Development of cementitious matrix materials, with improved performance, incorporating by-products from the steelmaking industry

Abstract

One of the most important industries in the north of Spain, especially in the BasqueCountry, is the steelmaking industry. The production of steel in electric arc furnaces,prevalent in this small region generates huge amounts of industrial steelmaking wastethat have to be properly managed. For many years, the Electric Arc Furnace (EAF) slagsgenerated by the steelmaking industry have been dumped as waste material in landfillsites. Nevertheless, many researchers have been investigating ways of standardizingthe use of EAF slag in construction and civil engineering, so that it can be used asaggregate in hydraulic and bituminous mixes. In this way, dumping sites will berelieved of this waste and the consumption of natural resources will be decreased.In this PhD thesis, a step forward has been taken towards standardization of the re-useof waste materials from the steelmaking industry as raw materials in the manufactureof hydraulic mixes for their use in the construction industry. It has been demonstratedthat, with the correct mix design, EAF slag concrete of the desired workability may bemanufactured and that it can even perform well in real-scale structural elements.Real-scale Reinforced Concrete (RC) beams with both pumpable and self-compactingconcretes have been manufactured to achieve this goal. Following the sustainabilityapproach, it was decided to manufacture the beams, not only with the standardPortland cement type I, but also with cement type IV with the addition of fly ash, inorder to manufacture more sustainable concretes.The decision to work firstly with small samples and to finish the work with real-scaleelements was taken to realize this objective. The experimental methods developed toreach the final objective have been divided into three chapters in this PhD thesis. Eachchapter has introduced a different level of scale that has broadened the investigation.The introductory chapters have presented the scope of the research and a fulldescription of the materials and methods used in the development of the thesis.In the first part (Chapter 4), the work performed on mortar mixes has been presented.Firstly, the interaction of the steel slags with cement type IV was analyzed bymanufacturing mixes with different dosages and developing mechanical and durabilitytests. The mechanical properties displayed an excellent behavior.Subsequently, the manufacture of self-compacting mortar mixes has been presented.It is essential to obtain a good mortar paste, in order to manufacture self-compactingconcretes. Several mortar mixes were manufactured and their fresh propertiesanalyzed. The hardened properties were also evaluated, achieving strengths of up to100 MPa, and the mixtures manufactured with EAF slag aggregate displayed superiorbehavior to mixtures manufactured with natural aggregates. This effect is aconsequence of a suitable mortar microstructure, as is evident from the MIP and CATanalyses. Accelerated aging tests were also performed on the self-compacting mortarmixes, demonstrating the innocuous effect of EAF slag.In the second part (Chapter 5), the studies on pumpable and self-compacting concretemixes with EAF slag added as aggregate have been presented. An in-depth analysis ofthe workability of self-compacting mixes has shown the essential need for carefulcontrol of the fine fraction and selection of a compatible chemical admixture to attainthe required flowability. A numerical simulation of the viscous flux of these selfcompactingmixes has been proposed, reporting very acceptable results. Themechanical properties of these concretes indicated good performance and the analysisof some SEM observations of the fracture surfaces on the SCC-EAFS concrete revealedsignificant features, which help us to understand their structure and mechanicalbehavior.An extensive testing regime to assess the durability of these concretes has also beendescribed. Some classical tests, such as freezing-thawing and drying-wetting, wereconducted until noteworthy deterioration was appreciated in the mixes. Singular tests,such as immersion in sea water in the tidal zone, and a study on reinforcementcorrosion in marine environment, were also performed to evaluate the quality andusefulness of this kind of concrete. The results have demonstrated that EAF slagconcretes behave in a satisfactory way in these environments.In the last part (Chapter 6), the manufacture and performance of real-scale EAF slagreinforced beams has been investigated. These beams were manufactured withpumpable and self-compacting concretes and, for each consistency, cement type I andcement type IV were used for manufacturing different mixes. All the mixes displayedgood fresh behavior during the casting period, producing beams without anyhoneycomb. The flexural behavior of the beams was analyzed and yielded resultssimilar to analytical values calculated with the existing formulation. The long-termdeflection of the beams was evaluated and all the beams performed well over the fullduration of the test.Analyzing the general conclusions drawn from this research, it can be stated that themain objective of the thesis has been achieved.