Microcalorimetric studies of some cementitious materials

The Al doped tricalcium silicate hydration in the presence of active silica was studied by means of calorimetry, DTA, TG, XRD and conductometry of the hydrating suspension.

---

Zusammenfassung Mittels Kalorimetrie, DTA, TG, Röntgendiffraktion und Konduktometrie der Hydratierungssuspension wurde in Gegenwart von aktivem Silizium-dioxid die Hydratation von mit Al versetztem Tricalciumsilikat untersucht.

     

Influence of fineness on the cementitious properties of steel slag

In this paper, the influence of fineness on the cementitious properties of steel slag and the properties of cement containing steel slag with different finenesses were investigated. The results show that increasing the fineness can significantly enhance the early as well as the late cementitious properties of steel slag. However, the early hydrations of cement and steel slag tend to hinder each other especially in the case of large steel slag replacement and high fineness of steel slag. Therefore, increasing fineness of steel slag cannot improve the early cementitious properties of the cement containing steel slag. At 28 days, the hydrations of steel slag and cement tend to promote each other. Increasing the fineness of steel slag enhances the late cementitious properties of the cement containing steel slag significantly.     

Thermogravimetrical analysis for monitoring carbonation of cementitious materials

Cement paste carbonation, i.e., the reaction between CO₂ and the hydrated cement phases, mainly calcium hydroxide or portlandite, can lead to a pH decrease, which in turn can give rise to steel corrosion in reinforced concrete. At the same time, the carbonation reaction contributes to combine CO₂ and fix it as calcium carbonate. It is a crucial phenomenon from the point of view of structure durability and also for cement-based materials sustainability. Cement paste specimens with two w/c ratios and eight types of cements were submitted to different environmental conditions for 4?years and the evolution of calcium carbonate formed or carbon dioxide bound was followed by TG performed in inert atmosphere. The amounts of calcium hydroxide, evaporable and C?CS?CH gel water were also measured. The CO₂ bound follows the same trend in all samples and environments: at the beginning there is a sharp increase followed by a very slow stretch and reaching a maximum after less than 2?years in most cases. The calcium hydroxide amounts evolve very differently in each environment. While outside it is almost consumed after 1?year, inside there is a decrease in the first year, but an increase in the next 3?years. The behavior of the C?CS?CH water in both environments is similar to that of the portlandite inside. The evaporable water diminishes in all cases to 1?%. From the data obtained by TG, the quantification of the C?CS?CH gel as well as the calculation of the Ca/Si ratio and the hydration of the gel formed by different type of binders has been possible.     

Cellulose nanofibrils for the performance improvement of ultra-high ductility cementitious composites

Cellulose - This study explored the effect of cellulose nanofibrils (CNFs) as a binder at different weight percentages, namely, 0%, 0.05%, 0.1%, and 0.15%, on the hydration, rheology, pore...     

Calorimetric studies of cementitious materials — chromium interaction

Cementitious systems based on portland cement are used for immobilization of toxic and hazardous wastes. The addition of waste material may impact the hydration reaction in cement matrix and consequently the setting and hardening process. The progress of reaction can be monitored by heat evolution measurements and the calorimetric results can indicate the declination from standard behaviour.In this study the microcalorimetry was used to evaluate the heat output during the hydration of cements in the presence of different chromium containing salts, viz. CrCl₃, Cr₂(SO₄)₃, Na₂CrO₄ and K₂CrO₄.     

Pozzolanic reactivity of the supplementary cementitious material pitchstone fines by thermogravimetric analysis

Thermogravimetric (TG) analysis was applied to the characterisation of the pozzolanic reaction in mortars containing the supplementary cementitious materials (SCMs) pitchstone fines (PF) and fly ash (FA) as partial replacements for Portland cement (PC). TG analysis was used to determine the proportion of calcium hydroxide (CH) present from the hydration of the PC based on the dehydroxylation of the CH present in the blended PC-SCM mortars. The consumption of CH indicated that both SCMs underwent the pozzolanic reaction and that PF was found to compare favourably in its pozzolanic reactivity of FA, the industry and globally accepted standard artificial pozzolan.     

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.     

Usage of supplementary cementitious materials: advantages and limitations

Journal of Thermal Analysis and Calorimetry - It is well known that cement production is not neutral for natural environment among others due to high CO2 emission. Different strategies of...     

Preparation and properties of cementitious composites for geothermal applications

Three fiber-reinforced cement composites were prepared and cured in an autoclave for up to 168 h at 2 MPa of steam pressure in order to investigate the effect of hydrothermal curing on the alteration of pore structure, density, and formation and stability of hydrated products with time. Compressive strength was reviewed in connection with sample porosity. It was found that the time of autoclaving plays a crucial role in objective assessment of the durability of composites as potential candidates for geothermal applications. A mercury intrusion porosimeter Quantachrome Poremaster 60GT was used for the estimation of the pore structure parameters of composites. The thermal analysis method was used to identify different temperature ranges of cured samples?? thermal decomposition and to characterize the nature of hydrated products. Two kinds of products were formed. The first group consisted of calcium-silicate-hydrate (C-S-H), calcium-silicate-aluminate-hydrate (C-S-A-H), calcium-aluminate-hydrate (C-A-H), and calcium-carbonate (C-C) as a product of carbonation. The second group are chemically bond products, e.g. hydroxyapatite (Ca₅(PO₄)₃(OH)) and gibbsite (Al(OH)₃). These two hydroceramic products formed under hydrothermal conditions act also as binders and they can be useful as geothermal cement binders.     

