From the article:

  Rudenko therefore resorted to including rebars in the bottom and top walls. 'They
  are needed during the pouring of a variety of cementitious filling materials inside
  the printed walls.' The cement used, however, is just a regular cement mix with a
  few additives. 'It is possible to use a special quick-setting concrete to speed
  up the process, but it will affect the cost, and I don't see much reason to build
  a house extremely fast at the expense of higher cost and lower quality.'