Blended Cement

Chemical composition of the glass aggregate and its vitreous texture suggested that the material may possess latent cementitious properties and be used as an active mineral admixture in cements and concretes. Ordinary portland cement is produced by pulverizing clinker, the fused product of high-temperature treatment consisting essentially of hydraulic calcium compounds, with a small amount of gypsum. Blended cements may contain, besides clinker, supplementary cementitious materials or active mineral admixtures. The latter are siliceous or siliceous and aluminous materials that possess little or no cementitious value but which chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.

These materials are beneficial to the performance of concrete, in that by their physical and chemical composition, they take part in the hydration reaction of portland cement. They consume some less desirable byproducts of cement hydration, and produce desirable calcium silicate hydrate, thus improving the microstructure and properties of the paste.

The following tests were conducted on ground glass aggregate samples to determine performance as a cementitious material:

• Slag activity index according to ASTM C 989-99, “Standard Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars”
• Strength Activity Index according to ASTM C 311-00, “Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use as a Mineral Admixture in Portland-Cement Concrete”

Mortar cubes were prepared to assess the performance of blends with 0%, 20% and 50% of the cement mixture being replaced by glass aggregate. The water required to achieve equivalent workability to the control mix decreased slightly with increasing glass content. This is considered beneficial because less chemical admixture will be required in a concrete mixture containing glass aggregate.

The results of the testing were very positive. The strength of the mortar mixes containing 20% glass were within the limits required by ASTM C 618. The results therefore indicated that the material had potential to perform as material at dosages of about 20% cement replacement. The great majority of the intended cement replacement market would be at 20% substitution. However, the glass aggregate displayed suitable 28-day strengths for as high as 50% replacement.