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Time-resolved in-situ synchrotron X-ray microtomography reveals new levels of detail about the chemical and physical processes that take place as Portland cement hardens. The conversion of a fluid paste into a hardened product can be monitored on a sub-minute time-scale, and with sample movement/settlement corrections applied to enable individual particles to be tracked as they react, hydrate, and become interconnected into a single strong monolith. The growth of the strength-giving hydrate phases surrounding cement grains, and of the fluid-filled pore network that surrounds them, is able to be directly viewed at the level of individual cement particles through the application of this tracking protocol. When cement is brought into contact with water, a layer which differs in density from the bulk of the cement grains becomes observable on the grain surfaces during the induction period (during which time the heat evolution from the paste is relatively low). As hydration continues, reaction products grow both from particle surfaces into the initially fluid-filled region, and also into the space originally occupied by the cement particles, forming a density gradient within the microstructure. As the reaction accelerates and larger volumes of solid phases precipitate, the newly-formed solid structure percolates via interconnection of agglomerated low-density outer hydrates, which then densify as hydration continues. This eventually leads to solidification of the structure into a hardened porous matrix.

This paper assesses the use and valorisation of two industrial wastes generated at a large scale, which are currently disposed in landfills, as raw materials to produce geopolymers. Specifically, a kaolinitic sludge from the mining industry (CKS), and bottom ash (BA) generated during coal combustion in a thermal power station, were used as aluminosilicate precursors in geopolymer synthesis. The geopolymers were synthesised at 50°C, with a sodium oxide/aluminium oxide (Na2O/Al2O3) molar ratio of 1.0, and different silica/aluminium oxide (SiO2/Al2O3) molar ratios adjusted by manipulating the content of the soluble silicate solution used as the activator. The mechanical strength and reaction products formed during the geopolymerisation process were assessed up to 90 days of curing. The use of CKS as the main component of the precursor blend provides a geopolymer with better mechanical properties due to its higher reactivity than BA. The content of soluble silicates in the alkali activator plays an important role during geopolymerisation, improving the mechanical properties due to the formation of a more reticulated and dense structure. The mortars show a compressive strength higher than 55 MPa after 28 days and low water absorption by capillarity. This elucidates the feasibility of valorising these industrial residues as precursors for geopolymer cements.

Alkali-activated materials are promising low-carbon alternatives to Portland cement; however, there remains an absence of a fundamental understanding of the effect of different activator types on their reaction products at the atomic scale. Solid-state 27Al and 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and 1H–29Si cross-polarization MAS NMR spectroscopy are used to reveal the effect of the activator anion on the nanostructure, cross-linking, and local hydration of aged alkali-activated slag cements. The main reaction product identified is a mixed cross-linked/non-cross-linked sodium-substituted calcium aluminosilicate hydrate (C–(N)–A–S–H) gel with a structure comparable to tobermorite 11 Å. Analysis of cross-polarization kinetics revealed that a higher content of soluble silicate in the activator promoted the incorporation of Al into the aluminosilicate chains of C–(N)–A–S–H gels, charge-balanced preferentially by protons within the gel interlayer. In sodium carbonate-activated slag cements, aluminosilicate chains of C–(N)–A–S–H gels are instead charge-balanced preferentially by Ca2+ or AlV ions. Hydrotalcite was observed as a secondary reaction product independent of the activator used and in higher quantities as the content of sodium carbonate in the activator increases. The presence of soluble silicates in the activator promotes the formation of an Al-rich sodium aluminosilicate hydrate (N–A–S–H) gel which was not identified when using sodium carbonate as the activator. These results demonstrate that the anion type in the activator promotes significant differences in the nanostructure and local hydration of the main binding phases forming in alkali-activated slag cements. This explains the significant differences in properties identified when using these different activators.

