Hydration Products Of Cement Paste Research Articles

Residual stress on ettringite crystals in mature Type K shrinkage-compensating cement pastes

This study is mainly focused on investigation of the relative content, morphology, chemical composition, microstructural parameters, and the deformation state (mechanical strains and stresses) of ettringite [Ca6Al2(SO4)3(OH)12·26H2O] crystals in 1-year-old Type K shrinkage-compensating cement pastes depending on variation of the relative content of ye’elimite admixture (YA) up to 12 wt%. The XRD, SEM-EDS, and optical microscopy techniques were used as characterization methods. Amongst the identified main hydration products in cement pastes: ettringite, calcium-silicate-hydrate (C-S-H), and calcium hydroxide (CH), with increase of the content of YA from 3 wt% to 12 wt% the relative content of the C-S-H and CH phases strongly decreases, whereas the content of ettringite considerably increases. In YA-containing cement pastes, it is revealed formation of near-spherical “ye’elimite–ettringite” clusters (a cluster is composed of a partially hydrated ye’elimite particle/core covered by small-sized, ≈1 μm, radially oriented ettringite crystallites) that exhibit toroid-like agglomerations. For reference sample (prepared from Portland cement with no YA), the average stress components σa and σc on etteringite crystals (along the a and c crystallographic axes, respectively) are determined to be −6.6 and −40 MPa, respectively. With increase of the content of YA in cement paste the stress components on etteringite crystals strongly increase reaching maximum values σa=-163.9 MPa and σc=-76.9 MPa at 12 wt% of the YA. According to a discussed model, the activation of the shrinkage-compensating effect in YA-containing cement pastes is attributed to formation of the “ye’elimite–ettringite” clusters with a sufficiently high concentration. It is predicted that, amongst the examined samples, the shrinkage-compensating effect is achieved in cement pastes with the YA contents of 9 and 12 wt%.

Effect of electrical field on the stability of hydration products of cement paste in different liquid media

Electrical fields accelerate the sulfate attack, which induces detrimental effects on the durability of cement-based materials. However, the role of the electrical field and aggressive liquid media in the degradation process is still unclear. Investigation of the role of the electrical field and aggressive liquid media during the degradation allows a better understanding of the mechanism of accelerated technique. Four different liquid media were used in this work to study the effect of the electrical field and liquid media in terms of the stability of hydration products. The evolution of hydration products was experimentally identified using multiple techniques, including X-ray diffraction (XRD), thermogravimetric analysis (TG), transmission electron microscopy (TEM), nitrogen adsorption/desorption (NAD), pH and CaO content measurements. The results indicated that the CSH structure was stable under the electrical field and saturated Ca(OH)2 solution. The calcium loss under the electrical field was more likely to leach out from portlandite even though the mixed Ca(OH)2-Na2SO4 solution was present. A severe degradation occurred under the electrical field and low-pH liquid media (pure water, Na2SO4-MgSO4 solutions). The Ca/Si ratio significantly decreased from 1.34 to 0.82 under the coupling effect of the electrical field and Na2SO4-MgSO4 solutions. Based on the fractal dimension results derived from NAD and TEM, the variations of pore structure and CSH gel morphology were quantitatively characterized. The coupled electrical field and low-pH liquid media dramatically altered the microstructure of hydration products, resulting in coarser pore networks and a shift to the disordered CSH gel.

Elucidating the mechanical property-enhancement mechanism of ferrite in oil-well cement using spherical ferrite

High-ferrite oil-cement (HFOC) has attracted extensive research interest owing to its excellent mechanical properties and corrosion resistance. However, it is difficult to distinguish ferrite and its hydration products in cement paste. Therefore, establishing the mechanism of improvement of the mechanical properties of Portland cement upon the addition of ferrite is challenging. In this study, spherical ferrite (SF) was prepared via jet granulation to explore the mechanism of hydration and improvement of mechanical properties of oil-well cement pastes upon the addition of ferrites. XRD results showed that the crystal structure of the SF is consistent with that of ferrite. Phase analysis of the hydration products revealed that the hydration products in the AFm phase gradually covered the SF particles entirely during the early hydration reaction and transformed into cubic crystals C3(A, F)H6 during the subsequent hydration reaction. The assessment of mechanical properties demonstrated that the compressive and tensile strengths of the oil-well cement paste increased by 11.7 % and 17.5 %, respectively, upon the addition of 4 % SF. Moreover, the addition of SF reduced the elastic modulus of the cement paste. The mechanical properties enhancement mechanism of SF in oil well cement involves participating in the hydration process of oil-well cement and consuming fracture energy. This study provides theoretical support for the development mechanism of mechanical properties of oil-well cement.

