Role Of Oven Dry Curing Health Essay

Ordinary Portland cement (OPC) is conventionally used as primary binder to produce conventional OPC concrete. The amount of carbon dioxide released during the manufacture of OPC and the extent of energy required to produce OPC are the matters of environmental concern and poor availability of power. The efforts are being made on various fronts to develop alternative binding material to replace cement. Fly-ash, abundantly available byproduct of coal fired thermal power stations, having no binding properties of its own, is now being widely used as an additive binding material in manufacture of concrete .Fly-ash with alkali activators can produce effective binding material, geo-polymer, through polymerization process which can be used to develop geo-polymer concrete.

The characteristics of alkali activator, decides the quality of geo-polymer and therefore affects the important properties of plastic and hardened concrete. In the present work a mixture of sodium hydroxide solution and sodium silicate solution is used as alkali activator. The present paper deals with studying the role of oven dry curing in comparison with ambient curing at room temperature, in the development of geo-polymer concrete.  Role Of Oven Dry Curing Health Essay.The laboratory investigations were performed by considering all important parameters of geo-polymer concrete, samples were cured under various temperature and durations. Under hot air oven dry curing conditions, the compressive strength of geo-polymer concrete depend on the day of curing from the date of casting. It is optimum on fourth day of oven dried curing and oven dried curing substantially enhance the compressive strength of geo polymer concrete even in the last week of testing of sample as compared to ambient curing condition.

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Keywords:-Geo-polymer concrete, fly-ash, oven dry curing, delayed curing, compressive strength

1. INTRODUCTION

Ordinary Portland cement (OPC) is conventionally used as primary binder to produce concrete. Worldwide the production of cement is increasing about 7% annually. The environmental issues associated with the production of OPC are well known. The extent of energy required to produce OPC is only next to steel and aluminum. The production of one ton of cement liberates about one ton of CO2 to the atmosphere, as the result of de-carbonation of limestone in the kiln during manufacturing of cement and the combustion of fossil fuels (Roy 1999). The contribution of Portland cement production worldwide to the greenhouse gas emission is estimated to be about 1.35 billion tons annually or about 7% of the total greenhouse gas emissions to the earth’s atmosphere (Malhotra2002). The global warming is being seriously considered at national and international level. The greenhouse effect created by the industrial emissions is increasing the global temperature that is resulting in climate changes. Therefore, any action or attempt made to reduce the effect should be encouraged and given more attention. In order to produce environmental-friendly concrete, Mehta (2002) suggested the use of fewer natural resources, less energy, and minimize carbon dioxide emissions. McCaffrey (2002) suggested that the amount of carbon dioxide (CO2) emissions by the cement industries can be reduced by decreasing the amount of calcinated material in cement, by decreasing the amount of cement in concrete, and by decreasing the number of building elements using cement.Role Of Oven Dry Curing Health Essay.  In Eastern part of Vidharbha region of the state of Maharashtra (India), there are four major thermal power plants established in the vicinity of Nagpur City and ill-effects of disposal and storage of fly-ash are well known. The relevance of the study lies in exploring the possibility of using abundantly available fly-ash in the manufacture of concrete as an alternative to cement in an attempt to reduce the emission of greenhouse gases, reduction in energy requirement and disposing off the byproducts in an environment-friendly way.

The present work is aimed at evaluating the role of oven dry curing in the development of geo polymer concrete. Therefore study the effect of delayed oven dried curing on fresh casted sample as well as on the samples which are first ambient cured at room temperature and then oven dry cured in the last week of compressive strength testing. And observe the effect of delayed oven dry curing on compressive strength of geo polymer concrete. Also compare the test results with ambient condition cured samples.

1.1 Geopolymers

Geo-polymers are members of the family of inorganic polymers. The chemical composition of the geo-polymer material is similar to natural zeolitic materials, but the microstructure is amorphous instead of crystalline (Palomo et al. 1999; Xu and van Deventer 2000). Unlike ordinary Portland / pozzolonic cements, geo-polymers do not form calcium silicate-hydrates (C-S-H) for matrix formation, but utilize the poly-condensation of silica and alumina and a high alkali content to attain structural strength. Therefore, geo-polymers are sometimes referred to as alkali activated alumino silicate binders. Geo-polymerization involves the chemical reaction of alumino-silicate oxides (Si2O5, Al2O2) with alkali polysilicates yielding polymeric Si-O-Al bonds. ( J. Davidovits- 1985). Role Of Oven Dry Curing Health Essay.

