A positron annihilation study on the hydration of cement pastes

Positron annihilation lifetime spectroscopy experiments were carried out in various ordinary Portland cement pastes, in an attempt to monitor the porosity of the pastes. It is found that positronium intensity is well correlated to the time evolution of the total porosity and it is influenced by the water-to-cement ratio. This parameter is also sensitive to the delayed hydration process induced by adding methanol to the water–cement mixture.     

Alongterm study of MgO hydration in cement pastes

The effect of various additives on the longterm hydration of Magnesium Oxide, (MgO) in cement pastes cured in water at 18±2°C up to 12 years was studied by X-ray diffraction (XRD), seanning electron microscopy (SEM) and EPMA. It was found that cement with high MgO content, is stabilized even after a longterm period of hydration by active pozzolans.     

Drying/hydration in cement pastes during curing

As concrete cures in the field, there is a constant competition for the mixing water between evaporation and hydration processes. Understanding the mechanisms of water movement in the drying/hydrating cement paste is critical for designing curing systems and specialized rendering materials, as well as for selecting repair materials and methodologies. In this work, X-ray absorption measurements indicate that fresh cement paste dries uniformly throughout its thickness, as opposed to exhibiting the sharp drying front observed for most porous materials. Furthermore, in layered composite cement paste specimens, water always flows from the coarser-pore layer to the finer one, both when coarser pores are produced by using an increased water-to-cement ratio (w/c) and when they are present due to using a cement with a coarser particle size distribution at a constant w/c. Conversely, no clear differential water movement is observed between layers of cement paste and mortar of the same nominal w/c. Based on the results of these experiments, drying has been introduced into the NIST CEMHYD3D cement hydration and microstructure development model, by emptying the largest water-filled pores present at any depth in the model specimen at a user-specified (drying) rate. With this addition, the CEMHYD3D model produces results in good agreement with experimental observations of both the drying profiles and the hydration kinetics of thin cement paste specimens.     

Preliminary investigations into the supercritical carbonation of cement pastes

Interactions between supercritical carbon dioxide (scCO₂) and hydrated cement pastes, of various water/cement ratio, have been investigated. The carbonation process was greatly accelerated in the scCO₂ compared to that in natural or CO₂ enriched environments. The nature of the reactions was dependent on the amount of water present in the paste. Thus carbonation of samples dried prior to treatment resulted in the reaction of all the unhydrated C₃S and C₂S, but little conversion of calcium hydroxide to calcium carbonate. In contrast, carbonation of samples containing moisture resulted in the conversion of most of the calcium hydroxide whilst the amounts of C₃S and C₂S reacted increased as the water/cement ratio increased. During the carbonation treatment, the pore structure of the cement pastes was altered and substantial reductions in porosity were achieved. The process may be used to improve the durability of glass fibre reinforced cement by lowering the alkalinity and calcium hydroxide content of the matrix.     

The nature of the hydration products in hardened cement pastes

An understanding of the performance of portland cement-based materials requires knowledge at the microstructural level. Developments in the instrumentation of several techniques have led to improved understanding of the composition, morphology, and spatial distribution of the various products of cement hydration. In particular, our understanding of the nature of the nearly amorphous calcium silicate hydrate (C–S–H) phases – which are the principal binding phases in all portland cement-based systems – has been advanced by developments in solid-state NMR spectroscopy and analytical TEM. This paper presents an overview of the nature of the hydration products formed in hardened portland cement-based systems. It starts with the most straightforward cementitious calcium silicate systems, C₃S and β-C₂S, and then considers ordinary portland cement and blends of portland cement with silica fume, ground granulated iron blast-furnace slag, and finally alkali hydroxide-activated slag cements.     

