Rate Of Ettringite Formation Research Articles

Properties and hydration characteristics of an iron-rich sulfoaluminate cementitious material under cold temperatures

In this study, an iron-rich steel-slag-based sulfoaluminate cementitious material (SSCM) was prepared using steel slag, flue gas desulphurisation gypsum, aluminium ash, and limestone tailings. The basic and cold-temperature characteristics of SSCM were studied. The results show that Fe2O3 introduced by steel slag promotes the transformation of C4A3S¯-o to C4A3S¯-c, making SSCM more resistant to freezing injuries than Portland cement. As the curing temperature decreases, the formation rate of ettringite in the early stage slows down obviously. With the hydration progressing, the ettringite content in pastes cured at low temperatures (0–5 °C) becomes similar to that cured at 20 °C, and the proportion of harmful pores is decreased. Although the pastes cured at negative temperatures still contain a large amount of unhydrated phase and a significantly increased proportion of harmful pores, the hydration reaction can still be maintained. The performance of SSCM shows a promising application prospect for construction in cold regions.

Structurization of cement systems with nanodispersed silica stabilized with acetate ions

The influence of a modifying additive in the form of silica sol stabilized with acetate ions on the structure and properties of cement stone has been studied. The mechanism of structure formation in the system of “cement-nanodispersed silica (NDS) stabilized with acetate ions” is proposed. It includes two aspects. In the initial period of hydration the crystallization of calcium hydroacetoaluminates occurs, the rate of their formation being higher than the rate of ettringite formation. Their crystals have some micro-reinforcing effect on cement stone, increasing its density and strength. The nanodisperse silica in combination with the calcium hydroacetoaluminates is directly involved in the structurization of cement stone, colmataging the pores. Si4+ atoms, building-in in the structure of portlandite phases, increase the impermeability of cement stone. The generation of calcium hydroacetoaluminates leads to the formation of the primary carcass, providing the strength development of cement stone in the early stages of hardening. It is established that adding the modifier results in pore redistribution in size: a decline in the amount of macropores and an increase in the quantity of the gel pores and submicropores (5-0.1 µm) at a constant micropore volume (0.1-20 µm).

The synthesis and characterization of an expansive admixture for M-type cements I. The influence of free CaO to the formation of ettringite

The hydration process of expansive admixtures for M-type of expansive or shrinkage compensation cements was studied by thermal analysis supplemented by infrared spectroscopy and X-ray diffraction. The main attention was focused on ettringite formation and on the influence of lime content upon the process. The behaviour of expansive admixture during hydration is significantly affected by the content of lime. The increasing content of lime slows down the rate of ettringite formation and supports the formation of monosulphate (AFm phase) that can be transformed afterwards into ettringite during hardening process. This process may produce better expansion effect because of better transfer of expansion stress after the setting of the paste.

Kinetic and morphological differentiation of Ettringites in plain and blended Portland cements using Metakaolin and the ASTM C 452-68 test. Part I: kinetic differentiation

In this first part of the study, the results obtained in prior research with XRD and SEM, as well as the Le Chatelier-Ansttet test were confirmed with the ASTM C 452-68 test. To this end, 20%, 30% and 40% metakaolin (MK) was added to ten Portland cements, six OPCs and four SRPCs. Both the ten plain PCs and the 30 metakaolin (MK) blends were tested for two years under ASTM C 452-68 specifications, determining not only the percentage increase in length, ΔL(%), of the specimens, but also the sulphate content in the curing water. Other parameters studied included: chemical analysis of the cementitious materials used and specific properties of some of the cements tested.The experimental results, ΔL(%) versus time, re-confirmed that the formation rate of ettringite from the reactive alumina, Al2O3r-, present in the pozzolan must be substantially higher than the formation rate of ettringite from the C3A present in the PC. This was verified by the variation of the sulphate content in the specimen curing water throughout the test. In light of those findings, in this article these two types of ettringite are denominated rapid forming ettringite or ett-rf, and slow forming ettringite or ett-lf.

