The effect of glass composition on the reactivity of synthetic glasses

The reactivity of synthetic glasses depends on their chemical compositions. In far from equilibrium dissolution experiments, the reactivity of Ca‐rich glasses with compositions similar to blast‐furnace slag is found to be much higher (up to ~60 wt.% after 7 days) compared to Si‐rich glasses with compositions similar to type F fly ash (up to ~20 wt.% after 7 days). Isothermal calorimetry and TGA experiments conducted on model systems containing portlandite and calcite and on glass‐blended Portland cement confirmed the higher reactivity of the Ca‐rich glasses. The degree of glass reaction after 91 days ranged from 7 to 20 wt.%. The results showed also a higher reactivity of the glasses containing more aluminum (both for Ca‐rich and Si‐rich glasses) indicating that not only calcium but also aluminum acted rather as network modifier than as network former. The results confirm a strong dependence of the glass reactivity on the degree of polymerization of the glass network.     

Physical performances of blended cements containing calcium aluminosilicate glass powder and limestone

This work explores the suitability of calcium aluminosilicate (CAS) glass particles as an alternative to conventional supplementary cementitious materials (SCMs) such as fly ash and blast furnace slag. The reason for adding CAS glass particles to the cement blend is to reduce the CO₂ emission of cement production at the same level of performance. For this purpose, blended cement mortars containing 30 wt.% clinker replacement are characterized with respect to workability and mechanical performance. The results indicated that real emission reductions are possible, particularly for combinations of finely ground limestone and CAS glasses which resulted in significantly higher strengths than would be predicted from the individual contributions of each constituent.     

Early stage dissolution characteristics of aluminosilicate glasses with blast furnace slag‐ and fly‐ash‐like compositions

Supplementary cementitious materials (SCM) have been used by the cement industry for decades to partly replace the portland cement fraction of concrete binders. This is particularly important today in addressing CO₂ emissions from the cement manufacturing process. However, defining the reactivity of these mainly aluminosilicate‐based materials and their influence on portland cement hydration chemistry has challenged the research community and has limited SCM replacement levels in cementitious binders. In this study, aluminosilicate glasses as models for blast furnace slag and fly‐ash systems were synthesized and exposed to different activator solutions in a continuously stirred closed system reactor for a period up to 3 hours. Solution compositions were measured from the very first minutes of dissolution and correlated with results from complementary solid surface analysis. Initial Ca concentration maxima in the first 30 minutes of exposure to the activating solution was a common feature in most dissolution profiles with a subsequent rapid decline attributable to Ca‐reincorporation on the reacting surface. Surface‐specific analysis confirmed Ca and Al enrichment at the surface, suggesting the formation of a Ca‐modified aluminosilicate layer, supporting a dissolution‐reprecipitation mechanism for SCM reactivity. Differing chemistries are thought to be responsible for the Ca and Al reintegration on the reacting surface depending on the pH of the solution; near‐neutral conditions favor Ca‐readsorption and surface condensation reactions, whereas alkaline solutions favor Ca‐reintegration via covalently bound phases.     

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.     

Analytical Study of Pure and Extended Portland Cement Pastes: II, Fly Ash- and Slag-Cement Pastes

A range of pastes of portland cement interground with low-calcium fly ash or granulated blast furnace slag was studied by X-ray diffraction, analytical electron microscopy, thermo-gravimetry, and determinations of CO₂ and of unreacted fly ash or slag. Partial replacement of clinker by fly ash results in increased reaction of the alite from at least as early as 3 d. The amount of Ca(OH)₂ formed from a given weight of clinker increases at 3 d due to the enhanced reaction, but from 28 d onward, it decreases due to the pozzolanic reaction. The mean Ca/SI ratio of the C-S-H decreases with time or fly ash content, toward a lower limit of ∼1.4. Partial replacement of clinker by slag also decreases both the amount of Ca(OH)₂ formed from a given weight of clinker and the mean Ca/Si ratio of the C-S-H, but to a lesser extent. Using the methods described in Part I of this paper, the results of the experimental methods were tested for mutual consistency, and volume percents of phases, porosities, and related quantities were calculated.     