In this study, a multi-technique approach was utilised to determine the high temperature performance of magnesium potassium phosphate cement (MKPC) blended with fly ash (FA) or ground granulated blast furnace slag (GBFS) with respect to nuclear waste immobilisation applications. Conceptual fire conditions were employed (up to 1200 °C, 30 min) to simulate scenarios that could occur during interim storage, transportation or within a final geological disposal facility. After exposure up to 400 °C, the main crystalline phase, struvite-K (MgKPO4·6H2O), was dehydrated to poorly crystalline MgKPO4 (with corresponding volumetric and mass changes), with MgKPO4 recrystallisation achieved by 800 °C. XRD and SEM/EDX analysis revealed reaction occurred between the MgKPO4 and FA/GBFS components after exposure to 1000–1200 °C, with the formation of potassium aluminosilicate phases, leucite and kalsilite (KAlSi2O6 and KAlSiO4), commensurate with a reduced relative intensity (or complete elimination) of the dehydrated struvite-K phase, MgKPO4. This was further supported by solid-state NMR (27Al and 29Si MAS), where only residual features associated with the raw FA/GBFS components were observable at 1200 °C. The high temperature phase transformation of blended MKPC binders resulted in the development of a glass/ceramic matrix with all existing porosity infilled via sintering and the formation of a vitreous phase, whilst the physical integrity was retained (no cracking or spalling). This study demonstrates that, based on small-scaled specimens, blended MKPC binders should perform satisfactorily under fire performance parameters relevant to the operation of a geological disposal facility, up to at least 1200 °C.

Struvite-K (MgKPO4·6H2O) is a magnesium potassium phosphate mineral with naturally cementitious properties, which is finding increasing usage as an inorganic cement for niche applications including nuclear waste management and rapid road repair. Struvite-K is also of interest in sustainable phosphate recovery from wastewater and, as such, a detailed knowledge of the crystal chemistry and high-temperature behavior is required to support further laboratory investigations and industrial applications. In this study, the local chemical environments of synthetic struvite-K were investigated using high-field solid-state 25Mg and 39K MAS NMR techniques, alongside 31P MAS NMR and thermal analysis. A single resonance was present in each of the 25Mg and 39K MAS NMR spectra, reported here for the first time alongside the experimental and calculated isotropic chemical shifts, which were comparable to the available data for isostructural struvite (MgNH4PO4·6H2O). An in situ high-temperature XRD analysis of struvite-K revealed the presence of a crystalline–amorphous–crystalline transition that occurred between 30 and 350 °C, following the single dehydration step of struvite-K. Between 50 and 300 °C, struvite-K dehydration yielded a transient disordered (amorphous) phase identified here for the first time, denoted δ-MgKPO4. At 350 °C, recrystallization was observed, yielding β-MgKPO4, commensurate with an endothermic DTA event. A subsequent phase transition to γ-MgKPO4 was observed on further heating, which reversed on cooling, resulting in the α-MgKPO4 structure stabilized at room temperature. This behavior was dissimilar from that of struvite exposed to high temperature, where NH4 liberation occurs at temperatures >50 °C, indicating that struvite-K could potentially withstand high temperatures via a transition to MgKPO4.

This study reports the development and application of a digital image cross correlation based approach to resolve contiguous microstructural volumes of interest in X-ray microtomography data, collected in fluid suspensions that undergo significant microstructural changes over time, using fresh cementitious pastes as an example. This computational method provides a high precision both for cementitious pastes that sediment only slightly (i.e. are cohesive), and for those that undergo significant sedimentation and/or settlement within the first few minutes of reaction. The normalised cross correlation algorithm presented here enables the observation of an identical volume of interest, i.e., one which contains a contiguous particle group, from the first seconds of observation onwards with excellent accuracy. This method enables segmentation of the same cluster of particles to be almost entirely automated and resolved in large sets of sequentially collected data, therefore enabling particle reaction to be observed directly while removing effects due to sedimentation.

The service-life of steel-reinforced concrete structures is primarily determined by the ability of the concrete cover to resist chloride ingress. With limited literature available on the ingress of chloride into alkali-activated slags (AAS) under service conditions, it is critical that this is described by appropriate models. This paper describes an interactive software framework to relate chloride ingress into AAS with the chemistry of the concrete cover, by considering the chloride binding capacity and porosity of the binder as a function of time, based on thermodynamic calculations of the phase assemblage as a function of slag and activator composition. This provides a major step towards developing the ability to predict the ingress of chlorides in alkali-activated concretes from a sound theoretical basis, which is essential in providing confidence in the durability of these materials in essential infrastructure applications.

