NEA TDBIV project : preparation of a state-of-the-art report on thermodynamic data for cement

The program of work of the fourth phase of the OECD NEA Thermochemical Database Project (TDB-IV) contemplates a line of activity on the preparation of a state of the art report on cements. The present work aims at presenting the project, its aims and its limits

Determination of the degree of reaction of fly ash in blended cement pastes

This paper gives a review over methods to determine the degree of reaction for supplementary cementitious materials (SCMs) with focus on Portland cement - fly ash blends only and summarizes and highlights the most important findings which are detailed in a parallel paper published in Materials and Structures. Determination of the extent of the reaction of SCMs in mixtures is complicated for several reasons: (1) the physical presence of SCMs affects the rate and extent of the reaction of the ground clinker component – the so called “filler effect”; (2) SCMs are usually amorphous with complex and varied mineralogy which make them difficult to quantify by many classical techniques such as X-ray diffraction; (3) the rate of reaction of SCMs in a cement blend may be quite different from its rate of reaction in systems containing simply alkali or lime. From this review it is clear that measuring the degree of reaction of SCMs remains challenging. Nevertheless progress has been made in recent years to offer alternatives to the traditional selective dissolution methods. Unfortunately some of these – image analysis and EDS mapping in the scanning electron microscope, and NMR - depend on access to expensive equipment and are time consuming. With regard to fly ashes, NMR seems to be reliable but limited to fly ash with low iron content. New methods with quantitative EDS mapping to segment fly ash particles from the hydrated matrix and to follow the reaction of glass groups of disparate composition separately look very promising, but time consuming. Sources with a high proportion of fine particles will have higher errors due to lower limit of resolution (1-2 μm). Whereas for SCMs which react relatively fast (e.g. slag, calcined clay) the methods based on calorimetry and chemical shrinkage seem promising on a comparative basis, the very low reaction degree of fly ashes before 28 days means that the calorimetry method is not practical. There is a lack of data to assess the usefulness of long term chemical shrinkage measurements. The possibility to quantify the amorphous phase by XRD is promising as this is a widely available and rapid technique which can at the same time give a wealth of additional information on the phases formed. However, the different reaction rates of different glasses in compositionally heterogeneous fly ashes will need to be accounted for and may strongly reduce the accuracy of the profile decomposition method. This paper is the work of working group 2 of the RILEM TC 238-SCM “Hydration and microstructure of concrete with supplementary cementitious materials”

Synthesis of Giorgiosite [Mg5(CO3)4(OH)2·5–6H2O], further light on a new hydrated magnesium carbonate for MgO-based cement

Abstract Giorgiosite is a relatively unknown hydrated magnesium carbonate (HMC) without a clear understanding of its characteristics and synthesis pathway. The phase has a nano-wire morphology with high surface area, and hence, attracts immediate interests for various applications including as early-strength-giving phase in HMC-based binder. However, there had been no clear and successful pathway in the past to synthesize the phase. The present work addresses this research gap and reports an effective protocol to obtain giorgiosite in high purity. We found that giorgiosite can be synthesized via the conversion of pure nesquehonite [MgCO3·3H2O] in a 1M Mg-acetate solution at 50 °C. In contrast, nesquehonite converted to dypingite [Mg5(CO3)4(OH)2·5H2O] in the absence of acetate. Here, the characteristics of giorgiosite as determined by XRD, TGA/FTIR, SEM, and Raman spectroscopy are reported. The better understanding of the characteristics of giorgiosite will contribute to the development of HMC-based binders, which have the potential to be a carbon-negative construction material. Further work is needed to shed light on the conversion pathway in the presence of organic ligand (e.g., acetate) and to determine the thermodynamic properties of giorgiosite.Abstract Giorgiosite is a relatively unknown hydrated magnesium carbonate (HMC) without a clear understanding of its characteristics and synthesis pathway. The phase has a nano-wire morphology with high surface area, and hence, attracts immediate interests for various applications including as early-strength-giving phase in HMC-based binder. However, there had been no clear and successful pathway in the past to synthesize the phase. The present work addresses this research gap and reports an effective protocol to obtain giorgiosite in high purity. We found that giorgiosite can be synthesized via the conversion of pure nesquehonite [MgCO3·3H2O] in a 1M Mg-acetate solution at 50 °C. In contrast, nesquehonite converted to dypingite [Mg5(CO3)4(OH)2·5H2O] in the absence of acetate. Here, the characteristics of giorgiosite as determined by XRD, TGA/FTIR, SEM, and Raman spectroscopy are reported. The better understanding of the characteristics of giorgiosite will contribute to the development of HMC-based binders, which have the potential to be a carbon-negative construction material. Further work is needed to shed light on the conversion pathway in the presence of organic ligand (e.g., acetate) and to determine the thermodynamic properties of giorgiosite

