would the three elements which make up cement ever come together to form cement naturally?

Answer 1

Cement, known as Portland cement, which is used in concrete is not calcium carbonate and is in fact a silicate compound. There is no similarity between the reaction of cement and the hydration of calcium oxide that is described in another answer here and the reaction that takes place when cement is mixed with water. The statement "If you add water to the CaO... it will form again CaCO3 capturing the CO2 dissolved in the water" is absolutely incorrect and does not describe the reaction of cement with water.

The elements of dry cement that you are refering to are:
tricalcium silicate (3CaO × SiO2)
dicalcium silicate (2CaO × SiO2)
tricalcium aluminate (3CaO × Al2O3)
tetra-calcium aluminoferrite (4CaO × Al2O3Fe2O3)

As you can see the only thing they have in common with calcium carbonate (CaCO3) is the CaO. In fact calcium carbonate is a raw material used in manufacturing cement, but the resulting chemical compound is NOT calcium carbonate and has no similarity. The raw materials used in manufacturing cement are limestone, shale or clay, and gypsum. These are heated in kilns to about 1550 degrees F.

The main chemical reaction that chemically combines cement and water (the water is incorporated into the molecular structure- it does not 'dry' out) is as follows:
2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ


To answer your question, yes cement is known to occur in nature. Geologists discovered a natural deposit in the 1960's that is about 12 million years old, that formed where an oil shale reacted with a limestone. The Greeks were probably the first to discover cement similar to what we know today. Here is a timeline:
http://matse1.mse.uiuc.edu/concrete/hist.html

Answer 2

Absolutely Yes. Calcite or calcium carbonate is a very common natural cement of many sedimentary rocks. Its chemical formula is CaCO3. So, you need CO2 from the atmosphere to be dissolved in the water (from the rain) to form the acid H2CO3 That's the main reason why rain is acid and not neutral (it has a pH about 5.5, but it gets even more acid when there is pollution). When this acidic rain reacts with the rocks, it leaches out Ca2+ (among many other elements) from the rock. When this water reaches saturation it will precipitate calcium carbonate, cementing the rock. So you have all this substances naturally occurring in the ground water. Note that all this chemical reactions occur at room temperatures.

What the man does to produce cement is to heat up Calcium carbonate (CaCO3) (at very high temperatures, above 900 C) to produce CaO and CO2. The CO2 is lost to the atmosphere and you have a powder of CaO. If you add water to the CaO (which is very reactive) it will form again CaCO3 capturing the CO2 dissolved in the water. That's why if your concrete stays wet for long periods of time (when first mixed) it will be of a better quality and also it will not form if you are close or below the freezing point of water.

Answer 3

There is no proof of cement occuring naturally.The chemistry of cement is very complex.The problem comes in the manufacture of the clinker. The raw mixture for cement is heated in a kiln to a temperature of about 1500 degree Celcius to produce clinkers and that is not possible in nature to regulate that temperature.This temperature has to be introduced wisely otherwise temperatures lower than this will produce cement of poor adhering quality and temperature higher than the required temperature might give glass instead of cement.Moreover, exposure to water decreases reactivity of cement.In nature this cannot be achieved due to rainfalls and moisture in air.

Answer 4

It would be difficult because the one ingredient has to be heated up to create it. Although there are rocks that are kind of created in a cement-like type process such as sandstone. They are called sedimentary rocks.