This work aims to develop materials for flexible concrete pavements as an alternative to asphalt concrete or polymer-bound rubber surfaces and presents a study on steel fibre reinforced rubberised concrete (SFRRuC). The main objective of this study is to investigate the effect of steel fibres (manufactured and/or recycled fibres) on the fresh and mechanical properties of rubberised concrete (RuC) comprising waste tyre rubber (WTR). Free shrinkage is also examined. The main parameters investigated through ten different mixes are WTR and fibre contents. The results show that the addition of fibres in RuC mixes with WTR replacement substantially mitigates the loss in flexural strength due to the rubber content (from 50% to 9.6% loss, compared to conventional concrete). The use of fibres in RuC can also enable the development of sufficient flexural strength and enhance strain capacity and post-peak energy absorption behaviour, thus making SFRRuC an ideal alternative construction material for flexible pavements.

This study assesses the durability and transport properties of low water/binder ratio (0.35) steel fibre reinforced rubberised concrete (SFRRuC) mixes, which are proposed to be used as flexible concrete pavements. Waste tyre rubber is incorporated in concrete as fine and coarse aggregate replacement and blends of manufactured steel fibres and recycled tyre steel fibres are used as internal reinforcement. The fresh, mechanical and transport properties of plain concrete are compared with those of SFRRuC mixes having different substitutions of rubber aggregates (0, 30 and 60% by volume). The chloride corrosion effects due to exposure to a simulated accelerated marine environment (intermittent wet-dry cycles in 3% NaCl solution) is also evaluated. The results show that, although water permeability (e.g. volume of permeable voids and sorptivity) and chloride ingress increase with rubber content, this increase is minor and water and chlorides permeability are generally within the range of highly durable concrete mixes. No visual signs of deterioration or cracking (except superficial rust) were observed on the surface of the concrete specimens subjected to 150 or 300 days of accelerated chloride corrosion exposure and a slight increase in the mechanical properties is observed. This study shows that the examined low water/binder SFRRuC mixes promote good durability characteristics, making these composite materials suitable for flexible concrete pavement applications.

An updated calcium silicate hydrate (C–S–H) model incorporating aluminium-containing end-members was used for thermodynamic modelling of blended cements using blast-furnace slag and Portland cement (BFS:PC) with ratios of 1:1, 3:1 and 9:1, using GEMSelektor. Selective dissolution and magic angle spinning nuclear magnetic resonance (MAS NMR) studies were performed to determine the degree of hydration (DoH) of the anhydrous material as an input parameter for the modelling work. Both techniques showed similar results for determining the DoH of the BFS within each sample. Characterisation of the hardened cement pastes over 360 days, using X-ray diffraction analysis and MAS NMR, demonstrated that the use of the updated C–S–H model can highlight the effect of different blend ratios and curing ages on the phase assemblages in these cements. Validation using this modelling approach was performed on 20 year old specimens from the literature to highlight its applicability for modelling later-age blended cements.

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)₂ was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na₂O/slag ratios. Particles of Mg(OH)₂ do not chemically react within these cements. Instead, Mg(OH)₂ acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)₂ replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)₂ still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)₂ cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)₂-bearing wastes such as Magnox sludge.

The mineralogy and structure of cements in the system Mg(OH)2–NaAlO2–SiO2–H2O are investigated, with a view toward potential application in the immobilization of Mg(OH)2-rich Magnox sludges resulting from historic United Kingdom nuclear operations. The reaction process leading to the formation of these aluminosilicate binders is strongly exothermic, initially forming zeolite NaA (LTA structure), which is metastable in low SiO2/Al2O3 binders, slowly evolving into the more stable sodalite and faujasite framework types. Notable chemical reaction of Mg(OH)2 was only identified in the formulation with SiO2/Al2O3 = 1.3 (the lowest molar ratio among those tested) after extended curing times. In this case, some of the Mg(OH)2 reacted to form an Mg–Al–OH layered double hydroxide. These results demonstrate that encapsulation of Magnox sludge waste streams could be carried out in these alternative binders but that the binders would encapsulate rather than chemically incorporate the Mg(OH)2 into the wasteform unless low SiO2/Al2O3 ratios are used.

We recently reported the first comprehensive investigation of magnesium potassium phosphate cements (MKPCs) blended with supplementary cementitious materials (pulverized fuel ash and granulated blast furnace slag) for the encapsulation of radioactive wastes [Gardner et al., Cem. Concr. Res. 74 (2015) 78-87]. Using a combination of characterization techniques, we demonstrated the important role of the reaction of the supplementary cementitious materials in contributing to the development of the microstructure and strength of MKPC composites. Here, we clarify aspects of our experimental design, and elaborate on the interpretation of our data, following discussion by Ma and Li.

