Effects of silicafume and fly ash on properties of alumina cement

s. Alumina cement which was used in this investigation contains about 56% of Al2O3 in the component. Early compressive strength alumina cement at 1 and 3 days can be achieved of 85% compressive strength value at 28 days. After a long period of hydration, the compressive strength of alumina cement harder decreased due to the releasing process of aluminum hydroxide [Al(OH)3] to the outside environment [1, 4,11]. To improve and maintain the long lasting compressive strength of alumina cement harden, new binders would be created SiO2Al2O3 and among CaO-SiO2-Al2O3. The new binders would exist sustainably in the cement harden as a result of the chemical reaction between the product of hydrated cement called gel [Al(OH)3] with micro silica (amorphous SiO2) [4]. This report demonstrates the result of the investigation which is about the effect of silica fume and fly ash on alumina cement.


Introduction
Alumina Cement is investigated and manufactured by many countries all over the world with the purpose of producing refractory mortars and concretes [1,11,13] . Furthermore, it is used to make rapid refractory mortars, rapid concretes and anti-shrinkage concretes. Besides, it can also be made use of in the places which have to be quickly constructed; therefore, the construction would not take much time it can put the works into use in a short time [ 1,3,11,12 ].
Alumina cement is mentioned in this report, that is manufactured from resources of Viet Nam domestic as: Ha Nam limestone, Tan Rai alumina oxide, Dong Anh factory's mud alumina and PhuTho kaolin. The components were properly calculated, ground and heated in 1400 Celsius degrees to make alumina cement clinker [4].
The proportion of silica fume and fly ash accounted for 5-20% in the components while 0.8% polycacboxylate was used.

The research methods and materials 2.1 The research methods
This study used standard methods and physicochemical methods to determine the properties of materials. The methods of investigation are as follows: -Chemical composition of Alumina cement is determined according to Viet Nam standard TCVN 6533 -Refractory alumosilicate -Test method for chemical analysis.
-Chemical composition of fly ash is determined according to Viet Nam standard TCVN 8262 -Fly ash: Test method for chemical analysis.
-Chemical composition of silica fume is determined according to Viet Nam standard TCVN 141 -Portland Cements -Test method for chemical analysis.
-Specific gravity, fineness of fly ash, silica fume and alumin cement is determined according to Viet Nam standard TCVN 4030 -Cements: Test method for determination of fineness.
-Setting time of alumina cement is determined according to Viet Nam standard TCVN 6017 -Cements: Test method for determination of setting time.
-Compressive strength of alumina cement is determined according to Viet Nam standard TCVN 6016 -Cements: Test method for determination of strength.
-Properties of chemical admixture is determined according to Viet Nam standard TCVN 8826 -Chemical admixtures for concrete.
-Particle size distribution of fly ash and silica fume is determined by Couter particle size (Laser Particle Size Analyzer) Model: LA-960A -Manufacturer: Horiba -JAPAN.
-Minerals (phases) composition of alumina cement is determined by the XRD method (X Ray Diffraction) by D2 PHARSER -Brucker.
-Particle shape of fly ash and silica fume is determined by scanning electronmicroscope (SEM). S-4800, HITACHI

Alumina cement
The chemical composition, minerals (phases) composition and physical properties of alumina cement areshown on

Fly ash
The chemical composition and mechanical properties of fly ash is shown on

Silica fume
The chemical and physical properties of Silica fume which was imported from Germany to show on Table 2.6. Particle size, particle shape of silica fume is shown on Fig. 2

Results and discussions
Alumin cements were replaced from 5 to 20% fly ash or silica fume and used 0.8% polycarboxylate by weight content, which have component ratios shown on Table 3.1.

Study results affected fly ash content to alumina cement properties
Fly ash and polycarboxylate affected consistence, setting time and compressive strength of alumina cement, which is shown on Table 3.2. The diagrams which were shown relation between fly ash content and cement properties on Fig. 3.1 and 3   Polycarboxylate used in alumina cement content that is reduced water requirement of cement paste (reducing 30% water compared to ordinary alumina cement). However, alumina cement contains polycarboxylate did not increase its compressive strength at all.
The cause of this phenomenon is that polycarboxylatelatches on to surface of cement particles, which helps slippery cement particles and reduces the need water in the cement paste.Polycarboxylate also reduced initial setting time of alumina cement paste (reducing 12.5% compared ordinary alumina cement paste), but final setting time of alumina cement paste reduced no more than with controlled alumina cement.
Fly ash on alumine cement component reduced water requirement and setting time of cement paste (reducing from 10 to 15% water requirement compared water requirement of ordinary alumina cement). The reducing setting time is on surface of fly ash particles keeping water, so it is limited flexible cement paste.
Alumina cement which used the combination of fly ash and polycarboxylateto would reduce water requirement, setting time of cement paste. (Reducing from 17.5% to 25% compared ordinary alumina cement paste).This phenomenon reduced water requirement and setting time of alumina cement paste the same as above explained.
Alumina cement contains from 5 to 10% fly ash which were increasing more compressive strength than ordinary alumina cement. Increasing ratio of compressive strength at 6 hour was reached maximum that is of 54%. However alumina cement contains to 20% fly ash which was reducing less compressive strength than ordinary alumina cement.

Study results affected silica fume to cement properties
The results affected Silicafume to consistence, setting time and compressive strength of alumina cement shown on Table 3.3.  Silica fume on alumina cement component also reduced water requirement and setting time of cement paste (reducing from 28.5% to 31% water requirement compared by control alumina cement). It is the same as alumina cement which contains polycarboxylate. Silicafume cements can reduce more water requirement than fly ash cements cause all silica fume particles are sphericities they can be created effectivity roller of silica fume and cement particles together.
Alumina cement contains from 5 to 15% silica fume which were increasing more compressive strength than ordinary alumina cement. Increasing ratio of compressive strength at 6 hours was reached maximum that is 48% of sample contains 10% silica fume in component. However, alumina cement contains to 20% silica fume which was reducing less compressive strength than ordinary alumina cement      However, XRD analysis of alumina cement harden contained fly ash or silica fume (Table 3.10) which shown Gibbite and Aluminum oxide content reduced as: Ordinary alumina cement is 18% Gibbsite and 8% Aluminum Oxide. , this phenomenon it has been proved that there was a reaction between amorphous silica of fly ash and silicafume with Gibbsite and Aluminum oxide of cement harden.