Fly ash is a pozzolan, meaning it is a siliceous and aluminous material that, in the presence of water, will combine with an activator (lime, portland cement or kiln dust) to produce a cementitious material, according to Fly Ash Facts for Highway Engineers, a publication of the FHWA, and authored by the American Coal Ash Association (ACAA).
Initially, this ash went right up the stack, but as awareness of the problems with air pollution increased, technologies were developed to make it easier to remove the ash from the stack stream. This material mostly was landfilled, but high landfill costs – along with the need to develop profit centers – spurred electric utilities to find markets for the pozzolanic material.
The American Society for Testing and Materials (ASTM) classifies fly ash into Class C and Class F categories. Class C, with a higher calcium oxide content (CaO, or “lime”), comes from the burning of western sub-bituminous or lignite coals, and Class F is derived from bituminous eastern coals. Research indicates that Class F fly ash is better suited for fighting alkali-silica reactivity, the common malady of concrete.
Silica Fume for HPC
Another industrial “waste” byproduct that would otherwise be landfilled – silica fume – has a most useful role as a component of durable concrete.
Silica fume is a byproduct of the reduction of high-quality quartz with coal or coke and wood chips in an electric arc furnace, during the production of silicon metal or ferrosilicon alloys. The fume is condensed from gases escaping from the furnace and mostly consists of superfine, spherical silicon dioxide particles.
From 1977 on, silica fume began to find its way into U.S. high-rise construction as an additive to concrete, to which it imparts durability and strength. In the 1980s, it was used extensively in high-profile high-rise projects, in some cases producing concrete that exceeded 14,000-psi compressive strengths.
Silica fume is used in bridge structures and decks, where it enhances compressive strengths while boosting durability by blocking migration of chloride ions to reinforcing steel.
One such bridge is the eight-mile-long Confederation Bridge connecting Prince Edward Island with New Brunswick in Canada’s Maritimes Region. This signature bridge is exposed to some of the world’s most extreme weather conditions, including significant amounts of ice that are constantly moving, high winds that result in salt water splash and spray zones on the piers, and frequent cycles of freezing and thawing.
To protect the structure against corrosion and to achieve a 100-year service life, the bridge was constructed with seven different concrete mix designs incorporating various supplementary cementitious materials – including silica fume and fly ash – to achieve low permeability, high early strength, low heat rise, and resistance to freezing and thawing.
GGBF Slag Cement
Like fly ash, use of blast-furnace slag in concrete has been practiced in Western Europe and Japan since the early 1940s. Blast furnace slag is the byproduct of the manufacture of molten iron, resulting from the fusion of limestone and other fluxes with the ash from coke and silica and alumina from iron ore.
While air-cooled slag has been used for decades for noncritical applications such as railroad track ballast, or landfilled or left in heaps at steel mills, in a processed state as ground granulated blast-furnace slag (GGBF) it takes on much higher value as an admixture to concrete.
According to ASTM, GGBF slag is a cementitious, glassy, granular material formed when molten blast-furnace slag is rapidly chilled by immersion in water. This chilling creates a granular product that is then ground to spec and used as an admixture in concrete where it provides improved performance over conventional concrete.
A major mass pour for a bridge pier spread footing was a major application of GGBF slag cement for the Missouri DOT. There, GGBF slag cement – in a 70-percent substitution for Type II portland cement – was being used to lower the heat of hydration in a 2,600-cubic-yard pour for a pier footing for the Phase 1 extension of Page Avenue (S.R. D) in St. Louis County. Ultimately more than 100,000 yards of the mix – representing about 10,000 tons of slag cement – would go on the job over a year and a half.
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