Silica-rich Gel Research Articles

Synchronous vacuum hot pressing ultra-high performance geopolymer: Ultrafast polymerization and early strength development

To overcome the application bottleneck of geopolymer in high-end industries, this study aims to develop ultra-high performance geopolymer products. By employing Synchronous Vacuum Hot Pressing, we have significantly enhanced the rate of polymerization and mechanical properties of Geopolymer based on Metakaolin-Fly Ash. The influence of Si/Al ratio, alkali equivalent, and molding time on the geopolymerization process, mechanical properties, and microstructure of geopolymers was investigated, while elucidating the underlying mechanisms. The compressive strength of SVHP-MFG reaches 90.92 MPa when the Si/Al ratio is 2, alkali equivalent is 8 %, and molding time is 30 min, owing to increased production of aluminosilicate gels and matrix structure densification. The ongoing reaction of unreacted particles and the subsequent transformation from "alumina-rich gel" to "silica-rich gel" in later stages further enhance the mechanical properties of the material. Additionally, the mean pore diameter of Synchronous vacuum hot pressing of metakaolin-fly ash geopolymer measures below 13.24 nm, thereby significantly enhancing the pore structure of the geopolymer. The synchronous vacuum hot pressing metakaolin fly ash-based geopolymers synthesized in this study represents a novel category of ultra-high performance ceramic-like geopolymers, exhibiting exceptional early strength and early stability. The research aims to promote the application of geopolymer in high-end industry and provide technical and theoretical support for the development of ultra-high performance geopolymer based products.

Effect of wet grinding carbonation of sintering red mud on the performance of carbon sequestered mortar

The use of alkaline solid waste to CO2 capture is a potential way to upgrade the recycling of solid waste and reduce CO2 emission. In previous work, we proposed a novel wet grinding carbonation technique to achieve rapid carbonation of sintering red mud (SRM). Herein, the effect of wet grinding carbonation of SRM (WGCS) replacement rate (0 %, 5 %, 10 %, 15 %, 20 % and 25 %) on the hydration and hardening of Portland cement was further explored, such as rheology, compressive strength, hydration products and water transfer properties. The plastic viscosity and yield stress of carbon sequestered paste rose together with the WGCS replacement rate, which was due to the decreased D50 of SRM after wet grinding carbonation. When the WGCS replacement rate was 15 %, the 56 d compressive strength of carbon sequestered mortar was increased by 14.1 %. The microscopic characterization showed that the nucleation of calcite crystals and the pozzolanic reaction of silica-rich gels clearly improved the microstructure compactness. The electric flux and porosity of cement mortar was decreased by 25.7 % and 11.4 %, respectively. In terms of environmental and economic benefits, the CO2 sequestration of carbon sequestered mortar reached as high as 24.7 kg/m3 and the CO2 emission was reduced by 30.9 %.

Degradation of alkali-activated Fe-rich slag in sulfuric acid

Alkali-activated Fe-rich non-ferrous metallurgical slag (AA-NFS), within the FeOx-SiO2-Al2O3-CaO-MgO system, were tested in 2 wt% sulfuric acid for 6 months. Corrosion depth and rate were measured non-destructively by micro X-ray computed tomography. Both intact and corroded areas were characterized by XRD, FTIR, SEM-EDX and NMR. AA-NFS with higher Ca/Si showed a higher resistance to sulfuric acid. The presence of Mg negatively affected the resistance, whereas higher Al concentrations slightly enhanced the resistance in the investigated chemical composition range. The corrosion degree is closely related to the NFS reaction extent in AA-NFS, and this corrosion process is diffusion-controlled. Leaching of the elements (Al, Ca, Mg, Fe and Na) leaves behind mostly silica-rich gels, causing the failure of the framework structure after long-term exposure. The leached Ca2+ reacted with SO42− in acid solution and formed expansive sulfate products, which possibly clog pores and offer an additional protective barrier for AA-NFS against sulfuric acid.