Percolation Phenomenon for Dissolution of Sodium Borosilicate Glasses in Aqueous Solutions

A percolation phenomenon has been discovered for the dissolution of sodium borosilicate glasses in aqueous solutions under low-pH conditions (pH ∼2). The glasses fall into one of two categories which are markedly different in the dissolution behavior, in terms of whether the Si/B ratio in the glass network exceeds a threshold value or not. The transition from one of the categories to the other is very sharp when the Si/B ratio is changed as the key parameter. The qualitative aspects of the experimental observation can be explained very well by a simple model combined with the site percolation theory.     

几种火山灰质掺合料的火山灰活性研究

采用酸溶法测定并比较几种典型的火山灰质掺合料的火山灰活性,探讨其火山灰反应程度对浆体强度的影响。试验结果表明,硅灰的火山灰活性最大,明显高于煤矸石和粉煤灰;煤矸石和Ⅱ级磨细粉煤灰的火山灰活性比Ⅰ级分选粉煤灰较大;酸溶法测定的煤矸石或硅灰的火山灰活性误差较大,活性偏低;掺合料火山灰的活性与其浆体强度有一定相关性。     

海水环境下矿物掺合料对硫铝酸盐水泥的水化影响

研究了海水环境下掺入硅灰、粉煤灰、矿渣对硫铝酸盐水泥抗压强度、化学收缩和水化产物的影响规律.结果表明:当硅灰的掺量为2.5%时,水泥浆体的抗压强度比空白组高.矿渣掺量为10%的水泥浆体28 d抗压强度明显超过掺入硅灰和粉煤灰时的强度,60 d强度高于空白组.掺入2.5%硅灰后,水泥浆体的化学收缩增大;在水化早期,粉煤灰和矿渣的火山灰活性很低,导致水泥浆体的化学收缩降低.掺入10%硅灰加快了硫铝酸盐水泥3 d水化反应,钙矾石生成量增多,水泥浆体早期强度比掺其它掺合料有所提高,但体积过快膨胀会破坏其内部结构,对水泥浆体的强度发展不利.     

Dissolution Rates of Commercial Soda–Lime and Pyrex Borosilicate Glasses: Influence of Solution pH

Dissolution rates of two commercial soda—lime silicate glasses and Pyrex borosilicate glass (Corning 7740) have been measured as a function of pH. The dissolution rate was constant with time and increased with increase in pH from 4 to 9. The activation energy for dissolution was nearly constant with pH for the Pyrex borosilicate glass, and decreased somewhat as pH increased for the soda—lime glasses. The openness of the glass surface influences strongly the rate of dissolution. The solubility of silica as a function of pH increases at pH above 8 but does not seem to be quantitatively related to dissolution rate. Other possible influences on this rate are briefly considered.     

Dissolution Rates of Silicate Glasses in Water at pH 7

Dissolution rates of different glasses in buffered solutions of constant pH of 7 were measured by weight change, profilometry, and ion implantation with Rutherford backscattering. Rates with different techniques agreed within experimental error. Natural obsidian glass dissolved most slowly, at a rate comparable with those of quartz and crystalline aluminosilicate minerals. Commercial soda–lime glass containing alumina dissolved slowly, at about the same rate as vitreous silica; soda–lime silicate glasses both commercial and laboratory without alumina dissolved much more rapidly. Pyrex borosilicate glass dissolved at a rate intermediate between those of soda–lime silicate glasses with and without alumina; at room temperature Pyrex borosilicate glass dissolved about 100 times faster than a commercial soda–lime glass containing alumina. We suggest that surface structure is the main factor determining the relative dissolution rates of silicate glasses. Glasses with transformed surface layers caused by hydration dissolve most rapidly; phase separation and openness of the glass structure are also important factors.     

Dissolution kinetics of calcined kaolinite and montmorillonite in alkaline conditions: Evidence for reactive Al(V) sites

Calcined clays are attracting significant research attention because of their potential to partially replace CO₂-intensive portland cement. This potential depends largely on their reactivity, especially dissolution under alkaline conditions. Identification and characterization of reactive sites in these amorphous calcined clays has so far not been reported. Here, we investigate kaolinite (1:1 clay) and montmorillonite (2:1 clay) calcined at different temperatures under alkaline conditions (0.1 mol/L NaOH). Solution compositions are determined in batch dissolution experiments, whereas ²⁷Al and ²⁹Si MAS and CP/MAS NMR are used to investigate the structure of the undissolved residues to identify sites that are reactive and undergo preferential dissolution. The highest Si and Al dissolution rates for kaolinite are observed for calcination at 700°C, corresponding to the temperature of optimum reactivity, whereas the rates decrease and become increasingly incongruent at higher temperatures. The Si and Al dissolution rates for optimally calcined kaolinite are 4 and 12 times larger than the corresponding rates for optimally calcined montmorillonite, in accord with the much higher reactivity of calcined kaolinite compared to calcined 2:1 clays. This superior performance of kaolinite is explained by novel ²⁷Al NMR results, which show strong evidence for preferential dissolution of highly reactive pentahedral aluminum sites.     