Rheological and hydration characterization of calcium sulfoaluminate cement pastes

Calcium sulfoaluminate (CSA) cements are currently receiving a lot of attention because their manufacture produces less CO₂ than ordinary Portland cement (OPC). However, it is essential to understand all parameters which may affect the hydration processes. This work deals with the study of the effect of several parameters, such as superplasticizer (SP), gypsum contents (10, 20 and 30 wt.%) and w/c ratio (0.4 and 0.5), on the properties of CSA pastes during early hydration. This characterization has been performed through rheological studies, Rietveld quantitative phase analysis of measured X-ray diffraction patterns, thermal analysis and mercury porosimetry for pastes, and by compressive strength measurements for mortars. The effect of the used SP on the rheological properties has been established. Its addition makes little difference to the amount of ettringite formed but strongly decreases the large pore fraction in the pastes. Furthermore, the SP role on compressive strength is variable, as it increases the values for mortars containing 30 wt.% gypsum but decreases the strengths for mortars containing 10 wt.% gypsum.     

Degradation of cement pastes by organic acids

Agriculture produces effluents, like liquid manure and ensilage effluents, that cause serious environmental problems. In order to limit this pollution, manure needs to be stored in water-tight silos often made of concrete. Manure contains organic acids which constitute a severe chemical threat for concrete. This research aims to analyze the degradation mechanisms of cement-based materials stored in organic acids. The results are used to identify the composition parameters of binders that influence durability. Ordinary and blended Portland cement specimens were immersed for 18 weeks in a mixture of 5 organic acids found in liquid manure at a pH of 4. Physical, chemical and mineralogical modifications in the pastes were explored by water intrusion porosity tests, electron microprobe and X-ray diffraction analyses. Analyses were run from the sound to the altered zone. The altered depths and the mass loss variations of the samples were monitored over time. The degradation of the matrix occurs by almost total decalcification, the vanishing of the crystallized or amorphous hydrated phases and the probable formation of a silica gel, which limits the kinetics of further degradation. In the altered zone, the anhydrous silico-calcic grains are chemically modified but C₄AF and slag grains keep their initial composition.     

Effect of PCs superplasticizers on the rheological properties and hydration process of slag-blended cement pastes

The effect of polycarboxylate (PC) superplasticizers with different structure on the rheological properties and hydration process of slag-blended cement pastes with a slag content between 0 and 75% has been studied. Fluidizing properties of PCs admixtures are significantly higher in slag-blended cement with respect to non-blended Portland cement. Also, it has been observed that the rise of the fluidity induced by the PCs on the cement pastes increases with the slag content. This effect is mainly attributed to a decrease in the amount of C₃A available to adsorb and consume admixture to form an organo-mineral phase. Consequently, the PC admixtures are absorbed onto the silicate phases of the clinker and onto the slag particles, inducing a repulsion and the concomitant reduction in yield stress despite a reduction in the zeta potential. The rheological results allow us to conclude that the highest increase of the fluidity is caused by the admixtures with highest molecular weight due to the higher steric repulsion induced. As a consequence of the adsorption of the PCs, a delay of the hydration process of the pastes has been observed.     

碳酸化钢渣和矿渣作硅酸盐水泥混合材水化性能研究

通过对钢渣碳酸化前后的硅酸盐相提取及水化放热性能和将碳酸化钢渣和矿渣作为混合材的硅酸盐水泥的胶砂强度和水化产物种类的测定,以及对它们微观形貌的观察,研究了碳酸化钢渣对胶凝体系水化性能的影响.结果表明,碳酸化使钢渣中硅酸盐相的含量由47.06％下降至14.38％;碳酸化促进了钢渣的早期水化,抑制其后期水化;在配比相同的条件下,碳酸化钢渣-矿渣-硅酸盐熟料体系试样的3、28d抗压强度较未碳酸化钢渣-矿渣-硅酸盐熟料体系试样的高;碳酸化生成的CaCO3促进了熟料的水化;碳酸化钢渣促进了胶凝体系中AFt的生成,且生成水合碳铝酸钙.     

