Alkali-activated slag cements produced with a blended sodium carbonate/sodium silicate activator

An alkali-activated slag cement produced with a blend of sodium carbonate/sodium silicate activator was characterised. This binder hardened within 12 h and achieved a compressive strength of 20 MPa after 24 h of curing under ambient conditions, which is associated with the formation of an aluminium substituted calcium silicate hydrate as the main reaction product. Carbonates including pirssonite, vaterite, aragonite and calcite were identified along with the zeolites hydroxysodalite and analcime at early times of reaction. The partial substitution of sodium carbonate by sodium silicate reduces the concentration of carbonate ions in the pore solution, increasing the alkalinity of the system compared with a solely carbonate-activated paste, accelerating the kinetics of reaction and supplying additional silicate species to react with the calcium dissolving from the slag as the reaction proceeds. These results demonstrate that this blend of activators can be used effectively for the production of high-strength alkali-activated slag cements, with a microstructure comparable to what has been identified in aged sodium-carbonate-activated slag cements but without the extended setting time reaction usually identified when using this salt as an alkali activator

Alkali Activated MaterialsState-of-the-Art Report, RILEM TC 224-AAM /

XIV, 388 p. 91 illus.online resource

The dependence of sorbed copper and nickel cyanide speciation on ion exchange resin type.

The present study investigates the influence of functional group structure and resin matrix on the speciation of copper and nickel cyanides sorbed onto two commercially available ion exchange resins. Batch experiments were performed using synthetic copper and nickel solutions containing 50 and 200 mgrL free cyanide, respectively. Despite the presence of CuŽCN.32y and CuŽCN.43y in solution, it has been found using Raman spectroscopy that the Imac HP555s resin, which has a polystyrene–divinylbenzene matrix, loads predominantly the CuŽCN.32y complex. In contrast, the polyacrylic resin, Amberlite IRA958, sorbed significant amounts of both CuŽCN.32y and CuŽCN.43y. It has been found that the speciation of nickel cyanide sorbed onto each resin was the same. A recently developed mathematical model based on statistical thermodynamic principles has been used as a tool to understand further the equilibrium sorption of copper and nickel cyanide complexes onto each resin studied. A higher sorption energy for nickel compared to copper has been observed for the sorption onto Imac HP555s. In contrast, the sorption energy for copper was found to be higher than for nickel for the polyacrylic resin, Amberlite IRA958. The values of the model parameters obtained were correlated with the chemical features of each complex in solution as well as sorbed onto the resins

Microstructural changes in alkali activated fly ash/slag geopolymers with sulfate exposure

Sulfate attack is recognized as a significant threat to many concrete structures, and often takes place in soil or marine environments. However, the understanding of the behavior of alkali-activated and geopolymer materials in sulfate-rich environments is limited. Therefore, the aimof this study is to investigate the performance of alkali silicate-activated fly ash/slag geopolymer binders subjected to different forms of sulfate exposure, specifically, immersion in 5 wt% magnesium sulfate or 5 wt% sodium sulfate solutions, for 3 months. Extensive physical deterioration of the pastes is observed during immersion inMgSO4 solution, but not in Na2SO4 solution. Calcium sulfate dihydrate (gypsum) forms in pastes immersed in MgSO4, and its expansive effects are identified as being particularly damaging to the material, but it is not observed in Na2SO4 environments.A lowerwater/binder (w/b) ratio leads to a greatly enhanced resistance to degradation by sulfate attack. Infrared spectroscopy shows some significant changes in the silicate gel bonding environment of geopolymers immersed in MgSO4, attributed mostly to decalcification processes, but less changes upon exposure to sodium sulfate. It appears that the process of ‘sulfate attack’ on geopolymer binders is strongly dependent on the cation accompanying the sulfate, and it is suggested that a distinction should be drawn between ‘magnesium sulfate attack’ (where both Mg2? and SO4 2- are capable of inducing damage in the structure), and general processes related to the presence of sulfate accompanied by other, non-damaging cations. The alkali-activated fly ash/slag binders tested here are susceptible to the first of these modes of attack, but not the second