Stoichiometrically controlled alkali-activated materials within the system CaO-MgO-Na2O-Al2O3-SiO2 are produced by alkali-activation of high-purity synthetic powders chemically comparable to the glass in ground granulated blast furnace slag, but without additional minor constituents. Mg content controls the formation of hydrotalcite-group and AFm-type phases, which in turn strongly affects C-(N)-A-S-H gel chemistry and nanostructure. Bulk Mg content and the Mg/Al ratio of hydrotalcite-group phases are strongly correlated. With sufficient Ca, increased bulk Mg promotes formation of low-Al C-(A)-S-H and portlandite, due to formation of hydrotalcite-group phases and a reduction in available Al. Hydrotalcite-group phase formation is linked to increased C-(N)-A-S-H gel polymerisation, decreased gel Al uptake and increased formation of the ‘third aluminate hydrate’. These findings highlight the importance of considering available chemical constituents rather than simply bulk composition, so that the desired binder structure for a particular application can be achieved.

The gamma irradiation resistance of early age Ba(OH)2-Na2SO4-slag cementitious grouts, formulated for the immobilisation of sulfate bearing nuclear waste, was assessed. The observable crystalline phases were not modified upon heating (50 °C) or upon gamma irradiation up to a total dose of 2.9 MGy over 256 h, but the compressive strengths of the irradiated samples increased significantly. 27Al and 29Si MAS NMR spectroscopy showed that the main binding phase, a calcium aluminosilicate hydrate (C-A-S-H) type gel, had a more ordered and polymerised structure upon heating and irradiation compared to that identified in reference samples. This is associated with a higher degree of reaction of the slag. Samples formulated with the waste simulant Na2SO4, but without Ba(OH)2, became porous and cracked upon heating and irradiation, but still retained their compressive strength. The Ba(OH)2-Na2SO4-slag grouts evaluated in this work withstand gamma irradiation without showing identifiable damage, and are thus a technically feasible solution for immobilisation of sulfate-bearing nuclear wastes.

The structural changes induced by accelerated carbonation in alkali-activated slag/metakaolin (MK) cements were determined. The specimens were carbonated for 540 h in an environmental chamber with a CO2 concentration of 1.0 ± 0.2%, a temperature of 20 ± 2°C, and relative humidity of 65 ± 5%. Accelerated carbonation led to decalcification of the main binding phase of these cements, which is an aluminum substituted calcium silicate hydrate (C-(N-)A-S-H) type gel, and the consequent formation of calcium carbonate. The sodium-rich carbonates trona (Na2CO3·NaHCO3·2H2O) and gaylussite (Na2Ca(CO3)2·5H2O) were identified in cements containing up to 10 wt.% MK as carbonation products. The formation of these carbonates is mainly associated with the chemical reaction between the CO2 and the free alkalis present in the pore solution. The structure of the carbonated cements is dominated by an aluminosilicate hydrate (N-A-S-H) type gel, independent of the MK content. The N-A-S-H type gels identified are likely to be derived both from the activation reaction of the MK, forming a low-calcium gel product that does not seem to undergo structural changes upon CO2 exposure, and the decalcification of C-(N-)A-S-H type gel. The carbonated pastes present a highly porous microstructure, more notable as the content of MK content in the cement increases, which might have a negative impact on the durability of these materials in service.

There is a growing global interest in maximising the re-use and recycling of waste, to minimise the environmental impacts associated with waste treatment and disposal. Use of high-volume wastes in the production of blended or novel cements (including alkali-activated cements) is well known as a key pathway by which these wastes can be re-used. This paper presents a critical overview of the urban, agricultural, mining and industrial wastes that have been identified as potential precursors for the production of alkali-activated cement materials, or that can be effectively stabilised/solidified via alkali activation, to assure their safe disposal. The central aim of this review is to elucidate the potential advantages and pitfalls associated with the application of alkali-activation technology to a wide variety of wastes that have been claimed to be suitable for the production of construction materials. A brief overview of the generation and characteristics of each waste is reported, accompanied by identification of opportunities for the use of alkali-activation technology for their valorisation and/or management.

