Hydration,mechanical properties and durability of high-strength concrete under different curing conditions

The properties of high-strength concrete under standard curing condition (20 °C, 95% RH), high-temperature curing condition (50 °C) and temperature match curing condition were comparatively investigated. The cumulative hydration heat of composite binder containing fly ash and silica fume is lower than that of composite binder containing the same amount of slag. Addition of fly ash and silica fume clearly reduces the adiabatic temperature rise of concrete, but adding slag leads to higher adiabatic temperature rise than Portland cement concrete. High-temperature curing condition and temperature match curing condition lead to the sustainable increase in compressive strength of concrete containing mineral admixture, but they hinder the later-age strength development of Portland cement concrete. For cement–slag paste and cement–fly ash–silica fume paste, the non-evaporable water contents increase significantly and the pore structures are much finer under high-temperature curing condition and temperature match curing condition, which negatively affect the pore structure of Portland cement paste. The differences in properties of concrete among three curing conditions become smaller with time. The properties obtained under standard curing condition can approximately reflect the long-term properties of high-strength concrete in the real structure. The concrete prepared with cement–fly ash–silica fume composite binder has the highest compressive strength, finest pore structure and best resistance to chloride permeability under any curing condition. This composite binder is very suitable to prepare the high-strength concrete with large volume.     

Optimization of cementitious composite for heavyweight concrete preparation using conduction calorimetry

The present work investigates the hydration heat of different cement composites by means of conduction calorimetry to optimize the composition of binder in the design of heavyweight concrete as biological shielding. For this purpose, Portland cement CEM I 42.5 R was replaced by a different portion of supplementary cementitious materials (blast furnace slag, metakaolin, silica fume/limestone) at 75%, 65%, 60%, 55%, and 50% levels to obtain low hydration heat lower than 250 j g^?1. All ingredients were analyzed by energy dispersive X-ray fluorescence (EDXRF) and nuclear activation analysis (NAA) to assess the content of major elements and isotopes. A mixture of two high-density aggregates (barite and magnetite) was used to prepare three heavyweights concretes with compressive strength exceeding 45 MPa and bulk density ranging between 3400 and 3500 kg m^?3. After a short period of volume expansion (up to 4 h), a slight shrinkage (max. 0.3°/°°) has been observed. Also, thermophysical properties (thermal conductivity, volumetric specific heat, thermal diffusivity) and other properties were determined. The results showed that aggregate content and not binder is the main factor influencing the engineering properties of heavyweight concretes.

     

A comparison of early hydration properties of cement–steel slag binder and cement–limestone powder binder

The early hydration properties of cement–steel slag composite binder and cement–limestone powder composite binder were compared in this study by determining the hydration heat of binder within 3 days, the pore structure of paste and the compressive strength of mortar at the age of 3 days. Results show that at the curing temperature of 25 °C, the early hydration heat of the binder containing steel slag is smaller, and the early pore structure of the paste containing steel slag is coarser, but the early compressive strength of the mortar containing steel slag is higher compared with the mix containing limestone powder. Though the early reaction degree of steel slag is low, its chemical contribution to the strength of mortar cannot be neglected. At the curing temperature of 50 °C, the early hydration heat of the binder containing steel slag is larger, and the early pore structure of the paste containing steel slag is finer, and the early compressive strength of the mortar containing steel slag is even higher compared with the mix containing limestone powder. Raising curing temperature can enhance the role played by steel slag more significantly than that played by limestone powder in the hydration and hardening of the composite binder.     

Synthesis of Pure Cementitious Phases by Sol‐Gel Process as Precursor

Calcium silicates and aluminates are the main constituents of ordinary Portland cement (OPC) and calcium aluminate cements (CAC) and therefore the pure phases are of great importance for the investigation of interactions between binder and additives or admixtures. Additionally, investigations on clinker phases doped with foreign ions enable the improvement of the performance of cements. For this purpose great amounts of pure phases are needed. These phases are hard to synthesize via a solid state reaction of solid educts. Thus there is a need for a new, more efficient route to synthesize these phases. The sol‐gel process as precursor provides an alternative to the conventional method. In this paper experimental evidence is presented for an improved synthesis of calcium silicates and aluminates via sol‐gel processes, the characterisation of these clinker phases and their hydration behaviour.     

