INTRODUCTION:

The primary source of food for billions of people globally is rice Globally, approximately 600 million tons of rice paddies are produced each year. The ash is 92 to 95% silica (SiO₂), highly porous and lightweight, with a very high external surface area. Its absorbent and insulating properties are useful to many industrial applications, and the ash has been the subject of many research studies. If a long term sustainable market and price for rice husk ash (RHA) can be established, then the viability of rice husk power or co-generation plants are substantially improved. Apart from the socioeconomic implications of durability, there is also a clear link between durability and the environment. By extending the life cycle of construction materials, we conserve valuable natural resources.The worry on the use of RHA concrete is not limited to structural stability, but also its durability in designing concrete structures.

Many researchers^1-4 explained the several key properties of high strength concrete using rice husk ashes (RHAs). Key properties of concrete including slump, density, compressive strength, water and chloride permeability resistances, were investigated in comparison between samples without using RHA and samples using two types of RHAs.

Muhammad Shoaib Ismail et al⁵ studied the effect of RHA as a 10-30% replacement of cement. Moayad N. Al-Khalaf et al⁶.shown that Rice husk ash was prepared as a pozzolana by a special process such that the final product conformed to engineering requirements in terms of physical and chemical properties, and the silica remained in an amorphous form with a minor amount of unburnt carbon. Bui D., Hu J. et al⁷ reported that RHA has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Ahmadi M. A. et al⁸.studied the development of mechanical properties up to 180 days of self compacting and ordinary concretes with rice-husk ash (RHA) Two different replacement percentages of cement by RHA 10%, and 20%, and two different water/cementicious material ratios (0.40 and 0.35) and it is concluded that 20%RHA concrete provides a positive effect on the Mechanical properties at age after 60. The studies on RHA concretes are very few. The studies of RHA concretes are mostly considering the percentage of replacements of 0, 5, 10, 15, 20. Research studies on the above materials like Fly Ash, metakaoline etc. are abundant. However, research in the field of Rice Husk Ash concrete which is used as potential mineral admixture in concrete is scanty. Disposal of RHA coupled with environmental problem necessitated to select a study on RHA concretes. However, it was also found from the literature that earlier studies pertaining to the fire resistance of RHA concrete was very limited. Hence, in the present study the basic properties like Strength, Durability, Fire Resistance and other aspects of the RHA concretes were proposed for investigation. Therefore in this communication the author reported the studies on RHA concrete samples exposed to sea water.

EXPERIMENTAL:

Rice Husk Ash:

Rice Husk Ash used in the present experimental study was obtained from Orissa, INDIA. General specifications, Physical Properties and Chemical Composition of this RHA used in this study which is furnished by the supplier are given in Table-1, Table-2 and Table-3.

Table 1 : Specifications of Rice Husk Ash
Silica	90% minimum
Humidity	2% maximum
Mean Particle Size	25 microns
Color	Grey
Loss on Ignition at 800⁰C	4% maximum
Table 2: Physical Properties of Rice Husk Ash
Physical State	Solid – Non Hazardous
Appearance	Very fine powder
Particle Size	25 microns – mean
Color	Grey
Odour	Odourless
Specific Gravity	2.3
Table 3: Chemical Properties of Rice Husk Ash
SiO₂	93.80%
Al₂O₃	0.74%
Fe₂O₃	0.30%
TiO₂	0.10%
CaO	0.89%
MgO	0.32%
Na₂O	0.28%
K₂O	0.12%
Loi	3.37%

Super Plasticizers:

Conplast SP430A2 which complies with IS: 9103⁹ Type ‘G’ as a high range water reducing admixture for obtaining a workable mix was used.