Geopolymers functionalised by antibacterial zeolite against biocorrosion

Aluminosilicate geopolymer with remarkable chemical and biological resistance is a promising material solution to the persistent challenge of microbial induced concrete corrosion (MICC) in sewers. In this work, novel Sor-zeolite, assembled by using microporous 13X zeolite as a host of antimicrobial ion sorbate, is incorporated into fly ash-slag geopolymer to form bio-resistant materials with long-lasting efficacy and densified microstructure. The inclusion of pure zeolite and Sor-zeolite increase the strength and refine the pore structure of geopolymer pastes owing to the seeding nuclei effects during the geopolymerisation process. The enhanced dissolution of FA and slag by the seeding effects of pure zeolite and Sor-zeolite, accelerates and enriches the precipitation of geopolymeric gels, mainly a mixture of K-A-S-H and C-A-S-H (abbreviated as K-(C)-A-S-H). The Sor-zeolite-functionalised geopolymer possesses outstanding antibacterial properties against sulphur oxidising bacteria Acidithiobacillus thiooxidans, inhibiting biofilm formation on the surface and reducing corrosion depth. The main biodegradation products of geopolymers are gypsum and silica-rich gels, resulted from decalcification and dealumination of Si–O–Al and Ca–O bonds in K-(C)-A-S-H.

Thermokinetic and Chemorheology of the Geopolymerization of an Alumina-Rich Alkaline-Activated Metakaolin in Isothermal and Dynamic Thermal Scans.

Alkaline sodium hydroxide/sodium silicate-activating high-purity metakaolin geopolymerization is described in terms of metakaolin deconstruction in tetrahedral hydrate silicate [O[Si(OH)3]]- and aluminate [Al(OH)4]- ionic precursors followed by their reassembling in linear and branched sialates monomers that randomly copolymerize into an irregular crosslinked aluminosilicate network. The novelty of the approach resides in the concurrent thermo-calorimetric (differential scanning calorimetry, DSC) and rheological (dynamic mechanical analysis, DMA) characterizations of the liquid slurry during the transformation into a gel and a structural glassy solid. Tests were run either in temperature scan (1 °C/min) or isothermal (20 °C, 30 °C, 40 °C) cure conditions. A Gaussian functions deconvolution method has been applied to the DSC multi-peak thermograms to separate the kinetic contributions of the oligomer's concurrent reactions. DSC thermograms of all tested materials are well-fitted by a combination of three overlapping Gaussian curves that are associated with the initial linear low-molecular-weight (Mw) oligomers (P1) formation, oligomers branching into alumina-rich and silica-rich gels (P2), and inter- and intra-molecular crosslinking (P3). The loss factor has been used to define viscoelastic behavioral zones for each DMA rheo-thermogram operated in the same DSC thermal conditions. Macromolecular evolution and viscoelastic properties have been obtained by pairing the deconvoluted DSC thermograms with the viscoelastic behavioral zones of the DMA rheo-thermograms. Two main chemorheological behaviors have been identified relative to pre- and post-gelation separation of the viscoelastic liquid from the viscoelastic solid. Each comprises three behavioral zones, accounting for the concurrently occurring linear and branching oligomerization, aluminate-rich and silica-rich gel nucleations, crosslinking, and vitrification. A "rubbery plateau" in the loss factor path, observed for all the testing conditions, identifies a large behavioral transition zone dividing the incipient gelling liquid slurry from the material hard setting and vitrification.

Investigation on the influential mechanism of FA and GGBS on the properties of CO2-cured cement paste

In the present work, the synergetic effect of fly ash (FA) and ground granulated blast furnace slag (GGBS) on the early compressive strength and microstructure development of CO2-cured mortars was investigated. A rim of several micrometers was found around carbonated cement particles, which contained not only silica-rich gel but also crystal calcium carbonate. The calcium carbonate formed around the cement particles were surrounded by an amorphous layer of around 3 nm, while no layers were observed around the calcite formed on FA or GGBS particles. The calcite formed on FA particles were hexagonal plate shaped, while the one on GGBS particles were rhombohedral shaped. The use of FA resulted in the increase of crystal size and crystallinity of calcite, while GGBS decreased the crystal size of calcite. The incorporation of FA and GGBS increased the calcite content and polymerization of silica-rich gel. However, this didn't result in a higher compressive strength. This was due to the looser microstructure and nanopores within the carbonation products compared to the pure ordinary Portland cement sample. The compressive strength of the ternary binder system showed linear relationship with capillary pores and the crystal size of calcite. Moreover, the effect of GGBS on the compressive strength reduction was more obvious than that of FA.

