Hydrothermal transformation of the calcium aluminum oxide hydrates CaAl2O4·10H2O and Ca2Al2O5·8H2O to Ca3Al2(OH)12 investigated by in situ synchrotron X-ray powder diffraction

The hydrothermal transformation of calcium aluminate hydrates were investigated by in situ synchrotron X-ray powder diffraction in the temperature range 25 to 170 °C. This technique allowed the study of the detailed reaction mechanism and identification of intermediate phases. The material CaAl₂O₄·10H₂O converted to Ca₃Al₂(OH)₁₂ and amorphous aluminum hydroxide. Ca₂Al₂O₅·8H₂O transformed via the intermediate phase Ca₄Al₂O₇·13H₂O to Ca₃Al₂(OH)₁₂ and gibbsite, Al(OH)₃. The phase Ca₄Al₂O₇·19H₂O reacted via the same intermediate phase to Ca₃Al₂(OH)₁₂ and mainly amorphous aluminum hydroxide. The powder pattern of the intermediate phase is reported.     

Reactions of fly ash with calcium aluminate cement and calcium sulphate

The hydration processes in the ternary system fly ash/calcium aluminate cement/calcium sulphate (FA/CAC/C$) at 20 °C were investigated; six compositions from the ternary system FA/CAC/C$ were selected for this study. The nature of the reaction products in these pastes were analysed by X-ray diffraction (XRD) and infrared spectroscopy (FTIR). At four days reaction time, the main hydration reaction product in these pastes was ettringite and the samples with major initial CAC presented minor ettringite but calcium aluminates hydrates. The amount of ettringite developed in the systems has no direct relation with the initial components.     

Quantitative Rietveld analysis of CAC clinker phases using synchrotron radiation

The quantitative Rietveld analyses of twenty samples of CAC from four different manufacturers over the world, one synthetic mixture and a NIST standard were performed using synchrotron radiation. As compared with conventional XRD, synchrotron powder diffraction permitted to find new minor phases, improve the characterization of solid solutions of iron rich CAC phases and reduce preferential orientation and microabsorption effects. Diffraction data were complemented with XRF and TG/DT analyses. Synchrotron results were used as a reference test to improve the performance of conventional powder diffraction, by an accurate selection of refinable profile and structural parameters, and permitted to extract several recommendations for conventional quantitative Rietveld procedures. It is shown that with these recommendations in mind, conventional XRD based Rietveld analyses are comparable to those obtained from synchrotron data. In summary, quantitative XRD Rietveld analysis is confirmed as an excellent tool for the CAC cement industry.     

The work of Powers and Brownyard revisited: Part 1

Powers and Brownyard [Studies of the physical properties of hardened Portland cement paste. Bull. 22, Res. Lab. of Portland Cement Association, Skokie, IL, U.S. J. Am. Concr. Inst. (Proc.), 43 (1947) 101-132, 249-336, 469-505, 549-602, 669-712, 845-880, 933-992 (reprint)] were the first to systematically investigate the reaction of cement and water and the composition of cement paste. They introduced the concept of nonevaporable (water retained in P-dried state) and gel water (additional water retained upon saturation). Their specific volumes (ν_n and ν_g) are lower than that of free water, causing chemical shrinkage. The retained water was furthermore related to the content of the four most abundant clinker phases, viz alite, belite, aluminate and ferrite.Their work is recapitulated here. Major aspects, such as the specific volume of nonevaporable and gel water, are addressed, as well as the issue of gel water being “compressed”. Subsequently, it will be demonstrated that their water retention data enable the study of the molar reactions of the calcium silicate phases and the reaction products C-S-H (C_1.7SH_3.2 when saturated) and CH, which represents a principal innovation. Using the molar reactions and the specific volumes of nonevaporable (ν_n) and gel water (ν_g), the density of saturated C-S-H and its porosity are derived.     

The hydration products of Portland cement in the presence of tin(II) chloride

The hydration products of Portland cement pastes cured using water containing tin(II) chloride have been compared with those using distilled water. In the latter case, the expected products—portlandite, ettringite and calcite—were observed. The X-ray diffraction patterns of the cement pastes cured in the presence of tin(II) chloride showed several additional peaks that have been attributed to the formation of calcium hydroxo-stannate, CaSn(OH)₆, and Friedel's salt (tetracalcium aluminate dichloride-10-hydrate), Ca₃Al₂O₆·CaCl₂·10H₂O. The amount of portlandite formed was reduced in the presence of tin(II) chloride. Calcium hydroxo-stannate contains tin in the +IV oxidation state and equations are presented to account for the oxidation of Sn(II) to Sn(IV) preceding the formation of CaSn(OH)₆ and Friedel's salt.     

