Many standardised durability testing methods have been developed for Portland cement-based concretes, but require validation to determine whether they are also applicable to alkali-activated materials. To address this question, RILEM TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ carried out round robin testing of carbonation and chloride penetration test methods, applied to five different alkali-activated concretes based on fly ash, blast furnace slag or metakaolin. The methods appeared overall to demonstrate an intrinsic precision comparable to their precision when applied to conventional concretes. The ranking of test outcomes for pairs of concretes of similar binder chemistry was satisfactory, but rankings were not always reliable when comparing alkali-activated concretes based on different precursors. Accelerated carbonation testing gave similar results for fly ash-based and blast furnace slag-based alkali-activated concretes, whereas natural carbonation testing did not. Carbonation of concrete specimens was observed to have occurred already during curing, which has implications for extrapolation of carbonation testing results to longer service life periods. Accelerated chloride penetration testing according to NT BUILD 443 ranked the tested concretes consistently, while this was not the case for the rapid chloride migration test. Both of these chloride penetration testing methods exhibited comparatively low precision when applied to blast furnace slag-based concretes which are more resistant to chloride ingress than the other materials tested.@en

Alkali-activated materials (AAMs) have high potential as alternative binder to ordinary portland cement (OPC), because of their high performance beside lower CO2 emissions. While there is a general consensus about their strength advantages over OPC, there is a widespread debate regarding their durability. Some groups believe that the availability of wide scientific/technical background, together with the already-known OPC durability problems, are sufficient for their commercialization; however, others consider the durability of AAMs to be an unproven issue. This controversy represents one of the limitations facing their bulk applications. The present work provides an overview of the latest developments on durability of fly ash/slag-based AAMs with the aim to update recent findings regarding their behavior under aggressive conditions (sulfates, freeze-thaw, chloride, carbonation, acid, efflorescence). This review will provide a better understanding of the durability issues of AAMs, which will stimulate further research to develop the appropriate testing methods and help to promote their commercialization.@en

Rhombohedral NASICON compounds with general formula Li1+xTi2-xScx(PO4)3 (0 ≤ x ≤ 0.5) have been prepared using a conventional solid-state reaction and characterized by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and impedance spectroscopy. The partial substitution of Ti4+ by Sc3+ and Li+ in pristine LiTi2(PO4)3 increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and6Li/7Li,31P, and45Sc MAS NMR spectra confirms the presence of secondary LiScO2 and LiScP2O7 phases that reduce the amount of lithium incorporated in the NASICON phase. In samples with x <0.3, electrostatic repulsions between Li ions located at M1 and M3 sites increase Li mobility. For x ≥ 0.3, ionic conductivity decreases because of secondary nonconducting phases formed at grain boundaries of the NASICON particles (core-shell structures). For x = 0.2, high bulk conductivity (2.5 × 10-3 S·cm-1) and low activation energy (Ea = 0.25 eV) measured at room temperature make Li1.2Ti1.8Sc0.2(PO4)3 one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.@en

Rhombohedral NASICON compounds with general formula Li1+xTi2-xScx(PO4)3 (0 ≤ x ≤ 0.5) have been prepared using a conventional solid-state reaction and characterized by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and impedance spectroscopy. The partial substitution of Ti4+ by Sc3+ and Li+ in pristine LiTi2(PO4)3 increases unit-cell dimensions and the number of charge carriers. In Sc-rich samples, the analysis of XRD data and6Li/7Li,31P, and45Sc MAS NMR spectra confirms the presence of secondary LiScO2 and LiScP2O7 phases that reduce the amount of lithium incorporated in the NASICON phase. In samples with x <0.3, electrostatic repulsions between Li ions located at M1 and M3 sites increase Li mobility. For x ≥ 0.3, ionic conductivity decreases because of secondary nonconducting phases formed at grain boundaries of the NASICON particles (core-shell structures). For x = 0.2, high bulk conductivity (2.5 × 10-3 S·cm-1) and low activation energy (Ea = 0.25 eV) measured at room temperature make Li1.2Ti1.8Sc0.2(PO4)3 one of the best lithium ionic conductors reported in the literature. In this material, the vacancy arrangement enhances Li conductivity.@en