Microstructure determination of cement pastes by NMR and conventional techniques

Applicability of NMR relaxation analysis to the characterization of pore structure in hydrating white cement pastes has been investigated. Measurements of transverse magnetization relaxation were made in specimens saturated and partially filled with water for periods of hydration between 1 and 28 days. The fast diffusion model successfully accounts for all relaxation measurements. The transverse surface relaxation rate was determined. A two-component pore volume size distribution was found. Comparison was made with mercury porosimetry and nitrogen sorption experiments and the results were correlated with compressive strength.     

A conduction calorimetric study of early hydration of ordinary Portland cement/high alumina cement pastes

Conduction calorimetry was applied to an investigation of the early hydration of ordinary Portland cement (OPC)/high alumina cement (HAC) pastes. Three different rate of heat-evolution profiles were observed, depending on the HAC/OPC ratio. Relevant processes affecting heat development include ettringite formation, HAC and OPC hydration. Results from SEM examination and X-ray diffraction studies are also presented. An acceleration of OPC hydration was observed in pastes containing less than 12.5% HAC. A similar acceleration effect on HAC hydration was also obtained with the addition of OPC. A large amount of ettringite was formed and OPC hydration delayed in the pastes containing 15%–30% HAC. The latter could be one of the factors attributed to poor strength development in these HAC/OPC systems. Early hydration mechanisms of OPC/HAC systems are also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Attack of cement pastes exposed to organic acids in manure

Manure such as silage effluents and liquid manure contains organic acids which constitute a severe chemical threat toward the concrete of agricultural structures. The purposes of this study were to identify the chemical composition parameters that influence durability by analysing the behaviour of the chemical elements of the cement paste (Ca, Si, Al, Fe and Mg) in organic acid solutions and to compare the intensity of the chemical attack by the different acids found in liquid manure. This study was carried out on cement pastes made from four binders (ordinary Portland cement, slag cement, OPC blended with silica fume and OPC blended with fly ash). The hardened cement pastes were first crushed, then immersed in solutions made of five organic acids with an initial pH of 4 and constantly stirred. The pH and the concentrations of major elements were monitored over time.

The results show that Si, Al, and Fe appear to be favourable elements for the chemical resistance of binders whereas the amount of Ca should be limited. Moreover, it is shown that the four acids found in liquid manure (acetic, propionic, butyric, iso-butyric) are equally aggressive. Lactic acid, present with acetic acid in silage effluent, is more aggressive according to the value of its pK_a.     

Use of admixtures in organic-contaminated cement-clay pastes

In this work microstructure, porosity and hydration degree of cement-based solidified/stabilized wasteforms were studied before assessing their leaching behaviour. 2-Chloroaniline was chosen as a model liquid organic pollutant and included into cement pastes, which were also modified with different admixtures for concrete: a superplasticizer based on acrylic-modified polymer, a synthetic rubber latex and a waterproofing agent. An organoclay, modified with an ammonium quaternary salt (benzyl-dimethyl-tallowammonium, BDMTA), was added to the pastes as pre-sorbent agent of the organic matter. All the samples were dried up to constant weight in order to stop the hydration process at different times during the first 28 days of curing, typically, after 1 day (1d), 7 days (7d) and 28 days. Then, the microstructure of the hardened cement-clay pastes was investigated by powder X-ray diffraction (XRD). The hydration degree and porosity were studied by thermal analysis (TG/DTA) and mercury intrusion porosimetry (MIP), respectively. For samples cured for 28 days a short-term leach test set by Italian regulation for industrial waste recycling (D.M. 5 February 1998) was performed. The best results showed a 5% release of the total initial amount of organic pollutant.     

