Hydrated Portland Cement Paste Research Articles

Recycling of hydrated Portland cement paste into new clinker

The recycling of concrete is essential because it reduces the environmental impact of concrete construction and demolition waste. For recycling, concrete waste is crushed and the coarse fraction of crushed material can be recycled as a coarse aggregate. Concrete waste fines, which are a mix of fine aggregates, coarse aggregate debris, and the hydrated cement paste, are currently not a part of the recycling process. Since hydrated cement paste has all the necessary chemical elements for clinker production but without heavy carbon dioxide emissions associated with traditional clinker raw material, it would be beneficial to recycle concrete fines for the production of clinker. However, the data available in the literature about the transformations of hydrated cement paste upon heating are contradictory. It is not clear whether hydrated cement paste can be converted back to clinker. In this research, the phase transformations in hydrated cement paste upon heating in the temperature range from 600 to 1450 °C were investigated using thermal analysis and X-ray diffractometry. The results show that hydrated Portland cement paste can be completely recovered into a new clinker using a thermal treatment. The main phase transformations during heating are described and compared to the literature data. The results indicate a great potential for the complete recycling of concrete fines for the production of cement with a low carbon footprint.

Elucidating the Effect of Accelerated Carbonation on Porosity and Mechanical Properties of Hydrated Portland Cement Paste Using X-Ray Tomography and Advanced Micromechanical Testing.

Carbonation of hydrated cement paste (HCP) causes numerous chemo–mechanical changes in the microstructure, e.g., porosity, strength, elastic modulus, and permeability, which have a significant influence on the durability of concrete structures. Due to its complexity, much is still not understood about the process of carbonation of HCP. The current study aims to reveal the changes in porosity and micromechanical properties caused by carbonation using micro-beam specimens with a cross-section of 500 μm × 500 μm. X-ray computed tomography and micro-beam bending tests were performed on both noncarbonated and carbonated HCP micro-beams for porosity characterization and micromechanical property measurements, respectively. The experimental results show that the carbonation decreases the total porosity and increases micromechanical properties of the HCP micro-beams under the accelerated carbonation. The correlation study revealed that both the flexural strength and elastic modulus increase linearly with decreasing porosity.

Influence of Polyepoxysuccinic Acid on Solid Phase Products in Portland Cement Pastes

The influence of polyepoxysuccinic acid(PESA) on the solid phase products in hydrated Portland cement pastes was investigated by isothermal calorimetry, X-ray diffraction(XRD), 29Si and 27Al nuclear magnetic resonance(NMR). The results indicated that PESA bonds Ca2+ ions in pore solution to prevent portlandite formation, and also combines with Ca2+ ions on the surface of silicate minerals to prolong the control time of phase boundary reaction process, leading to the retardation of silicate mineral hydration. Meanwhile, the interlayer Ca2+ ions in Jennite-like structure bridging PESA and C-S-H gels prevent silicate tetrahedron and aluminum tetrahedron from occupying the sites of bridging silicate tetrahedron, which causes the main existence of dimer in C-S-H structure, deceases the degree of Al3+ substituting for Si4+ and promotes the transformation from 4-coordination aluminum to 6-coordination aluminum. Furthermore, the -Ca+ chelating group from reacting PESA with Ca2+ ions combines easily with SO42- ions, resulting in transformation from ettringite, AFm to TAH(Third aluminum hydrate). However, with the higher addition of PESA, it will bridge the excess PESA by Ca2+ ions to form a new chelate with ladder-shaped double chains structure, which not only reduces the amount of PESA bonding Ca2+ ions, but also decreases its solidifying capability for SO42- ions, leading to the transformation from TAH to AFm or ettringite. Meanwhile, at later hydration, the inhibition effect of PESA on cement hydration is weakened, and the transformation degree from TAH to AFm is higher than that to AFt with the addition of PESA.

Are calcium silicate hydrates (C-S-H) present in alkali-activated glass cullet cement?

This contribution analyzes the structure of sodium-lime-silica (SLS) glass alkali-activated with NaOH. The mix forms an alkali-activated cement (AAC), which has proven to be mechanically resistant when immerged under water. The aim is to determine whether the presence of calcium in the glass powder may have contributed, together with alkali-activation, to forming calcium silicate hydrates (C-S-H), which are responsible for water resistance in hydrated Portland cement pastes. This is investigated by SEM, where needles similar to C-S-H in Portland paste are observed, then by X Ray Diffraction (XRD), but the pastes are too amorphous to allow any phase identification. Finally, 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) shows that C-S-H are at most 2–3% in the 4 M and 8 M NaOH AAC pastes, so that they cannot be considered responsible for the water resistance of SLS glass AAC pastes and mortars. The latter are rather formed of sodium silicate hydrates and/or silica gel.

Fitting Function for Flexural Strength of Cement Paste

There are many analytical expressions for compressive strength as functions of porosity of cement-based materials but only a few such expressions exist for flexural strength of these materials. In the present paper a new functional candidate for fitting the data of flexural strength of hydrated Portland cement paste has been tested. The functional candidate has been initially derived for porous polymeric materials on the basis of the percolation theory. The parameters of this function have been optimized for the cement paste by using the Levenberg-Marquardt iterative fitting procedure. The optimized function has been capable of accurate reproducing all the measured flexural data. This fact has been confirmed by a high value of the correlation coefficient and rather low values of statistical uncertainties. It has been shown that this modified fitting function is well applicable to the pastes of ordinary Portland cements and probably to other cementitious materials, too.

Quantification of synthesized hydration products using synchrotron microtomography and spectral analysis

The use of x-ray computed tomography (CT) as a standalone method has primarily been used to characterize pore structure, cracking and mechanical damage in cementitious systems due to low contrast in the hydrated phases. These limitations have resulted in the inability to extract quantifiable information on such phases. The goal of this research was to address the limitations caused by low contrast and improving the ability to distinguish the four primary hydrated phases in portland cement; C-S-H, calcium hydroxide, monosulfate, and ettringite. X-ray CT on individual layers, binary mixtures of phases, and quaternary mixtures of phases to represent a hydrated portland cement paste were imaged with synchrotron radiation. Known masses of each phase were converted to a volume and compared to the segmented image volumes. It was observed that adequate contrast in binary mixing of phases allowed for segmentation, and subsequent image analysis indicated quantifiable volumes could be extracted from the tomographic volume. However, low contrast was observed when C-S-H and monosulfate were paired together leading to difficulties segmenting in an unbiased manner. Quantification of phases in quaternary mixtures included larger errors than binary mixes due to histogram overlaps of monosulfate, C-S-H, and calcium hydroxide.

Immobilization of Cr(VI) by hydrated Portland cement pastes with and without calcium sulfate

This work aims to illustrate the impact of high concentrations of Cr(VI) (based on Na2CrO4) on the hydration assembly and microstructural development of hydrated Portland cement, and the results also present the role of calcium sulfate on the immobilization of Cr(VI) in Portland cement. The results showed that the immobilization of Cr(VI) in hydrated Portland cement was attributed to the formation of CrO4-U phase, an analogue of SO4-U phase (3CaO·Al2O3·CaSO4·0.5Na2SO4·15H2O). The growth of CrO4-U phase on the surface of clinker particles formed a diffusion barrier and hence increased the setting time. Increasing the calcium sulfate dosage impaired the Cr(VI) immobilization due to the competition between CrO42− and SO42− integrated into the U phase. The generalized acid neutralization capacity (GANC) test indicated that the Cr(VI) leaching behavior was a function of the leachate pH value. As the pH decreased to 11.8, the CrO4-U phase was converted quickly to CrO4-ettringite, which generated a slight increase in Cr(VI) concentration. The most leaching sector, approximately 89.3% of added Cr (1wt% of cement), was found in the pH range 11.8–10.5 due to the dissolution of secondary CrO4-ettringite. It could also be shown that the C–S–H had little chemical binding for Cr(VI).

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.

Strätlingite: compatibility with sulfate and carbonate cement phases

Abstract The silicate AFm, strätlingite, has been shown to be stable in high aluminosilicate cement systems but its stability with respect to the anion content of hydrated Portland cement paste is unknown. The stability of strätlingite in the presence of sulfate and carbonate phases relevant to cement systems are reported. Results show that strätlingite persists at the sulfate activity conditioned by gypsum, ettringite and at carbonate activity conditioned by the presence of calcite, carbonate AFm, or carbonate AFt. Structural incorporation of anions such as carbonate or sulfate in strätlingite was not observed in the temperature range 20–85 °C.

Drying of calcium-silicate-hydrates: regeneration of elastic modulus

Dynamic mechanical thermal analysis (DMTA) measurements for layered calcium silicate systems (1·4 nm tobermorite, jennite, synthetic calcium-silicate-hydrate and hydrated Portland cement paste) are reported. Elastic modulus results were obtained for samples initially conditioned to 11% relative humidity (RH) and dried during the DMTA test and for samples subjected to prolonged vacuum and thermal drying prior to a DMTA test. Decreases in elastic modulus relative to the 11% RH starting condition typically observed during a DMTA test were recovered subsequent to the prolonged thermal treatment. A possible ‘regeneration' mechanism for the elastic modulus is discussed. The ‘regeneration' occurs for all the calcium silicate systems studied. The results are consistent with the notions expressed for nanostructural models of calcium-silicate-hydrate in cement paste that comprise structural units based on 1·4 nm tobermorite and jennite.

Quantitative X-Ray Diffraction Analysis of Anhydrous and Hydrated Portland Cement – Part 1: Manual Methods

Portland cement is one of most important construction and building materials and its properties depend strongly on the mineralogical composition. Consequently, accurate analysis of the mineralogical composition of anhydrous Portland cement is crucial for both product quality control and optimisation of performance following initial hydration. In the latter sense, analysis of the mineralogical composition of hydrated Portland cement paste is critical to understand (1) the mechanism and kinetics of hydration of unmodified pastes and those modified with additives and (2) the resultant properties of cement pastes, mortars, and concretes. Such analyses typically are undertaken by quantitative X-ray diffraction (XRD).The present work reviews current practices in quantitative XRD analysis of anhydrous and hydrated Portland cement. To this end, Part 1 of this two-part work briefly mentions the point-counting method and the Bogue calculation method. The more commonly applied internal standard method and reference intensity ratio (RIR) method are discussed in more detail.

Dimensional stability of 1·4 nm tobermorite, jennite and other layered calcium silicate hydrates

Length-change measurements of 1·4 nm tobermorite, jennite and calcium silicate hydrate preparations (with varying calcium/silicon ratio) subjected to drying from the 11% relative humidity condition are reported. The dimensional stability investigation is focused on the role of adsorbed and interlayer water as shrinkage mechanisms involving menisci effects in capillary pores are eliminated in this humidity range. Length change against mass loss curves and the corresponding d002 basal spacing change (obtained by X-ray diffraction) with mass loss are assessed and inferences drawn regarding nanostructural models for calcium silicate hydrates in hydrated Portland cement paste. Length changes of the pure phases were significantly greater than those for hydrated cement paste due to the restraining effect of unhydrated cement particles and calcium hydroxide crystals in the paste. The dimensional stability and the role of the removal of ‘structural water' from the systems studied are discussed in terms of its relevance to cement-based materials.

Dynamic mechanical thermoanalysis of layered calcium silicate hydrates

Dynamic mechanical thermoanalysis (DMTA) was conducted on compacted specimens of calcium silicate hydrates (C-S-H), 1.4 nm tobermorite, jennite, and compacted hydrated Portland cement paste powders, as well as hardened cement paste. The synthetic silicates are key elements for compositional models of the hydrated calcium silicates present in cement paste. The study focuses on the nanostructural effects due to the removal of water from the 11 % RH condition. The DMTA results (E′ and tan∂ versus temperature curves) in the 25–110 °C range mimicked those of DMA (E′ and tan∂ versus mass loss curves) conducted at room temperature for C-S-H and cement paste. In addition, the DMTA curves for 1.4 nm tobermorite and jennite in the temperature range 110–300 °C were sensitive to phase changes including the transition of 1.4 nm tobermorite to 1.1 nm tobermorite and other forms, as well as the transition of jennite to metajennite. The DMTA curves of a 50/50 mixture of 1.4 nm tobermorite and jennite exhibit similarities and differences to that of hydrated cement paste that are influenced by porosity and the amorphous nature of C-S-H in the cement paste. The study provides useful data for evaluating Taylor’s concept of a possible tobermorite-jennite model for the C-S-H present in hydrated cement paste.