Effects of wet grinding combined with chemical activation on the activity of iron tailings powder

Utilizing iron tailings powder (ITP) as supplementary cementitious materials (SCMs) has significant economic and environmental benefits, while the low pozzolanic activity of ITP limits its incorporation to some extent. This paper aims to study the effect of compound activation (wet grinding with a chemical activator) on the activity of ITP. Two kinds of sulfate (CaSO4·2H2O and Na2SO4) were used as chemical activators, and the optimal dosages of CaSO4·2H2O and Na2SO4 were determined by the mechanical strength tests of cement-ITP pastes. Results demonstrate that adding 3 % CaSO4·2H2O (wet grinding with ITP) caused a slight increase in the early and later strength of cement pastes containing 30 % ITP, while adding 3 % Na2SO4 (wet grinding with ITP) resulted in a significant improvement in the early strength and a slight increase in the later strength. Compared to a single wet grinding activation, compound activation could produce more amorphous substances in ITP and cause a more prominent decrease in the surface binding energy of internal chemical bonds of ITP, thereby further improving the activity of ITP. Moreover, the hydration products of cement pastes containing 30 % compound activated ITP were compared with those of cement pastes containing 30 % wet ground ITP, and the activation mechanism of two sulfates on ITP was studied.

Belite-calcium sulphoaluminate cement prepared by EMR and BS: Hydration characteristics and microstructure evolution behavior

Belite-calcium sulphoaluminate (B-CSA) cement is regarded as one of the most promising alternatives to ordinary Portland cement (OPC). The production of B-CSA cement from industrial solid waste is a critical technology for maximising mineral resource utilisation. The morphology and microstructure of hydration products produced during the hydration process of B-CSA cement have a significant impact on the mechanical properties of the cement. In this paper, B-CSA cement prepared by electrolytic manganese residue (EMR) and barium slag (BS) was taken as the research object, and the changes of PH, turbidity and electrical conductivity in the early hydration process were monitored. Furthermore, the phase composition and microstructure of hydration products of cement paste were analysed using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TG), solid state MAS-NMR spectroscopy, nitrogen adsorption test, scanning electron microscope (SEM) and electron microprobe analyzer (EPMA). The results show that with the sharp change of pH value, turbidity and electrical conductivity in the early hydration process of B-CSA cement, a large amount of ettringite is formed during the rapid hydration of ye’elimite, the metastable AFt part is transformed into AFm and the hydration reaction of belite was inhibited. The hydration products are mainly ettringite, monosulfate and AH3, which are responsible for the strength development of B-CSA cement. The hydration rate of belite in the early stage is slow, and the C-S-H gel phase can be formed in an aluminum-rich environment. The microstructure evolution of cement paste was revealed by nitrogen adsorption tests and SEM. The crystal state of ettringite changes from columnar to fibrous to construct the hardened framework of disorderly distributed cement paste, and the gel phase fills in the void formed by the ettringite skeleton to form a dense structure and reduce the porosity. The interface between the unreacted belite phase and the AFt skeleton is clear, and the gel phase plays a role in connecting them. Mn2+ was enriched on the C-S-H gel phase to form the Ca-Si-Mn compound and solidified. Ba2+ preferentially forms BaSO4 with SO42− which precipitates at the interface between the ettringite phase and C-S-H gel phase. Based on the analysis of the test results, the hydration characteristics and microstructure evolution of B-CSA cement were determined.

