In this article we will discuss about:- 1. Introduction to High Alumina Cement 2. Manufacture of High Alumina Cement 3. Hydration 4. Effect of Conversion 5. Effect of Curing Temperature 6. Physical Properties 7. Refractory Properties.
Introduction to High Alumina Cement:
This cement is quite different from ordinary Portland cement in its composition and also in some properties. Hence its structural use is very limited. The search for a solution to the attack of gypsum bearing waters on Portland cement concrete structures led Jules Bied of France to the discovery of the high alumina cement.
As its name suggests it contains about 40% alumina 40% lime, and upto 8% silica with some ferrous and ferric oxides. The oxide composition is shown in table 3.10 below. However alumina should be less than 32% by weight and Alumina/Lime ratio between 0.85 and 1.3.
The knowledge of compound composition of high alumina cement is very little in comparison to Portland cement and no simple method of calculation is available. The main cementitious compounds are calcium aluminates of low basicity; CA and C5A3. Other compounds are present in few percent, but no free lime exists. Hence there is no problem of unsoundness in high alumina cement.
Manufacture of High Alumina Cement:
The raw materials used are lime stone or chalk and bauxite. Bauxite is a residual deposit formed by weathering under tropical conditions of rocks containing aluminium. It consists of hydrated alumina, oxides of iron and titanium, with small amounts of silica.
The two materials i.e., bauxite and lime are crushed into lumps not larger than 100 mm. Dust and small particles of bauxite formed during crushing are cemented into briquettes of smaller size as dust would damp the furnace. The crushed materials in the required proportions are fed into an open hearth furnace which is a combination of the cupola (vertical stack) and reverberatory (horizontal type) furnace. Pulverized coal is used for firing. The weight of coal is about 22% by weight of the cement produced.
The moisture and carbon dioxide in the furnace are driven off and the materials are heated by the hot gases of the furnace at about 1600°C, which is the fusion point of the materials. The fusion takes place at the lower end of the stack and the molten material falls in the reverberatory furnace and from there through a spout into steel pans. The molten material solidifies in steel pans in the form of pigs. It is fragmented in a rotary cooler and ground in a tube mill.
During grinding all particles of iron are separated by magnetic separators. The fineness of this cement is kepi of the order of 2500 cm2/gram to 3200 cm2/gram, but in no case its specific surface area should be less than 2250 cm2/gram.
The hardness of high alumina cement clinker is very high. Hence the power consumption and wear of the grinding mill is more than ordinary Portland cement. Hence due to high cost of power for grinding, bauxite high cost of fuel for firing materials upto high temperature, the cost of this cement is very high (about three times) in comparison of ordinary Portland cement.
The materials used in the manufacture of high alumina cement are fused completely in the kiln. Due to this fact the properties of ordinary Portland cement and high alumina cement are quite different. While discussing ordinary Portland cement, it was stated that the alumina compounds in ordinary Portland cement are mainly responsible for the attack of sulphate solutions, but high alumina cement containing 40% alumina is free from such an attack. It is due to the fact that in high alumina cement all materials are fused and no free lime is available, while it is not so in the case of ordinary Portland cement i.e. in ordinary Portland cement free lime is available.
The high alumina cement should never be mixed or stored together with ordinary Portland cement as the combination of high alumina cement, will cause flash set of the ordinary cement. The free lime of ordinary cement combines with the aluminates of high alumina cement, forming calcium aluminate. This compound expands and causes cracks in the concrete. This cement is known by different names in different countries as Fondu in France, lightening in U.K. and Lumonite in U.S.A.
Hydration of High Alumina Cement (HAC):
During the setting of the high alumina cement (HAC) the important reaction is the formation of mono- calcium aluminate deca-hydrate (CAH10), dicalcium aluminate octa-hydrate (C2AH8) and alumina gel (AHn). These aluminates develop high strength in the high alumina cement concrete but these aluminates are unstable and convert gradually to tri-calcium alumina hexa-hydrate (C3AH6) and gibbsite, which are more stable. The change in composition of high alumina cements results in loss of strength.
The change in crystal form from hexagonal to cubical form releases much water, resulting in increased porosity of concrete. The manner in which these changes take place depends on water/cement ratio, temperature and chemical environment. The process of change in composition resulting in loss of strength and change in crystal form from hexagonal to cubical form in called as CONVERSION.
