Search

An intense solar energetic particle (SEP) event was observed on 2021 October 9 by multiple spacecraft distributed near the ecliptic plane at heliocentric radial distances R ≲ 1 au and within a narrow range of heliolongitudes. A stream interaction region (SIR), sequentially observed by Parker Solar Probe (PSP) at R = 0.76 au and 48° east from Earth (ϕ = E48°), STEREO-A (at R = 0.96 au, ϕ = E39°), Solar Orbiter (SolO; at R = 0.68 au, ϕ = E15°), BepiColombo (at R = 0.33 au, ϕ = W02°), and near-Earth spacecraft, regulated the observed intensity-time profiles and the anisotropic character of the SEP event. PSP, STEREO-A, and SolO detected strong anisotropies at the onset of the SEP event, which resulted from the fact that PSP and STEREO-A were in the declining-speed region of the solar wind stream responsible for the SIR and from the passage of a steady magnetic field structure by SolO during the onset of the event. By contrast, the intensity-time profiles observed near Earth displayed a delayed onset at proton energies ≳13 MeV and an accumulation of ≲5 MeV protons between the SIR and the shock driven by the parent coronal mass ejection (CME). Even though BepiColombo, STEREO-A, and SolO were nominally connected to the same region of the Sun, the intensity-time profiles at BepiColombo resemble those observed near Earth, with the bulk of low-energy ions also confined between the SIR and the CME-driven shock. This event exemplifies the impact that intervening large-scale interplanetary structures, such as corotating SIRs, have in shaping the properties of SEP events., Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The STEREO SECCHI data are produced by a consortium of RAL (UK), NRL (USA), LMSAL (USA), GSFC (USA), MPS (Germany), CSL (Belgium), IOTA (France), and IAS (France). SOHO is a mission of international cooperation between ESA and NASA. The SDO/AIA data are provided by the Joint Science Operations Center (JSOC) Science Data Processing (SDP). Parker Solar Probe was designed, built, and is now operated by the Johns Hopkins University Applied Physics Laboratory as part of NASA's LWS program (contract NNN06AA01C). We thank the German Federal Ministry for Economic Affairs and Energy and the German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt, e.V., (DLR)) for their unwavering support of STEP, EPT, and HET under grants Nos. 50OT0901, 50OT1202, 50OT1702, and 50OT2002. N.W. acknowledges funding from the Research Foundation—Flanders (FWO–Vlaanderen, fellowship No. 1184319N). This project has received funding from the European Union's Horizon 2020 research and innovation programs under grant agreement No. 870405 (EUHFORIA 2.0). These results were also obtained in the framework of the ESA project "Heliospheric modeling techniques" (contract No. 4000133080/20/NL/CRS) and the projects C14/19/089 (C1 project Internal Funds KU Leuven), G.0D07.19N (FWO–Vlaanderen), SIDC Data Exploitation (ESA Prodex-12), and Belspo project B2/191/P1/SWiM. E.P. acknowledges support from NASA's PSP-GI (grant No. 80NSSC22K0349) and O2R (grant No. 80NSSC20K0285) programs. B.S.-C. acknowledges support through UK-STFC Ernest Rutherford Fellowship ST/V004115/1 and STFC grants ST/W00089X/1 and ST/V000209/1. R.G.H. acknowledges the financial support by the Spanish MICIU (project PID2019-104863RB-I00/AEI/10.13039/501100011033). R.V. and N.D. acknowledge funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 101004159 (SERPENTINE). N.D. also acknowledges support from the Turku Collegium for Science, Medicine and Technology of the University of Turku, Finland. A.A. acknowledges the support by the Spanish Ministerio de Ciencia e Innovación (MICINN) under grant PID2019-105510GB-C31 and through the "Centre of Excellence María de Maeztu 2020-2023" award to the ICCUB (CEX2019-000918-M). D.L. and I.G.R. acknowledge support from NASA Living With a Star (LWS) programs NNH17ZDA001N-LWS and NNH19ZDA001N-LWS, the Goddard Space Flight Center Internal Scientist Funding Model (competitive work package) program, and the Heliophysics Innovation Fund (HIF) program. I.G.R. also acknowledges support from the ACE mission. The data used in this paper can be downloaded from spdf.gsfc.nasa.gov, www.srl.caltech.edu/ACE/ASC/, soar.esac.esa.int/soar/, stereo.gsfc.nasa.gov, gong.nso.edu/data/magmap/, sdo.gsfc.nasa.gov/data/aiahmi/. See also 10.48322/7gr7-1791, 10.48322/97te-0132, 10.48322/wpk2-yq48, and 10.48322/c0zj-xf76. BepiColombo data used in Figures 11 and 12 can be downloaded from 10.25392/leicester.data.19447259.v1.

