Mechanical activation of clays as an alternative to thermal activation for ternary blended cement production

Abstract

[eng] The construction industry significantly contributes to climate change and global warming, especially due to the environmental footprint of Portland cement (PC) production, which is the second most widely used material worldwide. PC manufacture involves CO2 emissions from the limestone reaction during calcination and the fossil fuel burning to reach the high temperatures needed to produce clinker, the main component of PC. Although several strategies are being implemented to decarbonise the PC industry, the most promising short-term solution is partially replacing clinker with supplementary cementitious materials (SCMs). SCMs can replace clinker in blended cement, improving its long-term properties. However, SCMs can only replace clinker to a limited extent without affecting cement performance. Nevertheless, combining two or more SCMs to produce ternary blended cement allows for higher clinker replacements. One of the most promising ternary blended cement is limestone calcined clay cement (LC3), which uses widely available SCMs like clay and limestone, thereby reducing the environmental impact and cost of production and enabling local production of LC3. However, clays require a prior activation process to enhance their reactivity, typically achieved by calcining them in industrial rotary kilns that burn fossil fuels and emit a significant amount of CO2 during the thermal activation process. In contrast, mechanical activation is an electrically driven milling process that induces the amorphisation of clay minerals by breaking the chemical bonds within the crystalline structure, improving their reactivity. This process has the potential to reduce the carbon footprint associated with clay activation. The main goal of this PhD thesis is to explore the potential of using mechanically activated clay in ternary blended cements, aligning with the need for sustainable and affordable solutions to decarbonise the cement and construction industries. This thesis is presented as a compendium of articles, comprising a total of seven papers published, under review, or submitted to high-impact scientific journals. These scientific papers provide detailed descriptions of the results obtained from the mechanical activation of clays and the formulation of alternative ternary blended cements, consistently comparing them with those of LC3 containing metakaolin. In this regard, the first step of this thesis involved evaluating the current status of LC3 technology through a bibliometric analysis. The analysis confirmed the potential of LC3 to reduce the carbon footprint of the cement industry, especially in developing countries needing affordable and locally produced materials. The experimental work initially focused on evaluating the effect of mechanical activation on kaolinitic and muscovitic clays, which were selected due to their high and limited reactivity after thermal activation, respectively. The research revealed that the clays were successfully activated under optimal milling conditions, leading to increased reactivity and changes in particle size and morphology. The reactivity of mechanically activated kaolinitic clay was comparable to that of thermally activated kaolinitic clay, while the reactivity of mechanically activated muscovitic clay showed superior reactivity compared to its thermally activated counterpart. The promising results validated the potential of using mechanically activated clays as SCM or alternative cementitious materials precursors. Following the activation process evaluation, mechanically activated clays were introduced into ternary blended cements, replacing thermally activated clays in LC3. The resulting ternary blended cements exhibited properties comparable to traditional LC3 containing thermally activated kaolinitic clay and superior to those of LC3 containing thermally activated muscovitic clay. Furthermore, the incorporation of mechanically activated clay improved the workability of alternative ternary blended cements and induced an acceleration of hydration kinetics, enhancing early age performance. This PhD thesis outlines the potential of mechanical activation as an alternative method for producing activated clay SCMs, contributing to the reduction of the environmental impact associated with ternary blended cements.

[cat] La indústria de la construcció és una de les indústries que més negativament contribueix al canvi climàtic. Una de les principals estratègies per reduir el seu impacte ambiental és substituir parcialment el clínquer en el ciment per addicions que tinguin una menor petjada de carboni. El ciment ternari LC3 incorpora argiles calcinades i pedra calcària, reduint el cost de producció i l’impacte ambiental del ciment. Les argiles han de ser activades prèviament per incrementar la seva reactivitat, normalment mitjançant un tractament tèrmic. Una possible alternativa és l’activació mecànica, que augmenta la reactivitat de les argiles mitjançant un procés de molturació altament energètic que en trenca l’estructura. La implementació d’aquest procés podria reduir la petjada de carboni deguda a l’activació d’argiles. L’objectiu principal d’aquesta tesi doctoral és investigar la incorporació d’argiles activades mecànicament en ciments ternaris cercant alternatives per retallar les emissions de CO2 de la producció de ciment. El primer pas d’aquesta tesi doctoral ha estat la realització d’un estudi bibliomètric per tal d’avaluar l’estat de la investigació sobre els LC3, confirmant el potencial d’aquests ciments per reduir la petjada de carboni de la indústria de la construcció. La part experimental s’ha centrat en l’estudi de l’efecte de l’activació mecànica en argiles caolinítiques i moscovítiques. El procés de molturació en unes condicions òptimes causa l’amorfització de les argiles i genera canvis en la mida i morfologia de les seves partícules, augmentant la reactivitat del material. Aquests resultats confirmen el potencial de les argiles activades mecànicament per ser utilitzades com a addicions o bé com a precursors de ciments alternatius. Després d’avaluar la seva reactivitat, les argiles activades mecànicament s’han incorporat als ciments ternaris substituint les argiles activades tèrmicament. Els ciments ternaris formulats amb argiles activades mecànicament presenten propietats comparables o superiors a les dels ciments LC3 formulats amb argiles activades tèrmicament. A més, l’addició d’argiles activades mecànicament al ciment millora les seves propietats a curt termini. En conclusió, aquesta tesi doctoral emfatitza el potencial de l’activació mecànica com a mètode alternatiu d’activació d’argiles, fet que pot contribuir a reduir l’impacte ambiental del ciment i la indústria de la construcció.