IIA. Starting materials and Preparation of the Hardened Mortar Cement Pastes:
IIA.1. Starting Materials:
The materials used in this investigation are:
– Ordinary Portland cement clinker (OPCC); clinker collected from Helwan Cement Company, Egypt. The clinker used in this study was collected as one batch with large amount (about 65 Kg) which crushed in clinker crusher then ground for 20 seconds to be ready for grinding in lab bond ball mill with the gypsum alone or with gypsum and limestone in the presence and absence of various types and dosages from the applied grinding aids at a constant grinding time (about 5 minutes).
– Limestone dust (LS); collected from Helwan quarries.
-Gypsum (G); which are collected from contractors who bring them from different places in Egypt.
Table (2) shows the chemical oxide composition of various starting materials.
– Grinding aids (GA): The grinding aids used in this study were ethylene glycol (EG) and propylene glycol (PG) (as examples for glycol-based grinding aids), triethanolamine (TEA) (as an example for amine based grinding aids). All the used chemicals are provided from Aldrich-sigma. Commercial grinding aids (CG) supplied from Mapai company (density 1.05) was used as reference grinding aids used in the market. Table (3) shows the physical properties of the various grinding aids used in this study.
Table (2): Chemical oxide composition of the starting materials, wt. %.
IIA.2. Preparation of dry mixes
Various dry mixtures were prepared. The blank mixture was composed of clinker and gypsum in the presence and absence of limestone, Partial replacement of OPC by different ratios of limestone (LS) 5% and 10. The weight of each dry mix is (2 Kg) that homogenized and grinded for 45 minutes in ” Lab Bond Ball Mill” having Mill container from 57mm up to 420 mm outside diameter can be accommodated A standard range of Pascall ball mill pots from 0.5 liters up to 35 liters is available in porcelain, steel or stainless steel. In this experimental study, 25 systems were studied that some of them were studied in absence of limestone and the others were in presence of limestone. These systems can be shown as the Table (5&6).
Table (5) various grinding aids types with 100 % OPC
IIA.3.1.Preparation of mortar pastes
Different mortar specimens were prepared using the composition of 1:2:6 (water/cement/standard sand). Mortar preparation and casting were conducted according to the BS/EN 196-1:2005 using the standard mortar prism with 40 mm × 40 mm × 160 mm size. Water-to-binder ratio was fixed at 0.5 for all of the specimens (Lai, F.C. et al, 2013). The molds were then vigorously vibrated (by a Jolting apparatus) for a few minutes to remove air bubbles and to give a better compaction of the mortar. The surface of the mortar was smoothed by the aid of thinly edged trowel.
Prepared mortar prisms were taken from the mold after one day and placed into the water tank for curing at room temperature of 20± 2° C and will be kept inside a cabinet (aralab cabinet-model: Climatic chamber FITOCLIMA)for 24 hours at 95±2 % RH until the desired age of testing was achieved.
Finally, in the following day, the prisms were used for the compressive strength test of mortar at the ages of 2, 7, 28, and 90 days. The curing water was renewed every 2 weeks to avoid the alkaline media for a long time that may cause an error in the compressive strength measured.
IIB. Methods of physicochemical measurements:
IIB.1. The Water of consistency and setting time measurements
The water of standard consistency is known as the quantity of water required to give a paste which permits the settlement of the Vicat plunger to a point 5-7 mm from the bottom of the Vicat mold. The plunger was replaced by the needle of initial setting time. The needle was lowered gently to contact with the surface of the paste and released to measure its penetration depth. The Vicat apparatus consists of a frame bearing a movable plunger of 300 grams weight. The Plunger is 10 mm diameter at one end and is fitted with a removable 1 mm diameter needle at the other end. Needle Steel; mounted in plunger; 1 mm diameter x 50 mm long.
The plunger is reversible and carries an indicator which moves over the scale graduated in millimeters. The apparatus is supplied complete with plunger, needle, and hard rubber mold. The final setting time was determined by the aid of the needle with an annular attachment. The period of time between the addition of water and the moment at which the needle only made an impression on the paste surface is defined as the final setting time.
