X-ray Total Scattering Study of Phases Formed from Cement Phases Carbonation

Carbonation in cement binders has to be thoroughly understood because it affects phase assemblage, binder microstructure and durability performance of concretes. This is still not the case as the reaction products can be crystalline, nanocrystalline and amorphous. The characterisation of the last two types of components are quite challenging. Here, carbonation reactions have been studied in alite-, belite- and ye’elimite-containing pastes, in controlled conditions (3% CO2 and RH = 65%). Pair distribution function (PDF) jointly with Rietveld and thermal analyses have been applied to prove that ettringite decomposed to yield crystalline aragonite, bassanite and nano-gibbsite without any formation of amorphous calcium carbonate. The particle size of gibbsite under these conditions was found to be larger (~5 nm) than that coming from the direct hydration of ye’elimite with anhydrite (~3 nm). Moreover, the carbonation of mixtures of C-S-H gel and portlandite, from alite and belite hydration, led to the formation of the three crystalline CaCO3 polymorphs (calcite, aragonite and vaterite), amorphous silica gel and amorphous calcium carbonate. In addition to their PDF profiles, the thermal analyses traces are thoroughly analysed and discussed.This research was funded by Ministry of Science (Spain), grant number PID2019-104378RJI00, and Junta de Andalucía (Spain), grant number P18-RT-720

Belite calcium sulfoaluminate cement early hydration: citric acid sensitivity.

Three buckets, corresponding to three selected samples, of the same type of belite calcium sulfoaluminate (BCSA) cements have been studied. These cements show very similar elemental and mineralogical compositions, and textural properties. Mortars, for mechanical strength characterisation at 3 and 24 hours, were prepared by two different methodologies: i) with w/c of 0.40 without citric acid and ii) with w/c=0.40 and adding 0.375 wt% by weight of cement (bwc) of citric acid used as a retarder. On the one hand, the mechanical strengths, at 24 hours, obtained by the three mortars prepared by both methodologies are almost coincident, 61(5) MPa without citric acid and 61(4) MPa for the mortars prepared with citric acid. On the other hand, the early mechanical strengths at 3 hours for the mortars prepared without citric acid are also almost coincident among the three cements, i.e. ~49(3) MPa. However, when the citric acid is added, the mechanical strengths at 3 hours are quite different among the three buckets, i.e. 34(1), 42(1) and 48(1) MPa, respectively. The main aim of this study is to understand the reasons causing the different early mechanical strengths. First of all, a full characterization of the three cements has been performed: textural properties, elemental composition by X-Ray Fluorescence (XRF) and mineralogical composition, including the amorphous content, by Laboratory X-Ray Powder Diffraction (LXRPD) combined with the Rietveld method. Moreover, the soluble sulphate contents within the first minutes of hydration are being determined. An isothermal calorimetry study at 20ºC for pastes without and with different amounts of citric acid will be presented as well as in situ X-Ray powder diffraction data analysis. The results will be discussed to give a picture of the early hydration behaviour of these BCSA cements.CTS cement corporation, CSA research Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Synchrotron Pair Distribution Function analyses of Ye’elimite-based pastes

The study of nanocrystalline phase atomic ordering is intrinsically complicated. The presence of additional crystalline phase(s) exacerbates this challenge. Here, we use the synchrotron pair distribution function (PDF) to characterize the local atomic order of the nanocrystalline phases in ye’elimite-containing pastes. Here, a multi r-range analysis approach has been used, where the 30–50 Å r-range allows determining the crystalline phase contents and the 1.6-35 Å is used to characterize the atomic ordering in the nanocrystalline components. Quantitative phase contents have been also derived from these analyses. Specifically, we have prepared five stoichiometric ye’elimite pastes with variable anhydrite and water contents. For the pastes without anhydrite addition, ettringite was not present and two calcium aluminium monosulfate phases were required for good PDF fits. In addition to crystalline AFm (c =28.6 Å), a nanocrystalline AFm phase with a significantly smaller c-unit cell parameter value, c =26.8 Å, was required having about 60 Å of particle size. For a paste obtained with 31 wt% of anhydrite addition, the main crystalline phase was ettringite with the aluminium hydroxide having nano-gibbsite local structure with a quite small average particle size, =35 ÅThis work has been supported by Spanish MINECO through BIA2014-57658-C2 and BIA2017-82391-R, which are co-funded by FEDER. PDF experiment was performed at ID15A beamline of ESR

Effect of alkanolamines in kaolinitic calcined clays pozzolanic reactivity.