The difference among the effects of high-temperature curing on the early hydration properties of different cementitious systems

The difference among the effects of high-temperature curing on the early hydration properties of the pure cement, the binder containing fly ash, the binder containing GGBS, and the binder containing steel slag was investigated by determining the compressive strength, non-evaporable water content, hydration heat, and Ca(OH)₂ content. Results show that the order of the influence degrees of high-temperature on the early hydration of different binders is the binder containing GGBS > the binder containing steel slag > the binder containing fly ash > the pure cement. In the case of short period of high-temperature curing (only 1 day), the strength growth rate of the concrete containing GGBS is the greatest. Though the influence of increasing high-temperature curing period on the hydration degree of the binder containing fly ash is not the most significant, the strength growth rate of the concrete containing fly ash is the most significant due to the excessive consumption of Ca(OH)₂ by reaction of fly ash. In the case of high-temperature curing, the Ca(OH)₂ content of the paste containing steel slag is much higher than those of the paste containing GGBS and the paste containing fly ash, so though high-temperature curing promotes the hydration of the binder containing steel slag significantly, its influence on the strength growth rate of the concrete containing steel slag is not so significant.     

Investigation on mechanical properties, durability and micro-structural development of steel slag blended cements

To improve the properties of steel slag blended cements, a chemical activator was added into blended cements, the mechanical properties and durability of steel slag blended cements were investigated. The results show that steel slag in blended cement pastes presents low hydraulic activity and makes practically no contribution to strength development. After the addition of chemical activator, the mechanical properties and durability of ternary blended cements are increased significantly. The hydration process and micro-structural development of blended cement was investigated by isothermal calorimeter and scanning electric microscope, respectively. Steel slag started hydration in the first 3?days in the presence of chemical activator, steel slag and granulate blast furnace slag reacted with Ca(OH)₂ to form a dense microstructure as curing proceeded. Therefore, both early and late compressive strengths of steel slag blended cement with 35% cement clinker and 30% steel slag can be comparable with those of Portland cement.     

Simultaneous measurements of heat of hydration and chemical shrinkage on hardening cement pastes

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.     

Middle stage of Portland cement hydration influenced by different portions of silica fume,metakaolin and ground granulated blast-furnace slag

Present study deals with the influence of metakaolin (MK), silica fume (SF) and ground granulated blast-furnace slag (BFS) on middle hydration of ordinary Portland cement replaced by 45 mass% of particular supplementary cementitious materials (SCMs). Acceleration of cement hydration by SF and MK was proved up to the first 12 h by isothermal calorimetry as well as by thermogravimetric analyses. From the beginning of deceleratory period, when SCMs stopped to act as accelerators, more evident influence of the dilution effect was observed. Nevertheless, the presence of pozzolanic reactions was demonstrated already after 15 h of curing and even when SF and MK were used in the amount equal to 5 mass%. Synergic effect of the used SCMs allowed to increase the quantity of BFS up to 35 mass% without significant changes in their positive action.

     

Application of differential thermogravimetry to the hydration of expansive cement pastes

Differential thermogravimetric analysis was used to determine the hydration kinetics of expansive cement and its products at various ages of hydration. Analytical grade reagents, kaolin and Portland cement were used to prepare an expansive cement on the basis of calcium sulphoaluminate. Two mix compositions having the stoichiometric composition of trisulphate and monosulphate were synthesized from pure reagents. Three clinkers were also prepared from kaolin, gypsum and calcium carbonate with different compositions.The hydration of expansive cement prepared from the stoichiometric composition of trisulphate and Portland cement gives ettringite as the stable phase after seven days of hydration. The presence of more CaO than the stoichiometric composition of trisulphate favours the conversion of some ettringite to the monosulphate hydrate. The hydration of expansive cement prepared from the stoichiometric composition of monosulphate and Portland cement shows the presence of ettringite and the monosulphate phase. Ettringite is formed initially, and then transformed to the monosulphate form.     