Preparation of concrete specimen:

All ingredients were placed in the mixer except water and mixed in the dry condition. Initially 80% of water is added and mixed for 75 seconds. The remaining quantity of water is then added to the concrete mix replaced with RHA in different percentages by weight of cement and mixed for 45 seconds. Super plasticizer dosages added to maintain the workability of 75 mm for all the mixes. Specimens were cast in 100x100x100mm cube moulds. The specimens were compacted using table vibrator. For all specimens a constant compaction time of 50 seconds was adopted. All Samples were water cured for 28 days before carrying out all investigations. A total of 150 samples were cast and tested in the laboratory to study the sea water effect and rate of water absorption of RHA Concretes

Sea water attack:

M 20and M 40 grade RHA Concrete cubes of size 100x100x100mm were cast and cured for 28days in water. The air dried specimens were immersed in 5% strength seawater solution and tested for compressive strength at the end of 28days, 56days and 90days of seawater curing.

RESULTS AND DISCUSSIONS:

The results and discussions of the above tests were presented below.

The compressive strength for M20 grade on sea water exposure for 28, 56 and 90 days were given in tables 1-3.

Table 1: COMPRESSIVE STRENGTH FOR M 20 GRADES CONCRETE AFTER 28 DAYS SEAWATER EXPOSURE.

S.NO	Percentage of replacement	Weight before seawater EXPOSURE	Weight after seawater EXPOSURE	Compressive strength (N/mm2)
1	0%	2.461	2.467	30
2	5%	2.574	2.581	31.6
3	7.5%	2.493	2.535	32.33
4	10%	2.433	2.437	32
5	12.5%	2.487	2.498	31.5

Table 2: COMPRESSIVE STRENGTH FOR M 20 GRADES CONCRETE AFTER 56 DAYS SEAWATER EXPOSURE.

S.NO	Percentage of replacement	Weight before seawater EXPOSURE	Weight after seawater EXPOSURE.	Compressive strength (N/mm2)
1	0%	2.541	2.549	30
2	5%	2.577	2.586	31.33
3	7.5%	2.494	2.501	35.16
4	10%	2.483	2.499	30
5	12.5%	2.525	2.531	29.8

Table 3: COMPRESSIVE STRENGTH FOR M 20 GRADES CONCRETE AFTER 90 DAYS SEAWATER EXPOSURE.

S. NO	Percentage of replacement	Weight before seawater EXPOSURE	Weight after seawater EXPOSURE	Compressive strength (N/mm2)
1	0%	2.538	2.547	35
2	5%	2.581	2.586	33
3	7.5%	2.540	2.555	31.6
4	10%	2.485	2.491	32.16
5	12.5%	2.479	2.488	32

Similarly the compressive strength for M40 grade on sea water exposure for 28, 56 and 90 days were given in tables 4-6

Table 4: COMPRESSIVE STRENGTH FOR M 40 GRADES CONCRETE AFTER 28 DAYS SEAWATER EXPOSURE.

S.NO	Percentage of replacement	Weight before seawater EXPOSURE	Weight after seawater EXPOSURE.	Compressive strength (N/mm2)
1	0%	2.505	2.514	39.3
2	5%	2.506	2.519	44.6
3	7.5%	2.572	2.598	37.6
4	10%	2.527	2.532	38
5	12.5%	2.572	2.578	41

Table 5: COMPRESSIVE STRENGTH FOR M 40 GRADES CONCRETE AFTER 56 DAYS SEAWATER EXPOSURE.

S.NO	Percentage of replacement	Weight before seawater EXPOSURE	Weight after seawater EXPOSURE.	Compressive strength (N/mm2)
1	0%	2.531	2.542	39.3
2	5%	2.503	2.511	41
3	7.5%	2.630	2.632	38
4	10%	2.565	2.576	37
5	12.5%	2.610	2.618	36

Table 6: COMPRESSIVE STRENGTH FOR M 40 GRADES CONCRETE AFTER 90 DAYS SEAWATER EXPOSURE.

