Rice Husk Ash Concrete Characteristics – Workability, Strength and Admixtures

This article is about 3 characteristics Workability, Strength and Admixtures of Rice Husk Ash Concrete. The early characteristics of rice husk ash (RHA) concrete are influenced by several factors. One important factor is the water cement ratio, which refers to the ratio of water to cement used in the concrete mix. The water cement ratio directly affects the workability of the concrete, with a higher ratio generally resulting in higher workability.

Rice Husk Ash Concrete Characteristics

Another crucial factor is the amount of rice husk ash added to the concrete mix. The inclusion of RHA affects the strength and setting times of the concrete. The strength of RHA concrete tends to increase with higher amounts of RHA, up to an optimal level, beyond which excessive RHA may negatively impact the strength. Similarly, the setting times of RHA concrete can be influenced by the amount of RHA incorporated.

The amount of paste used in the mix is also significant in determining the early characteristics of RHA concrete. The paste, consisting of cement and water, provides the binding matrix for the concrete. The quantity of paste affects the workability and strength development of the RHA concrete.

The type and amount of admixture used in the RHA concrete mix play a crucial role in modifying its properties. Admixtures are added to enhance specific characteristics of the concrete, such as workability, strength, or setting times. The selection and dosage of admixtures need to be carefully considered to achieve the desired performance of RHA concrete.

Lastly, the mix proportion designed for RHA concrete is an essential factor in determining its early characteristics. The mix proportion refers to the specific ratios of various components, including cement, RHA, aggregates, water, and admixtures. The optimized mix proportion ensures the desired workability, strength, and setting times of the RHA concrete.

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1. Workability of Rice Husk Ash Concrete

Adding a small amount of rice husk ash (RHA) to a given water-cement ratio can be highly beneficial for enhancing both the stability and workability of concrete. This addition does not lead to any problems related to bleeding or segregation. The unique property of RHA lies in its large surface area, typically ranging from 50 to 60m2/g. This expansive surface area enables RHA to positively impact the concrete mixture. However, it is important to note that adding excessive amounts of RHA can result in a dry mixture due to the higher water demand caused by the increased surface area. To overcome this issue, the inclusion of superplasticizers or suitable admixtures becomes necessary. By incorporating these additives, the potential negative effects of a higher water demand can be effectively mitigated. For a visual representation of this concept, please refer to Figure-1.

The surface texture and area of rice husk ash contribute to its higher adsorbing character, which requires more water to maintain a desired consistency in concrete. Figure-1 illustrates that at a specific water cement ratio, the workability of the concrete tends to increase. The addition of sand helps reduce the overall spread of the concrete. By maintaining a consistent mixture while reducing the water demand, the overall engineering properties of the concrete mix can be improved.

The granulometry of cement, coarse aggregates, and fine aggregates plays a significant role in determining the presence or absence of voids in fresh and cured concrete. The size of any mineral admixture within the range of 20 micrometers, when added to the mix, helps decrease the overall voids present in the concrete. This mineral admixture supplements the cement grains and results in a mix that requires less water to achieve the desired consistency.

The densification of rice husk ash concrete improves its workability by using a combination of cement, rice husk ash, and water to fill the pores within the compacted aggregates. Concrete made using this method exhibits higher density compared to conventional mixes that utilize more cement. Consequently, this method allows for better control of slump, achieving an excellent rheological properties with a slump of 250 ±20mm.

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 utilization of rice husk ash in concrete production has been found to have a significant impact on the setting time of the material. This effect can be attributed to two main factors: the increased water absorbing demand and the larger surface area provided by the rice husk ash. These properties facilitate a faster reaction process, leading to a decrease in setting time. Another contributing factor is the unique structure of the rice husk ash, which consists of a rigid silica cellular skeleton.

This structure plays a role in reducing the setting time of the concrete. However, it is important to note that when the water cement ratio is high, there may not be substantial contact between the silica structure and the open matrix of the concrete. This lack of contact can potentially slow down the setting time. Consequently, this may hinder the early-stage strength development of the concrete. Therefore, careful consideration of the water cement ratio is necessary to ensure optimal performance and strength gain in the early stages of concrete curing.

Effect of Air Entrainment on Rice Husk Ash Concrete

In their research conducted in 1996, Zhang and Malhotra examined the impact of replacing air entraining admixture in concrete with rice husk ash. They reached the conclusion that as the percentage of rice husk ash replacement increased, the amount of air entrainment admixture also increased. This correlation was attributed to the larger surface area of rice husk ash, which led to a greater demand for admixture. The study encompassed various replacement values, including 0%, 5%, 8%, 10%, and 15%, as depicted in figure-3.

Fig.3: Graph representing the variation of air-entraining admixture (AHA), with the rice husk ash content in percentage

2. Effect of Admixtures on Rice Husk Ash Concrete

The water consistency of rice husk ash concrete was studied in relation to the varying quantity of admixtures, as presented in Table 1. These values were obtained from the research conducted by Singh et al. in 2002. The study focused on investigating the impact of two specific admixtures, lignosulfonate (LS) and calcium chloride (CaCl2), on the pozzolanic reaction, setting time, and consistency of the concrete made with rice husk ash.

The researchers examined how the addition of LS, which is a mixture of superplasticizer, and CaCl2, known as an accelerator, affected the properties of the concrete. By varying the quantity of these admixtures, they were able to assess their influence on the water consistency of the rice husk ash concrete. The results presented in Table 1 provide valuable insights into the relationship between the admixture quantities and the resulting water consistency of the concrete.

This research contributes to a better understanding of the factors influencing the performance of rice husk ash concrete. By exploring the effects of LS and CaCl2 on the pozzolanic reaction, setting time, and consistency, the study sheds light on how these admixtures can be utilized to optimize the properties of the concrete. The findings of Singh et al. provide important data for engineers and researchers working on the development and application of sustainable construction materials.

Table.1: Variation of water consistency with the varying quantity of admixtures in the rice husk ash concrete

The provided table clearly demonstrates that incorporating 1% of LS (specific substance) and 2% of calcium chloride into the given mix results in a significantly greater reduction in water demand. These specific percentages of LS and calcium chloride have proven to be effective in achieving this desired outcome.

FAQs about Rice Husk Ash Concrete