Alkali-activated materials are a new class of sustainable materials that can help supplant highly CO2-intensive ordinary Portland cement (OPC). Chemical admixtures and additives that manipulate the hydration and setting of OPC are readily utilized in the construction industry. However, for alkali-activated materials, the impact of these additives on the evolution of the atomic structure of the binder gel is largely unknown. Here, we utilize nano-ZnO (nanoparticles of zinc oxide), a known retarder for OPC hydration, and investigate its influence on the alkali-activation reaction in high and low calcium alkali-activated materials (slag and metakaolin systems, respectively). Using isothermal calorimetry and in situ X-ray pair distribution function analysis, the mechanism of nano-ZnO retardation in alkali-activated materials is uncovered, revealing that calcium plays a pivotal role in dictating whether nano-ZnO has an impact on the alkali-activation reaction. These results also provide important insight on the ability of slag and metakaolin-based alkali-activated materials to effectively immobilize zinc within the binder gel, which is of relevance to the waste solidification/stabilization community.

Due to the nanocrystallinity of the calcium–silicate–hydrate (C–S–H) gel in ordinary Portland cement-based paste combined with the presence of nanoscale heterogeneities such as varying calcium-to-silicon ratios and incorporation of aluminum in the structure, standard characterization techniques fail to fully capture the complex atomic structure and nanoscale morphology of this important binder phase. Here, neutron pair distribution function (PDF) analysis is applied to a range of deuterated C–S–H gels with varying Ca/Si ratios (denoted C–S–D). In situ temperature measurements reveal that the local atomic bonding environments in C–S–D gel undergo large structural rearrangements due to exposure to elevated temperature (above ~ 200 °C), including the collapse of the C–S–D gel interlayer spacing to 9.6 Å and the emergence of a disordered dicalcium silicate phase (similar to larnite). At lower elevated temperatures, the atom–atom correlations are dominated by scattering from deuterium atoms and therefore can be used to quantify the dehydration kinetics.

Helium ion microscopy is used to provide new insight into the nano- and microscale morphology of cleaved surfaces of alkali-activated slag (AAS), revealing that AAS contains several types of gel with varying surface morphologies after being subjected to D-drying. It is seen that the bulk C-(N)-A-S-H gel is globular in nature in a silicate-activated slag paste while the gel covering the slag particles is foil-like.

Alkali-activated materials (AAMs) are currently being pursued as viable alternatives to conventional ordinary Portland cement because of their lower carbon footprint and established mechanical performance. However, our understanding of the mesoscale morphology (∼1 to 100 nm) of AAMs and related amorphous aluminosilicate gels, including the development of the three-dimensional aluminosilicate network and nanoscale porosity, is severely limited. This study investigates the structural changes that occur during the formation of AAM gels at the mesoscale by utilizing a coarse-grained Monte Carlo (CGMC) modeling technique that exploits density functional theory calculations. The model is capable of simulating the reaction of an aluminosilicate particle in a highly alkaline solution (sodium hydroxide or sodium silicate). Two precursor morphologies have been investigated (layered alumina and silica sheets mimicking metakaolin and spherical aluminosilicate particles reminiscent of coal-derived fly ash) to determine if the precursor morphology has an impact on the structural evolution of the resulting alkali-activated aluminosilicate gel. The CGMC model can capture the three major stages of the alkali-activation process—dissolution, polycondensation, and reorganization—revealing that the dissolved silicate and aluminate species, ranging from monomers to nanoprecipitates (100s of monomers in size), exist in the pore solution of the hardened gel. The model also reveals that the silica concentration of the activating solution controls the extent of dissolution of the precursor particle. From the analysis of the aluminosilicate cluster size distributions, the mechanisms of AAM gel growth have been elucidated, revealing that Ostwald ripening occurs in systems containing free silica at the start of the reaction. On the other hand, growth of the hydroxide-activated systems (metakaolin and fly ash) occurs via the formation of intermediate-sized clusters in addition to continual growth of the largest particle. The simulation results indicate that the nature of the gel growth is not influenced by the precursor particle morphology.

