Composition of C–S–H in pastes with increasing levels of silica fume addition

New results show that the microstructure development of cement–silica fume blends is very different from plain cement. Portlandite (CH) tends to precipitate as platelets and even around clinker grains as “CH rims” and is consumed by pozzolanic reaction with silica fume. The Ca/Si ratio in the inner product (IP) C–S–H decreases as CH is consumed to reach Ca/Si ≈ 1.40–1.50 at the point when CH has disappeared, and then drops down to 1.00 in absence of CH. At later ages, the IP C–S–H is often composed of two distinct regions. The outermost (formed first) consists of originally high Ca/Si C–S–H, which Ca/Si slowly decreases. The second (formed later) forms only once CH is no longer present and has a lower Ca/Si. Between 10 and 38 °C, the main effect of increasing the temperature is to accelerate the reaction of cement and increase the reactivity of silica fume. The changes in Ca and Si in the pore solution of similar systems suggest that the composition of the solution and the solids reciprocally influence each other.     

The pozzolanic reactivity of monodispersed nanosilica hydrosols and their influence on the hydration characteristics of Portland cement

This study reveals that the nanosilica hydrosols with higher specific surface areas had faster pozzolanic reactivity, especially at early ages; moreover, the results are indicative of the accelerating influence of nanosilicas and silica fume on the hydration of cement. Faster initial and final setting times observed for cement pastes containing nanosilicas are consequence of these mechanisms. However, less hydration degree of cement compared to the plain paste was observed at age of 7 days and after. This can be attributed to the entrapment of some of mix water in the aggregates of nanosilicas formed in cement paste environment, making less water available for the progress of cement hydration. The same mechanism is believed to be responsible for the reduction of flowability of cement pastes.     

Carbonation of C–S–H and C–A–S–H samples studied by 13C, 27Al and 29Si MAS NMR spectroscopy

Synthesized calcium silicate hydrate (C–S–H) samples with Ca/Si ratios of 0.66, 1.0, and 1.5 have been exposed to atmospheric CO₂ at room temperature and high relative humidity and studied after one to 12 weeks. ²⁹Si NMR reveals that the decomposition of C–S–H caused by carbonation involves two steps and that the decomposition rate decreases with increasing Ca/Si ratio. The first step is a gradual decalcification of the C–S–H where calcium is removed from the interlayer and defect sites in the silicate chains until Ca/Si = 0.67 is reached, ideally corresponding to infinite silicate chains. In the seconds step, calcium from the principal layers is consumed, resulting in the final decomposition of the C–S–H and the formation of an amorphous silica phase composed of Q³ and Q⁴ silicate tetrahedra. The amount of solid carbonates and of carbonate ions in a hydrous environment increases with increasing Ca/Si ratio for the C–S–H, as shown by ¹³C NMR. For C–A–S–H samples with Ca/Si = 1.0 and 1.5, ²⁷Al NMR demonstrates that all aluminium sites associated with the C–S–H are consumed during the carbonation reactions and incorporated mainly as tetrahedral Al(–OSi)₄ units in the amorphous silica phase. A small amount of penta-coordinated Al sites has also been identified in the silica phase.     

Chemical acceleration of a neutral granulated blast-furnace slag activated by sodium carbonate

This paper presents results of a study on chemical acceleration of a neutral granulated blast-furnace slag activated using sodium carbonate. As strength development of alkali-activated slag cements containing neutral GBFS and sodium carbonate as activator at room temperature is known to be slow, three accelerators were investigated: sodium hydroxide, ordinary Portland cement and a combination of silica fume and slaked lime. In all cements, the main hydration product is C–(A)–S–H, but its structure varies between tobermorite and riversideite depending on the accelerator used. Calcite and gaylussite are present in all systems and they were formed due to either cation exchange reaction between the slag and the activator, or carbonation. With accelerators, compressive strength up to 15 MPa can be achieved within 24 h in comparison to 2.5 MPa after 48 h for a mix without an accelerator.     