The research on mechanism of C-S-H nanocrystal improving early properties of shotcrete at low temperature by thermodynamic modelling

In this study, the effect of CSH nanocrystals on the performance of shotcrete are investigated in a low-temperature environment. Setting time and early compressive strength were evaluated for mixtures with different dosages of CSH nanocrystal at 5 ℃, 10 ℃, and 20 ℃. The results show that CSH nanocrystal can shorten the setting time of cement paste mixed with the accelerator, improve the 1 d compressive strength of shotcrete. Meanwhile, the typical samples hydration products and microstructure were determined using XRD, TG/DTG, and SEM, indicate that combined CSH nanocrystal and Alkali free liquid accelerator (AS) can accelerate the cement hydration reaction and promote hydration products AFt, CH, and C-S-H. Moreover, the thermodynamic model (GEMS) simulates the composition change of hydration products of cement paste mixed with CSH nanocrystal and AS at 5 ℃, 10 ℃, and 20 ℃; Confirmed that CSH nanocrystal significantly influences the early hydration products CH, AFt, C-S-H at different curing temperatures, which is similar to the experimental results. Therefore, based on the experimental and simulation results, the synergistic effect of CSH nanocrystal and AS was proposed to solve the problems of a weak early performance of shotcrete and provide a reference for the construction of low-temperature shotcrete.

Activation of Blast Furnace Slag with CFB Fly Ash as a Supplementary Binder Material: Hydration Products and Effects of Sulfate Attack

Circulating Fluidized Bed (CFB) combustion is a clean technology for burning, with advantages of adapting to a large variety of fuel, high combustion efficiency, lower NOx emissions, and stable operation. The residue collected from the ash-hoppers of the electrostatic precipitator of the CFB boiler is called CFB fly ash. This paper presents the hydration development on the application of CFB fly ash to activating blast furnace slag (BFS) as a supplementary binder material (SBM) for replacement of Portland cement in making concrete. Investigation of the hydration products of cement pastes prepared with combinations of BFS and CFB fly ash were conducted by means of X-ray diffraction, thermal gravimetric analysis, and scanning electronic microscope. Test results show that the main hydration products of the CFB fly ash-BFS blended pastes were found to be hydrated calcium silicate (C-S-H), ettringite, gypsum, and some portlandite. Considering that CFB fly ash produced from the combustion of high-sulfur coke has high SO3 contents, the volume stability of mortar made from CFB fly ash-activated BFS was subjected to tests in accordance with ASTM C1012 and ASTM C1038 for evaluating the internal and external sulfate attack, respectively. The results indicate that, due to the high sulfur (SO3) content of CFB fly ash, the expansion caused by internal sulfate attack (ISA) increased with increasing proportion of CFB fly ash in the mixture. In contrast, no significant expansion was observed in the external sulfate attack (ESA) test, regardless of the proportion of CFB fly ash in the mixture. In order for the CFB fly ash to serve as a supplementary binder material and to maintain adequate volume stability, the amount of CFB fly ash used for the activation of BFS is recommended to be no more than 20% of the SBM.

Effects of the Curing Conditions on the Carbonation Curing Efficiency of Ordinary Portland Cement and a Belite-Rich Cement Mortar

In the present study, the efficiency of five different carbonation and/or water curing conditions on the properties of belite-rich cement mortar and ordinary Portland cement mortar was investigated. The hybrid curing of samples was carried out by submerging samples at different levels in water or in a lime-saturated solution kept under carbonation curing conditions. The compressive strength was measured to compare the physical properties of the cement mortars, and X-ray diffraction and thermogravimetric analysis results were analyzed to compare the physicochemical properties. The results revealed that the supply of additional moisture during carbonation curing tends to decrease carbonation curing efficiency and that the hydration products of cement paste are predominantly affected by the depth at which the specimen was immersed in the liquid rather than the type of liquid used.

