Artificial pozzolanic cement pastes containing burnt clay with and without silica fume

Pozzolanic cement blends were prepared by the partial substitution of ordinary Portland cement (OPC) with different percentages of burnt clay (BC), Libyan clay fired at 700 °C, of 10, 20, and 30%. The pastes were made using an initial water/solid ratio of 0.30 by mass of each cement blend and hydrated for 1, 3, 7, 28, and 90 days. The pozzolanic OPC–BC blend containing 30% BC was also admixed with 2.5 and 5% silica fume (SF) to improve the physicomechanical characteristics. The hardened pozzolanic cement pastes were subjected to compressive strength and hydration kinetics tests. The results of compressive strength indicated slightly higher values for the paste made of OPC–BC blend containing 10% BC The results of DSC and XRD studies indicated the formation and later the stabilization of calcium silicates hydrates (CSH) and calcium aluminosilicate hydrates (C₃ASH₄ and C₂ASH₈) as the main hydration products in addition to free calcium hydroxide (CH). Scanning electron microscopic (SEM) examination revealed that the pozzolanic cement pastes made of OPC–BC mixes possesses a denser structure than that of the neat OPC paste. Furthermore, the addition of SF resulted in a further densification of the microstructure of the hardened OPC–BC–SF pastes; this was reflected on the observed improvement in the compressive strength values at all ages of hydration.     

Effect of metakaolin pozzolanic activity in the early stages of cement type II paste and mortar hydration

The cement industry is one which most emits polluting gases to the environment, due to the calcium carbonate calcination, as well as to the burning of fossil fuels during the manufacturing process. Metakaolin (MK), in partial substitution to cement in its applications, is having a special worldwide growing role, for the technological increment due to its pozzolanic activity and mainly to the reduction of those emissions. In the present paper, the effect of pozzolanic activity of metakaolin was analyzed by thermal analysis in pastes and mortars of type II Portland cement in the first three days of the hydration, during which, relevant initial stages of the hydration process occur. By non-conventional differential thermal analysis (NCDTA), paste and mortar samples containing 0, 10, 20, 30 and 40% of metakaolin in cement mass substitution and using a 0.5 water/(total solids) mass ratio, were evaluated. The NCDTA curves, after normalization on cement mass basis and considering the heat capacity of each reactant, indicate that the pozzolanic activity behavior of metakaolin is different in pastes and mortars. Through the deconvolution of the normalized NCDTA curve peaks, it can be seen that ettringuite formation increases as cement substitution degree (CSD) increases, in both cases. Tobermorite formation is more enhanced in mortars than in pastes by MK, with a maximum formation at 30% of CSD. In the pastes, tobermorite formation increases as CSD increases but it is practically the same at 30 and 40% of CSD.     

Evaluation of pozzolanic activity and physicomechanical characteristics in metakaolin-lime pastes

In order to evaluate the pozzolanic activity of metakaolin, several pastes were prepared, by mixing metakaolin with hydrated lime, in different ratios. The pastes were stored in standard conditions (RH=99±1%, T=25±1°C) and evaluated using thermal analysis (DTA/TG), X-ray diffraction (XRD), compressive strength tests and mercury intrusion porosimetry (MIP), in time. The obtained results revealed that the compounds formed are CSH, C₂ASH₈ and C_{4 11} while C₄AH₁₃ was not detected up to 270 days of curing. The calcium hydroxide consumption increases as the initial amount of the metakaolin in the paste augments. The maximum strength development is obtained for metakaolin/lime ratio:1.     

Effect of substitution of blast-furnace slag by fired drinking water sludge on the properties of pozzolanic cement pastes

Journal of Thermal Analysis and Calorimetry - The effect of fired drinking water sludge (FDWS) as a mineral admixture on the physico-mechanical properties and the fire resistance of pozzolanic...     

Thermal resistance of alkali-activated metakaolin pastes containing nano-silica particles

Journal of Thermal Analysis and Calorimetry - In the current investigation, the opportunity of employing nano-SiO2 (NS) to modify fire resistance of metakaolin-based geopolymer pastes has been...     

