Modelling early age hydration kinetics of alite

The modelling platform μic [1] has been used to investigate the mechanisms occurring during the hydration of alite. It is shown that it is possible to obtain a good simulation of the hydration kinetics through the implementation of two mechanisms: a dissolution mechanism combined with nucleation and growth of products. The dissolution rate is varied according to the ratio β, between the ion activity product and the equilibrium solubility product according the theory published by Juilland et al. [2]. The solution concentrations are computed directly from the amount of alite dissolved taking into account the amount of water present and the amount of products formed, with activities and complex ion formation calculated according to standard methods. Saturation index calculations are implemented to compute the time of precipitation of C–S–H and portlandite (CH) individually. For the main heat evolution peak, the rate controlling mechanism switches to a modified form of boundary nucleation and growth. C–S–H grows in a diffuse manner in which the density of packing of the C–S–H phase increases with hydration [3]. The rate of heat evolution obtained from the simulations is compared with isothermal calorimetry data and good agreement is found.     

Silicate polymerization during the hydration of alite

The influence of admixtures and curing temperature on silicate polymerization during the hydration of alite was studied. Trimethylsilyl derivatives were separated by gel permeation chromatography. The major species identified in the hydrated pastes were dimer, linear pantamer, and linear octamer; at later ages, particularly at higher temperature, higher polymers are formed. A polymerization scheme is proposed. The effect of admixtures on silicate polymerization exactly parallels their effects on hydration.     

A remastered external standard method applied to the quantification of early OPC hydration

A promising external standard method, first described by O'Connor [15], was used to determine the quantitative phase composition of a hydrating cement paste. On the basis of the data produced we can conclude that the method used is absolutely to be recommended for the examination of OPC pastes, since it displays many advantages in comparison to internal standard methods and other methods. No reaction of the phase alite could be detected during the initial and the induction periods of the cement hydration. Additionally it was found that the cement phases involved in the aluminate reaction (bassanite, gypsum, anhydrite and C₃A) react successively. The changes detected in the phase composition of the OPC paste could be assigned to the different periods of OPC hydration.     

The influence of sodium and potassium hydroxide on alite hydration: Experiments and simulations

The basic nature of alkali hydroxides (NaOH, KOH) when added to mixing water, increases the pH in proportion to the level of salt addition. For alite (impure tricalcium silicate; MIII-Ca₃SiO₅) hydration, this pH increase accelerates the rate of hydration and reduces the duration of the induction, acceleration and deceleration regimes. This study evaluates alite hydration in solutions of varying compositions and alkalinities (0.1 M, 0.2 M and 0.5 M NaOH and KOH) in context of their heat release behavior and analysis of the solid/liquid phases. The modeling platform, μic, is used to simulate, describe and discriminate the impact of the pore solution chemistry and reaction product formation parameters on alite hydration (Bishnoi and Scrivener, 2009 [1]). Numerical predictions of the solid and liquid phase compositions and the heat release response show good agreement with experimental determinations. The simulations indicate that the effects up to the end of the induction period follow directly from a change in the pore solution composition under a solution controlled dissolution mechanism, which leads to the faster precipitation of portlandite. The changes in the main heat evolution peak appear to be related to an increase in the nucleation density of C–S–H in alkali hydroxide solutions. Examination under the SEM did not indicate significant difference in C–S–H morphology and composition in the presence of NaOH/KOH.     

碱对阿利持—硫铝酸盐水泥熟料矿物形成及水泥性能的影响

以石灰石、粉煤灰、粘土、石膏等为原料,研究了K2O、Na2O对阿利特-硫铝酸盐水泥熟料矿物形成及性能的影响,结果表明,少量碱能改善水泥生料的易烧性,促进fCaO的吸收,过多的碱使C4A3∧-S矿物难以形成;当碱掺加量约1.2%时,NaO有利于C3S矿物的形成,并提高水泥的早期强度,而K2O则使C3S的形成量减少,水泥的强度降低;掺加碱使水泥的凝结时间延长。     

Studying nucleation and growth kinetics of alite hydration using μic

This paper investigates the applicability of nucleation and growth mechanisms to the hydration of alite. Various possible mechanisms of nucleation and growth were simulated using the recently-developed microstructural modelling platform μic. Comparison with the Avrami equation and the boundary nucleation model demonstrate the limitations of these equations. Experimental measurements of the rates of hydration of alite powders with different particle size distributions were then simulated with a boundary nucleation and growth model in μic. The results show that while the nucleation and growth of C–S–H having bulk densities in the currently accepted range can explain the acceleration during the first few hours of hydration, it cannot explain the later deceleration. It was also found that the resistance to flow of ions offered by the layer of hydrates forming over the surface of the alite particles (diffusion control) cannot explain the deceleration. The deceleration could be reproduced when C–S–H was assumed to be loosely packed in the beginning with its density of packing increasing with hydration. It is proposed that during the early hours of hydration a loosely-packed C–S–H fills a large fraction of the microstructure and the further development of its microstructure occurs due to an increase in its packing.     

Effect of polysaccharides on the hydration of cement paste at early ages

This work deals with the relative efficiency of polysaccharides and their influence on cement hydration. Several parameters such as the structure, concentration, average molecular weight, and soluble fraction value of polysaccharides were examined. Cement hydration was monitored by isothermal calorimetry, thermogravimetry (TGA), and Fourier transform infrared (FTIR) spectroscopy. Results clearly show that retardation increases with higher polysaccharide-to-cement weight ratio (P/C). Low-molecular-weight starch showed enhanced retarding effect on the hydration of cement. The retardation effect of polysaccharides is also dependent on the composition of cement.     

水泥水化热对混凝土早期开裂的影响

对于预拌混凝土应用过程出现的早期开裂现象，有些混凝土专家归因于水泥比表面积太大和早期强度太高；而水泥界则认为，我国目前水泥的比表面积和早期强度并不比国外的高，混凝土的早期开裂主要是混凝土施工和养护不当所致。笔者认为，必须通过混凝土生产者和水泥生产商沟通，对早期裂缝的成因达成共识，在水泥生产、混凝土配制及施工养护等环节共同采取措施加以解决。“高强早强、高比表面积”及“水泥磨得太细”，这些都是表面现象，其本质是早期水化热太高及混凝土温度应力大的缘故。     

Effect of early hydration temperature on hydration product and strength development of magnesium phosphate cement (MPC)

This paper investigates the effect of early hydration temperatures on hydration products and strength development of magnesium phosphate cement (MPC). MPC paste specimens with borate contents of 3%, 6%, 9% and 12% are prepared and cured in different air temperatures and in the adiabatic condition. The internal hydration temperatures are measured by pre-embedded temperature probes. MPC samples with different hydration temperatures are also obtained by using thin slice samples. The hydration products in MPC samples with different hydration temperatures are analyzed by X-ray diffractometer (XRD) and scanning electron microscope (SEM) and the strength development is also measured. The results show that NH₄MgPO₄·6H₂O is the major hydration product and beneficial to strength development of MPC at hydration temperature below 70 °C. NH₄MgPO₄·H₂O is another major product, which significantly decreases the strength, when the temperature is higher than a critical temperature between 70 °C and 75 °C. NH₄MgPO₄·H₂O can directly form in the MPC paste, and comes from the decomposition of NH₄MgPO₄·6H₂O when the temperature is above 75 °C. With temperature elevation and duration extension, NH₄MgPO₄·6H₂O decomposes rapidly, and even entirely when the temperature is above 100 °C. The borate content has no effect on the types of major hydration products and the critical temperature.     

Effect of recrystallization on the characters of alite in Portland cement clinker

Alite, as well as belite, undergoes recrystallization during clinkering and grows large at the expense of small crystals. The growth is more noticeable in the direction perpendicular to (0001) than in the direction parallel to it. The crystals that are initially basal tablets become massive granules with the pyramidal faces well-developed. The recrystallization causes a considerable decrease in the concentration of the foreign ions in solid solution and thus occasionally lowers the crystal symmetry of elite to triclinic. These observations give evidence that the chemical composition of alite depends on its kinetics of crystallization from the interstitial melt.     

利用高铝煤矸石和盐石膏低温烧制阿利特—硫铝酸盐水泥熟料的研究

实验室及工业性试验研究结果表明,利用高铝煤矸石和盐石膏等工业废渣,低温烧制的阿利特－硫铝酸盐水泥熟料,不仅使水泥具有凝结硬化快,早后期强度的均高于硅酸盐水泥的特点,而且具有微膨胀和抗蚀性强的特性,文中对熟料率值和矿物组成算式进行了修正,通过分析认为,在实际生产中控制掺入生料中的石膏较配方设计高出20％左右,熟中C4A2S含量为4－6％,煅烧温度控制在1250－1300度,同时指出了影响水泥质量的工艺隐患。     

分别粉磨与共同粉磨对石灰石硅酸盐水泥颗粒分布及水化性能的影响

试验采用分别粉磨工艺和共同粉磨工艺配制石灰石硅酸盐水泥,分析了石灰石和熟料的粉磨特性差异,并研究了在单位能耗相同情况下,两种粉磨制度下石灰石硅酸盐水泥各组份颗粒分布及水化情况。结果表明,分别粉磨制度能合理控制石灰石硅酸盐水泥颗粒的分布,使熟料处于更细的颗粒范围内,提高了粉磨效率;细的石灰石粉能促进水泥的早期水化反应,而中后期的水化速率主要取决于熟料颗粒的细度;采用合理的分别磨粉工艺配制的石灰石硅酸盐水泥,在相同龄期下,其水化产物含量较多,水化产物之间的连结更紧密,水泥石整体结构更密实。     

Effect of various parameters on the level of mixing in continuous flow systems

Variations of the level of mixing in a flow system have been investigated as a function of the following parameters: diameter, shape and speed of the agitator, feed rate and position of the feed inlet. At agitator speeds inferior to a critical value, a minimum level of mixing is observed which depends on the position of the feed inlet. At speeds superior to the critical value, the level of mixing increases linearly with agitator speed, at a given feed rate, until conditions of perfect mixing are reached. The critical agitator speed and the position of each straight line depend on the agitator size. A modified Reynolds number makes possible a more general correlation. At a given agitator speed, superior to the critical value, the level of mixing decreases as the feed rate increases, due to the smaller amount of energy imparted by the agitator per unit mass of feed. The effect of agitator shape on the level of mixing is rather pronounced as it affects the flow patterns inside the reservoir.     

The effect of temperature on the hydration rate and stability of the hydration phases of metakaolin-lime-water systems

The paper presents the results of a study carried out to determine the effect of curing temperature on the kinetics of reaction of a metakaolin (MK)/lime mixture. MK and analytical grade Ca(OH)₂ were mixed in a ratio of 1:1 by weight and with a water/binder ratio of 2.37. Specimens were cured at 20 and 60 °C. In the first case, the curing time varied from 2 h up to 180 days and, in the second case, from 2 h up to 123 days. A mathematical model was applied to calculate the rate constant for the hydration reaction. The identity and the amount of the phases present were determined from thermal analysis (TG and DTA) data. The results showed that the rate constant for the samples cured at 60 °C was 68 times greater than the rate constant at 20 °C for the same curing period (up to 9 days). At 20 °C, the sequence of appearance of the hydrated phases was C-S-H, C₂ASH₈ and C₄AH₁₃; while at 60 °C, the sequence was C-S-H, C₂ASH₈, C₄AH₁₃ and hydrogarnet (C₃ASH₆). There is no evidence of further C₂ASH₈ and C₄AH₁₃ transformation into hydrogarnet in the mixture studied for 123 days at 60 °C.     

A note on early hydration of cement

In this note it is shown that the hydration of portland cement and of blastfurnace slag cement can be traced using the flotation of cement from mortars and concretes. The various stages of the hydration reaction, i.e. initiation, induction period, formation of an initial gel structure and the final hardening process can be detected clearly.     

Hardened alite pastes of various porosities. I. Degree of hydration,compressive strength and total porosity

The alite used in this investigation was synthesised from the stoichiometric mixture at 1550°C. The hardened alite pastes were made using initial water/alite ratios of 0.20, 0.30, 0.45 and 0.60. Degree of hydration, compressive strength and total porosity were estimated at various hydration time intervals of 0.5 h, 2 h, 6 h, 1 day, 3 days, 7 days and 28 days. A meaningful relation between compressive strength and water/alite ratio was established at constant values of degree of hydration, total porosity and Powers' gel-space ratio.     

Dilation of portland cement grains during early hydration and the effect of applied hydrostatic pressure on hydration

An optical interference method shows that at early stages of hydration there is a significant increase in size of individual cement grains. The effect of applied hydrostatic pressure on cement pastes is an increase in the degree of hydration. Explanations for these effects are given in terms of the osmotic gel membrane model of hydration.     

Effects of mixing energy on technological properties and hydration kinetics of grouting mortars

During slurry preparation, effects of certain phenomena on fluidity and hardening characteristics of cement-based grouts have been reported. Deterioration of fluidity and hardening will affect the slurry performance, quality of workmanship and result in subsequent structural defects. There has been little research conducted on the effects of mixing energy during slurry preparation which has focused on the reasons or mechanisms for changes in characteristic properties. This work describes and measures the effects of several mixing parameters on properties of grouting materials, such as fluidity, hardening characteristics, shrinkage, heat of hydration, ion elution and crystallographic structure using X-ray diffraction and SEM. The results indicate that long mixing processes cause deterioration in fluidity and setting properties. These observations can be explained by acceleration in hydration kinetics and changes in microstructures and subsequent changes in dispersion states due to different mixing durations.     

Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes

In this investigation, two mixes were used: ordinary Portland cement (OPC) and a blended cement prepared with the partial substitution of OPC by 10 mass% silica fume (SF). The setting and hardening characteristics were monitored by the aid of electrical conductivity as a function of curing time. The shear stress and electrical conductivity were studied at different temperatures, namely, 20, 35, 45 and 55 °C. As the temperature increases, the shear stresses decrease with the increase of shear rate. The height of electrical conductivity peaks of superplasticized cement pastes increases due to the increase of the paste fluidity. In the presence of 1.0% polycarboxylate (PC), the electrical conductivity of cement pastes decreases from 1 to 28 days. PC retards the hydration of cement pastes. The presence of PC extended the setting times of cement pastes at 35 °C than at 20 °C due to the increase in the adsorption capacity at this temperature. PC extends the dormant stage of the hydration process and delays the onset of the accelerating stage, without affecting its rate.     

Effect of first drying upon the pore structure of hydrated alite paste

Water losses and gains, nitrogen adsorption, and imbibition of organic fluids were measured in a study of the pore structure of two, well-hydrated alite pastes that had original water/alite ratios of 0.4 and 0.6 by weight. Drying from saturation to a relative humidity in the range 0.3 to 0.8 was a particularly slow process probably because some pores emptied via smaller pores. The nitrogen adsorption measurements, on pastes in which the water has been removed by organic fluid replacement, suggested that micropores were closing as a result of capillary tensions developed at relative humidities between 0.7 and 0.4. The loss of surface area was recoverable with resaturation of the pastes with water. Imbibition of water and organic fluids indicated that there was an irrecoverable pore volume loss after drying and rewetting that was dependent upon the relative humidity of drying.