Anyone know concrete mix design?

[brainanator]

Well I know this is really off, but you guys are mostly pretty smart with lots a diverse knowledge base, so I figured I'd ask for opinions before trying my ideas.

My reinforced concrete design professor just sprung a little project on us. We need to design a 7000psi concrete mix. We're making three 2" cubes that weigh 270g each (128pcf).
So it basically needs to be stronger than standard concrete (3-4000psi) and lighter (145pcf). We haven't done a lick of mix design in the class (we design beams and stuff).
So far my plan is to use plastic odd shaped beads as the coarse aggregate, sand as the fine aggregate, portland cement, and just enough water to make it workable (like a clay almost, little thinner). I'll also add a little extra cement to the mix for the extra strength. I'm not sure on the exact ratios, but I think we'll go with 1:1:1 (cement/coarse/fine) to keep the cement content high.

For a full list of rules see here: http://www.concrete.org/students/st_concretecube.htm

This is NOT for that competition, just a little one in class. Pretty sure if you do it and back your reasoning you'll get a good grade, I just want to maximize my performance.

Thanks for any advice you guys have.