239 research outputs found
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 yeâelimite-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 yeâelimite 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 AndalucĂa (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 Yeâelimite-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 yeâelimite-containing pastes. Here, a multi r-range analysis approach has been used, where the 30â50 Ă
r-range allows determining the crystalline phase contents and the 1.6-35 Ă
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 yeâelimite 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 Ă
), a nanocrystalline AFm phase with a significantly smaller c-unit cell parameter value, c =26.8 Ă
, was required having about 60 Ă
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 Ă
This 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 MĂĄlaga. Campus de Excelencia Internacional AndalucĂa 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, âŒ100 nm/h, is approximately four
times faster than that of large alite grains in the deceleration stage, âŒ25 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 PTA2019-017513âI
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 (”CT) 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 MĂĄlaga. Campus de Excelencia Internacional AndalucĂa 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 MĂĄlaga. Campus de Excelencia Internacional AndalucĂa Tech
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 (ÎŒCT) 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. ÎŒCT 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 ÎŒCT 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 MĂĄlaga. Campus de Excelencia Internacional AndalucĂa Tech
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