S. NO	Percentage of replacement	Weight before seawater EXPOSURE	Weight after seawater EXPOSURE	Compressive strength (N/mm2)
1	0%	2.539	2.548	38.6
2	5%	2.461	2.496	38
3	7.5%	2.580	2.582	36.16
4	10%	2.539	2.551	37.1
5	12.5%	2.571	2.578	36.5

The variation of the compressive strength in different replacements M20 and M40 grades for sea water exposure were given in tables7-8

TABLE 7 COMPRESSIVE STRENGTHS FOR DIFFERENT REPLACEMENTS AT- M 20 GRADE

Replacement → Age ↓	0%	5%	7.50%	10%	12.50%
28	30	31.6	32.33	32	31.5
56	30	31.33	32.02	30	29.8
90	35	33	31.6	32.16	32

TABLE 8. COMPRESSIVE STRENGTHS FOR DIFFERENT REPLACEMENTS AT M 40 GRADE

Replacement → Age ↓	0%	5%	7.50%	10%	12.50%
28	39.3	44.6	37.6	38	41
56	39.3	41	38	37	36
90	38.6	38	36.16	37.1	36.5

It was observed from the above discussions that RHA Concretes in the range of 5% to 7.5% replacement shows higher strengths than normal concrete at all ages in sea water / marine environment. It may be due to extensive dissolution of the ingredients of the specimens with ingredients of sea water to form complex chemical compounds which impart strength.

CONCLUSION:

The studies on Rice Husk Ash concretes (RHA) exposed to marine atmosphere for various ages, showed that an RHA replacement range of 5% to 7.5 % showed better compressive strengths than other replacements at all ages and for all three grades of concretes.

REFERENCES:

1 Dao Van Dong, Prof. Pham Duy Huu, Engineer-Nguyen Ngoc Lan. “Effect of Rice Husk Ash on Properties of High Strength Concrete”. The 3^rd ACF International Conference ACF/VCA. 2008, 442-449.

2 Alireza Naji Givi, Suraya Abdul Rashid, Farah Nora Aziz A, Mohamad Amran Mohd Salleh.“Contribution of Rice Husk Ash to the Properties of Mortar and Concrete:A Review”. Journal of American Science, 6(3), 2010, 157-165.

3 Zhang M.H, Lastra R and Malhotra V.M, “Rice – Husk Ash Paste and Concrete: Some Aspects of Hydration and the Microstructure of the Interfacial Zone Between the Aggregate and Paste”. Cement and Concrete Research, Vol 26, No. 6, 1996,963-977.

4 Raoul Jauvertgue, Frank Rendell, Seni tamba, Ibrahima Khlil Cisse. “Properties of Cement –Rice Husk Mixture”.Construction and Building Materials.17, Vol, 2003, 239- 243.

5 Muhammad Shoaib Ismail and Waliuddin A.M. “Effect of Rice Husk Ash on high strength concrete”. Construction and building materials, Vol 10, No. 7, 1996, .521-526.

6 Moayad N, Al-Khalaf and Hana A.Yousift, “Use of Rice Husk Ash in Concrete”.The International Journal of Cement Composites and Lightweight Concrete, 6, 4.1984, 241-248.

7 Bui D.D., Hu J, Stroeven P. “Particle size effect on the strength of Rice Husk Ash blended gap-graded Portland cement concrete”. Cement & Concrete Composites, 27, 2005, 357-366.

8 Alireza Naji Givi, Suraya Abdul Rashid, Farah Nora Aziz A, Mohamad Amran Mohd Salleh. “Contribution of Rice Husk Ash to the Properties of Mortar and Concrete:A Review”. Journal of American Science, 6(3), 2010, 157-165.

9 IS:9103, BS: 5075 and ASTM-C-494, Bereau of Indian Standards, Manak Bhavan, 9 Bahadur Shah Zafar Marg, New Delhi 110002, India

Received on 04.11.2013 Modified on 08.12.2013

Asian J. Research Chem 7(1): January 2014; Page 52-54