Silica fume as porogent agent in geo-materials at low temperature

The synthesis of geopolymers based on alkaline polysialate was achieved at low temperature (～25–80 °C) by the alkaline activation of raw minerals and silica fume. The materials were prepared from a solution containing dehydroxylated kaolinite and alkaline hydroxide pellets dissolved in potassium silicate. Then the mixture was transferred to a polyethylene mold sealed with a top and placed in an oven at 70 °C for 24 h. For all geopolymer materials, following dissolution of the raw materials, a polycondensation reaction was used to form the amorphous solid, which was studied by FTIR-ATR spectroscopy. The in situ inorganic foam based on silica fume was synthesized from the in situ gaseous production of dihydrogen due to oxidation of free silicon (content in the silica fume) by water in alkaline medium, which was confirmed via TGA-MS experiments. This foam has potential as an insulating material for applications in building materials since the thermal measurement has a value of 0.22 W m^?1 K^?1.     

Influence of amorphous silica on the hydration in ultra-high performance concrete

Amorphous silica particles (silica) are used in ultra-high performance concretes to densify the microstructure and accelerate the clinker hydration. It is still unclear whether silica predominantly increases the surface for the nucleation of C–S–H phases or dissolves and reacts pozzolanically. Furthermore, varying types of silica may have different and time dependent effects on the clinker hydration. The effects of different silica types were compared in this study by calorimetric analysis, scanning and transmission electron microscopy, in situ X-ray diffraction and compressive strength measurements. The silica component was silica fume, pyrogenic silica or silica synthesized by a wet-chemical route (Stoeber particles). Water-to-cement ratios were 0.23. Differences are observed between the silica for short reaction times (up to 3 days). Results indicate that silica fume and pyrogenic silica accelerate alite hydration by increasing the surface for nucleation of C–S–H phases whereas Stoeber particles show no accelerating effect.     

Hydration of Portland cement with high replacement by siliceous fly ash

The effects of two different low calcium fly ashes on the hydration of ordinary Portland cement (OPC) pastes containing 50 wt.% of fly ash were investigated over a hydration time of 550 days. The results were compared with a reference blend of OPC containing 50 wt.% of inert quartz powder allowing the distinction between "filler effect" and pozzolanic reaction.Until 2 days, no evidence of fly ash reaction was measured and its influence on the hydration is mainly related to the “filler effect”. From 7 days on, the effects of the pozzolanic reaction were observed by the consumption of portlandite, the change of the pore solution chemistry, the formation of a presumably water-rich inner hydration product and the change of the C–S–H composition towards higher Al/Si ratio compared to the C–S–H of neat OPC. Additional strength due to the pozzolanic reaction developed after 28 days of hydration.     

Time-dependent behaviour of hardened cement paste under isotropic loading

The experimental results of isotropic compression tests performed at 20 °C and 90 °C on a class G hardened cement paste hydrated at 90 °C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424–1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60 °C. The time-dependent deformations are more pronounced for hydration at a higher temperature.     

Atomic and nano-scale characterization of a 50-year-old hydrated C3S paste

This paper investigates the atomic and nano-scale structures of a 50-year-old hydrated alite paste. Imaged by TEM, the outer product C–S–H fibers are composed of particles that are 1.5–2 nm thick and several tens of nanometers long. ²⁹Si NMR shows 47.9% Q₁ and 52.1% Q₂, with a mean SiO₄ tetrahedron chain length (MCL) of 4.18, indicating a limited degree of polymerization after 50 years' hydration. A Scanning Transmission X-ray Microscopy (STXM) study was conducted on this late-age paste and a 1.5 year old hydrated C₃S solution. Near Edge X-ray Absorption Fine Structure (NEXAFS) at Ca L_3,2-edge indicates that Ca^2 + in C–S–H is in an irregular symmetric coordination, which agrees more with the atomic structure of tobermorite than that of jennite. At Si K-edge, multi-scattering phenomenon is sensitive to the degree of polymerization, which has the potential to unveil the structure of the SiO₄^4 − tetrahedron chain.     

