Understanding the Filler Effect on the Nucleation and Growth of C‐S‐H

The “filler effect”, due to the physical presence of mineral additions in cement, is mainly known to accelerate the hydration of the clinker component. Previously, this was attributed to the surface of the filler providing nucleation sites for C‐S‐H as there is a clear dependence on the surface provided by the filler particles. Our results reveal that the increase in nucleation is quite low compared to the area provided. Based on the isothermal calorimetry experiments and SEM images, we demonstrate that the most important parameter is the interparticle distance. We propose that this is mainly the result of the shearing conditions rather than extra surface available for C‐S‐H as formerly assumed. Quantitatively slag and fly ash behave very similarly to quartz. Limestone, on the other hand, seems also to stimulate C‐S‐H nucleation giving it a higher efficiency in accelerating clinker hydration.     

Influence of pozzolanic additives on hydration mechanisms of tricalcium silicate

Pozzolanic mineral additives, such as silica fume (SF) and metakaolin (MK), are used to partially replace cement in concrete. This study employs extensive experimentation and simulations to elucidate and contrast the influence of SF and MK on the early age hydration rates of tricalcium silicate (triclinic C₃S), the major phase in cement. Results show that at low replacement levels (i.e., ≤10%), both SF and MK accelerate C₃S hydration rates via the filler effect, that is, enhanced heterogeneous nucleation of the main hydration product (C–S–H: calcium‐silicate‐hydrate) on the extra surfaces provided by the additive. The filler effect of SF is inferior to that of MK because of agglomeration of the fine particles of SF, which causes significant reduction (i.e., up to 97%) in its surface area. At higher replacement levels (i.e., ≥20%), while SF continues to serve as a filler, the propensity of MK to allow nucleation of C–S–H on its surface is substantially suppressed. This reversal in the filler effect of MK is attributed to the abundance of aluminate [Al(OH)₄^?] ions in the solution—released from the dissolution of MK—which inhibit topographical sites for C–S–H nucleation and impede its subsequent growth. Results also show that in the first 24 hours of hydration, MK is a superior pozzolan compared to SF. However, the pozzolanic activities of both SF and MK are limited and, thus, do not produce significant alterations in the early age hydration kinetics of C₃S. Overall, the outcomes of this study provide novel insights into the mechanistic origins of the filler and pozzolanic effects of SF and MK, and their impact on cementitious reaction rates.     

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.     

Influence of silica fume on the microstructure of cement pastes: New insights from 1H NMR relaxometry

¹H NMR has been used to characterise white Portland cement paste incorporating 10 wt.% of silica fume. Samples were measured sealed throughout the hydration without sample drying. Paste compositions and C–S–H characteristics are calculated based on ¹H NMR signal intensities and relaxation analysis. The results are compared with a similar study of plain white cement paste. While the presence of silica fume has little influence on C–S–H densities, the chemical composition is impacted. After 28 days of sealed hydration, the Ca/(Si + Al) ratio of the C–S–H is 1.33 and the H₂O/(Si + Al) ratio is 1.10 when 10% of silica fume is added to the white cement. A densification of the C–S–H with time is observed. There are no major changes in capillary, C–S–H gel and interlayer pore sizes for the paste containing silica fume compared to the plain white cement paste. However, the gel/interlayer water ratio increases in the silica fume blend.     

The Early Age Hydration Reactions of a Hybrid Cement Containing a Very High Content of Coal Bottom Ash

A study examining the complex hydration chemistry of a hybrid alkaline cement containing a high content of coal bottom ash (BA) (>70%) and a low content of portland cement clinker in the presence of an alkaline activator is presented. The use of a water reducing additive was found to significantly delay the overall hydration process, allowing an opportunity to more clearly distinguish the hydration reactions that take place. The results presented showed that both the cement clinker phases and the ash glassy phases are highly reactive for the first 3 d of hydration. In situ formed reaction products portlandite and gypsum were shown to be metastable and had disappeared within 3 d of hydration. Ettringite stability was limited in the hybrid system but unlike gypsum and portlandite, remained detectable for the first 3 d of hydration at least. SEM‐EDX and subtracted Fourier transform infrared evidence suggest the development of three different gel bond environments, tentatively attributed to C–(A)–S–H, C–A–S–H, and (N,C)–A–S–H type gels.     

