Search Results
Abstract
In this work, the hydration rate and products of blended zeolite cements were studied for periods up to 360 days. Thermoanalytical methods (TG/DTG and DTA) were applied in order to evaluate the hydration rate of blended cements, while. X-ray diffraction and FTIR spectroscopy were used in order to identify the hydrated products. As it is concluded the incorporation of zeolite in cement contributes to the consumption of Ca(OH)2 formed during the cement hydration and the formation of cement-like hydrated products. The pozzolanic reaction of the zeolite is rather slow during the first days of hydration but it is accelerated after the 28 days.
Abstract
When exposed to attack by moisture, macroscopic defect-free materials (MDFs) undergo mass and phase changes. The nature of such changes was studied thermoanalytically. Attacked samples differ from non-attacked samples in the degradation of classical cement hydrates (TG, below 200C) and calcium carbonate (TG, DTA, 550-650C). Quantitative assessment favours the hypothesis of the impregnation/barrier effect due to the incorporation of polyphosphate glass into the structure of the MDFs. The identity of the thermal decomposition of attacked and non-attacked samples in the range 250-400C demonstrates the resistance of cross-linked sections of polymer and clinker constituents to the effects of moisture.
Investigations of cement early hydration in the presence of chemically activated fly ash
Use of calorimetry and infrared absorption methods
Abstract
The paper describes an attempt of chemical activation of fly ash and claims the usefulness of combination of such investigation methods as calorimetry and infrared absorption for investigations of early periods of cement hydration. The research samples were cement pastes made with an addition of fly ash and admixtures of chemical activators, CaCl2, Na2SO4 and NaOH, whereas a cement paste without fly ash addition and a cement-fly ash paste (both without admixtures) were used as reference samples. In order to investigate early periods of cement pastes hydration, the amount and rate of heat release were registered, and IR spectrums were checked at appointed hydration moments. As a result, it was shown that the combination of calorimetric and IR absorption methods in the investigations of early periods of cement hydration was useful. It was confirmed that the use of chemical activators CaCl2, Na2SO4 and NaOH accelerated the hydration of cement pastes containing fly ash additive in early hours after adding water. The action of activators on hydrating cement system is different for each of investigated compounds.
Abstract
Isothermal calorimetry and chemical shrinkage measurements are two independent techniques used to study the development of hydration in cementitious systems. In this study, calorimetry and chemical shrinkage measurements were combined and simultaneously performed on hydrating cement paste samples. Portland cement pastes with different water to cement ratios and a cement paste containing calcium sulfoaluminate clinker and anhydrite were studied. The combined calorimetry/chemical shrinkage test showed good reproducibility and revealed the different hydration behavior of sealed samples and open samples, i.e., samples exposed to external water during hydration. Large differences between sealed and open samples were observed in a Portland cement paste with low water to cement ratio and in the calcium sulfoaluminate paste; these effects are attributed to self-desiccation of the sealed pastes. Once the setup is fully automatized, it is expected that combined calorimetry/chemical shrinkage measurements can be routinely used for investigating cement hydration.
Abstract
The physicochemical properties of spent fluidized bed cracking catalyst and its influence on hydration process of cement slurry were studied. The samples were cement slurries prepared with water/solid=0.5 and additions of used catalyst amounted to 0, 5, 10, 15, 20 and 25%with resp. to the solid. After definite time they were subjected to thermogravimetric analysis (TG, DTG, DTA) and, in order to determine the progress of reaction with water, the heat of hydration was measured by means of isotherm calorimetry. The studies disclosed that the spent cracking catalyst is not merely an inactive filler in cement slurries, but it modifies the course of the hydration process. The spent catalyst is a pozzolana additive and its presence leads to a decrease of calcium hydroxide contents in the system. The spent catalyst affect on the heat of cement hydration. Small amounts additive accelerate the process of binding.
Abstract
The effect of PbO on cement hydration kinetics by calorimetric method was evaluated as a first step in this project. Substantial retardation of reaction with water at early stages with subsequent intensification of the process was found. As the next step, the model systems covering pure cement minerals and their mixtures of various composition as well as soluble Pb salts were taken into account to elucidate the mechanism of delayed, by quite good formation of products in the so-called post-induction period. The precipitation of sulphate, forming very thin impermeable layer seems to be responsible for this delaying effect in case of cement, however the other reactions of Pb compounds in alkaline environment of hydrating calcium silicate are not out of importance. In order to prove this, the studies of chemical composition in small areas were also carried out.
