Papers by Paulo J.M. Monteiro
Cement and Concrete Research, 2015
The mineralogy and spatial distribution of nano-crystalline corrosion products that form in the s... more The mineralogy and spatial distribution of nano-crystalline corrosion products that form in the steel/concrete interface were characterized using synchrotron X-ray micro-diffraction (μ-XRD). Two types of low-nickel highchromium reinforcing steels embedded into mortar and exposed to NaCl solution were investigated. Corrosion in the samples was confirmed by electrochemical impedance spectroscopy (EIS). μ-XRD revealed that goethite (α-FeOOH) and akaganeite (β-FeOOH) are the main iron oxide-hydroxides formed during the chlorideinduced corrosion of stainless steel in concrete. Goethite is formed closer to the surface of the steel due to the presence of chromium in the steel, while akaganeite is formed further away from the surface due to the presence of chloride ions. Detailed microstructural analysis is shown and discussed on one sample of each type of steel.
List of Figures..................................................................................... more List of Figures............................................................................................................................... vii
Journal of the American Ceramic Society, 2020
Calcium silicate hydrate (C-S-H), is the principal hydration product of Portland cement that main... more Calcium silicate hydrate (C-S-H), is the principal hydration product of Portland cement that mainly contributes to the physical and mechanical properties of concrete. This paper aims to investigate the three-dimensional structure of C-S-H with Ca/Si ratios of 1.0 and 1.6 at the nanoscale using electron tomography. The 3D reconstructions and selected region of interest analysis confirm that the morphology of both C-S-H materials are foil-like structures. The difference between the two materials is the density of elongated structures. C-S-H with Ca/Si ratio 1.6 is clearly composed of denser particles compared to the other C-S-H material due to overlapping of the foil-like structure. Pore analysis shows that C-S-H 1.0 and C-S-H 1.6 have porosities 69.2% and 49.8% respectively. Pore size distribution also reveals that C-S-H 1.0 has pore size range between 0-250 nm and C-S-H 1.6 between 0-100 nm. The pore network's size of C-S-H 1.0 is significantly larger than 1.6. This study illustrates the capability of using electron tomography to determine the 3D nanoscale structure of cementitious products and to distinguish between C-S-H 1.0 and 1.6.
MRS Proceedings, 1987
ABSTRACTThis paper contains a review of the results from the studies at the University of Califor... more ABSTRACTThis paper contains a review of the results from the studies at the University of California at Berkeley on various factors influencing the microstructure of the transition zone in concrete. Two types of aggregate, two different cement, and three mineral admixtures were investigated. Using cement paste-polished aggregate composite specimens cured up to three years, X-ray diffraction, scanning electron microscopy, and microhardness testing techniques were used for characterization of the transition zone.Compared to the transition zone between a quartz aggregate and an ASTM Type I portland cement, transition zones with smaller and less preferentially oriented crystals of calcium hydroxide were obtained when using a Type K expansive cement, or limestone aggregate, or mineral additives, such as condensed silica fume, granulated blast-furnace slag, and fly ash.
Scientific reports, Jan 8, 2017
Calciuam-silicate-hydrate (C-S-H) is the principal binding phase in modern concrete. Molecular si... more Calciuam-silicate-hydrate (C-S-H) is the principal binding phase in modern concrete. Molecular simulations imply that its nanoscale stiffness is 'defect-driven', i.e., dominated by crystallographic defects such as bridging site vacancies in its silicate chains. However, experimental validation of this result is difficult due to the hierarchically porous nature of C-S-H down to nanometers. Here, we integrate high pressure X-ray diffraction and atomistic simulations to correlate the anisotropic deformation of nanocrystalline C-S-H to its atomic-scale structure, which is changed by varying the Ca-to-Si molar ratio. Contrary to the 'defect-driven' hypothesis, we clearly observe stiffening of C-S-H with increasing Ca/Si in the range 0.8 ≤ Ca/Si ≤ 1.3, despite increasing numbers of vacancies in its silicate chains. The deformation of these chains along the b-axis occurs mainly through tilting of the Si-O-Si dihedral angle rather than shortening of the Si-O bond, and conseq...
Materials (Basel, Switzerland), Jan 4, 2017
This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tri... more This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C₃S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C-S-H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C₃S hydration was significantly extended, the degree of hydration of C₃S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C₃S in the C₃S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C₃S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C-S-H in the C₃S-HVFA paste directly indicate that Al substitutes for Si in C-S-H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic appr...