Influence of limestone powder used as filler in SCC on hydration and microstructure of cement pastes

In recent years, self-compacting concrete (SCC) has gained wide application in the construction industry. As for high performance concrete (HPC) and traditional concrete (TC), the microstructural properties of SCC are the main factors, which determine the material properties, i.e. the mechanical properties, transport properties and the durability behaviour.In order to investigate the development of the microstructure of SCC, the microstructural parameters of the paste including porosity, pore size distribution and phase distribution are determined by means of mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The thermogravimetric analysis (TGA) and the derivative thermogravimetric analysis (DTG) are used to identify the phase constituents. These parameters as studied for self-compacting concrete are compared with high performance concrete and traditional concrete. The specimens of self-compacting cement paste (SCCP) are made with water/binder ratio 0.41 and 0.48, the high performance cement paste (HPCP) with w/c 0.33 and traditional cement paste (TCP) with w/c 0.48. The measurements are performed at different hydration stages, i.e. at 1, 3, 7, 14, 28 and 56 days.The result of this research shows that the pore structure, including the total pore volume, pore size distribution and critical pore diameter, in the SCCP is very similar to that of HPCP. The fact that limestone powder does not participate in the chemical reaction was confirmed both from thermal analysis and BSE image analysis.     

Strain and thermal expansion coefficients of various cement pastes during hydration at early ages

Cement pastes undergo elevated temperature histories due to hydration heat liberation at early ages. Thermal expansion coefficients of cement paste and concrete change with age, showing a decrease after mixing, a subsequent minimum and then a gradual increase. These changes contribute to thermal strain. In this study, effects of water–cement ratio and cement type on volume changes in early-age cement pastes were experimentally examined using a newly developed apparatus capable of simultaneously determining both thermal expansion coefficient and total strain of cement pastes. The dependence of the thermal expansion coefficient on hydration was affected by water–cement ratio, cement type, elevated temperature history and particularly by the free water content of the cement pastes, while the relationship between thermal expansion coefficient and free water content varied with water–cement ratio. A notable increase in thermal expansion coefficient at early ages was observed when water–cement ratio was low and alite content in cement was high. At a water–cement ratio of 0.30, low-heat Portland cement paste resulted in a small total strain while moderate-heat and ordinary Portland cement pastes showed larger strains. Because no particular difference was observed in the thermal strains, shrinkage in the low-heat Portland cement paste was attributed to autogenous strain. At a water–cement ratio of 0.40, self-desiccation had a significant influence upon autogenous shrinkage and dependence of thermal expansion coefficient on hydration, and the effect of the mineral composition of cements was notable. However, for cement pastes with a water cement ratio of 0.55, no significant effects of self-desiccation were observed, probably because considerable excess water was present.     

The role of phosphonates in the hydration of Portland cement

Six phosphonates, comprising the three acids aminotri(methylenephosphonic acid) (ATMP), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and diethylenetriaminepenta-(methylenephosphonic acid) (DTPMP) and their corresponding sodium salts Na₅ATMP, Na₄HEDP and Na₆DTPMP, were added to Portland cement in dosages ranging from 0.03 to 0.09%, at a constant water/cement ratio of 0.35 and their conduction calorimetric behaviour was investigated up to 72 h. The induction period, the time to attain the maximum heat effect and the integral heat developed at different times were determined. All phosphonates increased the induction period, from about 3 h to greater than 72 h, with respect to the reference cement with an induction period of 2 h. The acid phosphonates were more effective retarders than their corresponding salts. At a concentration of 0.05% the induction period extended from 10.1 to 21.1 h with the acids and only from 4.1 to 16.2 h with the salts. DTPMP was the most effective retarder among all the phosphonates, a concentration of 0.05% producing an induction period of 21.1 h and an exothermic inflection at 42.4 h compared with values of 2.2 and 7.9 h, respectively, for the reference. The corresponding salt (Na₆DTPMP) was the most efficient of all the salt retarders. At a concentration of 0.05%, the induction period was extended to 16.2 h and the exothermal inflection to 31.4 h. In most instances the degree of extension of the induction period increased with the dosage of retarder. Phosphonates appear to be much more efficient retarders than many other retarders used in concrete practice.