Kollhoff, A., et al., Context. On 2020 November 29, the first widespread solar energetic particle (SEP) event of solar cycle 25 was observed at four widely separated locations in the inner (≲ 1 AU) heliosphere. Relativistic electrons as well as protons with energies > 50 MeV were observed by Solar Orbiter (SolO), Parker Solar Probe, the Solar Terrestrial Relations Observatory (STEREO)-A and multiple near-Earth spacecraft. The SEP event was associated with an M4.4 class X-ray flare and accompanied by a coronal mass ejection and an extreme ultraviolet (EUV) wave as well as a type II radio burst and multiple type III radio bursts. Aims. We present multi-spacecraft particle observations and place them in context with source observations from remote sensing instruments and discuss how such observations may further our understanding of particle acceleration and transport in this widespread event. Methods. Velocity dispersion analysis (VDA) and time shift analysis (TSA) were used to infer the particle release times at the Sun. Solar wind plasma and magnetic field measurements were examined to identify structures that influence the properties of the energetic particles such as their intensity. Pitch angle distributions and first-order anisotropies were analyzed in order to characterize the particle propagation in the interplanetary medium. Results. We find that during the 2020 November 29 SEP event, particles spread over more than 230° in longitude close to 1 AU. The particle onset delays observed at the different spacecraft are larger as the flare-footpoint angle increases and are consistent with those from previous STEREO observations. Comparing the timing when the EUV wave intersects the estimated magnetic footpoints of each spacecraft with particle release times from TSA and VDA, we conclude that a simple scenario where the particle release is only determined by the EUV wave propagation is unlikely for this event. Observations of anisotropic particle distributions at SolO, Wind, and STEREO-A do not rule out that particles are injected over a wide longitudinal range close to the Sun. However, the low values of the first-order anisotropy observed by near-Earth spacecraft suggest that diffusive propagation processes are likely involved., The CAU Kiel team acknowledges support from the German Federal Ministry for Economic Affairs and Energy, the German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt e.V., DLR) under grants 50OT0901, 50OT1202, 50OT1702, and 50OT2002. A.K. acknowledges financial support from the ANR COROSHOCK project (ANR-17-CE31-0006-01). D.L. acknowledges support from NASA-HGI grant NNX16AF73G and the NASA Program NNH17ZDA001N-LWS. This study has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 101004159 (SERPENTINE). The U. Turku team acknowledges funding by the Academy of Finland (Grant No. 336809). The UAH team acknowledges financial support by the Spanish Ministerio de Ciencia, Innovación y Universidades FEDER/MCIU/AEI Projects ESP2017-88436-R and PID2019-104863RB-I00/AEI/10.13039/501100011033. [...] The Swedish National Space Agency, ESA-PRODEX and all the participating institutes. I.G.R. acknowledges support from NASA programs NNH19ZDA001N-HSR and NNH19ZDA001N-LWS, and the STEREO mission. IRFU team acknowledges support from the Swedish National Space Agency grant 20/136. A.A. acknowledges the support of the Spanish Ministerio de Ciencia e Innovación (MICINN) under grant PID2019- 105510GB-C31 and through the ‘Center of Excellence María de Maeztu 2020-2023’ award to the ICCUB (CEX2019-000918- M). F.C. acknowledges the financial support by the Spanish MINECO-FPI-2016 predoctoral grant with FSE. E.A. would like to acknowledge the financial support by the Academy of Finland (Postdoctoral Grant No 322455). V.K. acknowledges the support by NASA under grants 18-2HSWO218_2-0010 and 19-HSR-19_2-0143. Solar Orbiter magnetometer operations are funded by the UK Space Agency (grant ST/T001062/1). T.S.H. is supported by STFC grant ST/S000364/1. T.L. and S.D. acknowledge support from the UK Science and Technology Facilities Council (STFC; grant ST/R000425/1).

