Silica fume adds greatly to the strength of the concrete. It is a natural cement type mixture. It can be obtained from most admixture companies. When you go to buy it, bring lots of singles and fives so that you can make exact change for them. They are not a cash business, and will be more likely to help you out in the future if you help them.

I use silica because all the literature suggests that it will greatly increase the strength. It seems that to make super positive holds that you can do bog dynos off, you really do need to add silica to make it strong enough.

I tried making a positive hold (a giant flake you can get all your fingers in) out of just portland cement, sand and water, and it eventually broke in my hand on a dyno move. I made the same hold using silica and plasticizers, and it hasn't broken yet.

Master Builders offers a wealth of info for the budding concrete expert. I found this excellent .pdf document on their site. It illustrates the strength silica can add.

I may add more on silica in the future.