Rice husk ash blended cement: Assessment of optimal level of replacement for strength and permeability properties of concrete

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Abstract

In this study, rice husk ash (RHA) prepared from the boiler burnt husk residue of a particular rice mill has been evaluated for optimal level of replacement as blending component in cements. The physical, chemical and mineralogical characteristics of RHA were first analysed. The properties of concrete investigated include compressive strength, splitting tensile strength, water absorption, sorptivity, total charge-passed derived from rapid chloride permeability test (RCPT) and rate of chloride ion penetration in terms of diffusion coefficient. This particular RHA consists of 87% of silica, mainly in amorphous form and has an average specific surface area of 36.47 m2/g. Test results obtained in this study indicate that up to 30% of RHA could be advantageously blended with cement without adversely affecting the strength and permeability properties of concrete. Another interesting observation emanating from this study is the linear relationship that exists among water sorptivity, chloride penetration and chloride diffusion.

Introduction

Rice husk is an agricultural residue obtained from the outer covering of rice grains during milling process. It constitutes 20% of the 500 million tons of paddy produced in the world [1]. Initially rice husk was converted into ash by open heap village burning method at a temperature, ranging from 300 °C to 450 °C [2]. When the husk was converted to ash by uncontrolled burning below 500 °C, the ignition was not completed and considerable amount of unburnt carbon was found in the resulting ash [3]. Carbon content in excess of 30% was expected to have an adverse effect upon the pozzolanic activity of RHA [4]. The ash produced by controlled burning of the rice husk between 550 °C and 700 °C incinerating temperature for 1 h transforms the silica content of the ash into amorphous phase [5], [6]. The reactivity of amorphous silica is directly proportional to the specific surface area of ash [7], [8]. The ash so produced is pulverized or ground to required fineness and mixed with cement to produce blended cement.

Several papers have been published on the performance of RHA blended concrete. However, only limited information is available on the permeability characteristics. The objective of the present investigation is to evaluate the rice mill boiler burnt rice husk residue as supplementary cementitious material with reference to strength and permeability properties of hardened concretes and identify the optimal level of replacement. In the present context, optimal level refers to the maximum favorable percentage of replacement of OPC with RHA up to which the strength and permeability properties of blended concrete are equivalent or more than that of unblended OPC concrete. Towards this end, experiments were carried out in two phases as per standard test procedures. In the first phase, chemical composition, physical properties, and characterization of RHA were carried out. This included evaluation of standard consistency, initial setting time, final setting time and compressive strength of RHA blended cements. In the second phase, studies on concrete specimens were conducted. This included tests on compressive strength, splitting tensile strength, water absorption, coefficient of water absorption, sorptivity, resistance to chloride ion penetration and diffusion coefficient. All the experiments were carried out in triplicate and mean values are reported.

Section snippets

Materials used

Ordinary Portland cement (OPC) conforming to Indian standard code IS 8112-1995 was used. Graded river sand passing through 1.18 mm sieve with fineness modulus of 2.85 and specific gravity of 2.55 was used as fine aggregate. Locally available crushed granite aggregate, passing through 12.5 mm sieve while being retained on 4.75 mm sieve with fineness modulus of 6.26 and specific gravity of 2.7. (Conforming to IS 383-1970) was used as coarse aggregate.

Boiler-fired rice husk residue was collected from

Physical and chemical analysis of OPC and RHA

The particle size distribution curves of OPC and RHA are shown in Fig. 3. The particles of RHA are nearly six times finer than those of OPC and the finer particles of RHA are well graded in their distribution. Mineralogical analysis of RHA carried out by X-ray diffraction showed that the silica was mostly in amorphous form. Small quantities of crystal-phase as quartz, coesite (silicon dioxide) and crystobalite are also present.

The physical properties of OPC and RHA are compared in Table 3. The

Conclusions

  • (1)

    Rice husk ash obtained from Indian paddy when reburnt at 650 °C for a period of 1 h transforms itself into an efficient pozzolanic material rich in amorphous silica content (87%) with a relatively low loss on ignition value (2.1%).

  • (2)

    As high as 30% by weight of OPC can be replaced with reburnt rice husk ash without any adverse effect on strength and permeability properties.

  • (3)

    Replacement with 30% of reburnt rice hush ash leads to substantial improvement in the permeability properties of blended

References (21)

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