Aran, A., et al., [Context] In mid-June 2020, the Solar Orbiter (SolO) mission reached its first perihelion at 0.51 au and started its cruise phase, with most of the in situ instruments operating continuously. [Aims] We present the in situ particle measurements of the first proton event observed after the first perihelion obtained by the Energetic Particle Detector (EPD) suite on board SolO. The potential solar and interplanetary (IP) sources of these particles are investigated. Methods. Ion observations from ∼20 keV to ∼1 MeV are combined with available solar wind data from the Radio and Plasma Waves (RPW) instrument and magnetic field data from the magnetometer on board SolO to evaluate the energetic particle transport conditions and infer the possible acceleration mechanisms through which particles gain energy. We compare > 17-20 MeV ion count rate measurements for two solar rotations, along with the solar wind plasma data available from the Solar Wind Analyser (SWA) and RPW instruments, in order to infer the origin of the observed galactic cosmic ray (GCR) depressions. [Results] The lack of an observed electron event and of velocity dispersion at various low-energy ion channels and the observed IP structure indicate a local IP source for the low-energy particles. From the analysis of the anisotropy of particle intensities, we conclude that the low-energy ions were most likely accelerated via a local second-order Fermi process. The observed GCR decrease on 19 June, together with the 51.8-1034.0 keV nuc-1 ion enhancement, was due to a solar wind stream interaction region (SIR). The observation of a similar GCR decrease in the next solar rotation favours this interpretation and constitutes the first observation of a recurrent GCR decrease by SolO. The analysis of the recurrence times of this SIR suggests that it is the same SIR responsible for the He events previously measured in April and May. Finally, we point out that an IP structure more complex than a common SIR cannot be discarded, mainly due to the lack of solar wind temperature measurements and the lack of a higher cadence of solar wind velocity observations., EPD was built with funding from Ministerio de Ciencia e Innovación (Spain), Deutsches Zentrum für Luft- und Raumfahrt (Germany), and the European Space Agency (ESA); operations are funded by FEDER/MCI/AEI Projects ESP2017-88436-R and PID2019-104863RBI00/AEI/10.13039/501100011033 (Spain), 50OT 2002 (DLR, Germany), and NASA contract 80MSFC19F0002. The UB team acknowledges the support by the Spanish Ministerio de Ciencia e Innovación (MICINN) under grant PID2019-105510GB-C31 and through the “Center of Excellence María de Maeztu 2020-2023” award to the ICCUB (CEX2019-000918-M). The CAU Kiel team acknowledges support by the German Federal Ministry for Economic Affairs and Energy, the German Space Agency (Deutsches Zentrum für Luft- und Raumfahrt e.V., DLR) under grants 50OT0901, 50OT1202, 50OT1702, 50OT2002, and 50OC1702. The Solar Orbiter magnetometer was funded by the UK Space Agency (grant ST/T001062/1). M.L. and S.B. acknowledge financial support by the Italian MIUR-PRIN grant 2017APKP7T on Circumterrestrial Environment: Impact of Sun-Earth Interaction. N.W. acknowledges funding from the Research Foundation – Flanders (FWO – Vlaanderen, fellowship no. 1184319N). D.L. acknowledges the support from the NASA-HGI grant NNX16AF73G and the NASA Programs NNH17ZDA001N-LWS. I.G.R. and D.L. acknowledge support from NASA program NNH19ZDA001N-LWS. E.S. was supported by a PhD grant awarded by the Royal Observatory of Belgium. V.K. acknowledges the support by NASA under grants 18-2HSWO218_2-0010 and 19-HSR-19_2-0143. S.P. is supported by the projects C14/19/089 (C1 project Internal Funds KU Leuven), G.0D07.19N (FWO-Vlaanderen), SIDEX (ESA Prodex-12), and EUHFORIA 2.0 (funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870405). IRFU team acknowledges support from the Swedish National Space Agency grant 20/136. F.C. acknowledges the financial support by the Spanish MINECO-FPI-2016 predoctoral grant with FSE. The JHU/APL team is supported under NASA contract NNN06AA01C and thanks NASA headquarters and the NASA/GSFC Solar Orbiter project office for their continuing support. Solar Orbiter Solar Wind Analyser (SWA) data are derived from scientific sensors which have been designed and created, and are operated under funding provided in numerous contracts from the UK Space Agency (UKSA, most recently grant ST/T001356/1), the UK Science and Technology Facilities Council (STFC), the Agenzia Spaziale Italiana (ASI), the Centre National d’Etudes Spatiales (CNES, France), the Centre National de la Recherche Scientifique (CNRS, France), the Czech contribution to the ESA PRODEX programme and NASA.

The purpose of this study was to investigate the effects of age on calcium silicate hydrate (C–S–H) present in two cement pastes: one in which the C–S–H at a younger age had a high Ca/Si ratio and another that had a low ratio. The study involved characterising a 20 year old neat ordinary Portland cement (OPC) paste (high Ca/Si) and a blend of the OPC with 90% ground granulated blast furnace slag (GGBS; low Ca/Si); the results are compared with data on the same mixes at 14 months old. The morphology of the outer product (Op) C–S–H in the blend was essentially the same as at 14 months, whereas that in the neat OPC paste appeared to be finer. The mean aluminosilicate chain length of the C–S–H was considerably longer in both pastes than in similar systems at younger ages. The chemical composition of the C–S–H in both pastes was significantly different than at 14 months: in the neat OPC paste, the Ca/Si ratio of the outer product C–S–H had increased with age, whereas that of the inner product C–S–H had...

The C-S-H gels present in both water- and alkali-activated hardened pastes of white Portland cement/blast-furnace slag blends have been studied by solid-state 29Si magic angle spinning nuclear magnetic resonance (NMR) spectroscopy and analytical transmission electron microscopy (TEM). Structural data are obtained by NMR for the semi-crystalline C-S-H gels in the alkali-activated systems and extended to the nearly amorphous gels in the water-activated systems by peak broadening; unambiguous chemical analyses are determined in the TEM. The following conclusions apply to both the semi-crystalline and nearly amorphous C-S-H gels: (1) aluminium substitutes for silicon at tetrahedral sites; (2) aluminium only substitutes for silicon in the central tetrahedron of pentameric silicate chains; (3) the results strengthen confidence in dreierkette-based models for the structure of C-S-H. Compositional similarities suggest that these conclusions will be true for OPC/slag blends, and possibly also for OPC/pulverized fuel ash blends indicating that the same structural model applies to C-S-H gels in a wide range of hardened cement pastes.

In the alkaline environment of concretes, silica can react in two ways. In the pozzolanic reaction, it combines with free lime to generate additional CSH binding phase. Alternatively reaction with alkali (ASR) can form a potentially expansive alkali silicate gel. 29Si enrichment enabled a series of 29Si MAS and CP MAS NMR spectra to be acquired in situ from a single sample. The hydrate from pozzolanic reaction of lime with silica was relatively crystalline and had long silicate chains. It was similar to the hydrate formed around silica in blended cements. Without lime, silica reacted with alkali solution to give a highly cross linked gel with a range of silicate mobilities. Tri-calcium silicate hydration was accelerated by high levels of alkali (KOH) in solution; shorter silicate chains and a more crystalline hydrate was produced than for hydration in water. Hydration in alkali solution of a model blended cement comprising a mixture of tri-calcium silicate and silica gave two products, a long chain relatively crystalline CSH and an alkali silicate of low rigidity. The alkali silicate gradually polymerized; at later ages it underwent a phase change.

