Alkali-activated Slag Systems Research Articles

Physico-chemical, mechanical, microstructure and durability characteristics of alkali activated Egyptian slag

The aim of the present work is to study the characteristics of alkali activated Egyptian slag (GBFS) mixed with different contents of Na2O and SiO2. The rate of the kinetic of activation of GBFS has been studied by electrical conductivity, FTIR, DTA/TGA, XRD and SEM techniques. Electrical conductivity of alkali activated slag systems depends mainly on the binder composition, the activator type and its concentration. The increase in alkali activator contents affected the location and intensities of the conductivity maximum. As the hydration time increases the amount of hydration products increase, hence the chemically combined water and combined slag contents increase. SEM images showed the presence of (N,C)ASH gel with low porosity. With the increase of the content of Na2O leads to form a denser closed microstructure, leading to higher compressive strength values. The activated GBFS showed good durability in 5% MgSO4 or 5% MgCl2 solution, i.e., the compressive strength increased gradually with immersing time up to 180days. The total chloride and total sulphate contents decrease with alkali activated GBFS due to the forming hydrated products that fill some available open pores, thereby inhibiting Cl− or SO42− ions penetration; this effect leads to a decreased accessibility of Cl− or SO42− ions towards the more dense with low capillary pore structure. It can be concluded that alkali activated GBFS are more durable in 5% MgSO4 or 5% MgCl2 than OPC pastes.

Comparative Analysis of the Influence of Sodium and Potassium Silicate Solutions on the Kinetics and Products of Slag Activation

Abstract 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.

Gel nanostructure in alkali-activated binders based on slag and fly ash, and effects of accelerated carbonation

Binders formed through alkali-activation of slags and fly ashes, including ‘fly ash geopolymers’, provide appealing properties as binders for low-emissions concrete production. However, the changes in pH and pore solution chemistry induced during accelerated carbonation testing provide unrealistically low predictions of in-service carbonation resistance. The aluminosilicate gel remaining in an alkali-activated slag system after accelerated carbonation is highly polymerised, consistent with a decalcification mechanism, while fly ash-based binders mainly carbonate through precipitation of alkali salts (bicarbonates at elevated CO2 concentrations, or carbonates under natural exposure) from the pore solution, with little change in the binder gel identifiable by nuclear magnetic resonance spectroscopy. In activated fly ash/slag blends, two distinct gels (C–A–S–H and N–A–S–H) are formed; under accelerated carbonation, the N–A–S–H gel behaves comparably to fly ash-based systems, while the C–A–S–H gel is decalcified similarly to alkali-activated slag. This provides new scope for durability optimisation, and for developing appropriate testing methodologies.

Hydration of alkali-activated slag: thermodynamic modelling

The composition of solid and liquid phases of alkali-activated slag systems was investigated by thermodynamic modelling. The calculations predict the formation of C(–A)–S–H, stratlingite and hydrotalcite in different alkali-activated slag systems, which agrees well with experimental findings. In addition traces of FeS and ettringite are predicted to precipitate. The type of alkaline activator (Na-silicate, NaOH) has no influence on the type of hydrates formed but influences the Ca/Si ratio of the C–S–H. The addition of sodium silicate leads to the formation of C–S–H with a higher Ca/Si ratio (1.1–1.2), whereas NaOH results in a Ca/Si ratio of 0.8to1.1. In contrast, the kind and concentration of the activator has strong influence on the composition of the pore solution. Low concentrations of the alkaline activator result in lower Na, Si and Al concentrations and lower pH values. In all pore solutions a high concentration of sulphide (HS–) is expected to be present.

Sodium Sulphate Activated GGBS/PFA and Its Potential as a Nuclear Waste Immobilisation Matrix

ABSTRACTThe UK currently uses Portland cement composite blends to immobilise/encapsulate intermediate and low level radioactive wastes (ILW and LLW). However, among other things, the high pH of these systems causes the corrosion of some metals, which can lead to expansion and excess generation of hydrogen. Therefore, in order to immobilise nuclear waste where corrosion is an issue, a near neutral cementing system is desirable.Among the activators which can be used in the alkali-activated slag (AAS) systems, a solution of Na2SO4 is near neutral but the ground granulated blast-furnace slag (GGBS) itself has a pH about 11, increasing the pH within the Na2SO4 activated AAS. As low calcium Pulverized Fuel Ash (PFA) only has a pH about 9, using a GGBS/PFA blend activated by Na2SO4 offers the potential to develop a near neutral cementing system for nuclear waste immobilisation purposes.In this paper, the replacement of GGBS in the Na2SO4 activated AAS system with PFA at 0%, 10%, 20% and 30% by mass was examined. The pH and corrosion of Al were determined and used as primary criteria for judging the feasibility for further development of a Na2SO4 activated GGBS/PFA matrix for immobilising nuclear wastes. The microstructure of the matrices was studied by SEM. Leaching studies were carried out to examine the possibility of immobilising Cs+ within these Na2SO4 activated GGBS/PFA matrices. The potential of using Na2SO4 activated GGBS/PFA for immobilising nuclear wastes is discussed.