So, you mix some stuff together, including portland cement, and you make stuff out of it…basically.
It looks like a very expressive media, with variable moisture retention, expiration, depth, thickness, weight, color, usefulness. Cost is also variable, since you can exchange ingredients at will, based on what is available, and what the specific requirements are. Perlite and other porous aggregates can be used to reduce weight, but are generally more costly in larger quantities then the denser options.
Fibrous components, used to add structural integrity, can be obtained in bulk, or generated from waste materials. Weathering is required to add character and to ensure that any water soluble components are leeched out prior to selling.