Quantitative study of Portland cement hydration by X-ray diffraction/Rietveld analysis and independent methods

X-ray diffraction (XRD) is a powerful technique for the study of crystalline materials. The technique of Rietveld refinement now enables the amounts of different phases in anhydrous cementitious materials to be determined to a good degree of precision. This paper describes the extension of this technique to a pilot study of the hydration of a typical Portland cement. To validate this XRD-Rietveld analysis technique, its results were compared with independent measures of the same materials by the analysis of backscattered electron images (BSE/IA) and thermogravimetric analysis (TGA). In addition, the internal consistency of the measurements was studied by comparing the XRD estimates of the amounts of hydrates formed with the amounts expected to form from the XRD estimates of the amounts of anhydrous materials reacted.     

The mineralogy of the CaO-Al2O3-SiO2-H2O (CASH) hydroceramic system from 200 to 350 °C

We describe a quantitative mineralogical study of the hydrothermal reactions of an oil well cement with added silica and alumina, hydrated at temperatures from 200 to 350 °C. We compare the products with pure end member systems and find phase stability can be altered radically, even by small amounts of additive. The upper temperature limits of α-C₂SH (< 250 °C), and 1.1 nm tobermorite C₅S₆H₅ (< 300 °C) are increased. C₈S₅, reported in a cement-based system for the first time, is stable to 300 °C and is believed to prevent foshagite C₄S₃H formation below 350 °C. Hydrogarnet C₃AS_3−yH_2y is the only aluminum bearing phase at < 300 °C but it coexists with C₄A₃H₃ and bicchulite C₈A₄Si₄H₄ at higher temperatures. The presence of alumina increases the stability of 1.1 nm tobermorite greatly and also to a lesser degree of gyrolite.     

Liquid-phase self-diffusion in hydrating cement pastes — results from NMR studies and perspectives for further research

The changes in the pore structure of hydrating cement are accompanied by changes in the dynamics of liquid phases contained in the pore system of the hydrating matrix. Dynamic NMR methods (relaxometry, diffusometry) allow the non-destructive observation of these changes. Relaxometry can be performed using quite simple equipment and has been widely used in studies of the kinetics of cement hydration. Diffusion studies, by contrast, require much more sophisticated equipment. On the other hand, the diffusion coefficient has a direct relevance for the transport of moisture or contaminants in the cement matrix while relaxation time measurements provide more indirect information. The purpose of the present paper is to review the possibilities of field gradient NMR in diffusion studies on hydrating cement and to provide an outlook on how this information can be used for improving our understanding of the properties and microstructure of hydrating cement. As an example, new results on the relationship between the diffusive exchange length in the sample and non-exponential relaxation in cement are discussed at the end of the contribution.     

Indirect determination of the Ca/Si ratio of the C-S-H gel in Portland cements

An alternative method for the indirect determination of the Ca/Si ratio of the C-S-H gel in Portland cements is proposed. It is based on graphical correlations of experimental determinations of the degree of hydration of alite and the amount of calcium hydroxide formed with theoretical data based on the hydration reactions of the alite and belite present in the cement. The initial composition of the cements is of prime importance in these calculations; the phase composition obtained by use of the Bogue, Taylor's modified Bogue, and quantitative X-ray diffraction methods gave rise to some differences in the results obtained. Ca/Si ratios determined using the Bogue composition were consistently lower than those from Taylor's method and the quantitative X-ray diffraction analysis methods for both the Portland cements studied. The Ca/Si ratios estimated based on the mineralogical phase compositions determined from these latter methods agreed well with those from energy dispersive X-ray spectroscopy measurements.     

Hydration of polyphase Ca3SiO5-Ca3Al2O6 in the presence of gypsum and Na2SO4

Alite (Ca₃SiO₅: C₃S^*) and calcium aluminate (Ca₃Al₂O₆: C₃A) are the major phases in Portland cement, which have an essential role in the development of early age properties. The effects of gypsum content, fineness, and Na₂SO₄ addition on the early-stage hydration kinetics are investigated for polyphase (co-sintered) Ca₃SiO₅-Ca₃Al₂O₆ model systems using calorimetry, X- ray diffraction, thermal analysis, and solid-state ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy. The results demonstrate that the hydration of C₃A significantly affects the hydration of C₃S. The C₃S and C₃A hydration is hindered considerably in severely over-sulfated systems (where C₃A hydration is suppressed due to a very high gypsum content) and systems with additional Na₂SO₄. Although there is a considerable amount of Al incorporation in the C-S-H phase, no clear trends with respect to gypsum content, hydration age or Na₂SO₄ addition are observed for the Al_IV/Si ratios of the C-S-H phase determined from ²⁹Si NMR. With the addition of Na₂SO₄, recrystallization of ettringite from the AFm phases is postponed from 1 day to 7 days. *Cement chemistry notation: C-CaO, S-SiO₂, A-Al₂O₃, -SO₃, N-Na₂O.     

