Abstract
The paper investigates the potential for recovering granulated blast furnace slag after four to five years’ storage in a stockpile (weathered) for use as an addition in concrete. The initial research physically and chemically characterised fresh and weathered granulated slag. Thereafter, studies on ground materials in paste and mortar were carried out. The weathered granulated slag was similar to fresh slag in terms of particle size, shape and elemental composition. However, there was greater roughening of particle surfaces, with various weathering products forming. Following grinding, fresh slag comprised angular particles covering a range of sizes, while finer particles in weathered slag included fragmented reaction products. In cement paste, weathered slag gave reductions in chemically bound water. In mortar, this showed little difference in flow properties compared to fresh slag, with reductions in compressive strength and increases in porosity also noted. Further analyses suggest that, at equal Blaine fineness, weathered slag (a) is actually coarser than fresh material, affecting particle packing and giving larger capillary pores, and (b) has lower reactivity due to reduced surface area. The practical implications are examined and approaches to using weathered slag in concrete suggested.
Highlights
The use of additions in concrete, ground granulated blast furnace slag (GGBS), from iron production, and fly ash, from coal-fired power generation, with Portland cement (PC), mainly developed during the latter part of the 20th Century
The needle-shaped crystals are typical of those found in ettringite ([Ca3Al(OH)6 12H2O]2 (SO4)3 2H2O), which may be related to the chemical composition of blast furnace slag
WEATHERED SLAG BEHAVIOUR WITH PORTLAND CEMENT scanning electron microscopy (SEM) images, and mineralogical analysis of the slag materials investigated in this study, show that the exposure of granulated blast furnace slag to the external environment leads to chemical reactions at the surface and product formation, including amorphous material, as well as calcium carbonate phases and ettringite
Summary
The use of additions (cement components) in concrete, ground granulated blast furnace slag (GGBS), from iron production, and fly ash, from coal-fired power generation, with Portland cement (PC), mainly developed during the latter part of the 20th Century These materials can offer several benefits to concrete, including reduced heat evolution, and enhanced long-term strength and durability properties (Paine et al, 2006; Thomas, 2013; Dyer, 2014). These are typically stockpiled before grinding to a fine powder for use as an addition (CCA Australia, 2018) This period depends on market conditions, but with time wet-storage can lead to weathering – sometimes referred to as ‘pre-hydration’ – a process influenced by factors including temperature/moisture conditions and material properties (Matthes et al, 2017). Standard (CEN) sand to BS EN 196-1 (BSI, 2016) was used in the test mortars for measuring flow characteristics and compressive strength
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