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Characteristics of rice husk ash concrete such as workability, strength, setting times and effects of concrete admixtures on its properties are discussed. Characteristics of rice husk ash concrete (RHA) at the early stages mainly depend on following factors:- Water cement ratio
- Amount of rice husk ash added
- Amount of paste used in the mix
- Type and amount of admixture
- Mix proportion designed
Contents:
Workability of Rice Husk Ash Concrete
A small addition of RHA (lesser than two to three by weight of the cement), to a given water cement ratio, is sufficient and helpful to improve the stability as well as the workability. This would no lead to any bleeding or segregation problems. This property is gained by the rice husk ash due to its large surface area- in the range of 50 to 60m2/g. The addition of rice husk ash in larger amount will result in the dry mixture, due to higher water demanded by the larger surface area. This can be compromised by the incorporation of Superplasticizers or any adequate admixtures. This is explained in figure-1.
Fig.1: Flow spread as per the flow table test conducted on mortar with varying water cement ratio and rice husk ash content. (As per Hwang and Wu)
As mentioned before, the surface texture and area of rice husk ash have higher adsorbing character, that will ask for more water, to maintain a given consistency. From the figure-1, it can be seen that at a particular water cement ratio value, the workability tends to increase. The addition of sand has the influence in reducing the overall spread of the concrete. By maintaining a given consistency, and reducing the demand for water, will help in improving the overall engineering properties of the mix. How granular the cement, the coarse and the fine aggregates are, will influence a number of voids that would be present in the fresh as well as later stages of the concrete. This granulometric characteristic is a great factor influencing the presence or absence of the voids in the concrete. By the addition of any mineral admixture that has a size in the range of 20 micrometers will facilitate the mix, in decreasing the voids present in overall. This mineral admixture helps to supplement the cement grains. This would finally bring up a mix, that demands lesser water to have the required consistency. The densification of a mixture of rice husk ash concrete helps in improving the workability. This method makes use of cement and the rice husk ash to fill up the pores, with the help of water. This is undergone within the compacted aggregates. The density of concrete made by this method is higher when compared to concrete made of the conventional mix, that makes use of more cement. This helps to control the slump to 250 ±20mm, that reflects excellent rheological properties.Setting Time of Rice Husk Ash Concrete
Fig.2: The variation of setting time with the variation of rice husk ash amount in concrete
The rice husk ash will result in the decrease of setting time. This is justified by the reason higher water absorbing demand and larger surface area. This property promotes faster reaction to occur. The rigid silica cellular skeleton structure of the rice husk ash is also a reason behind the reduction of setting time. If the water cement ratio is high, there is no appreciable contact between the silica structure and the open matrix, which would slow down the setting time. This will not help in strength gaining at the early stages of the concrete.Effect of Air Entrainment on Rice Husk Ash Concrete
Zhang and Malhotra (1996) studied the effect of air entraining admixture in the concrete that is replaced with rice husk ash. They concluded that with the increase in replacement percentage of rice husk ash, the air entrainment admixture amount also increases. This is justified due to the increasing surface area of rice husk ash that they demand more admixture. The results were obtained for rice husk ash replacement under 0, 5, 8, 10 and 15% replacement values, as shown in figure-3.
Fig.3: Graph representing the variation of air-entraining admixture (AHA), with the rice husk ash content in percentage
Effect of Admixtures on Rice Husk Ash Concrete
The variation of water consistency with the varying quantity of admixtures in the rice husk ash concrete is shown in table-1. The values mentioned in table 1, is based on the results obtained by Singh et.al (2002). The effect of lignosulfonate (LS) (A mixture of Superplasticizer) and calcium chloride (CaCl2 - An accelerator) on the pozzolanic reaction, setting time and the consistency of concrete made of rice husk ash was examined. Table.1: Variation of water consistency with the varying quantity of admixtures in the rice husk ash concrete
From the table, it is clear that the 1% of LS and 2% of calcium chloride could bring a higher reduction in the water demand in the mix given.
Read More:
Rice Husk Ash in Concrete -Properties, Advantages and Uses in Construction
What is Workability of Concrete? Workability Vs. Strength of Concrete 
