Artificial pozzolanic cement pastes containing burnt clay with and without silica fume

Pozzolanic cement blends were prepared by the partial substitution of ordinary Portland cement (OPC) with different percentages of burnt clay (BC), Libyan clay fired at 700 °C, of 10, 20, and 30%. The pastes were made using an initial water/solid ratio of 0.30 by mass of each cement blend and hydrated for 1, 3, 7, 28, and 90 days. The pozzolanic OPC–BC blend containing 30% BC was also admixed with 2.5 and 5% silica fume (SF) to improve the physicomechanical characteristics. The hardened pozzolanic cement pastes were subjected to compressive strength and hydration kinetics tests. The results of compressive strength indicated slightly higher values for the paste made of OPC–BC blend containing 10% BC The results of DSC and XRD studies indicated the formation and later the stabilization of calcium silicates hydrates (CSH) and calcium aluminosilicate hydrates (C₃ASH₄ and C₂ASH₈) as the main hydration products in addition to free calcium hydroxide (CH). Scanning electron microscopic (SEM) examination revealed that the pozzolanic cement pastes made of OPC–BC mixes possesses a denser structure than that of the neat OPC paste. Furthermore, the addition of SF resulted in a further densification of the microstructure of the hardened OPC–BC–SF pastes; this was reflected on the observed improvement in the compressive strength values at all ages of hydration.     

Systematic analysis of natural pozzolans from Greece suitable for repair mortars

Natural pozzolans were used as pozzolanic cement in concrete mixtures to increase the long-term strength, the concrete durability, and other material properties of Portland cement many centuries ago. The most common pozzolan is the volcanic ash, a siliceous or aluminosiliceous material, which is highly vitreous, coming from volcanic eruptions. In the Greek territory, 39 reactive volcanoes have been recorded both in insular and in terrestrial areas. The reactivity of pozzolans, in the case of lime-based mortars, is attributed to their content in amorphous silica which reacts with Ca(OH)₂ from lime, in environmental conditions, and forms C–S–H compounds responsible for the strength gain. Their use in building materials was diachronic. The significant properties of mortars containing pozzolans derive from the mechanism of its gradual strengthening attributed to the reaction of silicates with lime to form secondary cementitious phases which increase the durability and the dense structure of the mortars. In the present paper, two natural pozzolans from Greece are analyzed in order to record their morphological and analytical microstructure as well as their thermal and physical properties. The results revealed that the pozzolans tested, are materials of high quality and can be used for the production of compatible repair mortars. Also, valuable criteria could be instituted for the selection of reactive pozzolans which could be used for conservation purposes. Among others, crucial parameters for compatibility between old mortars and new ones are the surface features (color, texture, and roughness), the composition (type of binder, granulometry of aggregate), and the pore size distribution.     

Calorimetric and thermal analysis studies on the influence of waste aluminosilicate catalyst on the hydration of fly ash–cement paste

Binders containing large amounts of cement substitutes have been a subject of interest for many years because of the possibility to reduce the amount of cement in concrete, and in consequence decrease negative influence of cement production on natural environment. In this work, studies related to hydration of binders where 80 % of cement was substituted by blended pozzolana were carried out. The aim of this work was to investigate activation of fly ash–cement system by addition of spent aluminosilicate catalyst, using calorimetry and thermal analysis as main methods of investigations. It was demonstrated that spent fine-grained fluidised catalytic cracking catalyst acts acceleratingly on early hydration of binder. It seems to be beneficial to use up to 10 mass% of this spent catalyst. Higher amounts may cause changes in the mechanism of early hydration. Because Ca(OH)₂ in such systems is quickly consumed due to pozzolanic reaction it seems beneficial to modify composition of binders by introducing additional amounts of Ca(OH)₂ or cement.     