1.1.1 Source Material of Geo-polymer

Any material that contains mostly Silicon (Si) and Aluminum (Al) in amorphous form is a possible source material for the manufacture of geo-polymer. Several mineral and industrial by-product materials have been investigated in the past. The calcinated source materials such as fly ash, slag, calcinated kaolin, demonstrated a higher final compressive strength when compared to those made using non-calcinated materials. Among the by-product materials, only fly ash and slag have been proved to be the potential source materials for making geo-polymers. Fly ash is considered to be advantageous due to its high reactivity that comes from its finer particle size than slag. Moreover, low-calcium fly ash is more desirable than slag for geo-polymer feedstock material.

Fly Ash

Fly ash, an abundantly available byproduct of thermal power stations, particles are typically spherical and finer than Portland cement and lime. The diameter ranges from 1 micron to 150micron. Fly ash does not have any cementing properties by itself. In general, the reactivity of fly ash depends upon

Chemical composition, fineness and percentage of amorphous or reactive silica present. It also depends on quality of coal used as fuel. The types and relative amount of incombustible matter in the coal determine the chemical composition of fly ash. Fly ash that results from burning sub-bituminous coals is referred as ASTM Class C fly ash or high-calcium fly ash. It is typically contains more than 20 percent of CaO .On the other hand, fly ash from the bituminous and anthracite coals is referred as ASTM Class F fly ash or low-calcium fly ash. It consists of mainly an aluminosilicate glass and has less than 10 percent of CaO. Role Of Oven Dry Curing Health Essay. The color of fly ash can be tan to dark grey depending upon the chemical and mineral constituents (Malhotra and Ramezanianpour 1994). The loss on ignition (LOI) is a measure of unburt carbon remaining in the ash. Fineness of fly ash mostly depends on the operating conditions of coal crushers and the grinding process of the coal itself. Finer gradation generally results in a more reactive ash and contains less carbon.

Low-calcium (ASTM Class F) fly ash is preferred as a source material than high-calcium (ASTM Class C) fly ash. The presence of calcium in high amount may interfere with the polymerization process and alter the microstructure (Gourley 2003). The spherical shape of fly ash often helps to improve the workability of the fresh concrete. Its small size particle acts as filler of voids in the concrete to produce dense and durable concrete.

Devodovits et.al.(2005)claimed that to produce optimal binding properties, the low-calcium fly ash should have the percentage of unburned material (LOI) less than 5%, Fe2O3 content should not exceed 10%, and, reactive silica should be between 40 – 50%, and 80 – 90% of particles should be smaller than 45 µm.

Alkaline activators

Alkaline liquid plays an important role in the polymerization process (Palomo et al.1999). Reactions occur at a high rate when the alkaline liquid contains soluble silicate, either sodium or potassium silicate, compared to the use of only sodium hydroxides (Xu and Van Deventer, 2000). Addition of sodium silicate solution to the sodium hydroxide solution to prepare the alkaline liquid enhanced the reaction between the source material and the solution. In general the NaOH solution caused a higher extent of dissolution of minerals than the KOH solution (Palomo et al. 1999) .The compressive strength and the workability of geo-polymer concrete are influenced by the proportions and properties of the constituent materials that make the geo-polymer paste. Role Of Oven Dry Curing Health Essay.

2. EXPERIMENTATION PROGRAMME

The laboratory investigation was focused at studying the role of timely and delayed oven dried curing on compressive strength of concrete. The methodology adopted for the present study is summarized in the following steps.

2.1 Reference Concrete Mix

Geo-polymer concrete reference mix is designed by considering the optimum combinations of parameters of geo polymer concrete such as the percentage of mass of combined aggregate , density of concrete, the mass ratios of alkaline activator to fly ash , and mass ratio of sodium silicate to sodium hydroxide, on the basis of previous studies.

2.2 Ingredient Materials of Geo-polymer Concrete:

Aggregate

Geologically, major portion of the Vidharbha region of Maharashtra is covered with black basalt. Therefore, coarse aggregates (CA) derived from basalt are used. Experimental trials suggested that CA of the size of 14 mm and less can produce a cohesive mix. A combination of 14 mm (20 %), 10 mm (40 %) and 7 mm (40 %) is proposed in the present work. The fineness modulus of CA is 5.20. Locally available good quality sand having fineness modulus 2.46 is used as fine aggregate (FA). The mass of combined aggregates (CA + FA) is taken as 77% of mass of concrete in the proportion of 65:35

Alkaline Liquid

Further for the laboratory investigations alkaline liquid is prepared using commercially available sodium silicate liquid and12 molar concentration sodium hydroxide liquid. .Sodium silicate liquid content Na2O = 14.61 %, SiO2 = 25.18% and water = 59.99 %. In alkaline liquid, mass ratio of sodium silicate to sodium hydroxide is maintained as 2.5, and mass ratio of alkaline liquid to fly-ash s is maintained as 0.35. and while preparing workable mix in laboratory, it was observed that water to geo solids ratio required to develop workable geo-polymer concrete is almost in the range of 0.24 to 0.28. It was maintained as 0.2559 throughout laboratory investigation. Role Of Oven Dry Curing Health Essay.

