Recycled materials for aggregates
The largest-volume of recycled material used as construction aggregate is blast furnace and steel furnace slag. Blast furnace slag is either air-cooled (slow cooling in the open) or granulated (formed by quenching molten slag in water to form sand-sized glass-like particles). If the granulated blast furnace slag accesses free lime during hydration, it develops strong hydraulic cementitious properties and can partly substitute for portland cement in concrete. Steel furnace slag is also air-cooled. In 2006, according to the USGS, air-cooled blast furnace slag sold or used in the U.S. was 7.3 million tonnes valued at $49 million, granulated blast furnace slag sold or used in the U.S. was 4.2 million tonnes valued at $318 million, and steel furnace slag sold or used in the U.S. was 8.7 million tonnes valued at $40 million. Air-cooled blast furnace slag sales in 2006 were for use in road bases and surfaces (41%), asphaltic concrete (13%), ready-mixed concrete (16%), and the balance for other uses. Granulated blast furnace slag sales in 2006 were for use in cementitious materials (94%), and the balance for other uses. Steel furnace slag sales in 2006 were for use in road bases and surfaces (51%), asphaltic concrete (12%), for fill (18%), and the balance for other uses.
Aggregate for concrete, a mix of colors crushed to a small size, is substituted for many construction and utility projects in place of pea gravel or crushed rock, oftentimes saving municipalities like the City of Tumwater, Washington Public Works, thousands of dollars (depending on the size of the project). Glass aggregate is not sharp to handle. In many cases, the state Department of Transportation has specifications for use, size and percentage of quantity for use. Common applications are as pipe bedding--placed around sewer, storm water or drinking water pipes to transfer weight from the surface and protect the pipe. Another common use would be as fill to bring the level of a concrete floor even with a foundation. Use of glass aggregate helps close the loop in glass recycling in many places where glass cannot be smelted into new glass.
Aggregates themselves can be recycled as aggregates. Unlike deposits of sand and gravel or stone suitable for crushing into aggregate, which can be anywhere and may require overburden removal and/or blasting, 'deposits' of recyclable aggregate tend to be concentrated near urban areas, and production from them cannot be raised or lowered to meet demand for aggregates. Supply of recycled aggregates depends on physical decay of structures and their demolition. The recycling plant can be fixed or mobile; the smaller capacity mobile plant works best for asphalt-aggregate recycling. The material being recycled is usually highly variable in quality and properties.
A recent report by the National Ready Mixed Concrete Association reached the following conclusions: 1. At a given water ratio, within the range employed in most structural concrete, smaller maximum fine aggregate size will tend to produce higher concrete strengths than larger ones. 2. The larger sizes will require less mixing water and hence, for a given cement factor, will produce concrete of lower water-ratio than the smaller sizes. 3. The advantage of small aggregate in the water-ratio strength relationship may or may not be sufficient to offset the effects of its higher mixing water demand. It appears that optimum maximum size, so far as strength is concerned, will vary for different aggregates, different cement factors, different test ages and probably other conditions. 4. A realistic appraisal of the recent data for several different aggregates must lead to the conclusion that size of aggregate, within a reasonable range, is of less importance to concrete strength than other aggregate characteristics. Even in the leaner mix, where the larger aggregate gave higher strength, the advantage was inconsequential in relation to strength differences between aggregates of the same size from different sources.