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, the filler particles of limestone improve the hydration rate of cement compounds and consequently increase the strength at the 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 carbo aluminate 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).
I.3. Grinding Aids:
Grinding aids are mostly organic compounds that are added to the clinker during grinding in the cement mill for increasing the grind ability of the cement clinker and therefore reduce the energy required to grind the clinker into a given fineness. As a consequence, the presence of grinding aids increases the efficiency of the cement mill. Grinding aids have been used for more than 50 years and the most common additives can be divided into groups according to their structure as glycols, alkanols, amines, and phenol type compounds. In addition, to increase the efficiency of the mill, some grinding aids also provide important positive effects on the final cement such as increasing rheology of the fresh cement paste or concrete and improved the strength development. Grinding aids which provides these “extra” properties are called “quality improvers” or “performance enhancer” (Engelsen, C.J. 2008).