There are a number of additives that can be added to the batch during the mixing process. Some of them help increase the strength of the finished concrete. Another class of additive will effect the look of the finished product.

Strength:
Concrete additives such as fiber are added to the dry ingredients to help produce a stronger concrete. There are many different recipes that fall under the umbrella of GFRC or Glass Fiber Reinforced Concrete. I like to keep it simple. For strength you can add a bonding/fortifier, also known as milk because of its white appearance. You should follow the directions and replace water in the mix with the milk. Small bags of concrete fibers are available from most masonry supply stores. The bag usually gives a suggested rate of application. I have found that a medium handful is suitable for concrete fiber reinforcement of a two bag mix. It is suggested to mix the concrete reinforcing fibers into the dry mix thoroughly before adding any of the liquid ingredients. One more strength improving additive comes to mind. Water reducers. They are powders that are added to the mix that will allow less water to plasticize the mix improving the water cement ratio. Concrete becomes stronger when mixed with the least possible amount of water.



Appearance:
If you are not into the look of pinholes in the final casting then you should consider using a product designed to get the air out of the mix. These powders help reduce the amount of vibration required to release the air bubbles.