Ettringite formation and behaviour in clayey soils

Failures in soil stabilization have been reported previously as being due to the formation of ettringite, an expansive mineral which develops in the presence of sulfate, calcium, and aluminum compounds of clay fraction in high pH levels between 10.36 and 14. By comparing the pattern of formation of ettringite, formed from different possible sources and specifically in stabilized soil, it is expected that a clearer picture of the kinetics of ettringite formation from different sources will be obtained. In this paper, a set of physico-chemical experiments and XRD tests were performed to investigate the process of ettringite formation and to explore its possible performance in clayey soils. The growth of the ettringite XRD peaks was used as a measure of the rate and its formation pattern. It is shown that the rate of ettringite formation in soil stabilization is much slower than that of artificial ettringite. Furthermore, it is shown that ettringite swells to the order of 50% and its fluid retention is significantly higher (by as much as 400%) than that of the soil sample studied in this research.

Hydration of calcium sulfoaluminate cement at less than 24 h

Calorimetry, X-ray diffraction, scanning electron microscopy, microprobe and porosimetry were used to investigate hydration processes occurring at 25°C, 55°C and 85°C for commercial calcium sulfoaluminate (CSA) cement during the first 24 h. CSA cement pastes harden through the formation of an initial ettringite skeleton and its subsequent infilling by mixtures of ettringite, AFm and C-S-H. At low water/cement ratios these yield a dense, rather featureless paste. The formation rate of ettringite is significantly increased at elevated temperatures. A ‘shrinking sphere’ model is proposed to explain the competitive kinetics of hydration processes with formation of inner and outer products which are shown to differ significantly in mineralogy.

Hydration of calcium sulfoaluminate cement at less than 24 h

Calorimetry, X-ray diffraction, scanning electron microscopy, microprobe and porosimetry were used to investigate hydration processes occurring at 25°C, 55°C and 85°C for commercial calcium sulfoaluminate (CSA) cement during the first 24 h. CSA cement pastes harden through the formation of an initial ettringite skeleton and its subsequent infilling by mixtures of ettringite, AFm and C-S-H. At low water/cement ratios these yield a dense, rather featureless paste. The formation rate of ettringite is significantly increased at elevated temperatures. A ‘shrinking sphere’ model is proposed to explain the competitive kinetics of hydration processes with formation of inner and outer products which are shown to differ significantly in mineralogy.

Effect of sodium salt of naphthalene-formaldehyde polycondensate on ettringite formation

The suspension hydration of C 3A with gypsum (in the molar ratio of 1:3) was investigated at room temperature and water/solid ratio of 4. The hydration was carried out in presence of 0, 1, and 3% sodium salt of naphthalene-formaldehyde polycondensate and the mixes were designated as I, II, and III, respectively. The only hydration product formed in the presence and absence of the superplasticizer was ettringite. The rate of ettringite formation was retarded by the presence of the superplasticizer. This effect was more pronounced at high dosage of the superplasticizer only during the first 24 h. The presence of sodium salt of naphthalene-formaldehyde polycondensate caused a decrease in the size of the formed ettringite crystals, and as the percentage of the admixture increased, the crystal size decreased. In addition, there was an interaction between the used superplasticizer and the formed ettringite as indicated from infrared analysis.

Comparative XRD analysis ettringite originating from pozzolan and from portland cement

The ettringite is often formed when concrete is attacked by sulphatic waters. In this investigation, a semi-quantitative analysis and study of the ettringite (2Θ = 9.08 °) in the solid phase of the Fratini test was made by XRD. The cements P-1 (14.11% C 3A) P-31 (7.6% C 3A) and PY-6 (0.00% C 3A), five pozzolans, D,O,A,C and M and five mixed cements PUZC 70/30, PY-6/D, O, A, C or M, respectively, were used. These PUZC were mixed with gypsum in proportions to give 7.0% or 21.0% SO 3. Identical analysis and study was made of the corresponding mixtures of each pozzolan plus Ca(OH) 2 and gypsum. The experimental results have demonstrated that 1. The formation rate of ettringite from pozzolan, is notably higher than the formation rate of ettringite from OPC. 2. The topochemical mechanism of the etrringite formation can proceed when ALL the necessary reactive materials, including the “reactive alumina,” Al 2O 3r- [or better perhaps, “aluminate anion” or “amorphous alumina” (tetra- or penta-coordinated alumina)], are in the solution.