石膏对改性硅酸盐水泥性能的影响

将磷铝酸盐水泥熟料掺入硅酸盐水泥中改性后,运用XRD和SEM等测试技术,研究了石膏对改性硅酸盐水泥性能的影响.结果表明,石膏的掺入可以改善改性硅酸盐水泥的力学性能和抗冻性;在石膏掺量为3.5%时,改性硅酸盐水泥水化速度最快,硬化浆体的结构最致密,强度最高,抗冻性最好.     

Phase development in conventional and active belite cement pastes by Rietveld analysis and chemical constraints

High belite cements may be an alternative to reduce CO₂ emissions. Although CO₂ emissions may be depleted up to 10%, unfortunately, the hydration reactivity of belite phases is slow which leads to low mechanical strengths at early ages. In order to enhance their hydraulic reactivity, the activation of these cements by doping with alkaline oxides has been proposed. Here, we have synthesised a laboratory belite clinker without activation (47 wt.% of β-C₂S and 19 wt.% of α_H′-C₂S) and two alkaline oxide activated clinkers (one with 13 wt.% of β-C₂S, 24 wt.% of α_H′-C₂S and 19 wt.% of α-C₂S; and the second with 12 wt.% of β-C₂S, 42 wt.% of α_H′-C₂S and 5 wt.% of α-C₂S). We have also developed a methodology to analyse quantitatively the phase evolution of cement pastes and we have applied it to these high belite cements. Rietveld quantitative phase analysis of synchrotron X-ray powder diffraction data, together with chemical constraints, is used to determine the phase development up to 1 year of hydration in the belite cement pastes. β-C₂S almost does not react during the first 3 months, meanwhile α_H′-C₂S reacts on average more than 50% in the same period. Moreover, the degree of reaction of α-C₂S is slightly larger (on average about 70% after three months) than that of α_H′-C₂S. Full phase analyses are reported and discussed including the time evolution of amorphous phases and free water.     

Standard reference materials for cements

The eight portland cements and two calcium aluminate cements in the Standard Reference Material (SRM) 1880 series are among the most popular SRMs in the catalog of the National Institute of Standards and Technology (NIST) Standard Reference Materials Program. Numerous laboratories rely on them for elemental analysis and qualification for ASTM C 114-00 Standard Test Methods for Chemical Analysis of Hydraulic Cement. NIST has collected new candidate materials from around the world and partnered with Construction Technology Laboratories (CTL) in their preparation and certification. This paper describes the procedures taken at NIST and CTL to prepare and test materials for certification including the homogeneity testing, the X-ray fluorescence methods and the statistical data analysis performed for value assignment.     

The hydration phase and pore structure formation in the blends of sulfoaluminate-belite cement with Portland cement

Sulfoaluminate-belite (SAB) cements are an attractive class of low-energy cements from the viewpoint of saving energy and releasing less CO₂ into the atmosphere during their production. Their hydraulic activity, however, does not match that of the ordinary Portland cement (PC) and needs improvement before they can be used on their own. However, SAB cements when blended with PC have the potential to be used effectively in traditional applications as shown by this study. Mortars made with blends of SAB cements and PC, and a cement-to-sand ratio of 1:3 by weight and a water-to-cement ratio of 0.5, indicate a superior protection against corrosion of steel to those made with blends of PC and blast-furnace slag (BFSPC). The prepared mortars were stored at 20 °C for 90 days under either a 60% relative humidity (RH)-dry air, or 100% RH-wet air conditions. With further improvement in the SAB cement quality through better understanding of their characteristics, a genuine competition between SAB/PC and BFSPC can be expected in practice.     

Class H cement hydration at 180 °C and high pressure in the presence of added silica

Under deep oil-well conditions of elevated temperature and pressure, crystalline calcium silicate hydrates are formed during Portland cement hydration. The use of silica rich mineral additives leads to the formation of crystalline hydrates with better mechanical properties than those formed without the additive. The effects of silica flour, silica fume (amorphous silica), and a natural zeolite mixture on the hydration of Class H cement slurries at 180 °C under externally applied pressures of 7 and 52 MPa are examined in real time using in-situ synchrotron X-ray diffraction. For some compositions examined, but not all, pressure was found to have a large effect on the kinetics of crystalline hydrate formation. The use of silica fume delayed both C₃S hydration and the formation of crystalline silicate hydrates compared to what was seen with other silica sources.     