Alkali-activated materials and related alternative cementitious systems are sustainable technologies that have the potential to substantially lower the CO2 emissions associated with the construction industry. However, these systems have augmented chemical compositions as compared to ordinary Portland cement (OPC), which may impact the evolution of the hydrate phases. In particular, calcium-silicate-hydrate (C–S–H) gel, the main hydrate phase in OPC, is likely to be altered at the atomic scale due to changes in the bulk chemical composition, specifically via the addition of alkalis (i.e., Na or K) and aluminum. Here, via density functional theory calculations, we reveal the presence of a charge balancing mechanism at the molecular level in C–S–H gel (as modeled using crystalline 14 Å tobermorite) when alkalis and aluminum atoms are introduced into the structure. Different structural representations are obtained depending on the level of substitution and the degree of charge balancing incorporated in the structures. The impact of these substitutional and charge balancing effects on the structures is assessed by analyzing the formation energies, local bonding environments, diffusion barriers and mechanical properties. The results of this computational study provide information on the phase stability of alkali/aluminum containing C–S–H gels, shedding light on the fundamental atomic level mechanisms that play a crucial role in these complex disordered materials.

•X-ray and neutron PDF analyses have been performed on five types of class F fly ash.•Bulk glassy structure was assessed through the complementarity of the two methods.•Amorphous aluminum, iron and carbon have major influence on the atomic ordering.This paper presents the comparative results obtained from X-ray and neutron pair distribution function (PDF) analysis aimed at determining the variability in aluminosilicate glass chemistry in five types of class F fly ash (FA). Results have been discussed in light of the complementary information provided by the two methods in order to give a comprehensive overview of FA structure at the nanoscale. The analysis of short range correlations reveals that the bulk glassy structure of FA sources differing in chemical composition are relatively similar, but some specific distinctions in atomic structure are visible in those containing high levels of amorphous VI-coordinated aluminum (e.g., amorphous mullite/alumina), iron and/or carbon (with similar local bonding environment to graphite). The obtained experimental results fill a deficit in literature in the atomic structure and associated variability for class F FA, which is extensively used in several industrial applications including as raw material in alkali-activated cements.

Oil well cements have received a significant amount of attention in recent years due to their use in high-risk conditions combined with their exposure to extremely aggressive environments. Adequate resistance to temperature, pressure, and carbonation is necessary to ensure the integrity of the well, with conventional cements prone to chemical degradation when exposed to CO2 molecules. Here, the local atomic structural changes occurring during the accelerated carbonation (100% CO2) of a sustainable cement, alkali-activated slag (AAS) have been investigated using in situ X-ray diffraction and pair distribution function analysis. The results reveal that the extent of carbonation-induced chemical degradation, which is governed by the removal of calcium from the calcium-alumino-silicate-hydrate (C-A-S-H) gel, can be reduced by tailoring the precursor chemistry; specifically the magnesium content. High-magnesium AAS pastes are seen to form stable magnesium-containing amorphous calcium carbonate phases, which prevents the removal of additional calcium from the C-A-S-H gel, thereby halting the progress of the carbonation reaction. On the other hand, lower-magnesium AAS pastes form amorphous calcium carbonate which is seen to quickly crystallize into calcite/vaterite, along with additional decalcification of the C-A-S-H gel. Hence, this behavior can be explained by considering (i) the solubility products of the various carbonate polymorphs and (ii) the stability of amorphous calcium/magnesium carbonate, where because of the higher solubility of amorphous calcium carbonate and associated saturation of solution with respect to calcium, additional C-A-S-H gel decalcification cannot proceed when this amorphous phase is present. These results may have important implications for the use of new cementitious materials in extremely aggressive conditions involving CO2 (e.g., enhanced oil recovery and geological storage of CO2), particularly because of the ability to optimize the durability of these materials by controlling the precursor (slag) chemistry.

Calcium–silicate–hydrate (C–S–H) gel is the main binder component in hydrated ordinary Portland cement (OPC) paste, and is known to play a crucial role in the carbonation of cementitious materials, especially for more sustainable alternatives containing supplementary cementitious materials. However, the exact atomic structural changes that occur during carbonation of C–S–H gel remain unknown. Here, we investigate the local atomic structural changes that occur during carbonation of a synthetic calcium–silicate–hydrate gel exposed to pure CO2 vapour, using in situ X-ray total scattering measurements and subsequent pair distribution function (PDF) analysis. By analysing both the reciprocal and real-space scattering data as the C–S–H carbonation reaction progresses, all phases present during the reaction (crystalline and non-crystalline) have been identified and quantified, with the results revealing the emergence of several polymorphs of crystalline calcium carbonate (vaterite and calcite) in addition to the decalcified C–S–H gel. Furthermore, the results point toward residual calcium being present in the amorphous decalcified gel, potentially in the form of an amorphous calcium carbonate phase. As a result of the quantification process, the reaction kinetics for the evolution of the individual phases have been obtained, revealing new information on the rate of growth/dissolution for each phase associated with C–S–H gel carbonation. Moreover, the investigation reveals that the use of real space diffraction data in the form of PDFs enables more accurate determination of the phases that develop during complex reaction processes such as C–S–H gel carbonation in comparison to the conventional reciprocal space Rietveld analysis approach.