The feasibility of a waste glass powder residue (GP) from glass recycling as partial mineral precursor to produce alkali-activated materials is investigated. GP served as powder coal fly ash (PCFA) replacement within a reference system composed of 50% PCFA and 50% ground granulated blast furnace slag (GGBS). Compared with PCFA, GP was better involved in the alkali activation process by having a higher silica and Ca dissolution. Furthermore, increasing GP replacement up to 30% prolonged the induction period, facilitated the gel formation and yielded a 35% higher 28-day compressive strength. These observations are similar to the effect of using both sodium hydroxide and sodium silicate as alkali activator in alkali-activated slag/fly ash systems. A higher polymerization of the gel network was also observed. Microstructure analysis indicated that the main reaction product is a calcium silicate hydrate type gel substituted with Al and Na (C-(N)-A-S-H type gel). This work largely contributes to the understanding of the reactivity and potential of GP and promotes its practical utilization as a mineral precursor in the production of alkaline cements.@en

A newly developed thermodynamic approach was introduced from the literature to investigate the influence of NaOH content on the hydrate assemblage and chemistry of aqueous solution of activated slag paste (Na2O/slag=4%, 6% and 8%). The ICP-OES test was performed to obtain the aqueous species profiles such as Si, Ca, Al and Na etc. The elemental concentrations in aqueous solution show a similar trend between thermodynamic calculations and experimental results. The modeling results showed that the influence of NaOH content on the uptake of Na into the primary reaction products and chemical shrinkage depends on slag reaction extent and NaOH content. The NaOH content does not show an obvious influence on the porosity development. The volume fraction of primary products in total products is between 40% and 60%, and particularly increases slightly after a certain degree of reaction for activated slag with Na2O/slag=4% and 6%. The volume ratio between total products and reacted slag is around 1.5~1.7, smaller than the value from 2.06 to 2.2 reported for Portland cement.@en

The performances of alkali activated materials (AAM) are strongly related to their microstructures. The microstructure development is mainly affected by chemistry of the primary raw materials. In the present study the influence of different proportions of fly ash and slag (100:0, 70:30, 50:50, 30:70, 0:100 wt.%, named S0, S30, S50, S70, S100, respectively) on the microstructure and mineralogy of the alkali activated fly ash and slag based pastes has been investigated through XRD, TGA/DTGA, ESEM/EDX techniques. XRD has been used for the qualitative and quantitative analysis of different reaction products. The Rietveld refinement method and TGA/DTGA analysis have demonstrated that the increase of slag content increased the amount of amorphous phase associated with gel formation. From ESEM results, a more refined and denser matrix has been deduced in slag rich mixtures. These results are in agreement with higher strength values observed for S70 and S100 mixtures (~100 MPa at 28d and ~120 MPa after 90d curing). Depending on the fly ash to slag ratio, two types of gel have been distinguished by EDX analysis, a N-A-S-H gel in the fly ash rich mixtures and a C-A-S-H gel in the slag rich pastes.@en

Alkali-activated materials (AAM) have received increasing interest during the past few decades due to their environmental and technological advantages. However the durability of these binders is still considered as an unproven issue that needs to be addressed before their commercial adoption and large-scale production. The present work is focussed on the investigation of durability performances of concretes manufactured through the alkaline activation of fly ash and slag blends using a low-concentrated activating solution (formulated by blending commercial sodium silicate and sodium hydroxide). The aim of this study is to provide new insights on durability properties of AAM and to assess their resistance under severe conditions. For that purpose, concrete specimens have been exposed to accelerated carbonation (varying the curing and the exposure times from 7 to 28 days). After 28 days curing, their chloride resistance has been assessed using the non-steady-state chloride migration experiments following the NordTest method (NT Built 492). Prior to durability test, the compressive strength of investigated mixtures was determined at early ages (1, 7 days) and after 28, 56, 90 days curing. The slag-rich mixtures have shown high compressive strength values (~45MPa after 1 day and ~80MPa after 28 days curing). Excellent mechanical properties have been also found for the fly ash-rich mixtures at early and late curing ages (15MPa after only 1 day and ~50MPa after 28 days curing). From carbonation test, it has been found that the carbonation depth increases as increasing fly ash/slag ratio or the exposure time. However when curing time is increased, a decrease on carbonation depth was observed. Slag-rich concretes were practically not carbonated after 7, 14 days exposure and showed 3-7mm carbonation depth after 28 days exposure. A higher carbonation depth (~18mm) was observed in fly ash-rich mixture. When the concrete specimens were exposed to chloride ingress, a high chloride permeability associated with higher concrete porosity was found for fly ash-rich mixtures with a chloride penetration depth of 18mm and chloride migration coefficient near 15-17×10-12m2s-1. These parameters decreased considerably as increasing slag content reaching values near 5mm and 2×10-12m2s-1, respectively. @en

