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You should add a High Range Water Reducing Admixture to it. Once you do this, you will also want to add a hydration stabilizer, shrinkage reducing admixture, and air entraining agent. You may be able to get your water to cement ratio down to anywhere between 0.30 - 0.35 but you will have to get a real high quality cement. This mix will also allow you to reduce the amount of cement needed.

The industry standard and essential reference for concrete mix design is "Design and Control of Concrete Mixtures" by the Portland Cement Association (PCA). A link to purchase the 15th edition is at the first link below. University libraries with civil engineering programs will usually have this publication. If not, go to a local library and ask if they can obtain the book through an inter-library loan program.

The second link is to the PCA's Cement and Concrete Basics home page, with links that may help you.

A classic reference is the US Bureau of Reclamation's Concrete Manual. The Manual was last published in 1981, so it does not address advances in concrete admixtures and processes since then. However, it discusses concrete mix design fundamentals in great detail. The Manual is at the third link below.

The fourth link is to the American Concrete Institute's Concrete Knowledge Center - Materials for Concrete Construction web page.

http://www.concrete.org/Technical/CKC/Materials_fo...