Comparative study of accelerated decalcification process among C3S, grey and white cement pastes

This study compared the resistance of Triclinic-C₃S, grey (OPC) and white (WOPC) Portland cement paste to decalcification induced by accelerated leaching in concentrated ammonium nitrate solutions. Paste microstructure was studied with scanning and backscattering electron microscopy (SEM and BSEM) and nitrogen BET surface area techniques. Ca²⁺ leached content was quantified by ICP, XRD and FTIR techniques were used to study phase mineralogy. The conclusions drawn from the findings were that calcium leaching-induced decay in the cementitious materials studied (C₃S, OPC and WOPC), accelerated by immersion in ammonium nitrate, affected the main calcium phases in the samples (CH, C-S-H gel and ettringite), i.e., both the anhydrous and the hydrated phases. The present study showed that the Ca/Si ratio of C-S-H gels declines on a gradient from the sample core outward. Specimen surface area and nanoporosity rose in cementitious materials after Ca leaching-induced decay and subsequently declined as a result of the collapse of the structure of the hydrated cement, and in particular of the C-S-H gel. C₃S paste was impacted more quickly and intensely by leaching than the WOPC and OPC pastes. Further to the findings of this study, the leaching resistance of these three materials, in descending order, is: OPC > WOPC > C₃S.     

Resistance of blended cement pastes subjected to organic acids: Quantification of anhydrous and hydrated phases

Concrete for agricultural construction is often subject to aggressive environmental conditions. Ground granulated blast furnace slag (GGBFS) or metakaolin (MK) largely improve the chemical resistance of the binder. Anhydrous particles seem particularly resistant to the acid solution. The purpose of this study is to quantify anhydrous particles in blended cement pastes as a function of acid exposition time in order to evaluate their acid resistance.Cement pastes were moist cured for 28 days and then immersed in an acetic acid solution for 2 months. The quantification of the anhydrous phases was carried out using ²⁹Si MAS NMR, selective dissolution and back-scattered electron (BSE) images analysis, while the hydrated phases content was evaluated by TGA. After 28 days of hydration, 60% of OPC, 44% of GGBFS and 76% of MK particles were hydrated. The amount of anhydrous particles drops for all materials during acid immersion. After 2 months of immersion, the amount of anhydrous particles drops by 49%, 23% and 15% for OPC, GGBFS, and MK respectively. This study confirms that GGBFS and MK anhydrous and hydrates phases present higher acid resistance than OPC.     

Dehydration and rehydration processes of cement paste exposed to high temperature environments

Microstructural changes of an OPC cement paste after being exposed at various elevated temperatures and further rehydration have been evaluated using ²⁹Si MAS-NMR. Thermogravimetry and XRD are also employed to complement the information. NMR studies of cement paste exposed to high temperatures demonstrate a progressive transformation of C-S-H gel that leads at 450°C, to a modified C-S-H gel. For temperatures above 200°C to a progressive formation of a new nesosilicate. At 750°C, the transformation of C-S-H is complete into the nesosilicate form with a C₂S stoichiometry close to larnite, but less crystalline. Also is observed an increase of portlandite that takes place up to temperatures of 200°C. A progressive increase of calcite formation up to 450°C is noticed. The ettringite disappearance below 100°C is confirmed and the portlandite and calcite are converted to lime at 750°C. The initial anhydrous phases as larnite and brownmillerite remain unaltered during heating. Rehydration of the heated samples (450 and 750°C) shows recrystallization of calcite, portlandite and ettringite, and the C-S-H reformation from the new nesosilicate. The larnite and brownmillerite remain unaltered during rehydration. The developing of damaged due to the formation of microcracking is detected and improved because of rehydration phenomena.     

Characterisation of pre-industrial hybrid cement and effect of pre-curing temperature

This study aimed to determine the physical-mechanical, mineralogical and microstructural properties of a pre-industrially manufactured hybrid cement (HYC) containing 5% alkaline activator and less than 30% clinker. The effect of the initial curing temperature (25 ± 1 or 85 °C for 20 h) on hydration kinetics and the development of compressive strength were also explored. The hydration products formed were characterised using XRD, SEM/EDX and ²⁷Al and ²⁹Si MAS-NMR. The findings showed that pre-industrial hybrid cement sets when hydrated with water and hardens to a 28-day mechanical strength of 35 MPa. The main reaction product formed was a mix of cementitious gels: C-(A)-S-H and C-A-S-H. Curing at 85 °C for 20 h, shows a behaviour similar to OPC, inhibited ettringite formation and generated more polymerised gels, enhancing 3-day but not 28- or 90-day mechanical strength.     

Long-term progressive deterioration following fire exposure of OPC versus slag blended cement pastes

The normal practice of repairing fire-damaged concrete structures is to remove the visibly damaged portions and restore them with new concrete. However, little attention has been given to the long-term performance of fire exposed concrete which is not removed from the structure. This paper addresses this issue. Ordinary Portland cement (OPC) pastes, when exposed to a critical temperature of 400°C, undergo complete breakdown. This behaviour was attributed to the dehydration of Ca(OH)₂, followed by the expansive rehydration of CaO. In contrast, partial replacement of the OPC binder with slag, had a beneficial effect in the mechanical properties of the paste after exposure to high temperatures, as slag significantly reduces the amount of available Ca(OH)₂ in the cement paste. The present work provides new data regarding the long-term (after the exposure event) effect of CaO rehydration in the OPC and OPC/slag pastes. After 1 year the ongoing effect of the CaO rehydration was severe in the OPC paste while OPC/slag blends were not affected by rehydration. Compressive strength and thermogravimetric results are presented to explain this behaviour.     

The formation,stability and microstructure of calcium sulpoaluminate hydrates present in hydrated cement pastes,usingin situ synchrotron energy-dispersive diffraction

Synchrotron radiation-energy dispersive diffraction has been used for the first time to study the formation and stability of the calcium sulphoaluminate hydrates in hydrated Portland cement pastes. By using this technique it has also been possible to investigate microstructural and compositional characteristics of the ettringite (AFt) phase. The longer term slow development of the monosulphate (AFm) phase has also been monitored, although the characterized content is quite low. Differences were detected between the microstructural characteristics of the AFt phase formed in the high ferrite sulphate-resisting-type cement pastes, as compared with the equivalent phase formed from the ordinary Portland cements. These differences were especially significant at later hydration times and have been ascribed to compositional differences between the ettringite (AFt) formed from the two different types of cement.     

The hydration of slag,part 1: reaction models for alkali-activated slag

Reaction models are proposed to quantify the hydration products and to determine the composition of C–S–H from alkali-activated slags (AAS). Products of the slag hydration are first summarized from observations in literature. The main hydration products include C–S–H, hydrotalcite, hydrogarnet, AFm phases (C₄AH₁₃ and C₂ASH₈) and ettringite. Then, three stoichiometric reaction models are established correlating the mineral composition of slag (the glass part) with the hydration products. Using the proposed models, quantities of hydration products and composition of C–S–H are determined. The models are validated with a number of experimental investigations reported in literature, yielding good agreement, i.e., these models can successfully predict the hydration reaction of AAS. The models are furthermore applied to calculate the retained water in the hydration products of AAS in different hydration states and a general hydration equation of AAS is derived. As an illustration to one of the model applications, chemical shrinkage of the AAS cement paste in different hydration states are predicted. The chemical shrinkage of AAS is shown to be remarkably higher than OPC. Furthermore, phase distribution in the hardened AAS paste and the porosity are calculated.     

