Usually our stone is laid dry. Patios are laid on a gravel and sand foundation with a soil/sand mixture in grouts to encourage natural greening over time. We will imbed stone dust in grouts for a more formal look if requested. We do not lay stone over old concrete slabs but will break up and remove the concrete and other rubble from the site before proceeding. We will mortar walls if necessary, although we tend to avoid cement because it may crack over time.