Hardened Portland Cement Paste Research Articles

Corrosion resistance of polymer-modified hardened cement paste and phosphoric acid-activated metakaolin geopolymer in carbonic acid solution

Corrosion resistance of hardened Portland cement paste (HCP), polymer-modified hardened cement paste (PHCP) and phosphoric acid-activated metakaolin (PMK) geopolymers in carbonic acid solution was comparatively evaluated by multiple techniques of thermogravimetry (TG), energy dispersive spectrometer (EDS), fourier transform infrared spectrometer (FTIR), inductively coupled plasma optical spectrometer (ICP), microindentation and compressive tests. The incorporated polymer kinetically slows down the corrosion process of Portland cement paste as the carbonation depth of PHCP is lower than those of HCP characterized by all techniques. Meanwhile, the addition of polymer slightly hinders leaching of substances from HCP by blocking the pores. Despite the increase of elastic modulus, decreases of compressive strength of both HCP and PHCP samples caused by carbonation are observed probably due to the generation of microcracks and uneven brittleness across the section. PHCP presents less mechanical loss after carbonation treatment than HCP. PMK geopolymer samples soaked in deionized water and carbonic acid solution have comparatively identical results of TG, EDS, FTIR and mechanical measurements, indicating that the PMK geopolymer is highly resistant to carbonic acid, thanks to its inherent acidic nature. In addition, the leaching amount of PMK geopolymer in carbonic acid solution is also much less than the HCP and PHCP.

Acid attack on hydrated cement: effect of organic acids on the degradation process

Owing to their ability to form buffer solutions, the attack of organic acids on concrete structural components can be highly aggressive. This work considers the changes in microstructure, chemical and phase composition in hardened Portland cement paste (hcp) exposed to acetic acid/sodium acetate or citric acid/sodium citrate buffer solutions. The degradation products were investigated using 29Si and 27Al NMR spectroscopy with XRD and ICP-OES. Exposure to acetic acid/sodium acetate at pH 3.9 ≤ pH ≤ 5.5 decalcifies hcp to produce aluminosilica gels (0.1 ≤ Al/Si ≤ 0.3) with Si predominately in Q3/Q4 sites and NBO values (non-bridging oxygen per Si atom) 0.6 ≤ NBO ≤ 0.9. Cross-linking processes causing the formation of the gel from C–A–S–H dreierketten incorporate Al, originally in crystalline phases and C–A–S–H phases. Degradation by citric acid/sodium citrate is governed by the precipitation of expansive calcium citrate which continuously removes degraded surface material. Pore-blocking at the degradation front inhibits acid transport deeper into the material. A new mathematical expression is presented which enables the calculation of NBO for aluminosilica gels of known Al/Si ratio from 29Si NMR spectra despite overlapping signals. The expression was verified by a stochastic computer model based on a Si quartz lattice with substituted Al and vacancies. The model simulated the measured 29Si NMR spectra of aluminosilica gels.

A Study on the Microstructure and Mechanical Properties of Portland Cement Incorporating Aluminosilicate Waste.

The influence of aluminosilicate pozzolanic waste, specifically spent fluid catalytic cracking waste (FCCW) and metakaolin waste (MK) from the expanded glass industry, on the properties of hardened Portland cement paste were analysed. The study involved replacing part of cement with FCCW and MK and observing their impact on the hydration, microstructure, density, and compressive strength of hardened cement paste. Various analysis methods were employed, including X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning electron microscopy (SEM), to understand the changes in the structure of the hardened cement paste during hydration. The findings revealed that FCCW tends to accelerate the cement hydration process due to its high surface area and pozzolanic activity. Notably, the formation of portlandite crystals was observed on FCCW particle surfaces in a specific direction. These crystals appeared smaller and developed in different directions in compositions containing a composite binder with mixture of FCCW and MK in a ratio 1:1. This could be influenced by pozzolanic reactions activated by fine particles of MK and the formation of calcium silicate hydrates (C-S-H) and calcium alumino silicate hydrates (C-A-S-H) in the presence of portlandite. The XRD and TG results indicated that the specimens containing a composite binder exhibited the least amount of portlandite. The compressive strength of these specimens increased compared to the control specimens, although the amount of cement was 9% lower.