Heat of hydration of high reactive pozzolans in blended cements: Isothermal conduction calorimetry

A study was carried out comparing silica fume (SF) and dealuminated kaolin (DK) as pozzolanic materials in blended cements. Ten, 20 or 30 wt% of SF or DK were substituted for Portland cement. The kinetics of hydration up to 45 h were studied using isothermal conduction calorimetry. Blends containing pozzolanic materials usually have decreased heats of hydration compared to pure cement during the period of C₃S hydration, i.e. during the main hydration peak. Depending on the chemical composition and the activity of the pozzolan, the reaction taking place with the lime typically contributes to the heat output after the main hydration peak.The pozzolanic activity of DK is the principal factor and heat evolution increases with respect to pure PC mortar, during the first 15 h. The presence of hydrated silica (silanol groups) in DK increases the pozzolanic activity especially before and during induction period. The acidic silanol sites are capable of a fast acid-base reaction with the alkalis and with any Ca(OH)₂ present in cement during the induction period.     

Thermogravimetric study of the reduction of CuO–WO<Subscript>3</Subscript> oxide mixtures in the entire range of molar ratios

The present study reports the results of investigation on the role of metakaolin in the formation of ettringite in a model relevant to Portland cement. The model consists of ternary system (Trio) metakaolin–lime–gypsum. Five samples of defined ternary system were cured at different temperatures 20, 30, 40, 50 and 60 °C. Conduction calorimeter TAM AIR was mainly used to capture heat evolution at different temperatures. Thermoanalytical (simultaneous TGA/DSC) and X-ray diffraction methods were used to identify different products after curing. It results that ettringite is the main hydration product supplemented by calcium silicate and calcium aluminosilicate hydrates according to sample composition. The mechanism and kinetics of hydration, as displayed by calorimetric curves, depend on composition of samples and curing temperatures. Two main types of processes have been elucidated: reaction of aluminum ions with sulfate ones in the presence of calcium ions in aqueous solution to form ettringite supplemented by pozzolanic activity leading to the formation of calcium silicate and calcium aluminosilicate hydrates. Concomitant condensation of alumina and silica species and carbonation have influenced the course of hydration. Activation energy E _a depends slightly on composition of ternary system.     

New Deposit of Mordenite–Clinoptilolite in the Eastern Region of Cuba: Uses as Pozzolans

This work describes the newly discovered zeolites in the eastern region of Cuba. In the researched area, there have been no previous studies of natural zeolite exploration. Therefore, the results shown here are new. The main object of this research is to analyse five samples of zeolites and demonstrate their pozzolanic capacity and the possibility of their usage in the industrial manufacturing of pozzolanic cements. The study of the samples was performed by X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy (SEM). A chemical analysis (CAQ) to determine the quality of the samples as pozzolans was performed, by determining the total SiO₂, reactive SiO₂, total CaO, reactive CaO, Al₂O₃, MgO and the insoluble residue (I.R.). Lastly, an eight-day pozzolanicity analysis (PA) was carried out to determine the pozzolanic reactivity of the samples. The results obtained by XRD, XRF and SEM established that the researched zeolite samples have two main zeolitic phases: mordenite and clinoptilolite. Altered volcanic glass, quartz and smectite (montmorillonite) are the secondary phases. The results of the chemical quality analysis (CAQ) showed that the samples contain a considerable amount of reactive SiO₂ and reactive CaO, as well as a low content of insoluble residue, which reinforces their properties as pozzolans. The results of the pozzolanicity analysis (PA) concluded that the analysed samples actively react with Ca(OH)₂ after eight days. Based on all the results mentioned above, it is established that both mordenite and clinoptilolite behave like pozzolans and can be recommended for the manufacture of pozzolanic cements, which have more effective properties than Portland cement, in terms of physical, chemical and mechanical strength, low heat of hydration, resistance to sulphates, low CO₂ emissions to the atmosphere and negligible impacts on the environment.     