Hydration of type K expansive cement paste and the effect of silica fume: II. Pore solution analysis and proposed hydration mechanism

This is the second part of a two-part paper on the hydration characteristics of Type K expansive cement pastes and the effect of silica fume addition. Data concerning expansion of pastes and quantitative analysis of the solid phases related to expansion were presented in Part I. Part II presents the portion of the study related to the pore solution analysis. The results presented in Parts I and II used to propose a hydration mechanism of expansive cement pastes and the effect of silica fume. In the expansive cement paste without silica fume, ettringite formation and the consequent expansion terminated due to the depletion SO 4 2− in the pore solution after 11 days of hydration. In the presence of silica fume, the initial formation of ettringite was accelerated leading to an increased rate and magnitude of expansion. After two days of hydration, a reduction in the concentration of alkali cations in the pore solution was observed. This resulted in a decrease in the OH-concentration in the pore solution and subsequent decrease in the rate of ettringite formation. The expansion stopped at about five days of hydration even though approximately 55% of the original C 4 A 3 S remained unhydrated and SO 4 2− still remained in pore solution. It is speculated that the reduction of the pH of the pore solution after two days of hydration may provide an explanation for the termination of ettringite formation in the expansive cement paste containing silica fume.

Mechanism of the hydration of tricalcium aluminate and gypsum of 1:3 mole ratio at low water/solid ratio and in suspension

The hydration kinetics of tricalcium aluminate, C 3A, with gypsum or anhydrite at 1:3 mole ratio were studied using hydration periods of 5 min up to 7 days. These studies were assessed with the aid of differential thermal analysis, thermogravimetric analysis and X-ray diffractometry as well as chemical analysis. The results revealed that hydration periods of up to 7 days of tricalcium aluminate with gypsum in paste form and in suspension, forms ettringite with unhydrated components of C 3A and gypsum. On the other hand, the only hydration product of the slurry of C 3A with anhydrite is ettringite after 7 days of hydration. The kinetics of hydration were studied by the quantitative determination of ettringite from the TG analysis. The results also illustrate that the rate of ettringite formation is slower in the presence of gypsum than in the presence of anhydrite and also that hydration of the slurry gives more ettringite than the hydration of the paste.

On the role of free calcium oxide in expansive cements

Results of research, tending to elucidate the effect of free CaO content in portland cement clinker upon the expansion of specimens of expansive cements, prepared in semi-commercial scale by intergrinding of portland cement clinker, C 4A 3 S -phase containing special clinker and gypsum, by means of examination of liquid phase composition and porosity of mortars, have been presented. Special clinker was obtained by burning a mix of limestone, fly ash with high Al 2O 3 content and gypsum. Obtained results confirm the advantageous effect of free lime upon the hydration process and properties of expansive cements. The concentration of CaO in the liquid phase seems to influence rather the rate of ettringite formation than the size of its crystals.

Effect of some liquefying agents on properties and hydration of portland cement and tricalcium silicate pastes

The effect of a melamine sulfonate resin, a naphthalene sulfonate resin and a sulfonated lignin on the rheological properties and the hydration of portland cement and tricalcium silicate pastes was studied. In addition to improving the flow properties of the pastes all three substances retarded the hydration of C 3S and altered the stoichiometric composition of the CSH-phase formed. The rate of ettringite formation was altered by the agents differently in two different cements studied.