Back to basics: Nanomodulating calcium silicate hydrate gels to mitigate CO2 footprint of concrete industry

To realize the sustainable development of concrete, it is vital to mitigate its consumption and environmental footprint (especially CO2 footprint) from prolonging the service life through upgrading mechanical and durable performances of concrete. Incorporating nanofillers can effectively tailor the microstructures and performances of bulk cement paste and cement paste at interfacial transition zone in concrete. The hydrated calcium silicate (C–S–H) gels account for half of the volume of hardened Portland cement pastes, and they are the fundamental source of overall properties of concrete. However, the underlying mechanisms of nanofillers on C–S–H gels remains unclear. Herein, this paper underpinned the role of 5 types of representative nanofillers in tailoring the nanostructure of C–S–H gels in cement composites. The research results demonstrated that through the nano-core effect, nanofillers induce the formation of two new C–S–H gels in outer hydration products, namely nano-core-shell element doped low-density C–S–H (NEDLD C–S–H) and nano-core-shell element doped high-density C–S–H (NEDHD C–S–H). The indentation modulus/hardness of NEDLD and NEDHD C–S–H reaches 25.4/0.80 GPa and 46.7/2.72 GPa, respectively. Such superior performances of NEDLD and NEDHD C–S–H derive from the existence of nano-core-shell elements in C–S–H gels rather than the increase in C–S–H packing density. In a short-range, nanofillers form nano-core-shell elements by adsorbing silica tetrahedrons during the hydration process, improving the mechanical properties of C–S–H basic building blocks. In the long-range, the nano-core-shell elements modify the nano-scale performances of C–S–H gels in outer hydration products due to the increase of C–S–H gels’ integrality.

Accelerated carbonation of hardened cement paste: Quantification of calcium carbonate via ATR infrared spectroscopy

AbstractIn context of carbon capture and storage in cement and concrete industry, there is a strong demand for fast, reliable, and low‐cost CO2 quantification methods. Attenuated total reflection infrared spectroscopy (ATR‐IR) in conjunction with multivariate calibration via partial‐least‐squares regression was applied to quantify CaCO3 in carbonated hardened Portland cement pastes, as this method shows great potential in the field of process control. Thermogravimetric analysis coupled with infrared spectrometry for the detection of the evolving gases was used as a reference for quantification. Three methods for the quantitative analysis with different partial‐least‐squares parameters were developed on a series of ground physical mixtures of slightly carbonated and highly carbonated hydrated cement pastes that had absorbed up to 77% of the theoretical capacity for CO2. Additional samples for optimization and validation of the method were prepared by accelerated carbonation of cylindrical slices of hardened cement paste as a function of exposure time. In these experiments, the major CO2 uptake occurs in the first 60 min until the formation of CaCO3 layers limits the diffusion of CO2 and Ca2+ ions. The developed partial‐least‐squares models provided low estimation errors of max. 1.5 wt% and high correlation coefficients above 99.5%. The validation covers a concentration range of 20–48 wt% of CaCO3. Limitations of the method are discussed.

Elastic constants of nano-scale hydrated cement paste composites using reactive molecular dynamics simulations to homogenization of hardened cement paste mechanical properties

Calcium-silicate-hydrates, portlandite and ettringite are the main hydrated cement paste phases. The study of these phases might lead to a better understanding of the mechanical properties of hardened cement paste. In this work, molecular dynamics simulations were used to simulate different composites of these main phases. The objective were to obtain their elastic constants and to use them as a reference to calculate the homogenization of simplified hardened cement paste using Mori-Tanaka scheme. The results of calcium-silicate-hydrate/calcium-silicate-hydrate, calcium-silicate-hydrate/portlandite and calcium-silicate-hydrate/ettringite composites with three different orientations and configurations were obtained via molecular dynamics simulations using a software called LAMMPS. The homogenization of simplified hardened Portland cement paste gave the value of Young’s modulus in the range from 10.1 GPa to 15.1 GPa. This method might be able to determine Young’s modulus of normal hardened cement paste by the calculations of elastic constants of different composites with other hydrated and unhydrated phases.

