Search Results
Abstract
The isothermal heat of hydration of MgSO4 hydrates was studied by humidity controlled calorimetry. Two hydrates, starkeyite (MgSO4·4H2O) and a mixture of MgSO4 hydrates with summary 1.3 mol H2O were investigated. The solid-gas reactions were initiated at 30°C and 85% relative humidity. The heat of hydration was determined in a circulation cell in the calorimeter C80 (Setaram). The crystal phases formed after the hydration process were analyzed by thermogravimetry (TG) and X-ray powder diffraction (XRD). Starkeyite reacted with the water vapour to the thermodynamic stable epsomite and the MgSO4 hydrate mixture with 1.3 mol water to hexahydrite. The hydration heats of starkeyite and the mixture were determined to be −169±3 and −257±5 kJmol−1, respectively.
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 use of active mineral additions is an important alternative in concrete design. Such use is not always appropriate, however, because the heat released during hydration reactions may on occasion affect the quality of the resulting concrete and, ultimately, structural durability. The effect of adding up to 20% silica fume on two ordinary Portland cements with very different mineralogical compositions is analyzed in the present paper. Excess gypsum was added in amounts such that its percentage by mass of SO3 came to 7.0%.
The chief techniques used in this study were heat conduction calorimetry and the Frattini test, supplemented with the determination of setting times and X-ray diffraction. The results obtained showed that replacing up to 20% of Portland cement with silica fume affected the rheology of the cement paste, measured in terms of water demand for normal consistency and setting times; the magnitude and direction of these effects depended on the mineralogical composition of the clinker. Hydration reactions were also observed be stimulated by silica fume, both directly and indirectly – the latter as a result of the early and very substantial pozzolanic activity of the addition and the former because of its morphology (tiny spheres) and large BET specific surface. This translated into such a significant rise in the amounts of total heat of hydration released per gram of Portland cement at early ages, that silica fume may be regarded in some cases to cause a synergistic calorific effect with the concomitant risk of hairline cracking. The addition of excess gypsum, in turn, while prompting and attenuation of the calorimetric pattern of the resulting pastes in all cases, caused the Portland cement to generate greater heat of hydration per gram, particularly in the case of Portland cement with a high C3A content.
Abstract
This paper analyzes the effect of fly ash chemical character on early Portland cement hydration and the possible adverse effects generated by the addition of gypsum. Behaviour was analyzed for pure Portland cements with varying mineralogical compositions and two types of fly ash, likewise differing in chemical composition, which were previously characterized under sulphate attack as: silicic-ferric-aluminic or aluminic-silicic ash in chemical character, irrespective if they are in nature, siliceous or siliceous and aluminous materials according to the ASTM C 618-94a. The experimental results showed that water demand for paste with a normal consistency increased with the replacement ratio in fly ash with a more aluminic than silicic chemical character, whereas it declined when silicic-ferric-aluminic ash was used. On the other hand, the differences between the total heat of hydration released at the first valley and the second peak also clearly differentiated the two types of ash. While the relative differences increased in the more aluminic than silicic ash, they declined in the more silicic than aluminic. In another vein, the findings indicate that within a comparable Blaine fineness range, the reactive alumina (Al2O3 r−) content in pozzolanic additions has a greater effect on mortar strength than the reactive silica (SiO2 r−) content, at least in early ages up to 28 days. Finally, the adverse effect generated in the presence of excess gypsum is due primarily to the chemical interaction between the gypsum and the C3A in the Portland cement and the reactive alumina (Al2O3 r−) in the fly ash.
Calorimetric comparison of portland cements containing silica fume and metakaolin
Is silica fume, like metakaolin, characterized by pozzolanic activity that is more specific than generic?