Consumption of calcium hydroxide and formation of C–S–H in cement pastes

The pozzolanic properties of the equilibrium catalyst (Ecat), an abundant waste from fluid catalytic cracking units of oil refineries, have been studied in cementitious matrices. The consumption of calcium hydroxide is usually taken as a means to assess the pozzolanicity of a material, ignoring the formation of C–S–H and other hydrates, the main product of the pozzolanic reaction. The use of thermal analysis methods allows the identification and quantification of the products of the hydration reaction, based on the loss of chemically combined water of phases present in a cementitious matrix. In the present study, TG/DTG techniques were used to quantify the amount of C–S–H + ettringite, C–A–S–H + C–A–H, and calcium hydroxide in cement–Ecat pastes, containing 0–40 % Ecat, aged for 1–28 days. The influence of the content of these phases on the mechanical properties of the resulting samples was also investigated by axial compression. The results further confirmed the pozzolanic activity of Ecat, so that the production of C–S–H and C–A–S–H increased with the aging time, but in a smaller amount than in the reference paste. The compressive strength test showed that the pastes containing 10 % Ecat, aged for 14 and 28 days, presented improved properties as compared to the corresponding references.     

Swelling effects in cross-linked polymers by thermogravimetry

A Brazilian coal power plant generates a waste composed by the fly and bottom ashes produced from coal combustion and by a spent sulfated lime generated after SO₂ capture from combustion gases. This work presents a study of the early stages of the hydration of composites formed by this waste and a type II Portland cement, which will be used for CO₂ capture. The cement substitution degrees in the evaluated composites were 10, 20, 30 and 40%, and the effect of the coal power unit waste on the hydration reaction was analyzed on real time by NCDTA, during the first 40 h of hydration. The results show that the higher is the substitution degree, the higher is the retarding effect on the cement hydration process. Actually, by respective thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis on initial cement mass basis, this effect is caused by double exchange reactions among Ca and Mg components of the waste, during the first 4 h of hydration, which promote a much higher exothermic effect in the NCDTA curve, simultaneously to respective induction periods. The pozzolanic reactions, due to the presence of the waste silica and alumina containing amorphous phases, consume part of the original Ca(OH)₂ content existent in the waste in the case of 30 and 40% substituted pastes, and also from part of the Ca(OH)₂ produced in cement hydration reactions, in the case of the 10 and 20% substituted pastes.     

Determination of pozzolanic activity of materials by thermal analysis

The results of pozzolanic activity determination using DTA-TG method are presented. This feature was characterised by Ca(OH)₂ residue determination in cement pastes admixtured with siliceous earth, consuming the calcium ions from hydrolysis of cement clinker minerals. The rate of pozzolanic reaction was thus estimated. Some results for fly ash containing pastes were also given. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermomechanical treatment of two Ecuadorian zeolite-rich tuffs and their potential usage as supplementary cementitious materials

Two Ecuadorian zeolite-rich tuffs, coded as Zeo1 (Mordenite) and Zeo2 (Clinoptilolite–Heulandite–Mordenite), were treated and used as supplementary cementitious materials (SCM) so as to study the effect of the thermal and mechanical treatments on pozzolanic reaction in mortars. The treatment was carried out by means of thermomechanical process according to a central composite-blocked cube-star experimental design. In this experimental design as independent factors the milling time (48, 60, 90, 120 and 132 min) and the heating temperatures (559, 600, 700, 800 and 841 °C) were used but keeping a constant heating time (5 h), and zeolite-rich tuffs as experimental blocks. The proportion of SCM was kept constant i.e. 15 %. On the other hand, the compressive strength, fixed lime, normalized water in hydrates and mg Ca(OH)₂ per mg Cement at 7, 28 and 45 days, as well as hydration products related to dehydration degree of the zeolitic tuffs (DOD) were used as dependent factors. In addition, Quantitative and High-temperature chamber XRD, TG-DSC, particle size distribution and SEM-EDS were also carried out. The most significant factor was calcination leading to increase in the compressive strength compared to control, but just up to 800 °C because of recrystallization. As a conclusion, a meaningful loss of crystal structure of zeolites was not observed; but instead, the treatment could only lead to removing the water in cages and voids, thus improving the reaction with Ca(OH)₂, producing more hydrates. In addition, lime was not only fixed by pozzolanic reaction, but also by carbonation.     