Effect of ultrasonication on carboxyl-functionalized multiwalled carbon nanotubes in fresh and hardened Portland cement pastes

abstract: The effects of chemical and physical methods on multiwalled carbon nanotube (MWCNT) dispersion in Portland cement pastes were investigated. Consistency, rheology, compressive strength, dynamic elastic modulus, flexural strength, water absorption, and void content tests were performed to evaluate these effects. Changes in the rheology of the cement pastes and surfactants resulted in a significant reduction in apparent viscosity and yield strength. Additionally, cement pastes containing carboxyl- functionalized MWCNTs (MWCNT-COOH) and surfactant showed higher compressive strengths at 28 d. However, the ultrasonic dispersion method did not significantly influence the properties of hardened Portland cement compared with Portland cement produced using the non-sonicated aqueous solution.

Influence of calcined medium grade clay on the properties of Portland cement pastes

ABSTRACT In the recent decades, the composition of Portland cements has undergone significant changes related to an increase in the range and content of supplementary cementitious materials following development of composite and multicomposite cements. The development of these binders that meet the requirements of sustainability requires the expansion and adaptation of the mineral resource base of cement industry to the production of low-emission binders with a gradual decrease of resource and energy intensities. Efforts to find the widespread sources of pozzolans providing high reactivity and physical–chemical stability have led to intensive studies on thermally treated clays of various chemical–mineralogical composition and their industrial adoption. Due to scarcity and high cost of kaolin clays, great attention is given to medium- and low-grade clays. This article reports on a study to evaluate the effect of calcined clay with 52% of clay minerals on the properties of fresh and hardened pastes, in particular on the resistance to sodium sulfate and nitric acid attacks. The reaction products assemblage of the blended Portland cement pastes was investigated by XRD and thermal analyses. The hardened Portland cement pastes incorporated with thermally treated clay demonstrated better durability performance compared to reference ones.

Monitoring the hydration behavior of hardened cement paste affected by different environmental pH regimes

The purpose of this paper is to investigate the hydration behavior of hardened Portland cement paste cured in different environmental pH values by compressive strength, XRD, TG-DTG and EIS. Meanwhile, a newly proposed equivalent circuit model is built to establish the correlation between the electrochemical parameters and compressive strength of cement paste. The results show that the matrix strength, hydration products and pore structure of hardened cement paste are significantly affected by different pH values. According to the in-situ nondestructive monitoring of EIS, the evaluating for the matrix strength of cement-based materials can be achieved by calculating the resistivity of discontinuous connected pores (Rcp) in the recommended equivalent circuit model.

Scaling of strength in hardened cement pastes - Unveiling the role of microstructural defects and the susceptibility of C-S-H gel to physical/chemical degradation by multiscale modeling

The scaling of strength across hierarchically structured composite materials is accented by the presence of defects with distinct morphology and origin appearing within the microstructure. In hardened Portland cement pastes, these defects manifest as capillary pores, microcracks, and air voids. The transition of material strength from the nano-to-microscale is bridged in the present paper by a validated computational model based on a hierarchical representation of the paste microstructure, resting solely on realistic microstructural features. The model unravels and quantifies the strength scaling mechanisms arising from the various types of defects and critically assesses the susceptibility of C-S-H gel to physical and/or chemical deterioration, quantifying their impact on the gel's structural integrity which subsequently projects as a decrease of the material's load-bearing capacity. The fully-developed model is subsequently used as a quantitative tool to assess the strength degradation of cement paste exposed to a magnesium sulfate attack occurring at low temperature.