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge, Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.

Improving sulphuric acid resistance of slag-based binders by magnesium-modified activator and metakaolin substitution

Alkali-activated slag (AAS) is an appealing alternative to ordinary Portland cement (OPC) in aggressive acid environment considering its relatively low calcium content and potential in retarding aggressive media penetration. This study evaluates the effect of reactive MgO–NaOH mixture as an activator on the performance of AAS with and without metakaolin substitution upon sulphuric acid exposure. The experimental results reveal an improved acid resistance of slag pastes with supplementary magnesium and the improvement is further enhanced with 20% metakaolin substitution. The improved effect by supplementary magnesium is not only attributed to its microstructure refinement for retarding acid penetration; but also contributed by the acid neutralisation capacity of the formed brucite and the potentially positive effect of magnesium on stabilising the AAS binding gel. Stratified silica-rich gels are formed in the metakaolin-substituted AAS, whose formation is primarily attributed to the intrinsic difference of the formed aluminosilicate gels and related to the pH value and the presence of Mg2+. Considering the dense structure of these gels, they might act as protective barriers for the AAS upon acid deterioration.

The in vivo dissolution of tricalcium silicate bone cement.

This study aimed to investigate the in vivo dissolution of tricalcium silicate (Ca3 SiO5 , C3 S) bone cement in the rabbit femoral defect. Results indicated that C3 S paste directly integrated with the bone tissue without the protection of the bone-like apatite. Calcium silicate hydrate gel (C-S-H gel) and Ca(OH)2 were the main components of C3 S paste. The dissolution model of C3 S paste was a mass loss rather than a decrease in volume. The initial dissolution of C3 S paste (0 ~ 6 weeks) was greatly attributed to the release of Ca(OH)2 , and the later dissolution (>6 weeks) was attributed to the decalcification of C-S-H gel. Although the mass of C3 S paste could decrease by more than 19 wt % after 6 weeks of implantation, the created pores (<1μm) were not large enough for the bone tissue to migrate into C3 S paste. The loss of Ca ions also resulted in the transformation of SiO4 tetrahedrons from Q1 and Q2 to Q0 , Q3 , and Q4 in C-S-H gel. Because only isolated SiO4 tetrahedrons (Q0 ) and Ca ions could be absorbed by the bone tissue, C3 S paste gradually transformed into a silica-rich gel. The fundamental reason for no decrease in volume of C3 S paste was that the SiO4 tetrahedron network still maintained the frame structure of C3 S paste during the implantation.

Effect of water-to-cement ratio induced hydration on the accelerated carbonation of cement pastes.

Recently, the use of accelerated carbonation curing has attracted wide attention as a promising method to reduce carbon dioxide (CO2) emission and improve the mechanical properties of cement-based materials. However, the diffusion mechanism of CO2 in the matrix and the content of hydration products are the key factors that restrict the carbonation reaction rate. To understand the combined behavior of hydration and carbonation reactions, this paper investigates the influence of cement hydration induced by water-to-cement ratio (w/c) (ranging from 0.25 to 0.45) on microstructure and microhardness properties of cement paste. The experimental results demonstrated that carbonation only occurred at the surface layer of cement paste samples and carbonation efficiency was significantly influenced by greater hydration due to higher w/c. The carbonation depth of the sample with 0.45 w/c was about 6 times higher than that of sample with 0.25 w/c after 28 days of CO2 curing. XRD results revealed that calcite-type calcium carbonate is the main carbonation product and consumption of clinker phases (C2S and C3S) during the hydration enhanced the calcite precipitation in the pores of the surface layer. According to FTIR, with increasing w/c, the position of Si-O-Si stretching bond of the carbonated surface changed from Q2 to Q3, confirming the formation of amorphous silica-rich gel, along with the appearance of CO32- bonds related to calcite. In overall, the micro-mechanical analysis in this study showed that the carbonation significantly improved the surface microhardness of cement paste samples, while the refinement of capillary pores due to carbonation also decreased the negative impact of large pores formed in the matrix of cement paste prepared with high w/c.

