Predicting the conditions for alkali-silica reaction (ASR) had been difficult for several decades due to the lack of in-depth knowledge of the ASR products. In this study, thermodynamic data for the ...synthesized ASR products (i.e., K-shlykovite, Na-shlykovite and ASR-P1) at 80 °C are determined. The effect of the initial Ca/Si ratio, from 0 to 0.5, on the formation of ASR products at 80 °C is investigated for the samples prepared with an initially fixed K/Si or Na/Si ratio of 0.5. The results show that the amount of ASR products formed first increases and then decreases with increasing the initial Ca/Si ratio. The reduced amount of ASR products at higher Ca/Si ratio is accompanied by formation of C-S-H, suggesting that a conversion of ASR products to C-S-H can occur at high Ca/Si ratio. The solid phases and aqueous chemistry predicted by thermodynamic modelling agrees very well with the experimental results.
This paper is the work of working group 3 of the RILEM Technical Committee on Hydration and Microstructure of Concrete with SCM (TC 238-SCM). The pore solution is an essential but often overlooked ...part of hydrated cements. The composition of the cement pore solution reflects the ongoing hydration processes and determines which solid phases are stable and may precipitate, and which phases are unstable and may dissolve. The study of the cement pore solution therefore contributes to the understanding of the mechanisms as well as of the kinetics of cement hydration. This paper reviews the impact of supplementary cementitious materials (SCMs) on the pore solution composition of blended cements. In a first part, the extraction and analysis methods of cement pore solutions are reviewed, leading to a set of practical guidelines and recommendations. In a second part, an extensive literature survey is used to document the effect of the addition of SCMs (blast furnace slag, fly ash and silica fume) on the pore solution. Finally, in a third part the collected literature data are compared to thermodynamic simulations. The performance and current limitations of thermodynamic modelling of blended cement hydration are demonstrated and discussed in view of future progress.
Both alkalis and calcium play essential roles in the formation of alkali-silica reaction (ASR) products. Investigation of their combined effect helps to better understand the conditions of ASR. In ...this study, samples with a constant Ca/Si ratio of 0.3 but different K(or Na)/Si and K/Na ratios have been synthesized at 80 °C. Experimental studies and thermodynamic modelling show that a sufficient amount of K or Na is essential to initiate ASR; at low alkali concentrations C-S-H is stabilized instead. However, too high alkaline concentrations (≥900 mM at K(or Na)/Si ≥ 1) also favor C-S-H formation and suppress ASR product formation. The results reveal a strong effect of the alkalis (K and/or Na) on calcium concentrations and on the formation of ASR products; a maximum ASR product formation is observed at Na or K concentrations between 200 and 500 mM and at initial Ca/Si ratio between 0.1 and 0.4.
The carbonation of portlandite, calcium silicate hydrate (C-S-H), and ettringite was investigated at 57% RH and 91% RH using X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, and ...the phenolphthalein spray test. The experiments show that the carbonation of portlandite, ettringite, and C-S-H with Ca/Si = 0.7 is significantly faster at 91% RH than at 57% RH. Little effect of RH is observed for C-S-H with higher Ca/Si. Portlandite and C-S-H with Ca/Si = 0.7 carbonate only partially at 57% RH; complete carbonation is observed if the relative humidity is increased to 91% RH. In contrast, the carbonation of C-S-H with Ca/Si = 1.2 and 1.5 is complete at both relative humidities. The carbonation rate of C-S-H decreases with decreasing Ca/Si ratio, both at 57% and 91%RH. Carbonation at 57% RH promotes the formation of vaterite and aragonite over calcite; the precipitation of amorphous calcium carbonate is observed for C-S-H with Ca/Si = 0.7.
The reaction mechanism of alkali-silica reaction (ASR) is poorly understood due to the difficulties to directly characterize ASR products in concrete. In this study, ASR products with initial Ca/Si ...of 0.25 and (K + Na)/Si ratio 0.5 with different K/Na ratios are synthesized at 80 °C. The synthesized ASR products are characterized by different techniques, also the solution chemistry is analyzed and saturation indices are calculated. The results show that crystalline and nano-crystalline phases are formed in the presence of both alkalis and calcium. No ASR product is present in the absence of calcium. All synthesized crystalline ASR products highly resembles the crystal structure of shlykovite, indicating that a substitution of K in shlykovite by Na can occur. Its silicate sheet structure has strong similarity to the ASR products formed in concrete according to Raman spectra, while some differences are observed in their morphologies and basal peak of the XRD patterns.
