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

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

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

Processing and hydration activation of limestone calcined clay belite rich cements.

Belite-rich limestone calcined clay cements, BR-LC3, could be an alternative for low carbon binders with potentially very good durability properties, given the high amount of C-S-H gel from the cement hydration with additional C-(A)-S-H from the pozzolanic reaction. Nevertheless, BR-LC3 phase hydration rates at early ages are slow and they must be enhanced, for instance by using C-S-H nucleation seeding admixtures. In this work, a BR-LC3 binder was prepared using a clinker-activated Belite-rich cement, BC (58 wt%), kaolinitic calcined clay (26 wt%), limestone (13 wt%) and gypsum (3 wt%). Pastes were prepared with a water-to-binder (w/b) ratio of 0.40 and superplasticizer. Mortars were prepared with the w/b=0.40 and having a target slump self-flow of 210±20 mm. Paste hydration characterization was carried out by thermal analysis, Rietveld quantitative phase analysis and mercury intrusion porosimetry. The compressive strengths of the mortars were also determined. Remarkable compressive strength improvements at 7 and 28 days are shown by using a C-S-H seeding admixture. The improvement of mechanical strengths is not related to belite phase hydration acceleration but mainly to lower porosity.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Supplementary cementitious material based on calcined montmorillonite standards

The most effective approach to mitigating the environmental impact of cement production is through the uti- lisation of pozzolanic materials to prepare blended cement. Calcined kaolinitic clays are one of the most promising materials to be used as supplementary cementitious materials (SCM) due to their final performances. However, there are locations where kaolinitic rocks are not available. This work presents the study of two dioctahedral smectite standards from the Clay Minerals Society (CMS), SWy-3 and SAu-1, to be used as refer- ences. After thermal activation optimisation, their pozzolanic performances are compared with those of low- grade kaolinitic clay. On the one hand, raw SWy-3 has an amorphous content of ~80 wt%, considered mont- morillonite, with quartz and feldspars. The optimum calcination temperature was 800ºC, at which the smectite is fully dehydroxylated (as determined by thermal analysis) but not completely amorphized (as shown by powder diffraction). After calcination (800ºC), with a median particle size by volume (Dv,50) of ~11 μm, ~480 J/g were released at 7 days in the R3 test. On the other hand, SAu-1 has a higher amount of amorphous content, ~96 wt%, which includes interstratified illite/smectite. The optimum calcination temperature was found to be 750ºC. This sample (Dv,50 ~10 μm) gave 452 J/g at 7 days in the R3 test. The optimised active smectites gave a pozzolanic activity comparable to clay with 30–35 % kaolinite, as determined, for the first time, by the R3 test. This work has established descriptors for the activation of natural smectites/bentonites and reference values for the resulting smectite-based SCMs.Funding for open access charge: Universidad de Málaga / CBU

Advanced 3D nanoimaging of cement paste hydration using synchrotron Xray ptychographic tomography

Hydration of Portland cements (PC) is a complex phenomenon caused by the presence of multiple components with multi-length scale variability. To investigate this complicated process, state-of-theart 3D nanoimaging techniques with excellent spatial resolution and the ability to scan large fields of view are required. In this work, synchrotron X-ray near-field ptychographic tomography is used to study four cement pastes within 0.2 mm thick capillaries. Synchrotron ptychotomography produces electron density and absorption coefficient tomograms that facilitate the characterisation of amorphous materials. By achieving a spatial resolution of around 220 nm with good contrast, nanofeatures such as the density and spatial distribution of amorphous constituents can be identified. Key findings include the revelation that a seeded C-S-H gel has a lower density than a CaCl2 accelerated PC paste. In addition, the density of the C-A-S-H gel in a PC blend containing metakaolin and limestone decreases when limestone is present. These density variations are critical in estimating the space filling in the paste. C-S-H nucleation seeding results in a reduced inner/outer C-S-H gel ratio, with different average sizes of high-density inner C-S-H gel found under different hydration conditions. Various additives, including nucleation seeding admixtures and CaCl2, alter the water/cement ratio and the overall density of the C-S-H gel. The addition of alkanolamine admixture increases C4AF hydration, resulting in the development of amorphous Al-rich iron-silicate hydrogarnet, which affects C4AF dissolution. The addition of limestone to the PC-MK blend changes the electron density of the C-A-S-H gel and increases the production of amorphous monocarboaluminate-like components, which influences porosity refinement.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

4D nanoimaging of early age cement hydration.

