The utilization of these materials has relied primarily on the dual benefits of reduced presence in landfills and cost. These have also proven to yield a higher service life as opposed to conventional ordinary portland cement (OPC) concrete due to an enhanced microstructure. The use of such materials however has not been readily acceptable due to detrimental early age behavior. The influence of design factors is studied to understand the reaction mechanism. Silicon polymerization at the molecular level is studied to understand the aluminosilicate interactions which are responsible for prevention of any leaching of ions. A comparative study between fly ash and slag binders is carried out to evaluate the stable states of sodium, aluminum and silicon in both these binders, since the likelihood of the sodium ions leaching out is high.
Compressive and flexural strength have been reported in previous literature, but the impact of crack resistance was unevaluated from an approach of characterizing the fracture process zone. Alternative routes of activation are explored with an intent to reduce the high alkalinity by use of neutral salts such as sodium sulfate. High volume OPC replacement by both class C and F fly ash is performed to evaluate the differences in hydration phase formation responsible for its pore refinement and strength. Spectroscopic studies have also allowed to study the fundamental material structure. Durability studies are also performed on these samples to understand the probability external sulfate attacks as opposed to OPC mixes.
Alkali-activated slag concretes are being extensively researched because of its potential sustainability-related benefits. For such concretes to be implemented in large scale concrete applications such as infrastructural and building elements, it is essential to understand its early and long-term performance characteristics vis-à-vis conventional ordinary portland cement (OPC) based concretes. This paper presents a comprehensive study of the property and performance features including early-age isothermal calorimetric response, compressive strength development with time, microstructural features such as the pore volume and representative pore size, and accelerated chloride transport resistance of OPC and alkali-activated binder systems. Slag mixtures activated using sodium silicate solution (SiO2-to-Na2O ratio or Ms of 1–2) to provide a total alkalinity of 0.05 (Na2O-to-binder ratio) are compared with OPC mixtures with and without partial cement replacement with Class F fly ash (20 % by mass) or silica fume (6 % by mass). Major similarities are noted between these binder systems for: (1) calorimetric response with respect to the presence of features even though the locations and peaks vary based on Ms, (2) compressive strength and its development, (3) total porosity and pore size, and (4) rapid chloride permeability and non-steady state migration coefficients. Moreover, electrical impedance based circuit models are used to bring out the microstructural features (resistance of the connected pores, and capacitances of the solid phase and pore-solid interface) that are similar in conventional OPC and alkali-activated slag concretes. This study thus demonstrates that performance-equivalent alkali-activated slag systems that are more sustainable from energy and environmental standpoints can be proportioned.
This paper reports the influence of activator type and concentration on the rheological properties of alkali-activated fly ash suspensions. A thorough investigation of the rheological influences (yield stress and plastic viscosity) of several activator parameters, including: (i) the cation type and concentration of alkali hydroxide and (ii) the alkali-to-binder ratio (n) and silica modulus (Ms), and (iii) the volume of the activation solution, on the suspension rheology is presented. The results indicate a strong dependence on the cation and its concentration in the activation solution. The viscosity of the activation solution and the volumetric solution-to-powder ratio are shown to most strongly influence the plastic viscosity of the suspension. The suspension yield stress is predominantly influenced by the changes in fly ash particle surface charge and the ionic species in the activator. A shift from non-Newtonian to Newtonian flow behavior is noted in the case of silicate-based suspensions for Ms ≤ 1.5. This behavior, which is not observed at higher MS values, or when the fly ash is dispersed in hydroxide solutions or pure water, is hypothesized to be caused by colloidal siliceous species present in this system, or surface charge effects on the fly ash particles. Comparisons of the rheological response of alkali-activated suspensions to that of portland cement-water suspensions are also reported.