Abstract
When cement hydrated compositions are analyzed by usual initial mass basis TG curves to calculate mass losses, the higher is the amount of additive added or is the combined water content, the higher is the cement ‘dilution’ in the initial mass of the sample. In such cases, smaller mass changes in the different mass loss steps are obtained, due to the actual smaller content of cement in the initial mass compositions. To have a same mass basis of comparison, and to avoid erroneous results of initial components content there from, thermal analysis data and curves have to be transformed on cement calcined basis, i.e. on the basis of cement oxides mass present in the calcined samples or on the sample cement initial mass basis. The paper shows and discusses the fundamentals of these bases of calculation, with examples on free and combined water analysis, on calcium sulfate hydration during false cement set and on quantitative evaluation and comparison of pozzolanic materials activity.
Abstract
To use flue gas desulfurization (FGD) gypsum and limestone as supplement of cement, conduction calorimetry was applied to investigate the early hydration of ternary binder of calcium aluminate cement (CAC), Portland-limestone cement (PLC), and FGD gypsum, supplemented with the determination of setting times and X-ray diffraction (XRD) analysis. Different exothermal profiles were presented in two groups of pastes, in which one group (group A) sets the mass ratio of FGD gypsum/CAC at 0.25 and the other group (group B) sets the mass ratio of PLC/CAC at 0.25. Besides the two common exothermal peaks in cement hydration, a third exothermal peak appears in the pastes with 5–15% FGD gypsum after gypsum is depleted. It is found that not PLC but FGD gypsum plays the key role in such ternary binder where the reaction of ettringite formation dominates the hydration process. PLC accelerates the hydration of ternary binder, which mainly attributes to the nucleating effect of fine limestone particles and PC clinker. The modified hydration process and mechanism in this case is well visualized by the means of calorimetry and it helps us to optimize such design of ternary cementitious material.
Abstract
The need for cements or other cementitious materials that afford high early age mechanical strength has led to the use of extremely reactive pozzolanic additions such as silica fume, nanosilica, metakaolin and similar. The inclusion of the right proportion of such pozzolanic additions stimulates portland cement hydration, i.e., directly, as they are initially moistened by the mixing water, non-directly when they act as “seed crystals”, and indirectly, because of the pozzolanic reaction between the addition particles and the portlandite forming from the portland cement components hydration; since this reaction is characterized by its intensity and speed, when its occurs it prevails over the other two. Indirect stimulation also causes the fraction of portland cement in the blend to release more heat of hydration than pure portland cement, and its does so on a scale consistent with the existence of a calorific synergic effect. Such greater heat is released in the early stages of hydration primarily by C3A and C3S that react with the mixing water to respectively generate ettringite and hydrated calcium silicates. When portland cements have a low to nil C3A content, less heat of hydration is released due to the absence of an AFt phase that could be transformed into AFm. However, when extremely active pozzolanic additions, such as silica fume, are used, ettringite forms from C4AF, further contributing to origin amounts of hydration heat released comparable to the above calorific synergic effect.
Summary This paper presents a study of a cement-based solidification/stabilization process of storm water runoff solid residuals by non-conventional differential thermal analysis (NCDTA). The study was used to investigate the early hydration stages of a type I Portland cement containing the raw residual, two fractions of the residuals (coarse and fine), and two additives (quicklime and sodium bentonite). During these stages the fine fraction of the residuals retards the hydration reactions more than the coarse one, and both fractions have components that are reactive during the hydration process. When sodium bentonite is present in the pastes, the higher the initial cement content of the pastes, the lesser is the reactivity of the residuals. The presence of quicklime, which undergoes simultaneous highly exothermal hydration, accelerates the cement hydration reactions as well as those due to the presence of the residual solids. In these quicklime-containing compositions, the effect of sodium bentonite is similar to that when no quicklime is added, except when the whole residuals are used.