The atomic structures of calcium silicate hydrate (C–S–H) and calcium (–sodium) aluminosilicate hydrate (C–(N)–A–S–H) gels, and their presence in conventional and blended cement systems, have been the topic of significant debate over recent decades. Previous investigations have revealed that synthetic C–S–H gel is nanocrystalline and due to the chemical similarities between ordinary Portland cement (OPC)-based systems and low-CO2 alkali-activated slags, researchers have inferred that the atomic ordering in alkali-activated slag is the same as in OPC–slag cements. Here, X-ray total scattering is used to determine the local bonding environment and nanostructure of C(–A)–S–H gels present in hydrated tricalcium silicate (C3S), blended C3S–slag and alkali-activated slag, revealing the large intrinsic differences in the extent of nanoscale ordering between C–S–H derived from C3S and alkali-activated slag systems, which may have a significant influence on thermodynamic stability, and material properties at higher length scales, including long term durability of alkali-activated cements.

Amorphous calcium/magnesium carbonates are of significant interest in the technology sector for a range of processes, including carbon storage and biomineralization. Here, the atomic structure of one hydrated amorphous magnesium carbonate (hydrated AMC, MgCO3·3D2O) is investigated using an iterative methodology, where quantum chemistry and experimental total scattering data are combined in an interactive iterative manner to produce an experimentally valid structural representation that is thermodynamically stable. The atomic structure of this hydrated AMC consists of a distribution of Mg2+ coordination states, predominately V- and VI-fold, and is heterogeneous due to the presence of Mg2+/CO32--rich regions interspersed with small ‘pores’ of water molecules. This heterogeneity at the atomic length scale is likely to contribute to the dehydration of hydrated AMC by providing a route for water molecules to be removed. We show that this iterative methodology enables wide sampling of the potential energy landscape, which is important for elucidating the true atomic structure of highly disordered metastable materials.

Calcium–silicate–hydrate (C–S–H) gel is the main binder component in hydrated ordinary Portland cement (OPC) paste, and is known to play a crucial role in the carbonation of cementitious materials, especially for more sustainable alternatives containing supplementary cementitious materials. However, the exact atomic structural changes that occur during carbonation of C–S–H gel remain unknown. Here, we investigate the local atomic structural changes that occur during carbonation of a synthetic calcium–silicate–hydrate gel exposed to pure CO2 vapour, using in situ X-ray total scattering measurements and subsequent pair distribution function (PDF) analysis. By analysing both the reciprocal and real-space scattering data as the C–S–H carbonation reaction progresses, all phases present during the reaction (crystalline and non-crystalline) have been identified and quantified, with the results revealing the emergence of several polymorphs of crystalline calcium carbonate (vaterite and calcite) in addition to the decalcified C–S–H gel. Furthermore, the results point toward residual calcium being present in the amorphous decalcified gel, potentially in the form of an amorphous calcium carbonate phase. As a result of the quantification process, the reaction kinetics for the evolution of the individual phases have been obtained, revealing new information on the rate of growth/dissolution for each phase associated with C–S–H gel carbonation. Moreover, the investigation reveals that the use of real space diffraction data in the form of PDFs enables more accurate determination of the phases that develop during complex reaction processes such as C–S–H gel carbonation in comparison to the conventional reciprocal space Rietveld analysis approach.

Alkali-activated materials are a new class of sustainable materials that can help supplant highly CO2-intensive ordinary Portland cement (OPC). Chemical admixtures and additives that manipulate the hydration and setting of OPC are readily utilized in the construction industry. However, for alkali-activated materials, the impact of these additives on the evolution of the atomic structure of the binder gel is largely unknown. Here, we utilize nano-ZnO (nanoparticles of zinc oxide), a known retarder for OPC hydration, and investigate its influence on the alkali-activation reaction in high and low calcium alkali-activated materials (slag and metakaolin systems, respectively). Using isothermal calorimetry and in situ X-ray pair distribution function analysis, the mechanism of nano-ZnO retardation in alkali-activated materials is uncovered, revealing that calcium plays a pivotal role in dictating whether nano-ZnO has an impact on the alkali-activation reaction. These results also provide important insight on the ability of slag and metakaolin-based alkali-activated materials to effectively immobilize zinc within the binder gel, which is of relevance to the waste solidification/stabilization community.