I've just purchased a new JD 4300 with 430 FEL and 72" mower deck. I opted for the 3ph ballast box rather than the loaded tires because I will be mowing some soft ground. The loader manual indicates that a ballast box filled with Portland Cement weighs 50% more than a ballast box containing concrete.

My question: Isn't concrete merely portland cement, water, sand and gravel? Is the manual suggesting that I only mix portland cement and water and fill the box with this mixture? If so, in what proportions?

I appreciate any information from someone who, unlike me, knows something about concrete.