Fresh Portland Cement Research Articles

Fresh cement as a frictional non-Brownian suspension

Cement is an essential construction material due to its ability to flow before later setting, however the rheological properties must be tightly controlled. Despite this, much understanding remains empirical. Using a combination of continuous and oscillatory shear flow, we compare fresh Portland cement suspensions to previous measurements on model non-Brownian suspensions to gain a micro-physical understanding. Comparing steady and small-amplitude oscillatory shear, we reveal two distinct jamming concentrations, ϕμ and ϕrcp, where the respective yield stresses diverge. As in model suspensions, the steady-shear jamming point is notably below the oscillatory jamming point, ϕμ<ϕrcp, suggesting that it is tied to frictional particle contacts. These results indicate that recently established models for the rheology of frictional, adhesive non-Brownian suspensions can be applied to fresh cement pastes, offering a new framework to understand the role of additives and fillers. Such micro-physical understanding can guide formulation changes to improve performance and reduce environmental impact.

Effect of Various Coarse Aggregates on the Compressive Strength of Concrete in Jalalabad City, Afghanistan

One of the most frequently used building materials on the globe is concrete. Concrete is composed of aggregate, water, and cement. This study investigated the influence of different coarse aggregates on the compressive strength of concrete. The coarse aggregates used in the experimental design consisted of crushed granite, crushed river stone, and natural gravel, with river sand as the fine aggregate. Sieve analysis and unite weight was performed on specimens frequently used by citizens. Fresh Portland cement and water-cement ratios of 0.55 and 0.7 were both used. To investigate the influence of various aggregates on concrete compressive strength a specimen for 20 N/mm2 concrete ratio of 1:1.5:3, and 48 concrete cubes measuring 15 cm x 15 cm x 15 cm were casted in the laboratory at Alfalah University. For fresh concrete, the slump test results were 63 mm for crushed granite, 65 mm for crushed river stone, and 50 mm for natural gravel. Crushing the concrete cubes after 7, 14, 21, and 28 days determined the compressive strength. Additionally, the highest compressive strength values achieved for crushed granite were 25.94 N/mm2, crushed river stone was 23.68 N/mm2, and natural aggregate concrete was 17.73 N/mm2. Based on these findings, crushed granite has been recommended for usage in reinforced concrete applications.

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.

Rheology and air entrainment of fresh Portland cement mortars in simulated low air pressure environments

Air entrainment in cement-based materials is believed to be affected by low air pressure (AP) which is the atmospheric characteristic in high plateaus, and thus the rheology may be affected. This paper attempts to study rheology and air entrainment of fresh Portland cement mortar in simulated low APs. The results indicate that the rheological responses with the varying AP gradients are relevant to the air entrainments in fresh mortar mixtures. The volume changes of air voids in fresh mortars are linearly related to the varying APs, which is owing to not only the AP difference between the inside and outside air voids but also the additional pressure on the liquid films of air voids stemming from the surface tension changes. Moreover, the Bingham capillary number is calculated to account for the air void deformability. Then rheological parameters are correlated to the content and deformability of air voids in fresh mortars.

A proposed method and monitoring system for evaluating workability of Portland cement concrete during mixing

The objective of this study is to address the problem that the traditional quality control method of fresh Portland cement concrete cannot allow the workability to be evaluated during the mixing process. Based on the rheological theory and observed linear relationship between the shear force and flow deformation of concrete mixtures, a mathematical model was established to characterize the workability of concrete using the stirring power of concrete mixers. In this study, a laboratory-scale two-shaft mixer with a rotation speed control system and a power monitoring system was designed and tested. A LabVIEW program was also developed to process and analyze changes of the stirring power data. Laboratory test results show that the optimal workability of fresh concrete with very different mix designs is usually obtained when the second wave trough of stirring power appears. The test results are also compared to the conventional slump test, which indicate the proposed approach and developed system can be used to evaluate the workability of fresh concrete during mixing and identify the optimal mixing time length.

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.

Research into the Long-Time Strength of Cement Stone with a Modified Structure

The problem of a time-dependent gain of a cement dispersed system’s strength remains the focus of both domestic and foreign researchers in terms of its practical significance. To study the above problem, we used non-additive sulfate-resistant Portland cement grade 400 made by Volsk city plant with a normal density of 24 %. 9 series of samples were made from fresh Portland cement. The kinetics of changes in the cement stone’s physical and mechanical properties was observed on series’ samples for 9.5 and 18 years. The paper imparts the values of the cement, water, superplasticizer consumption and compaction ration derived from compacted cement paste after forming test pieces, numerical values of properties in check dates as a result of natural moisture exchange with the environment and drying of test samples to a constant mass at 105°C, as well as the degree of its hydration at the age of 18 years, which was determined by calculation from non-evaporable, that is, chemically bound, water, taking into account that its amount in fully hydrated cement comprises 23% of the cement mass. Changing of the superplasticizer introduction procedure, even with a minimum consumption of S-3, led to a change in the yield value of the cement stone. The experiments targeting to reveal the long-time behavior of a superplasticized cement stone allow us to speak of a clear trend in the influence of SP and its introduction procedure on changes in long-time strength values. However, this is only a trend, since there are many other factors of influence than those discussed in this paper, which testifies to the necessity for a more detailed study of this problem due to its complexity considering the development and application of a new generation superplasticizer and composite binders to construction practice.

