ACI 232.1R-12 Report on the Use of Raw or Processed Natural Pozzolans in Concrete.
4.2—Calcined shale The raw material is shale or slate, which consists largely of alumino-silicate clay minerals. In addition, there may be varying amounts of calcite (limestone), quartz, feldspar, and mica. The shale or slate is quarried by conventional means, crushed to a maximum size of 1.5 in. (38 mm), and then calcined (heat-treated) in a rotary kiln. Calcining tempera- tures vary depending on the source, but are typically in the range of 1800 to 2000°F (980 to 1090°C) with a residence time in the kiln of approximately 45 minutes. This results in a clinker ranging in size from 0.25 to 2 in. (6 to 51 mm) that is air-quenched at the discharge of the kiln. The fnished product is achieved by grinding the clinker in a ball mill to a high Blaine fneness of 600 to 800 m 2 /kg. This will result in a fnely divided powder with a median particle size on the order of 5 microns, similar in fneness to Type III port- land cement. Calcined shale may have a typical elemental analysis of 50 percent silica, 20 percent alumina, 8 percent iron, and 8 percent calcium. The alumino-silicate clay minerals, derived from the shale or slate, provide the necessary elements needed for a pozzo- lanic reaction. Heat treatment alters the crystalline structure of the alumino-silica clay minerals, making them capable of participating in a pozzolanic reaction.
4.3—Diatomaceous earth Diatomaceous earth is composed of the siliceous skel- etal remains of microscopic aquatic plants called diatoms. The silica is present as an amorphous hydrous silica termed opal. Diatomite is diatomaceous earth suffciently pure to be mined and occurs as a fne, granular, lightweight, porous aggregate with an appearance similar to chalk. Diatomite is also known as kieselgur in Germany and tripolite in Libya. Tripolite should not be confused with the sedimentary rock tripoli, which is a residual silica resulting from the weath- ering of chert or siliceous limestone that is generally not opaline. Moler is a diatomaceous earth from Denmark containing volcanic ash and up to 30 percent smectite clay (Christensen et al. 2001). Most diatomite deposits commer- cially mined in the United States, which are highly pure, are found in California, Nevada, Oregon, and Washington (Founie 2006). All diatomite has pozzolanic reactivity to varying degrees. Diatomite is a highly reactive pozzolan due to its high content of amorphous silica and its high specifc surface area. It has a specifc surface area approximately 10 times higher than that of portland cement. Lacustrine, or freshwater deposits of diatomite, appear to have higher pozzolanic reactivity. Diatomite can be calcined and fnely ground to achieve optimum pozzolanic reactivity and performance characteris- tics. The temperature of calcination should be below 1650°F (900°C) to avoid loss of opal through cristobalite crystalliza- tion (Christensen et al. 2001).ACI 232.1R pdf download.