Formation and stability of magnesium silicate hydrate and hydromagnesite

Abstract The effect of carbonate on the magnesium silicate hydrate (M-S-H) formation was studied at high Mg/Si molar ratio of 1.5. M-S-H pastes were synthesized from silica fume and MgO or MgO plus hydromagnesite in a Na 2 CO3 containing solution. X-ray diffraction data and thermogravimetric analysis indicated that brucite is destabilized and M-S-H phases formed much faster in the presence of carbonates. Additionally, in the system containing hydromagnesite, the hydromagnesite reacted to form M-S-H. In a third experiment, the carbonation of M-S-H with Mg/Si=1.5 in a suspension was investigated. While a reference suspension of M-S-H with Mg/Si=1.5 kept under inert atmosphere still contained brucite and a pH about 10.1, the forced carbonation of M-S-H decreased the pH to 7.3 and destabilized the brucite. No evidence of the formation of crystalline or amorphous hydrated magnesium (hydroxy)carbonate phases was observed.Abstract The effect of carbonate on the magnesium silicate hydrate (M-S-H) formation was studied at high Mg/Si molar ratio of 1.5. M-S-H pastes were synthesized from silica fume and MgO or MgO plus hydromagnesite in a Na 2 CO3 containing solution. X-ray diffraction data and thermogravimetric analysis indicated that brucite is destabilized and M-S-H phases formed much faster in the presence of carbonates. Additionally, in the system containing hydromagnesite, the hydromagnesite reacted to form M-S-H. In a third experiment, the carbonation of M-S-H with Mg/Si=1.5 in a suspension was investigated. While a reference suspension of M-S-H with Mg/Si=1.5 kept under inert atmosphere still contained brucite and a pH about 10.1, the forced carbonation of M-S-H decreased the pH to 7.3 and destabilized the brucite. No evidence of the formation of crystalline or amorphous hydrated magnesium (hydroxy)carbonate phases was observed

Characteristics of Copper-based Oxygen Carriers Supported on Calcium Aluminates for Chemical-Looping Combustion with Oxygen Uncoupling (CLOU)

Eight different oxygen carriers (OC) containing CuO (60 wt %) and different mass ratios of CaO to Al2O3 as the support were synthesized by wet-mixing followed by calcination at 1000 °C. The method of synthesis used involved the formation of calcium aluminum hydrate phases and ensured homogeneous mixing of the Ca2+ and Al3+ ions in the support at the molecular level. The performance of the OCs for up to 100 cycles of reduction and oxidation was evaluated in both a thermogravimetric analyzer (TGA) and a fluidized bed reactor, covering a temperature range of 800 to 950 °C. In these cycling experiments, complete conversion of the OC, from CuO to Cu and vice versa, was always achieved for all OCs. The reactivity of the materials was so high that no deactivation could be observed in the TGA, owing to mass transfer limitations. It was found that OCs prepared with a mass ratio of CaO to Al2O3 in the support >0.55 agglomerated in the fluidized bed, resulting in an apparent deactivation over 25 cycles for all temperatures investigated. High ratios of mass of CaO to Al2O3 in the support resulted in CuO interacting with CaO, forming mixed oxides that have low melting temperatures, and this explains the tendency of these materials to agglomerate. This behavior was not observed when the mass ratio of CaO to Al2O3 in the support was ≤0.55 and such materials showed excellent cyclic stability operating under redox conditions at temperatures as high as 950 °C.The authors thank Mohammad Ismail and Matthew Dunstan for helping with the XRD analysis and Alex Casabuena-Rodriguez and for helping with the SEM. This work was supported by the Engineering and Physical Sciences Research Council (EPSRC grant EP/I010912/1).This is the final version of the article. It first appeared from ACS via http://dx.doi.org/10.1021/acs.iecr.5b0117

