Kaolin and calcium carbonate are both used as fillers or extenders, in other words they make something cheaper. By adding them the volume goes up and the cost per ton goes down. What music to our ears!
But add too much filler and you start to lose good properties, so it is always a trade-off of cost versus quality.
Fillers replace a bit of something that is much more expensive, such as titanium dioxide in paint, soap in soap bars, resin in plastic parts, pulp in paper or rubber in conveyor belts. Like it or not, there is a filler in nearly everything we see around us.
If kaolin is called china clay and the mineral is kaolinite, then calcium carbonate is called limestone, calcite or chalk, and the mineral is calcite. Limestone is the name of the sedimentary rock it comes from. However you also get calcium carbonate from marble, chalk and crushed sea shells.
The main difference in my mind between kaolin and calcium carbonate is that kaolin is a clay, while calcium carbonate is a salt. That means it can dissolve. Lucky for us, it won’t dissolve at high pH such as in paint or soap, or in organic media like plastics or rubber. It only really dissolves in acids.
You can be pretty sure that kaolin, though, will not dissolve whatever you do to it. (The only thing I know that can dissolve kaolin is a super-strong acid called hydrogen fluoride).
For many years, printing paper was made in acid conditions. This meant calcium carbonate could not be used as filler because it would dissolve. Now most paper mills have gone over to alkaline sizing. Thus calcium carbonate now beats kaolin as the main filler for paper.
Calcium carbonate is preferred for paper because it is much whiter than kaolin. Its ISO-Brightness is usually 96%, compared to around 83% for a typical kaolin. Quite a big difference!
Another big difference with these two fillers is the shape of their particles. Kaolin has plate-like particles (platelets) while calcium carbonate particles are irregular rhombohedral (or effectively round) in shape. This is why kaolin particles cut out the light better than calcium carbonate particles. In the paint industry we say kaolin has better hiding power than calcium carbonate.
Some clever scientists have now been able to make precipitated calcium carbonate with a platy shape for the paper industry.
I recently visited a lime factory and saw the age-old process of making lime from calcium carbonate. Not surprisingly, you will normally find a lime factory next to a limestone deposit. The limestone is crushed and calcined to 930 degrees C to form what is called burnt lime/quicklime or unslaked lime:
CaCO3 + heat = CaO + CO2
The burnt lime or calcium oxide is reactive to water (it heats up and gives off steam). This process is called slaking and the result is slaked lime or calcium hydroxide – Ca(OH)2. That’s the stuff that is sold as builder’s lime. It is also used in water treatment and chemical plants for pH control.
Calcium carbonate is useful to us humans as a source of calcium and as an antacid. It is also used as the carrier in many forms of tablets and pills. Kaolin’s main medicinal use is in diarrhoea remedies.
So these two useful fillers are not only all around us, they may also be inside us!