5This paper reports the influence of activator type and concentration on the rheological properties of alkali 6 activated fly ash suspensions. A thorough investigation of the rheological influences (yield stress and 7 plastic viscosity) of several activator parameters, including: (i) the cation type and concentration of alkali 8 hydroxide, and (ii) the alkali-to-binder ratio (n) and silica modulus (Ms), and (iii) the volume of the 9 activation solution, on the suspension rheology is presented. The results indicate a strong dependence on 10 the cation and its concentration in the activation solution. The viscosity of the activation solution and the 11 volumetric solution-to-powder ratio are shown to most strongly influence the plastic viscosity of the 12 suspension. The suspension yield stress is predominantly influenced by the changes in fly ash particle 13 surface charge and the ionic species in the activator. A shift from non-Newtonian to Newtonian flow 14 behavior is noted in the case of silicate-based suspensions for Ms ≤ 1.5. This behavior, which is not 15 observed at higher MS values, or when the fly ash is dispersed in hydroxide solutions or pure water, is 16 hypothesized to be caused by colloidal siliceous species present in this system, or surface charge effects 17 on the fly ash particles. Comparisons of the rheological response of alkali activated suspensions to that of 18 portland cement-water suspensions are also reported. Page 2 NOMENCLATURE 22 n ratio of Na2O in the activator to the total fly ash content Ms ratio of SiO2-to-Na2O in the activator (as/p)v Activation solution-to-powder ratio, by volume (Refer to the definition of activation solution in 2.1 (as/b)v Activation solution-to-binder ratio, by volume; binder implying fly ash here (w/s)m Water-to-solids ratio, mass-based (Refer to the definition of solids in 2.1) w/c Water-to-cement ratio, mass-based, for OPC systems  Shear stress, Pa y Yield stress, Pa p Plastic viscosity, Pa.s a Apparent viscosity, Pa.s ̇ Shear rate, s -1 23Page 3 INTRODUCTION 24Ordinary portland cement (OPC) based concrete is one of the most widely used materials globally, and 25 production of OPC has been shown to require a significant quantity of energy and release significant 26 quantities of CO2. One of the sustainable alternatives to OPC that has been gaining attention is the use of 27 geopolymeric or alkali activated materials, where alumino-siliceous wastes/by-products such as fly ash or 28 slag can be activated using alkalis to create a binding medium that is X-ray amorphous and has a three-29 dimensional network structure (Palomo et al. 1999;Puertas and Fernández-Jiménez 2003; Škvára et al. 30 2009). The formation of the binding gel is a complex process including the dissolution process where Si 31and Al from the source materials are dissolved into a highly alkaline solution, precipitation of 32 aluminosilicate gel, and further polymerization and condensation to develop the final microstructure 33 (Davidovits 1999;Davidovits 2005). Geopolymeric...

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This paper primarily explores the influence of the alkali cation (Na or K) on the reaction kinetics, product formation, gel structure, and mechanical properties of alkali activated slag systems. For the same activator Ms, i.e., molar SiO2–M2O ratio (M = Na or K), a shorter induction period, a larger acceleration peak, and consequently, a higher amount of total heat release under isothermal conditions is observed for the K-silicate activated slag pastes. The early-age compressive strengths in these systems roughly relate to the heat release response. The later-age (7 days and beyond) compressive strengths are observed to be higher for the Na-silicate activated systems, which is corroborated by: (1) higher amounts of C–(A)–S–H gel in this system indicated by a thermal analysis-based approximate quantification method, and (2) higher combined intensities of Q1 and Q2 structures that point to increased degrees of reaction, and lower amounts of unreacted slag obtained from 29Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra also show evidences of Al-substituted C–S–H gel, with a higher amount of substitution when Na-silicates are used.