Evaluation of pozzolanic activity and physico-mechanical characteristics in ceramic powder-lime pastes

Ceramic powder has been used as an artificial pozzolanic addition, in preparing pozzolanic mortars for the historic/traditional structures’ construction. In order to evaluate the pozzolanic activity of ceramic powder, several pastes were prepared, by mixing it with hydrated lime, in different ratios. The pastes were stored in standard conditions (RH=99±1%, T=25±1°C) and evaluated using thermal analysis (DTA/TG), X-ray diffraction (XRD), compressive strength tests and mercury intrusion porosimetry (MIP), in time. The obtained results revealed that the compounds formed were CSH and C₄ACH₁₁ (monocarboaluminate) after 270 days of curing. The calcium hydroxide consumption increases as the initial amount of the ceramic powder in the paste augments. The maximum strength development is obtained for ceramic powder/hydrated lime ratio 3:1.     

Hydration products in two aged cement pastes

The hydration products in two aged cement pastes (DTA/DTG/TG) were compared with those in fresh ones (static heating, SH) and were also studied by mass spectrometry (MS), IR and thermo XRD-analysis. The products considered here were: the sorbed water, the CSH gel including hydrates, portlandite, calcite, aragonite and vaterite. Except carbonates their content was higher in the stronger paste C-43, than in C-33, but lowered with ageing (only the CSH gel water remained approximately unchanged). The sorbed water content became with time lower and similar in both pastes (it evaporated up to 155-185°C in TG); the escape of the rest moved to higher temperatures (500-700°C). The three DTG peaks at 200-400°C indicated jennite-like phase in the CSH gel; the mass loss (155-460°C) was higher on ageing due to development of organic matter, especially in C-43 (DTA, TG, IR). Portlandite content changed little and carbonate content increased considerably. They decomposed in air at 470 and 720-740°C, in argon at 450 and 680-710°C and in vacuum at 400 and 630°C, respectively (DTG peak, XRD). Between 500 and 700°C the simultaneous evolution of H₂O and CO₂was observed by MS, which is attributed to dehydroxylation of jennite-like phase and/or to decomposition of some carbonate hydrate and/or hydrocarbonate (three peaks on CO₂evolution curve, MS). The d(001) peak of portlandite exceeded the nominal value and will be analyzed separately.     

Differential thermal studies of polymethyl methacrylate-impregnated cement pastes

Differential thermal curves have been obtained for two polymethyl methacrylate-impregnated cement pastes prepared at a water/cement ratio of 0.37 and 0.70. Complex thermal effects, including a substantial decrease in the endothermal peak for Ca(OH)₂ decomposition, were observed in samples heated in air. These effects originate in the portland cement paste, in the polymer, and from interactions between the polymer and the hydrated cement during heating. Less complex effects resulted when DTA was carried out in N₂. There was no evidence of a reaction between the hydrated cement and PMMA during impregnation.     

Low frequency dielectric behaviour of Portland cement pastes

Summary Dielectric constants and loss tangents of a Portland cement paste were measured during and after hardening in the frequency range 60 c/s-300 kc/s. One day after preparation two loss areas were observed; during hydration (at 20 °C; 65% R. H.) a h. f. maximum moved slowly to lower frequencies; a l. f. loss increased in magnitude during the first few days of curing, but disappeared during further hardening. Results for a cement paste cured under water for 6 weeks, showed that the h. f. maximum had come to a stop at about 10 kc/s. From the continuous shift of this maximum with temperature in passing the freezing-point it is concluded that the h. f. loss is caused by water of crystallization; the activation energy is 13 kcal/mole. The l. f. loss is believed to be caused by water in open pores. Cement exposed to air during hardening appears to have more open pores than cement hardened under water.

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Zusammenfassung Dielektrische Konstante und dielektrische Verluste von Portland-Zement Pasta wurden w?hrend und nach Aush?rtung im Frequenzbereich zwischen 60 und 3.10⁵ Hz gemessen und diskutiert. Ein Tag nach der Bereitung der Paste zeigt diese zwei Verlustgebiete. W?hrend der Aush?rtung verschiebt sich das hochfrequente Verlustmaximum nach niederen Frequenzen. Es kommt schlie?lich bei etwa 10⁴ Hz zum Stillstand, wie Messungen an unter Wasser ausgeh?rtetem Zement zeigen. Die Temperaturverschiebung dieses Maximums verl?uft stetig beim Durchgang durch den Gefrierpunkt mit einer Aktivierungsenergie von 13 kcal/Mol. Aus diesen Tatsachen ergibt sich ein Deutungsvorschlag dieses Maximums: Es wird Dipolverlusten von gebundenem Kristallwasser zugeschrieben. Das zweite, niederfrequente, Verlustgebiet erh?ht sich innerhalb der ersten Tage der Aush?rtung, um dann wieder zu verschwinden. Es wird als ein Maxwell-Wagner-Sillars-Maximum von in Poren absorbiertem Wasser gedeutet.