The decomposition in chemical reaction of conversion from CAH10 to C3AH6 and alumina hydrate is affected by temperature. The higher the temperature, the faster is the rate of conversion. It is also affected by higher concentration of lime or rise in alkalinity. Further, higher the water/cement ratio, greater the rate of conversion.
It is shown as:
CA + 10H → CAH10
3CAH10 → C3AH6 + 2AH3 + 18H
This reaction liberates the water required for the conversion process to continue. Thus conversion is a function of temperature and water/cement ratio both. The conversion reaction results in the reduction of volume of solids and an increase in the porosity as the overall dimensions of specimens of cement paste or concrete remain constant.
Effect of High Alumina Cement Conversion:
The conversion of high alumina cement leads to loss of strength due to the fact that converted cubic tri-calcium alumina hexa-hydrate (C3AH6) has a higher density than the un-converted mono calcium aluminate deca-hydrate (CAH10). Thus, if the overall volume of the body remains constant, then conversion results in an increased porosity of the paste, which has a tremendous effect on the strength of cement paste or cement concrete.
The loss of strength due to conversion is a function of temperature and water/cement ratio both. At moderate and high w/c ratio, the residual strength is so low which cannot be accepted for most of the structural purposes.
Even with low w/c ratio, conversion increases the porosity so much that chemical attack may take place easily. Thus due to conversion effects HAC is no longer used for structural concrete above or below ground level, but it is useful material for repair work of limited life and for temporary works.
The water of hydration of high alumina cement is about 50% of the weight of the dry cement, which is about 100 twice the water required for the hydration of ordinary Portland cement, but with high alumina cement low water cement ratio of the order of 0.35 is practical and indeed desirable.
Effect of Curing Temperature on High Alumina Cement:
Long term field studies and experiments carried at the laboratory have shown that:
(a) Concrete made with HAC with a free water/cement ratio less than 0.4 and stored in water at 18°C throughout its initial period of curing and its subsequent life, a minimum strength will be reached after about 5 years. However this minimum strength will not be appreciably less than the one day strength.
(b) If concrete made with HAC is stored in water at 18°C for 24 hours and then stored in water at 38°C, it converts rapidly to high limit and reaches a minimum strength in about 90 days. However this strength is very substantially less than 1 day strength.
(c) If concrete made with HAC is stored in water at 18°C for a long period say upto about 8½ years and then immersed in water at 38°C, it will convert rapidly and loose strength to the minimum reached for continuous storage at 38°C.
(d) The 38°C temperature is the upper limit of temperature likely to be developed during curing or normally heated buildings. Hence it is recommended that the design of high alumina concrete should be based on the minimum strength at 38°C temperature.
(e) Highly converted high alumina cement concrete is prone to chemical attack in the presence of long term wetness and a chemically aggressive agent. This risk may be more serious for concrete with higher water/cement ratio.
The influence of w/c ratio on loss of strength on conversion in shown in the following Table 3.11:
The minimum strength value after conversion may be adopted as shown in the following Table 3.12:
Physical Properties of High Alumina Cement:
The high alumina cement is black in colour and its rate of strength development is very high. About 80% of its ultimate strength is developed at the age of 24 hours Even 8 hours strength is sufficient for the removal of the form work. The high rate of gain of strength is due to its rapid hydration. Its rate of heat development also is very high.
Its rate of heat development is 9 calories/hour per gram of cement (9 cal/g/h), whereas for rapid hardening cement, rate of heat development is (3.5 cal/g/h) i.e., it is 2½ times that of rapid hardening Portland cement. However total heat of hydration is of the same order for both the cements. For ordinary Portland cement rate of heat development is 1.6 cal/h per gram.
Compressive strength of 1:2:4 concrete prepared with high alumina cement gives about 420 kg/cm2 after 24 hours and 490 kg/cm: after 3 days. Its ultimate strength is also higher than that of ordinary cement. As stated above its heat development is very rapid and takes place, about in the first 10 hours.
The high rate of heat development makes this cement unfit for mass concrete work, but rapid rate of heat development is of great advantage when concrete is placed in freezing weather.