Journal of the American Ceramic Society, 2015
Morphological details of calcium silicate hydrate (C–S–H) stemming from the hydration process of ... more Morphological details of calcium silicate hydrate (C–S–H) stemming from the hydration process of Portland cement (PC) phases are crucial for understanding the PC-based systems but are still only partially known. Here we introduce the first soft X-ray ptychographic imaging of tricalcium silicate (C3S) hydration products. The results are compared using both scanning transmission X-ray and electron transmission microscopy data. The evidence shows that ptychography is a powerful method to visualize the details of outer and inner product C–S–H of fully hydrated C3S, which have fibrillar and an interglobular structure with average void sizes of 20 nm, respectively. The high-resolution ptychrography image enables us to perform morphological quantification of C–S–H, and, for the first time, to possibly distinguish the contributions of inner and outer product C–S–H to the small angle scattering of cement paste. The results indicate that the outer product C–S–H is mainly responsible for the q−3 regime, whereas the inner product C–S–H transitions to a q−2 regime. Various hypotheses are discussed to explain these regimes.
Journal of the American Ceramic Society, 2015
Cement and Concrete Composites, 2015
This paper reports the composition and properties of highly flowable selfconsolidating concrete (... more This paper reports the composition and properties of highly flowable selfconsolidating concrete (SCC) mixtures made of high proportions of cement replacement materials such as fly ash and pulverized limestone instead of high dosage of a plasticizing agent or viscosity-modifying chemical admixtures. Self-consolidating concrete mixtures
ABSTRACT Six calcium sulfoaluminate-based cementitious systems composed of calcium sulfoaluminate... more ABSTRACT Six calcium sulfoaluminate-based cementitious systems composed of calcium sulfoaluminate, calcite, vaterite, and gypsum were cured as pastes and mortars for 1, 7, 28 and 84 days. Pastes were analyzed with X-ray diffraction, thermogravimetric and differential thermal analyses. Mortars were tested for compressive strength, dimensional stability and setting time. Furthermore, pastes with a water/cementitious material mass ratio of 0.80 were tested for heat evolution during the first 48 h by means of isothermal conduction calorimetry. It has been found that: (1) both calcite and vaterite reacted with monosulfoaluminate to give monocarboaluminate and ettringite, with vaterite being more reactive; (2) gypsum lowered the reactivity of both carbonates; (3) expansion was reduced by calcite and vaterite, irrespective of the presence of gypsum; and (4) both carbonates increased compressive strength in the absence of gypsum and decreased compressive strength less in the presence of gypsum, with vaterite's action more effective than that of calcite.
American Mineralogist, 2014
The elasticity of monocarboaluminate hydrates, 3CaO•Al 2 O 3 •CaCO 3 •xH 2 O (x = 11 or 8), has b... more The elasticity of monocarboaluminate hydrates, 3CaO•Al 2 O 3 •CaCO 3 •xH 2 O (x = 11 or 8), has been monocarboaluminate (x = 11) exhibits exceptionally low compressibility compared to other reported calcium aluminate hydrates. This stiff hydration product can contribute to the strength of concrete made with Portland cements containing calcium carbonates. In this study, full elastic tensors and mechanical principles methods based on density functional theory. The results indicate that the compressibility of monocarboaluminate is highly dependent on the water content in the interlayer region. The structure also becomes more isotropic with the addition of water molecules in this region. Since the monocarboalu minate is a key hydration product of limestone added cement, elasticity of the crystal is important to understand its mechanical impact on concrete. Besides, it is put forth that this theoretical calculation will of ion exchange on compressibility.
Ambiente Construído, 2010
O aluminato tricálcico (C3A) é a fase que reage mais rápido na hidratação do cimento quando em co... more O aluminato tricálcico (C3A) é a fase que reage mais rápido na hidratação do cimento quando em contato com a água. Geralmente, a forma cristalina sintetizada durante o processo de clinquerização é cúbica. No entanto, o íon Na+ pode ser incorporado na estrutura cristalina do C3A, em substituição ao Ca2+, levando à modificação da forma cristalina cúbica, para a ortorrômbica (Na- C3A). Embora alguns estudos tenham investigado a interferência desta fase modificada na hidratação do cimento, algumas lacunas de conhecimento ainda existem. O presente estudo utiliza fases puras sintetizadas em laboratório, para, desta forma, examinar as reações isoladamente. A hidratação destes aluminatos na presença de gipsita foi estudada utilizando calorimetria isotérmica, microscopia eletrônica de varredura (MEV), difração de raios X (DRX) e ensaio de reologia de modo dinâmico, para comprovar e observar o comportamento diferenciado de cada aluminato. Os resultados mostraram que existem grandes diferenças...