Five kaolinitic clays with different amounts of kaolinite, ranging between ~70 wt% and ~30 wt%, have been studied to unravel the possible activation effect of alkanolamines on the aluminate fraction of calcined clays. This is of interest for enhancing the reactivity of LC3 binders. These clays were calcinated at 860ºC for 4 h and ground to particle sizes of DV,50 ~10 μm. Three alkanolamines were selected: triisopropanolamine (TIPA), triethanolamine (TEA) and methyldiethanolamine (MDEOA), added in two dosages, 0.025 and 0.050 wt% (by weight of calcined clay, bwcc). The role of alkanolamines as activators in calcined clays was assessed following the ASTM c1897 standard bases on R3-tests. Concretely, first by measuring the heat evolved due to the pozzolanic reaction of the calcined clay and Ca(OH)2 (i.e. R3 mixture) by isothermal calorimetry at 40ºC during 7 days, and second, the bounded water by measuring the weight loss of R3 mixture after heating them at 350ºC.This study concludes that there is no significant activation of the pozzolanic activity of kaolinitic calcined clays just by adding alkanolamines. However, a mild activation was observed, i.e., higher heat evolved up to 7 days, by adding 0.05 wt% bwcc of TIPA, TEA and MDEOA to a high kaolinite content disordered metakaolin with high specific surface area calcined clay.PID2020-114650RB-I00 CTS cement corporation Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Effect of boron and water-to-cement ratio on the performances of laboratory prepared Belite-Ye'elimite-Ferrite (BYF) cements

The effect of superplasticiser, borax and the water-to-cement ratio on BYF hydration and mechanical strengths has been studied. Two laboratory-scale BYF cements—st-BYF (with -C2S and orthorhombic C4A3 S) and borax-activated B-BYF (with a’H-C2S and pseudo-cubic C4A3s) have been used, and both show similar particle size distribution. The addition of superplasticiser and externally added borax to BYF pastes has been optimised through rheological measurements. Optimised superplasticiser contents (0.3, 0.4 and 0.1 wt%for st-BYF, B-BYF and st-BYF with externally added 0.25 wt % B2O3, respectively) result in low viscosities yielding homogeneous mortars. The calorimetric study revealed that st-BYF is more reactive than B-BYF, as the values of heat released are 300–370 J/g and 190–210 J/g, respectively, after 7 days of hydration; this fact is independent of the water-to-cement ratio. These findings agree with the higher degree of hydration at 28 days of b -C2S in st-BYF (from 45 to 60%) than a’H-C2S in B-BYF (~20 to 30%). The phase assemblage evolution has been determined by LXRPD coupled with the Rietveld method and MAS-NMR. The formation of stratlingite is favoured by increasing the w/c ratio in both systems. Finally, the optimisation of fresh BYF pastes jointly with the reduction of water-to-cement ratio to 0.40 have allowed the achieving of mortars with compressive strengths over 40 MPa at 7 days in all systems. Moreover, the st-BYF mortar, where borax was externally added, achieved more than 70 MPa after 28 days. The main conclusion of this work does not support Lafarge’s approach of adding boron/borax to the raw meal of BYF cements. This procedure stabilises the alpha belite polymorph, but its reactivity, in these systems, is lower and the associated mechanical strengths poorer.UMA18-FEDERJA-095 y P18-RT-720, ambos cofinanciados FEDE

Multiscale understanding of tricalcium silicate hydration reactions

Tricalcium silicate, the main constituent of Portland cement, hydrates to produce crystalline calcium hydroxide and calcium-silicate-hydrates (C-S-H) nanocrystalline gel. This hydration reaction is poorly understood at the nanoscale. The understanding of atomic arrangement in nanocrystalline phases is intrinsically complicated and this challenge is exacerbated by the presence of additional crystalline phase(s). Here, we use calorimetry and synchrotron X-ray powder diffraction to quantitatively follow tricalcium silicate hydration process: i) its dissolution, ii) portlandite crystallization and iii) C-S-H gel precipitation. Chiefly, synchrotron pair distribution function (PDF) allows to identify a defective clinotobermorite, Ca11Si9O28(OH)2.8.5H2O, as the nanocrystalline component of C-S-H. Furthermore, PDF analysis also indicates that C-S-H gel contains monolayer calcium hydroxide which is stretched as recently predicted by first principles calculations. These outcomes, plus additional laboratory characterization, yielded a multiscale picture for C-S-H nanocomposite gel which explains the observed densities and Ca/Si atomic ratios at the nano- and meso- scales.This work has been supported by Spanish MINECO through BIA2014-57658-C2-2-R, which is co-funded by FEDER, BIA2014-57658-C2-1-R and I3 (IEDI-2016-0079) grants. We also thank CELLS-ALBA (Barcelona, Spain) for providing synchrotron beam time at BL04-MSPD beamline

Mix and measure - Combining in situ X-ray powder diffraction and microtomography for accurate hydrating cement studies.