Synergistic use of sodium bicarbonate and aluminum sulfate to enhance the hydration and hardening properties of Portland cement paste

A novel composite admixture (N-A admixture) including sodium bicarbonate (NaHCO3) and aluminum sulfate (Al2(SO4)3) was developed for the in situ generations of calcium carbonate (CaCO3) nano-reinforced particles in Portland cement paste. The effect of N-A admixture dosage (0%, 1%, 2%, 3%, 4%, 5%) on the hydration and hardening properties of Portland cement paste such as setting time and compressive strength was systematically investigated. The enhancement mechanism of the N-A admixture was also revealed by using XRD, TGA, SEM, MIP, and ICC. The results indicated that the N-A admixture can effectively accelerate the hydration reaction process of Portland cement paste and shorten its setting time. When the N-A admixture was added at 3%, the compressive strength at 1 day was 86.9% higher than that of the control Portland cement paste, and there was no inversion of strength at 90 days. N-A admixture promoted the formation of more hydration products. At the early stage of hydration, in situ CaCO3 with a grain size ranging from 25.53 to 42.98 nm was generated, the amount of which increased with the addition of N-A admixture. Moreover, the in situ CaCO3 also reacted with the aluminum phase to form mono-carbonate (Mc) in the late stage, which effectively reduced the proportion of large pores, resulting in a dense microstructure of the hardened Portland cement paste. The results of this work provide a novel and facile method for the generation of in situ CaCO3, which can be used to greatly improve the performance of cement concrete.

Mechanical properties and durability performance against fire, gamma ray and bio-fouling of hardened Portland cement pastes incorporating lead bearing wastes

Lead bearing glass waste (LGW) is a harmful waste produced from glass finishing industry. LGW was ground and utilized as alternative substitution for ordinary Portland cement (OPC). OPC-LGW hardened pastes were fabricated by replacement OPC by 0, 5, 10, 20 and 30% LGW then hydrated under tap water for 90 days. The performance of hardened OPC-LGW pastes was studied in various deterioration actions such as: shielding of γ-ray, firing up to 800 °C, attack by sulfate ions and cement bio-fouling behaviors. Microstructure and phase compositions of the developed hydrates were explored using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The obtained consequences showed that, up to 20% replacement of OPC by LGW, there was an improvement in mechanical properties after 28 and 90 days of hydrations. Also, there was a boosted in the thermal stability up to 800 °C as well as an improvement in γ-rays shielding compared to neat OPC. Besides, OPC-LGW hardened pastes showed high resistivity to attack by SO4−2 ions and biofilm formation.

Nanodispersed TiO2 hydrosol modified Portland cement paste: The underlying role of hydration on self-cleaning mechanisms

The self-cleaning performance of photocatalytic cement paste is related to the dispersion of nano-TiO2 in the hardened matrix. This work aims to study the influences of Portland cement hydration on the self-cleaning behaviour of acidic anatase TiO2 hydrosol modified hardened Portland cement paste (HPCP), and the working mechanisms. The presence of TiO2 hydrosol results in the retardation of hydration at early age and better self-cleaning performance of HPCP. The additional surface defects of TiO2 in HPCP are the main reason of self-cleaning performance enhancement. The morphology and the pore size distribution of hydration products also contribute to the enhancement of self-cleaning performance by the surface electron capture effect, which are supported by the analyses of Confocal Raman Microscopy and Scanning Electron Microscope. A new mechanism is suggested to explain the role of photocatalytic property and cement hydration on the enhancement of self-cleaning performance of HPCP with different concentration of TiO2 hydrosol.