Correlation between dynamic mechanical thermo-analysis and composition-based models for C-S-H in hydrated portland cement paste

A dynamic mechanical analysis (DMTA) technique was used to assess the mechanical performance of 1.4 nm tobermorite (T), jennite (J) and mixtures of tobermorite and jennite and tobermorite and calcium hydroxide (CH). A comparison of E′ (storage modulus) and tan∂ (internal friction) versus temperature curves for compacted solid bodies of these mineral systems with corresponding curves for cement paste hydrated for 2 month and 3 years (both referred to as ‘young’) and 45-year-old paste (referred to as ‘old’) was made. Original data was provided to assess the practical validity of the Richardson–Groves composition-based models for the C-S-H in cement paste. Differences in mechanical performance between the ‘young’ paste and the ‘old’ paste could be accounted for by application of a T/J dominant model (‘young’ paste) and a J–T/CH structural model with a minor amount of T (‘old paste’).

X-ray diffraction: a powerful tool to probe and understand the structure of nanocrystalline calcium silicate hydrates

X-ray diffraction (XRD) patterns were calculated and compared to literature data with the aim of investigating the crystal structure of nanocrystalline calcium silicate hydrates (C-S-H), the main binding phase in hydrated Portland cement pastes. Published XRD patterns from C-S-H of Ca/Si ratios ranging from ~ 0.6 to ~ 1.7 are fully compatible with nanocrystalline and turbostratic tobermorite. Even at a ratio close or slightly higher than that of jennite (Ca/Si = 1.5) this latter mineral, which is required in some models to describe the structure of C-S-H, is not detected in the experimental XRD patterns. The 001 basal reflection from C-S-H, positioned at ~ 13.5 Å when the C-S-H structural Ca/Si ratio is low (< 0.9), shifts towards smaller d values and sharpens with increasing Ca/Si ratio, to reach ~ 11.2 Å when the Ca/Si ratio is higher than 1.5. Calculations indicate that the sharpening of the 001 reflection may be related to a crystallite size along c* (i.e. a mean number of stacked layers) increasing with the C-S-H Ca/Si ratio. Such an increase would contribute to the observed shift of the 001 reflection, but fails to quantitatively explain it. It is proposed that the observed shift could result from interstratification of at least two tobermorite-like layers, one having a high and the other a low Ca/Si ratio with a basal spacing of 11.3 and 14 Å, respectively.

Microstructural Development of Hydrating Portland Cement Paste at Early Ages Investigated with Non-destructive Methods and Numerical Simulation

Microstructure development of hydrating cement paste at early ages is not only an indicator of the reactivity of cement, but also a factor on the workability of fresh concrete. In this study, the microstructure development of hydrating cement paste at early ages is investigated with non-destructive methods including ultrasound P-wave propagation velocity measurement and non-contact electric resistivity tests, together with conventional needle penetration depth and calorimetry tests. The hydration process and microstructural development of the cement paste is modeled with the three-dimensional computer model CEMHYD3D. Evolution of microstructural parameters including the volumetric fraction of phases and their percolation status are analyzed by using the results of the numerical simulation. Microstructural mechanisms of the two non-destructive techniques (ultrasound pulse propagation and electric resistivity measurements) are discussed. The main findings of this study are that the velocity of ultrasound P-wave propagation in hydrating cement paste is a function of the propagation routes in the material and inter-particle forces. The electric resistivity is controlled by the ionic concentrations in the pore solution during the early hours and later by the connectivity of pores. A model for the development of ultrasound P-wave propagation velocity is also proposed.