Rheological characterization of cement pastes with functional filler particles

Cement pastes were studied using conventional rheological methods. The effect of different types of functional fillers on the rheological properties and hydration rate of the pastes was analysed. The fillers were found to have varying chemistry and therefore surface properties. Porous titania particles raised the cohesive energy of the paste significantly already at low additions. Limestone particles raised the cohesive energy somewhat, but clearly less than titania. The zeta potential of the particles was found to somewhat affect the rheology. Reaction rates of the pastes were measured using oscillatory rheological measurement with constant amplitude and frequency. Titania particles were found to accelerate the hydration rate more than limestone particles. Of the limestone particles pure calcite increased the reaction rate more than dolomitic limestone. When the limestone particles were modified to provide C–S–H nucleation sites the cohesive energy was raised and the reaction rate was further increased.     

硅灰和减水剂对硅酸三钙水化调控机理的研究

采用XRD、精密水化微量热、ESEM、EDS和NMR等测试技术,研究了硅灰和聚羧酸减水剂对C3S水化的影响。结果表明：聚羧酸减水剂的掺入抑制了C3S的早期水化放热,而硅灰消耗了C3S水化产生的CH,促进了C3S的水化.两者都使C3S水化产物C-S-H凝胶的形貌由针棒状发生了转变,且其硅氧四面体的聚合状态有较大不同,尤其是硅灰显著影响了C-S-H凝胶硅氧四面体聚合状态中Q1、Q2的含量。     

Some New Perspective on the Reaction Mechanism of MgO–SiO2–H2O System

The application of MgO–SiO₂–H₂O system, one of the most popular basic refractories, is greatly limited because of the hydration of MgO. In this work, the phase and morphological development of MgO–SiO₂–H₂O system during aging were analyzed using various techniques. It is found that Mg(OH)₂ initially appears and the amount increases after 10 days aging at room temperature. At elevated temperature, the heat treatment can facilitate the hydration reaction. Mg(OH)₂ covers the surface of silica fume particles. The combination product connects with each other and distributes homogeneously around MgO particles to produce M–S–H gels, which inhibit the further hydration.     

Class H cement hydration at 180 °C and high pressure in the presence of added silica

Under deep oil-well conditions of elevated temperature and pressure, crystalline calcium silicate hydrates are formed during Portland cement hydration. The use of silica rich mineral additives leads to the formation of crystalline hydrates with better mechanical properties than those formed without the additive. The effects of silica flour, silica fume (amorphous silica), and a natural zeolite mixture on the hydration of Class H cement slurries at 180 °C under externally applied pressures of 7 and 52 MPa are examined in real time using in-situ synchrotron X-ray diffraction. For some compositions examined, but not all, pressure was found to have a large effect on the kinetics of crystalline hydrate formation. The use of silica fume delayed both C₃S hydration and the formation of crystalline silicate hydrates compared to what was seen with other silica sources.     

Dissolution rate of silica fume in very high strength concrete

A very high strength concrete, having a 91 day compressive strength of 113 MPa, was developed using Type III cement, limestone aggregates, sodium naphthalene superplasticizer and silica fume, with W/C ratio of 0.24. SEM-EDXA and AEM were used to study the rate of dissolution of silica fume in this concrete, with progressive hydration. The ultrafine particle size of silica fume (< 1 μm) makes it difficult to view the state of these particles in concrete under the SEM. With AEM, however, it was possible to observe the dissolution process of silica fume particles, which begin at an early stage. Within 28 days, most of the silica fume is consumed in the pozzolanic reaction. The initial reaction product is a silica rich gel which later transforms into different morphological types of C-S-H which are compacted together. This is a major contributory factor for the very high strength of this concrete. Some partly reacted silica fume particles, however, remain in the hardened paste; lack of water most probably inhibits their complete transformation.     

