Influence of limestone on the hydration of ternary slag cement

The hydration kinetics, microstructure and pore solution composition of ternary slag-limestone cements 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 followed by the implementation of QXRD/PONKCS method. The accuracy of the calibrated PONKCS phase was assessed on slag and corundum mixes of varying ratios, at different w/s ratios. Thus, the method was used to analyse hydrated cements 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 to the synergistic effects. The nucleation effects account for enhanced clinker hydration while the space available for hydrate growth plus lowering of the aluminium concentration in the pore solution led to the improved slag hydration

Reactive Limestone as a Strategy Towards Low-Clinker Factor Cements

Ordinary portland cement production accounts for at least 5% of global CO2 emissions. As the principal component in concrete formulations in addition to the expansion, modernization and retrofitting of infrastructure in developed and developing countries, the share of greenhouse emissions is certain to expand. This environmental burden has brought questions with regard to cement as a sustainable material in the coming decades. The use of low-clinker factor cements has been thought as a means to a reduced CO2 footprint material, one way of achieving this is through the use of secondary cementitious materials (SCM’s) with pozzolanic properties (i.e., fly ash, furnace slag, silica fume, metakaolin, etc…), these materials have been widely used to reduce cement fractions while maintaining, sometimes even improving, mechanical properties of binders. However, availability of SCM’s, particularly a local scales, as well as durability related problems (i.e, carbonation, reinforcement corrosion, etc…) have hindered the increase of SCM’s use in cement. An attractive alternative is the use of limestone (calcite, CaCO3) as part of the binder formulation with or without SCM’s, while part of cement production as raw material and also present in commercially available cements, limestone fractions remain limited to 15% in the United States due to detrimental effects on durability and mechanical properties mainly due to dilution. This research work investigates the possibility of including limestone as a reactive part of the contentious binder. Rendering limestone as a reactive agent in hydration reactions can potentially induces a large increase in limestone replacement of OPC leading to concrete formulations with reduced cement content while keeping competitive mechanical properties.Factors such as physical effects (i.e., filer effect), as well as chemical effects are analyzed in this research work while making emphasis in conditions that can increase or limit limestone reactivity in cementitious formulations, such as: quantity and/or characteristics of aluminous material present in the system, water-to-cement ratio, characteristics of the carbonate bearing source and ion exchange conditions for calcium carboaluminate mineral formation

Modelo configuración borde urbano (Cerros Orientales-Chapinero).

El presente proyecto va enfocado a la restauración urbana y forestal del sector de Pardo Rubio en la localidad de chapinero en Bogotá, como propuesta de configuración de borde para la conservación de los cerros orientales de Bogotá. El eje conductor del diseño es la integración del verde a la ciudad, el cual se enfocó en una intervención progresiva que incluye el diagnostico de la zona, la reubicación de los núcleos poblaciones, el desarrollo de una franja verde recreativa pasiva, el sendero ecológico como limite físico y el centro de investigación forestal “CIF”; proponiendo un modelo replicable para todo el límite urbano de los Cerros Orientales, con el fin de mejorar el estado físico del territorio y evitar el crecimiento ocupacional de la población residente. Por otra parte, este modelo propone educar por medio de un diseño urbano que fomente el cuidado del medio ambiente y tenga la capacidad de brindar seguridad, orden, cultura, turismo, educación y sobre todo, mejor calidad de vida para los que allí habiten. Finalmente, como punto activador y controlador del modelo de borde, se diseñó el proyecto puntual de un Centro de Investigación Forestal que tiene como intención principal convertirse en un hito arquitectónico no solo para la zona, si no la ciudad en general. De esta forma, el remate de este eje propiamente universitario (Calle 45) tendrá una doble funcionalidad: el encuentro de lo académico y lo ambiental donde se fomentará la investigación y el control de la biodiversidad

The Influence of Metakaolin on Limestone Reactivity in Cementitious Materials

International audienceRecent studies have demonstrated that in the presence of limestone (CaCO3), carbonate-AFm phases (i.e., hemi-and/or mono-carboaluminate) may be stabilized at the expense of sulfate-AFm, which is more commonly found in cement systems. This suggests that enhancing AFm phase formation may be a novel way of incorporating increased quantities of limestone as a reactive component in cement-based systems. Often, in an ordinary portland cement (OPC), the quantity of the AFm hydrates formed is limited by the availability of aluminum. Therefore, as means of enhancing AFm phase formation, this paper evaluates metakaolin addition to determine how it affects limestone reactions and carbonate-AFm formation in the OPC systems. The results of a multi-method study including: X-ray diffraction with Rietveld refinement (QXRD), strength measurements, thermogravimetric analysis, and thermodynamic calculations are used to quantify solid phase constitutions, and the extent of limestone that has been consumed in reaction. Obtained results suggest that pozzolanic reactions which occur when metakaolin is used as an aluminous source are observed to be beneficial in offsetting the dilutive effects of OPC replacement noted in blended cement formulations