Erratum to “Thermal Stability of the Cement Sheath in Steam Treated Oil Wells”

Erratum to “Thermal Stability of the Cement Sheath in Steam Treated Oil Wells”

Clinkering Reactions During Firing of Recyclable Concrete

Thermal transformations occurring during firing of recyclable concrete and hydrated Portland cement paste were monitored quantitatively by in situ and ex situ X‐ray diffraction, thermal analysis, and clinker microscopy. Reactions below 1150°C were dominated by either the decarbonation of calcium carbonates of the limestone aggregate fraction in the recyclable concrete or the low temperature decomposition of cement hydrates of the cement paste. Continued heating of the cement paste showed a rapid and extensive reaction of the free lime to form intermediate calcium silicate and aluminate phases such as belite, gehlenite, mayenite, and ye'elimite. The combination of free lime was less rapid in the dominantly limestone‐rich recyclable concrete. Similarly, subsequent melt and clinker phase formation (1250°C and higher) occurred more rapidly in the hydrated cement paste, indicating a positive contribution of cement hydrates to raw meal burnability. Reactions below typical cement clinkering temperatures were enhanced (1) by the intimate mixing of lime, silicate, and aluminate components in the hydrated cement and (2) by the elevated sulfate content that acted as flux and belite mineralizer. Lime formed in the thermal decomposition of portlandite (±450°C) was partially carbonated to calcite at temperatures below the onset of calcite decarbonation (±650°C).

Effects of Sample Preparation and Interpretation of Thermogravimetric Curves on Calcium Hydroxide in Hydrated Pastes and Mortars

Calcium hydroxide [Ca(OH)2] is one of the major constituents of hydrated portland cement paste. Its content can be used to trace the progress of cement hydration or serve as an indicator of the extent of pozzolanic reaction. The thermogravimetric analysis (TGA) method is often used to determine the Ca(OH)2 content because it is a relatively easy and fast procedure. However, no universally accepted method exists for the preparation of TGA specimens and for the interpretation of the resulting TGA curves. This paper presents an investigation on the contents of Ca(OH)2 in samples subjected to different preparation techniques. The results showed that a certain amount of calcium carbonate (CaCO3) was produced as a result of carbonation during the sample preparation process. The degree of carbonation was dependent on the sample preparation, and carbonated Ca(OH)2 was considered to determine the accurate total Ca(OH)2 content. In addition, a modified interpretation of the TGA curve for Ca(OH)2 was suggested. In this interpretation, the mass losses caused by the other hydration products, except for the Ca(OH)2 and the carbonated Ca(OH)2, were considered so that the accurate content of Ca(OH)2 could be determined. The interpretation technique was verified by comparing the results with those obtained by differential scanning calorimetry. Ultimately, the actual contents of Ca(OH)2 in pastes undergoing different sample preparation techniques were determined by using the modified interpretation of the TGA curve for the Ca(OH)2. The results showed that this interpretation yielded comparable contents of Ca(OH)2 in most of the sample preparation techniques used in this study.

Fracture surfaces and compressive strength of hydrated cement pastes

The properties of the fracture surfaces of hydrated Portland cement pastes were investigated. Analyses of fracture surfaces proved to be capable to provide information about the actual values of compressive strength. Since the compressive strength of cement pastes, σ c , is dependent mainly on capillary porosity and the porosity is a controlling factor of height irregularities of fracture surfaces, the compressive strength, as a consequence, shows a functional dependence on the height irregularities. As a measure of height irregularities the 3D profile and roughness parameters were employed. The 3D surface profiles of the fractured specimens were registered by the confocal microscope. The parameters SP a and SP q , i.e. the arithmetic average and the quadratic mean (rms) heights, respectively, were found to be statistically reliable indicators of surface irregularity and thus the dependences σ c ( SP a ) and σ c ( SP q ) were also shown to be relevant indicators of compressive strength. It is hypothesized that the height irregularities of fracture surfaces might become a basis for the microscopic assessment of the compressive strength of cement pastes.