The hydration of a mixture of tricalcium silicate and silica has been studied by 29Si solid-state nuclear magnetic resonance, using selective enrichment of the reactants with 29Si in order to follow and compare the behaviour of the silicon nuclei originating from either source. This approach shows for the first time that the silicon atoms from the two components are not equilibrated throughout the hydration products but are preferentially located in distinct species. In particular, from the distinctive spectra observed when the silica only is enriched, it is concluded that the part of the calcium silicate hydrate gel formed which incorporates silicon from this source has a longer chain length and a slightly better-ordered structure than the remainder. The spectra obtained with selective enrichment are interpreted in terms of a model based on a dreierkette chain structure for C-S-H.

In the current paradigm for the modulation of galactic cosmic rays (GCRs), diffusion is taken to be the dominant process during solar maxima while drift dominates at minima. Observations during the recent solar minimum challenge the pre-eminence of drift at such times. In 2009, the 2 GV GCR intensity measured by the Newark neutron monitor increased by 5% relative to its maximum value two cycles earlier even though the average tilt angle in 2009 was slightly larger than that in 1986 ( 20 vs. 14 ), while solar wind B was significantly lower ( 3.9 nT vs. 5.4 nT). A decomposition of the solar wind into high-speed streams, slow solar wind, and coronal mass ejections (CMEs; including postshock flows) reveals that the Sun transmits its message of changing magnetic field (diffusion coefficient) to the heliosphere primarily through CMEs at solar maximum and high-speed streams at solar minimum. Long-term reconstructions of solar wind B are in general agreement for the 1900-present interval and can be used to reliably estimate GCR intensity over this period. For earlier epochs, however, a recent 10Be-based reconstruction covering the past 104 years shows nine abrupt and relatively short-lived drops of B to _0 nT, with the first of these corresponding to the Sp rer minimum. Such dips are at variance with the recent suggestion that B has a minimum or floor value of 2.8 nT. A floor in solar wind B implies a ceiling in the GCR intensity (a permanent modulation of the local interstellar spectrum) at a given energy/rigidity. The 30 40% increase in the intensity of 2.5 GV electrons observed by Ulysses during the recent solar minimum raises an interesting paradox that will need to be resolved.

The local site symmetry of aluminium in Al-substituted calcium silicate hydrate phases, similar to those found in hardened Portland cement pastes, has been determined by experimental measurement and theoretical modelling of the electron energy loss near-edge structure associated with the aluminium K-edge measured using parallel electron energy loss spectroscopy in the transmission electron microscope. Changes in the local aluminium environment were observed between different regions of the microstructure which are explained in terms of aluminium substitution into both tetrahedral silicon and octahedral magnesium sites. This spatially-resolved information is in excellent agreement with27Al and29Si magic angle spinning nuclear magnetic resonance data on bulk samples as well as the predictions based upon compositional trends derived from the results of energy dispersive X-ray and electron energy loss elemental microanalysis. In a wider context we wish to stress the complementarity of both spatially-resolved and bulk spectroscopic techniques, which possess differing degrees of sensitivity, in the analysis of materials science samples.

