Performance of sodium carbonate/ silicate activated slag materials

Alkali-activated slag (AAS) materials are acknowledged as environmentally friendly due to the reduced embodied energy associated with their production. However, the use of highly alkaline solutions such as sodium silicate to promote the chemical reactions that lead to their hardening, poses potential human and environmental hazards that might constrain their utilization beyond specialized applications. It is possible to use less alkaline solution based on near-neutral salts as activators, such as sodium carbonate, to produce alkali-activated slag binders with desirable properties. However, to achieve this, the ‘right match’ between slag chemistry and activation conditions is required. The use of sodium carbonate presents several advantages compared with using sodium silicate when producing AAS, including reducing alkalinity to values comparable to that of Portland cement, and extending the setting time and improving workability, which facilitates the casting of these materials. Sodium carbonate-activated slag binders do not always meet the setting time and strength requirements for on-site concreting, which has limited the application of these materials. A recent study in pastes demonstrated that the addition of sodium silicate in these binders significantly improves the compressive strength development, while effectively controlling the kinetics of reaction, which makes AAS binders produced with a blend of activators an attractive candidate for producing concretes. In this study we report compressive strength, water absorption and durability properties of AAS concretes produced with a blended sodium carbonate/silicate activator. Shrinkage microcracking of these materials was also studied, by drying the specimens for 8 weeks at 65% relative humidity (RH) and 23ºC. The results obtained are compared with concretes produced solely using sodium silicate as alkali activator

Assessing the long-term structural changes of metakaolin geopolymers encapsulating strontium loaded ion-exchanger

Zeolite-type inorganic ion-exchangers are extensively used in the nuclear industry to remove fission product radionuclides from contaminated process water and in groundwater cleanup. A significant amount of ion-exchangers loaded with concentrated radioactive isotopes of Sr are generated every year, and this is a particularly pressing issue in the Fukushima Daiichi site, where minimising the environmental release of these radioisotopes is currently the focus of much work. Encapsulation of these granular radionuclide-loaded ion-exchangers, which are often stored as slurries, into a stable solid waste form (as required for disposal) with a low leaching rate of toxic ions is challenging but critical for the safety of long-term geological disposal. Metakaolin geopolymers are attracting interest in the immobilisation of nuclear wastes. However, only limited information is available from the literature regarding the stability of key ion-exchangers in geopolymer binders, and the potential modifications occurring in the binder materials as a function of interactions with the ion exchangers. In this study, an ion exchanger representing those used in the Fukushima Daiichi wastewater treatment process, loaded with inactive isotopes of Sr, was encapsulated using metakaolin-based geopolymers. Different alkali cations were used as activators and the effects of different reaction temperatures were also assessed. The phase evolution, dimensional stability, and changes in microstructure of the geopolymer binders containing Sr-loaded ion-exchanger were characterised up to 1 year, to provide important information for evaluating the partitioning of Sr between the pore solution, ion-exchangers, and the binder

Lightweight foamed geopolymer

Foamed cementitious materials are becoming more commonly used as an alternative to organic polymer foams in the insulation of buildings. Foamed geopolymers are a promising alternative to other foamed cement-based materials, potentially offering attractive performance with reduced environmental footprint in both manufacturing and operational phases of the material lifecycle. To produce a geopolymer foam derived from metakaolin with a very high strength/density ratio, flash calcined metakaolin was mixed with a sodium silicate activator solution, foamed using aluminum powder and with the addition of polyethylene glycol (PEG) as a bubble stabilising agent. After curing, the densities of the obtained materials ranged from approx. 997 kg/m3 to 1016 kg/m3, with 7-day compressive strengths of up to 14 MPa. The foamed geopolymers produced here have desirable mechanical properties and performance as a construction product, and could potentially be used as a lightweight material for walls or partitions

Effect of calcination method and clay purity on the performance of metakaolin-based geopolymers

