Composition

Kaolin is a clay composed primarily of the hydrated aluminosilicate mineral kaolinite (Al₂O₃^.2SiO₂^.2H₂O) with minor amounts of quartz, feldspar, mica, chlorite and other clay minerals. It is distinguished from other clays by its softness, whiteness and ease of dispersion in water. Primary kaolin deposits were formed by the alteration of in situ minerals such as feldspar and other aluminum silicates to kaolinite. Secondary deposits are laid down as sediments, usually in fresh water, far from the place of origin. Various types of secondary kaolins may be termed ball clays, fireclays or flint clays depending on their properties or use (New South Wales DMR, 1998).

Replacement

The replacement range for calcined kaolinite depends on the reactivity of the material. Calcined kaolinite may replace between 5 and 30% of the portland cement by mass.

Advantages

When blended with portland cement, calcined kaolinite offers the following advantages over unmodified portland cement:

• Increased resistance to sulfate attack
• Increased pore refinement
• Decreased permeability
• Decreased corrosion to reinforcement
• Increased resistance to alkali-silica reaction (ASR)
• Increased strengths
• Lower heat evolution
• Produces a light-colored cement

Research shows that concrete made with calcined kaolinite generates less bleed water than conventional portland cement concrete (Balogh, 1995). When replacing portland cement at 30% by mass, the calcined kaolinite acts as an accelerator, so set times should be noted (Ambroise, 1994). One research project showed that for optimum strength, the

calcined kaolinite replacement range is 5-10% by mass (Walters and Jones, 1991). When using calcined kaolinite or any alternative cementitious material as a portland cement replacement, it is necessary to create trial mixtures to ensure proper proportioning for the desired properties.

References