Using limestone aggregates and different cements for enhancing resistance of concrete to sulphuric acid attack

A research program was undertaken to improve concrete's resistance against sulphuric acid attack. Six concretes were investigated, four using calcareous limestone aggregates and two using silicious aggregates. Cements used in these concretes included a portland cement, a binary cement containing ground granulated blast furnace slag, and two ternary cements containing slag and silica fume or fly ash and silica fume. All the concretes had the same water/cement ratio of 0.4, with compressive strengths in the range of 45 MPa and 58 MPa at the age of 28 days. In the experiment, concrete cylinders were immersed in 1% sulphuric acid solution and they were periodically examined for appearance, measured for mass change and tested in compression up to 168 days. The concrete using limestone aggregates and the ternary cement containing silica fume and fly ash performed the best.     

Relationship Between Structure and Glass Transition Temperature in Low-silica Calcium Aluminosilicate Glasses: the Origin of the Anomaly at Low Silica Content

The anomalous behavior of the glass transition temperature ( T _g) in low silica calcium aluminosilicate glasses has been related to the structural modifications observed by neutron and X-ray diffraction. The diffraction data indicate that Al and Si are in tetrahedral sites and that Ca atoms are in distorted octahedral sites. By subtracting the correlation functions for glasses at constant SiO₂ or constant Al₂O₃ content, we have shown that Si and Al atoms are introduced in a different way within the glass structure. Si is present in various Q ⁿ sites, while Al resides in Q³ and Q⁴ sites for glasses with high CaO content and enters fully polymerized Q⁴ sites with increasing SiO₂ or Al₂O₃ content. The higher proportion of Al in Q³ positions at high CaO content yields a depolymerization of the network. The lower connectivity will contribute to a decrease of the viscosity, which may be at the origin of the decrease of T _g for glasses at low silica content.     

The effect of MgSO4 and HCL solutions on the strength and durability of pozzolan cement mortars

Pozzolan cements are produced by adding pozzolans such as silica fume, rice husk ash, blast furnace slag, fly ash, trass in 20% replacement for Portland cement. On the 28th day of production, the produced specimens are stored in water, in MgSO₄·7H₂O (5%) solution and in HCl (pH = 2) solution. The strengths and weights were determined after the mortars are stored in solutions for 56 days. Compressive strengths of the mortars stored in water for 28 days are silica fume, rice husk ash, and control, 43.3, 40.1, and 31.0 MPa, respectively. The highest loss of compressive strength is 20% and the highest gain of weight is 4.2%, occurring in blast furnace slag mortar in MgSO₄.     

Fly ash as an assemblage of model Ca–Mg–Na-aluminosilicate glasses

Model Ca–Mg–Na-aluminosilicate glasses based on compositions found in a calcareous fly ash were synthesised and their reactivity assessed in NaOH solution and in Portland cement paste. It was found that the reactivity followed the same trend in both systems and that the reaction of the glasses in pastes was very similar to that of the components of similar composition in the real fly ash. This finding indicates that the reactivity of glass in cement can be directly linked to the chemical composition of the glass. Further, when the reactivity of the glasses was normalized to their surface area, it was found that there exists a strong correlation with the NBO/T, the ratio of non-bridging oxygens and tetrahedral ions in the glass.     