The calcination of kaolinite clay to produce metakaolin can be achieved using a range of processes, including rotary, fluidised bed and flash calcination. Rotary calcination was the most popular of these processes for many years as it takes place in a rotary kiln, which is readily available, at easily attainable temperatures of 650 – 800 °C. However, in recent years’ flash calcination processes have become more widely used, and the technology has advanced to a point where commercial flash metakaolin-based geopolymers are now available. Flash calcination involves the rapid heating of clay at temperatures of around 1000 °C for less than a few seconds. The differences in these calcination methods can have a notable effect on the structural ordering of the metakaolin itself, as well as playing an important role in defining the chemical and physical properties of metakaolin-based geopolymers. The purity of the clay also plays a key role in the chemistry of the geopolymers produced. Calcined clay-based geopolymers can be used as construction materials or for the immobilisation of problematic wastes, among other applications, as they can offer desirable performance characteristics. The chemical and physical properties of these geopolymers, and thus the influence of the clay source on key performance parameters, will need to be fully understood when deciding how they can be used for many different applications. This study demonstrates the effect of the calcination method on the properties of calcined metakaolin geopolymer systems for waste immobilisation applications. A main focus of this study is the rheological properties, as the flow properties of these systems are one of the most important parameters for many geopolymer applications. The porosity, heat evolution and mineralogical development of these systems is also presented, with a view towards assessing performance in targeted applications for the immobilisation of nuclear waste

Impact of water content on the performance of alkali-activated slag concretes

In this study, we report the effect of varying the water/binder (w/b) ratio on the performance of sodium silicate activated concretes. Compressive strength development and water transport properties of these concretes were assessed, along with their resistance to carbonation. The results demonstrate that varying the water content within a reasonable range induced negligible changes in the compressive strengths of these concretes, when a constant paste content was used. A direct correlation between the w/b ratio and the amount of permeable voids in the concretes was not identified. The carbonation behaviour of these concretes changes prominently depending on the CO2 concentration of exposure, meaning that comparable accelerated carbonation rates were observed at varying w/b ratios, conversely to observations under natural carbonation conditions where w/b was significant in defining the carbonation rate

Effect of the activator dose on the compressive strength and accelerated carbonation resistance of alkali silicate-activated slag/metakaolin blended materials

The effects of activator dose on the properties of alkali-activated slag/metakaolin blends, were studied in fresh and hardened states: heat evolution, strength and accelerated carbonation. High activator concentrations affect the slag dissolution rate, reducing compressive strength when this is the sole precursor. An increased activator concentration favours metakaolin reaction, promoting high strengths and reduced permeability. Metakaolin addition, and increased activator concentrations reduce the susceptibility to carbonation, associated with the refinement of the pore network under extended CO2 exposure. The effect of adding an aluminosilicate precursor to an alkali-activated slag system is strongly dependent on the activator concentration

Effect of sulfides in the passive layer of steel reinforcement in alkali-activated slags

Steel reinforcing elements (rebars) embedded in Portland cement concretes are protected from corrosion by a thin passive film that is formed and maintained on rebar surfaces due to the high pH level of the surrounding concrete. Corrosion of reinforcing steel is frequently induced by its interaction with chloride ions, leading to local destruction of the passive layer. The nature and stability of this layer change when the surrounding concrete is produced with different cementitious materials, as the permeability of the matrix as well as the chemistry of pore solution can vary significantly. This is particularly the case when using non-Portland cements, such as alkali-activated slags (AAS). Ground granulated blast furnace slag can contains sulfur at levels between 1-2 wt.%, mostly in a reduced state. The sulfide ions are released during the alkali-activation of vitreous slag into the alkaline aqueous solution, and subsequently can alter the nature of the passive film formed on a steel surface. In this study the influence of sulfide on the stability, chemical composition and morphology of the passive layer forming in steel embedded in alkali-activated slags mortars, and in simulated alkali-activated slag pore solutions were investigated. The potential influence of sulfide in corrosion induced in the presence of chlorides was also assessed. This was carried out by combining electrochemical measurement with a detailed inspection of the rebars specimens using different analytical techniques. The outcomes of this study revealed that in absence of sulfides, corrosion initiation is governed by localised breakdown of the passive film, followed by metastable/stable pit growth. However, in sulfide containing pore solutions localised pitting induced by chlorides was not identified. The presence of sulfides in these systems alters the mechanism of corrosion initiation, and its influence is strongly dependent on sulfides concentration at the steel/solution interface