Three types of alkali-silica reaction (ASR) products, i.e., crystalline K-shlykovite, Na-shlykovite and nano-crystalline ASR-P1, have recently been synthesized at 80 °C. It is critical to verify the ...formation of these ASR products in concrete in order to transfer the knowledge of ASR gained from the synthesized products to the ones formed in field concrete. Therefore, ASR products formed in concrete exposed to KOH, NaOH or mixture of KOH/NaOH solution at 60 and 80 °C are analyzed using Raman spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy. The results show that at temperature between 60 and 80 °C K-shlykovite and ASR-P1 are formed in concrete exposed to KOH or a mixture of KOH/NaOH solution, whereas Na-shlykovite is formed in concrete exposed to NaOH solution. Moreover, K-shlykovite and Na-shlykovite do not co-precipitate in the same concrete, not even in the concrete exposed to the mixture of KOH/NaOH solution.
The binding of alkalis and calcium to calcium silicate hydrate (C-S-H) can be measured by cation exchange using cobalt hexamine solution with adapted pH. The effective cation exchange capacity (CEC) ...decreased at higher Ca/Si in C-S-H. A maximum of chargeexch/Si of 0.08–0.10 was measured at Ca/Si = 0.8 decreasing to 0.01–0.02 at Ca/Si = 1.6. At high Ca/Si, only a minor fraction of Ca was present at exchangeable sites, while most of the Ca2+ present in interlayer or surface was specifically bound and thus not exchangeable. In contrast, sodium was present only at exchangeable sites in the C-S-H, more at low than at high Ca/Si. An increase of pH in the presence of 100 mmol/L NaOH increased the effective CEC of C-S-H by 5–10% indicating a limited influence of pH on the deprotonation of the silanol groups.
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Alteration of widespread interfaces between cements and clays in geological time scales is essential to the safety assessment of radioactive waste repositories but not well understood partly due to ...the low reliability of thermodynamic data for zeolites. Here, we collected and full-scale characterized Ca-based zeolites with six types of frameworks that could possibly form in the interfaces. Besides, chemical interactions between degraded cementitious phases and rock forming minerals after equilibrating for ~6 years were investigated. By using the currently generated thermodynamic data of selected zeolites, chemical interactions in the cement/rock systems were studied, showing that no equilibrium was reached after 6 years and zeolites favored in thermodynamics, such as chabazite and gismondine rather than faujasite, can be expected to form at even longer timescale. The experimentally derived thermodynamic data was verified preliminarily by establishing consistent predominance diagrams of cement-zeolite-clay minerals, helping to predict zeolite stability domains in large timescales.
This study investigates the effect of wollastonite on the hydration and properties of magnesium potassium phosphate (MKP) cements. In MKP cements some efflorescence can occur; the presence of ...wollastonite suppresses efflorescence as the formation of Mg2KH(PO4)2·15H2O is prevented. The presence of wollastonite leads also to more heat (per g of MKP cement) due to the filler effect and due to the reaction of wollastonite, which increases strength, especially at low water-to-binder (w/b) ratio of 0.25 and at later ages, and lowers the pH values in the cement pore solution. The reaction of wollastonite does not lead to the formation of crystalline hydrates, both experimental and thermodynamic findings suggest the formation of amorphous hydroxyapatite and magnesium silicate hydrates (M-S-H).
The hydration kinetics, microstructure and pore solution composition of ternary slag-limestone cement have been investigated. Commercial CEM I 52.5 R was blended with slag and limestone; maintaining ...a clinker to SCM ratio of 50:50 with up to 20% slag replaced by limestone. The sulphate content was maintained at 3% in all composite systems. Hydration was followed by a combination of isothermal calorimetry, chemical shrinkage, scanning electron microscopy, and thermogravimetric analysis. The hydration of slag was also followed by SEM image analysis and the QXRD/PONKCS method. The accuracy of the calibrated PONKCS phase was assessed on slag and corundum mixes of varying ratios, at different water/solid ratios. Thus, the method was used to analyse hydrated cement without dehydrating the specimens. The results show that the presence of limestone enhanced both clinker and slag hydration. The pore volume and pore solution chemistry were further examined to clarify the synergistic effects. The nucleation effects account for enhanced clinker hydration while the space available for hydrate growth plus the lowering of the aluminium concentration in the pore solution led to the improved slag hydration.
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