海工水泥原料组成的优化试验研究*

根据国家标准对海工水泥原材料组成的要求,本文以粉煤灰、矿粉、硅灰为混合材与硅酸盐水泥熟料、石膏复合,通过水泥砂浆物理性能试验、抗渗性能试验、抗硫酸盐侵蚀试验和混凝土氯离子扩散系数试验,优化、确定了海工水泥合理的原材料组成范围。试验结果表明,当熟料掺量≥33%,硅灰掺量≤3%时,所制备的海工水泥的力学性能满足国家标准42.5级海工水泥的要求;以33%的熟料、7%的石膏、17%的粉煤灰、40%的矿粉和3%的硅灰制备的海工水泥具有较好的早期、后期强度和良好的耐久性能。XRD和SEM分析结果表明,与普通硅酸盐水泥相比,海工水泥水化体系中AFt含量多,可提高水泥石的致密度,减小孔隙率,使水泥硬化体具有优异的力学性能和耐久性能。     

LF精炼废渣水热浸出过程中主要矿相的溶解行为

通过水热浸出实验分别研究了精炼废渣及合成的废渣中2种主要单一矿相12CaO?7Al2O3和2CaO?SiO2的溶解行为,并将二者进行对比分析,探究了LF精炼废渣在水热浸出过程中的溶解行为。结果表明,废渣浸出过程中浸出液的pH?12,且随浸出时间增加,电导率和Ca浓度增加,Al浓度急剧下降,Si浓度低于0.1 mg/L且保持不变;12CaO?7Al2O3浸出过程中,随时间增加,浸出液pH值稳定在约11.3,浸出液中Al浓度增加,Ca浓度略微下降。2CaO?SiO2浸出液中主要为Ca2+,Si浓度低于0.6 mg/L;废渣与单一矿相浸出过程的pH值及Al, Si浓度较接近,可以通过单一矿相的溶解行为研究精炼废渣在水热浸出过程中的溶解行为,但废渣浸出液的Al和Si浓度均低于单一矿相,表明废渣中CaO等其它组分溶解抑制了12CaO?7Al2O3和2CaO?SiO2溶解。     

Rheological properties of cementitious materials containing mineral admixtures

The rheological properties of cementitious materials containing fine particles, such as mineral admixtures (MA), were investigated using a Rotovisco RT 20 rheometer (Haake) with a cylindrical spindle. The mineral admixtures were finely ground blast furnace slag, fly ash and silica fume. The cementitious materials were designed as one, two and three components systems by replacement of ordinary portland cement (OPC) with these mineral admixtures. The rheological properties of one-component system (OPC) were improved with increasing the dosage of PNS-based superplasticizer. For two-components systems, yield stress and plastic viscosity decreased with replacing OPC with blast furnace slag (BFS) and fly ash (FA). In the case of OPC-silica fume (SF) system, yield stress and plastic viscosity steeply increased with increasing SF. For three components systems, both OPC-BFS-SF and OPC-FA-SF systems, the rheological properties improved, compared with the sample with SF. In the two and three components systems, the rheological properties of samples containing BFS improved much more than with FA replacement alone.     

Hydration and reaction mechanisms on sodium silicate glass surfaces from molecular dynamics simulations with reactive force fields

Recent development of reactive force fields have enabled molecular dynamics simulations of interactions between silicate glasses and water at the atomistic scale. While multicomponent silicate glasses encompass a wide variety of compositions and properties, one common structural feature in these glasses is the combination of the network structure that is made up of silica tetrahedra linked through corner sharing interspersed with network modifiers like alkali and alkaline-earth ions that break up the Si–O–Si linkages by forming nonbridging oxygen. In reactions with water, ion exchange between alkali ions in the glass and proton or hydronium in the solution, as well as hydrolysis reaction of the Si–O–Si linkages and subsequent silanol formation, is observed and well documented. We have used a set of recently developed reactive force field to investigate the reactions between water and the surfaces of silica and sodium silicate glasses of different compositions for reactions up to 8 nanoseconds. Our results indicate sodium leaching into water and diffusion of water molecules up to 25 Å into the glass surface. We examined the structural and compositional changes inside the glass and around the diffused ions and use these to explain the rates of silanol formation at the surface. We also observed proton transport in the glass which has an indirect influence on the silanol formation rates. While the surface of the glass was rough to start with, it undergoes further modification into a hydrated gel-like structure in the glass for up to 5 Å in the higher alkali containing glasses. It was found that the leached sodium ions remain close to the interface and that fragments of silicate network from the surface is capable of dislodging from the bulk glass and enter the aqueous solution. These simulations thus provide insights into the formation and structure of an alteration layers commonly observed in multicomponent silicate glasses corroded in aqueous solutions.     

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.