Mechanical, microstructural and acid resistance aspects of improved hardened Portland cement pastes incorporating marble dust and fine kaolinite sand

The objective of this research is the implementation of low cost solid wastes like marble dust (MD) and fine kaolinite sand (FKS) aspiring to upgrade mechanical, microstructure and acid resistance aspects of the hardened Portland cement pastes. Diverse blends of (OPC-MD-FKS) were adapted and then were cured under tap water for hydration up to 90 days. Physico-chemical and mechanical aspects of the fabricated specimens were evaluated via determination of compressive strength values (MPa) and water absorption percentages (%) at diverse durations of hydration (1, 3, 7, 28 and 90 days). The deterioration actions of disparate acidic media towards selected hardened mixes were assessed via immersing the 28-days hydrated pastes in 5% sulfuric or 10% acetic acid solutions for 1, 2, 4 and 6 months. Microstructure and phase configuration of the developed hydrates were explored via scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques. The attained consequences acknowledged that partial substitution of OPC by up to 20% MD boosted compressive strength magnitudes and sorptivity of the hardened pastes. In addition, OPC-MD hardened pastes exhibited higher resistance to the deterioration action of sulfuric acid compared to those containing additional FKS. On the other hand, acetic acid ascertained to have severe deterioration action towards all investigated blends after 6 months of immersion due to the progression of soluble salts.

Gamma spectrometry and LabSOCS-calculated efficiency in the radiological characterisation of quadrangular and cubic specimens of hardened portland cement paste

Pursuant to European Directive 2013/59, materials additioned with NORM waste must be characterised radiologically to determine their acceptability for use in construction. The radionuclides studied to judge that acceptability, 232Th, 226Ra and 40K, are normally measured with gamma spectrometry. Gamma detectors are calibrated by using standards as similar as possible in dimensions and chemical composition as the matrix in the samples to be measured. In light of the broad spectrum of chemical and physical characteristics of NORM samples, experimental calibration is often beyond the means of gamma spectrometry laboratories. As a result, Monte Carlo-based methods are deployed to simulate the experimental setup consisting in detector and sample, both geometrically and chemically. Canberra Industries’ LabSOCS (Laboratory Sourceless Calibration Software) is one of the tools available for such calculations. This study verified the accuracy and precision of the counting efficiency delivered by LabSOCS, both with the standard powder geometry and a new geometry consisting in a 5 cm cubic specimen of Portland cement paste. The findings showed that in both geometries the accuracy and precision of LabSOCS-calculated efficiency (by specimen height and activity), across an energy range from 45.64 keV (210Pb) to 1460.82 keV (40K) met the acceptability criteria routinely applied in environmental radioactivity laboratories. The geometry proposed yielded activity values for the end construction material closer to the true indices than the conventional method consisting in summing the partial activities of the unreacted components.

Structure and nanomechanical characterization of synthetic calcium-silicate-hydrate with poly-methacrylic acid

ABSTRACT: The principal phase of hardened Portland cement pastes is calcium silicate hydrate (C-S-H), which influences the physical and mechanical properties of construction materials. In this work, calcium silicate hydrate (C-S-H) was synthesized, with the addition of poly-methacrylic acid with sodium (PMA), for the development of C-S-H/ polymer nanocomposites. Among the polymers available, PMA is indicated in the literature as one of those viable for producing C-S-H/polymer complexes. However, no consensus exists regarding the type of interaction this produces. The resulting compounds were characterized by XRD, FT-IR, TGA, carbon content (CHN), TEM, SEM and elastic modulus and hardness were measured by instrumented indentation. A significant change was verified in the nanomechanical properties of C-S-H with PMA, resulting in reduction in the elastic modulus and hardness. The set of results presented do not confirm the intercalation of PMA in the interlayer space of C-S-H, but presented evidence of the potential for intercalation, since changes in the microstructure clearly occurred.

