Mineralogical characteristics influence the structure and pozzolanic reactivity of thermally and mechano-chemically activated meta-kaolinites

\ua9 2024 The Royal Society of Chemistry. Increasing early age reactivity of cement replacements is a barrier to reducing the embodied carbon of blended Portland cements. Mechano-chemical activation is an emerging alternative to conventional thermal activation for clays, which can accelerate early age reactivity. Knowledge gaps on the structure and reactivity of mechano-chemically activated kaolinitic clays include the influence of Fe-bearing phases and the mineralogical characteristics of kaolinites from different sources. This study evaluated the effectiveness of mechano-chemical vs. thermal activation for an Fe-rich clay containing disordered kaolinite and 24 wt% goethite, and a low-Fe clay containing highly ordered kaolinite. In the Fe-rich clay, mechano-chemical activation simultaneously caused dehydroxylation of kaolinite to form meta-kaolinite, and dehydration of goethite to form hematite. Agglomerates of intermixed meta-kaolinite and goethite/hematite nanoparticles were shown to have similar Al and Si environments after thermal or mechano-chemical activation (as determined by STEM-EDX, 27Al and 29Si MAS nuclear magnetic resonance and electron energy loss spectroscopy). Mechano-chemical activation enhanced early age (<12 hours) reactivity for both clays. Evaluating early age reactivity by unit mass of anhydrous meta-kaolinite explains how surface-adsorbed moisture results in underperformance of mechano-chemical activation at later ageing times. External surface area alone does not predict reactivity acceleration well - edge : basal surface area of meta-kaolinite is proposed as a more relevant factor that governs early age performance of mechano-chemically activated clays. The structure-property-performance relations of mechano-chemically activated meta-kaolinites are explained through interactions of kaolinites\u27 intrinsic mineralogical characteristics (i.e. initial particle size, aspect ratio, structural order) and extrinsic processing effects (i.e. intensive milling on structural order and physical characteristics)

Mineralogical characteristics influence the structure and pozzolanic reactivity of thermally and mechano-chemically activated meta-kaolinites

Increasing early age reactivity of cement replacements is a barrier to reducing the embodied carbon of blended Portland cements. Mechano-chemical activation is an emerging alternative to conventional thermal activation for clays, which can accelerate early age reactivity. Knowledge gaps on the structure and reactivity of mechano-chemically activated kaolinitic clays include the influence of Fe-bearing phases and the mineralogical characteristics of kaolinites from different sources. This study evaluated the effectiveness of mechano-chemical vs. thermal activation for an Fe-rich clay containing disordered kaolinite and 24 wt% goethite, and a low-Fe clay containing highly ordered kaolinite. In the Fe-rich clay, mechano-chemical activation simultaneously caused dehydroxylation of kaolinite to form meta-kaolinite, and dehydration of goethite to form hematite. Agglomerates of intermixed meta-kaolinite and goethite/hematite nanoparticles were shown to have similar Al and Si environments after thermal or mechano-chemical activation (as determined by STEM-EDX, 27Al and 29Si MAS nuclear magnetic resonance and electron energy loss spectroscopy). Mechano-chemical activation enhanced early age (<12 hours) reactivity for both clays. Evaluating early age reactivity by unit mass of anhydrous meta-kaolinite explains how surface-adsorbed moisture results in underperformance of mechano-chemical activation at later ageing times. External surface area alone does not predict reactivity acceleration well – edge : basal surface area of meta-kaolinite is proposed as a more relevant factor that governs early age performance of mechano-chemically activated clays. The structure–property–performance relations of mechano-chemically activated meta-kaolinites are explained through interactions of kaolinites' intrinsic mineralogical characteristics (i.e. initial particle size, aspect ratio, structural order) and extrinsic processing effects (i.e. intensive milling on structural order and physical characteristics)

Correlations among the mineralogical and physical properties of halloysite nanotubes (HNTs)

Twenty one samples of relatively pure tubular halloysites (HNTs) from localities in Australia, China, New Zealand, Scotland, Turkey and the USA have been investigated by X-ray diffraction (XRD), infrared spectroscopy (IR) and electron microscopy. The halloysites occur in cylindrical tubular forms with circular or elliptical cross sections and curved layers and also as prismatic tubular forms with polygonal cross sections and flat faces. Measurements of particle size indicate a range from 40 to 12,700 nm for tube lengths and from 20 to 600 nm for diameters. Size distributions are positively skewed with mean lengths ranging from 170 to 950 nm and mean diameters from 50 to 160 nm. Cylindrical tubes are systematically smaller than prismatic ones. Features related to order/ disorder in XRD patterns e.g. as measured by a 'cylindrical/prismatic' (CP) index and IR spectra as measured by an' OH-stretching band ratio' are related to the proportions of cylindrical vs. prismatic tubes and correlated with other physical measurements such as specific surface area and cation exchange capacity. The relationships of size to geometric form, along with evidence for the existence of the prismatic form in the hydrated state and the same 2M 1 stacking sequence irrespective of hydration state (i.e. 10 vs. 7 Å) or form, suggests that prismatic halloysites are the result of continued growth of cylindrical forms

Determining aluminium co-ordination of kaolinitic clays before and after calcination with electron energy loss spectroscopy

Developing a greater understanding of kaolinite dehydroxylation upon calcination is crucial for several industrial applications, including cements. Aluminium coordination in meta-kaolinite indicates the extent of its dehydroxylation and its potential chemical reactivity, and it is typically determined using 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. This technique however presents limitations for Fe-rich materials, given the magnetic properties of Fe ions and minerals containing Fe. In this study, the effect of calcination on Al coordination was assessed in a low-Fe clay used as a reference system, and a Fe-rich clay. Al coordination in the low-Fe clay was quantified via 27Al MAS NMR spectra deconvolution, using data collected at 9.4 T and 11.7 T. Energy dispersive X-ray spectroscopy (EDX) maps and electron energy loss spectroscopy (EELS) measurements were carried out in a scanning transmission electron microscope (STEM) on both clays. Al K-edge EEL spectra showed distinguishable 4/5-fold Al and 6-fold Al sites in both clay types. Differences in line-profile indicated a higher proportion of 4/5-fold Al in kaolinite in the Fe-rich clay compared to the low-Fe clay. Conversely, the Fe-rich clay contained a lower proportion of 4/5-fold Al in meta-kaolinite after calcination, relative to the low-Fe clay. These differences are consistent with the greater structural disorder of the meta-kaolinite identified in the Fe-rich clay by X-ray diffraction and the geological origins of both clays. Overall, this study demonstrates the potential of EELS to provide information about Al coordination for individual kaolinite and meta-kaolinite particles