Comparing study on hydration properties of various cementitious systems

The hydration properties of slag sulfate cement (SSC), slag Portland cement (PSC), and ordinary Portland cement (POC) were compared in this study by determining the compressive strength of pastes, the hydration heat of binders within 72 h, the pore structure, the hydration products, and the hydration degree. The results indicated that main hydration products of PSC paste and POC paste are calcium hydroxide and C–S–H gel, while those of SSC paste are ettringite and C–S–H gel from the analyses of XRD, TG–DTA, and SEM. At the early curing age, the compressive strength depends on the clinker content in the cementitious system, while at the late curing age, which is related to the potential reactivity of slag. From hydration heat analysis, the cumulative hydration heat of PSC is lower than that of POC, but higher than that of SSC. Slag can limit chemical reaction and the delayed coagulation of gypsum, which also plays a role in the early hydration. So SSC shows the lowest heat release and slag can’t be simulated without a suitable alkaline solution. Based on MIP analysis, the porosity of POC paste is the smallest while the average pore size is the biggest. At the age of 90 days, the compressive strength of SSC can get higher development because of its relative smaller pore size than that of PSC and POC paste.     

Calorimetric determination of the effect of additives on cement hydration process

Possibilities of a multicell isoperibolic-semiadiabatic calorimeter application for the measurement of hydration heat and maximum temperature reached in mixtures of various compositions during their setting and early stages of hardening are presented. Measurements were aimed to determine the impact of selected components?? content on the course of ordinary Portland cement (OPC) hydration. The following components were selected for the determination of the hydration behaviour in mixtures: very finely ground granulated blast furnace slag (GBFS), silica fume (microsilica, SF), finely ground quartz sand (FGQ), and calcined bauxite (CB). A commercial polycarboxylate type superplasticizer was also added to the selected mixtures. All maximum temperatures measured for selected mineral components were lower than that reached for cement. The maximum temperature increased with the decreasing amount of components in the mixture for all components except for silica fume. For all components, except for CB, the values of total released heat were higher than those for pure Portland cement samples.     

Hydration of Portland cement with alkanolamines by thermal analysis

Four types of alkanolamines (i.e., traditional alkanolamines represented by TEA and TIPA and new alkanolamines represented by DEIPA and EDIPA) were added to Portland cement as chemical additives, and their effects on the cement properties and hydration process were investigated. An isothermal calorimeter was used to track the hydration heat flow of the cement pastes with or without alkanolamines. Thermogravimetric analyses were performed to measure the degree of hydration over the course of 28 days. In addition, X-ray diffraction, MIP analysis and SEM were used as auxiliary tests. The results indicated that alkanolamines improved the compressive strength of the cement mortars. It was found that TEA increased the rate of the second hydration of C₃A, and TIPA accelerated the hydration of C₄AF. DEIPA and EDIPA promoted the hydration of both the aluminum and ferrite phases as well as catalyzed the conversion of AFt to the AFm phase. By contrast, the new alkanolamines represented by DEIPA and EDIPA expressed more superior properties.     