Drying-induced changes in the structure of alkali-activated pastes

Drying of cement paste, mortar, or concrete specimens is usually required as a pre-conditioning step prior to the determination of permeability-related properties according to standard testing methods. The reaction process, and consequently the structure, of an alkali-activated slag or slag/fly ash blend geopolymer binder differs from that of Portland cement, and therefore there is little understanding of the effects of conventional drying methods (as applied to Portland cements) on the structure of the geopolymer binders. Here, oven drying (60 �C), acetone treatment, and desiccator/vacuum drying are applied to sodium silicateactivated slag and slag/fly ash geopolymer pastes after 40 days of curing. Structural characterization via X-ray diffraction, infrared spectroscopy, thermogravimetry, and nitrogen sorption shows that the acetone treatment best preserves the microstructure of the samples, while oven drying modifies the structure of the binding gels, especially in alkali-activated slag paste where it notably changes the pore structure of the binder. This suggests that the pre-conditioned drying of alkali activation-based materials strongly affects their microstructural properties, providing potentially misleading permeability and durability parameters for these materials when pre-conditioned specimens are used during standardized testing

Effects of different polycarboxylate ether structures on the rheology of alkali-activated slag binders

Polycarboxylate ether (PCE) admixtures are generally used in concrete at relatively low concentrations, enabling the reduction of mixture cost and enhanced flow properties due to reduction of cement content and significant enhancement of rheology, respectively. However, typical PCE polymer structures that are used in Portland cement have little or no effect on alkali activated slag (AAS) binder rheology due to ineffective consumption of polymer by a number of mechanisms, including degradation of the polymer chains within the high alkaline environments present in AAS systems. In this study, a range of PCEs with long and moderate PolyEthyleneGlycol (PEG) side chain lengths, and with high and low molecular weights (M), are examined. Co-polymers containing a higher density of backbone charges, as is typical for a Portland cement superplasticiser, increase the yield stress of alkali-activated slag. A co-polymer with longer side chains and lower M show a yield stress reduction, indicating a mild increase in workability compared to an unmodified AAS paste. It is suggested that in the high ionic strength environment of an AAS binder, a more charged polymer is consumed through interactions with other ions and charged particles, which can bring an increase in yield stress and plastic viscosity of AAS

Implication of microstructure and durability measures for commercial development of alkali-activated concretes

Abstract not available

New Structural Model of Hydrous Sodium Aluminosilicate Gels and the Role of Charge-Balancing Extra-Framework Al

A new structural model of hydrous alkali aluminosilicate gel (N-A-S-H) frameworks is proposed, in which charge-balancing extra-framework Al species are observed in N-A-S-H gels for the first time. This model describes the key nanostructural features of these gels, which are identified through the application of ¹⁷O, ²³Na, and ²⁷Al triple quantum magic angle spinning solid-state nuclear magnetic resonance spectroscopy to synthetic ¹⁷O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate gel predominantly comprises Q⁴(4Al), Q⁴(3Al), Q⁴(2Al), and Q⁴(1Al) Si units charge-balanced by Na⁺ ions that are coordinated by either 3 or 4 framework oxygen atoms. A significant proportion of Al³⁺ in tetrahedral coordination exist in sites of lower symmetry, where some of the charge-balancing capacity is provided by extra-framework Al species which have not previously been observed in these materials. The mean Si^IV–O–Al^IV bond angles for each type of Al^IV environments are highly consistent, with compositional changes dictating the relative proportions of individual Al^IV species but not altering the local structure of each individual Al^IV site. This model provides a more advanced description of the chemistry and structure of alkali aluminosilicate gels and is crucial in understanding and controlling the molecular interactions governing gel formation, mechanical properties, and durability