Effect of AlF3 production waste on the processes of hydration and hardening of the alkali-activated Portland cement with sodium silicate hydrate

Generally, the early strength of hardened Portland cement paste is controlled, by changing the mineral composition of clinker and grinding fineness. By using clinker (calcium silicates) and liquid glass (sodium silicate) together, these materials have high degree of coagulation and rapid hardening. For this reason, it is important to regulate the setting times. One solution of this problem is the use of chemical admixtures. In this research, the admixture made from liquid glass and AlF3 production waste was suggested (the waste from phosphates fertilizers production). The cement system “Portland cement clinker–sodium silicate hydrate–AlF3 production waste” was characterized by X-ray diffraction, differential scanning calorimetry, measurements of hydration temperature and the compressive strength of hardened cement paste specimens. The microstructure of hardened cement pastes was evaluated by scanning electron microscopy. Reference specimens were made from pure Portland cement clinker. A possibility to regulate hydration rates both at the early stage of hardening and at the later ones by addition of this admixture was demonstrated. The effect of AlF3 production waste was described. By using this admixture, the hydration temperature increased, the hydration process delayed, and compressive strength of hardened Portland cement clinker paste samples increased, especially at later ages. This gave the opportunity for composition: clinker–liquid glass–AlF3 production waste to create slowly curing cement; which is suitable for long transportation and for massive constructions.

Sulfate resistance of RFCC spent catalyst-blended Portland cement

The reuse of spent catalysts from residue fluid catalytic cracking (RFCC) units as pozzolanic materials in cement and concrete production offers a number of important benefits. In spite of all these benefits, the durability performance of the produced blended cement is an important issue to be considered. This study investigates the effects of RFCC spent catalyst on durability performance of hardened Portland cement paste in a highly aggressive sulfate environment. The 28-day cured paste specimens prepared from binary cement mixtures incorporating different replacement levels of 0, 10, 20, and 30% (by mass) RFCC spent catalyst at a constant water-to-cement ratio of 0.30 were exposed to 10 mass% solution of magnesium sulfate. The accelerated sulfate attack under alternative cycles of wetting and drying was studied by monitoring the changes in compressive strength, length, and mass of specimens and also by the application of XRD, SEM and EDX techniques. Based on the results and a comparison with plain Portland cement, binary cement mixtures exhibit a higher rate of deterioration in spite of their significantly improved compressive strengths resulted from pozzolanic reaction.

Ceramic waste as an efficient material for enhancing the fire resistance and mechanical properties of hardened Portland cement pastes

Utilization of industrial byproducts like ceramic waste (CW) for enhancing the microstructure and mechanical properties as well as the fire resistance of hardened ordinary Portland cement (OPC) pastes is the main goal of this research. Different cement blends were prepared by partial replacement of OPC with 5, 10 and 20CW (mass%). Besides, the effect of addition of 0.05 and 0.1 (mass%) of carbon nano-tubes (CNTs) on the mechanical properties and thermal resistance of these composites was investigated. The compressive strength values of different composites were determined after 1, 3, 7, 28 and 90days of hydration. The thermal resistance test of different hardened composites was evaluated. For thermal resistance test, the specimens hydrated for 28days under tap water were fired at 300, 600 and 800°C for 3h. After that, cooling of the fired specimens was done via gradual and rapid cooling methods. The compressive strength test was performed for all fired specimens at each firing temperature. The compressive strength results revealed that, from the economic point of view the optimum replacement of OPC with ceramic waste is 10%, while the optimum addition of CNTs is 0.05 by mass% of the composite. Phase composition and microstructure of some selected samples were studied using XRD, TG/DTG and SEM analyses, the results indicated that, the main hydration products formed are nearly amorphous calcium silicate hydrates, calcium sulphoaluminate hydrate and calcium hydroxide.

WITHDRAWN: Biofouling characteristics of algae on hardened Portland cement pastes admixed with Bi(OH)3, TiO2 and Al2O2 in fresh and marine environments

WITHDRAWN: Biofouling characteristics of algae on hardened Portland cement pastes admixed with Bi(OH)3, TiO2 and Al2O2 in fresh and marine environments