The Effect of Magnesium and Zinc Ions on the Hydration Kinetics of C3S

Impure tricalcium silicate (C₃S) in portland cement may contain various foreign ions. These ions can stabilize different polymorphs of C₃S at room temperature and may affect its reactivity. In this paper, the effects of magnesium and zinc on the polymorph type, hydration kinetics, and the hydrate morphology of C₃S were investigated. The pure C₃S has the T1 structure while magnesium and zinc stabilize polymorphs M3 and T2/T3, respectively. The two elements have distinct effects on the hydration kinetics. Zinc increases the maximum heat released. Magnesium increases the hydration peak width. The C–S–H morphology is modified, leading to longer needles in the presence of zinc and thicker needles in the presence of magnesium. Zinc is incorporated into C–S–H, while magnesium is only incorporated slightly, if at all, but rather seems to inhibit nucleation. Implementing experimentally measured parameters for C–S–H nucleation and growth in the μic hydration model captured well the observed changes in hydration kinetics. This supports C–S–H nucleation and growth to be rate controlling in the hydration of C₃S.     

垃圾焚烧飞灰煅烧阿利尼特水泥熟料的形成过程及其水化性能研究

以城市垃圾焚烧飞灰(以下简称焚烧飞灰)为主要原料,在实验室电炉里成功研制了阿利尼特水泥熟料。本文主要研究水泥熟料煅烧形成过程及其水化性能,分析了阿利尼特水泥的适宜石膏掺量、水化放热特征、水化产物及其显微结构。研究结果表明:利用垃圾焚烧飞灰为主要原料可以成功烧制阿利尼特水泥熟料,煅烧过程中首先出现C2S、C12A7和C2S·CaCl2,随后与MgO和CaCl2反应生成阿利尼特;掺加5%二水石膏可以促进阿利尼特水泥水化,较普通硅酸盐水泥更快,阿利尼特水泥可以作为一种早强快硬型水泥使用;阿利尼特水泥主要水化产物除含有硅酸盐水泥中常见的CSH凝胶、棒状AFt和Ca(OH)2晶体外,还含有C3A·CaCl·210H2O晶体。     

利用同步辐射X射线小角散射研究超低水灰比水化水泥早期微结构

利用同步辐射X射线小角散射技术表征超低水灰比水泥水化早期产物分形结构,研究水化水泥早龄期微结构及演变。水泥水化早期水化产物双分形结构表明：水泥水化硅酸钙纳米凝胶颗粒随机堆积,内外层水化产物中纳米凝胶粒子结构分别具备不同的自相似性;内层水化产物中高密度凝胶堆积紧密,早龄期时随水化时间增加,堆积密度降低。外层水化产物中低密度凝胶堆积较为松弛,早龄期时随水化时间增加,堆积密度也在降低;水化诱导期生成结构最为致密的一层凝胶产物,诱导期后,凝胶产物层堆积密度随水化龄期的增加而减小,水化反应平缓后,凝胶堆积密度的演变也趋于平缓。     

The filler effect: The influence of filler content and type on the hydration rate of tricalcium silicate

