Contribution of Recycled Alumina Waste to Cement Strength and Microstructure Development

The role of alumina waste as a reactive pozzolan for the local Portland cement has been thoroughly investigated. The results showed that the initial and final setting times of the hardened blended pastes were reduced significantly by increasing the amount of the added alumina. In addition, the compressive strength and the hydraulic property measures of the hardened blends showed progressive improvement reaching approximately 42% and 23% after the early ages of hydration (1 and 3 days), respectively. According to such conditions, these cement blends could be applied in many concrete applications, such as high speed construction, rapid repair, frost prevention, tunneling, shoring, gas and oil well cementing, that require concrete to have rapid setting and strength development abilities. The improvement of the physico-mechanical characteristics and the hydration kinetics of the hardened blended pastes with the recycled alumina were explained after the microstructure study included the examinations of hydration product morphology and type, the thermal gravimetric (TG), the differential scanning calometry (DSC) and the differential thermo-gravimetric (DTG) thermographs analyses. The scanning electron microscope (SEM) was used to examine the microstructure and morphology of hydration products.


Introduction
Portland cement mainly consists of four clinker materials: two silicates (calcium silicates hydrate C 3 S and β-C 2 S) and two aluminates (calcium aluminates hydrate C 3 A and C 4 AF), and gypsum as setting additive regulator.When Portland cement is mixed with water, its phases start to hydrate.The reaction of the tri-calcium aluminate (C 3 A) and the ferrite phase (C 4 AF) predominates at the early ages of hydration.The hydrous calcium aluminium sulfate, ettringite (AFt), is the usual hydrated product in the early ages of the hydration.The reaction of calcium silicate phases (C 3 S and β-C 2 S) predominates from about the time of initial set onward forming calcium silicate hydrates and Ca(OH) 2 , (Park, Sun, Lee,& Shin, 1999;Nehdi & Hayek, 2005;Mobili, Belli, Giosuè, Bellezze & Tittarelli, 2016;Federica, Elena, & Luca, 2016;Amer & El-Hoseny, 2017).
Durability of Portland cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion, fire or another process of deterioration.In other words, cement concrete will be termed durable, when it keeps its form and shape within the allowable limits, while exposed to different environmental conditions, (Elena et al., 2016).Durability of concrete has been a major concern of civil engineering professionals.Also, it has been of considerable scientific and technological interest over the last few decades, (Park et al., 1999;Nehdi & Hayek, 2005;Alessandra et al., 2016;Federica et al., 2016;Amer & El-Hoseny, 2017).
Pozzolanic materials added to blended cements are siliceous or non-siliceous and aluminous materials, which by themselves have little or no cementitious value.But these materials on reacting with calcium hydroxide Ca(OH) 2 in presence of moisture at ordinary temperature possess cementitious properties.Due to hydration of tri-calcium silicate and di-calcium silicate, calcium silicate hydrate (CaO, SiO 2 , H 2 O) and calcium hydroxide Ca(OH) 2 are formed.Calcium silicate hydrate is known as C-S-H gel and results to significant strength gain.On the other hand, Ca(OH) 2 is an unwanted product having no cementitious value.It is soluble in water and leaches out making the concrete porous.Calcium hydroxide causes many other negative features in the concrete.Among these are the "efflorescence" and the "alkali silica reaction" (ASR).However this unimportant, non-cementitious water soluble calcium hydroxide can be converted to insoluble cementitious materials by adding finely divided pozzolanic material.Natural pozzolans have been used since antiquity with excellent results for production of durable concrete.Industrial by-products as pozzolanic additives are now used for the production of high-performance concrete.The fly ash, silica fume and the blast furnace slag are used to improve the resistance of concrete in aggressive environments, (Zelie, Rusie, Veza,& Krestulovie, 2000;Morsy & Shebl, 2007).
The progress of hydration of cement can be determined by different means:  The measurement of the amount of Ca(OH) 2 in the paste resulted from the hydration of the silicates.
 The heat evolved by hydration.
 The specific gravity of the paste.
 The amount of chemically combined water.
 The amount of un-hydrated cement present using X-ray quantitative analysis.
 Also indirectly from the strength of the hydrated paste.
In the present work, a recycled reactive alumina was added as a cement substituent with 5% and 10% additions.The effect of the added alumina was followed up through compressive strength measures of the hydrated blended paste and through the hydration kinetics including the amount of the chemically combined water and the amount of the free lime in the paste which resulted from the hydration of the silicates.In addition, these results were explained after the microstructure study included the examinations of hydration product morphology and type, the thermal gravimetric (TG), the differential scanning calometry (DSC) and the differential thermo-gravimetric (DTG) thermographs analyses.The scanning electron microscope (SEM) was used to examine the microstructure and morphology of the produced hydration products.