Thermodynamic modelling of phase evolution in alkali-activated slag cements exposed to carbon dioxide

Carbonation of cementitious materials induced by their interaction with atmospheric CO2 is one of the main degradation mechanisms threatening their durability. In this study, a novel thermodynamic model to predict the phase evolution of alkali-activated slags exposed to an accelerated carbonation environment is presented. This model predicts the phase assemblages of carbonated alkali-activated slag cements, as a function of CO2 uptake under 1 v/v % CO2 conditions, considering the bulk slag chemistry and activators used. The changes taking place during the carbonation process regarding the physicochemical properties of the main binding gel, an alkali calcium aluminosilicate hydrate (C-(N)-A-S-H), the secondary reaction products CaAl and MgAl layered double hydroxides, and amorphous aluminosilicate gels, were simulated and discussed. The predictions of the thermodynamic model are in good agreement with experimental data retrieved from the literature, demonstrating that this is a valuable tool for predicting long-term performance of alkali-activated slag cements

Structure of Portland Cement Pastes Blended with Sonicated Silica Fume

[EN] Application of power ultrasound to enhance dispersion of commercial densified silica fume leads to increased compressive strengths and refinement of the pore structure in mortars, compared with those that are untreated. This was attributed to the enhanced pozzolanic reactivity achieved by particle dispersion through sonication, leading to higher consumption of portlandite during curing, and formation of a calcium silicate hydrate gel with a higher degree of cross-linking than is identified in specimens with densified silica fume. This suggests that with the use of sonicated silica fume, it is possible to reduce the required quantity of admixture in blended cements to achieve specified performance, with the additional advantage of the formation of a highly densified structure and refined pore network, contributing to potential improvements in durability.This study was sponsored by the Ministerio de Ciencia e Innovacion of Spain (Project SILISONIC BIA-2007-63252 and research scholarships BES-2008-002440 and EEBB-2011-43847), the European regional development fund (FEDER), and the Universitat Politecnica de Valencia (Spain). Participation of SAB and JLP was funded by the Australian Research Council (ARC), including partial funding through the Particulate Fluids Processing Centre, a Special Research Centre of the ARC. The authors thank Dr. John Gehman for support in conducting the NMR experiments at the Bio21 Institute, University of Melbourne, Australia.Rodriguez Martinez, ED.; Bernal, SA.; Provis, JL.; Paya Bernabeu, JJ.; Monzó Balbuena, JM.; Borrachero Rosado, MV. (2012). Structure of Portland Cement Pastes Blended with Sonicated Silica Fume. Journal of Materials in Civil Engineering. 24:1295-1304. https://doi.org/10.1061/(ASCE)MT.1943-5533.0000502S129513042

Symbolic powers of monomial ideals and Cohen-Macaulay vertex-weighted digraphs

In this paper we study irreducible representations and symbolic Rees algebras of monomial ideals. Then we examine edge ideals associated to vertex-weighted oriented graphs. These are digraphs having no oriented cycles of length two with weights on the vertices. For a monomial ideal with no embedded primes we classify the normality of its symbolic Rees algebra in terms of its primary components. If the primary components of a monomial ideal are normal, we present a simple procedure to compute its symbolic Rees algebra using Hilbert bases, and give necessary and sufficient conditions for the equality between its ordinary and symbolic powers. We give an effective characterization of the Cohen--Macaulay vertex-weighted oriented forests. For edge ideals of transitive weighted oriented graphs we show that Alexander duality holds. It is shown that edge ideals of weighted acyclic tournaments are Cohen--Macaulay and satisfy Alexander dualityComment: Special volume dedicated to Professor Antonio Campillo, Springer, to appea