Portland Cement - Cement Grinding

Cement Grinding

In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate (usually gypsum or anhydrite) is added to the clinker and the mixture is finely ground to form the finished cement powder. This is achieved in a cement mill. The grinding process is controlled to obtain a powder with a broad particle size range, in which typically 15% by mass consists of particles below 5 μm diameter, and 5% of particles above 45 μm. The measure of fineness usually used is the "specific surface area", which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface area. Typical values are 320–380 m2·kg−1 for general purpose cements, and 450–650 m2·kg−1 for "rapid hardening" cements. The cement is conveyed by belt or powder pump to a silo for storage. Cement plants normally have sufficient silo space for 1–20 weeks production, depending upon local demand cycles. The cement is delivered to end-users either in bags or as bulk powder blown from a pressure vehicle into the customer's silo. In industrial countries, 80% or more of cement is delivered in bulk.

Typical constituents of Portland clinker plus Gypsum
Clinker CCN Mass %
Tricalcium silicate (CaO)3 · SiO2 C3S 45-75%
Dicalcium silicate (CaO)2 · SiO2 C2S 7-32%
Tricalcium aluminate (CaO)3 · Al2O3 C3A 0-13%
Tetracalcium aluminoferrite (CaO)4 · Al2O3 · Fe2O3 C4AF 0-18%
Gypsum CaSO4 · 2 H2O 2-10%
Typical constituents of Portland cement
Cement CCN Mass %
Calcium oxide, CaO C 61-67%
Silicon dioxide, SiO2 S 19-23%
Aluminum oxide, Al2O3 A 2.5-6%
Ferric oxide, Fe2O3 F 0-6%
Sulfate 1.5-4.5%

An alternative fabrication technique EMC (Energetically modified cement) uses very finely ground cements that are made from mixtures of cement with sand or with slag or other pozzolan type minerals which are extremely finely ground together. Such cements can have the same physical characteristics as normal cement but with 50% less cement particularly due to their increased surface area for the chemical reaction. Even with intensive grinding they can use up to 50% less energy to fabricate than ordinary Portland cements.

Chemical composition of EMC (50/50 OPC/FA - Fly Ash)
Compound OPC % FA % EMC %
CaO 62.4 15.0 40.9
SiO2 17.8 49.4 33.2
Al2O3 4.0 19.6 6.3
Fe2O3 3.9 5.2 4.1
SO3 3.2 0.8 1.6
Na2O <0.1 0.3 0.1
K2O 0.3 1.2 1.2
Insolubles 0.5 51.3 21.6

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