A novel upcycling technique of recycled cement paste powder by a two-step carbonation process

Abstract The construction industry, being a major consumer of natural resources and energy, is eager to develop novel upcycling techniques for converting secondary resources derived from concrete waste into new and value-added products. However, upcycling of demolished concrete waste is limited by the shortage of practical and economical techniques. This paper presents the development of an innovative upcycling technique to convert recycled fine cement waste to a Ca-rich residue and a Si-rich gel by using a two-step carbonation process. This two-step process involves i) fine recycled cement powder reacting with a Na2CO3 solution to precipitate a calcium-rich residue and ii) after filtration, the filtrate containing Na2SiO3 and NaOH was subjected to a flow-through CO2 gas carbonation to obtain the suspension with a silica-rich gel and the Na2CO3 solution. The physical and chemical properties of the precipitated products from both steps were analyzed by a range of techniques, including particle size distribution, Fourier-transformed infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray fluorescence spectroscopy (XRF), thermogravimetric analysis (TGA), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM). The results indicated that the proposed technique was able to successfully convert recycled cement paste powder to two new value-added reaction products, containing calcite and silica gel.

Immobilization forms of ZnO in the solidification/stabilization (S/S) of a zinc mine tailing through geopolymerization

Solidification/stabilization (S/S) of a zinc mine tailing through geopolymerization has been studied. The mechanical property and microstructure of the mine tailings based-geopolymers were investigated through compressive strength measurements, FTIR, NMR and SEM characterizations. The stabilization of zinc in the geopolymers was analyzed through toxicity characteristic leaching procedure (TCLP) and the immobilization forms were characterized through XPS. With the content of metakaolin in raw materials increased from 0 to 50%, the amount of geopolymer gel in the binders and the compressive strength of geopolymer increased. As a ratio of metakaolin to mine tailings of 1:1, the mine tailings promoted the formation of silica-rich gel and the compressive strength of geopolymer was 30.1 MPa which was similar to pure metakaolin-based geopolymer. The immobilization efficiency of Zn in the geopolymers is correlated to their mechanical property. Zinc is immobilized in the geopolymers through physical encapsulation and adsorption of the leached Zn2+ by geopolymer gel.

Integrated analysis of a black-glazed porcelain bowl in Tushan Kiln dated back to Song Dynasty, China

Research on corrosion of archaeological porcelain glaze has rarely been carried out in the past, although corrosion damage precious heritage items continuously. A group of black-glazed porcelains from the Song Dynasty was excavated at a poor conservation site in Chongqing, and one of their fragments was investigated with optical microscope, SEM-EDX, FTIR and Raman spectroscopy. The results showed that the alteration crust of the glaze mainly consisted of silica-rich gel and contained a variety of heterogeneous phases, including the liquid phase separation structure and different crystals. The crystals mainly include hematite, magnetite, rutile, pseudobrookite and silicon dioxide. The inter-diffusion reaction between alkalis in glaze and hydrogenated species in contact solution was the main reaction, which led to the formation of silicon-rich hydrate layers. The amorphous layers eventually transformed into acicular silica crystals under long exposure to environment.

