Portland cement can be defined as an inorganic material, which, when mixed with water, forms a paste which hardens by means of hydration reactions and after hardening, retains its strength and stability even under water. The quality of cement is usually evaluated by many factors such as the rate of strength development, heat liberated during hydration and its durability in a various corrosive medium. Chemical structure, fineness and particle size distribution of the cement have a strong effect on the cement compressive strength. European and American Standards accept fineness, which has considerable effects on cement strength and hydration rate, as a vital parameter (Avsar, H., 2006).
During grinding of cement clinker, about 5-15% of the gypsum is added for proper retardation because increasing fineness makes more tricalcium aluminate available for early hydration. The higher early rate of hydration leads to a higher early rate of heat liberation, which may cause cracking in concrete constructions. However, grinding feed to very fine particles requires more energy which increases the production cost (Avsar, H., 2006).
On the other hand, smaller particle size permits area to be available for water-cement interaction per unit volume. The finer particles dominate the early strength development of the cement (up to 2 days) while the larger particles dominate the strength after this time (PCA, 1988). Therefore, the variation of cement fineness should be well controlled and monitored during the cement milling process. The cement milling process is a complex process that involves many parameters affecting the quality parameter of weight percentage of product residue on the sieve (or fineness) with a definite size of holes (Avsar, H., 2006).
I.1.1 Hydration of Portland cement
The hydration process refers to the changes occurring when anhydrous cement particles are mixed with water leading to setting and hardening of cement. The mixture of cement and water with various proportions where setting and hardening occurs is called a ‘ paste’. The meaning of this term extends to include the hardened material that later produced. The setting is stiffing without significant development of compressive strength and typically occurs within a few hours. Hardening is the significant development of compressive strength that can occur through different days of hydration (Hewlett, 1998 and Ramachandran, 1995).
The main parameters that affect the development of the compressive strength of the cement are calcium silicates (C3S and ?-C2S) in fine cement which react with water to produce calcium silicate hydrate (CSH) gel (called tobermorite gel), and calcium hydroxide (commercially known as free lime). The hydration reaction of the two calcium silicates represents the largest percentage of Portland cement. However, tricalcium silicate hydrates harden rapidly to provide high early strengths, while the reaction of ?-dicalcium silicate is far slower and is responsible for late strength (Erdogan, T.Y, 2003)
2 C3S + 6H C3S2H3 + 3CH
Tricalcium Water C-S-H gel Calcium hydroxide
2 C2S + 4H C3S2H3 + CH
Dicalcium Water C-S-H gel Calcium hydroxide
The calcium silicate hydrate (C-S-H) gel represents about 60% of the total solids in the hydrated cement system. However, its exact chemical composition is variable (Neville; A.M., 1993). Due to its poorly crystalline structure, CSH develops tiny irregular particles and high surface area. The surface area of calcium silicate hydrates which is larger than the unhydrated cement which greatly affects the physical properties of the CSH (Erdogan; T.Y., 1997 and Neville; A.M., 1993). The growth of C-S-H particles is forcing the adjacent particles like the remaining unhydrated cement grains and aggregates to interlock to form a dense and compact structure. The development of this structure is responsible for setting and hardening, and strength development. The calcium hydroxide (CH) formed after the hydration reaction has thin hexagonal crystalline plates that later on merge into a large deposit (Neville, A.M., 1993).
The tri-calcium aluminate (C3A) is one of the most important phases in Portland cement. Although the average C3A content is little about 3-10% if it was compared to the other phases of C3S and C2S, it’s significantly affecting the early reactions. The hydration reaction of C3A with water is very rapid due to the electrophile behavior of aluminum oxide but does not contribute to the strength of cement considerably (Erdogan, T.Y., 2002). The hydration of C3A occurs with sulfate ions supplied by the dissolved gypsum. The result of the reaction is called “ettringite”. The formation reaction for the hexagonally-shaped prism crystals of ettringite causes great expansion in volume (Erdogan, T.Y., 2003).