Edited for clarity.
Existing 3D printers for concrete use a premixed 'build material' consisting of dry sand and cement powder, and the print head is a water sprayer. Successive layers of premix are layed down, and the 3D part is made by selective spraying of water. I think this system would result in a lot of waste, because water has a way of diffusing everywhere and the unused build material would become prone to clumping and spoilage, and not store well after a single use. Also the machine itself and the surroundings are vulnerable to cloggage, because of the dry cement powder dust that inevitably floats around.
The idea is that the build material would be simply sand or gravel (or other aggregate such as perlite, etc), layed down in layers, and the print head would spray a cement slurry to bind the aggregate.
It may seem like a small improvement, but I think it would make the difference between an experimental printer that sort-of-works, and a practical everyday machine. So go ahead, (-) for practicality :)-- afinehowdoyoudo, Apr 13 2013random, halfbakery