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

Examination of alkali-activated material nanostructure during thermal treatment

The key nanostructural changes occurring in a series of alkali-activated materials (AAM) based on blends of slag and fly ash precursors during exposure to temperatures up to 1000 °C are investigated. The main reaction product in each AAM is a crosslinked sodium- and aluminium-substituted calcium silicate hydrate (C-(N)-A-S-H)-type gel. Increased alkali content promotes the formation of an additional sodium aluminosilicate hydrate (N-A-S-(H)) gel reaction product due to the structural limitations on Al substitution within the C-(N)-A-S-H gel. Heating each AAM to 1000 °C results in the crystallisation of the disordered gels and formation of sodalite, nepheline and wollastonite. Increased formation of N-A-S-(H) reduces binder structural water content after thermal treatment and correlates closely with previous observations of improved strength retention and reduced microcracking in these AAM after heating to 1000 °C. This provides new insight into thermally induced changes to gel atomic structure and thermal durability of C-(N)-A-S-H/N-A-S-H gel blends which are fundamental for the development of new fire-resistant construction materials

Application of the Rietveld method to the analysis of anhydrous cement

X-ray powder diffraction allows direct measurement of the phase content in cement. More recently, whole pattern approaches such as the Rietveld method show an improvement in both within (repeatability) and between laboratory (reproducibility) precision. The aim of this paper is to discuss the influence of the different parameters involved in the Rietveld method and review the most recent quantitative X-ray powder diffraction studies on anhydrous cement. Comparisons with Bogue calculations, scanning electron microscopy and nuclear magnetic resonance are also discussed. (C) 2010 Elsevier Ltd. All rights reserved

Microstructure of recycled concrete

International audienceInterfacial Transition Zone (ITZ) between old and new cement paste in recycled mortars and concretes is characterized. The microstructure is investigated during the first phase of hardening (2 and 28 days) for mortars and up to 1 year for concretes. Mortars and concretes made with over-saturated fine and coarse recycled aggregates develop more porous ITZ with lower anhydrous profiles. Transport of water from the over-saturated recycled aggregates to the cement paste, and water lens formation by micro-bleeding effect, are expected to explain this phenomenon. On the other hand, mortars made with dried fine recycled aggregates develop “denser” ITZ with anhydrous profile similar to that observed on mortars made with dried natural fine aggregates with the same targeted average W/C ratio. Transport of portlandite from the new cement paste to the old cement paste is observed during the first 28 days of hardening. This phenomenon is stronger for mortars made with over-saturated fine aggregates. The reduction of W/C ratio to obtain C25, C35 and C45 recycled concretes improves slightly the porosity profiles. But concretes based on 100% of recycled coarse aggregates have higher porous ITZ than concretes based on 30% of recycled fine and coarse aggregates. The low porosity of the ITZ of the C45 concrete based on 30% of fine and coarse recycled aggregates could be the consequence of a positive curing effect, with a contribution of the recycled aggregate water to cement hydration because of the partial desaturation during the hardening of the cement paste with a low initial water cement ratio (W/C close to 0.41). A treshold value for paste porosity which separates porous networks wealkly interconnected (low increase of permability with porosity) and porous network very interconnected (high increase of permeability with porosity) has been identified. The value is closed to 17%–18% of porosity

Durability of Hardened Portland Cement Paste used for Oilwell Cementing

Durability of materials used for the completion of oilwell is of utmost importance for oil and gas industry. We carried out ageing tests on a hardened cement paste in two types of fluid by varying the experimental procedure. We show that the observed alterations are highly dependent on the way of conducting the tests. To correctly assess the long-term durability of cement-based materials, it is necessary to renew periodically the ageing fluid. By doing so, a severe impairment of the macroscopic properties of an hardened cement paste aged in a monthly-replaced brine can be observed

Characterization of 3D Printing Mortars Made with OPC/CSA Mixes

International audienc

Characterization and modeling of major constituent equilibrium chemistry of a blended cement mortar

Cementitious materials containing ground granulated iron blast furnace slag and coal combustion fly ash as admixtures are being used extensively for nuclear waste containment applications. Whereas the solid phases of ordinary Portland cement (OPC) have been studied in great detail, the chemistry of cement, fly ash and slag blends has received relatively less study. Given that OPC is generally more reactive than slag and fly ash, the mineralogy of OPC provides a logical starting point for describing the major constituent chemistry of blended cement mortars. To this end, a blended cement mortar containing Portland cement, granulated blast furnace slag, fly ash and quartz sand was modeled using a set of solid phases known to form in hydrated OPC with the geochemical speciation solver LeachXS/ORCHESTRA. Comparison of modeling results to the experimentally determined pH-dependent batch leaching concentrations (USEPA Method 1313) indicates that major constituent concentrations are described reasonably well with the Portland cement mineral set; however, modeled and measured aluminum concentrations differ greatly. Scanning electron microscopic analysis of the mortar reveals the presence of Al-rich phyllosilicate minerals heretofore unreported in similar cementitious blends: kaolinite and potassic phyllosilicates similar in composition to illite and muscovite. Whereas the potassic phyllosilicates are present in the quartz sand aggregate, the formation of kaolinite appears to be authigenic. The inclusion of kaolinite in speciation modeling provides a substantially improved description of the release of Al and therefore, suggests that the behavior of phyllosilicate phases may be important for predicting long-term physico-chemical behavior of such systems