of wet cement collapses by more than approximately 10 percent, add more cement to your mix.
Sprinkle water into your form, especially if you are pouring cement onto bare soil. Dampen the soil before pouring in the cement.
Pour cement. Tamp or use a vibrator to raise any air bubbles to the surface of the cement. Trapped air bubbles are known as "voids" and will weaken your cement.
Wet the cement after one hour of drying. Place burlap or another breathable material over the concrete and keep it wet. Do not be afraid of putting too much water on the concrete. Keep the concrete and the ground touching the concrete wet for the first 14 days.
Remove the burlap after 14 days but continue to spray the concrete with water several times each day (depending on weather conditions) for another 14 days. Do not be afraid of putting too much water on the concrete.
Stop spraying the concrete after 28 days. At this point the concrete will have completely set and will be totally rock hard.