The effect of sulfur- and carbon-codoped TiO2 nanocomposite on the photocatalytic and mechanical properties of cement mortars

This study has established the impact of a nano-ТіО 2 P25 modifier and a nanocomposite based on titanium dioxide, doped with sulfur and carbon dioxide (ТіО 2 /S,C), on the photocatalytic, mechanical properties and the structural formation of cement mortars. The paper reports the results of the particle size distribution of the Portland composite cement and the ТіО 2 nano additives; a comprehensive assessment of the particle size distribution has been performed both in terms of volume and specific surface. It has been proven that the ТіО 2 /S,C nanocomposite is characterized by the extremely high surface activity, which determines the photocatalytic properties of the surface of cement mortars. The comparison of the mechanical properties of cement mortars modified by titanium dioxide nano additives has been carried out. An experimental study has confirmed the improved photocatalytic properties of the cement mortar surface in the visible spectrum through the doping of the nano-sized titanium dioxide with carbon and sulfur. A combination of the ТіО 2 nano additives and the superplasticizers of polycarboxylate type leads to the increased strength of the modified samples in proportion to a hardening age. Given the high surface activity of the ТіО 2 /S,C nanocomposite's particles, the cement paste hydration products deposit at their surface, thereby forming such conglomerates with them that seal the microstructure of the cement matrix. It has been shown that using a nanocomposite based on the modified titanium dioxide decreases the indicators of free energy while the surface of the cement mortar acquires hydrophobic properties, which contributes to the processes of self-cleaning. Thus, there is a reason to argue about the feasibility of using the ТіО 2 /S,C nanocomposite to improve the photocatalytic, self-cleaning, mechanical, and hydrophobic properties of cement mortars

Small scale tests of cement with focused ion beam and nanoindentation

Nanoindentation is used for characterization of small scale material properties of hydrated cement. It is employed as a precise loading tool on samples fabricated with Focused Ion Beam milling (FIB). The effect of heat on the microstructure of cement during different FIB energy loads is studied. Milling currents as low as 0.1 nA can be considered as save and not damaging. Micrometer sized beams were bent to reveal strength and fracture characteristics. Small scale elastic properties, tensile strength and fracture energy of individual low scale microstructural constituents of cement paste like C-S-H rich phases and Portlandite were assessed. Very high tensile strengths at the micrometer scale were observed for cement paste hydration products (200-700 MPa) with fracture energies 4-20 J/m2 The results are consistent with atomistic simulations and multi-scale modeling from available literature.

Effect of cementitious capillary crystalline waterproofing coating on the gas permeability of mortar

AbstractThis study presents an experimental investigation in the effect of cementitious capillary crystalline waterproofing (CCCW) coating on the gas permeability of mortar and the mechanism how the gas permeability is affected. The gas permeability coefficient of mortar coated with or without CCCW material was measured through modified Cembureau method. The pore structure, micromorphology and hydration products of cement paste with or without CCCW coating were analyzed by the means of mercury intrusion porosimetry (MIP), scanning electron microscopy‐energy disperse spectroscopy (SEM‐EDS), X‐ray diffraction (XRD) and thermogravimetry‐differential scanning calorimetry (TG‐DSC). The results showed that under the effect of CCCW coating, the gas permeability of mortar got decreased and the pore structure changed towards smaller pore sizes. SEM‐EDS results exhibited that there was a large amount of ettringite filling in the pores in the paste with CCCW coating. XRD and TG‐DSC analysis indicated that there was more calcium carbonate formed in the paste with CCCW coating than blank paste. The formation of ettringite in pores and increased calcium carbonate production refines the pore size distribution and densifies the cement matrix, which induces the decrease in gas permeability of mortar coated with CCCW material.

Effects of Polycarboxylate Superplasticizers with Different Functional Units on Early Hydration Behavior of Cement Paste

AbstractEffects of polycarboxylate superplasticizers (PCEs) with different functional units on the flowability, hydration heat, and hydration products of cement paste were investigated. Two kinds o...

Microscale Tests of Cement Paste Performed with FIB and Nanoindentation

This study deals with experimental determination of tensile properties of cement paste hydration products at micro-scale. Cantilever micro-beams with length of about 16 µm and pentagon cross section with micrometer dimensions were fabricated by focused ion beam milling on hydrated cement paste samples. Nanoindentation was used for evaluating elastic properties while tensile properties were derived from beam bending tests. Displacement controlled micro-scale tests give access to both tensile strength and estimates of fracture energy based on the load-displacement curves measured with the nanoindenter. The mean tensile strength and the fracture energy of inner hydration product were assessed as 791 MPa and 16.7 J/m2, respectively. The huge difference of the micro-scale properties when compared to macroscopic values comes from the scaling properties of concrete.