Influence of nano-modification on mechanical and durability properties of cement polymer anticorrosive coating

In the present study performance evaluation of nano-modified cement polymer anticorrosive coating (CPAC) was undertaken by conducting the Chemical Resistance Test (CRT), Applied Voltage Test (AVT), Bond Strength Test (BST), Accelerated Corrosion Test (ACT) and Coating Flexibility Test (CFT). The site oriented coating comprises of nitrite, styrene-butadiene polymer and other additives. The anticorrosive polymer solution is compatible with concrete or cement when uniformly mixed with fresh ordinary portland cement (OPC). Totally forty-five specimens were subjected to various performance evaluation tests. In CRT observations were made on drilled and undrilled specimens after 45 days test period in liquid and vapour phase. The coating did not blister, soften and lose bond in all the tested medium during CRT and meet the requirement of BIS 13620-1993 and ASTM A775/A775M.The coating has the ability to withstand the electrochemical stresses during one-hour AVT. In the BST, single and double coated rebars showed +126.96% and +46.08% greater usable bond strength respectively than uncoated rebar. In the ACT, there is a significant escalation in time of cracking of specimens of double-coated reinforced rebars by 2 times as compared to uncoated rebars. Cracking time for single coated reinforced rebars was found 1.6 times more than uncoated rebars. In the CFT, coating completely in the inner and the outer radius of the 180° bend rebar fails to meet the requirements of BIS and ASTM standards. Thus the coating has to be applied subsequent to cutting and rebar twisting is finished.

Determining the Rheological Properties of Fresh Concrete using Concrete Shear Box

A test method which is accurate, viable and is based on the fundamental scientific description and at the same time the results are independent of the test device can go a long way in better understanding of the workability of fresh concrete. The principal objective of this research study is to assess the feasibility of concrete shear box to measure the rheological properties of fresh Portland cement concrete using Bingham model. Study assesses the repeatability and reproducibility of the results. The repeatability values of relative yield stress and relative plastic viscosity are 97.63% and 91.49% and reproducibility values are 91.68% and 95.51% respectively and the results reinforce the effectiveness of using concrete shear box.

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.

Influence of dry ice on the performance of Portland cement and its mechanism

This study reports on the effect of dry ice on the hydration and hardening behavior of Portland cement. Dry ice was directly added to fresh Portland cement paste, with its dosage fixed at 0 wt%, 0.3 wt%, 0.6 wt%, 0.9 wt%, 1.2 wt% and 1.5 wt%. Results showed that dry ice had little effect on the standard consistency and setting time of cement paste when its dosage was less than 0.9 wt%, but otherwise the standard consistency as well as the setting time was increased. The compressive strength of the paste at 7 days increased slightly when the dosage of dry ice was less than 0.9 wt%, but the compressive strength was reduced with the addition of 0.9 wt% to 1.5 wt% dry ice. For the compressive strength at 28 days, the cement paste with 0.6 wt% dry ice showed the greatest value, being increased by 30.9% compared to the control cement paste, and the compressive strength of the samples with dry ice was all higher than that of the control. In addition, hydration heat, XRD, DTA-TG and SEM results showed that the incorporation of dry ice retarded the early hydration of Portland cement, improved its later hydration, and optimized the hardened structure of the cement paste. These results can provide a reference for the future application of dry ice in Portland cement-based materials.

Protection of Steel Rebar in Salt-Contaminated Cement Mortar Using Epoxy Nanocomposite Coatings

Epoxy reinforced with two kinds of nanoparticles dealing with nano-SiO2 and nano-Fe2O3 was coated on steel rebar embedded in a chloride contaminated cement mortar. NaCl was added to the fresh Portland cement paste (at 0.3% and 0.5% by weight of cement) to simulate the chloride contamination at the critical level. The effect of incorporating nanoparticles on the corrosion resistance of epoxy-coated steel rebar was investigated by linear potentiodynamic polarization and electrochemical impedance spectroscopy. For the 0.3 wt.% chloride mortars, the electrochemical monitoring of the coated steel rebars during immersion for 56 days in 0.1 M NaOH solutions suggested the beneficial role of nano-Fe2O3 particles in significantly improving the corrosion resistance of the epoxy-coated rebar. After 56 days of immersion, the nano-Fe2O3 reduced the corrosion current of epoxy-coated rebar by a factor of 7.9. When the chloride concentration in the cement mortar was 0.5 wt.%, the incorporation of nanoparticles into the epoxy matrix did not enhance the corrosion resistance of epoxy coating for the rebar. At this critical level, chloride ions initiated rebar corrosion through nanoparticles at the epoxy/rebar interface.