It is reported an innovative methodology based on in situ MoKα1 laboratory X-ray powder diffraction (LXRPD) and microtomography (μCT) avoiding any sample conditioning. The pastes are injected in 2.0 mm capillaries and the extremes are just sealed. The measurements take place in the same region of the hydrating paste. Thick capillaries are key to avoiding self-desiccation, which dictates the need of high-energy X-ray radiation for the diffraction study. This approach has been tested with a PC 42.5 R paste having w/c = 0.50. μCT data were collected at 12 h and 1, 3, 7 and 79 days. LXRPD data were acquired at 1, 3, 7 and 77 days. In this proof-ofprinciple research, the same paste was also cured ex situ. Portlandite contents obtained by thermal analysis, ex situ powder diffraction, in situ mass balance calculation and in situ powder diffraction were 13.8, 13.1, 13.1 and 12.5 wt%, respectively. From the μCT study, the grey value histogram evolution with time showed a crossing point which allowed us to distinguish (appearing) hydrated products from (dissolving) unhydrated cement particles. Segmentations were carried out by global thresholding and the random forest approach (one type of supervised Machine Learning). The comparison of the segmented results for the unhydrated cement fraction and the Rietveld quantitative phase analysis outputs gave an agreement of 2 %. The potential of this methodology to deal with more complex binders is also presented.This research was partly supported by the research grant PID2020- 114650RB-I00 of Agencia Estatal de Investigacion which is co-funded by ERDF

Activation of LC3 binders by C-S-H nucleation seeding with a new tailored admixture for low-carbon cements.

The use of supplementary cementitious materials is currently the most favorable strategy for reducing CO2 emissions in cements. Limestone Calcined Clay Cements, LC3, are a type of cement that allows the reduction of CO2 emissions up to 40%. The proportions of the mixtures can vary, but the most investigated combination, LC3-50, contains about 50 wt% clinker, 30 wt% calcined kaolinitic clay, 15 wt% limestone and an optimised calcium sulphate content. However, the mechanical strengths of LC3 at early ages are not good enough and they should be improved. One way of doing this is by employing commercial strength-enhancing (accelerator) admixtures based on C-S-H nucleation seeding. For this work, LC3-50 cements were prepared with clays with varying kaolinite contents. Mortars and pastes were fabricated using a new PCE-based superplasticizer developed to avoid the loss of fluidity at early ages typical of LC3 binders. The selected accelerator for this study was Master X-Seed STE53. The results show that the loss of fluidity of LC3 mortars during the first hours could be solved by a recently developed PCE-based superplasticizer. The compressive strengths at 1 day for LC3 mortars strikingly improved by using the C-S-H seeding admixture and this behavior was maintained for up to 28 days.Partial funding from PID2020-114650RB-I00 research grant, which is co-funded by ERDF, is gratefully acknowledged

Activation of LC3 binders by C-S-H nucleation seeding with a new tailored admixture for low-carbon cements.

The use of supplementary cementitious materials is currently the most favorable strategy for reducing CO2 emissions in cements. Limestone Calcined Clay Cements, LC3, are a type of cement that allows the reduction of CO2 emissions up to 40%. The proportions of the mixtures can vary, but the most investigated combination, LC3-50, contains about 50 wt% clinker, 30 wt% calcined kaolinitic clay, 15 wt% limestone and an optimised calcium sulphate content. However, the mechanical strengths of LC3 at early ages are not good enough and they should be improved. One way of doing this is by employing commercial strength-enhancing (accelerator) admixtures based on C-S-H nucleation seeding. For this work, LC3-50 cements were prepared with clays with varying kaolinite contents. Mortars and pastes were fabricated using a new PCE-based superplasticizer developed to avoid the loss of fluidity at early ages typical of LC3 binders. The selected accelerator for this study was Master X-Seed STE53. The results show that the loss of fluidity of LC3 mortars during the first hours could be solved by a recently developed PCE-based superplasticizer. The compressive strengths at 1 day for LC3 mortars strikingly improved by using the C-S-H seeding admixture and this behavior was maintained for up to 28 days.Partial funding from PID2020-114650RB-I00 research grant, which is co-funded by ERDF, is gratefully acknowledged. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Pozzolanic materials to reduce CO2 emissions: local solutions for a global issue.

Abstract de una keynote invitada.In recent decades, the cement sector has been looking for solutions to reduce the carbon footprint, being one of the most promising strategies, the replacement of clinker with supplementary cementitious materials, SCMs. However, the main limitation of this approach is the availability of suitable SCMs. This work presents the study of three families of pozzolanic materials, Spanish calcined clays (CC), Natural Pozzolans (NP) and fly ashes (FA). The characterization of each family will be presented, with emphasis on the kaolinite content of the original clays and the amorphous contents of the natural pozzolans and fly ashes. The results of the pozzolanic prediction tests will be compared: strength activity index, SAI, and R3 test according to ASTM C1897-20. The SAI test has two important limitations: i) it gives false positives at 28 days, as does the addition of quartz (Qz) and ii) a minimum of 28 days is required to obtain the pozzolanic activity results. In addition, the R3 test has proved to be useful in ruling out inert additions, such as quartz. Moreover, it presents a very good correlation between the heat emitted and the combined water at 7 days and the amount of kaolinite in clays or amorphous in ashes. However, the absolute values of heat and combined water cannot be compared between different families. That is, in the calcined clay family, it can be inferred that if the kaolinite content is higher than 50 wt%, the heat released should be between 500-700 J/g, whereas a fly ash with an amorphous content of around 70 wt% will release between 200-250 J/g.PID2020-114650RB-I00 grant from Spanish government, Master Builders Solutions Deutschland GmbH (Germany) and Buzzi Unicem SpA (Italy) are thanked for the funding. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech