Observations on the rheological response of alkali activated fly ash suspensions: the role of activator type and concentration

Abstract

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...

The Impact of the Amount of Water Used in Activation Solution and the Initial Temperature of Paste on the Rheological Behaviour and Structural Evolution of Metakaolin-Based Geopolymer Pastes

“…The highest viscosity was observed for geopolymer pastes with an initial temperature of 5 • C. The significant decrease of viscosity (approximately two times, from 350 to 170 mPa•s) was observed for pastes with a water/10 M NaOH ratio in the AS from 0.6 to 1.2, respectively. The same results are presented in several papers [17,34,35] and it is concluded that viscosity is increased by increasing the concentration of NaOH in the activation solution. This is related to the higher OH− concentration in the AS and is confirmed by other researchers [36].…”

“…The dynamic viscosity was studied in order to evaluate viscosity behavior of the AS and its impact on the viscosity of geopolymer pastes. The increase in temperature from 5 °C to 35…”

Reproducible mini-slump test procedure for measuring the yield stress of cementitious pastes

“…the same water/binder ratio and same slag/fly ash ratio, a higher activator dose also increased the spread diameter due to the dispersion effect of the sodium metasilicate added as activator. Sodium silicate is known to act as a dispersant in particulate suspensions [24,25], lowering the yield stress of the paste [26] which resulted in a greater spread in the mini-slump test. This dispersion effect is particularly remarkable in the relatively sticky slag-based pastes; the spread diameter is only 72.5 mm in the pure slag paste with 4 wt% activator dose at w/b 0.40, while a 12 wt% activator dose in the equivalent paste increased the spread diameter to 120.9 mm.…”

Mechanical Properties Improvement Mechanism of Silica Fume‐Modified Ultrafine Cement Used to Repair Pavement Microcracks

“…Wang et al studied the mechanical properties of geopolymers mixture when fly ash was replaced by mineral powder, whose results showed that the mechanical properties of the synthesized geopolymers are significantly improved [13]. Vance et al studied the effect of different alkali activators and concentration on the rheological properties of fly ashbased polymer mixtures [14]. e study of the main synthetic materials in the world's polymer research had changed from traditional kaolin to fly ash [13][14][15][16].…”

“…Vance et al studied the effect of different alkali activators and concentration on the rheological properties of fly ashbased polymer mixtures [14]. e study of the main synthetic materials in the world's polymer research had changed from traditional kaolin to fly ash [13][14][15][16]. Yang et al's results showed that the compressive performance of the synthetic alkali activator material has excellent pressure resistance when the water-cement ratio was less than 0.3 [17].…”