Wallboards, as well as other building materials, are produced by methods which use significantly reduced embodied energy, generating far less greenhouse gases when compared with the energy used to fabricate gypsum wallboard. A novel cementitious core, consisting in one embodiment of post-industrial waste such as slag and combined with pH modifiers, provides a controlled exothermic reaction to create a gypsum-wallboard-like core which can be wrapped in a selected material such as recycled paper and manufactured on a conveyor system to appear, weigh and handle similar to gypsum wallboard, but without the large amounts of energy required to make gypsum wallboard. The manufacturing process results in lower greenhouse gas emissions than the processes used to make gypsum wallboard.