Alkali activation of a novel calcium‐silicate hydraulic binder with CaO/SiO2 = 1.1

A quasi‐amorphous low‐calcium‐silicate hydraulic binder, with an overall CaO/SiO₂ (C/S) molar ratio of 1.1, was produced. This cementitious material was then hydrated with aqueous solutions containing 3 wt% alkalis (either NaOH, Na₂CO₃ or Na₂SiO₃). The evolution of the hydration processes of the samples were monitored by compressive strength testing, XRD, FTIR, ²⁹Si and ²⁷Al MAS NMR, isothermal calorimetry and TGA. It was found that the nearly exclusive hydration product formed was a C‐S‐H phase with a semi‐crystalline structure. More importantly, the paste prepared with the Na₂SiO₃ solution developed compressive strength values similar to those of ordinary portland cements (OPC) with faster early age kinetics. In addition, the isothermal calorimetry results indicated that these new hydraulic binders present much lower heat of hydration values compared with a traditional OPC. The results presented here open the possibility of producing cement with a compressive strength comparable to that of OPC but with lower CO₂ emissions during the production process and with lower hydration heat related problems during the production of concrete structures.     

Implications for C3S kinetics from combined C3S/CA hydration

This study was performed to understand the influence of a critical amount of CA on the kinetics of C₃S hydration. For this purpose, monoclinic C₃S was blended with 15 wt.% CA and investigated by heat flow calorimetry and in situ XRD at 23°C at a water to cement ratio of 0.5. The binary mixture shows 3 distinct heat flow maxima where the underlying C₃S dissolution is proceeding stepwise. The C₃S dissolution rates during the 3 steps are varying strongly, depending on the hydrate phase precipitated during the respective reaction step. Comparison of these dissolution rates with a pure C₃S reference sample allows the conclusion that the dissolution rate of pure C₃S after the heat flow maximum might be governed by either the remaining available reactive surface of C₃S or a diffusion‐controlled process, which would both be influenced by the respective hydrate phase precipitating on the C₃S surface.     

Hydration of C3S in presence of CA: Mineral-pore solution interaction

C₃S and CA are the main phases of OPC and Fe-rich CAC, respectively. The objective of this research was to investigate the influence of CA on C₃S hydration, representing an under sulfated OPC-rich binder, and to shed light on the underlying hydration mechanisms. To this end, C₃S was blended with 1-30 wt-% CA and the pastes (w/c 0.5) were investigated by heat flow calorimetry, in situ X-ray diffraction and analysis of the pore solution chemistry. CA additions ≥5 wt-% reveal a separation into three distinct heat flow maxima, whereas additions ≤3 wt-% just retard the start of the main reaction. The silicate reaction (dissolution of C₃S and precipitation of C–S–H with or without CH) can be retarded for 4 to ≥22 hours in comparison to pure C₃S depending on the admixed CA content. The start of the silicate reaction seems to be related to a decrease in Al- and increase in Ca-concentration in the pore solution. However, it can be shown in this study that C₃S is able to dissolve even at high Al concentrations in the pore solution.     

Aqueous Solubility Diagrams for Cementitious Waste Stabilization Systems: II, End-Member Stoichiometries of Ideal Calcium Silicate Hydrate Solid Solutions

Solubility in the fully hydrated CaO–SiO₂–H₂O system can be best described using two ideal C-S-H-(I) and C-S-H-(II) binary solid solution phases. The most recent structural ideas about the C-S-H gel permit one to write stoichiometries of polymerized C-S-H-(II) end-members as hydrated precursors of the stable tobermorite and jennite minerals in the form of 5Ca(OH)₂·6SiO₂·5H₂O and 10Ca(OH)₂·6SiO₂·6H₂O, respectively. For thermodynamic modeling purposes, it is more convenient to express the number of basic silica and portlandite units in these stoichiometries using the coefficients n _Si and n _Ca. Thermodynamic solid-solution aqueous-solution equilibrium modeling by applying the Gibbs energy minimization (GEM) approach shows the best generic fits to the available experimental solubility data at solid 0.8 < Ca/Si < 2.0 if both stoichiometry and thermodynamic constants of the end-members are normalized to n _Si= 1.0 ± 0.3. Recommended stoichiometries and thermodynamic data for the C-S-H end-members provide a reliable basis for the subsequent multicomponent extension of the ideal C-S-H solid solution model by incorporation of end-members for the (radio)toxic elements or trace metals.     