In this study, the effect of sodium sulfate content and curing duration (from fresh paste up to 18 months) on the binder structure of sodium sulfate activated slag cements was evaluated. Isothermal calorimetry results showed an induction period spanning the first three days after mixing, followed by an acceleration-deceleration peak corresponding to the formation of bulk reaction products. Ettringite, a calcium aluminium silicate hydrate (C-A-S-H) phase, and a hydrotalcite-like Mg-Al layered double hydroxide have been identified as the main reaction products, independent of the Na2SO4 dose. No changes in the phase assemblage were detected in the samples with curing from 1 month up to 18 months, indicating a stable binder structure. The most significant changes upon curing at advanced ages observed were growth of the AFt phase and an increase in silicate chain length in the C-A-S-H, resulting in higher strength.

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

The effect\ud of modifying the water content of an alkali\ud -\ud activated slag binder\ud was\ud assessed, in terms of the\ud kinetics of reaction and the structural development of the material.\ud There\ud is not a s\ud ystematic correlation\ud between the water content of the mix and the rate of reaction, indicating that there is an optimal value that\ud favours dissolution of the slag and precipitation of reaction products.\ud A h\ud igher water content reduce\ud d\ud the\ud crystallinity and\ud density of the reaction products, especially at advanced age. Small changes in the water\ud content can have a significant impact on the compressive strength development of alkali\ud -\ud silicate activated\ud slag mortars, suggesting that when producing\ud materials base\ud d on\ud alkali\ud -\ud activated\ud binders\ud , it is essential to\ud carefully control\ud the\ud water content.

This paper addresses the effects of relative humidity (RH) and carbon dioxide (CO2) concentration on the rate and effects of accelerated carbonation in alkali-activated slag/metakaolin (MK) concretes. Strength and water absorption are used alongside phenolphthalein measurements to monitor carbonation, and the effects of drying at different RHs are particularly significant in controlling carbonation rates. Different trends in the carbonation rate as a function of MK content are observed when varying the CO2 concentration, further revealing that the carbonation rates of these materials under accelerated conditions are influenced strongly by the testing protocol. The standard phenolphthalein method for testing carbonation depth appears only to be capturing the change in alkalinity with pore solution carbonation, meaning that it does not correlate well with other performance parameters at high CO2 concentrations.

Multi-technique characterisation of sodium carbonate-activated blast furnace slag binders was conducted in order to determine the influence of the carbonate groups on the structural and chemical evolution of these materials. At early age (

The main reaction product in Ca-rich alkali-activated cements and hybrid Portland cement (PC)-based materials is a calcium (alkali) aluminosilicate hydrate (C-(N-)A-S-H) gel. Thermodynamic models without explicit definitions of structurally-incorporated Al species have been used in numerous past studies to describe this gel, but offer limited ability to simulate the chemistry of blended PC materials and alkali-activated cements. Here, a thermodynamic model for C-(N-)A-S-H gel is derived and parameterised to describe solubility data for the CaO–(Na2O,Al2O3)–SiO2–H2O systems and alkali-activated slag (AAS) cements, and chemical composition data for C-A-S-H gels. Simulated C-(N-)A-S-H gel densities and molar volumes are consistent with the corresponding values reported for AAS cements, meaning that the model can be used to describe chemical shrinkage in these materials. Therefore, this model can provide insight into the chemistry of AAS cements at advanced ages, which is important for understanding the long-term durability of these materials.

Soluble sulfate ions in nuclear waste can have detrimental effects on cementitious wasteforms and disposal facilities based on Portland cement. As an alternative, Ba(OH)2–Na2SO4–blast furnace slag composites are studied for immobilisation of sulfate-bearing nuclear wastes. Calcium aluminosilicate hydrate (C–A–S–H) with some barium substitution is the main binder phase, with barium also present in the low solubility salts BaSO4 and BaCO3, along with Ba-substituted calcium sulfoaluminate hydrates, and a hydrotalcite-type layered double hydroxide. This reaction product assemblage indicates that Ba(OH)2 and Na2SO4 act as alkaline activators and control the reaction of the slag in addition to forming insoluble BaSO4, and this restricts sulfate availability for further reaction as long as sufficient Ba(OH)2 is added. An increased content of Ba(OH)2 promotes a higher degree of reaction, and the formation of a highly cross-linked C–A–S–H gel. These Ba(OH)2–Na2SO4–blast furnace slag composite binders could be effective in the immobilisation of sulfate-bearing nuclear wastes.