Fly ash

Flyash is procured from Khaperkheda Thermal Power Station (KTPS) — Nagpur in Vidharbha region (Maharashtra/India). The Table 1 provides thechemical composition of the fly ash used in the present study

Table 1: Chemical composition of flyash.

Composition

SiO2

Al203

Fe203

CaO

MgO

SO3

Na2O

K2O

TiO2

P2O5

LOI

percentage

60.02

34.25

1.19

1.05

1.30

0.36

0.26

0.82

1.62

0.48

2.17

Table 2: Quantities of ingredient materials of geo-polymer concrete for 1 cum

Ratio of

alkaline

liquid to

fly ash

Mass of Alkaline liquid

in Kg

Mass of

NaOH

Liquid

In Kg

Mass of Sodium

silicate liquid in Kg.

Mass of Flyash

in Kg.

Fine

aggregate

in Kg.

Coarse

Aggregate

in Kg.

Super plasticizer

in Kg.

0.35

143

40.85

102.15

408.90

646.80

1201.20

nil

2.3 Casting and testing of cubes.

In order to study the effect of oven dry curing on compressive strength of geo-polymer concrete on fresh samples and delayed oven dry curing of ambient cured samples at room temperature in last week of testing,. Samples are casted as cubes (size 100×100 mm) and cylinders (size 100 x200 mm) for particular combination of mix. The specimens are cured in ambient condition at room temperature and hot air oven condition at 75 Co temperature for 24 hrs duration.Role Of Oven Dry Curing Health Essay.  Fresh specimens are tested for 28 days characteristic strength. And first ambient cured and then oven dried in last week samples are tested at certain weeks. Fresh samples as cube and cylinder are casted, cured and tested simultaneously under the same environment. A test samples of three specimens is considered to calculate average compressive strength of geo-polymer concrete

3. RESULTS AND DISCUSSION

In line with the objectives of the present work, concrete samples are casted and tested as per requirement and the test results are presented as follows. Figure 1 to figure 9 depicts the variation of compressive strength for various parameters.

Fig1 Effect of ovendry curing in initial period of casting of concrete

Fig 2 cylinder to cube strength ratio

Fig 1 &2 represents that 28days average compressive strength of cylindrical sample is less than Cubical sample. And ratio of cylindrical sample compressive strength to cubical samples compressive strength is almost same as conventional concrete. It is in the range of 0.80.

Fig.3. Effect of oven dry curing in molar concentration of sodium hydroxide

Fig.3 shows the effect of oven dry curing in parameter III that is the molar concentration of NaOH solution in alkaline activator. As the molar concentration of sodium hydroxide liquid increases, compressive strength of concrete also increases in both ambient and oven dry curing condition. There is substantial increase in compressive strength in oven dried condition as compared to ambient curing condition. Role Of Oven Dry Curing Health Essay.

Fig 4 effect of oven dry curing in variation of age of concrete.

Fig 4 represents effect of oven dry curing in variation of age of concrete from 1 week to 36 week. It is observed that for same mix concrete which is casted in various batches and oven dried in initial period at 75 0 -24 hrs. It is observed that compressive strength of concrete is variable

Fig. 5 Effect of oven dry curing in mass ratio of alkaline liquid to fly ash.

Fig 5 shows the effect of oven dry curing in variation of mass ratio of alkaline activator to fly ash on the compressive strength under different temperatures for 24 &48 hrs duration. It is observed that optimum compressive strength is observed for oven dry curing at 750 C for the ratio 0.30 and 0.35, and 600c for the ratio 0.25.It is also observed that strength is not marginally increased by increasing duration of curing.

Fig 6 effect of oven dry curing in variation of mass ratio of sodium silicate to sodium hydroxide

Fig.6 shows the effect of oven dry curing in variation of mass ratio of sodium silicate to sodium hydroxide from 1.75 to 3.0, on the compressive strength of concrete. The ratio of 2.5 provides almost optimum strength at all temperatures. Maximum compressive strength is observed in oven dry curing at 750 for 24hr. It is also observed that strength is not marginally increased by increasing duration of curing.

