ACI Materials Journal/July-August 2007 397
ACI MATERIALS JOURNAL
TECHNICAL PAPER
ACI Materials Journal, V. 104, No. 4, July-August 2007.
MS No. M-2006-268.R1 received December 15, 2006, and reviewed under Institute publication policies. Copyright © 2007, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including authors’ closure, if any, will be published in the May-June 2008 ACI Materials Journal if the discussion is received by February 1, 2008.
The possibility of using granulated blast-furnace slag (GBFS), furnace bottom ash (FBA), and their combination as fine aggre-gates in concrete was studied by performing experiments. These materials were used without applying any preprocesses such as sieving and grinding. The compressive, flexural, and split tensile strengths of concretes with natural sand replaced with GBFS, FBA, and GBFS plus FBA at 10, 20, 30, 40, and 50% were examined at a fixed water-cement ratio (w/c). The percentages represent the replacement percentage of fine aggregate by GBFS, FBA, or their combination and were evaluated depending on weight basis. Also, microstructure and water absorption capacity of concrete were researched. Test results showed that concrete strength decreases with increasing replacement ratio with respect to reference concrete. In addition, FBA decreases the strength of concrete more than GBFS. In particular, the strength of concrete was detrimentally affected when the replacement ratio was beyond 40%. The microstructure studies showed that different pore structures were formed in the concrete depending on the replacement material, that is, GBFS or FBA. It is concluded that the main reason for the strength reduction in new concrete is the formation of a porous concrete structure. Moreover, an increase trend in water absorption capacity was observed for both replacement materials.
Keywords: compressive strength; fly ash; slag.
INTRODUCTION
The environmental protection and cost-effective considerations bring about the era of using the industrial by-products such as fly ash (FA), granulated blast-furnace slag (GBFS), and furnace bottom ash (FBA) in concrete. GBFS is a by-product obtained in the production of pig iron in the blast furnace and is formed by combining the earthy constituents of iron ore with limestone flux. When the molten slag is swiftly quenched with water, it forms a fine, granular, almost fully noncrystalline, glassy form known as granulated slag, having latent hydraulic properties. Due to the presence of high contents of silica and alumina in a noncrystalline state, GBFS is used as a cementitious ingre-dient in mortars containing limes, as an addition in producing blending cements or as mineral admixtures in concrete.1,2 The strength of concrete containing GBFS depends on the fineness of slag, the activity index, and the slag-cement ratio of the mixture.3 GBFS is obtained from molten slag of higher temperatures in every modern blast furnace. Therefore, it should be noted that GBFS contains some friable particles even when it has a good gradation, which can inversely affect concrete strength. Furnace bottom ash (FBA) is a by-product in coal-fired power plants that agglomerates and settles down at the bottom of the furnace. It usually has no pozzolanic property in the nonground form, which makes it unsuitable to be used as a cement replace-ment material in concrete. Its particle size distribution, however, which is similar to that of sand, makes it attractive
to use as a sand replacement material.4 Previously, many studies have been conducted for materials that are mixed with FA, GBFS, FBA, cement, and sand. In most of these studies, GBFS, or FBA, is used as mineral admixture by grinding it into very fine powder. In the present study, however, GBFS and FBA are used as fine aggregate (sand) replacement material.
Yüksel et al.5 investigated the possibility of using GBFS as fine aggregate without grinding. In the study, they analyzed the properties of concrete by changing the GBFS replacement percentage from 0 to 100% with a step change of 25%. The results showed that GBFS can be used as fine aggregate at a certain replacement percentage range. It was observed that beyond 50% GBFS replacement, the decrease in strength was very high. Therefore, the maximum replace-ment percentage studied in the present study was taken as 50% for GBFS and FBA.
The objective of this paper is to compare strength properties of GBFS and/or FBA replaced concrete with reference concrete. GBFS, FBA, or their combination was used as fine aggregate instead of sand and replaced depending on weight basis. Neither sieving nor grinding was applied to the GBFS or FBA. They were added in the concrete as they were taken from the plants. Proportions of 10 to 50% replacement were considered and compressive, flexural, and split strength tests were conducted on the produced concrete specimens. In addition, water absorption tests were performed on the same concrete specimens and their microstructures were analyzed using scanning electron microscope (SEM) micrographs.
RESEARCH SIGNIFICANCE
Although sieving and grinding these by-products improves the properties of concrete a great deal, it should be noted that each preprocess puts an additional burden of cost on concrete production. New utilization forms, which are environmentally friendly, should be investigated in addition to current usage forms for GBFS and FBA without addi-tional costs or processes. Using GBFS and FBA as partial fine aggregate in concrete on the weight basis without grinding doesn’t result in any additional costs or processes. At present, FBA is thought of as solid waste and GBFS is an industrial by-product. If they can be used as a substitute for fine aggregates (sand) in concrete, they will be raw materials for concrete rather than being a waste or by-product. This can create economical values for GBFS and FBA and help protect the environment by reducing the amount of solid waste. Title no. 104-M44