A Comparison of Intergrinding and Blending Limestone on Reaction and Strength Evolution in Cementitious Materials

The use of powdered limestone is a promising approach to reduce the clinker factor of portland cements. Recent regulatory actions in the United States and Canada have allowed for portland cements to contain up to 15% limestone (mass basis). This action allows for the replacement of cement by limestone through: (1) intergrinding the cement clinker and limestone through the production process or (2) by blending the cement and limestone through the concrete batching process. While both avenues appear feasible, there is a need to compare and contrast the performance features of one approach versus the other. This paper evaluates cement pastes containing interground and blended limestone in terms of their hydration and strength evolution behavior. Experiments and numerical simulations performed within a boundary nucleation and growth (BNG) model indicate that the reaction response of interground cements can be achieved or exceeded by blended systems, depending on the characteristics of the cement and the limestone used, i.e., Type I/II, Type III or blend of Type I/II and Type III. Thus, by adjusting the cement or limestone fineness, blended systems can be proportioned to display strengths which are superior to the interground case at early ages. However, by later ages all binders show similar strengths. The results do suggest that for replacement levels up to 15% (mass-basis), intergrinding or blending are both viable strategies to reduce the clinker factors of portland cements, while maintaining early-age properties similar to pure cement formulations

The Influence of Filler Type and Surface Area on the Hydration Rates of Calcium Aluminate Cement

The addition of finely pulverized materials such as limestone and quartz has been observed to increase the reaction rates of ordinary portland cement. This study describes the effects of mineral fillers including: limestone, dolomite and quartz on the hydration rates of calcium aluminate cements. By detailed analysis of isothermal calorimetry data and application of a phase boundary nucleation and growth model, it is shown that finely ground minerals enhance reaction rates in relation to: (1) their surface area, which increases with increasing proportion of CAC replacement by a fine mineral filler, and (2) the increase in the amount of water available for CAC hydration, i.e., dilution, that occurs as the CAC is replaced by a filler. Unlike in the case of OPC, CAC hydration rates, and enhancements therein in the presence of fillers are independent of the mineral type and are only a function of available surface area

Elucidating the Role of the Aluminous Source on Limestone Reactivity in Cementitious Materials

International audienceWhen limestone (CaCO3) is present in ordinary portland cement (OPC), carbonate‐AFm phases (i.e., hemi‐ and/or mono‐carboaluminate) are stabilized at the expense of the sulfate‐AFm, which is more commonly found in cement systems. In OPC, the quantity of AFm hydrates formed is often limited by the availability of aluminum. Therefore, as a means of enhancing AFm phase formation, this study elucidates the role of aluminous sources including: calcium aluminate cements, metakaolin, and a hydratable alumina to determine if their addition would enhance limestone reactions and carbonate‐AFm formation in cement systems. The results of a detailed study including: X‐ray diffraction, strength measurements, thermogravimetric analysis, and thermodynamic calculations are used to quantify solid phase constitutions, and the extent of limestone reacted. The results suggest that, the amount of limestone reacted and the specific carbonate‐AFm formed is sensitive to both, the nature of the aluminous source and limestone content. Pozzolanic reactions which occur when metakaolin is used as an aluminous source are noted to be especially beneficial in offsetting the effects of OPC replacement. It is noted that although the different aluminous materials react with different quantities of CaCO3 during hydration, enhanced carbonate‐AFm formation alone is insufficient to ensure strength equivalence, when OPC is replaced by limestone

The Influence of Slightly and Highly Soluble Carbonate Salts on Phase Relations in Hydrated Calcium Aluminate Cements

The addition of slightly (CaCO3) and highly soluble (Na2CO3) carbonate salts is expected to favor the formation of carboaluminate phases in hydrated calcium aluminate cements (CACs). A multi-method approach including X-ray diffraction, thermogravimetric analysis, and thermodynamic calculations is applied to highlight that the conversion phenomena in CACs cannot be mitigated by the formation of carboaluminate phases (monocarboaluminate: Mc and hemicarboaluminate: Hc) which are anticipated to form following the addition of carbonate salts. Here, carboaluminate phase formation is shown to depend on three factors: (1) water availability, (2) carbonate content of the salts, and their ability to mobilize CO32- species in solution, and (3) lime content associated with the carbonate salt. The latter two factors are linked to the composition and solubility of the carbonate agent. It is concluded that limestone (CaCO3), despite being a source of calcium and carbonate species, contributes only slightly to carboaluminate phase formation due to its low solubility and slow dissolution rate. Soluble carbonate salts (Na2CO3) fail to boost carboaluminate phase formation as the availability of Ca2+ ions and water are limiting. Detailed thermodynamic calculations are used to elucidate conditions that affect the formation of carboaluminate phases