The Intemational Journal of Cement Composites and Lightweight Concrete, Volume 9, Number3 August 1987 Rice husk and fly ash concrete blocks Julibn Salas, Marina Alvarez and Janer Veras* Synopsis This paper forms part of a project on materials, technology and prototypes of very low cost housing, Carried out at the Instituto Eduardo Torroja (Spain). Results are presented of 40 x 20 x 20 cm blocks, with a concrete made up of an agricultural waste product, rice husk, and an industrial by-product, fly ash, and formed by means of a manual placement machine. The blocks have a density of about 11 O0 kg/m 3, a K value of 0-20W/m°C and failure loads of 7000 to 8000kp. Keywords Masonry, lightweight concrete, insulation, rice husk, fly ash, agricultural wastes, concrete elements, compressive strength, thermal conductivity, appropriate technology, strength of materials, mix design INTRODUCTION Based on previous research [1-4] on the properties of concrete with a rice-husk base, the present work which is of an experimental nature, expounds the use of this range of materials (lightweight, insulating, and structural) in the form of building components that are easy to produce even in developing countries. For the use of these materials, the following prerequisites were estab- lished: (1) adequate manageability, without the need to resort to mechanical means; (2) technology to put this into effect that may be easily assimilated; (3) minimal investment in equipment; (4) flexibility of use (neutrality of the components); and (5) use of abundant waste materials (rice husk, and fly ash) and those that may be easily acquired (cement). Two basic elements, capable of satisfying the above requirements, were selected: a hollow block measuring 40 x 20 x 20cm and a 90 x 60 x 6cm panel. The present work describes the development and study of the properties of the hollow blocks. HOLLOW BLOCKS Production equipment The production equipment for these blocks involves minimal investment; it is solid and easy to handle and therefore represents an important step in the industriali- sation and rationalisation of the building process in developing countries. It involves a layer machine of the * Instituto Eduardo Torroja (C.S.I.C.), Apartado 19002, Madrid, Spain. Received 25 June 1980 Accepted 28 January 1987 (~) Longman Group UK Ltd 1987 0262-5075/87/09306177/$02.00 manual placement variety (Figure 1)in which two speci- fic modifications have been introduced. (1) To provide wall thicknesses that are greater than normal so that they adapt better to the range of low resistance concretes. (2) Longitudinal tongue and groove, to improve behav- iour with respect to horizontal action. The block designed consists of a modular frame measuring 40 x 20 x 20cm with an overall horizontal section area of 791.3cm 2 and a net or resistant section of 448.5cm 2 giving an apparent volume of 8.521dm 3 (Figure 2). Each placement carried out by the machine pro- duces six blocks. The machine runs on electricity and allows for alterations in the mould for the shape of such elements. Mix proportioning Work has been conducted with three families of basic mixes referred to as L, C and M. Later on, a further mix B was definea; detads of these mixes are gwen later for comparative purposes. The basic or standard mix L is the same as that used by the authors in making up insulating concretes [1]. In order to obtain concretes with greater resistance and a more homogeneous texture, a 'filler' was added; fly ash in a natural state, originating from the Soto de Ribera plant [2] was used in a quantity equal to 25% of the weight of the cement (mix C) and 54% by weight (mix M) (Table 1). Five water/(cement + ash) ratios were used for each mix. Since vibration time appeared to be a fundamental parameter, all the mixes were vibrated for one minute; and four mixes were selected to be vibrated again for two and three minutes. Thus altogether twenty-four mixes were tested, with six blocks being made from 177