Fig.7 Effect of oven dry curing in variation of percentage of mass of combined aggregate

Fig.7 shows the effect of oven dry curing in variation of percentage of mass of combined aggregate on the compressive strength at variable age of concrete such as 7,24,&45 days. At 73% provides almost optimum strength at all ages. Role Of Oven Dry Curing Health Essay. And for the range of 73 to 77 % , compressive strength of geo-polymer concrete is maximum..

Fig. 8 Effect of Delayed Oven Dry curing on Compressive Strength of first ambient cured and then oven dry cured in last week of testing of samples

Fig 8 represents the effect of delayed oven dry curing in last week of testing of samples at different ages of concrete such as 9 & 11 week at temperature 45o & 60oC. It is observed that even delayed oven dry curing can substantially increase the compressive strength of geo-polymer concrete as compared to ambient curing at room temperature. Also the increase in duration of curing can improve the strength of

Fig.9. Effect of oven dry curing duration in mass ratio of alkaline liquid to fly ash.

It is observed from fig.9 that optimum compressive strength corresponds to ratio of 0.30 and 0.35 under oven dry curing condition at 750 C for 48hr duration. There is not marginal increase in compressive strength with increase in duration of curing from 24hrs to 48hrs.

Fig.10. Effect of oven dry curing duration in mass ratio of sodium silicate to sodium hydroxide

Fig10 represents for ratio 2.5, gives optimum strength at all temperatures. As the curing duration is increased from 24hrs to 48hrs strength is not marginally increased. It is not substantial, therefore 24hrs curing is sufficient. Maximum compressive strength is observed in oven dried curing condition for 750-24hr.Role Of Oven Dry Curing Health Essay.

.4. CONCLUSIONS

From the laboratory investigations, following conclusions are drawn:

(1)Locally available fly ash is found to be suitable for development of geo-polymer concrete. Since it is low Calcium fly ash as CaO content is 1.05%.

(2) In view of role of oven dry curing, for parameter I, mass ratio of alkaline activator to fly ash for 0.25, 0.40, the optimum compressive strength of geo polymer concrete is observed at 60°C; it is similar to previous studies. However, for ratio of 0.30, 0.35 the maximum strength is observed at 75°C. There is no substantial improvement in strength of concrete by increasing duration of oven dry curing.

(3) For parameter II, mass ratio of sodium silicate to sodium hydroxide as 2.50, optimum compressive strength of geo polymer concrete is observed. It is also observed that there is no substantial improvement in strength of concrete by increasing duration of oven dry curing.

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(4 )For parameter III, molar concentration of sodium hydroxide solution, it is observed that compressive strength of geo-polymer concrete increases with increase in molar concentration in both ambient and oven dry curing conditon. Substantial increase in compressive strength in oven dry curing condition as compared to ambient curing.. Role Of Oven Dry Curing Health Essay.

(5) For all parameters in oven dry curing condition at various temperatures for increased duration from 24hr to 48hr., it is observed that compressive strength of geo polymer concrete did not increase substantially by increasing oven dry curing duration. Therefore, 24 hrs curing is considered sufficient and cost effective for all combinations of mix. This is similar to the recommendations of researchers.

(6) The optimum combination of mix for development of geo polymer concrete is, mass ratio of alkaline liquid to fly ash as 0.35; and mass ratio of sodium silicate to sodium hydroxide as 2.50 in oven dry curing condition at 75°C for 24hrs. Similar ratios for these parameters are recommended in literature.

(7) In oven dry curing, it is observed that even delayed oven dry curing by 8 days, also increase the compressive strength of concrete as compared to ambient curing; but it is less than regular oven dry curing.

(8) In the initial period oven dry curing it is observed that 28 days average compressive strength is optimum at third and fourth day oven dry curing from the date of casting. If delayed oven dry curing period increases from 5th day onwards then compressive strength of geo polymer concrete go on decreasing.

(9) 28days average compressive strength of cylindrical sample is less than Cubical sample. And ratio of cylindrical to cubical samples compressive strength is almost same as conventional concrete. It is in the range of 0.80.

(11) Even the delayed oven dry curing in last week of testing also substantially increase the compressive strength of concrete as compared to ambient curing.

(12) For all the parameters study it is observed that oven dry curing compressive strength is substantially high, it is 2-3 times more than ambient curing condition for the same concrete mix. It is proved that geo polymer concrete is not suitable for insitu concrete , it can be applicable for precast concrete member which are developed in controlled process of curing. Role Of Oven Dry Curing Health Essay.