A study of high-performance slag-based composite admixtures

To promote the large-scale application of ground granulated blast furnace slag powder (GGBFS) in the cement and concrete industry, high-performance slag-based composite admixtures (SCAs) were prepared with GGBFS as the main raw material and converter steel slag powder (SS), limestone powder (LP) and fly ash (FA) as auxiliary raw materials. The effects of the fineness and content of each auxiliary raw material on the performance of the SCA were investigated. The experimental results showed that SS had multiple beneficial effects on mortar and concrete; it can increase the strength, reduce the bleeding and compensate for the shrinkage of the concrete. The best performance of medium- and low-strength concrete with a high SCA content was achieved using the SCA with the following composition: GGBFS: 64%; SS: 20%; LP: 6%; and FA: 10%. The fresh Portland cement (PC)+SCA concrete specimens had better fluidity and stability than the PC+GGBFS concrete specimen, and the compressive strengths of the PC+SCA concrete specimens were the same or slightly higher than that of the PC+GGBFS concrete specimen. The results of scanning electron microscopy, X-ray photoelectron spectroscopy and pore size analysis showed that the concrete specimens prepared with PC+SCA had significantly smaller and fewer large pores and a more uniform and dense interfacial structure than the concrete specimen prepared with PC+GGBFS. The exceptional performance of SCAs results from the advantageous synergistic effects of GGBFS and SS.

Role of the filler on Portland cement hydration at very early ages: Rheological behaviour of their fresh cement pastes

The rheological behaviour of fresh Portland cement (PC) pastes with different chemical and potential mineralogical compositions was analysed when were blended with crystalline mineral additions (fillers). These two fillers were very different: a siliceous, Q, α-quartz type, and another limestone, C. Trials were conducted during latent hydration at 25°C. The findings showed that apparent viscosity was higher in the PC with low C3A and high C3S content, consequently, the Na2Oeq.(%) content of everyone also contributed very significantly. Moreover, it was also observed that the incorporation of each filler affects differently the rheological behaviour of fresh cement pastes. The different influence is due to the different physical and chemical properties of each filler (nature, chemical composition and character and texture intimate of the particles), and also of the type of PC to interact. Finally and at these very early ages, both fillers stimulated the hydration of the PC which they were mixed by direct and non-direct way, and in the case of C filler by indirect way as well, but without any pozzolanic activity. Despite the fact that blended cement P1/Q 60/40 has proved to be a “false positive” in Frattini test.

CARBONATION RESISTANCE OF SLAG MORTARS ACTIVATED BY DIFFERENT ALKALI ACTIVATORS

In this study, the carbonation depths of slag mortars activated by sodium silicate (water glass), sodium hydroxide and sodium carbonate were investigated and the results were compared to those of control mortar with CEM I 42.5 R normal Portland cement. In the mixture, the sand/binder ratio was 2.75 and the water/binder ratio was 0.50. Na concentrations in the mixture proportions were determined as 4%, 6%, and 8% for all the activators. For liquid sodium silicate activator, SiO2/Na2O ratios (Ms) of 0.75, 1, 1.25, and 1.5 were also chosen. Prismatic specimens having 40 x 40 x 160 mm dimensions were prepared from both fresh Portland cement and slag mortar mixtures for the measurements. The day after the mortar casting, the prisms were demolded and placed in a humidity cabinet at 65% ± 5 relative humidity and 22 ± 2 °C temperature. The carbonation tests of mortars were conducted at 7, 28, 90 and 180 days. It was observed that the carbonation resistance of slag mortars activated by alkalis was lower than that of control mixture with Portland cement. However, it was also seen that an increment in the Na dosage of activator improved the resistance of the activated slag mortars to carbonation.

Role of Adsorption Phenomena in Cubic Tricalcium Aluminate Dissolution.