Experimental, Materials and Methods
The materials that were used in this study were the ordinary Portland cement CEM I 52.5R (OPC) and aluminum hydroxide (industrial waste).The OPC was provided by EL Arish Cement Company, North Sinai, Egypt.Aluminum hydroxide was supplied from Meet Ghamr industrial wastes congregation.The aluminum hydroxide grab sample was subjected to calcination tests at different temperatures from 300°C to 900°C in a laboratory electric furnace with heating rate 15 o C /min. for various firing times.The produced alumina was ground using stirring mill at 1700 rpm for 3 hrs to reach D 50 and D 90 , the mean particle size of 4.6 and 24.4 micron, respectively.OPC-alumina blends were prepared by substituting 5% and 10% alumina at the expense of cement.The dry blending samples were mechanically stirred for 3 hrs.using a porcelain mixer to attain complete homogeneity, and then they were kept in airtight containers for further investigation.
The needed water for the normal consistency as well as the initial and final setting times were measured using a Vicat apparatus, (El-Diadamony, Amer, Sokkary, & El-Hoseny, 2016).The dry mixtures were mixed with water for about 3 min.The pastes were then molded into cubic specimens (2.54 cm x2.54 cm x2.54 cm) by using stainless steel cubic moulds.Immediately after molding, the samples were cured at about 100% relative humidity at room temperature for the first 24 hours in order to attain the final setting of the specimens, (Abo-El-Enein, Abbas, & Ezzat, 2010;Abo-El-Enein et al., 2014).The hardened cement pastes were then removed from the moulds and cured under tap water.The applied curing time intervals were 1, 3, 7, 14 and 28 days of hydration.The stopping of the hydration process was performed on the crushed cubic specimens after the compressive strength determination.The stopping solution was composed of methyl alcohol and acetone (1:1 by volume), (The Egyptian specification, 2010).The samples were then dried at 80°C for three hrs. in CO 2 -free atmosphere and were kept in desiccators for further investigation.Kinetics of the hydration process was studied throughout the determination of the chemically-combined water and the free lime contents meas 2010; Abo-El diffractometer instrument fro coated with a pictures using

Samples C
The chemical aluminum hyd Table 1.The Figure 1

The Stand
The results of showed increa hardened OPC needed more w reduction from Figure 8. Thes produced after

The Hydra
The results of OPC-A5 and hardened neat Table 4.At th

Conclusions
• Significant improvements in the physico-mechanical properties and the hydration characteristics of the cement pastes were recorded after blending them with 5% and 10% recycled alumina waste.
• The initial and final setting times of the blended cement pastes were reduced significantly after alumina addition.
• The optimum replacement of OPC for the improvement of hydraulic properties of blended cement pastes is 5% alumina.The replacement of alumina causes a marked improvement of the strength and the microstructure of the blended pastes, especially at the early stages of hydration.
• The DSC thermo-grams and XRD analysis for the investigated cement blended pastes indicated the formation of nearly amorphous calcium silicates, calcium sulpho-aluminate hydrates, calcium aluminate hydrates and portlandite.
• The SEM micrographs showed that the partial substitution of OPC by 5% alumina in the early stages of hydration leads to the formation of the denser structure calcium sulpho-aluminate hydrate (AFm) which strengthens the cement paste body.
• The improvement in the physical and the mechanical characteristics in addition to the hydraulic properties of the cement paste body in the early ages of hydration after 5% alumina addition is important in many concrete applications, such as high-speed construction, rapid repair, frost prevention, tunneling, shoring, gas and oil well cementing, that require concrete to have rapid setting and strength development abilities.

Figure
Figure Figure 10.Th Figure 19