Study of the hydration of Portland cement blended with blast-furnace slag by calorimetry and thermogravimetry

The hydration of ordinary Portland cement (OPC) blended with blast-furnace slag (BFS) is a complex process since both materials have their own reactions which are, however, influenced by each other. Moreover, the effect of the slag on the hydration process is still not entirely known and little research concerning the separation of both reactions can be found in the literature. Therefore, this article presents an investigation of the hydration process of mixes in which 0–85% of the OPC is replaced by BFS. At early ages, isothermal, semi-adiabatic and adiabatic calorimetric measurements were performed to determine the heat of hydration. At later ages, thermogravimetric (TG) analyses are more suitable to follow up the hydration by assessment of the bound water content w _b. In addition, the microstructure development was visualized by backscattered electron (BSE) microscopy. Isothermal calorimetric test results show an enhancement of the cement hydration and an additional hydration peak in the presence of BFS, whilst (semi-)adiabatic calorimetric measurements clearly indicate a decreasing temperature rise with increasing BFS content. Based on the cumulative heat production curves, the OPC and BFS reactions were separated to determine the reaction degree Q(t)/Q _∞ (Q = cumulative heat production) of the cement, slag and total binder. Moreover, thermogravimetry also allowed to calculate the reaction degree by w _b(t)/w _b∞. The reaction degrees w _b(t)/w _b∞, Q(t)/Q _∞ and the hydration degrees determined by BSE-image analysis showed quite good correspondence.     

Effect of addition of nano-magnetite on the hydration characteristics of hardened Portland cement and high slag cement pastes

In this investigation the effect of addition of magnetite nanoparticles on the hydration characteristics of both ordinary Portland cement (OPC) and high slag cement (HSC) pastes was studied. The cement pastes were prepared using a water/solid (W/S) mass ratio of 0.3 with addition of 0.05, 0.1, and 0.3 % of magnetic fluid Fe₃O₄ nanoparticles by mass of cement. An aqueous stable magnetic fluid containing Fe₃O₄ nanoparticles, with a mean diameter in the range of super-paramagnetism, was prepared via co-precipitation method from ferrous and ferric solutions. The admixed magnetite-cement pastes were examined for compressive strength, chemically combined water content, X-ray diffraction analysis, and differential scanning calorimetry. The results of compressive strength revealed that the hardened pastes made from OPC and HSC admixed with different amounts of magnetic fluid have higher compressive strength values than those of the neat cement OPC and HSC cement pastes at almost all ages of hydration. The results of chemically combined water content for the admixed cement pastes showed almost the same general trend and nearly comparable values as those of the neat cement pastes. From the XRD diffractograms obtained for the neat OPC and HSC cement pastes, the main hydration products identified are calcium silicate hydrates, portlandite, and calcium sulfoaluminate hydrates. Addition of magnetic fluid nanoparticles to both of OPC and HSC did not affect the main hydration products of the neat OPC or HSC cement in addition to one main basic difference, namely, the formation of calcium iron hydroxide silicate as a new hydration product with a reasonable hydraulic character.     

Determination of Kinetic Equations of Alkaline Activation of Blast Furnace Slag by Means of Calorimetric Data

The alkaline activation of blast furnace slag promotes the formation of new cement materials. These materials have many advantages over ordinary Portland cement, including high strength, low production cost and good durability. However, many aspects of the chemistry of alkaline activated slags are not yet very well understood. Some authors consider that these processes occur through a heterogeneous reaction, and that they can be governed by three mechanisms: a) nucleation and growth of the hydrated phase; b) phase boundary interactions and c) any diffusion process though the layer of hydration products. The aim of this paper was to determine the mechanism explaining the early reaction of alkaline activation of a blast furnace slag through the use of calorimetric data. A granulated blast furnace slag from Avilés (Spain) with a specific surface of 4450 cm²> g^-1 was used. The alkaline activators used were NaOH, Na₂CO₃ and a mix of waterglass (Na₂SiO₃·nH₂O and NaOH. The solution concentrations were constant (4% Na₂O with respect to the slag mass). The solutions were basic (pH 11-13). The mixes had a constant solution/slag ratio of 0.4. The thermal evolution of the mixes was monitored by conduction calorimetry. The test time was variable, until a rate of heat evolution equal to or less than 0.3 kJ kg^-1 h^-1 was attained. The working temperature was 25°C. The degree of hydration (α) was determined by means of the heat of hydration after the induction period. The law governing the course of the reaction changes at a certain degree of hydration. From a generally accepted equation, the values of α at which the changes are produced were determined. These values of α depend on the nature of the alkaline activator. Nevertheless, for high values of α, the alkaline activation of slag occurs by a diffusion process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calorimetric Studies of Hydration of Cementitious Materials Varying in Basicity