The NASICON series, with formula Bax/2Li1-xTi2(PO4)3 (0.4 ≤ x ≤ 1), has been prepared by solid-state reaction and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman, nuclear magnetic resonance (NMR) and impedance spectroscopy (IS). XRD patterns of samples indicated the formation of single phases with rhombohedral structure (space group R-3c). The Rietveld analysis of XRD patterns was performed to deduce location of Li and Ba ions. FTIR, Raman, and NMR techniques showed the only presence of isolated PO4 groups in analyzed phosphates. 31P MAS-NMR spectra were used to investigate Li and Ba distribution and 7Li MAS-NMR spectra to discriminated Li ions with different mobility in conduction paths. A maximum total conductivity of 2.5 × 10−7 S cm−1 and a minimum activation energy of 0.47 eV were obtained at room temperature for Ba0.3Li0.4Ti2(PO4)3 (x = 0.6).@en

The NASICON series, with formula Bax/2Li1-xTi2(PO4)3 (0.4 ≤ x ≤ 1), has been prepared by solid-state reaction and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman, nuclear magnetic resonance (NMR) and impedance spectroscopy (IS). XRD patterns of samples indicated the formation of single phases with rhombohedral structure (space group R-3c). The Rietveld analysis of XRD patterns was performed to deduce location of Li and Ba ions. FTIR, Raman, and NMR techniques showed the only presence of isolated PO4 groups in analyzed phosphates. 31P MAS-NMR spectra were used to investigate Li and Ba distribution and 7Li MAS-NMR spectra to discriminated Li ions with different mobility in conduction paths. A maximum total conductivity of 2.5 × 10−7 S cm−1 and a minimum activation energy of 0.47 eV were obtained at room temperature for Ba0.3Li0.4Ti2(PO4)3 (x = 0.6).@en

The carbonation-induced degradation in alkali-activated materials (AAMs) is unknown. This is due to the current level of understanding of the carbonation mechanism in accelerated conditions, whereas the results from the long-term performance of AAMs in service are not available. In this paper, the natural laboratory carbonation of alkali-activated fly ash (FA) and blast furnace slag (BFS)-based pastes was studied. The aim of experiments was to investigate the influence of the binder composition on the carbonation mechanism. The microscope techniques, i.e. polarization, fluorescent and Environmental Scanning Electron Microscope (ESEM) enabled the identification of the carbonation front and local defects due to carbonation, such as microcracks, and pore structure changes. The pH of the simulated pore solution in carbonated and noncarbonated samples was analyzed. The pore structures after carbonation were characterized in terms of capillary and gel pores by Mercury Intrusion Porosimetry (MIP) and nitrogen sorption. The effect of the change in the porosity on the mechanical properties was examined by Nanoindentation tests. Results show that the natural carbonation did not reduce the alkalinity of the pore solution to below the pH 9. The pH was kept above 10.5 in all the mixtures. The samples with 50 wt% and less BFS content, were uniformly carbonated. The increase of the total porosity in those samples has been attributed to the simultaneous effect of the carbonation and drying shrinkage of the pastes. The change in the porosity for a consequence has a reduction in the modulus of elasticity. The samples containing more than 50 wt% of BFS were fully resistant to carbonation or carbonation was induced along the cracks as observed from the microscopic analysis.@en