     

Characterisation of cement pastes by inverse gas chromatography

Two cement pastes, commonly used in concrete formulations, were characterised by IGC at 35-80 degrees C before and after coating with an epoxy resin and a hardener. The cements are mixtures of hydrates in various proportions, such as calcium silicate hydrate (CaO-SiO2-H2O) and calcium hydroxide Ca(OH)2. Apolar and polar probes were used to determine the dispersive and acid-base characteristics of the cement pastes. These materials have high surface energy as judged from the dispersive contribution to the surface free energy (gamma(s)d) values lying in the 50-70 mJ/m2 range at 60-80 degrees C. Examination of the specific interactions permitted to show that the cement pastes are strongly amphoteric species with a substantial predominant Lewis basicity that is in line with the basic pH of their aqueous suspensions. Following coating with an epoxy resin (DGEBA) and a hardener (triethylene tetramine), the surface energy of the cements decreases substantially with the mass loading of the organic material. The surface thermodynamic properties were also correlated with the surface chemical composition as determined by X-ray photoelectron spectroscopy.     

NMR control of aging and durability of hardened cement pastes

We report on proton NMR relaxation of hardened and aged grey and white CEM I paste with several controlled degree of relative humidity. A proton NMR relaxation study of these two CEM I cement pastes shows very different pore size hierarchies for these two materials. The good sensitivity of the proposed method for analysing the evolution of the pore size dependencies under drying and wetting history of the material could be very useful for studying the durability of building material under severe conditions of use.     

Thermal resistance of hardened cement pastes containing vermiculite and expanded vermiculite

The durability and thermal stability of hardened Portland cement pastes containing vermiculite (V) and expanded vermiculite (EV) exposed to high temperatures were studied. Different mixtures were prepared using 2.5, 5, and 10 wt% of both types of V. Each mixture, after 28?days of hydration, was heated at 300, 600, and 800?°C for 3?h. Two modes of cooling were used; gradual cooling in air and rapid cooling in cold water. The percentage of residual strength, chemically combined water content, change in phase composition, and the thermal stability of the heated specimens were studied. The specimens cooled in water showed greater loss in strength than the air-cooled specimens. The presence of V improved the heat resistance of ordinary type I Portland cement (OPC) pastes. 5 wt% replacement revealed the best performance at all heating temperatures. The EV showed better thermal resistance than the nonexpanded one. Addition of silica fume (SF) with V in OPC pastes lead to superior performance. This can be explained as result of the combined effects of insulation properties of V and pozzolanic reactivity of SF which accounts for the notable increase in the residual strength for these mixes.     

Microgravimetric measurements of water vapour sorption on hydrated cement pastes

To study the pore structure of hydrated cement pastes, differing in pretreatment and chemical composition, water vapour sorptlon experiments have been used. The experiments were carried out at 25.0°C in a pure water vapour atmosphere, up to relative vapour pressures of 0.98. The amount of water adsorbed or desorbed was determined gravimetrically using a Cahn 2000 microbalance. The set-up developed for this purpose and some experimental results are presented.     

Immobilization of Zn(II) in Portland cement pastes

The aim of this paper is to study the solidification/stabilization potential of cementitious matrices on the immobilization of Zn(II) before its disposal into the environment by determining the mechanisms of interaction between the Zn(II) ions and the binder. The results of structural and mineralogical characterization of cement pastes formed with different amounts of immobilized Zn(II) ions are presented and the study includes results from thermogravimetric analysis (TG), scanning electron microscopy, X-ray diffraction, and leaching performance. Zn(II) ions delay the hydration reaction of Portland cement due to the formation of mainly CaZn₂(OH)₆·2H₂O , as well as Zn₅(CO₃)₂(OH)₆, Zn(OH)₂, and ZnCO₃ in minor proportion. Correlations between total mass loss in TG analysis and leached Zn(II) ions in long-term curing pastes have been obtained. This result is important because in a preliminary approach from a TG on an early-aged cement paste containing Zn(II), it could be possible to perform an estimation of the amount of Zn(II) ions that could be leached, thus avoiding costly and time-consuming tests.     

