Utilization of natural zeolite in aerated concrete production
Introduction
Lightweight concrete (LWC) has been widely used in buildings as masonry blocks, wall panels, roof decks and precast concrete units. LWC offers design flexibility and substantial cost savings as a consequence of its lower unit weight. The required air pores for the production of LWC can be obtained by using expanded lightweight aggregate (perlite, blast furnace slag, volcanic ash, etc.) or the air pores can be formed in cement paste by the addition of gas-generating agents such as zinc or aluminum powder [1], [2]. Although different types of lightweight aggregates have commercially been introduced into the market, the use of these aggregates is limited [3].
Autoclaved aerated concrete (AAC) is a well-known lightweight concrete and consists of a mixture of sand, lime, cement, gypsum, water and an expanding agent. AAC can be molded and cut into precisely dimensioned units and cured in an autoclave. During the production process, the ingredients combine to form the calcium silicate hydrate gels that establish the special properties of the finished product. Air is entrapped artificially by the addition of metallic powders, such as Al and Zn, or foaming agents. The chemical reaction caused by the addition of aluminum makes the mixture expand to about twice its volume, resulting in a highly porous structure. Curing it in an autoclave under pressure considerably reduces drying shrinkage and water movements [4]. Consequently the final products with an average compressive strength of 2.5–7.5 MPa and an oven-dry unit weight of 400–600 kg/m3 (G2–G6 according to TS 453) offer considerable advantages over other construction materials, such as improved high-thermal and sound insulation, excellent fire resistance, high resource and energy efficiency and outstanding structural performance [5]. AAC products were first developed in Europe in the early 1920s as an alternative building material to lumber. A Swedish architect, Axel Johanson, introduced the product to Europe. Since then, AAC has become widely used in building construction throughout the world. As in the United States, China, India and the EU, the share of AAC in the total domestic concrete market has also progressively increased in Turkey over the last few decades. In 2008, the total amount of production capacity was reported as 2.1 million m3, which made Turkey second biggest AAC producer in the EU. In recent years, the use of pozzolanic materials for the preparation of lightweight concrete, e.g., natural zeolite, silica fume, coal fly ash and slags, has gained attention owing to stringent environmental directives to recycle waste material.
Zeolites are crystalline alumina silicates with uniform pores, channels and cavities. They possess special properties such as ion exchange, molecular sieves, a large surface area and catalytic activity, which makes them a preferable material for tremendous industrial applications [6]. About 40 natural zeolites have been identified during the past 200 years; the most common ones are analcime, chabazite, clinoptilolite, mordenite and philipsite. Worldwide production of natural zeolite was estimated at about 3–4 M ton on the basis of recorded production and production estimates [7]. Rather than the known application areas, a considerable amount of research [8], [9], [10], [11], [12], [13] concerning the use of natural zeolite, especially clinoptilolite in concrete applications as pozzolanic cement, lightweight aggregates and dimension stone has been conducted in recent years. However, its use in AAC as an aggregate and foaming agent is still limited. Furthermore, it should be considered that the physicochemical properties of raw materials, mixture formulation and employed methods are significantly affected by the final properties of concrete. Zeolite surface can be activated to have a high energy after calcination at temperatures greater than 400 °C. When subsequently immersed in water, the surface generates a large amount of air and heat due to adsorption. This heat increases the temperature of the air in the pores, or it is absorbed on the surface of zeolite particles. The expansion of the air volume results in the foaming and volume expansion of concrete during mixing and pre-storage periods. Turkey has 50 billon tons of natural zeolite, mainly clinoptilolite reserve [14]. Only 40,000 tons of this reserve was mined and consumed in 2005 mainly as soil conditioner and feed additives. This necessitates finding new alternative usage areas to increase this lower utilization ratio.
The aim of the present study is to test the use of natural zeolite in AAC production as an aggregate in place of silica sand and as a bubble-generating agent. Natural and calcined forms of clinoptilolite specimens were used as aggregates with different replacement amounts to determine the effects of clinoptilolite usage on the final properties of the products. The compressive strength, flexural strength and thermal conductivities of cured concretes and microstructural properties of the raw and final products were examined.
Section snippets
Materials and method
The zeolite-aerated concrete (ZAC) mixture was produced by using ordinary Portland cement (CEM I 42.5R), limestone, quartz sand, zeolite and water. The zeolite, clinoptilolite rich tuff (CLN), sample used in the AAC mixture was supplied by the Enli Mining Company, Gördes region, Turkey. The average clinoptilolite content of this material has been reported as nearly 80% by an earlier publication [15]. The grainy raw materials were mixed and ground in a ceramic ball mill to ensure the required
Chemical and mineralogical analyses
The chemical analyses obtained from XRF results that identify the main oxide compositions of the major ingredients of ZAC specimens are given in Table 1. As shown in Table 1, natural zeolite can be classified as K-rich clinoptilolite. The mineralogical analysis results of zeolite with the XRD technique are given in Fig. 1. As seen in Fig. 1, zeolite consists of clinoptilolite, quartz and feldspar phases. The glassy phase in zeolite represents the pozzolanic property of this material.
Unit weight
The
Conclusions
This article demonstrates the practicality of using natural zeolite in lightweight concrete production as an aggregate and a bubble-generating agent. The following conclusions can be drawn from this study:
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It was found that replacement of silica sand with zeolite decreases the unit weight of aerated concrete specimens. However, use of fine zeolite compared with a coarse sample increases the water requirement of the mixture because of the higher surface area. This has negatively affected the
Acknowledgement
This study forms part of an Eskişehir Osmangazi University (ESOGU) sponsored research project 2005/150/8. The authors wish to express their gratitude to the ESOGU for its financial assistance.
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