New developments in low clinker cement paste mineralogy

The use of industrial waste as a cement addition often changes the composition and development of the hydrated phases and with them matrix performance and durability, in particular at later ages. The effect of the presence in blended cement of 20% to 50% kaolinite based activated carbon waste (ACW) on paste hydration has been characterized by means of XRD, SEM/EDX, TG/DTG, NMR and FTIR to identify and monitor the mineralogical phases forming in materials at ages of up to 180 d. The results showed that the main reaction products forming in the first 7 d included C-S-H gels, C4ACH12 and C4AH13 (hydroxy-AFm). Whilst monocarboaluminate (Mc) content declined with rising percentages of ACW, the amount of hexagonal phase hydroxy-AFm rose. Then microstructure of the C-S-H gels developing in the OPC and the 50% additioned paste differed. Compact C-S-H gel plates, and phyllosilicate-like laminar spongy microplates with high polymerised C-S-H gel formed in the blended cement paste.Peer ReviewedPostprint (author's final draft

Use of coal mining waste as pozzolanic material in new blended cement matrixes

Research and eco-innovation geared to obtain alternative sources of raw materials from waste constitute pathways for enhancing the competitiveness of resource-intensive industries. Cement and concrete manufacture calls for new sources of new, highly pozzolanic products to improve the mechanical properties and durability of the resulting matrices, while at the same time reducing production costs and environmental impact. Spanish coal mining wastes generated in the extraction and washing steps from a mine in the Castilla-León region were investigated. Mineralogically, these wastes are composed by kaolinite (20-30%), illite (45-70%) and quartz (5-15). This composition is very interesting in order to activate, by controlled thermal activation, the present kaolinite that generates metakaolin, a highly pozzolanic product. Morphological, textural and microstructural changes affect the activity and reactivity of activated wastes. These first studies open up a new research line, practically unknown to the international research community, and stand out the important economic and environmental benefits associated with the recycling of these wastes as supplementary cementing materials for future commercial blended cements.Peer ReviewedPostprint (published version

Coal mining waste: an alternative for the design of metakaolinite-based eco-efficient cements

Los materiales arcillosos de naturaleza caolinítica han despertado un gran interés en los últimos tiempos como materia prima para la producción de metacaolinita y su posterior uso como adición puzolánica al cemento. No obstante, la explotación de yacimientos de caolinita lleva asociada un importante impacto medioambiental y paisajístico, lo que ha impulsado la búsqueda de fuentes alternativas para su obtención. En este trabajo se propone la reutilización de residuos industriales procedentes de la minería del carbón para la producción de metacaolinita reciclada. De esta forma se mejoraría la sostenibilidad y el coste del proceso, fomentando al mismo tiempo el reciclaje de residuos y contribuyendo a la Economía Circular. Con este objetivo, se ha evaluado la aptitud del residuo como adición puzolánica en un sistema residuo/cal, encontrándose para el mismo una actividad puzolánica comparable a la mostrada por metacaolinitas comerciales. A continuación se ha estudiado el efecto de la adición del residuo sobre las propiedades físicas y mecánicas en pastas y morteros con sustituciones parciales de cemento del 20 y 50 %. Finalmente, se han investigado la resistencia de estos materiales frente a un ambiente de 10% de CO2.Postprint (published version

Coal mining waste: an alternative for the design of metakaolinite-based eco-efficient cements

Los materiales arcillosos de naturaleza caolinítica han despertado un gran interés en los últimos tiempos como materia prima para la producción de metacaolinita y su posterior uso como adición puzolánica al cemento. No obstante, la explotación de yacimientos de caolinita lleva asociada un importante impacto medioambiental y paisajístico, lo que ha impulsado la búsqueda de fuentes alternativas para su obtención. En este trabajo se propone la reutilización de residuos industriales procedentes de la minería del carbón para la producción de metacaolinita reciclada. De esta forma se mejoraría la sostenibilidad y el coste del proceso, fomentando al mismo tiempo el reciclaje de residuos y contribuyendo a la Economía Circular. Con este objetivo, se ha evaluado la aptitud del residuo como adición puzolánica en un sistema residuo/cal, encontrándose para el mismo una actividad puzolánica comparable a la mostrada por metacaolinitas comerciales. A continuación se ha estudiado el efecto de la adición del residuo sobre las propiedades físicas y mecánicas en pastas y morteros con sustituciones parciales de cemento del 20 y 50 %. Finalmente, se han investigado la resistencia de estos materiales frente a un ambiente de 10% de CO2

New developments in low clinker cement paste mineralogy

The use of industrial waste as a cement addition often changes the composition and development of the hydrated phases and with them matrix performance and durability, in particular at later ages. The effect of the presence in blended cement of 20% to 50% kaolinite based activated carbon waste (ACW) on paste hydration has been characterized by means of XRD, SEM/EDX, TG/DTG, NMR and FTIR to identify and monitor the mineralogical phases forming in materials at ages of up to 180 d. The results showed that the main reaction products forming in the first 7 d included C-S-H gels, C4ACH12 and C4AH13 (hydroxy-AFm). Whilst monocarboaluminate (Mc) content declined with rising percentages of ACW, the amount of hexagonal phase hydroxy-AFm rose. Then microstructure of the C-S-H gels developing in the OPC and the 50% additioned paste differed. Compact C-S-H gel plates, and phyllosilicate-like laminar spongy microplates with high polymerised C-S-H gel formed in the blended cement paste.Peer Reviewe