Despite a century of research, our understanding of cement dissolution and precipitation processes at early ages is very limited. This is due to the lack of methods that can image these processes with enough spatial resolution, contrast and field of view. Here, we adapt near-field ptychographic nanotomography to in situ visualise the hydration of commercial Portland cement in a record-thick capillary. At 19 h, porous C-S-H gel shell, thickness of 500 nm, covers every alite grain enclosing a water gap. The spatial dissolution rate of small alite grains in the acceleration period, ∼100 nm/h, is approximately four times faster than that of large alite grains in the deceleration stage, ∼25 nm/h. Etch-pit development has also been mapped out. This work is complemented by laboratory and synchrotron microtomographies, allowing to measure the particle size distributionswith time. 4D nanoimagingwill allow mechanistically study dissolution-precipitation processes including the roles of accelerators and superplasticizers.Financial support from PID2019-104378RJ-I00 research grant, which is co-funded by FEDER, is gratefully acknowledged. ToScA (United Kingdom) is gratefully acknowledged for awarding Jim Elliott Award to Shiva Shirani, I.R.S. is thankful for funding from PTA2019-017513–I

X-ray near-field ptychographic nanoimaging of cement pastes

The hydration processes of Portland cements (PC) and blends are complicated as there are many components with great heterogeneity at different length scales. Thus, 3D nanoimaging techniques with high spatial resolution and scanning large fields of view are needed. Here, synchrotron X-ray near-field ptychographic tomography is used to investigate four pastes within 0.2 mm thick capillaries: PC with CaCl2, PC hydration enhanced by C-S-H nucleation seeding, PC partly substituted with metakaolin, and PC partly substituted with metakaolin and limestone. Data analysis emphasis has been placed on the characterization of amorphous components: (i) C-S-H and C-A-S-H gels; (ii) iron aluminium siliceous hydrogarnets; (iii) metakaolin; and (iv) aluminium carboaluminate, AFm-like. Synchrotron ptychotomography yields electron density and absorption coefficient tomograms and the resulting bivariate plots are instrumental for characterising these amorphous components. The attained spatial resolution, ∼220 nm, with very good contrast allowed us to determine nanofeatures including mass densities and spatial distributions of amorphous components. For instance, the C-S-H gel mass density differences between the two type of accelerated pastes are detailed.Funding for open access charge: Universidad de Málaga / CBUA. This research has been partly supported by the research grant PID2019-104378RJ-I00 which is co-funded by ERDF. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland for provision of synchrotron radiation beamtime at cSAXS beamline of the SLS, proposal number 20210147

Accuracy in cement hydration investigations: Combined X-ray microtomography and powder diffraction analyses

Cements are multi-phase materials that can be well understood using a multi-technique approach. Furthermore, the hydration of cement pastes is a very complex set of processes. In this work, we focus on the accuracy of the results of the laboratory computed tomography (µCT) analysis by quantifying three sets of components based on their attenuations: porosity, hydration products (HP) and unhydrated cement phases (UHP), and comparing with laboratory X-ray powder diffraction data (LXRPD).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

In situ synchrotron powder diffraction study of LC3 cement activation at very early ages by C-S-H nucleation seeding.

Limestone Calcined Clay Cements, LC3, are being widely researched as low-carbon binders. However, the hydration reactions during the first day are key for their performances and they were not well known. Here, we employ in situ synchrotron X-ray powder diffraction to understand the hydration reactions that take place during the first day. The influence of two superplasticisers and three strength-enhancing admixtures have been investigated. The diffraction data were analysed by the Rietveld method and the results compared to mass balance calculations. For LC3 binders and in the absence of strength-enhancing admixtures, the hydration rates of the clinker phases, i.e. C3S, C3A and C4AF, are accelerated because of the filler effect. The C-S-H based-admixtures further accelerate the hydration of tricalcium aluminate and ferrite. Chiefly, it is firmly proved that the pozzolanic reaction takes place from 7 h onwards in the studied experimental conditions. The estimated degree of hydration of metakaolin at 22 h, in the studied binders, was ∼10 %.This research was partly supported by the research grant PID2020-114650RB-I00 which is co-funded by ERDF. ALBA synchrotron is thanked for providing beamtime at the BL04-MSPD beamline. We thank thorough discussions with Peter Schwesig and Sebastien Dhers from Master Builders Solutions. Funding for open access charge: Universidad de Malaga/CBU