Journal of the American Ceramic Society, 2013
The material characteristics and elastic properties of aluminumsubstituted 11 A tobermorite in th... more The material characteristics and elastic properties of aluminumsubstituted 11 A tobermorite in the relict lime clasts of 2000year-old Roman seawater harbor concrete are described with TG-DSC and 29 Si MAS NMR studies, along with nanoscale tomography, X-ray microdiffraction, and high-pressure X-ray diffraction synchrotron radiation applications. The crystals have aluminum substitution for silicon in tetrahedral bridging and branching sites and 11.49(3) A interlayer (002) spacing. With prolonged heating to 350°C, the crystals exhibit normal behavior. The experimentally measured isothermal bulk modulus at zero pressure, K 0 , 55 ±5 GPa, is less than ab initio and molecular dynamics models for ideal tobermorite with a double-silicate chain structure. Even so, K 0 , is substantially higher than calcium-aluminum-silicate-hydrate binder (C-AS -H) in slag concrete. Based on nanoscale tomographic study, the crystal clusters form a well connected solid, despite having about 52% porosity. In the pumiceous cementitious matrix, Al-tobermorite with 11.27 A interlayer spacing is locally associated with phillipsite, similar to geologic occurrences in basaltic tephra. The ancient concretes provide a sustainable prototype for producing Al-tobermorite in high-performance concretes with natural volcanic pozzolans.
Journal of Materials in Civil Engineering, 2008
In concrete pavements, steel dowels are exposed to a particularly aggressive environment that lea... more In concrete pavements, steel dowels are exposed to a particularly aggressive environment that leads to depassivation and greatly reduces the corrosion initiation stage. Aggressive agents such as chlorides and CO 2 have easy access to the dowels through pavement joints and, consequently, the corrosion performance of the system depends largely on the properties of the steel dowel being used. This study investigates the corrosion performance of several types of steel dowels embedded in concrete and subjected to accelerated ...
Corrosion Science, 2014
ABSTRACT In a X-ray microcomputed tomography study, active corrosion was induced by galvanostatic... more ABSTRACT In a X-ray microcomputed tomography study, active corrosion was induced by galvanostatically corroding steel embedded in cement paste. The results give insight into corrosion product build up, crack formation, leaching of products into the cracks and voids, and differences in morphology of corrosion attack in the case of carbon steel or stainless steel reinforcement. Carbon steel was homogeneously etched away with a homogeneous layer of corrosion products forming at the steel/cement paste interface. For ferritic stainless steel, pits were forming, concentrating the corrosion products locally, which led to more extensive damage on the cement paste cover.
Construction and Building Materials, 2014
The purpose of this study is to compare the effects of Portland cement replacement on the strengt... more The purpose of this study is to compare the effects of Portland cement replacement on the strength and durability of self-consolidating concretes (SSC). The two replacement materials used are high-volume natural pozzolan (HVNP), a Saudi Arabian aluminum-silica rich basaltic glass and high-volume Class-F fly ash (HVFAF), from Jim Bridger Power Plant, Wyoming US. As an extension of the study, limestone (LS) is also used to replace Portland cement, alongside HVNP or HVFAF, forming ternary blends. Along with compressive strength tests, non-steady state chloride migration and gas permeability tests were performed, as durability indicators on SCC specimens. The results were compared to two reference concretes; 100% ordinary Portland cement (OPC) and 85% OPC-15% LS by mass. The HVNP and HVFAF concrete mixes showed strength and durability results comparable to the reference concretes; identifying that both can effectively be used to produce of low-cost and environmental friendly SCC.
Cement and Concrete Research, 2014
The present study focuses on identification and micro-structural characterization of the hydratio... more The present study focuses on identification and micro-structural characterization of the hydration products formed in high-volume fly ash (HVFA)/portland cement (PC) systems using monochromatic scanning x-ray micro-diffraction (μ-SXRD) and SEM-EDS. Pastes with up to 80% fly ash replacement were studied. Phase maps for HVFA samples using μ-SXRD patterns prove that μ-SXRD is an effective method to identify and visualize the distribution of phases in the matrix. μ-SXRD and SEM-EDS analysis shows that the C-S-H formed in HVFA system containing 50% or more of fly ash has a similar structure as C-S-H(I) with comparatively lower Ca/Si ratio than the one produced in PC system. Moreover, coexistence of C-S-H(I) and strätlingite is observed in the system containing 80% of fly ash, confirming that the amount of alumina and silicate phases provided by the fly ash is a major factor for the formation of C-S-H(I) and strätlingite in HVFA system.
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Papers by Paulo J.M. Monteiro