C–(N)–S–H and N–A–S–H gels: Compositions and solubility data at 25°C and 50°C

Calcium silicate hydrates containing sodium [C–(N)–S–H], and sodium aluminosilicate hydrates [N–A–S–H] are the dominant reaction products that are formed following reaction between a solid aluminosilicate precursor (eg, slags, fly ash, metakaolin) and an alkaline activation agent (eg NaOH) in the presence of water. To gain insights into the thermochemical properties of such compounds, C–(N)–S–H and N–A–S–H gels were synthesized with compositions: 0.8≤Ca/Si≤1.2 for the former, and 0.25≤Al/Si≤0.50 (atomic units) for the latter. The gels were characterized using thermogravimetric analysis (TGA), scanning electron microscopy with energy‐dispersive X‐ray microanalysis (SEM‐EDS), and X‐ray diffraction (XRD). The solubility products (K_S0) of the gels were established at 25°C and 50°C. Self‐consistent solubility data of this nature are key inputs required for calculation of mass and volume balances in alkali‐activated binders (AABs), and to determine the impacts of the precursor chemistry on the hydrated phase distributions; in which, C–(N)–S–H and N–A–S–H compounds dominate the hydrated phase assemblages.     

Alkaline Activation of Metakaolin: Effect of Calcium Hydroxide in the Products of Reaction

The alkali activation of metakaolin (MK) leads to the production of high-mechanical-performance network-structure materials. Adding calcium hydroxide (Ca(OH)₂) to the raw MK produces a somewhat different reaction: a network structure and C-S-H gel form. In the present study, MK and (MK + Ca(OH)₂) mixes were activated with 5 M and 12 M NaOH solutions and cured at 45°C. A 5 M concentration, in the absence of Ca(OH)₂, did not produce MK activation within the test time. An activator concentration of 12 M resulted in complete activation and the formation of a network structure. When Ca(OH)₂ was present in the raw mix, a small amount of C-S-H gel formed.     

Interactions between Polymeric Dispersants and Calcium Silicate Hydrates

To better understand the mechanism of interaction between hydrating silicate-based cements and polymeric dispersants of the type used as superplasticizers in modern construction concretes, two different types of polymeric dispersant were added (at concentrations of 1 and 10 g/L) during the synthesis of calcium silicate hydrate (C-S-H) via the pozzolanic reaction in dilute slurries of lime and reactive silica, at Ca/Si ratios in the range of 0.66–1.50. Although both polymers gave degrees of adsorption of >79% in all cases studied, no significant structural modifications of the resulting C-S-H products were observed via X-ray diffraction or ²⁹Si magic angle spinning–nuclear magnetic resonance. These results differ from recent work in which it was shown that similar types of polymer could intercalate into the interlayers of C-S-H that was made using an alternative process. It is suggested that the process by which the C-S-H is formed may have a strong influence on whether C-S-H can intercalate polymers. This observation is relevant to understanding the fate of such polymers in concrete.     

协同水化制备水硬性材料及其水化产物的研究

按钢渣、矿渣和粉煤灰以5∶3∶2的比例组成复合废渣,在改性水玻璃等激发剂的协同作用下,制备得到的浆体28 d抗压强度高达53.56 MPa,凝结时间及安定性均合格。通过X射线衍射(XRD)分析、电子扫描电镜(SEM)形貌观察等对浆体中的水化产物进行分析,结果表明,在协同水化作用下,水化产物中存在网络状的水化硅酸钙(C-S-H)。     

Incorporation of Al and Na in Hydrothermally Synthesized Tobermorite

Calcium silicate hydrate and its Al‐substituted form synthesized by a hydrothermal process were investigated by X‐ray diffraction, compositional analysis, and magic‐angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, in order to determine the mechanism of Al and Na incorporation in the tobermorite structure with varying molar ratios of Ca/Si and Al/Si. At a high molar ratio of Ca/Si, the silicate chains of tobermorite are ruptured, the degree of polymerization of the silicate chains is lowered, and the high calcium concentration lowers the content of Na₂O in the structure. Solid‐state ²⁹Si and ²⁷Al MAS NMR spectroscopy confirm that all Al atoms were incorporated in the silicate chains of tobermorite. The tetrahedrally coordinated Al (Al(IV)) could either act as the bridging tetrahedron () for the dreierketten chain of tobermorite, or be present in Q³ sites that link two dreierketten chains together. Therefore, the degree of polymerization of the silicate chains of tobermorite is increased at high molar ratio of Al/Si. Furthermore, the greater charge deficit due to the replacement of Si⁴⁺ by Al³⁺ ions is compensated by increased adsorption or binding of Na⁺.     