Influence of elevated curing temperature on the properties of cement paste and concrete at the same hydration degree

Three different curing temperatures (20 °C, 40 °C, and 60 °C) were set, so that the nonevaporable water (wn) contents of plain cement pastes cured at these three temperatures were measured to determine the hydration degree of cement. Tests were carried out to compare the pore structure and strength of cement paste, as well as the strength and permeability of concrete under different temperature curing conditions when their cements were cured to the same hydration degree. The experimental results show that either at a relatively low hydration degree (wn=15%) or high hydration degree (wn=16.5%), elevated curing temperature has little influence on the hydration products of cement paste, while it has a negative influence on the pore structure and compressive strength of cement paste. However, this negative effect is weaker at high hydration degree. The large capillary pore (>100 nm) volumes of cement pastes remain almost the same at high hydration degree, regardless of curing temperatures. As for the concrete, elevated curing temperature also has negative influence on its compressive strength development, at both low hydration degree and high hydration degree. And this negative effect is stronger than that on cement paste’s compressive strength at the same hydration degree. On the whole, elevated curing temperature has little influence on the resistance of concrete to chloride ion penetration.

A multi-scale approach of Mode I Crack in ettringite

Ettringite, as one of the hydration products of cement paste, has attracted the attention of pavement engineers recently. In this paper, a concise and comprehensive multi-scale modelling approach is presented to study the Mode I fracture based on the continuum level tool Phase-field Theory (PFT) and the atomistic level tool Molecular Dynamics (MD) Simulation. The PFT is a new modelling and computational tool used to predict the crack propagation using a phase-field variable, which assumes to be positive one in the intact solid and negative one in the crack region. Realising the fact that at the crack tip, the PFT is unable to deal with the singularity of stress and strain, and thus we use the MD Simulation to further investigate mechanical performances at a finer level. By the energy coupling the two different approaches at the crack tip, we present the two-way multi-scale coupling simulation of ettringite to simulate Mode I cracking fracture. The comparison between our multi-scale results with results from the traditional fracture mechanics proves that our multi-scale model is reasonable.

Effect of protogenetic anhydrite on the hydration of cement under different curing temperature

Compared with the added gypsum, protogenetic anhydrite is the anhydrite in clinker and cement admixture. And using protogenetic anhydrite as the setting retarder instead of added gypsum to make cement could be an effective way to make full use of clinker and wastes with high sulphur content. However, the high temperature sensitivity of solubility property of protogenetic anhydrite may result in unhydrated anhydrite exist in cement paste under high curing temperature at early age, and that may make a bad influence on the properties of cement. This work investigated the effects of protogenetic anhydrite on the long-term strength, volume stability and hydration products of cement paste under different curing temperature. It is shown that higher curing temperature caused the greater value of optimum content of protogenetic anhydrite in cement, and protogenetic anhydrite could not make the bigger volume expansion and worse strength of cement than gypsum when the high curing temperature being used. But curing temperature caused the dissolution of protogenetic anhydrite change and influenced the concentration of sulfate ion in the pore solution of hardened cement paste. The results of XRD patterns showed that the risk of delayed ettringite formation in hardened cement paste cured at high temperature had been increased. The results will be helpful to understand the role of protogenetic anhydrite in cement, and further promote the use of clinker and wastes with high sulfate.

A numerical method for predicting Young’s modulus of heated cement paste

The evaluation of the mechanical properties of heated cement paste is essential to the safety assessment of concrete structures exposed to elevated temperatures. A numerical method is developed in this paper for predicting the Young’s modulus of heated cement paste with or without silica fume up to 600°C. The initial volume fractions of various constituents and the thermal decomposition of hydration products in cement paste are approximately estimated. A two-phase composite sphere model is then built and a two-step approach is applied to evaluating the Young’s modulus of heated cement paste with thermal decomposition and microcracking effects. Finally, the validity of the proposed numerical method is verified with three sets of experimental data collected from the literature.