Resistance of alkali-activated grouts to acid leaching

At very low pH values materials based on hydraulic binders cannot be directly exposed to acid solutions due to leaching, hence the need for additional coatings. As these are generally characterized by relatively high cost and environmental impact, new solutions are being developed. In this study, grouts based on alkali-activated materials (activated slag and activated metakaolin with and without fly ash) have been considered as an alternative to be directly exposed to acid solutions at pH = 3.5, and compared with slag cement grout. The direct monitoring of mass and volumes was combined with the chemical analysis of the leachate and the observations of cross sections through SEM/EDS and X-ray tomography in order to understand the mechanisms involved in the leaching of studied materials. The cumulated amount of leached ions was converted into an equivalent mortar volume loss and compared with the direct measurement of the volume loss. A good correlation was found for slag cement and activated metakaolin, as leaching mainly affected the binding phase. For activated slag the volume loss was lower than expected due to the formation of new phases such as silica-rich gel. The higher volume loss of metakaolin-fly ash grout was attributed to the influence of partially reacted fly ash particles.

Interfacial chemistry of a fly ash geopolymer and aggregates

The environmental and accessibility concerns associated with the increasing use of river sand in construction have diverted the attention of scientists to alternative materials. Waste glass has been widely investigated as a replacement for sand in geopolymer concretes, and the chemical and mechanical properties of the resulting materials are investigated. Since glass is rich in amorphous silica, it is believed that the surface of glass aggregates can take part in the reaction and make a stronger geopolymer matrix compared to that of sand aggregates. An improvement of 30% in compressive strength was obtained by replacing sand aggregates with glass aggregates. While the mechanical properties support interfacial reaction with glass aggregates, the nature of the chemical bonding structure at the interface has not been revealed. In this research, synchrotron Fourier transform infrared (SR-FTIR) microspectroscopy has been used for the first time to map the distribution of the chemical bonding structures at the geopolymer interface with aggregates. While in-situ FTIR shows the development of geopolymer gel over time, SR-FTIR results show clear differences in gel chemistry around the glass compared to the sand. The study reveals that there is a relatively higher degree of silica-rich gels close to the sand aggregates and a higher extent of geopolymeric gels around glass aggregates. The alkaline content of glass makes the area surrounding the glass aggregates more suitable for maintaining monomeric silica. These small silica species help to develop a higher degree of geopolymer networks in the vicinity of the glass aggregates compared to the sand. This level of visibility is very useful for understanding the behaviour of geopolymer concretes, which is critically important for nano-engineering of materials towards a desirable distribution of the binding gels.

Strong mineralization ability of strontium zinc silicate: Formation of a continuous biomorphic mineralized layer with enhanced osteogenic activity.

Silica gel plays an important role in the formation of some biomorphic minerals (e.g. silica-carbonates) with morphologically complex micro/nanostructures in a pure inorganic system. Herein, we demonstrate the potential of strontium zinc silicate (Sr2ZnSi2O7, SZnS) bioceramics as a biomorphic mineral "garden" due to its incongruent dissolution behavior. Briefly, the preferential release of Sr ions from SZnS leaves behind a silica-rich gel on the ceramic surface and leads to an alkaline pH in the localized area close to the silica-rich gel, providing a growth condition similar to that for the conventional synthesis of biomorphic minerals. Based on this unique characteristic of SZnS, a continuous and integrated carbonated calcium-phosphate mineralized layer was formed on 3D porous SZnS scaffolds with the purpose of enhancing scaffold's bioactivity. The mineralized layer not only provides numerous nanotopographic cues for guiding cell behavior, but also avoids burst ion release, thus overcoming side effects caused by the overdose of bioactive ions and the over-high pH. In vitro cell culture experiments and in vivo animal studies demonstrate that the scaffold with nanostructured mineralized layers promotes osteogenic differentiation of osteoblasts and induce more new bone tissues compared to the non-mineralized scaffold.