IIB.2. Compressive strength determination:
At each time interval, compressive strength tests were performed on the hardened cement pastes using three cylindrical specimens at each hydration time and the average value recorded as Kg/cm2. This test was performed using a Ton industrial machine (West Germany) for a maximum load of 60 tons.
IIB.3. Stopping of hydration:
The stopping of the hydration process was performed on the crushed mortar specimens after the compressive strength determination. The stopping solution (El-Diamony and khalil, 1981) was composed of methyl alcohol and acetone (1:1 by volume). A representative sample of about 10 gm of the ground specimens was placed into a 250 ml beaker containing about 100 ml of stopping solution and stirred magnetically for about 1 hour, then filtered through sintered glass funnel, washed three times with the stopping solution and finally dried at 80 0C for three hours in CO2-free atmosphere and maintained in a desiccators containing soda lime and CaCl2 until the time of testing was reached .
IIB.4. Kinetics of hydration:
Kinetics of hydration process was studied by the determination of non-evaporable (chemically-combined) water content (Wn, %) and free lime (CaO, %) contents for the hardened specimens at a different time of hydration.
IIB.4.1. Determination of chemically combined water content (Wn,%):
Two representative samples of the dried specimens, exact about 1g each, were weighted in porcelain crucibles and ignited for one hour at 1000?C in an adjustable muffle furnace, cooled in a desiccator and then weighted. The chemically-combined water content (i.e., the amount of water retained after drying) was calculated as Wn (%) using the following equation:
Wn (%) = ( W1 – W2 ) / W2 x 100
W1: is the weight of the dried sample before ignition (g) and
W2: is the ignited weight of the sample (g)
IIB.3.2 Determination of the free lime content (CaO, %):
The free CaO content was determined by the following method (Abo-El-Enein et al., 1974): free CaO present in the hardened sample will react with dry glycerol with the formation of calcium–glycerate. The glycerate is treated with an alcoholic solution of ammonium acetate. In the solution only calcium glycerate would be present, free CaO, will react with glycerol and consequently would not present in solution. The reactions are as follows:
The sample (exact about 0.5g) was dispersed in 40 ml of glycerol/ethanol mixture (1: 5) by volume; together with small amount of anhydrous barium chloride (about 0.5g) as a catalyst, and phenolphthalein as indicator. This mixture was kept in a conical flask, fitted with an air – reflux, on a hot plate for 30 minutes (the color becomes pink). The contents were titrated with a standardized alcoholic ammonium acetate solution until the pink color was disappeared. Heat again, if the pink color reappears, complete the titration with ammonium acetate solution until no further appearance of pink color takes place.
IIB.5. Phase composition and Microstructure:
The phase composition of the formed hydrates was investigated by means of X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The microstructure of the formed hydrates is studied using Scanning Electron Techniques (SEM).
IIB.5.1. X-ray diffraction analysis (XRD):
X-ray diffraction technique was carried on some selected hydrated and dried cement pastes. The sample was ground in an agate mortar as gently as possible to 10 – 15 µm. It was then packed into the specimen holder, which was a metal plate 1mm. thick, having a rectangular hole larger than the area irradiated by the X-ray beam and backed by a glass plate. It was finally pressed using a glass microscope slide. A stabilized X-ray generator was used, using Bruker. D8 Advanced Cu K? target with secondary nano chromate and fitted with a copper target X-ray tube, Geiger Muller tube. The setting used was tube run at 30 KV, 15 mA divergence, receiving and scatter slides, 1,1 cm and 1 respectively and chart speed 100 c.p.s.
IIB.5.2. Differential Thermal Analysis (DTA):
The thermal behavior of the specimens of different mixes was tested by DTA technique. DTA is a technique which monitors the temperature difference exciting between a sample and a reference material as a function of time and /or temperature assuming that both sample and a reference are subject to the same environment at a selected heating or cooling rate. DTA curve gives information about both exothermic and endothermic processes that occur within the material upon heat treatment.
IIB.5.3 Scanning electron microscopy (SEM):
Scanning electron microscope, (SEM), has been used to study the morphology and microstructure of some selected cement pastes. For SEM examination, freshly fractured specimens were coated with a thin film of gold under vacuum evaporator with cathode rays. JSM-5410 Scanning electron microscope was used in this investigation.