The partial replacement of ordinary portland cement (OPC) by fine mineral fillers accelerates the rate of hydration reactions. This acceleration, known as the filler effect, has been attributed to enhanced heterogeneous nucleation of C‐S‐H on the extra surface provided by fillers. This study isolates the cause of the filler effect by examining how the composition and replacement levels of two filler agents influence the hydration of tricalcium silicate (T1‐Ca₃SiO₅; C₃S), a polymorph of the major phase in ordinary portland cement (OPC). For a unit increase in surface area of the filler, C₃S reaction rates increase far less than expected. This is because the agglomeration of fine filler particles can render up to 65% of their surface area unavailable for C‐S‐H nucleation. By analysis of mixtures with equal surface areas, it is hypothesized that limestone is a superior filler as compared to quartz due to the sorption of its aqueous CO₃^2? ions by the C‐S‐H—which in turn releases OH^? species to increase the driving force for C‐S‐H growth. This hypothesis is supported by kinetic data of C₃S hydration occurring in the presence of CO₃^2? and SO₄^2? ions provisioned by readily soluble salts. Contrary to prior investigations, these results suggest that differences in heterogeneous nucleation of the C‐S‐H on filler particle surfaces, caused due to differences in their interfacial properties, have little if any effect on C₃S hydration kinetics.     

硅灰改性压实水泥材料的组成和微观结构

用ＤＴＡ、ＸＲＤ及ＳＥＭ分析了不同养护条件下硅灰改性前后压实水泥的组成和微观结构，并探讨了结构与耐久性的关系。结果表明：在常温水养护条件下，除Ｃａ（ＯＨ）２外，硅灰改性前后压实水泥的组成无明显差异；压蒸条件下，随压蒸时间延长伴有产物的转化和晶化，掺入硅灰后，产物向低ＣａＯ／ＳｉＯ２方向转化和晶化；材料结构中未水化水泥颗粒进一步水化以及产物的转化和晶化对材料的结构和性能产生不利影响，甚至会导致结构破坏     

Solution‐Controlled Dissolution of Supplementary Cementitious Material Glasses at pH 13: The Effect of Solution Composition on Glass Dissolution Rates

Supplementary cementitious materials (SCMs) are widely used to partially replace portland clinker in blended cements. Reducing clinker contents further without compromising the development of early strength necessitates a better assessment and enhancement of the reactivity of the available SCMs. To this purpose, the reactivity of synthesized calcium aluminosilicate glasses covering a compositional range from blast‐furnace slags (BFS) over fly ashes to silica fume was analyzed by dissolution experiments. Initial glass dissolution rates were measured at 20°C and pH 13, and with varying initial concentrations of aqueous Al, Ca, and Si. At pH 13, glass dissolution rates were observed to scale linearly with the glass Ca/(Al + Si) molar ratio. Ca‐rich blast‐furnace type glass dissolution was shown to be up to one order of magnitude faster than tectosilicate fly ash and silica fume type glass dissolution, supporting different pathways to dissolution. In solutions that are strongly undersaturated with respect to hydrous glass and hydration products, glass dissolution rates are independent of changes in solution undersaturation and aqueous Si activity. In contrast, dissolution rates decrease with aqueous Ca concentration for all glasses and with aqueous Al concentration for tectosilicate‐type glasses. The insights gained are instrumental in finding ways to enhance SCM reactivity.     

Effect of particle size distribution of metakaolin on hydration kinetics of tricalcium silicate