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Resume On a ajouté au ciment Portland six phosphonates, comprenant les trois acides aminotri (acid méthylènephosphonique) (ATMP), 1-acide hydroxyéthylidène-1,1-acide diphosphonique (HEDP) et diéthylènetriaminepenta(acide méthylènophosphonique) (DTPMP) et les sels de sodium correspondants Na₅ATMP, Na₄HEDP et Na₆DTPMP, à des dosages allant de 0,03 à 0,09%, avec un rapport eau/ciment constant de 0,35. On a étudié leur comportement sous conduction calorimétrique pendant une durée de 72 h. On a déterminé la période d’induction, le temps nécessaire pour atteindre l`effet de chaleur maximal et la chaleur intégrale produite à certains moments. Tous les phosphonates ont augmenté la période d’induction de 3 h à plus de 72 h, par rapport au ciment de référence qui présente une période d’induction de 2 h. Les phosphonates sont des retardateurs plus efficaces que les sels qui leur correspondent. Pour une concentration de 0,05%, la période d’induction est passée de 10 à 21,1 h avec les acides, et seulement de 4,1 à 16,2 h avec les sels. Le DTPMP a été le retardateur le plus efficace parmi les phosphonates: une concentration de 0,05% a déterminé une période d’induction de 21,1 h et une manifestation exothermique à 42,4 h, par rapport à des valeurs respectivement de 2,2 et 7,9 h pour le ciment de référence. Le sel correspondant (Na₆DTPMP) a été le plus efficace de tous les sels retardateurs. Pour une concentration de 0,05, la période d’induction a atteint 16,2 h et la manifestation exothermique 31,4 h. Dans la plupart des cas, le degré d’allongement de la période d’induction était en relation avec le dosage de retardateurs. Les phosphonates semblent être bien plus efficaces que beaucoup d’autres retardateurs utilisés dans la pratique du béton.

     

Preliminary investigations of the phase composition and fine pore structure of super-critically carbonated cement pastes

Super-critical carbonation of cement-based materials can lead to significant improvements in their properties. Preliminary investigations suggested that processing should be aimed at producing a matrix material with minimal amounts of Ca(OH)₂, anhydrous material and C—S—H gel along with a controlled pore structure. Using ²⁹Si MAS NMR and TGA as the principal investigative techniques it has been shown that moisture content during carbonation is a major factor in determining the phase composition and pore structure of the resulting matrix. Of the drying regimes studied, 60% DOD gave the greatest amount of conversion to calcium carbonate and silica gel.     

Influence of nano-SiO2 on the Portland cement pastes

The evolution of nanotechnology provides materials with new properties and over the last years a lot of effort has been put to introduce nano-particles into cement pastes in order to improve their properties and to produce materials of better performance. In the present research work, nano-SiO₂ produced by pyrolysis and with specific area of 200 m²/g has been added at different percentages (0%, 0.5%, 1%, 2% and 5%) to high-strength cement pastes. These pastes were tested for their mechanical and structural properties at different ages. Nanoparticles act as nuclei for crystallization and large, idiomorphic crystals of Ca–Si composition were formed assisting, up to a certain percentage, in producing materials with dense structure, reduced porosity and improved strength.     

Properties of polyvinyl alcohol cement pastes

Properties of cement pastes containing varying amounts of each of polyvinyl alcohol (PVAL), mixtures of polyvinyl alcohol and phenol formaldehyde (PF) and mixtures of poly vinyl alcohol and borax were studied in this paper. Though the strength parameters of the PVAL-cement pastes are comparable to virgin cement paste their resistance to acid is far superior. Soxhlet extraction with water, done to determine leachability of the polymer from the polymer cement paste, revealed that the percentage of polyvinyl alcohol leached out was less when borax or PF resin was added to the PVAL cement paste. The compressive strength of the poly vinyl alcohol–phenol formaldehyde cement paste was found to be inferior to the other two cases but the retention of compressive strength after immersing in each of acid, base and kerosene was much better. In general, polyvinyl alcohol when added to cement pastes improves the chemical resistance properties in terms of retention of compressive strength after exposure to chemicals.