Structural changes in C–S–H gel during dissolution: Small-angle neutron scattering and Si-NMR characterization

Flow-through experiments were conducted to study the calcium–silicate–hydrate (C–S–H) gel dissolution kinetics. During C–S–H gel dissolution the initial aqueous Ca/Si ratio decreases to reach the stoichiometric value of the Ca/Si ratio of a tobermorite-like phase (Ca/Si = 0.83). As the Ca/Si ratio decreases, the solid C–S–H dissolution rate increases from (4.5 × 10^− 14 to 6.7 × 10^− 12) mol m^− 2 s^− 1. The changes in the microstructure of the dissolving C–S–H gel were characterized by small-angle neutron scattering (SANS) and ²⁹Si magic-angle-spinning nuclear magnetic resonance (²⁹Si-MAS NMR). The SANS data were fitted using a fractal model. The SANS specific surface area tends to increase with time and the obtained fit parameters reflect the changes in the nanostructure of the dissolving solid C–S–H within the gel. The ²⁹Si MAS NMR analyses show that with dissolution the solid C–S–H structure tends to a more ordered tobermorite structure, in agreement with the Ca/Si ratio evolution.     

Hydration of water- and alkali-activated white Portland cement pastes and blends with low-calcium pulverized fuel ash

Pastes of white Portland cement (wPc) and wPc-pulverized fuel ash (pfa) blends were studied up to 13 years. The reaction of wPc with water was initially retarded in the presence of pfa particles but accelerated at intermediate ages. Reaction with KOH solution was rapid with or without pfa. A universal compositional relationship exists for the C-A-S-H in blends of Pc with aluminosilicate-rich SCMs. The average length of aluminosilicate anions increased with age and increasing Al/Ca and Si/Ca; greater lengthening in the blends was due to additional Al^3 + at bridging sites. The morphology of outer product C-A-S-H was always foil-like with KOH solution, regardless of chemical composition, but with water it had fibrillar morphology at high Ca/(Si + Al) ratios and foil-like morphology started to appear at Ca/(Si + Al) ≈ 1.2–1.3, which from the literature appears to coincide with changes in the pore solution. Foil-like morphology cannot be associated with entirely T-based structure.     

Effect of temperature on the hydration of Portland cement blended with siliceous fly ash

The effect of temperature on the hydration of Portland cement pastes blended with 50 wt.% of siliceous fly ash is investigated within a temperature range of 7 to 80 °C.The elevation of temperature accelerates both the hydration of OPC and fly ash. Due to the enhanced pozzolanic reaction of the fly ash, the change of the composition of the C–S–H and the pore solution towards lower Ca and higher Al and Si concentrations is shifted towards earlier hydration times. Above 50 °C, the reaction of fly ash also contributes to the formation of siliceous hydrogarnet. At 80 °C, ettringite and AFm are destabilised and the released sulphate is partially incorporated into the C–S–H. The observed changes of the phase assemblage in dependence of the temperature are confirmed by thermodynamic modelling.The increasingly heterogeneous microstructure at elevated temperatures shows an increased density of the C–S–H and a higher coarse porosity.     

Formation of magnesium silicate hydrate (M-S-H) cement pastes using sodium hexametaphosphate

Magnesium silicate hydrate (M-S-H) gel is formed by the reaction of brucite with amorphous silica during sulphate attack in concrete and M-S-H is therefore regarded as having limited cementing properties. The aim of this work was to form M-S-H pastes, characterise the hydration reactions and assess the resulting properties. It is shown that M-S-H pastes can be prepared by reacting magnesium oxide (MgO) and silica fume (SF) at low water to solid ratio using sodium hexametaphosphate (NaHMP) as a dispersant. Characterisation of the hydration reactions by x-ray diffraction and thermogravimetric analysis shows that brucite and M-S-H gel are formed and that for samples containing 60 wt.% SF and 40 wt.% MgO all of the brucites react with SF to form M-S-H gel. These M-S-H cement pastes were found to have compressive strengths in excess of 70 MPa.     