Alteration of synthetic basaltic glass in silica saturated conditions: Analogy with nuclear glass

This study investigates the analogy between basaltic and borosilicate glasses of nuclear interest, by focusing on mechanisms controlling glass dissolution under silica saturation conditions. These conditions are representative of a non- or slowly renewed contacting solution, favouring the formation of a potentially passivating silica rich gel layer and secondary phases. Laboratory batch experiments were performed with synthetic basaltic glass altered at 90 °C, at pH 7 in a saturated 29Si-doped aqueous solution for more than 600 days. Using elemental and isotopic solution analysis and solid characterizations by SEM, TEM and ToF-SIMS, we show that basaltic glass corrodes at an unexpectedly high and constant dissolution rate of 4 × 10−3 g m−2 d−1 associated with the absence of passivating gel. Our results highlight the fact that the dissolution rate is controlled by the hydrolysis of the glassy network, sustained by the precipitation of clay-type minerals and amorphous silica. When tested in similar conditions, the International Simple Glass (ISG), a six oxide borosilicate glasses of nuclear interest displays a much lower rate limited by water diffusion through a passivating layer. The different behavior of the two glasses is explained by their ability to form secondary crystalline phases at the expense of an amorphous passivating film.

Nanoporous silica gel structures and evolution from reactive force field-based molecular dynamics simulations

Nanoporous silica-rich gel formed on silicate glass surfaces during dissolution in aqueous environment is critical in elucidating the corrosion mechanisms and the long-term residual dissolution behaviors. Silica gel models were created using two types of methods with reactive force field-based molecular dynamics simulations. The results show that the remnant silica gels created from the ISG bulk structure have a more isolated and closed pore morphology and slightly higher glass network connectivity. This contrasts with the gel structures created by hydrogarnet defect formation that exhibit more connected pore morphologies. The remnant gel structures show lower water diffusivity which was explained by the nano-confinement effect of water molecules due to frequent interactions of water molecules with adjacent silica walls and the more isolated pore morphology in the remnant gel structures. These results reveal the complexity in terms of micro and atomic structures of these silica gels, and both structure features have impact on water transport in the gel layer hence the passivating effect that controls the long-term dissolution behavior of these glasses.

Use of Coal Fly Ash or Glass Pozzolan Addition as a Mitigation Tool for Alkali-Silica Reactivity in Cement Mortars Amended with Recycled Municipal Solid Waste Incinerator Bottom Ash

Municipal solid waste incinerator bottom ash was evaluated for its alkali aggregate reactivity in cement mortars. Subsequently, two wastes were evaluated as supplementary cementitious materials for their efficacy in mitigating expansive alkali-silica reactions in cement mortars amended with bottom ash. Three bottom ashes were incorporated into mortar bars as 15, 30 and 50% replacements of non-reactive sand. Control MSWI amended mortars were compared to those containing coal fly ash and glass pozzolan (synthetic glass). Dimensional expansion of the mortars was measured over 14 days to quantify the extent to which the recycled coal fly ash and glass pozzolan mitigated the alkali reactivity of the ash-amended mortar specimens. Average expansions of the ash-amended mortars ranged from 0.10 to 0.40%, suggesting that ash components reacted expansively in an alkaline environment. Microscopic analysis was used to complement the data and confirm the presence of silica-rich gel within glass fragments of the ash, as well as cracking of the aggregates. Supplementary cementitious materials (class F coal fly ash, glass pozzolan) mitigated expansion of the mortars by 90% on average, likely through consumption of the alkalis in the paste pore water.

Nanoscale Chemical Degradation Mechanisms of Sulfate Attack in Alkali-activated Slag

Chemically induced material degradation is a major durability issue facing many technologically important materials systems, including conventional and new sustainable cementitious materials. In this study, the nanoscale chemical degradation mechanisms have been elucidated for an amorphous sodium hydroxide-activated slag paste (one type of sustainable cement) exposed to different types of sulfate-bearing solutions (i.e., Na2SO4, MgSO4, and H2SO4), by combining synchrotron-based X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and X-ray pair distribution function (PDF) analysis. The XRD, FTIR, and PDF results show that the chemistry and structure of the paste is essentially immune to Na2SO4 attack, whereas exposure to 5–10 wt % MgSO4 and H2SO4 cause complete disintegration of the main binder gel (i.e., sodium-containing calcium–(alumino)–silicate–hydrate), along with formation of magnesium–silicate–hydrate or silica-rich gels and extensive precipitation of gypsum. These differences a...