Examination of the jarosite-alunite precipitate addition in the raw meal for the production of portland cement clinker

The aim of the present research work was to investigate the possibility of adding a jarosite-alunite chemical precipitate, a waste product of a new hydrometallurgical process developed to treat economically low grade nickel oxides ores, in the raw meal for the production of Portland cement clinker. For that reason, two samples of raw meals were prepared, one with ordinary raw materials, as a reference sample ((PC)_Ref) and another with 1% jarosite-alunite precipitate ((PC)_J/A). Both raw meals were sintered at 1450 °C. The results of chemical and mineralogical analyses as well as the microscopic examination showed that the use of the jarosite-alunite precipitate did not affect the mineralogical characteristics of the so produced Portland cement clinker. Furthermore, both clinkers were tested by determining the grindability, setting time, compressive strength and expansibility. The hydration products were examined by XRD analysis at 2, 7, 28 and 90 days. The results of the physico-mechanical tests showed that the addition of jarosite-alunite precipitate did not negatively affect the quality of the produced cement.     

An in situ synchrotron energy-dispersive diffraction study of the hydration of oilwell cement systems under high temperature/autoclave conditions up to 130 °C

The technique of synchrotron energy dispersive diffraction has been developed for in situ studies of cement hydration under autoclave conditions. This has been applied to oilwell cements hydrating at typical oilwell temperatures up to 130 °C. The results show clearly the detailed interplay between 11 detectable phases, from which a phase transformation scheme has been derived; this illustrates the progression of hydration up to 130 °C for two extreme cases, with and without conservation of water content and autoclave pressure. The monosulphate hydrate phases are found to exhibit different stability bounds, with a surprising sequence of the 14-water, 10-water then 12-water monosulphate as temperature/time increases; the latter form is particularly associated with conditions of water/pressure loss. The effect of retarders on C₃S dissolution and CH formation is negligible above 70 °C, whereas the effect on the calcium sulphoaluminate hydrates is more complex, and possible reasons for this are discussed.     

Synthesis of cement based CaO-Al2O3-SiO2-H2O (CASH) hydroceramics at 200 and 250 °C: Ex-situ and in-situ diffraction

Hydroceramic compositions in the CaO-Al₂O₃-SiO₂-H₂O (CASH) system have potential as geothermal well sealants as well as autoclaved construction materials. We report new data on phase compositions and reaction rates in hydrothermal syntheses at 200 °C and 250 °C using a commercial API Class G oilwell cement alone, and at 200 °C with additions of silica flour and of corundum (alumina). Curing times were in the range 1-240 h. We use both ex-situ laboratory X-ray diffraction and in-situ synchrotron energy-dispersive X-ray diffraction to track rates of reaction. When cement only is hydrated, jaffeite, α-C₂SH and portlandite are formed. When silica flour is added a precursory gel forms prior to the crystalline calcium silicate hydrate phases xonotlite and gyrolite. Both XRD and EDD data suggest that the addition of silica flour retards the hydration of the cement at early times (< 24 h). In alumina-containing systems the rate of consumption of clinker phases is the same as in cement only systems. Jaffeite and α-C₂SH occur as intermediates but the major end product is a siliceous katoite-type hydrogarnet. Quantitative phase analysis using Rietveld refinement of ex-situ diffraction data gives results which are mostly consistent with stoichiometric constraints in all three systems examined here.     

Hydration of anhydrite of gypsum (CaSO4.II) in a ball mill

The hydration of an anhydrite of gypsum (CaSO₄.II) in a ball mill was studied as a function of time and temperature. The amount of gypsum formed at different intervals of time was determined by weight loss method and powder X-ray diffraction technique. Specific surface area at different time intervals was determined by LASER granulometric method. The results showed that the maximum rate of formation of gypsum was at a longer time than the time for the development of maximum specific surface area. In the presence of activators, the time for maximum rate of gypsum formation and maximum specific surface area shifted towards lower hydration time. Morphological changes during the course of hydration have been studied by the scanning electron microscopic (SEM) technique. A mechanism of hydration has been proposed.     

Hydrothermal synthesis of calcium sulfate whisker from flue gas desulfurization gypsum

Plenty of flue gas desulfurization (FGD) gypsum generated from coal-fired power plants for sulfur dioxide se-questration caused many environmental issues. Preparing calcium sulfate whisker (CSW) from FGD gypsum by hydrothermal synthesis is considered to be a promising approach to solve this troublesome problem and uti-lize calcium sulfate in a high-value-added way. The effects of particle size of FGD gypsum, slurry concentration, and additives on CSW were investigated in this work. The results indicated that fine particle size of FGD gypsum and moderately high slurry concentration were beneficial for crystal nucleation and growth. Three additives of magnesium chloride, citric acid, and sodium dodecyl benzene sulfonate (SDBS) were employed in this study. It was found that mean length and aspect ratio of CSW were both decreased by the usage of magnesium chloride, while a small quantity of citric acid or SDBS could improve the CSW morphology. When multi-additives of citric acid-SDBS were employed, the mean length and aspect ratio increased more than 20%. Moreover, surface morphology of CSW went better, and the particle size and crystal shape became more uniform.