While the recycling of low added-value residual materials constitutes a present-day challenge in many engineering branches, attention has been given to cost-effective building materials with similar constructive features as those presented by materials traditionally employed in civil engineering. Bearing in mind their properties and performance, this chapter addresses prospective applications of some elected agro-industrial residues or by-products as non-conventional building materials as means to reduce dwelling costs. Such is the case concerning blast furnace slag (BFS), a glassy granulated material regarded as a by-product from pig-iron manufacturing. Besides some form of activation, BFS requires grinding to fineness similar to commercial ordinary Portland cement (OPC) in order to be utilized as a hydraulic binder. BFS hydration occurs very slowly at ambient temperatures while chemical or thermal activation (singly or in tandem) is required to promote acceptable dissolution rates.

The fibrous wastes originating from sisal and banana agroindustry as well as from eucalyptus cellulose pulp mills have been evaluated as raw materials for reinforcement of alternative cementitious matrices, based on ground BFS. Production and appropriation of cellulose pulps from collected residues can considerably increase the reinforcement capacity by means of vegetable fibers. Composites are prepared in a slurry dewatering process followed by pressing and cure under saturated-air conditions. Exposition of such components to external weathering leads to significant long-term decay of mechanical properties while micro-structural analysis has identified degradation mechanisms of fibers as well as their mineralization. Nevertheless, these materials can be used indoors and their physical and mechanical properties are discussed aiming at achieving panel products suitable for housing construction whereas results obtained thus far have pointed to their potential as cost-effective building materials. Phosphogypsum rejected from phosphate fertilizer industries is another by-product with little economic value. Phosphogypsum may replace ordinary gypsum provided that radiological concerns about its handling are properly overcome as it exhales radon-222, a gaseous radionuclide whose indoor concentration should be limited and monitored. Some phosphogypsum properties of interest (e.g., bulk density, consistency, setting time, free and crystallization water content, and modulus of rupture) have indicated its large-scale exploitation as a surrogate building material.