The solid state chemistry of metakaolin-blended ordinary Portland cement

The hydration of ordinary Portland cement (OPC) pastes containing 0 and 20% metakaolin was monitored by differential thermal analysis (DTA) and solid state magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The presence of hydrated gehlenite and a relative reduction in calcium hydroxide content of the metakaolin-blended OPC pastes observed by DTA are indicative of the pozzolanic reaction of metakaolin. An increase in the capacity of metakaolin-blended OPC pastes to exclude chloride ions from the pore electrolyte phase, via solid phase binding, has been reported. It is proposed that this increase in chloride binding capacity could be attributed to the participation of calcium aluminate species in the formation of Friedel's salt which would otherwise be engaged in the formation of hydrated gehlenite and other AFm phases. The accelerating effect of replacement additions of metakaolin has been shown by ²⁹Si NMR and was denoted by a comparative increase in the intensity of resonances arising from Q¹ and Q² species compared with that of Q⁰ species for metakaolin-blended specimens. The primary reactive centres of the pozzolan have been shown to be the 5-coordinate aluminium and amorphous silica. The spreading of the Q⁴ resonance of the amorphous silica of metakaolin through the Q³ and into the Q² and Q¹ regions of the NMR spectrum during pozzolanic reaction has been observed.     

Effects of slag and cooling method on the progressive deterioration of concrete after exposure to elevated temperatures as in a fire event

In the present work OPC and OPC/slag concretes were exposed to elevated temperatures, 400 and 800°C. The critical temperature of 400°C has been reported for OPC paste. Above 400°C, the paste hydrate Ca(OH)₂ dehydrates into CaO causing the OPC paste to shrink and crack. After cooling and in the presence of air moisture, CaO rehydrates into Ca(OH)₂, resulting in disintegration due to re-expansion of OPC paste. Therefore, the present work assessed whether this also applies to OPC concretes. Two cooling methods were used: furnace and water cooling. Following the heat treatment/cooling method, compressive tests and Infrared (IR) spectroscopic studies were conducted. Results showed that after 400°C, water cooling caused all concrete, regardless of the type of blended cement binder, a further 20% loss in the residual strength. After 800°C, water cooling caused OPC concrete a further 14% loss while slag blends presented around 5% loss. IR indicated that the further loss observed in the OPC concrete is due to the accelerated CaO rehydration into Ca(OH)₂. Afterward, the non-wetted furnace cooled specimens were exposed to air moisture for one week, resulting in further strength loss of 13%. IR results suggested that slow rehydration of CaO occur with exposure to air moisture. In conclusion, water cooling caused more damage in OPC concrete, while the concrete that has not been wetted undergoes progressive deterioration. This indicates a need to monitor the non-wetted concrete after a fire event has occurred for potential further deterioration.     

自燃煤矸石胶凝材料中钙矾石形成研究

采用XRD分析了自燃煤矸石胶凝材料中活性Al2O3在不同因素影响下水化形成钙矾石的情况.研究结果表明,在相同条件下芒硝存在,时自燃煤矸石中活性Al2O3水化生成钙矾石,而含等量硫的石膏存在时水化浆体中不但有水化产物钙矾石还有未反应的石膏;胶凝体系中不含熟石灰时,活性Al2O3水化首先生成石膏,随着水化的进行再转化成钙矾石,而在有熟石灰时水化则直接生成钙矾石;矿渣促进了自燃煤矸石中Al2O3水化形成钙矾石.     

Characterization of a novel calcium phosphate composite bone cement: Flow, setting, and aging properties

The flow, setting, and aging characteristics of a newly developed calcium phosphate/calcium aluminate composite orthopaedic cement were studied. The effect of vibration on the flow of the cement paste was studied and found to greatly enhance placement. The setting times of this cement were dependent on temperature and decreased with increasing temperatures. At 37^∘C, the working and setting times were 6.3 ± 0.3 and 12.8 ± 0.4 minutes, respectively. Hydration and conversion of the cement phases continued while specimens were stored under simulated, physiological conditions. A cumulative increase in mass of 8.23 ± 0.65% was observed over a 14 month test period. During this time, the cement was found to expand slightly, 0.71 ± 0.39%. X-ray diffraction was used to characterize the crystalline phases present during hydration and conversion. The calcium aluminate in the cement hydrated and formed calcium aluminate chloride hydrates, while no changes were observed in the β-tricalcium phosphate during the testing period.     