The present study investigated the feasibility of the immobilisation of sulphate bearing radioactive wastes in blast furnace slag (BFS) based binders. BaSO4–BFS composites were produced via two methods using Na2SO4 as a waste simulant, along with Ba(OH)2 to promote precipitation of BaSO4 in an insoluble sulphate form and the consequent activation of the BFS. BaSO4 was effectively formed by both methods, and solid wasteforms were successfully produced. Although both methods produced BaSO4 embedded in the cement-like composites, different reaction products including ettringite and witherite were produced, depending on the order Ba(OH)2 was mixed with the system. These results show that the immobilisation of soluble sulphate-bearing aqueous wastes is achievable in Na2SO4–Ba(OH)2–BFS composites.

This paper presents an overview examining the microstructural and macrostructural properties of alkali-activated binders based on granulated blast furnace slags, metakaolin and their blends, developed by the Composite Materials Group of Universidad del Valle over the past decade. Durability results of activated binders when exposed to aggressive agents such as chlorides, and carbon dioxide are reported. The results of this research have elucidated the great feasibility of adopting alkali-activation technology in Colombia for producing high strength concretes based on industrial by-products, with a wide range of properties that can be suitable for different civil infrastructure applications, and contribute to the valorization of low-cost industrial by products through production of more environmentally friendly building materials. Our research highlights the fact that a deep understanding of the chemistry of these systems allows the manipulation of the microstructure and therefore the performance of the final products, toward the production of sustainable and versatile materials.

This paper presents the outcomes of a series of beamline-based studies, the results of which are combined to provide a more detailed multiscale understanding of the structure and chemistry of geopolymer binders.\ud \ud The range of beamline-based characterization techniques which have been applied to the study of geopolymer binders is increasing rapidly; although no single technique can provide a holistic view of binder structure across all the length scales which are of importance in determining strength development and durability, the synergy achievable through the combination of multiple beamline techniques is leading to rapid advances in knowledge in this area. Studies based around beamline infrared and X-ray fluorescence microscopy, in situ and ex situ neutron pair distribution function analysis, and nano- and micro-tomography, are combined to provide an understanding of geopolymer gel chemistry, nano- and microstructure in two and three dimensions, and the influences of seeded nucleation and precursor chemistry in these key areas.\ud \ud The application of advanced characterization methods in recent years has brought the understanding of geopolymer chemistry from a point, not more than a decade ago, when the analysis of the detailed chemistry of the aluminosilicate binder gel was considered intractable due to its disordered (“X-ray amorphous”) nature, to the present day where the influence of key compositional parameters on nanostructure is well understood, and both gel structure and reaction kinetics can be manipulated through methods including seeding, temperature variation, and careful mix design.\ud \ud This paper therefore provides a review outlining the value of nanotechnology – and particularly nanostructural characterization – in the development and optimization of a new class of environmentally beneficial cements and concretes. Key engineering parameters, in particularly strength development and permeability, are determined at a nanostructural level, and so it is essential that gel structures can be analyzed and manipulated at this level; beamline-based characterization techniques are critical in providing the ability to achieve this goal.

The RILEM technical committee TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ conducted a round robin testing programme to determine the validity of various durability testing methods, originally developed for Portland cement based-concretes, for the assessment of the durability of alkali-activated concretes. The outcomes of the round robin tests evaluating sulfate resistance, alkali-silica reaction (ASR) and freeze–thaw resistance are presented in this contribution. Five different alkali-activated concretes, based on ground granulated blast furnace slag, fly ash, or metakaolin were investigated. The extent of sulfate damage to concretes based on slag or fly ash seems to be limited when exposed to an Na2SO4 solution. The mixture based on metakaolin showed an excessive, very early expansion, followed by a dimensionally stable period, which cannot be explained at present. In the slag-based concretes, MgSO4 caused more expansion and visual damage than Na2SO4; however, the expansion limits defined in the respective standards were not exceeded. Both the ASTM C1293 and RILEM AAR-3.1 test methods for the determination of ASR expansion appear to give essentially reliable identification of expansion caused by highly reactive aggregates. Alkali-activated materials in combination with an unreactive or potentially expansive aggregate were in no case seen to cause larger expansions; only the aggregates of known very high reactivity were seen to be problematic. The results of freeze–thaw testing (with/without deicing salts) of alkali-activated concretes suggest an important influence of the curing conditions and experimental conditions on the test outcomes, which need to be understood before the tests can be reliably applied and interpreted.