Influence of ZnO on the activation of kaolinite-based coal waste: pozzolanic activity and mineralogy in the pozzolan/lime system

One inconvenience presented by the thermal activation of kaolinite-based wastes is their low content of metakaolinite, a highly pozzolanic product listed in current standards for the manufacture of commercial cements. The addition of a chemical activator during the thermal activation process is a priority line of research to increase the reactivity of the recycled metakaolinite. In this paper, an additional chemical activator, ZnO, is studied and its effect on both pozzolanic properties and the evolution of mineralogical phases in the thermal activation of coal waste with a reaction time of up to 90¿days in the pozzolan/lime system. To do so, activation temperatures of between 550¿°C/650¿°C were selected and additions of chemical activator (ZnO) in percentages of between 0.0% and 3.0% by weight of coal waste, because it is an activator with a positive effect on a 100% natural kaolinite. The results showed that the incorporation of ZnO inhibited the reactivity of the recycled metakaolinite and in consequence, the capacity of the metakaolinite to react with the surrounding lime; even more so when the content of added chemical activator was raised, albeit with some exceptions, in the samples activated at 550¿°C and 650¿°C with 0.5% of chemical activator. In none of the cases under analysis was the chemical activator able to improve the properties of the metakaolinite in comparison with the properties of the reference sample activated only with temperature. The hydrated phases that appeared in the pozzolanic reaction were tetracalcium aluminate hydrate, stratlingite, monosulfoaluminate hydrate and LDH (phyllosilicate/carbonate).Peer Reviewe

Cements based on kaolinite waste

The cement industry involves high-energy consumption that generates high CO2 emissions into the atmosphere. Environmental concerns can be addressed by replacing parts of Portland cement clinkers with pozzolanic materials in mortars and concrete. Slag, fly ash and silica fume are materials considered for the planned replacement. Research studies on clay minerals, such as kaolinite, are being followed with special attention by the scientific community and the cement industry. It is well known that these minerals require an activation process to transform kaolinite (K) into metakaolinite (MK). MK is an amorphous material from the transformation of K with high pozzolanic activity, which is its capacity to react with the portlandite released during the hydration of Portland cement, generating compounds such as C–S–H gels and some aluminum-phase hydrates. One of the MK production methods is heat treatment controlled by kaolinite at temperatures in the range of 600–900°C. Different residues have been used (coal mining, paper sludge and waste from a drinking water treatment plant) activated at 600°C for 2h to elaborate blended cements. Due to their good behaviour as future eco-efficient additions, this research is a study by x-ray fluorescence (XRF), x-ray diffraction (XRD) and scanning electron microscopy (SEM) of their influence on the performances of blended cement mixtures (binary and ternary one), with substitutions of pozzolan ratio at 28 days of hydration. The porosity of pozzolanic cements decreases because of the formation of hydrated phases during pozzolanic reaction.Peer Reviewe

Carbonation-induced mineralogical changes in coal mining waste blended cement pastes and their influence on mechanical and microporosity properties

The worldwide pursuit of new eco-efficient pozzolans is ongoing. Kaolinite-based waste is an eco-friendly source of recycled metakaolinite, a highly pozzolanic product. In this study, a blended cement paste containing 20% activated coal waste (ACW) was exposed to a 100% CO2 atmosphere at 65% RH for 7 days. The variations in its phase composition and strength were studied and compared to an OPC control. Both pastes were cured for 28 days prior to the carbonation test. Reaction kinetics were assessed using XRD, SEM/EDX, TG/DTG, FT-IR, Micro-Raman spectroscopy, pore solution pH and the cumulative carbonated fraction. The blended cement carbonated 68% faster than the control. While portlandite carbonation was the main reaction in both cements, decalcification was also observed (more intensely in the 20% ACW paste) in other hydraulic calcium phases (C-S-H gel, monocarboaluminate (C4AcH12), ettringite and tetracalcium aluminate (C4AH13). The end product of this reaction was calcium carbonate, mainly in the form of calcite, although traces of aragonite and amorphous carbonate were also detected. Compressive strength values rose with accelerated carbonation time and pore size reduction in both cement pastesPeer Reviewe

Mineralogical study of calcined coal waste in a pozzolan/Ca(OH)(2) system

Activated carbon mining waste influences the formation of hydrated phases, their saturation indexes and mineral stability fields during pozzolanic reactions. The behavior of these reactions is predicted in this study by examining the influence of carbon waste at 600 degrees C over 2 h, by means of a thermodynamic model running on a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations (PHREEQC program: pH-REdox-Equilibrium in Program C). Experimental analysis of the solid phases shows early formation of monosulfoaluminate hydrate, C-S-H phases and subsequent precipitation of laminar minerals, tetracalcium aluminate hydrate, layered double hydroxide compounds (phyllosilicate/carbonate) (LDH) and stratlingite. Monosulfoaluminate hydrate was formed on day one of the reaction for samples with activated coal mining waste. The thermodynamic calculations confirmed the experimental observations.Peer ReviewedPostprint (published version