Influence of temperature on the hydration products of low pH cements

The chemical evolution of two hydrated “low pH” binders prepared from binary (60% Portland cement + 40% silica fume) or ternary (37.5% Portland cement + 32.5% silica fume + 30% fly-ash) mixtures was characterized over one year at 20 °C, 50 °C, and 80 °C. The main hydrates were Al-substituted C–S–H. Raising the temperature from 20 to 80 °C caused a lengthening and cross-linking of their silicate chains. Ettringite that formed in pastes stored at 20 °C was destabilized. Only traces of calcium sulfate (gypsum and/or anhydrite) reprecipitated after one year in some materials cured at 50 °C and 80 °C. The sulfates released were therefore partially adsorbed on the C–A–S–H and dissolved in the pore solution. The pore solution pH dropped by about 2 units as the temperature increased. Conversely, the soluble alkali fractions did not change significantly. Only the ternary binder resulted in a pore solution pH below 11 at the three temperatures studied.     

The effect of pressure on tricalcium silicate hydration at different temperatures and in the presence of retarding additives

The hydration of tricalcium silicate (C₃S) is accelerated by pressure. However, the extent to which temperature and/or cement additives modify this effect is largely unknown. Time-resolved synchrotron powder diffraction has been used to study cement hydration as a function of pressure at different temperatures in the absence of additives, and at selected temperatures in the presence of retarding agents. The magnitudes of the apparent activation volumes for C₃S hydration increased with the addition of the retarders sucrose, maltodextrin, aminotri(methylenephosphonic acid) and an AMPS copolymer. Pressure was found to retard the formation of Jaffeite relative to the degree of C₃S hydration in high temperature experiments. For one cement slurry studied without additives, the apparent activation volume for C₃S hydration remained close to ~ ? 28 cm³ mol^? 1 over the range 25 to 60 °C. For another slurry, there were possible signs of a decrease in magnitude at the lowest temperature examined.     

Leaching behaviour of low Ca:Si ratio CaO–SiO2–H2O systems

A dynamic leaching study of the dissolution of low calcium to silicon ratio (C/S) calcium–silicate–hydrate (C–S–H) systems with initial C/S ranging from 0.2 to 0.6 has been undertaken. Dissolution was studied in demineralised water at 25 °C to a degree of leaching of 2.5 m³ kg^− 1. These C–S–H gels show remarkably similar behaviour during early leaching stages, giving an equilibrated pH of ~ 9.9 and a solution phase C/S of ~ 0.29. Over longer times, C–S–H gels with C/S > 0.29 evolve, on leaching, towards a congruent dissolution point with a solid C/S close to 0.84 (consistent with tobermorite) and pH ~ 10.8. C–S–H gels with C/S < 0.29 become increasingly silica-rich on leaching but maintain an alkaline pH > 9.5 down to at least C/S = 0.07 (the lowest ratio reached). For C/S < 0.7, chemical modelling and X-ray diffraction data support an explanation of the incongruent dissolution behaviour of the low C/S C–S–H gels based on the congruent dissolution of distinct amorphous silica and tobermorite-like C–S–H phases. Above C/S of 0.7, the dissolution data are well described by an ideal solid solution model for the C–S–H phases. These results are of relevance to the consideration of the disposal of silica-rich vitrified intermediate-level radioactive wastes in cement-based concepts for geological disposal, where maintenance of alkaline pH values forms a key component of the chemical barrier to radionuclide migration. The implications are that the long-term pH buffering capacity provided by cementitious backfill materials would not be significantly affected by interactions with silica-rich wasteforms, which may lower the net C/S ratio of C–S–H phases, due to the natural tendency of these systems to restore congruent dissolution at pH 10.8.     

