A system for obtaining a photopolymerized prepolymer for use as a component of a material suitable for manufacturing buildings or building components by 3D printing processes. The system contains a flexible closed loop conveyor stretched between a precursor loading station and a prepolymerization material receiver from which the product is unloaded to a construction 3D printing machine. The conveyor carries a plurality of flexible trays capable of looping around the pulleys of the closed loop conveyor. The trays are shallow troughs that have open tops and carry dosed portions of the precursor, which is photopolymerized on its way from the loading station to the unloading station by sequentially passing under light sources of two photopolymerization stations. When the trays pass through the unloading position, they are turned upside-down and allow the precured material to fall into a receiver.