An alternative thermal method for identification of pozzolanic activity in Ca(OH)2/pozzolan pastes

Pozzolans play an important role in the industry of cement and concrete. They increase the mechanical strength of cement matrices and can be used to decrease the amount of cement in concrete mixtures, thus decreasing the final economic and environmental cost of production; also, as some of them are byproducts of industrial processes (such as silica fume and fly ash) and their use can be seen as a solution for some residues, that otherwise would be disposed as a waste. Pozzolans fixate the Ca(OH)₂ generated during cement’s hydration reactions to form calcium silicate hydrates (C–S–H), calcium aluminate hydrates (C–A–H), or calcium aluminosilicate hydrates (C–A–S–H), depending on the nature of the pozzolan. Traditionally, the pozzolanic activity is identified using the Ca(OH)₂ fixation percentage which is quantified by thermogravimetric (TG) analysis, using the mass loss due to the Ca(OH)₂ dehydroxylation around 500 °C. An alternative method to identify pozzolanic activity at lower temperatures using a standard issue moisture analyzer (MA) is presented in this paper, using the mass loss due to hydrate’s dehydration generated by pozzolans in the pozzolanic reaction. Samples of Ca(OH)₂ blended with different pozzolans were prepared and tested at different hydration ages. Using TG analysis and an MA, a good correlation was found between the total mass loss of the same sample, using the two methods at the same temperature. It was concluded that the MA method can be considered a less expensive and less time-consuming alternative to identify pozzolanic activity of siliceous or aluminosiliceous materials.     

Thermogravimetry on calcined mass basis — hydrated cement phases and pozzolanic activity quantitative analysis

When cement hydrated compositions are analyzed by usual initial mass basis TG curves to calculate mass losses, the higher is the amount of additive added or is the combined water content, the higher is the cement ‘dilution’ in the initial mass of the sample. In such cases, smaller mass changes in the different mass loss steps are obtained, due to the actual smaller content of cement in the initial mass compositions. To have a same mass basis of comparison, and to avoid erroneous results of initial components content there from, thermal analysis data and curves have to be transformed on cement calcined basis, i.e. on the basis of cement oxides mass present in the calcined samples or on the sample cement initial mass basis. The paper shows and discusses the fundamentals of these bases of calculation, with examples on free and combined water analysis, on calcium sulfate hydration during false cement set and on quantitative evaluation and comparison of pozzolanic materials activity.     

CO2 sequestration by high initial strength Portland cement pastes

During the formation of pastes, mortar and concretes have been used to capture CO₂. This work presents a methodology to estimate the carbon dioxide (CO₂) sequestered by high strength and sulfate-resistant Portland cement pastes during their early stages of hydration, by Thermogravimetry and Derivative Thermogravimetry. Water to cement ratio equal to 0.50 and 0.70 were evaluated and the captured CO₂ amount was determined through TG/DTG curve data on initial cement mass basis, obtained during accelerated carbonation from the fluid state and accelerated carbonation after a first hydration process. The experiments were performed in a controlled chamber, maintaining the CO₂ content at 20 vol % and the temperature at 25 °C, at different relative humidity (RH) (60 and 80 %) ambient. The procedure allows one to estimate the amount of CO₂ sequestered by the initial cement mass of a given volume of paste, as well as to evaluate the RH and W/C ratio influence on the amount of hydrated formed products, mainly on the Ca(OH)₂, important for CO₂ fixation.     

Application of differential thermogravimetry to the hydration of expansive cement pastes

Differential thermogravimetric analysis was used to determine the hydration kinetics of expansive cement and its products at various ages of hydration. Analytical grade reagents, kaolin and Portland cement were used to prepare an expansive cement on the basis of calcium sulphoaluminate. Two mix compositions having the stoichiometric composition of trisulphate and monosulphate were synthesized from pure reagents. Three clinkers were also prepared from kaolin, gypsum and calcium carbonate with different compositions.The hydration of expansive cement prepared from the stoichiometric composition of trisulphate and Portland cement gives ettringite as the stable phase after seven days of hydration. The presence of more CaO than the stoichiometric composition of trisulphate favours the conversion of some ettringite to the monosulphate hydrate. The hydration of expansive cement prepared from the stoichiometric composition of monosulphate and Portland cement shows the presence of ettringite and the monosulphate phase. Ettringite is formed initially, and then transformed to the monosulphate form.     

