Replacement of Cement with Rice Husk Ash in Concrete

In the present investigation, a feasibility study is made to use Rice Husk Ash as an admixture to an already replaced Cement with fly ash (Portland Pozzolana Cement) in Concrete, and an attempt has been made to investigate the strength parameters of concrete (Compressive and Flexural). For control concrete, Indian Standard (IS) method of mix design is adopted. Five different replacement levels namely 5%, 7.5%, 10%, 12.5% and 15% were chosen for the replacement study. A range of curing periods starting from 3 days, 7 days, 28 days and 56 days are considered in the present study. Series ranging from 5% to 10% RHA concrete but better compressive strengths at later ages though showing lower compressive strengths initially. However, split tensile strengths are lower for RHA concrete when compared to normal concrete.


Introduction
From the middle of 20 th century, there had been an increase in the consumption of mineral admixtures by the cement and concrete industries. The increasing demand for cement and concrete is met by partial cement replacement.
Substantial energy and cost savings can result when industrial by-products are used as a partial replacement for the energy intense Portland cement. The use of by-products is an environmentally friendly method of disposal of large quantities of materials that would otherwise pollute land, water and air. Most of the increase in cement demand will be met by the use of supplementary cementing materials [2]. Rice milling generates husk as a by-product. During milling of paddy about 78 % of weight is received as rice, broken rice and bran. Rest 22 % of the weight of paddy is received as husk . This husk is used as fuel in the rice mills to generate steam for the parboiling process. This husk contains about 75 % organic volatile matter and the balance 25 % of the weight of this husk is converted into ash during the firing process, which is known as rice husk ash ( RHA ). Rice husk is usually burnt approximately 48 hours under uncontrolled combustion process. The burning temperature is within the range of 600 to 850 degrees C. The ash obtained is ground in a ball mill for 30 minutes and its appearance in color is grey. This RHA in turn contains around 85% -90% amorphous silica. So for every 1000 kgs of paddy milled , about 220 kgs ( 22 % ) of husk is produced, and when this husk is burnt in the boilers , about 55 kgs (25 %) of RHA is generated [3] [5]. India is a major rice producing country, and the husk generated during milling is mostly used as fuel in the boilers for processing paddy, producing energy through direct combustion and / or by gasification . About 20 million tones of RHA is produced annually. This RHA is a great environmental threat causing damage to the land and the surrounding area in which it is dumped. Several ways are being thought of for disposing it by making commercial use of this RHA. In the present investigation, Portland cement was replaced by rice husk ash at various percentages to study compressive and flexural strength [4].

Experimental Programme
Materials Used. Cement: Cement used in the experimental work is PORTLAND POZZOLONA CEMENT (PPC) conforming to IS: 1489 (Part1)-1991 [10]. The chemical and physical properties of the cement obtained on conducting appropriate tests as per IS: 269/4831 [11] and the requirements as per IS 1489-1991 [10] are satisfied.
Rice Husk Ash: Rice Husk Ash used in the present experimental study was obtained from N.K Enterprises Jharsuguda, Orissa. Specifications, Physical Properties and Chemical Composition of this RHA as given by the Supplier are given in Tables 1, 2  Super Plasticizers: Super plasticizers are usually highly distinctive in their nature, and they make possible the production of concrete which, in its fresh or hardened state, is substantially different from concrete made using water-reducing admixtures. The superplasticizer used in this project is Conplast SP430A2 manufactured by "FOSCROC Chemicals". The main objective of using this super plasticizer is to produce workable concrete requiring little or no vibration during placing.   Chloride content: Less than 0.05% Air entrainment: Less than 1% over control Water: Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. Since it helps to form the strength giving cement gel, the quantity and quality of water is required is important. Mixing water should not contain undesirable organic substances or inorganic constituents in excessive proportions. In this project clean potable water was obtained for mixing and curing of concrete.
Mix Design for M20-Grade Concrete: The mix proportions considered for each replacement by RHA are presented in table 4.

Casting of Test Specimens
All the materials were brought to room temperature and the samples were prepared by hand mixing in a manner to avoid loss of water or any material .The mix is placed into 150x150x150 mm cube moulds with necessary compaction carried out using table vibrator. The sample moulds were water cured for different curing periods after demoulding the hardened samples.

Tests Conducted Test for compressive strength and flexural strength of RHA concrete specimen
Standard 200 T compression testing machine is used for testing of samples of size 150x150x150 mm. The samples are placed in the machine with neat capping done to ensure uniform transmission of load.
Similarly, standard 40T universal testing machine was used for testing the flexural beams of size 500mmx100mmx100mm.
Pre samples are mounted over roller supports and the load is applied at the centre to note the failure .The strength is calculated using formulae as per BS 5075 -1982 [9].

Results and Discussions
Effect of age on Compressive Strength of RHA Concrete: Table 5 and Figure 1 represent the variation of compressive strength with age for M20 grade RHA concrete.
In each of these variations, it can be clearly seen that, as the age advances, the compressive strength also increases. The highest strength obtained at a particular age for different replacement levels with RHA is reported in table 5 for the ages of 3 days, 7 days, 28 days and 56 days respectively. Variation of flexural strength with respect to age and percentage of RHA and effect of RHA is depicted in Table 6 and   At all the cement replacement levels of Rice husk ash; there is gradual increase in compressive strength from 3 days to 7 days. However there is significant increase in compressive strength from 7 days to 28 days followed by gradual increase from 28 days to 56 days.
At the initial ages, with the increase in the percentage replacement of both Rice husk ash, the flexural strength of Rice husk ash concrete is found to be decreasing gradually till 7.5% replacement. However as the age advances, there is a significant decrease in the flexural strength of Rice Husk ash concrete.
The compressive strength of 7.5% RHA Concrete showed 3.46% increase in its strength when compared to that of normal concrete at 56days.
The Flexural strength was reported to have decrease by 8.7% at 28days and 6.66% at 56days for 7.5% RHA Concrete when compared to normal concrete.
By using this Rice husk ash in concrete as replacement the emission of green house gases can be decreased to a greater extent. As a result there is greater possibility to gain more number of carbon credits.
The technical and economic advantages of incorporating Rice Husk Ash in concrete should be exploited by the construction and rice industries, more so for the rice growing nations of Asia.