Effect of rice husk ash on the strength and durability characteristics of concrete Hwang Chao-Lung ⇑ , Bui Le Anh-Tuan, Chen Chun-Tsun Department of Construction Engineering, National Taiwan University of Science and Technology, 43, Keelung Rd., Sec. 4, Taipei 106, Taiwan article info Article history: Received 23 December 2010 Received in revised form 14 March 2011 Accepted 11 April 2011 Available online 4 May 2011 Keywords: Rice husk ash (RHA) Concrete Compressive strength Durability abstract This work investigates the effects of adding residual rice husk ash (RHA) from South Vietnam, generated when burning rice husk pellets in the boiler, to cement. To improve pozzolanic reactivity, RHA was ground for 1 h. The non-ground RHA and ground RHA were used to test strength activity index according to ASTM C311. The properties of the concrete were investigated, including compressive strength, concrete electrical resistivity, and ultrasonic pulse velocity. Results show that the non-ground RHA can be applied as a pozzolanic material. Decreasing the non-ground RHA average particle size provides a positive effect on the compressive strength of mortar. Compressive strength of cylindrical concrete in the 47–66 MPa range was obtained in this study. The results also indicate that up to 20% of ground RHA could be advan- tageously blended with cement without adversely affecting the strength and durability properties of concrete. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Vietnam is one of the largest exporters of rice in the world, pro- ducing large volumes of husk in a three-season year. The Mekong Delta region alone produces 3.6 million tons of rice husks a year. Because of the high energy capacity of rice husk (about 3200 kcal/kg, two barrels of oil by rice husk ton) [1,2] a large amount of rice husk has been used as bio fuel to power boilers, pro- ducing steam for drying and the parboiling process. In the future, rice husk from the Delta will be used to fuel thermoelectric plants. The rice husk has a large dry volume due to its low bulk density (90–150 kg/m 3 ) [3]. To save storage space and transport rice husk economically, most rice husk was made into pellets as shown in Fig. 1. However, residual RHA, generated when burning rice husk pellets, should be considered for its environmental effects. For several decades, the use of RHA as a highly reactive pozzo- lanic material in concrete production has been researched, espe- cially in developing countries. The ash properties vary due to the differences in incinerating conditions, rate of heating, geographic location, and fineness [1,4–6]. Highly reactive RHA is obtained when RHA is burnt under controlled conditions. This RHA contains a high silica content in the form of non-crystalline or amorphous silica of up to 95% [7]. The reactivity is attributed to the high con- tent of amorphous silica, and to the very large surface area gov- erned by the porous structure of the particles. Generally, the reactivity is also favored by increasing the fineness of the RHA [4,5,8,9]. However, Mehta [10] has considered that since RHA’s pozzolanic activity derives mainly from the internal surface area of the particles, the grinding of RHA to a high degree of fineness should be avoided. Hwang and Chandra [3] have suggested that the particle size of RHA in the 10–75 lm range exhibits satisfactory pozzolanic behavior. In other conditions, a residual RHA is produced with a lower quality due to high carbon content. The high carbon content leads to an increase in water demand and produces a darker color in mortar and concrete. However, the filler effect has been demon- strated as being even more pronounced than the pozzolanic effect [11]. Therefore, by grinding up to an appropriate particle size, the pozzolanic reactivity of residual RHA can be improved, reducing the adverse effect of the high carbon content in the ash, and increasing the homogeneity of the material, but the process means a considerable cost [1,6,12]. The optimized RHA, by controlled burn and/or grinding, has been used as a pozzolanic material in cement and concrete. Using it provides several advantages, such as im- proved strength and durability properties, and environmental ben- efits related to the disposal of waste materials and to reduced carbon dioxide emissions [1,8,13,14]. Up to now, little research has been done to investigate the use of RHA as supplementary material in cement and concrete produc- tion in Vietnam. For example, in 2005 Bui et al. [8] published a pa- per in which they burnt Vietnam rice husk in a drum incinerator for RHA production and researched the particle-size effect on the strength of RHA blended gap-graded Portland cement concrete. For this reason, this study investigates the strength activity index of mortars containing residual RHA that is generated when burning rice husk pellets and RHA as received after grinding residual RHA. The effect of partial replacement of cement with different percent- ages of ground RHA on the compressive strength and durability of concrete is examined. 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.04.009 ⇑ Corresponding author. Tel.: +886 2 27353950; fax: +886 2 27354893. E-mail address: cmrl@seed.net.tw (H. Chao-Lung). Construction and Building Materials 25 (2011) 3768–3772 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat