Mechanically activated alite: New insights into alite hydration

For superior understanding of alite hydration an investigation of mechanically activated alite (M3 modification) was performed by XRD and heat flow calorimetry. Activation resulted in reduced particle size, a decreased mean crystallite size and partial amorphization. For the samples of activated alite a significantly accelerated and intensified hydration was observed and complete conversion of alite was found after 24 h. The enthalpy of reaction for crystalline alite was determined to be − 548 J/g from measured heat of hydration after 24 h. The enthalpy of reaction of amorphous “alite” was found to be less exothermic (− 386 J/g). The main hydration period is controlled by nucleation of C–S–H, while the transition from acceleration to deceleration period takes place after consumption of the small alite particles. XRD amorphous C–S–H phase is indicated to precipitate in considerable amount even in the highest activated alite before “long-range ordered”, XRD detectable C–S–H was observed.     

Evolution of elastic behavior of alite paste at early hydration stages

Elastic behavior is an important aspect of fresh cement-based materials. In this paper, the elasticity development of fresh pure alite paste within 5 hours was monitored in situ by small amplitude oscillation shear. Isothermal microcalorimetry, X-ray diffraction, scanning electronic microscopy, ¹H nuclear magnetic resonance, zeta potential measurements were used to investigate the underlying mechanisms. It was found that the evolution of storage modulus can be characterized by three different stages. The first stage stems from the compression of electrical double layer and the slight increase of solid volume fraction; the second stage is only controlled by the colloidal interaction; and the third stage is associated to the significant growth of solid volume fraction, the enhancement of short-range attraction and the rigidification of calcium silicate hydrate (CSH) network. Moreover, the relationship between the formation of CSH and the growth of elasticity was also quantified.     

Effect of mixing on the early hydration of alite and OPC systems

The kinetics of hydration of cementitious materials is sensitive to the mixing procedure. High shear mixing conditions lead to an increase in the kinetics of hydration at early age compared to low shearing conditions such as hand mixing. In this study the effect of mixing speed and procedure was studied on alite and Portland cement in the presence or not of aggregates. The kinetics of hydration was monitored using isothermal calorimetry at 20 °C. The early reactivity was enhanced both with an increase in the speed of mixing and the shearing conditions. The principal features are a shortening of the induction period; a higher rate of hydrate precipitation during the acceleration period as well as an increase in the height of the main heat evolution peak. Analysis of the results in terms of dissolution theory, coupled with quantitative simulation with the μic modelling platform indicate different effects of mixing prior to and after the end of the induction period. Before the end of the induction period mixing has an impact on the rate of dissolution in the fast dissolution regime and high undersaturation, which appears to be (at least partially) controlled by the rate of transport of ions away from the alite surface. After the end of the induction period the main effect of mixing appears to be the production of more C-S-H nuclei, due to the possible detachment of the primary C-S-H (metastable) by mechanical action. This higher nucleation density leads to a denser microstructure for systems mixed at high intensities.     

Investigation into relations among technological properties, hydration kinetics and early age hydration of self-leveling underlayments

Technological properties such as flow value, setting times, compressive strength and early age dimensional stability as linear shrinkage and expansion have been studied for two types of self-leveling underlayments (SLU) in which the kind of calcium sulfates was varied. The influence on hydration kinetics has been measured by isothermal heat flow calorimetry. The results obtained for the technological properties change significantly when different kinds of calcium sulfates are used. The basic trend for the results changes when the composition changes from a calcium aluminate cement system to a Portland cement system. Additionally, there was an interesting relationship to final dimensional stability and shape of heat evolution curve. Moreover the time to reach plateau of dimensional stability was related to the development of compressive strength. In the meanwhile, using hemihydrate in Portland cement systems caused low compressive strength and significant expansion. On the other hand, in the results of XRD measurements, the first genesis of Ettringite corresponded to the first shrinkage of SLU. Interesting results related with technological properties and hydration kinetics or the results of XRD other than these above results were obtained.     

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.     

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.     

Influence of the reactivity of the amorphous part of mechanically activated alite on its hydration kinetics

The hydration of one mechanically activated alite passing through different drying procedures is examined by heat flow calorimetry and quantitative in-situ XRD analysis. The reactivity of the alite powders is strongly affected by the drying technique. It is shown that the reactivity of the amorphous part of the activated alite sample is particularly affected. Due to the fast initial dissolution of the amorphous “alite” part, the hydration progression is speeded up significantly. However, if the hydration is not speeded up by the amorphous “alite” dissolution, as in the case of surface passivation, the heat released until the transition to the deceleration period will increase. It is discussed that a crystalline alite dissolution by etch pit opening could increase the reactive alite surface and therefore increase the reaction degree at the transition to the deceleration period.     

Dissolution theory applied to the induction period in alite hydration

The early hydration of alite, in particular the reason for the onset of the induction period, has been a subject of controversy for many decades. Several theories have been proposed, principally the formation of a protective phase inhibiting dissolution or delayed nucleation and growth, but none have successfully taken into account all the experimental data available. This paper proposes a new mechanism, based on a geochemical approach to crystal dissolution that fully explains the origin of the induction period. It implies that during cement hydration, dissolution is initially dominated by the formation of etch pits on surfaces and later becomes limited to step retreat from such pits. This change in mechanism alone can account for the rapid decrease in reaction after first contact with water, without the need to invoke the formation of a protective phase. Furthermore it can explain all the experimental findings in the literature. While this geochemical view of dissolution explains many features of the induction period it does not account for its end. This remains a question for further research, but the most probable explanation appears to be the onset of rapid growth of C-S-H.     

Microstructural development of early age hydration shells around cement grains

An important microstructural aspect of the early hydration of Portland cement (PC) is the formation of a shell of hydration products around cement grains. There is, at present, limited information on the mechanism of formation of the shell and of the chemistry of the phases that constitute the shells. Through the use of STEM imaging of early age hydrated cement pastes as early as 2 h, the present work shows that the shells correspond to the first C-S-H type product formed which has a distinct morphology compared to C-S-H formed later when the main reaction occurs (nucleation and growth stage at setting time). The shells form only around the silicate part of the grain and are not empty but filled with a fragile fibrous C-S-H which appears to have a lower (packing) density than the rest of the hydration products. The cement grains underneath the shells are seen to react unevenly and the hydration seems to follow a reaction front, leaving striations up to 1 µm deep on the grains. Over the long term, the original fragile product seems to densify and gives rise to the usual inner C-S-H. High resolution EDS chemical analysis and mappings were used to get insight into the chemistry associated with the formation of these early age products. The C/S ratio of all C-S-H (inner and outer shell) is the same (within the limits of the analysis accuracy) and evolves insignificantly over the first 24 h of hydration. High concentrations of sulfate are associated with the C-S-H formed during the early development of the microstructure, but these decrease later, the sulfate being mainly incorporated into ettringite.     

水泥浆体早期收缩的试验研究

开发了一套采用高精度LVDT位移传感器测定水泥浆体长度变化的仪器,并用该仪器对不同水灰比和不同混合材的水泥浆体的收缩行为进行了研究。结果表明,该仪器能够准确、连续地测量水泥浆体终凝后的自收缩和干燥收缩的发展过程。早期自收缩和干燥收缩随着水灰比降低而明显增加,在20℃相对湿度50%的环境下,水泥浆体的自收缩占干燥收缩的10%左右;掺粉煤灰可以少量降低水泥浆体的自收缩和干燥收缩,而掺入石灰石和矿渣则不同程度的增大了水泥浆体早期收缩率。     

Aluminum-rich belite sulfoaluminate cements: Clinkering and early age hydration

Belite sulfoaluminate (BSA) cements have been proposed as environmentally friendly building materials, as their production may release up to 35% less CO₂ into the atmosphere when compared to ordinary Portland cements. Here, we discuss the laboratory production of three aluminum-rich BSA clinkers with nominal mineralogical compositions in the range C₂S (50-60%), C₄A₃$ (20-30%), CA (10%) and C₁₂A₇ (10%). Using thermogravimetry, differential thermal analysis, high temperature microscopy, and X-ray powder diffraction with Rietveld quantitative phase analysis, we found that burning for 15 min at 1350 ºC was the optimal procedure, in these experimental conditions, for obtaining the highest amount of C₄A₃$, i.e. a value as close as possible to the nominal composition. Under these experimental conditions, three different BSA clinkers, nominally with 20, 30 and 30 wt.% of C₄A₃$, had 19.6, 27.1 and 27.7 wt.%, C₄A₃$ respectively, as determined by Rietveld analysis. We also studied the complex hydration process of BSA cements prepared by mixing BSA clinkers and gypsum. We present a methodology to establish the phase assemblage evolution of BSA cement pastes with time, including amorphous phases and free water. The methodology is based on Rietveld quantitative phase analysis of synchrotron and laboratory X-ray powder diffraction data coupled with chemical constraints. A parallel calorimetric study is also reported. It is shown that the β-C₂S phase is more reactive in aluminum-rich BSA cements than in standard belite cements. On the other hand, C₄A₃$ reacts faster than the belite phases. The gypsum ratio in the cement is also shown to be an important factor in the phase evolution.     

Impact of hydroxypropylguars on the early age hydration of Portland cement

Hydroxypropylguars (HPG) are used as admixtures in factory-made mortars. These molecules present water retention properties comparable to those obtained with commonly used cellulosic water-retaining agent.The influence of HPG on cement hydration was investigated in order to improve understanding on the delayed effect induced by polysaccharides. Hydration kinetics were characterized by means of conductivity and isothermal calorimetry measurements. The influence of polymer concentration and predissolution was studied. A weak influence of HPG on the germination of hydrates was observed. In contrast, HPG induced a significant decrease in the hydrates growth rate. Strong effects of the polymer concentration and predissolution were noticed too. From these results, we supported the hypothesis that HPG adsorption on hydrated phases via polar interactions should be responsible for the delayed effect observed.     

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.     

Parameters controlling early age hydration of cement pastes containing accelerators for sprayed concrete

The objective of this work is to parametrize the early age hydration behavior of accelerated cement pastes based on the chemical properties of cement and accelerators. Eight cements, three alkali-free and one alkaline accelerators were evaluated. Isothermal calorimetry, in situ XRD and SEM imaging were performed to characterize kinetics and mechanisms of hydration and the microstructure development. The reactivity of all accelerators is directly proportional to their aluminum and sulfate concentrations and to the amount and solubility of the setting regulator contained in cement. Alite hydration is enhanced if a proper C₃A/SO₃ ratio (between 0.67 and 0.90) remains after accelerator addition and if limestone filler is employed, because undersulfated C₃A reactions are avoided. Combinations of compatible materials are recommended to enhance the performance of the matrix and to prevent an undesirable hydration behavior and its consequences in mechanical strength development.     

Thermal behavior of cement matrix with high-volume mineral admixtures at early hydration age

Thermal cracks that usually occur in mass concrete are closely related to the thermal behavior of cement matrix, such as heat liberation, temperature rise and thermal shrinkage. Cement pastes added with large-volume mineral admixtures that are usually used for thermal controlling were cast into well-sealed plastic cylinder and covered by heat insulation materials to simulate the pseudo-adiabatic condition of mass concrete. The deformation and temperature rise of cement specimens under the heat insulation condition have been examined at early hydration age. Results show that with addition of fly ash, coal gangue and blast furnace slag the heat liberation and peak temperature of cement paste decrease, while its total shrinkage increases.There is no shrinkage but expansion of the pastes during the temperature rise process, which may be ascribed to the complete compensation of the shrinkage by thermal dilation of the pastes. The thermal dilation coefficient (TDC) of cement paste changes drastically with the hydration duration, and it is also related to the addition of mineral admixtures.     

Analysis of pentasodium triphosphate hydration kinetics

The present work analyzes the effect of various factors on the hydration of pentasodium triphosphate. The experimental method is based on application of the hydration test. Technical-quality products with different proportions of phase I and phase II have been used. The variables studied are phase I/phase II ratio, initial temperature, particle size, stirring rate and composition of the slurry (presence of hexahydrate crystals and water hardness). The results have been discussed according to a kinetics model that includes a series of stages of a physical nature (dissolution of anhydrous salt and the crystallization of the hexahydrate), as well as of a chemical nature (solvation of the ions in solution). Crystallization of the hexahydrate may be the controlling stage in the process.     

Influence of curing temperature on the process of hydration of supersulfated cements at early age

In this paper, the influence of curing temperature was studied for supersulfated cements made with two slags having different chemical compositions. Supersulfated cements (SSC) made with low-alumina slag developed lower porosity, higher compressive strength and degree of hydration at higher temperatures. SSC made with high-alumina slag resulted in higher strengths and presented a more complex mechanism of hydration that was strongly influenced by the solubility of anhydrite.     

Properties of alite cements

For carrying out a comprehensive investigation on physical and mechanical properties of alite mortars and concretes, large quantities of monoclinic alite were produced at the University of California at Berkeley. Laboratory-size specimens were employed to determine strength, drying shrinkage, and sulfate resisting characteristics of mortars and concretes made with alite cements. Small amounts of gypsum (3%) addition accelerated the setting and hardening of the alite cements, however, large amounts (6%) resulted in strength deterioration. Drying shrinkage of alite concretes was significantly lower than portland cement concretes of the same fineness. Long term sulfate immersion of concrete specimens made with an alite cement caused serious spalling and strength loss.     

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.     

Gel formation during early hydration

It is well know that gel formation and, in particular, gel-space ratio within cement paste has a considerable bearing on the strength and mechanical properties of concrete. This paper describes investigations into a method for monitoring the temporal changes in gel-space ratio within cement paste employing wide-band frequency response techniques.