29Si isotopic enrichment was used for acquisition of multiple 29Si magic-angle spinning (MAS) and cross-polarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra, in situ in an NMR probe, from a single sample of hydrating Ca3SiO5 (C3S). Data with excellent signal-to-noise ratios were obtained at 20, 50 and 75 °C, with minimal use of spectrometer time, and without the need for the quenching of multiple samples. Spectral line widths and polymer-chain lengths derived from the spectra had no detectable differences from experiments in which the quenching was carried out with propan-2-ol. Furthermore, the effects of the MAS technique on the hydration reaction appeared to be minimal. At 20 °C, the bulk hydrate initially produced was dimeric; at later stages of the reaction, polymerization occurred. Arrhenius energies of 35 and 100 kJ mol−1, respectively, were calculated for these two reactions. The cross-polarization (CP) spectra acquired throughout the hydration showed that at 20 °C, 2% of the hydrated monomeric Q o (H) species persisted from after the induction period through to the late stages of the hydration reaction; this indicates that this species is unlikely to result from surface hydroxylation of C3S; an upfield shift of this species occurred with increasing hydration, indicating a possible change of environment for the silicate species. The amount of Q o (H) produced was found to increase at higher temperatures. Potential mechanisms for polymerization were assessed and a model in which dimeric-silicate units are linked together by insertion of monomers (dimer → pentamer → octomer) was found to give the best fit to the observed data; these results support a dreierketten model for the structure of the hydrate.

Selective isotopic enrichment of SiO 2 with 29 Si in a mixture with tricalcium silicate (C 3 S) has allowed the Si from this phase to be effectively labeled during the course of the hydration reaction, thus isolating its contribution to the reaction. A double Q 2 signal has been observed in 29 Si solid-state MAS NMR spectroscopy of C-S-H gels of relatively low Ca/Si ratio, prepared by hydration or by carbonation of a C 3 S paste. The origin of the weaker, downfield peak is discussed and tentatively attributed to bridging tetrahedra of a dreierkette silicate chain structure

A microcrystalline form of calcium hydroxide consisting of clusters of lamallae, ∼10nm in thickness, intimately mixed with CSH gel, has been observed in a lime-silica cement paste by transmission electron microscopy. The volumes of calcium hydroxide and CSH in the clusters are comparable. The implications of this are discussed.

The C-S-H gels present in commercial blast-furnace slag and synthetic-slag glass pastes produced by hydrating with 5M KOH solution have been studied by a combination of transmission electron microscopy (TEM) and 29 Si and 27 Al nuclear magnetic resonance (NMR) spectroscopy. They are related by both composition and morphology to the C-S-H gels present in slag-OPC pastes but are more crystalline. The inner product C-S-H is intermixed on a fine scale with a Mg,Al-rich phase with a Mg/Al ratio of ≈2.5. The C-S-H in both inner and outer product contains substituted Al in tetrahedral co-ordination sites. The data are analysed in terms of a model for the structure of C-S-H gel.

The local environment of aluminium in Al-substituted calcium silicate hydrate phases in cement pastes has been determined by measurement and modelling Al K-ELNES. The results are in agreement with bulk 27Al and 29Si MAS NMR data and predictions derived from compositional trends.

Solid-state 27 Al and 29 Si magic angle spinning NMR spectroscopy has been combined with electron energy loss spectroscopy carried out in the transmission electron microscope to determine the location of Al substituting in a semicrystalline C-S-H gel present in a hydrated synthetic slag glass. The gel is found to contain mainly pentameric silicate chains in which the central silicon is substituted by aluminum

Slices of hardened pastes of OPC and an OPC/silica fume blend of the same water: solids ratio of 0·5 were carbonated in air for 116 days at a fixed relative humidity. The blended paste slices were found to have carbonated less overall, and to a lesser depth, than the OPC slices. The microstructures of the partially carbonated slices were examined by transmission electron microscopy, and in the case of the OPC the silicate structure was investigated by 29Si NMR.