Mechanically activated alite: New insights into alite hydration

For superior understanding of alite hydration an investigation of mechanically activated alite (M3 modification) was performed by XRD and heat flow calorimetry. Activation resulted in reduced particle size, a decreased mean crystallite size and partial amorphization. For the samples of activated alite a significantly accelerated and intensified hydration was observed and complete conversion of alite was found after 24 h. The enthalpy of reaction for crystalline alite was determined to be − 548 J/g from measured heat of hydration after 24 h. The enthalpy of reaction of amorphous “alite” was found to be less exothermic (− 386 J/g). The main hydration period is controlled by nucleation of C–S–H, while the transition from acceleration to deceleration period takes place after consumption of the small alite particles. XRD amorphous C–S–H phase is indicated to precipitate in considerable amount even in the highest activated alite before “long-range ordered”, XRD detectable C–S–H was observed.     

The effect of gyrolite additive on the hydration properties of Portland cement

The influence of gyrolite additive on the hydration properties of ordinary Portland cement was examined. It was found that the additive of synthetic gyrolite accelerates the early stage of hydration of OPC. This compound binds alkaline ions and serves as a nucleation site for the formation of hydration products (stage I). Later on, the crystal lattice of gyrolite becomes unstable and turns into C–S–H, with higher basicity (C/S ~ 0.8). This recrystallization process is associated with the consumption of energy (the heat of reaction) and with a decrease in the rate of heat evolution of the second exothermic reaction (stage II). The experimental data and theoretical hypothesis were also confirmed by thermodynamic and the apparent kinetic parameters of the reaction rate of C₃S hydration calculations. The changes occur in the early stage of hydration of OPC samples and do not have a significant effect on the properties of cement stone.     

Effects of temperature on the atomic structure of synthetic calcium–silicate–deuterate gels: A neutron pair distribution function investigation

Due to the nanocrystallinity of the calcium–silicate–hydrate (C–S–H) gel in ordinary Portland cement-based paste combined with the presence of nanoscale heterogeneities such as varying calcium-to-silicon ratios and incorporation of aluminum in the structure, standard characterization techniques fail to fully capture the complex atomic structure and nanoscale morphology of this important binder phase. Here, neutron pair distribution function (PDF) analysis is applied to a range of deuterated C–S–H gels with varying Ca/Si ratios (denoted C–S–D). In situ temperature measurements reveal that the local atomic bonding environments in C–S–D gel undergo large structural rearrangements due to exposure to elevated temperature (above ~ 200 °C), including the collapse of the C–S–D gel interlayer spacing to 9.6 Å and the emergence of a disordered dicalcium silicate phase (similar to larnite). At lower elevated temperatures, the atom–atom correlations are dominated by scattering from deuterium atoms and therefore can be used to quantify the dehydration kinetics.     

Hydration kinetics of cements by Time-Domain Nuclear Magnetic Resonance: Application to Portland-cement-derived endodontic pastes

Time-Domain Nuclear Magnetic Resonance (TD-NMR) of ¹H nuclei is used to monitor the maturation up to 30 days of three different endodontic cement pastes. The “Solid–liquid” separation of the NMR signals and quasi-continuous distributions of relaxation times allow one to follow the formation of chemical compounds and the build-up of the nano- and subnano-structured C–S–H gel. ¹H populations, distinguished by their different mobilities, can be identified and assigned to water confined within the pores of the C–S–H gel, to crystallization water and Portlandite, and to hydroxyl groups. Changes of the TD-NMR parameters during hydration are in agreement with the expected effects of the different additives, which, as it is known, can substantially modify the rate of reactions and the properties of cementitious pastes. Endodontic cements are suitable systems to check the ability of this non-destructive technique to give insight into the complex hydration process of real cement pastes.