Fresh Portland Cement Pastes Research Articles

Study of the Hydration Mechanism of Portland Cement with Raman Spectroscopy Applying CO2 Laser Radiation

This work presents the results by irradiating fresh Portland cement pastes with CO2 laser, at powers of 1.6, 2.0, and 2.4 W, at different water-to-cement (w/c) ratios 0.4, 0.45, and 0.5. Raman spectra show the evolution and accelerated growth of the main hydration products through the intensity changes of the Raman signal. The Raman shifts reveal that the C-S-H, CH, AFt, and CaCO3 bands are detected in a shorter time, which means that the setting times are also reduced. The detected band of calcium carbonate, which is due to the incorporation of limestone during the grinding process, shows that it is responsible largely for the acceleration in the reactions during the hydration of the cement. The Raman signals of the hydration products of nonirradiated cement pastes present a delay in their evolution with respect to the irradiated pastes. The curves obtained from the Raman spectra can be used to model the mechanism of hydration as well as to understand the setting process.

Evaluation of microstructural changes in fresh cement paste using AC impedance spectroscopy vs. oscillation rheology and 1H NMR relaxometry

AC impedance spectroscopy (ACIS) is a promising technique for monitoring the microstructure evolution of fresh cement paste in real-time. This paper compared the change of bulk electric resistivity (ρbulk) obtained from ACIS with the developments of storage modulus (G′) and the mean transverse relaxation times (T2) of fresh Portland cement pastes within 5 h. It was found that the three different phases (Phase I, II, and III) on the microstructural build-up process of fresh paste can be accurately distinguished by analyzing ρbulk, as well as G′ and T2. The use of ρbulk fails to characterize the microstructural changes in Phase I due to the great sensitivity on the electrical conductivity of the interstitial solution. However, it can successfully reflect the developing features of microstructure in Phases II and III, and reliably evaluate the impacts of water to cement ratio, superplasticizer, and supplementary cementitious materials on the microstructural development.

Calcined clays for low carbon cement: Rheological behaviour in fresh Portland cement pastes

In this paper the rheological behavior of fresh Portland cement pastes mixed with different calcined clays was analyzed. For this purpose, two samples of Portland cement with different mineral composition (one with low C3A and high C3S content and another with low C3S and high C3A) were used combined with different replacement percentages of two calcined clays (two different metakaolin). Grounded α-quartz was used as control as well. Both mineral admixtures had different crystallinity and morphology: the α-quartz is fully crystalline while metakaolin is completely amorphous with a very small crystalline fraction giving very high pozzolanic properties in the vitreous state. All determinations were performed in the hydration latency period. The results show that the fresh Portland cement paste with low C3A(%) content and high C3S(%) content present great shear strength and the replacement with calcined clays affects the rheological behavior of the fresh pastes depending on the greater or lower pozzolanic reactivity of the mineral addition.

Determination of characteristic rheological parameters in Portland cement pastes

The standard use of admixtures in Portland cements (with and without mineral additions)-based concrete manufacture today has made the physical–chemical interaction among the various components a highly topical issue.One way to analyse the resulting effects is discussed in this paper, which substantiates the importance of a careful selection of the parameters used to evaluate the rheological behaviour of plain fresh pastes of Portland cements. To this end, the pastes studied in this research contained no mineral additions.The initial analysis explored the variations in rheometer response depending on angular velocity and phase duration. Ramp direction, number of angular velocities and the size of the inter-velocity steps were also studied. Lastly, the importance of allowing the cement paste sample to rest during the measurement sequence was also analysed.These trials were conducted on two types of Portland cements with completely different mineralogical compositions. They were repeated every 20min prior to the initial setting time, more exactly up to the first nadir on their heat of hydration release curves, determined in earlier conduction calorimetry studies.The results showed that parameter selection in designing a scheme to evaluate the rheological behaviour of fresh Portland cement pastes had a substantial effect on the response obtained. That response was also highly dependent upon the degree and type of hydroxy-induced alkalinity in the liquid phase of the pastes, in turn a result of the mineralogical and chemical composition of the PCs, in particular their N2O and K2O (%) contents, which therefore proved to play a prominent role in rheological behaviour.

