Over the years, the concrete industry increasingly has used mineral admixtures with cement-like characteristics that are cheaper than cement and can enhance concrete performance greatly. The most commonly used mineral admixtures are fly ash, ground granulated blast-furnace slag (typically called slag), and silica fume.
Using the St. Louis metro area for a general cost comparison between mineral admixtures and cement illustrates the savings. Fly ash costs 30% as much as cement by weight; slag, 75% as much; and silica fume, 800% as much. [Note that silica fume greatly enhances hardened concrete properties and is used on many High Performance Concrete (HPC) projects. Although the cost of the material is higher, many times it is the best material to use to produce HPC]. Research and experience have shown that mineral admixtures can lower the cost of the concrete because they can replace part of the cement in a concrete mix.
The producer who wants to use mineral admixtures effectively needs a procedure to evaluate mix design by focusing on three areas.
Area 1: Floors. The American Concrete Institute publication Specifications for Structural Concrete (ACI 301-99) allows the producer to use a lower cementitious materials content, provided a successful history of use is provided or a demonstration floor of at least 8X8 feet is performed. Details of this are listed in ACI 301-99, 4.2.2.1.
Area 2: Admixture amount. Concrete used as flowable fill may have a fly ash-to-cement ratio of 3:1 or higher. Concrete mixtures that will not be in a deicing chemical environment do not have a limit on mineral admixture amount, although obviously the project specifications may have limitations or requirements. ACI 301-99 does present limitations for concrete exposed to deicing chemicals.
Area 3: Effectiveness of mineral admixtures. Last but certainly not least, the producer must ask how effective the addition of the mineral admixture is. One approach to answering this question is to use a "Cement Efficiency Approach." (A calculation to use in this approach is shown in the article.) Many factors affect the strength of concrete; however, when you compare mixture designs, the CE can give you a simple basis of comparison.
The article includes tables showing minimum cementitious materials content requirements for floors, requirements for concrete exposed to deicing chemicals, and the relative cement efficiencies of various mixtures.