High alumina cement is slow setting cement. Its initial setting time is 4 to 5 hours and final setting time about 30 minutes later after initial setting. The setting time of high alumina cement is greatly affected by the addition of plaster of paris, lime, Portland cement, and organic matter. Thus no additives should be used. To lower down the setting time of this cement 1 to 2% hydrated lime may be added to it. When rapid setting is of vital importance i.e. for stopping the ingress of water or temporary construction between the tides etc. mixtures of ordinary Portland cement and high alumina cement in suitable proportions are used, but the ultimate strength of such pastes is quite low.
In normal concrete construction, the two cements should not be allowed to come in contact with each other as in such cases flash set will occur. The flash set or accelerated setting time is due to the formation of a hydrate of C4A by the addition of lime from the ordinary Portland cement to calcium aluminate from the high alumina cement. Also gypsum contained in Portland cement may react with hydrated calcium aluminates and cause flesh set.
If the layers of two cements are to be placed, then they should be laid at different times. If the first layer is laid of high alumina cement, then the layer of Portland cement should be laid at least 24 hours later. In case first layer is made with Portland cement, then the concrete made with high alumina cement should be laid after 3 to 7 days. Contamination through plant or tools should also be avoided.
For the same water/cement ratio and equal mix proportions concrete made with high alumina cement exhibits more workability than Portland cement concrete due to the fact that particles of high alumina cement have smoother surface than Portland cement particles, as the raw materials of high alumina cement fuse fully in the hearth. Secondly high alumina cement has lower total surface area 2500 cm2/to 3200 cm2/g.
Resistance to Chemical Attack:
1. It is highly resistant to sulphate attack. This is due to the absence of Ca(OH)2 in hydrated high alumina cement and secondly due to the protective influence of the relatively inert gel, formed during hydration.
2. This cement is free from the attack of CO2 dissolved in pure water.
3. This cement is not acid resistant, but can withstand well very dilute solutions of acids (PH value greater than 3.5 to 4.0) found in industrial effluents.
4. This cement is attacked readily by nitric, hydrochloric, or hydrofluoric acids.
5. Caustic alkalies even in dilute solutions attack this cement with great vigour by dissolving the alumina gel.
6. Though this cement stands extremely well to sea water, but sea water should never be used as mixing water. By the use of sea water as mixing water, the setting and hardening of the cement are affected very badly. Similarly calcium chloride should never be added to this cement.
Refractory Properties of High Alumina Cement:
High alumina cement concrete is one of the foremost refractory materials but its performance varies with the range of temperature. Between room temperature and about 500°C, concrete made with high alumina cement loses strength more than concrete made with Portland cement, then upto 800°C the two concretes are comparable but above 1000°C, high alumina cement gives excellent performance.
Between 700°C to 1000°C, solid reactions between the cement and fine aggregate take place. This reaction is known as ceramic bond, which is responsible for the increase in the strength of high alumina cement concrete between 800°C to 1000°C. The rate of reaction increases with rise of temperature.
Thus concrete made with high alumina cement and crushed fired brick as aggregate can with stand very high temperatures say upto 1350°C. Temperatures above 1350°C upto 1500°C can be withstood by using special aggregates such as fused alumina or Carborundum. Temperature upto 1800°C can be with stood over a prolonged period of time by concrete made from special white calcium aluminate cement with fused alumina aggregate. This cement contains 70-80% Al2O3 (Alumina) 20-25% lime and about 1.0% iron and silica. Its composition reaches to C3A5. However its price is very high.
Refractory concrete made with high alumina cement has a good resistance to acid attack. The chemical resistance increases by firing at 900°C to 1000oC. As soon as the concrete hardens, it can be put to use, that is it should not be pre fired. The refractory brick work expands on heating thus it needs provision of expansion joints, while high alumina cement concrete can be cast monolithically or with but joints only exactly to the required size and shape. Thus refractory high alumina cement concrete can withstand considerable thermal shocks. Refractory linings can be made by shot creating high alumina cement mortar.
For insulation purposes when temperature rise is expected upto 950°C, light weight concrete made with high alumina cement and light weight aggregates can be used with advantage. The density and thermal conductivity of this concrete are of the order 500 to 1000 kg/m3 and 0.21 to 0.29 j/m2s.°C/m.