Portland Cement is a mixture of Limestone (CaCO 3 ), mixed with a second material containing clay as source of alumino-silicate. Because clay is needed, usually an impure limestone which already contains some clay is used. The impure Limestone must be made of at least 80% Limestone.
Depending on the purity of the limestone and the amount of clay it contains, a second raw material is added. Some of the second raw materials used are clay, shale, sand, iron ore, bauxite, fly ash and slag.
This mixture of raw materials is then ground together in a “Rawmill” into very fine pieces, the final mixture is known as the “Raw Mixture” or “Rawmix”. The proportions of all ingredients in Portland Cement are formulated to be within 0.1% of the proportion specifications.
The Rawmix is then fired in a slowly-rotating Cement Kiln, where the temperature increases over the length of the Kiln. The hottest part of the Kiln is 1400-1450°C, which causes melting and fusing of the Rawmix into individual round stones known as “Clinker Nodules”
The “Clinker” is then mixed with Calcium Sulfate, usually from the soft Gypsum or Anhydrite Mineral Crystals. The amount of Gypsum (2-8%, usually 5%) controls the set time of the concrete.
- More Gypsum = slower set time
- No Gypsum = flash (instant) setting of cement
The mixture of Clinker and Gypsum Crystals is then put into a “Cement Mill” where it is ground into a very fine powder
So, what happens is that the Alumino-Silicate (clay etc) is mixed with the Lime and heated, changing their compounds, then gets mixed with Calcium Sullfate (Gypsum Crystals), and their crystals interlock and harden when water is added
When water is added, it causes complex chemical reactions to occur which harden Concrete. How and Why the series of Chemical Reactions occurs is only partially understood by scientists today.
The heated Lime creates a new chemical compound, and, when mixed with water it creates the exact replica of the mineral known as “Portlandite”. Portlandite is actually a rare naturally occurring mineral found in volcanic and other heated rocks.
So the Heated Limestone rock mixed with water becomes the same composition as the naturally occurring Portlandite mineral
Carbon Dioxide (CO 2 ) is slowly absorbed into the water-mixed cement while it is mixed and throughout its curing process which reacts with the Portlandite to create an insoluble Calcium Carbonate. Calcium Carbonate is the principle ingredient in many rocks, including Lime, Marble, Tufa, Calcite, Argonite, and many other Rocks
So, when water is added to the mixture of heated and grounded ingredients, the different ingredients crystallize, and their crystals interlock with each other and, with the absorption of CO 2, the mixture becomes insoluble Calcium Carbonate. Calcium Carbonate is the the main part of many kinds of Rocks, but Naturally occurring Calcium Carbonate-based Rocks are often Soluble, meaning some can be quite easily worn away
by water, Whereas the mixed and cured Portland Cement has created an insoluble Calcium Carbonate Rock which cannot be easily worn away by water. It is as hard and durable as some very strong naturally- occurring rocks, and when mixed mixed with aggregate (sand, hard-rock gravel, etc) it becomes the rock- solid binding agent of the suspended aggregate and the resulting concrete is as rugged and durable as some of the hardest of rocks
Hardened & Cured Cement is not a ‘Man-made’ rock, per se, in that no part of it is synthetic, but it is a man-recomposed rock, having taken rocks, Crystals, and other minerals, changing their construction through heating, and grinding them to a wide-fiber powder which changes to a new rock when mixed with water and exposed to CO 2 . When Cement is mixed with rocks and sand to create Concrete, the entire mass becomes much stronger than just cement.
The chemical process of adding water creates a very alkali material which can cause serious burns if left on your skin, so always use gloves & goggles when handling Cement mixed with water, and always wear a safety breathing filter mask when handling the raw unmixed cement powder
Because water is what causes the chemical reaction which causes cement to harden, cement will harden into a rock even when completely underwater, drawing from the CO2 in the water