Report of TC 238-SCM: hydration stoppage methods for phase assemblage studies of blended cements—results of a round robin test

For many microstructural studies it is necessary to “stop” cement hydration—to remove free water. This paper describes the results of a round robin test on the impact of hydration stoppage methods on the composition of hydrated cements. A regular and a fly ash blended Portland cement hydrated for 90 days were selected. Ten laboratories participated in the round robin test. Four common hydration stoppage methods were studied: (1) oven drying at 105 °C, (2) solvent exchange by isopropanol, (3) vacuum drying and (4) freeze drying. After the stoppage of hydration powder samples were studied by thermogravimetry (TG) and X-ray diffraction (XRD). Bound water and Ca(OH)₂ content were determined based on the TG data. Portlandite and ettringite content were quantified by Rietveld analysis of the XRD data. The goal was to establish interlaboratory reproducibility and to identify the best available protocols for research and standardization purposes. Based on the results of the round robin test three recommendations are made. (1) Oven drying at 105 °C is not recommended. This dehydrates, alters and decomposes calcium aluminate hydrates significantly more than other methods and often produced carbonation artefacts. (2) Isopropanol exchange is the most appropriate hydration stoppage method for the study of the complete hydrate assemblage of cements, including calcium aluminate hydrates such as ettringite and AFm phases. (3) For quantification of portlandite (Ca(OH)₂) all tested hydration stoppage protocols are satisfactory, with the exception of oven drying.     

Phase transformations and mechanical strength of OPC/Slag pastes submitted to high temperatures

Ground granulated blast furnace slag (GGBFS or “slag”) is a by product of the steel industry and is often used in combination with ordinary Portland cement (OPC) as a binder in concrete. When concrete is exposed to high temperatures, physical and chemical transformations lead to significant loss of mechanical strength. Past studies have reported changes in concrete where OPC is 100% of the binder, but there is a lack of published data on slag blended cements. This work provides better understanding of how slag blended cement pastes behave when exposed to high temperatures, when the critical transformations occur, and what the consequences in the structure of these pastes are. Thermogravimetric analysis made it possible to identify when the transformations occurred and the changes in mechanical strength in the cement paste. A unique outcome of this work is the lower damage presented by slag blended cements after exposure to high temperatures     

Shrinkage characteristics of alkali-activated fly ash/slag paste and mortar at early ages

The purpose of this study is to investigate the shrinkage characteristics of alkali-activated fly ash/slag (henceforth simply AFS) and the factors affecting it. A series of tests were conducted to determine the chemical shrinkage, autogenous shrinkage and drying shrinkage. The microstructures and reaction products were also characterized through XRD and SEM/EDS analyses. An increase in the slag content from 10% to 30% resulted in a denser matrix and showed a higher Ca/Si ratio of C–N–A–S–H in the microstructure. Higher sodium silicate and slag contents in a mixture caused more chemical, autogenous, and drying shrinkage, but led to a higher compressive strength. From the test results, it can be concluded that the autogenous shrinkage of AFS mortar occurs mainly due to self-desiccation in hardened state rather than volume contraction by chemical shrinkage in fresh state. The AFS paste showed higher drying shrinkage than ordinary Portland cement (OPC), which may be caused by the higher mesopore volume of the AFS paste compared to that of OPC paste.     

Nanosilica effects on composition and silicate polymerization in hardened cement paste cured under high temperature and pressure

Cement pastes of water to cement ratio (w/c) of 0.45 with and without nanosilica are hydrated under two conditions, room condition (20 °C with 0.1 MPa pressure) and an oil well condition (80 °C with 10 MPa pressure) for 7 days. For the cement pastes with nanosilica, 1% and 3% of cements weights were replaced by nanosilica. The composition of the hardened cement pastes is investigated using X-ray diffraction (XRD). Nuclear magnetic resonance (NMR) experiments are used to quantify the silicate polymerization in hydrated cement paste. Microstructural phases are identified according to the corresponding mechanical property using nanoindentation. The results showed that under room curing conditions, hardened cement paste with 1% nanosilica has the highest level of calcium silicate hydrate (C–S–H) polymerization. However, under high temperature and pressure curing conditions, hardened cement paste with 3% nanosilica has the highest level of C–S–H polymerization. A new relatively stiff microstructural phase is observed in cement pastes incorporating nanosilica and cured under elevated pressure and temperature conditions. The significance of curing conditions and nanosilica content on the polymerization and stiffness of hydrated cement pastes are discussed.     

