Production and Use of Calcined Natural Pozzolans in Concrete
Authors: Barger GS , Hansen ER , Wood MR , Neary T , Beech DJ , Jaquier D 
Published: January 2010

The goal of producing concrete that provides longterm durability with regard to properties such as improved sulfate resistance and reduced susceptibility to alkali-silica reactions (ASR) has led to the development of several high-performance materials. While the use of fly ashes and ground granulated blast-furnace slags (GGBFS) in concrete is gaining acceptance in various applications, the mineralogical composition of such byproduct materials cannot be as easily controlled as a manufactured pozzolan. Since 1993, Ash Grove has developed and directed the manufacture of pozzolans to improve concrete durability, while avoiding the potential problems of byproduct pozzolan availability and uniformity. Processing of these performance-enhancing pozzolans is accomplished by thermally treating and converting crystalline clay materials to amorphous alumino silicates. These pozzolans can be interground with portland cement clinker and gypsum to produce a Type IP blended cement or they can be ground separately as mineral admixtures for concrete. These two products are produced under the specification requirements of ASTM C 1157 or ASTM C 595 for hydraulic cements and ASTM C 618 for mineral admixtures, respectively. In ASTM C 618, the classification for raw or calcined natural pozzolans is Class N. The data contained in this paper describe some of the choices that are available to manufacturers of blended hydraulic cements to “engineer” into the cementitious system the desired properties for concrete such as improved sulfate resistance, reduced permeability, and the ability to strongly mitigate ASR.

Raw materials for pozzolan manufacture are chosen using criteria such as alkali content, silica-to-alumina ratio, and the presence of minor constituents. Development of the required thermal treatment profile is conducted in the laboratory prior to full-scale kiln processing of the clay. Laboratory grinds of the kiln-processed pozzolan are conducted to balance and optimize surface area as it relates to water demand characteristics. Short- and long-term paste, mortar, and concrete testing is conducted on samples from full-scale production mill test grinds.

The shales that are calcined for use as pozzolans (while not containing significant amounts of kaolinite) are converted to amorphous phase materials during the thermal treatment processing. Most of the clays that have been used for pozzolan production contain significant amounts of kaolinite (Al2O3∙2SiO2∙2H2O) that, when thermally treated, convert to an amorphous phase called metakaolinite. Pozzolans containing metakaolinite are manufactured at several sites across the United States under the supervision of Ash Grove Cement Company. These pozzolans may have different but uniform amounts of metakaolinite depending upon geographic location and geologic formation. They have been shown to reduce ASR when tested by ASTM C 227, C 441, C 1293, and C 1260. As pozzolan composition increases from the normal silicaalumina ratio to a significantly higher silica-alumina ratio, the sulfate-resistance characteristics of hydrated paste have been shown to improve significantly when tested by ASTM C 1012. The calcined clays also contribute to microstructure changes that result in a very low permeability cementitious matrix. This reduced permeability has been demonstrated by testing using chloride ponding, electrical conductivity, and mercury intrusion porosimetry methods. X-ray diffraction and differential thermal analysis (DTA) characterization helps define the type and quantity of hydration products that are formed and that explain the improved performance.