エトリンガイトの生成量および生成起源が長期間硫酸塩に浸漬した高炉セメント系材料の耐硫酸塩性に及ぼす影響

Sulfate attack is known as one of the deterioration phenomena of concrete structures. Sulfates react chemically with hydration products of cement pastes and lead expansion or weakening of cement matrix. Deterioration cases of this phenomenon have been reported in a variety of environments all over the world. However, most of studies about sulfate resistance of hardened cementitious materials were mainly considered at 23℃ in Na₂SO₄ solution immersion. Thus, this study was conducted to investigate the effect of sulfate species (Na₂SO₄ and MgSO₄) and high temperature (40℃) on the mechanisms of sulfate attack of cementitious materials containing Ordinary Portland Cement (OPC) with Blast Furnace Slag (BFS), Anhydrite (AH) and Limestone Powder (LSP), and to evaluate the expansion characteristics of specimens immersed sulfate solutions considering “Amount of ettringite”, “Ettringite generation rate” and “Ettringite generation origin”. From the results of sulfate immersion test, the expansion was caused due to Na₂SO₄, however not only expansion but also mass decrease were caused due to MgSO₄. This was considered that hydration products in each sulfate solution were different because of pH differences of sulfate solutions were occurred by the difference of sulfate species. And the expansion characteristics of the specimens were not evaluated by only simple amount of ettringite, however it became able to evaluate considering the ettringite generation rate and ettringite generation origin. In addition, the paste specimen of 70% BFS-cement with 10% AH and 10% LSP (BFS 59.5%-OPC 25.5%-AH 10%-LSP 10%) was proposed a material design against sulfate attack. 本研究は、普通ポルトランドセメントに高炉スラグ微粉末-無水せっこう-石灰石微粉末を用いた高炉セメント系材料について、硫酸塩種および温度条件が硫酸塩劣化機構に及ぼす影響を明らかにすること、さらにエトリンガイトの生成量、生成速度および生成起源に着目して検討することにより、膨張性状の評価を行うことを目的とした。その結果、硫酸塩種によって溶液のpHに差が生じ、水和生成物が変化することで劣化形態が異なるものと考察した。また、単純なエトリンガイト生成量だけでは膨張性状を評価できなかったが、単位週当りのエトリンガイト生成量や、特に生成起源別エトリンガイト量を考慮することによって、膨張性状評価が可能となることが示唆された。

Molecular Dynamic Simulation of Failure of Ettringite

Ettringite is an important component in the hydration products of cement paste. To better understand the failure modes under tensile loading of cement-based materials, mechanical properties of each individual hydration product needs to be evaluated at atomic scale. This paper presents a molecular dynamic (MD) method to characterize and understand the mechanical properties of ettringite and its failure modes. The molecular structure of ettringite is established using ReaxFF force field package in LAMMPS. To characterize the atomic failure modes of cement paste, MD simulations were conducted by applying tensile strain load and shear strain load, respectively. In each MD failure simulation, the stress-strain relationship was plotted to quantify the mechanical properties at atomic scale. Then elastic constants of the ettringite crystal structure were calculated from these stress-strain relationships. MD simulations were validated by comparing the mechanical properties calculated from LAMMPS and those acquired from experiments. Future research should be performed on bridging-relationships of mechanical properties between atomic scale and macroscale to provide insights into further understanding the influence of mechanical properties at atomic scale on the performance of cement-based materials at macroscale.

Study of the pozzolanic activity and hydration products of cement pastes with addition of natural zeolites

AbstractThis paper presents results from comparative thermogravimetric, calorimetric and pozzolanic activity analyses of five natural zeolite samples from Bulgaria, Slovakia, Philippines, USA and North Korea. The zeolites actively participate in the hydration processes of cement. Their activity in the early stage of hydration is based mainly on the large surface area of the particles while, in the later stages of activation, chemical reactions occur between the products of the hydration of cement and the soluble SiO2 that is present in the bulk of the zeolites. It has been shown that in all cement pastes which contain zeolite additives, the quantity of portlandite is lower than that in pure cement paste or is even totally absent. The amounts of hydration products are greater when 30% zeolite is used than when 10% zeolite is added (excluding the sample with chabazite). The lowest pozzolanic activity is shown by chabazite, which possessed the lowest SiO2/Al2O2 ratio.