Influence of slag composition on the stability of steel in alkali-activated cementitious materials

Among the minor elements found in metallurgical slags, sulfur and manganese can potentially influence the corrosion process of steel embedded in alkali-activated slag cements, as both are redox-sensitive. Particularly, it is possible that these could significantly influence the corrosion process of the steel. Two types of alkali-activated slag mortars were prepared in this study: 100% blast furnace slag and a modified slag blend (90% blast furnace slag? 10% silicomanganese slag), both activated with sodium silicate. These mortars were designed with the aim of determining the influence of varying the redox potential on the stability of steel passivation under exposure to alkaline and alkaline chloride-rich solutions. Both types of mortars presented highly negative corrosion potentials and high current density values in the presence of chloride. The steel bars extracted from mortar samples after exposure do not show evident pits or corrosion product layers, indicating that the presence of sulfides reduces the redox potential of the pore solution of slag mortars, but enables the steel to remain in an apparently passive state. The presence of a high amount of MnO in the slag does not significantly affect the corrosion process of steel under the conditions tested. Mass transport through the mortar to the metal is impeded with increasing exposure time; this is associated with refinement of the pore network as the slag continued to react while the samples were immersed

Magnesia-Based Cements: A Journey of 150 Years, and Cements for the Future?

This review examines the detailed chemical insights that have been generated through 150 years of work worldwide on magnesium-based inorganic cements, with a focus on both scientific and patent literature. Magnesium carbonate, phosphate, silicate-hydrate, and oxysalt (both chloride and sulfate) cements are all assessed. Many such cements are ideally suited to specialist applications in precast construction, road repair, and other fields including nuclear waste immobilization. The majority of MgO-based cements are more costly to produce than Portland cement because of the relatively high cost of reactive sources of MgO and do not have a sufficiently high internal pH to passivate mild steel reinforcing bars. This precludes MgO-based cements from providing a large-scale replacement for Portland cement in the production of steel-reinforced concretes for civil engineering applications, despite the potential for CO2 emissions reductions offered by some such systems. Nonetheless, in uses that do not require steel reinforcement, and in locations where the MgO can be sourced at a competitive price, a detailed understanding of these systems enables their specification, design, and selection as advanced engineering materials with a strongly defined chemical basis

Generalized Structural Description of Calcium–Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model

Structural models for the primary strength and durability-giving reaction product in modern cements, a calcium (alumino)silicate hydrate gel, have previously been based solely on non-cross-linked tobermorite structures. However, recent experimental studies of laboratory-synthesized and alkali-activated slag (AAS) binders have indicated that the calcium–sodium aluminosilicate hydrate [C-(N)-A-S-H] gel formed in these systems can be significantly cross-linked. Here, we propose a model that describes the C-(N)-A-S-H gel as a mixture of cross-linked and non-cross-linked tobermorite-based structures (the cross-linked substituted tobermorite model, CSTM), which can more appropriately describe the spectroscopic and density information available for this material. Analysis of the phase assemblage and Al coordination environments of AAS binders shows that it is not possible to fully account for the chemistry of AAS by use of the assumption that all of the tetrahedral Al is present in a tobermorite-type C-(N)-A-S-H gel, due to the structural constraints of the gel. Application of the CSTM can for the first time reconcile this information, indicating the presence of an additional activation product that contains highly connected four-coordinated silicate and aluminate species. The CSTM therefore provides a more advanced description of the chemistry and structure of calcium–sodium aluminosilicate gel structures than that previously established in the literature

Influence of the synergy between mineral additions and Portland cement in the physical-mechanical properties of ternary binders

The paper deals with the synergistic effect of mineral additions on the physical-mechanical performance of ternary blends prepared with different Portland cements (PC). The effect in setting and heat flow release is also analyzed. The mineral additions used are blast furnace slag (BFS), fly ash (FA) and limestone filler (LF). PCs with different C3A and alkali content have been tested to study the synergy in ternary blends. Ternary binders with PC low in C3A and alkali content achieve similar mechanical strength gain as plain PC and refinement of pore size distribution from early hydration ages due to the acceleration of PC hydration induced by the mineral additions. In contrast, ternary binders with PC higher in C3A and alkali content have a delayed in mechanical strength at early hydration ages, but significantly higher at long hydration times