Hardened ordinary Portland cement pastes of various ages have been examined by analytical transmission electron microscopy (TEM) and electron microprobe analysis (EMPA). The stability of the various hydrate phases in the electron microscope is discussed. Although all are subject to damage in varying degrees, even the least stable phase, AFt, can be recognized in relict form in the TEM. The basic framework of the microstructure and the differentiation into inner and outer hydration product are well-established at 24 h hydration. Although the dominant inner product formed within the boundaries of the original anhydrous grains is C-S-H gel, particles of AFt, AFm, Ca(OH)2, a magnesium-rich phase and an iron-rich phase are occasionally observed within the inner product. The Ca∶Si ratio of the C-S-H gel determined by TEM shows significant variation from one region to another in a given paste. There is no relationship between the average Ca∶Si ratio of the C-S-H and the maturity of the paste, although young pastes appear to show a bimodal distribution. Microanalysis by EMPA gives Ca∶Si ratios in substantial agreement with those found by TEM but it is essentially impossible to obtain by EMPA analyses of outer product C-S-H without admixture of other phases, particularly sulphoaluminate phases. Despite the presence of small amounts of embedded phases as revealed by TEM, single-phase inner product C-S-H can be analysed by EMPA. The compositions of AFt and AFm phases have been obtained by TEM and the results do not require the substitution of silicon in the formulae.

The interstitial phase of an oil-well cement production clinker was examined by analytical transmission electron microscopy. The predominant interstitial component was a ferrite of varying composition, the Fe: Al ratio varying from a low value to 2, and values most commonly in the range O·7-2. A substantial variation in composition sometimes took place over quite short distances. Displacement faults were found in the ferrite, and these appeared to be associated with relatively iron-rich regions of the ferrite. Small regions of C2S, MgO and non-cubic C3A were also found in the interstitial phase of the clinker.

The General model for CSH gel described by Richardson and Groves (1) has been extended to incorporate elements other than Ca, Si, O and H which have been detected by X-ray microanalysis of gels in hardened Portland cement and blended cement pastes.

Models for the structure of CSH gels occuring in hardened C3S cement pastes are considered and compared to some examples in which composition and silicate anion structure have been investigated experimentally.

The microstructure and composition of hardened cement pastes of a wide range of blends of ground granulated blast-furnace slag with ordinary Portland cement have been studied, using techniques of transmission electron microscopy with microanalysis combined with electron microprobe analysis. Throughout the range, a calcium silicate hydrate gel (C-S-H) is the dominant cementing phase, present in the “inner product” within the space originally occupied by either slag grains or alite or belite grains originating from the Portland cement, or in the “outer product” in the originally water-filled spaces. The morphology and composition of the outer product C-S-H and the composition of inner product C-S-H change with blend composition. Inner product of slag grains contains C-S-H of the same composition as the outer product C-S-H, intimately mixed with a Mg, Al-rich hydroxide phase whose fineness shows considerable variation. Inner product C-S-H of alite or belite does not differ significantly in Ca∶Si ratio from that of slag. The reduction of Ca∶Si ratio of all forms of C-S-H with increasing slag loading may have implications for the pH-buffering capacity of blends of large slag loading.

The structures of partially carbonated hardened C3S cement pastes have been investigated by a combination of 29Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X-ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO2 for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C-S-H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C-S-H was, in the main, retained. A cross-linked silica-rich gel formed at later times in paste carbonated in CO2 but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.

The microstructure of hardened pastes of C3S and a C3S/silica fume blend have been examined by transmission electron microscopy before and after partial carbonation in pure CO2 at a relative humidity of 72·6%. Fibrillar outer product C-S-H and the outer regions of inner product C-S-H gel carbonate without change of morphology to silica gel. Calcium carbonate is formed mainly in outer product regions as microcrystalline vaterite or calcite. Residual CH embedded in vaterite microcrystals is observed in the carbonated C3S pastes. It is proposed that outer product C-S-H and CH surfaces carbonate relatively rapidly, compared with inner regions of CH or the inner regions of inner product C-S-H, leading to a substantial level of carbonation of thin slices (0·4 mm) of paste within one day, with little further carbonation in the next few days. The implications for the theory of the kinetics of cement paste carbonation are discussed.