Use of nanocomposites as permeability reducing admixtures

Permeability is one of the fundamental properties of concrete structures as it is strictly related to durability. Mitigation of the degradation processes induced by aggressive solutions can be achieved by controlling water penetration through the pore network. In this study, we test the potential use of nanocomposites as waterproofing agents in concrete. Macroscopic measurements show that the addition of a small amount of nanoparticles effectively reduces the extent of the water permeation front. A combination of experiments, based on X‐ray tomography, mercury intrusion porosimetry, and BET nitrogen adsorption, and of numerical simulations, are used to interpret the macroscopic observations. These investigations show that C–S–H precipitation away from cement surfaces, induced by the presence of nanoparticles, leads to a refinement of the pore network. Such a microstructural change in the cement matrix results in a net reduction in the overall concrete permeability.     

Modeling the effect of fineness and filler in early‐age hydration of tricalcium silicate

Alite (impure C₃S) being the major and most reactive phase in ordinary portland cement has been studied extensively. This paper focuses on the mathematical modeling of new hypothesis developed, densified volumetric growth, applied to the main hydration peak in alite hydration. This hypothesis assumes a time and particle size‐dependent growth rate and densification rate of C–S–H controlled by its internal surface area. To test this hypothesis, a new microstructural modeling platform (Cementitious Reactions Simulator) has been developed. The model was used to calibrate and predict the isothermal calorimetric results from the literature and two sets of original experimental results with alite replaced with different replacement fractions of quartz, fly ash, and slag of different fineness. The model is able to capture and predict the effect of fineness and fillers without the need of varying the parameters for the particular set of simulations. The values of the parameters of simulation reflect the current experimental evidence from the literature and thus provide validation of the hypothesis which suggests that the main hydration period including deceleration period in the alite hydration is induced by the densification and reducing outer growth rate of C–S–H.     

Effect of the densification of C–S–H on hydration kinetics of tricalcium silicate

Effect of water to cement (w/c) ratio and temperature profiles on the densification of C–S–H (calcium silicate hydrate gel) and hydration kinetics of triclinic tricalcium silicate (C₃S) is studied beyond the first day of hydration. Calorimetry and quantitative X‐ray diffraction/Rietveld analysis show that degree of hydration is unaffected by w/c up to 7 days and marginally thereafter. Coupling the degree of hydration with the portlandite content measured from thermal analysis indicate that C/S ratio of C–S–H decreases with increasing w/c. There is a clear increase in the portlandite content with increasing w/c, even though the degree of hydration is unchanged, due to the variations in C/S ratio of C–S–H. On the other hand, when C₃S is initially cured at a lower temperature (20°C) and then at a higher temperature (40°C), there is a significant increase in the reactivity even until 28 days and vice versa. These experimental results were explained using the densified volumetric growth hypothesis, which assumes that hydration kinetics are dependent on the internal surface area of C–S–H.     

Recycling kraft pulping chemicals: cyclic voltammetry of molten salt mixtures containing Na2CO3, Na2SO4, Na2S/Na2S x and Na2O/Na2O2

Cyclic voltammetry (CV) has been used to investigate molten salt mixtures in the temperature range of 820–840 °C between the initial and anticipated final compositions for the proposed electrolytic recausticizing process of inorganic pulping chemicals. A mixture simulating the initial conditions for the proposed process (sodium carbonate, sodium sulfide/polysulfide and sodium sulfate) exhibited carbonate oxidation and sulfate reduction at the limits of the potential window. Additional oxidation of sulfide to sulfur occurs at potentials inside the positive limit of the potential window with subsequent chemical reaction to form polysulfide. To simulate the final composition sodium oxide and peroxide were added to the mixtures; the resulting CVs had an additional oxidation peak attributed to oxide and peroxide oxidation. We conclude the electrolytic recycle process is feasible, producing sulfide and oxide from carbonate and sulfate of sodium in the molten state, yet separation is necessary between the anolyte and catholyte so the reduction products are not consumed by oxidation.     

