Mix-design and Properties of Mortars from Alkali-activated Fly Ashes Containing High Amounts of Unburned Carbon Matter
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International Journal of Concrete Structures and Materials Open Access
RESEARCH
Mix‑design and Properties of Mortars from Alkali‑activated Fly Ashes Containing High Amounts of Unburned Carbon Matter Stefania Manzi1* , Andrea Saccani1, Luca Baldazzi1 and Isabella Lancellotti2
Abstract Alkali-activated materials are a promising type of binder candidate as a substitute to Portland cement. Fly ashes can be used as binder precursors giving higher environmental benefits. In the present research, fly ashes (Type F) containing different amounts of unburned carbonaceous matter have been used to formulate mortars. Serious problems concerning the workability in the fresh state have been found when high carbon content are reached. An attempt to avoid the preliminary treatments used to eliminate the unburned matter is carried out by exploiting different mixdesign receipts obtained by changing the water/binder ratio, the ratio of the alkaline activators and using different types of superplasticizer additives. Data so far collected underline that a high amount of unburned carbonaceous matter can not only compromise the mechanical properties of the materials, but also the rheological ones and underline the necessity to develop ad hoc additives for this type of binders. Keywords: fly ashes, alkali-activated materials, unburned carbon matter, mortars mix-design, workability, mechanical properties, microstructure 1 Introduction Environmental issues are steadily and earnestly driving the building industry towards the production of low impact materials. This goal can be achieved in different ways, such as casting new composites using recycled aggregates (Manzi et al. 2013, 2017; Tam et al. 2018; Chen et al. 2019) deriving from demolished concrete or exploiting wastes of different origin as fine aggregates (Bursi et al. 2017; Saccani et al., 2017; Gayana and Chandar 2018; Yang and Lee 2019). A further approach is based on the formulation of binders that can be either traditional Portland mixed with wastes having pozzolanic activity (Karim et al. 2011; Hanif et al. 2017; Hemalatha and Ramaswamy 2017; Juenger et al. 2019) or alkaliactivated wastes containing silica and alumina (Mustafa *Correspondence: [email protected] 1 Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna, Via Terracini 28, 40131 Bologna, Italy Full list of author information is available at the end of the article Journal information: ISSN 1976-0485 / eISSN 2234-1315
Al Bakri et al. 2011; Bignozzi et al. 2014; Carabba et al. 2016,2017a; Monticelli et al. 2016; Cui et al. 2019; Hwang and Shahsavari 2019; Matsuda et al. 2019; Zhang et al. 2019; Saccani et al. 2020). In both cases, fly ashes are usually found to provide positive effects. However, the chemical composition of fly ashes can largely change because of the different chemical composition of the starting fuels as well as the different combustion conditions. It has already been underlined that high amounts of unburned matter can compromise the use of f
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