Portland cement can be defined as an inorganic material

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. The 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 retardation purpose. Also, it increases the fineness of cement and 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 provide 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 involves many parameters that affect 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 term “paste” can be defined as a mixture of cement that react with water in various proportions where setting and hardening occurs. 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, while the 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 both 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 un-hydrated cement 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 un-hydrated 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).
I.2. Portland Limestone Cement (PLC)

I.2.1 Limestone Nature

Limestone can be defined as “A sedimentary rock consisting mostly of calcium carbonate, CaCO3, primarily in the form of the mineral calcite” (Atlas of Water Resources, 2002) and often contains variable amounts of silica in the form of flint, as well as varying amounts of clay and silt. The texture of limestone varies from coarse to fine depending on the method of formation. Because of impurities, many limestones exhibit different colors, especially on weathered surfaces. When limestone partially replaces clinker to produce Portland- limestone cement, the compressive strength can be expected to decreases. In these cases, the ultimate strengths may be slightly reduced in the systems with limestone replacement; this is attributed to limestone powder being weaker than clinker (Bentz; D.P. et al, 2009). Due to the physical nature of limestone filler, it causes a better packing of cement granular skeleton and high dispersion of cement grains (Helmuth, 1980; El-Alfi et al, 2004).

Besides, limestone filler acts as the crystallization nucleus for the precipitation of cement hydration and produce an acceleration of the cement hydration (European Committee for Standardization, 2000; Gutteridge and Dalziel, 1990).

In addition, the amount of limestone increases the heat of hydration and the free lime contents increase slightly. On the other hand, the total porosity will be decreased and the compressive strength enhances at an early age while reduces at late ages (Neville, 1996).

I.2.2 Incorporation of Limestone with Cement

The limestone filler is added to the Portland cement in different levels (about 5%), and when added from 6% to 20 % and from 21% to 35%, the cement will be called “Portland limestone cement” (Hawkins et al, 2003). Portland limestone cement is considered to be in common use in Europe for certain relatively low strength as (32.5 N) general construction applications, (Moir; G. K., 2003). Limestone nowadays used as partially substitute in cements produced throughout the world, ASTM C 150, “Standard Specification for Portland Cement,” to allow up to 5% ground limestone in Portland cement, The CSA (Canadian Standard Association) standard allows up to 5%