Falck - without stepping back and putting in a little due diligence, I would not recommend using grout instead of concrete. I'm not saying it can't be done, I'm just saying you should make sure it can be done. The essence of basic concrete design is based on various combinations of specific components (cement, water, fine and coarse aggregate). As I am sure you are aware, the components and relative proportions are different for grout than that of concrete. The design of concrete is more than just getting "the PSI rating". To support my statements above, ACI has very specific requirements and procedures for accepting mix designs. Some of those requirements include historical data of the proposed mix, number of samples and breaks, and the strength required when you don't have historical data. There is a reason for such data...it provides a minimum degree of reliability and a precedence that the particular combination and proportion of ingredients will result in the desired performance. I would be surprised if you are able to find historical data for the success of using grout instead of concrete. It may be worth contacting ACI to get their opinion.