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I.1. Portland cement

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).

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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
Silicate

2 C2S + 4H C3S2H3 + CH
Dicalcium Water C-S-H gel Calcium hydroxide
Silicate

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).
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. A decrease in compressive strength can be expected when limestone partially replaces clinker to produce cement. 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). The main effects of limestone filler are due to its physical nature. 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 (European Committee for Standardization, 2000; Gutteridge and Dalziel, 1990).
These effects produce an acceleration of the cement hydration. 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 the 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% limestone in Portland cement, so they define the Portland cement as “The product obtained by pulverizing clinker which essentially made of hydraulic calcium silicates to which calcium sulfate, limestone, water, and processing additions may be added at the option of the manufacturer”.

In the European standard EN 197-1, limestone could be added to cement in three different dosage levels. The level (1) is CEM I, “Portland cement,” may contain up to 5% minor additional constituents, of which limestone is one possible material. The level (2) and (3) are for the CEM II that can be called “CEM II/A-L and CEM II/B-L”, are called “Portland-limestone cement,” contain 6% to 20% and 21% to 35% ground limestone, respectively. The effect of limestone filler addition to Portland cement has been widely studied in cement pastes, mortars, and concretes (Baron, J. and Douvre, C., 1987; Tsivilis et al, 2002; Taoufik; L. A. et al, 2008; Bentz, D. P., 2006). In general, limestone fillers are incorporated for increasing the distribution of cement which decreasing the water demand. Also, for enhancing the cement granular packing factor and to block up capillary pores. Moreover, filler particles of limestone improve the hydration rate of cement compounds and consequently increases the strength at early ages (Bachiorrini 1985; European Committee for Standardization, 2000). In fact, limestone filler hasn’t any pozzolanic effect when it was added with cement. However, its effect on cement strength could be related to its reaction with C3A phase (one of the phases constitute cement) to form an AFm phase (calcium mono carboaluminate hydrate) (Guemmadi; Z. and Houari; H., 2002; Bachiorrini, 1985; Bonavetti 1976).

Using of limestone as a partial substitution for Portland cement and fine aggregates in concretes and mortars has an increasing attention recently. Its use contributes for cost savings through replacement of cement or sand by a product of the limestone extraction industry, and for reduction of the environmental pollution through elimination of dust disposal and reduction of CO2 emissions associated with cement production (Meddah, M.S. et al, 2014)( Torkaman, J. et al, 2014). The filling effect of limestone beside its chemical reaction with alumina phases are the reasons for the improvement of the properties of mortars and concretes. It reacts with tricalcium aluminate (C3A) in cement to form calcium carbo-aluminate; promotes early hydration of cement and interacts with aluminate formed during hydration, leading to a decrease of porosity and increase of the gained strength of concrete, within certain amounts of limestone (Knop, Y. et al, 2014) (Bizzozero, J. and Scrivener, K.L. , 2015). When 10% of Portland cement had been replaced by limestone, an increase of the compressive strength at earlier curing ages (1, 7 and 14 days) and comparable values by 28 days curing age, in compositions was observed. While the replacement increasing of Portland cement to up 20%, the compressive strength reduced at all curing ages (Menéndez, G. et al, 2003). (Meddah, M.S. et al, 2014) worked with contents of limestone varying from 15 to 45% and registered a decrease in the entire interval; even at the different water to cement ratios are used. Lollini, F. et al, 2014 showed a significant improvement of properties of concrete, but their results shall be discussed carefully since they used a synthetic binder that might have a significant contribution in the improved properties, contrary to the work where no synthetic binder had been used.

The limestone is easily ground than clinker and becomes concentrated in the finest particles. Many benefits can be achieved by using PLC concrete. PLC concrete shows better workability and less bleeding than control concrete. When the replacement ratio of limestone is less than 5%, the performance of concrete is not affected. In addition, ecological advantages, such as reductions in CO2 and NOx emissions from cement manufacturing, can be obtained by using PLC concrete (Tennis, P.D. et al, 2011) (Hooton, R.D. et al, 2007). Several studies focused on PLC concrete rather than limestone blended self-consolidating concrete. Hydration, compressive strength development, and carbonation resistance are the main factors for the practical use of PLC concrete. Many experimental studies have been performed and many theoretical models have been developed for studying PLC concrete. Bonavetti, V. et al, 2003 found that limestone replacement (replacing a portion of the Portland cement with limestone) could increase the water-to-cement (W/C) ratio and the degree of hydration of cement. The properties related to the pore structure of concrete remained unaffected up to a 25% replacement of limestone-to-binder materials, above 25% replacement, the pore structure begins to deteriorate (Elgalhud, A.A. et al, 2016).

(Bentz, D.P. et al, 2009) found that the early-age strength of PLC concrete is higher than that of control concrete. It was found that limestone replacements increase the carbonation depth of concrete, with increasing the curing periods, the carbonation resistance of concrete increases. Cement is the most widely used binder to produce concrete and the most common construction material today (Parrott, L.J., 1996 and Balayssac, J.P et al, 1995). Though concrete is a material with the lowest greenhouse emission, cement has the highest. With the carbon footprint of cement accounting for over 7% of total world emissions, it becomes the single most important material of environmental concern around the world. This concern has led to a search for lower CO2 emitting binders and use of blended cement, incorporating a large number of natural and industrial by-products (Ashok, K.T. and Subrato, C., 2016).

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