This study extracted pore solution from the hardened blast furnace slag pastes activated with sodium silicate and/or sodium hydroxide through the steel-die method and analyzed the aqueous phase composition via ICP-OES technique. The results show that the concentrations of Si, Al, Na and OH" decrease with progress of slag reaction and increase with increase of Na20, while the concentration of Ca shows no substantial change. The concentrations of Na, Al and Ca are positively correlated to the concentration of OH" in a quasi-linear relationship, while the concentration of Si has a great dependence on the initial concentration of 0H'. Two regions were identified in the plots between the concentrations of Si and Ca, by which two kinds of C-S-H gels are expected. The Al/Si is linearly correlated to Na/Si for both sodium hydroxide and sodium silicate activated systems. @en

This paper presents the characterization of two biomass ashes and investigates their feasibility to be utilized as binder precursor in alkali-activated materials. The investigated Biomass ash (BA1 and BA2), which has CO32- and SO42- containing phases, have a high amount of Ca but a relatively low SiO2 and Al2O3 content with respect to class F fly ash. Both of them shows adequate reactivity for alkaline activation, although the reactive amount of Si and Al is limited. A promising route for the valorisation of BA is using it as a replacement of slag or fly ash for the production of alkaline cements. In this study, two biomass ashes have been used as replacement for fly ash inside a reference alkali-activated system containing 50 wt.% fly ash/50 wt.% blast furnace slag as binder precursor. Mortar samples prepared with original BA revealed severe expansions due to the existence of metal Al phase within the ashes. Minor Al content in both BA were determined by the gas release potential tests, suggesting elimination of metallic phases in both BA is necessary. For this purpose, both ashes were pre-treated using low concentration NaOH solution in an earth moist state before sample preparation, which has been proved to be highly effective to eliminate expansions producing around 77 % increase in compressive strength compared with sample prepared with original BA ashes. This study largely promotes the understanding on the physical and chemical properties of biomass ash and provides a potential solution to this kind of waste material. In addition, it contributes to a theoretical basis for mixt design in BA as binder precursor in geopolymer concrete. @en

This paper presents the characterization of two biomass ashes and investigates their feasibility to be utilized as binder precursor in alkali-activated materials. The investigated Biomass ash (BA1 and BA2), which has CO32- and SO42- containing phases, have a high amount of Ca but a relatively low SiO2 and Al2O3 content with respect to class F fly ash. Both of them shows adequate reactivity for alkaline activation, although the reactive amount of Si and Al is limited. A promising route for the valorisation of BA is using it as a replacement of slag or fly ash for the production of alkaline cements. In this study, two biomass ashes have been used as replacement for fly ash inside a reference alkali-activated system containing 50 wt.% fly ash/50 wt.% blast furnace slag as binder precursor. Mortar samples prepared with original BA revealed severe expansions due to the existence of metal Al phase within the ashes. Minor Al content in both BA were determined by the gas release potential tests, suggesting elimination of metallic phases in both BA is necessary. For this purpose, both ashes were pre-treated using low concentration NaOH solution in an earth moist state before sample preparation, which has been proved to be highly effective to eliminate expansions producing around 77 % increase in compressive strength compared with sample prepared with original BA ashes. This study largely promotes the understanding on the physical and chemical properties of biomass ash and provides a potential solution to this kind of waste material. In addition, it contributes to a theoretical basis for mixt design in BA as binder precursor in geopolymer concrete. @en

This paper presents the characterization of two biomass ashes and investigates their feasibility to be utilized as binder precursor in alkali-activated materials. The investigated Biomass ash (BA1 and BA2), which has CO32- and SO42- containing phases, have a high amount of Ca but a relatively low SiO2 and Al2O3 content with respect to class F fly ash. Both of them shows adequate reactivity for alkaline activation, although the reactive amount of Si and Al is limited. A promising route for the valorisation of BA is using it as a replacement of slag or fly ash for the production of alkaline cements. In this study, two biomass ashes have been used as replacement for fly ash inside a reference alkali-activated system containing 50 wt.% fly ash/50 wt.% blast furnace slag as binder precursor. Mortar samples prepared with original BA revealed severe expansions due to the existence of metal Al phase within the ashes. Minor Al content in both BA were determined by the gas release potential tests, suggesting elimination of metallic phases in both BA is necessary. For this purpose, both ashes were pre-treated using low concentration NaOH solution in an earth moist state before sample preparation, which has been proved to be highly effective to eliminate expansions producing around 77 % increase in compressive strength compared with sample prepared with original BA ashes. This study largely promotes the understanding on the physical and chemical properties of biomass ash and provides a potential solution to this kind of waste material. In addition, it contributes to a theoretical basis for mixt design in BA as binder precursor in geopolymer concrete. @en