Carboxylates and sulfates of polysaccharides for controlled internal water release during cement hydration

Calcium and aluminium carboxylates and sulfates of polysaccharides form hydrogels. Four Al- and three Ca-salt hydrogels of such polysaccharides, which were found to be stable enough to be mixed into fresh Portland cement pastes of low initial water-to-cement ratio of 0.275, were compared with respect to their water retardation potential using non-destructively operating low-field 1H NMR relaxometry. All of the investigated hydrogels release their water to the cement mainly during the accelerated period of cement hydration. At a degree of hydration of about 0.7 the water in the hydrogels is consumed completely by the hydration reactions. Based on Powers’ hydration model the development of the volumetric phase distribution of the investigated hydrating cement with internal curing by the hydrogels was quantified. Within the hardening cement the Ca-salt hydrogels are more stable than the Al-hydrogels which is concluded from the changes of the T2 relaxation times during the cement hydration.

Rheometric and ultrasonic investigations of viscoelastic properties of fresh Portland cement pastes

The paper investigates the possibility of using a shear wave reflection technique to monitor the viscoelastic behavior (represented by storage shear modulus and viscosity) of Portland cement paste at very early age. Three cement pastes with water/cement ratios equal to 0.4, 0.5 and 0.6 cured under water at a constant temperature of 25 °C were studied. By measuring the wave reflection coefficients and the phase angles of reflected ultrasonic waves, the dynamic storage shear moduli and the viscosity of the cement paste can be calculated. The calculated results of the storage modulus were compared with the results obtained directly from the oscillatory rheometric measurement. In addition, the viscosity calculated from the wave reflection measurements was compared with results obtained directly from the step rheometric method and a qualitative agreement was found. The results show that as a non-destructive method, the ultrasonic wave reflection method provides useful information about both the elastic and viscous behavior of cement pastes at very early age.

Flow Behavior of Fresh Portland Cement Pastes

Portland cement pastes prepared at various water/cement ratios were studied using the rate-controlled coaxial cylinder viscometer Rotovisko-Haake 20, system M5-oscillatory measuring device MV2P with serrated surfaces. The tests were performed under both continuous and oscillatory flow conditions. In this article, experimental shear stress and shear rate data were fitted very satisfactorily with a model proposed by the authors. Excellent results were achieved also by applying the Quemada equation. Oscillatory flow data gave further information on the workability of the pastes investigated. In addition, the influence of two different commercial superplasticizing agents (Concretan RX and Ergomix 1000, the first based on polycyclic copolymers with modified structures carrying hydroxylated side chains, the second on a modified polyacrylic resin) was studied with the aim of determining their optimum dosage. In the case of Ergomix 1000, agreement between the optimum dosage determined rheologically and that recommended by the producer was obtained.

Setting behaviour of cement pastes containing admixtures determined from shear modulus tests

Synopsis The rates of development of shear modulus of three fresh Portland cement pastes, two containing accelerating admixtures and one containing a retarding admixture, were compared with the shear modulus of a control mix during the, first few hours after mixing. It was found that the shear modulus equipment used was sensitive enough to detect differences in structure development between the mixes. The initial setting times were determined using the Vicat apparatus and it was observed that the mixes had diferent shear modulus values at their Vicat initial setting time. However, average shear moduli of at least 1970 k Pa had been reached in all cases by the Vicat initial setting time, with the maximum shear modulus of 3270 k Pa being measured for the control mix without additives.

Saturation Factors for Calcium Hydroxide and Calcium Sulfates in Fresh Portland Cement Pastes

Aqueous extracts from five cement pastes at w/c = 0.5 were analyzed for Ca2+, SO2–4, Na+, and K+. Degrees of saturation with respect to portlandite, gypsum, and syngenite were determined at ages from 6 min to 3 h, taking into account ionic association effects. Portlandite saturation is usually attained within a few minutes of mixing, but rates of saturation vary. The maximum degree of saturation is 2 to 3 times the equilibrium value and is attained within 2 h. Initial super‐saturation with respect to gypsum, and in some cases syngenite, occurs at very early ages and declines to the saturation level within 12 min. Results are interpreted by use of a solution‐precipitation model for cement hydration. The calculated critical size of portlandite nuclei at early ages is ∼0.1 μm. Slow growth of portlandite nuclei suggests a surface poisoning phenomenon.

Thixotropic behaviour of cement pastes

In the present work the thixotropic behaviour of fresh Portland cement pastes is examined on the basis of the stress transients determined at constant shear rate. Some significant quantities are considered: the difference between maximum and equilibrium shear stresses, the breakdown area and the density of the energy absorbed by thē material for the breakdown of the thixotropic structure. A distance parameter is proposed in order to evaluate the influence of the specific surface and water/cement ratio on these quantities.