The focus of this study is to elucidate the role of particle size distribution (PSD) of metakaolin (MK) on hydration kinetics of tricalcium silicate (C₃S–T₁) pastes. Investigations were carried out utilizing both physical experiments and phase boundary nucleation and growth (pBNG) simulations. [C₃S + MK] pastes, prepared using 8%_mass or 30%_mass MK, were investigated. Three different PSDs of MK were used: fine MK, with particulate sizes <20 µm; intermediate MK, with particulate sizes between 20 and 32 µm; and coarse MK, with particulate sizes >32 µm. Results show that the correlation between specific surface area (SSA) of MK's particulates and the consequent alteration in hydration behavior of C₃S in first 72 hours is nonlinear and nonmonotonic. At low replacement of C₃S (ie, at 8% _mass), fine MK, and, to some extent, coarse MK act as fillers, and facilitate additional nucleation and growth of calcium silicate hydrate (C–S–H). When C₃S replacement increases to 30% _mass, the filler effects of both fine and coarse MK are reversed, leading to suppression of C–S–H nucleation and growth. Such reversal of filler effect is also observed in the case of intermediate MK; but unlike the other PSDs, the intermediate MK shows reversal at both low and high replacement levels. This is due to the ability of intermediate MK to dissolve rapidly—with faster kinetics compared to both coarse and fine MK—which results in faster release of aluminate [Al(OH)₄⁻] ions in the solution. The aluminate ions adsorb onto C₃S and MK particulates and suppress C₃S hydration by blocking C₃S dissolution sites and C–S–H nucleation sites on the substrates’ surfaces and suppressing the post-nucleation growth of C–S–H. Overall, the results suggest that grinding-based enhancement in SSA of MK particulates does not necessarily enhance early-age hydration of C₃S.     

Evolution of C–S–H in lime mortars with nanoparticles: Nanostructural analysis of afwillite growth mechanisms by HRTEM

This work studies the formation of calcium–silicate–hydrates (C–S–H) in lime mortars prepared with additions of nano-Ca(OH)₂ and nano-SiO₂ at 23.3°C. Mineral identification was carried out by X-ray diffraction after 10, 30, 90, and 120 days of curing. The nanoscale study starts from the generation of amorphous phases until the development of crystalline phases. Observations of binder mortar made by transmission electron microscopy (low magnification and high resolution TEM) after 30 and 110 days of curing showed the formation of two types of C–S–H with different degrees of crystallinity depending on the curing time. The development of short-range order C–S–H globular phases was visible after 30 days. C–S–H evolved into lamellar crystalline phases visible after 110 days of curing. The crystalline phase corresponds to the C–S–H known as afwillite (Ca₃(SiO₃OH)₂·2H₂O), first reported to affect cement and concrete's mechanical and hydration properties. It appears as isolated fibers, growing epitaxially along the edges of the calcite (product of the carbonation of Ca(OH)₂), and advancing inward aided by atomic defects (grain boundaries, stacking faults). In addition, high-resolution transmission electron microscopy tools and electron diffraction simulation confirmed a monoclinic symmetry for afwillite crystals. These results contribute to analyzing the presence of crystalline/amorphous C–S–H in lime mortars, providing information on the structure of afwillite and its possible effects on the binder materials.     

煤矸石对水泥熟料水化促进作用及机理(英文)

通过测试化学结合水量及用背散射电子图像分析法测试水泥熟料水化程度,研究了煤矸石–水泥混合体系中煤矸石对水泥熟料水化的促进作用,并通过对Ca(OH)2含量测试分析了其作用机理。结果表明:煤矸石的掺入促进了混合体系中水泥熟料的水化进程,且活性越高,掺量越大,促进水泥效果越明显;水化早期煤矸石对水泥熟料水化的促进作用主要来源于简单"稀释"作用(物理作用),水化中后期具有较高活性的活化煤矸石通过自身反应活性发挥吸收体系中Ca(OH)2促进水泥熟料水化(化学作用)。     

The hydration of Portland cement,C3S and C2S in the presence of a calcium complexing admixture (EDTA)

The effect of EDTA, a calcium chelating agent, on the early hydration of Portland cement, C₃Sand β-C₂S has been studied by solution analysis and electron microscopy. EDTA is a retarded of cement hydration. Under normal conditions of hydration, the silica levels in solution are very low (<0.05 M) but in the presence of EDTA an initial flush of silica appears in the bulk aqueous phase. On continued hydration, following the saturation of EDTA with calcium, the appearance of ‘free’ calcium causes precipitation of C-S-H gel from the bulk solution and changes in microstructure of the colloidal gel around clinker particles in C₃S and β-C₂S pastes are observed. The action of EDTA as a retarding admixture is explained in terms of the membrane model of cement hydration.