Effect of addition of nano-magnetite on the hydration characteristics of hardened Portland cement and high slag cement pastes

In this investigation the effect of addition of magnetite nanoparticles on the hydration characteristics of both ordinary Portland cement (OPC) and high slag cement (HSC) pastes was studied. The cement pastes were prepared using a water/solid (W/S) mass ratio of 0.3 with addition of 0.05, 0.1, and 0.3 % of magnetic fluid Fe₃O₄ nanoparticles by mass of cement. An aqueous stable magnetic fluid containing Fe₃O₄ nanoparticles, with a mean diameter in the range of super-paramagnetism, was prepared via co-precipitation method from ferrous and ferric solutions. The admixed magnetite-cement pastes were examined for compressive strength, chemically combined water content, X-ray diffraction analysis, and differential scanning calorimetry. The results of compressive strength revealed that the hardened pastes made from OPC and HSC admixed with different amounts of magnetic fluid have higher compressive strength values than those of the neat cement OPC and HSC cement pastes at almost all ages of hydration. The results of chemically combined water content for the admixed cement pastes showed almost the same general trend and nearly comparable values as those of the neat cement pastes. From the XRD diffractograms obtained for the neat OPC and HSC cement pastes, the main hydration products identified are calcium silicate hydrates, portlandite, and calcium sulfoaluminate hydrates. Addition of magnetic fluid nanoparticles to both of OPC and HSC did not affect the main hydration products of the neat OPC or HSC cement in addition to one main basic difference, namely, the formation of calcium iron hydroxide silicate as a new hydration product with a reasonable hydraulic character.     

Correlation between pore structure and low-temperature dilatation of hydrated cement pastes and mortars

The pore structure and low-temperature dilatation behavior of traditional hydrated Portland cement paste compacted by different methods were investigated. The aim of the investigation was to demonstrate the influence of the water-cement ratio and the compacting conditions on the developing pore structure and the frost dilatation during the early stage of the hydration process. A low water-cement ratio and a high compacting pressure resulted in initially low porosity, but in coarser pore sizes. Vibration resulted in lower pore volumes as compared with those of cast cement pastes, but the pore size distributions were similar. In accordance with the pore size distribution, two frost dilatation effects were measured when macro- and mesopores also occurred in the hydrated cement pastes. In the samples compacted by high pressure, a single frost dilatation effect occurred in connection with the macropores present in the sample. The magnitude of the frost dilatation effect decreased with increasing curing time. The decrease is caused by a decrease in the volume of the pores and also by an increase in matrix strength.

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Zusammenfassung Es wurde die Porenstruktur und das Tieftemperaturdilatationsverhalten von herkömmlichem, hydratiertem, durch verschiedene Methoden verdichtetem Portland-Zementleim untersucht. Das Ziel der Untersuchung bestand im Nachweis des Einflusses des Wasser-Zement-Verhältnisses und der Verdichtungsbedingungen auf die entstehende Porenstruktur und die Frostdilatation in der frühen Periode des Hydratationsprozesses. Ein niedriges Wasser-Zement-Verhältnis und ein hoher Verdichtungsdruck liefert eine geringe Porösität, aber größere Porenmaße. Vibration liefert im Vergleich zu gegossenem Zementleim ein geringeres Porenvolumen, aber die Beiträge zur Porengröße waren ähnlich. In Übereinstimmung mit dem Porengrößeeinfluß wurden zwei Frostdilatationseffekte gemessen, wenn sowohl Makroals auch Mesoporen in den hydratierten Zementleimen vorkamen. In denjenigen Proben, die unter Hochdruck verdichtet wurden, tritt in Verbindung mit der Gegenwart von Makroporen in der Probe ein einfacher Frostdilatationseffekt auf. Die Höhe des Frostdilatationseffektes sinkt mit steigender Aushärtungszeit. Dieses Sinken wird durch eine Abnahme des Volumens der Poren verursacht und ebenfalls durch ein Ansteigen der Matrixfestigkeit.