Superstructure in a Triclinic Phase of Tricalcium Silicate

A triclinic phase of tricalcium silicate (TI) was investigated using transmission electron microscopy (TEM). Electron diffraction patterns were analyzed by introducing a subcell with the cell parameters of a = 7.081 Å, b = 7.043 Å, c = 25.230 Å, α= 89.97°, β= 90.37°, and γ= 119.44°. It was proven that the coordinates of all the reflections can be expressed to be ha *+ kb *+ lc *± m /6( a *+ 2 b *+ 2 c *), where m = 0, 1, 2, and 3. The result indicates that the structure modulation in TI is a one-dimensional type with a structural modulation normal to (122). The modulated structure could be observed in a high-resolution TEM image as wavy contrast streaking parallel to the plane with an interval of six times the (122) spacing.     

Calcium-silicate-hydrates/polycarboxylate ether nanocomposites doped by magnesium: Enhanced stability and accelerating effect on cement hydration

C-S-H/PCE (calcium-silicate-hydrates/polycarboxylate ether) nanocomposites have a significant accelerating effect on early hydration and early strength development of cementitious materials. In this study, magnesium ions are doped into C-S-H/PCE nanocomposites, and it is found that C-S-H/PCE nanocomposites doped by appropriate magnesium ions have much better stability. Besides, the accelerating effects of the modified nanocomposites on early cement hydration also get improved. The doped magnesium ions displace calcium ions and promote the absorption of PCE. Moreover, the incorporation of magnesium ions results in two unique morphologies of nanocomposites, that is, globule and gel. The globules are semi-crystalline similar to C-S-H, while the gels are amorphous and have more branched silicate chains.     

SO3掺杂对高镁熟料Alite晶型和水化性能的影响

文章探讨了SO3对高镁水泥熟料中C3S晶型的调控以及对该熟料水化性能的影响。通过X射线衍射对熟料矿物含量和C3S的晶型进行表征并测试了熟料的净浆强度及结合水。研究表明:当SO3的掺量为1%、2%时不会对C3S的形成带来阻碍,样品中f-CaO含量均小于1%。在该熟料中,阿利特以M3型为主,多为细小的晶体,SO3的掺入能够稳定M1型的阿利特。1%~2%SO3的掺入能促进熟料的水化,提高熟料的中后期强度,当掺量为2%时3、28d净浆抗压强度分别提高8%和5%。水化程度随着SO3掺量的增加而增加,水化产物中含有大量的钙钒石。     

NaOH、Na2CO3含量的简易分析法

介绍了NaOH、Na2CO3含量的简易分析法在烧碱生产中的应用情况,采用本方法提高了化验分析速度和生产效率。     

硝酸锶对水热合成硬硅钙石球形团聚体的影响

硬硅钙石是一种用途广泛的保温材料，本工作以硝酸锶作为矿化剂，研究了在动态水热合成硬硅钙合成中，硝酸锶对硬硅钙石球形团聚体的影响。研究表明：在合成时添加硝酸锶，可以使水化硅酸钙（C－S－H）凝胶的ξ电位的绝对值减小，从而C－S－H胶团的凝聚长大，C－S－H凝胶粒度的增大是硬硅钙石球形团聚体增大的基础，Sr^2 进入到硬硅钙石晶体内引起其晶胞常数a和c的增大，促进硬硅钙石的纤维状晶体以及球形团聚体的长大，从而有利于得到轻质硬硅钙石材料。     

十水合硫酸钠及十水合碳酸钠热导率的测量研究

用热敏电阻作加热元件和测温元件,首次系统测定了适合用作相变储热材料的十水合硫酸钠、十水合碳酸钠在温度范围10～80℃的热导率,两者实验热导率的偏差率分别为:±2.56%和±2.41%。同时,报道了十水合硫酸钠、十水合碳酸钠在实验中观测到的熔点分别为30℃和30℃,这些测量值与文献值基本吻合。     

Effects of chitosan and zirconia on setting time,mechanical strength,and bioactivity of calcium silicate‐based cement

In this research, we aimed at improving the setting properties and biocompatibility of the mineral trioxide aggregate‐like cements while maintaining the main chemical formula. Consequently, chitosan and zirconium oxide were added to the cement instead of bismuth oxide to improve the mechanical behavior, limit the possible toxicity, and enhance the bioactivity of the cements. Adding zirconia resulted in a shorter setting time and adding chitosan contributed to the setting time, mechanical strength, and biocompatibility at the same time. Thus, cements containing both chitosan and zirconia had the shortest setting time, highest compressive strength, and apatite‐forming ability.