Abstract
This new study must be regarded to be a direct outcome of two previous studies published by these same authors, which were conducted to respond to interesting questions brought out about the effect of silica fume, SF and metakaolins, M and MQ, on the heat of hydration of portland cements, PC, with very different C3A and C3S contents. The answer to these so interesting questions has been the primary objective of the present research. For this purpose, the same PC, PC1 (14% C3A) and PC2 (≈0% C3A), metakaolins, silica fume and blended cements were once again used more 60/40 for sulphate attack, and the same analytical techniques (CC, pozzolanicity and XRD analysis) and parameters determined as well. In this new research, the sulphate attack was determined by two accelerated methods: Le Chatelier-Ansttet and ASTM C 452-68. The experimental results of sulphate attack mainly, have demonstrated definitively that the high, rapid and early pozzolanic activity exhibited by SF also is, as in the case of the two metakaolins, more specific than generic, for it indirectly stimulated greater C3A than C3S hydration, but only in the first 16 h monitored in this study. Thereafter it is the contrary, i.e., anti- or contra-specific for the same purpose. And the longer the hydration time, the more anti- or contra-specific it became, since, when exposed to sulphate attack, SF blended cements resisted or even prevented the aggressive attack against PC1 which, with a higher C3A content than PC2, was the more vulnerable of the two. By contrast, metakaolin MQ not only failed to hinder or prevent the attack, but heightened its effects, rendering it more intense, aggressive and rapid, leading to what could be called a rapid gypsum attack.
techniques suitable for the heat of hydration evaluation monitoring solution calorimetry and isothermal calorimetry has been selected. Solution calorimetry is convenient for long-term monitoring of the heat of hydration and also, e.g., for rapid determination
Abstract
In this study the calorimeter was applied to follow the early hydration of mixtures produced using three different synthetic slag vitreous materials, differing with alumina to silica ratio and mixed or not with synthetic two different metakaolin additions (kaolin heated with sodium containing admixture or without admixture). These mixtures were processed with sodium/potassium hydroxide solutions and placed inside the chamber of calorimeter. The kinetics of hydration process was thus characterized and the hydraulic properties of slag–metakaolin mixtures were very well assessed. Substantial heat evolution was found in the presence of activators, in many cases exceeding 100 J g–1 for 24 h process; heat evolved on hydration with water only was very poor, below 20 J g–1 after 24 h. The rate of heat evolution vs. time plots showed specific shape, more or less similar to the typical heat evolution curves reported for cements. Finally, some conclusions were drawn and the amount/concentration of activators was selected for further studies.
Abstract
In this study a calorimeter was applied to investigate the hydration of cements with fly ash (pulverised fuel ash – PFA) admixture. Four cements were used to produce the binders containing from 5 to 60% fly ash. The process of hydration in cementitious systems with fly ashes is slower than in reference pastes without admixtures. However, the calorimetric calculations and the shape of heat evolution curves seem to indicate a complex interaction between the components of cement and ash resulting in the increasing total heat evolved values per unit of cement. At higher fly ash content the accelerating effect of alkalis and alumina should be taken into account and discussed in terms of the composition of initial cement. The modifications of hydration kinetics and mechanism in this case is very well visualised by means of calorimetry.
Abstract
Calorimetry has been used in the investigations of cementitious systems with different set controlling admixtures. The kinetics and mechanism of hydration process was thus characterized on two different cement clinkers mixed with calcium sulphate containing materials. These admixtures were collected as a residue in the fluidised bed combustion (FBC) of coals with simultaneous desulphurisation process - so-called bottom ash. Apart from anhydrite/gypsum, they were composed mainly of alumina and silica containing material of disordered structure, originating from the coal contaminations of clay character. Anhydrite/gypsum acts as set controlling admixture. The aluminosilicate component reacts with calcium ions released to the solution from the calcium silicate clinker minerals. It has been found that fluidised bed combustion wastes can be successfully used as set controlling admixture. There is no other harmful effects; those could be easily detectable by calorimetry. However the effect is dependent upon the composition of cement clinker. At low calcium aluminate content a slight acceleration of hydration process can be easily observed, particularly at higher amount of admixture. In the mixtures with high calcium aluminate clinker the heat evolved is slightly reduced in the presence of admixture. The dominating role of aluminate phase in heat evolution process within the first hours of hydration process has been thus proved.
Abstract
Calorimetry has been used in the investigations of calcium aluminate materials produced as a binder for aluminate-corundum composites of high refractoriness. The kinetics and of hydration process was thus characterized and the optimum compositions of initial binders and cement-corundum refractory filler blends could be selected for further tests. The acceleration of heat evolution - the shortening of so-called induction period and relatively high heat output in the presence of corundum was observed. It means the acceleration of hydration process, that is early crystallisation of hydration products and subsequent further dissolution of initial anhydrous aluminate phases. In the presence of fine grained corundum particles these phenomena should be attributed to the nucleating effect of fine corundum particles.