This paper presents the characterization of two biomass ashes and investigates their feasibility to be utilized as binder precursor in alkali-activated materials. The investigated Biomass ash (BA1 and BA2), which has CO32- and SO42- containing phases, have a high amount of Ca but a relatively low SiO2 and Al2O3 content with respect to class F fly ash. Both of them shows adequate reactivity for alkaline activation, although the reactive amount of Si and Al is limited. A promising route for the valorisation of BA is using it as a replacement of slag or fly ash for the production of alkaline cements. In this study, two biomass ashes have been used as replacement for fly ash inside a reference alkali-activated system containing 50 wt.% fly ash/50 wt.% blast furnace slag as binder precursor. Mortar samples prepared with original BA revealed severe expansions due to the existence of metal Al phase within the ashes. Minor Al content in both BA were determined by the gas release potential tests, suggesting elimination of metallic phases in both BA is necessary. For this purpose, both ashes were pre-treated using low concentration NaOH solution in an earth moist state before sample preparation, which has been proved to be highly effective to eliminate expansions producing around 77 % increase in compressive strength compared with sample prepared with original BA ashes. This study largely promotes the understanding on the physical and chemical properties of biomass ash and provides a potential solution to this kind of waste material. In addition, it contributes to a theoretical basis for mixt design in BA as binder precursor in geopolymer concrete. @en

The technology of alkaline activated materials (AAMs) has become a sustainable alternative to cement production with respect to CO2 emission and energy consumption. This technology is based on by-products usage from industrial energy and manufacturing processes, such as fly ash or blast furnace slag. The long-term performance of AAMs is decisive for their structural application. Generally, physical properties, such as porosity, pore size distribution, internal pore surface and connectivity determine material long-term behavior. Characterization of these properties in activated fly ash, slag and their blends was performed by Mercury intrusion porosimetry (MIP), Nitrogen adsorption and ESEM. Since the microstructure is greatly influenced by dissolution rate of fly ash and slag in selected activator, the pore solution was characterized by inductively coupled optical emission spectrometry (ICP-OES). Mixtures with the following fly ash and slag content (100:0, 70:30, 50:50, 30:70, 0:100 wt.%) have been prepared using sodium hydroxide/sodium silicate solutions. The increase of slag amount produces highly dense and refined microstructure, where only gel pores were identified and measured by Nitrogen adsorption. ICP-OES analysis has shown high concentrations of sodium and sulfur in solutions of slag rich mixtures, while Al, Si, Ca were almost completely absent from solution suggesting their total incorporation into reaction products of pastes. @en

Carbonation of the pore solution in alkali-activated materials (AAMs) produces alkali and/or alkali-earth carbonates. When the carbonate solubility in the water is very high (case of the most alkali carbonates), it is very hard to determine the carbonation depth in AAMs with the phenolphthalein indicator frequently used in Ordinary Portland Cement (OPC)-based materials. Carbonation gradually decreases the alkalinity of the pore solution, while the color after spraying phenolphthalein changes from colorless to pink when pH< 13 and changes back to colorless when pH< 8.2. The color change with phenolphthalein indicator may still exist in the less alkaline areas where carbonation may have already occurred. Therefore, using the indicator test is likely to underestimate the depth to which carbonation reaction has occurred in AAMs and more complete assessment is required. This study investigates the carbonation front in alkali-activated fly ash (FA) and blast furnace slag (BFS) pastes in natural laboratory conditions. Monitoring carbonation front in the samples after one year of exposure has been carried out under polarized light microscope (PLM), and environmental scanning electron microscope (ESEM). The carbonation products were sharply distinguished from the other constituents of the paste, by their crystallographic and optical characteristics under PLM, and characterized by X-Ray diffraction (XRD). @en