ok, it is not really adult lego, but ICF which stands for Insulated concrete forms.  Basically you stack they like you would lego, putting rebar as you go and tie the blocks as needed.  Once you have them ready to go you fill them with concrete making sure they are level both vertical and horizontal and your good to go.  We looked at a couple different manufactures of ICF blocks and settles on the Advantage ICF building System by Plasti-Fab.   We are using their 6″ block which is really 11″ wide with a 6″ concrete core.