Hydration characteristics of municipal solid waste incinerator bottom ash slag as a pozzolanic material for use in cement

This study investigated the pozzolonic reactions and engineering properties of municipal solid waste incinerator (MSWI) bottom ash slag blended cements (SBC) with various replacement ratios. The 90-day compressive strengths developed by SBC pastes with 10% and 20% cement replacement by slags generated from the bottom ash were similar to that developed by ordinary Portland cement pastes. Thermal analyses indicated that the hydrates in the SBC pastes were mainly portlandite (Ca(OH)₂) and calcium silicate hydrate (C–S–H) gels, similar to those found in ordinary Portland cement paste. It is also indicated that the slag reacted with Ca(OH)₂ to form C–S–H. The average length (in terms of the number of Si molecules) of linear polysilicate anions in C–S–H gel, as determined by ²⁹Si nuclear magnetic resonance, increased in all the SBC pastes with increasing curing age, which outperformed that of ordinary Portland cement at 90 days. It can thus be concluded from the study results, that municipal solid waste incinerator bottom ash can be processed by melting to obtain reactive pozzolanic slag, which may be used in SBC to partially replace the cement.     

The structure of the calcium silicate hydrate phases present in hardened pastes of white Portland cement/blast-furnace slag blends

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. This revised version was published online in November 2006 with corrections to the Cover Date.     

Carbonation of ternary building cementing materials

The carbonation processes of ettringite and calcium aluminate hydrates phases developed by hydration of calcium aluminate cement, fly ash and calcium sulphate ternary mixtures have been studied. The hydrated samples were submitted to 4% of CO₂ in a carbonation chamber, and were analysed, previous carbonation and after 14 and 90 days of carbonation time, by infrared spectroscopy and X-ray diffraction; the developed morphology was performed with the 14 days carbonated samples. The results evidenced that ettringite reacts with CO₂ after 14 days of exposition time and evolves totally at 90 days; the developed hydrated phases C₃AH₆ in samples with major CAC content, also reacts with CO₂. Due to carbonation, calcium carbonate – mainly vaterite but also aragonite-, depending on the initial formulation, aluminium hydroxide and gypsum were detected.     

Microindentation creep of secondary hydrated cement phases and C–S–H

Microindentation creep measurements were obtained on compacted specimens of several secondary hydrated cement phases in equilibrium with water vapor at 11%RH. Values of creep modulus, indentation modulus and indentation hardness for calcium hydroxide, ettringite, gypsum and calcium carbonate are reported. The porosity dependence of these parameters was established and the significance of porosity on the time-dependent deformation of these materials was discussed. In addition the microindentation creep behavior of pure C–S–H and C₃S paste hydrated 32 years was determined. The discussion focuses on the relative importance of the contribution of the secondary phases in hydrated cement-based materials to creep with respect to the more ‘active’ C–S–H phase.     

Solid state NMR investigations on the role of organic admixtures on the hydration of cement pastes

The influence of different inorganic additives and organic admixtures on the hydration and hardening of Portland cement (CEM I 42.5R) were studied on a nanometer scale by advanced solid state NMR methods. Added quartz was found to be partially attacked by the alkaline media of the cement paste. Even small amounts of organic admixtures strongly influence the hydration and crystallization process of the cement paste. Methyl cellulose, poly(vinyl acetate co vinyl alcohol), poly(ethylene oxide), poly(acrylic acid) and poly(acrylamide) modify the hydration of the calcium aluminum oxides. Major changes in the inorganic structure were detected for low amounts of citric acid and tartaric acid which suppress silicate condensation and strongly alter calcium aluminum oxide hydration. Within this study several solid state NMR methods like 1D magic angle spinning (MAS), 2D exchange and 2D double quantum NMR were applied for the detection of ¹H, ²⁷Al and ²⁹Si nuclei. Thus, cement pastes, inorganic additives and organic admixtures could be monitored individually. The findings on a molecular level as provided by NMR are related to changes in the mechanical properties of the cement pastes.