This paper presents results of thermochemical and thermokinetic studies of the hydration of portland cement, alkaline cements with mineral additives and slag alkaline binder. The influence of the modulus of basicity of the binders on the thermochemical and thermokinetic characteristics of hydration was estimated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hydration of shrinkage-compensating binders with different compositions and water-binder ratios

The calorimetric data of blended shrinkage-compensating binders with different compositions were measured at 25°C at different water-binder ratios using an isothermal calorimeter. The hydration characteristics of shrinkage-compensating binders were evaluated and their influence on the expansive properties of blended shrinkage-compensating binders was determined. Composition and w/b ratio significantly affect the hydration rate and degree of shrinkage-compensating binders, as well as their expansive and mechanical properties. The total heat of hydration of binders decreases with w/c ratios. Its final hydration degree also decreases with w/c ratio. The ternary binders composed with Portland cement, mineral admixture and expansive agent show low hydration heat and rate of heat evolution, but their total heat of hydration increases continuously and surpasses that of binary binder in later period at low w/b ratio. This revised version was published online in August 2006 with corrections to the Cover Date.     

Application of isothermal and isoperibolic calorimetry to assess the effect of zinc on cement hydration

The amount of zinc in the clinker or in the secondary raw materials has been increasing in recent years. Zinc can get to Portland cement from solid waste or tires which are widely used as a fuel for burning in a rotary kiln. The aim of this work was to determine the effect of zinc on Portland cement hydration. This effect was studied by isothermal and isoperibolic calorimetry. Both calorimetry methods are suitable for measurements during the first days of hydration. Isoperibolic calorimetry monitors hydration process in real-life conditions, while isothermal calorimetry does it at a defined chosen temperature. Zinc was added to the cement in the form of two soluble salts of Zn(NO₃)₂, ZnCl₂ and a poorly soluble compound ZnO. The concentration of zinc added was chosen as 0.05, 0.1, 0.5 and 1 mass%. The results show that increasing amounts of zinc ions in cement pastes lead to hydration retardation and reduce both the maximum temperature and the maximum heat flow due to the retarding effect of zinc. The newly formed compounds during hydration were identified by X-ray diffraction method.     

Calorimetry of portland cement with silica fume and gypsum additions

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.     

Calorimetry and other methods in the studies of expansive cement hydrating mixtures

In this study, the calorimeter was applied to follow the hydration of special cement mixtures exhibiting expansion or shrinkage compensation. The shrinkage-less and expansive binders were produced by mixing of Portland cement with an expansive additive produced by sintering and composed of calcium sulfoaluminate (yeelimite), calcium sulfate (anhydrite) and lime. The studies were focused on the synthesis of this aluminate??sulfate??lime additive (temperature of burning process as a parameter controlling the relative activity of components) from the materials being the by products and subsequently on the mixture proportions to ensure the hydration process resulting in non-shrinkage or expansion effect. In the experiments the proportions of expansive mixture and cementitious materials were variable. The investigations with aim to find the relationship between the volume changes and composition of initial mixtures in cement pastes and mortars (with sand) were also carried out. The phase composition and microstructure of products were characterized. The expansive additive in the environment of hydrating cement transforms into ettringite and gives an increase of volume when the plastic material transforms to the more rigid matter but before the ultimate hardening takes place. Proper, moderate setting and hardening in strongly modified mixtures is achieved when the calorimetric curve corresponding to the heat evolution on hydration is analogous to that for the basic Portland cement. The rate of heat evolution data are well compatible with the other results related to the other methods of hydration kinetics assessment (e.g. chemical shrinkage) and discussed in terms of the phase composition of hydration products.