Influence of Cu powders on the properties and characteristics of nano-MgO based aluminate cementitious materials

From the perspective of practical application, the development of desirable thermal and mechanical performance of solid sensible materials for thermal energy storage (TES) is highly needed. Here, we report the improved properties of nano-MgO optimized aluminate cementitious materials incorporated with Cu powders for TES. The composite TES materials were heated at 105, 350, and 900 °C, respectively. The results show that as the Cu powders content increases the thermal conductivity and volume heat capacity significantly increase, but there is a gradual decrease in compressive strength. Through the characterizations such as calorimetric test, XRD, FESEM, TG-DSC, and MIP, a significant feature of mass compensation also has been obtained, which might result from the oxidation reaction of the Cu powder at elevated temperatures.     

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.

     

Thermal hardening and structure of a phosphorus containing cementitious model material

The reaction of phosphoric acid with wollastonite has been studied for reaction mixtures with a molar ratio r=P/Ca from 0.39 up to 2.9. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) reveal the formation of the crystalline products brushite (when r is smaller than 1.4) and monetite and calcium dihydrogenphosphate monohydrate (when r is above 1). Magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and FTIR show that amorphous silica is also formed which contains some residual calcium and hydroxyl groups. The proposed reactions are confirmed by the evolution of the reaction enthalpy measured with differential scanning calorimetry (DSC). The reaction was monitored with temperature modulated DSC (TMDSC) and dynamic mechanical analysis (DMA). The sharp increase of the elasticity modulus coincides with the onset of decrease in heat capacity. The setting of the reaction mixture does not slow down the reaction rate.     

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.     

Determination of the optimum parameters in the high resolution thermogravimetric analysis (HRTG) for cementitious materials

In this article, the methodology to implementation of high resolution thermogravimetric analysis (HRTG) for construction materials like Portland cement pastes is presented. The aim of this technique is to make easier the identification of the decomposition reactions that frequently are overlapping on conventional thermogravimetric analysis (TG) like is the case of some mineral phases in the cement pastes. The optimum parameters related to sample mass and purge flow gas were established. It is necessary carried out the analysis with high quantity of sample (60 mg in this case) and without purge gas in order to get better results and excellent reproducibility. The tests have average heating rate higher than 3 °C min⁻¹ in the temperature range studied (35–300 °C), showing that the HRTG is not time-expensive technique.     

Studies of cementitious systems with new generation by-products from fluidised bed combustion

The rate of heat evolution as well as total heat output are strongly affected by other components of hydrating mixture, apart from neat portland cement, such as slag, fly ash and other industrial by-products; among them the wastes from fluidised bed combustion (FBC) has been taken into account recently. In this study the calorimeter was applied to follow the early hydration of cements produced with these materials. They interact with cement paste in a few ways: as set controlling agent and as active pozzolanic admixtures. Thus the rate of heat evolution/hydration is modified, depending on the composition of clinker and percentage of waste in the mixture. After the series of measurements for clinker-waste mixture hydrated systems also some ‘model’ mixtures were investigated to separate the effects from particular waste components. This revised version was published online in July 2006 with corrections to the Cover Date.     

Understanding of bonding and mechanical characteristics of cementitious mineral tobermorite from first principles

This paper reports density functional theory study of the structural and mechanical properties of tobermorite mineral (9 Å phase) as one of the main component of cementitious materials in a concrete chemistry. Calculated bulk modulus and elastic constants reflect a relatively high resistance of the tobermorite structure with respect to external isostatic compression. Moreover, the elastic constants proved the anisotropic character of the tobermorite structure. The directions parallel to the axb plane are more resistant to the compression than the perpendicular direction. The largest contribution to this resistance comes from the “dreierketten” silicate chains. The bonding analysis linked macroscopic mechanical properties and the atomic structure of the tobermorite. It was found that polar covalent Si? O bonds are stiffer than iono‐covalent Ca? O bonds. The SiO₄ tetrahedra are resistant with respect to the compression and the effect of external pressure is reflected by the large mutual tilting of these tetrahedra as it is shown by changes of the Si? O? Si bridging angles. Polyhedra with the seven‐fold coordinated Ca²⁺ cations undergo large structural changes. Especially, axial Ca? O bonds perpendicular to the axb plane are significantly shortened. Besides, it was shown that structural parameters, more or less in parallel orientation to the axb plane, are mainly responsible for the high resistance of the tobermorite structure to external pressure. The main mechanism of a dissipation of energy entered to the structure through the compression is proceeded by the tilting of the tetrahedra of the silicate chains and by large shortening of the axial Ca? O distances. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

Initial hydration of cementitious systems using a simple isothermal calorimeter and dynamic correction

A simple isothermal calorimeter ideal to study hydration of cementitious systems is described together with an ampoule design to allow addition of water and mixing with the ampoule inside the calorimeter. An overview of dynamic corrections is given, and the utilisation of the different dynamic corrections on the calorimeter output is discussed. Correction of data on b-hemihydrate hydration to form gypsum has shown good kinetic agreement with data from synchrotron X-ray diffraction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heat evolution in hydrated cementitious systems admixtured with different set controlling components

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. This revised version was published online in July 2006 with corrections to the Cover Date.