Abstract
The sustainability of resources is becoming a worldwide concern, including construction and building materials, especially with the alarming increase rate in global population. Alternative solutions to ordinary Portland cement (OPC) as a concrete binder are being studied, namely the so-called alkali-activated cements (AAC). These are less harmful to the environment, as lower CO2 emissions are associated with their fabrication, and their mechanical properties can be similar to those of the OPC. The aim of developing alkali-activated materials (AAM) is the maximization of the incorporated recycled materials, which minimises the CO2 emissions and cost, while also achieving acceptable properties for construction applications. Therefore, various efforts are being made to produce sustainable construction materials based on different sources and raw materials. Recently, significant attention has been raised from the by-products of the steelmaking industry, mostly due to their widespread availability. In this paper, ladle slag (LS) resulting from steelmaking operations was studied as the main precursor to produce AAC, combined with phosphating bath sludge—or phosphate sludge (PS)—and aluminium anodising sludge (AS), two by-products of the surface treatment of metals, in replacement rates of 10 and 20 wt.%. The precursors were activated by two different alkaline solutions: a combination of commercial sodium hydroxide and sodium silicate (COM), and a disposed solution from the cleaning of aluminium extrusion steel dies (CLE). This study assesses the influence of these by-products from the steelmaking industry (PS, AS and CLE) on the performance of the alkali-activated LS, and specifically on its fresh and hardened state properties, including rheology, heat of hydration, compressive strength and microstructure and mineralogy (X-ray diffraction, scanning electron microscopy coupled with energy dispersive spectroscopy and Fourier transform infra-red. The results showed that the CLE had no negative impact on the strength of the AAM incorporating PS or/and AS, while increasing the strength of the LS alone by 2×. Additionally, regardless of the precursor combination, the use of a commercial activator (COM) led to more fluid pastes, compared with the CLE.
Highlights
There is an increasing concern regarding the sustainability of the planet’s natural resources
The results indicated that a suitable geopolymer was obtained with an aluminium waste content of 40%, which was activated by a 10 M NaOH solution, cured at 80 ◦ C
L-COMand andLA-COM, LA-COM,the thespread spreadvalues valuespractically practicallydoubled, doubled, when compared to the counterparts, the alkali when compared to the L-CLE and LA-CLE counterparts, the alkali activator/solid activator/solid ratios are the similar orthe very close, in the L, case pastes ratios are the similar or very close, in case of pastes
Summary
There is an increasing concern regarding the sustainability of the planet’s natural resources. The United Nations have set 17 sustainability goals, including sustainable industrialisation, to increase the use of the current resources to their maximum extent and encompass newer and cleaner technologies by 2030 [1]. Sustainability 2021, 13, 9938 alleviate the strain on resources, including the discovery of new raw materials for concrete, especially those based on wastes and residues. Concrete is fabricated globally with ordinary Portland cement (OPC), due to the wellknown secular mechanical strength and durability it can provide. The production of OPC is a highly intense process, which includes a calcination step with temperatures up to 1500 ◦ C, resulting in significant carbon dioxide (CO2 ) emissions and, a very harmful environmental impact [2,3]. Partial replacements of OPC with industrial wastes are common practice nowadays [4,5], using, for instance, ground granulated blast-furnace slag [6], coal fly ash [7], ladle slag [8,9], ceramic waste [10], among others
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