Effect of gypsum on C3S hydration

The effect of gypsum on the early stages of C₃S hydration is explored by means of DTA, high-resolution STEM, and liquid phase studies.     

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.     

Physicochemical effects of nanosilica on C3A/C3S hydration

In this paper, C₃A-gypsum and C₃A-C₃S-gypsum model cement systems with and without nanosilica were studied. The effects of nanosilica on the early stage cement hydration, particularly C₃A hydration, were assessed through the heat of hydration (isothermal calorimetry), phase assemblage (quantitative X-ray diffraction), zeta potential, ion concentration measurements, and morphology (scanning electron microscopy) examinations. The results indicate that while promoting C₃S hydration, nanosilica retarded C₃A hydration in both the systems studied. The retardation was caused by the adsorption and coverage of nanosilica on C₃A surfaces through the electrostatic interaction, thus decreasing the C₃A dissolution rate and hindering the precipitation of hydration products. Consequently, the reduced gypsum consumption rate and the seeding effect of nanosilica further promoted C₃S hydration. These findings suggest that nanosilica and other silica-based nanoparticles can physicochemically influence hydration of cement-based materials, and a better understanding of these influencing mechanisms can help optimize performances of nanoparticle-modified cement-based materials.     

C3S and C2S hydration in the presence of Na2CO3 and Na2SO4

This study analyses the behavior of calcium silicates C₃S and C₂S hydrated in two alkaline media, Na₂CO₃ and Na₂SO₄. The silicates were synthesized with laboratory reagents and hydrated in water, to which solid‐state alkaline activators with 4 wt% Na₂CO₃ or 4 wt% Na₂SO₄ were added. Two‐ and 28‐day mechanical strength values were determined and the reaction products were characterized with XRD, SEM/EDX, and ²⁹Si and ²³Na MAS NMR. The findings showed that the presence of Na₂CO₃ hastened hydration kinetics and stimulated early‐age mechanical strength development in both silicates. The most significant effect of sodium sulfate, however, was observed in the 28‐day material in both silicates, in which it raised strength by stimulating the precipitation of C–S–H gels with a high percentage of Q² units.     

The effect of ionic polymers on the hydration of C3S

Composite materials were prepared from C₃S and 12% (1) vinyl sulfonic acid (2) p-styrene sulfonic acid. The hydration of the composite systems were compared to that of neat C₃S for different ages up to 90 d, by DTA-TG, IR and XRD. The polymers were found to retard the hydration of the silicate due to the interaction between the Ca²⁺ ions, released during the hydration of C₃S, and the sulfonate group of the polymers.     

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。     

Effect of BaCO3 on C3A hydration

Ba ions are known to immobilise sulfates by forming BaSO₄. The use of BaCO₃ as a full or partial substitute for gypsum to regulate C₃A (3CaO·Al₂O₃) hydration was consequently studied with a view to establishing its correct dosage in sulfate-resistant cements presently under development.The hydration rate of synthetic C₃A was determined in the presence of varying percentages of gypsum, BaCO₃, and gypsum + BaCO₃ by running conduction calorimetry analyses on early age (up to 20 h) samples. The hydration products were subsequently identified with XRD, FTIR and DTA/TG.The addition of (20–42 wt.%) BaCO₃ to C₃A neither regulated the speedy reaction of the latter with water nor reacted with the aluminate. Gypsum + BaCO₃ blends proved able to regulate C₃A hydration; the heat flow curves for the mixes studied exhibited an induction period, an indication that gypsum acted as a C₃A hydration regulator whilst at the same time reacting with BaCO₃ to form barite.     

Alkaline Hydration Of C2S and C3S

This study analyzed the behavior of two laboratory‐synthesized calcium silicates, C₃S and C₂S, after hydration in 8‐M NaOH and in water as a control. Two‐ and 28‐d mechanical strength values were determined and the products were characterized with XRD, TEM, and ²⁹Si and ²³Na MAS NMR. The results showed that hydrating C₃S in a highly alkaline medium had no significant effect on the mechanical development of the material, whereas in C₂S hydration, that medium hastened hydration substantially, impacting setting and hardening times. This finding has technological implications, given the low early‐age reactivity of dicalcium silicate under normal hydration conditions.     

Recent advances in the field of cement hydration and microstructure analysis

This paper is a bibliographic tool reviewing experimental and theoretical studies related to cement hydration and microstructure development that have been published within the four years of the interim period between the 12th and 13th International Congress on the Chemistry of Cement.     

Cement hydration and microstructure formation in the presence of water-soluble polymers

Hardening of cement mortars modified with small amounts of water-soluble polymers implies both cement hydration and polymer film formation. In this paper, the effect of the presence of water-soluble polymers on the cement hydration reactions is investigated by means of isothermal calorimetry, thermal analysis, FT-IR spectroscopy and SEM investigation. In spite of an initial retardation of the hydration reactions, a higher degree of hydration is found after 90 days for 1% PVAA, MC and HEC modified mortars, due to a better dispersion of the cement particles in the mixing water. MC also affects the morphology of the Ca(OH)₂ crystals. Polymer bridges are detected between the layered crystals, gluing the layers together and strengthening the microstructure. Additionally, the internal cohesion of all bulk polymer modified cement pastes is improved. In the presence of the polymers, a more cohesive microstructure with a smaller amount of microcracks is created.     

Study on the hydration and microstructure of Portland cement containing diethanol-isopropanolamine

Diethanol-isopropanolamine (DEIPA) is a tertiary alkanolamine used in the formulation of cement grinding-aid additives and concrete early-strength agents. In this research, isothermal calorimetry was used to study the hydration kinetics of Portland cement with DEIPA. A combination of X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC)–thermogravimetric (TG) analysis and micro-Raman spectroscopy was used to investigate the phase development in the process of hydration. Mercury intrusion porosimetry was used to study the pore size distribution and porosity. The results indicate that DEIPA promotes the formation of ettringite (AFt) and enhances the second hydration rate of the aluminate and ferrite phases, the transformation of AFt into monosulfoaluminate (AFm) and the formation of microcrystalline portlandite (CH) at early stages. At later stages, DEIPA accelerates the hydration of alite and reduces the pore size and porosity.     

Modeling and simulation of cement hydration kinetics and microstructure development

Efforts to model and simulate the highly complex cement hydration process over the past 40 years are reviewed, covering different modeling approaches such as single particle models, mathematical nucleation and growth models, and vector and lattice-based approaches to simulating microstructure development. Particular attention is given to promising developments that have taken place in the past few years. Recent applications of molecular-scale simulation methods to understanding the structure and formation of calcium–silicate–hydrate phases, and to understanding the process of dissolution of cement minerals in water are also discussed, as these topics are highly relevant to the future development of more complete and fundamental hydration models.     

Understanding the effect of MPEG-PCE's microstructure on the adsorption and hydration of OPC

Polycarboxylic ethers or polycarboxylate (PCEs) are one of the most employed superplasticizers in construction. However, the understanding of their microstructure–property relationship is still incomplete. Recently, a theoretical model was proposed that relates the microstructure–conformation of the PCE to its effect on the adsorption onto cement particles and cement hydration time. In this work, the effects of a wide range of PCEs with different side chain lengths (P = 5, Group 1; P = 20, Group 2; and P = 45 and 113, Group 3) having flexible backbone worm conformation except one which has stretch backbone worm conformation (P = 113) were experimentally investigated for their effect on adsorption and cement hydration. It is found that PCEs from Group 1 show electrostatic repulsion as dispersing mechanism, unlike PCEs from Groups 2 and 3. Furthermore, the prediction of the theoretical model is also assessed for all the studied PCEs. Only Group 1 PCEs (shortest side chains) showed deviation from the theoretical predictions, and it was attributed to their different behaviors from the standard PCEs for which the theoretical model was developed.     

Direct observation of the formation of linear C chain/carbon nanotube hybrid systems

Multi-wall carbon nanotubes (MWCNTs) containing linear C chains have been synthesised by arc discharge in liquid nitrogen. The experimental conditions used allow one to obtain nanotubes with a very thin innermost diameter, as evidenced by the radial breathing mode (RBM) features in the Raman spectra. A correlation between the RBM and the features of the C chains is reported, which gives a direct indication of how these linear carbon chain/carbon nanotube hybrid systems form. C chains are inserted only in CNTs having the innermost diameter equal to 0.7 nm, as expected.     

掺杂对高C3S水泥熟料烧成及C3S形成动力学的影响

利用铁尾矿作为作硅铝质原料进行生料配料(熟料率值KH =0.97,SM =2.5,IM =1.5),其中C3S含量超过70％.在此空白生料的基础上分别掺入氟、硫和氟硫复合阴离子,各试样分别在1300℃,1350℃,1400℃和1450℃下煅烧30 min,然后测定熟料中f-CaO的含量,并计算C3S的形成活化能.通过差热分析和XRD分析,研究高C3S熟料的烧成过程.结果表明:氟、硫的掺入能够显著改善生料的易烧性,大幅降低C3S的活化能,促进熟料的烧成.单掺时硫比氟的效果要好,氟硫复掺比单掺效果好.生料的差热分析表明,氟、硫能够使石灰石的分解温度降低10 ～20℃,同时能促进熟料的液相烧结.在1450℃时复掺氟硫熟料的主要矿相是C3S,还有少量的C2S,C3A和C4AF,表明高C3S水泥熟料已经烧成.     

Influence of the C/S and C/A ratios of hydration products on the chloride ion binding capacity of lime-SF and lime-MK mixtures

The mechanisms of chloride ion binding in cementitious systems when supplementary cementing materials are present are not completely understood, although it is believed to relate to the alumina content of the mixture. This relationship is investigated through the use of lime-silica fume and lime-metakaolin mixtures. It was found that while the alumina content does have an important influence, the chloride binding capacity was also controlled by the calcium-to-alumina and calcium-to-silica ratios. At a high C/A ratio, the formation of monocarboaluminate is favoured, which has a high ability to form Friedel's salt, and does so at low chloride concentrations (less than 0.1 M). With a low C/A ratio, the formation of stratlingite is favoured, with little formation of monocarboaluminate. If alumina is not present, chloride is bound by the C-S-H phase. The binding capacity of the C-S-H was found to depend on its calcium-to-silica ratio, C-S-H with a higher C/S having a greater binding capacity.     

Ionic complexation and adsorption of small organic molecules on calcium silicate hydrate: Relation with their retarding effect on the hydration of C3S

The ion complexing power and the adsorption of set-retarders such as sugar derivatives (d-glucitol, d-gluconate…), amino-carboxylates and phosphonates (EDTA, EDTMP, …) on C-S-H were identified and compared. The complexation equilibria of the predominant calcium and/or hydroxide complex and its relative constant were determined after measuring the solubilizing effect of the molecules in suspensions buffered by portlandite. The adsorption of the molecules on C-S-H as well as their possible complexation with silicate ions in solution were also investigated. All the molecules studied are shown to complex calcium and/or hydroxide ions but only amino-phosphonates, d-gluconate and d-glucitol complex silicate ions. They all adsorb on portlandite and C-S-H but to different extents. These organic compounds are also revealed to be sensitive to the presence of calcium counter-ions at the surface of C-S-H. Finally, the relation between their ionic complexing powers, adsorption properties and retarding effects on the hydration of C₃S are discussed.