Fresh and hardened properties of ambient cured one-part alkali-activated pastes along with superplasticizers

Abstract

Alkali-activated materials (AAMs) have been developing rapidly in the past decade as environmentally friendly alternatives to ordinary Portland cement (OPC), aiming to reduce the CO2 environmental burden accompanied by OPC production. Typically, AAMs are produced through chemical activation (either alkali or acidic) of aluminosilicate materials (naturally available or industrial byproducts) to produce binders with outstanding mechanical strength and durability. The majority of research related to AAMs focused on the terminology of "two-part" using liquid-based activators; however, some concerns were raised regarding the applicability in large-scale in-situ construction. From here came the idea of "one-part" or "just add water" AAMs using solid-based activators, which could facilitate the production of AAMs as bagged materials with simple operation similar to OPC. The use of high range water reducing admixtures, or else termed as superplasticizers (SPs), is crucial in the OPC industry. Accordingly, several studies evaluated the suitability of commercial SPs in the two-part AAMs. The chemical working groups vary in existing SPs, including lignosulphonates, naphthalene, melamine,and polycarboxylates. According to researchers' extensive efforts to expand the use of AAMs, understanding the interactions and mechanisms of SPs in the one-part AAMs may pave the road toward such a purpose. Considering the above background, this thesis focuses on the fresh and hardened properties of the one-part AAMs along with several commercial SPs. Chapter 3 of this thesis investigates the effectiveness of several admixtures in the one-part ambient-cured alkali-activated pastes. The solid activator used was anhydrous sodium metasilicate along with three SPs, including naphthalene, melamine, and polycarboxylate, with different water/precursor ratios. The fresh properties (flowability and setting time) and compressive strength development (age of 1, 3, 7, and 28 days curing) were assessed for all AAM mixes. The chemical stability of the SPs was evaluated in the alkali medium of sodium metasilicate using the attenuated total reflectance fourier transform infrared (ATR-FTIR) test. Finally, the effect of reducing water content (w/b) using different superplasticizers on the properties of one-part AAMs was examined. In chapter 4, a similar experimental program is carried out to evaluate the same SPs in the one-part AAMs activated using Ca(OH)2/Na2SO4 powder combination. Following, the most suitable SP for the Ca(OH)2/Na2SO4 AAMs was used to lower the water content of the one-part pastes. The effect of such an approach on the porosity, microstructure, compressive strength, and reaction kinetics was studied and reported. Furthermore, a comparison between the achieved Ca(OH)2/Na2SO4 AAMs and the conventional Na2SiO3-anhydrous AAMs was carried out in terms of compressive strength, flowability,and setting time. Chapter 5 focuses on the mixing methods of AAMs along with superplasticizers. Three mixing methods were evaluated, including one-part, two-part, and a newly proposed mixing method termed as "hybrid mixing". The effects of different mixing methods on the reaction kinetic, mineralogy, fresh, hardened, and microstructural properties of the AAMs were evaluated. The mechanisms that govern the overall observations were discussed in detail. Chapter 6 deals more with the rheological behavior of the one-part AAMs. In this chapter, the effect of solid activator content on the overall rheological behavior of one-part AAMs was evaluated considering thixotropy. Further, the use of different fly ash types and contents was studied to understand their effect on the fly ash/ slag one-part AAMs. Such a chapter discusses in detail all the rheological parameters, including plastic viscosity, dynamic yield stress, static yield stress, and thixotropic index of the one-part AAMs. The particles packing parameters were also calculated and then tied with the thixotropic index aiming for a better understanding of the origin of thixotropy. The final part of this thesis presents an experimental study on the effect of delayed addition of polycarboxylate ether (PCE) on the one-part AAMs. The adsorption isotherms, reaction kinetics, and thixotropic behavior were all evaluated. The mechanisms that dominate all the observations were discussed in detail. In summary, this thesis has provided a systematic study for the overall behavior of the one-part AAMs with superplasticizers. All the above-mentioned chapters and their corresponding discussion and analyses lay a solid foundation for the practical applications of one-part AAMs in engineering practice, including repairing, strengthening, and most importantly, additive manufacturing (3D printing).