The workability of fresh Portland cement (PC) concrete critically depends on the reaction of the cubic tricalcium aluminate (C3A) phase in Ca- and S-rich pH >12 aqueous solution, yet its rate-controlling mechanism is poorly understood. In this article, the role of adsorption phenomena in C3A dissolution in aqueous Ca-, S-, and polynaphthalene sulfonate (PNS)-containing solutions is analyzed. The zeta potential and pH results are consistent with the isoelectric point of C3A occurring at pH ∼12 and do not show an inversion of its electric double layer potential as a function of S or Ca concentration, and PNS adsorbs onto C3A, reducing its zeta potential to negative values at pH >12. The S and Ca K-edge X-ray absorption spectroscopy (XAS) data obtained do not indicate the structural incorporation or specific adsorption of SO42- on the partially dissolved C3A solids analyzed. Together with supporting X-ray ptychography and scanning electron microscopy results, a model for C3A dissolution inhibition in hydrated PC systems is proposed whereby the formation of an Al-rich leached layer and the complexation of Ca-S ion pairs onto this leached layer provide the key inhibiting effect(s). This model reconciles the results obtained here with the existing literature, including the inhibiting action of macromolecules such as PNS and polyphosphonic acids upon C3A dissolution. Therefore, this article advances the understanding of the rate-controlling mechanism in hydrated C3A and thus PC systems, which is important to better controlling the workability of fresh PC concrete.

Influence of bleeding on properties and microstructure of fresh and hydrated Portland cement paste

The influence of bleeding on slump flow and solid phase distribution of fresh Portland cement paste, as well as the hydration product distribution, splitting behavior, and porosity of the hydrated cement paste were investigated. It is manifested that bleeding results in heterogeneous distribution of solid phases in fresh cement paste, clinker particle is descended due to gravity and the suspension made of liquid and gypsum particle is impelled to ascend. This results in the abundance of gypsum particle and ettringite crystals in the upper part of fresh and hydrated cement paste respectively. Meanwhile, bleeding can impair mechanical property and anti-penetration ability of the hydrated cement paste.

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.

Fingerprinting of Chemical Admixtures in Fresh Portland Cement Concrete by Portable Infrared Spectrometer

The ongoing second SHRP 2 R06-B project entitled Evaluating Applications of Field Spectroscopy Devices to Fingerprint Commonly Used Construction Materials conducted at the University of Connecticut targets, among other objectives, the identification and quantification of chemical admixtures in portland cement concrete (PCC). The two specific objectives for the PCC study are (a) the applicability of portable infrared spectrometers for identification of the admixtures in fresh concrete under field conditions and (b) the feasibility of tracking interaction between hydration products during PCC setting. In the first phase of the study, four admixtures (air entrainer, retarder, accelerator, and super-plasticizer) were tested to establish their chemical signature in the infrared and ultraviolet regions of the electromagnetic spectrum. The fingerprinting of the admixtures was performed with a portable attenuated total reflection (ATR) Fourier transform infrared spectrometer. The second phase concerned evaluating freshly mixed PCC samples prepared with each admixture separately in the amount prescribed by the manufacturer. Chemical changes in the PCC samples were tracked over the 6-h period by the portable ATR spectrometer. Quantitative analysis of the ATR spectra allowed for evaluating the effect of chemical admixtures on hydration activity in PCC.

Cytotoxicity of Portland Cement with Different Radiopacifying Agents: A Cell Death Study

Introduction The aim of this study was to investigate the cytotoxicity of white Portland cement (PC) alone or associated with bismuth oxide (PCBi), zirconium oxide (PCZir), and calcium tungstate (PCCa) in 2 cell lineages. Methods Murine periodontal ligament cells (mPDL) and rat osteosarcoma cells (ROS 17/2.8) were exposed for 24 hours to specific concentrations of fresh PC and PC associations with radiopacifiers. Zinc oxide–eugenol cement and hydrogen peroxide treatment were applied as cytotoxic positive controls. Cell viability after incubation with the cements was assessed by mitochondrial dehydrogenase enzymatic assay. Cell morphology was microscopically analyzed by cresyl violet staining, and the mechanism of cell death was determined by acridine orange/ethidium bromide methodology. All data were analyzed statistically by analysis of variance and Tukey post hoc test ( P < .05). The correlation among cell death by apoptosis or necrosis and pH values was established by Pearson linear coefficient. Results The mitochondrial dehydrogenase enzymatic assay only revealed significant cell death rate at high concentrations of cement elutes. PC alone was not cytotoxic, even at 100 mg/mL. Microscopic images showed that none of the PC formulations caused damage to any cell lines. Statistical analysis of apoptosis/necrosis data demonstrated that PC and PC plus radiopacifying agents promoted significant necrosis cell death only at 100 mg/mL. Conclusions The mPDL cells were more sensitive than ROS17/2.8. The results showed that PC associated with bismuth oxide, zirconium oxide, or calcium tungstate is not cytotoxic to mPDL or ROS17/2.8. Zirconium oxide and calcium tungstate might be good alternatives as radiopacifying agents.