Investigation of the cellulose ethers effect on the Portland cement hydration by thermal analysis

Cellulose ethers (CE) are introduced in almost all cement-based dry mortars in order to retain water in mortar mass avoiding losing it too quickly by substrate absorption or water evaporation. In this way the workability of the fresh material, the adherence to the substrate and internal-strength characteristics of mortar, render or tile adhesive are improved. One of the side effects of cellulose ethers is the Portland cement hydration delaying. The influence of six commercial cellulose ethers, hydroxyethylmethyl cellulose (HEMC) type, on the hydration of Portland cement CEM I 42.5 R, was followed by thermal analysis (TG and DTA curves). Three of these cellulose ethers are unmodified, and have different viscosities, while three of them have the same viscosity but differ in the degree of modification (unmodified, one with medium modification and one with high modification). The interest of dry mortars producers for the effects of these cellulose ethers, is generated by the wide offer available on the market and by the absence of systematic data on the effect of different viscosities and degrees of modification on dry mortars properties. In order to quantify the effect of the CE on the cement hydration, the surface area of the endothermic effect corresponding to the dehydration of portlandite (Ca(OH)₂), formed after 1, 3, and 7 days of hydration, was defined. It was noted that the proportion of Ca(OH)₂ in samples containing CE after 1 day was 30–40 % lower than in reference sample. After 3 and 7 days of hydration the proportion of Ca(OH)₂ in samples containing CE approaches that of reference sample (10–20 % less). For the same period of hydration, the different viscosity, and different degree of modification of cellulose ethers cause variations in narrow limits of the proportion of Ca(OH)₂, and the degree of cement hydration, respectively.     

Reactivity of fly ash in cement paste studied by means of thermogravimetry and isothermal calorimetry

Four paste mixtures with varying replacement level of the cement content by fly ash have been studied. Due to fly ash, the acceleration period decreased and a third hydration peak was noticed with isothermal calorimetry. The total heat after 7 days increased with increasing fly ash content. From 1 to 7 days, thermogravimetry showed a higher chemically bound water and Ca(OH)₂-content for the pastes with fly ash. Between 7 and 14 days the calcium hydroxide started to be depleted due to the pozzolanic reaction. A unique relation was found between calcium hydroxide and total heat development.     

Pozzolanic properties of a residual FCC catalyst during the early stages of cement hydration

The catalyst used in fluidized catalytic cracking (FCC) units of refineries after several recovery cycles in regeneration units, reduces its activity and it is partially substituted by new catalyst in the process. As it has a high silicon and aluminum oxides content, the pozzolanic properties of a Brazilian FCC spent residual catalyst, used in different substitution degrees to cement, were evaluated by three thermal analysis techniques during the early stages of hydration of a type II Portland cement. NCDTA curves show in real time that the residual catalyst, accelerates the stages of cement hydration. TG and DSC curves of respective pastes after 24 h of hydration evidence the pozzolanic activity of the waste, respectively, by the lower water mass loss during the dehydroxylation of the residual calcium hydroxide and by the lower dehydroxylation endothermal effect. Within the analyzed period, the higher is the cement substitution degree, the higher is the pozzolanic activity of the residual catalyst.     

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.     

Synthesis,characterization and TG-DTA study of diethyl 5-(4-hydroxyethoxyphenylazo)isophthalate

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.     

Lessons learned from fires of the wood caused by the spontaneous combustion of coal dust in underground mines

The capture of CO₂ and SO₂ from industrial gas effluents has been done usually by lime-containing products. For this purpose, cement pastes also can be used, due mainly to their calcium hydroxide content formed during hydration. To select the best cement for this purpose, TG and DTG curves of different Portland cement pastes (types I, II, III and G), prepared with a water-to-cement ratio (W/C) equal to 0.5, were analyzed at different ages, at same operating conditions. The curves were transformed into respective cement calcined and initial mass basis, to have a common and same composition reference basis, for a correct quantitative hydration data comparison. This procedure also shows that there is an unavoidable partial drying effect of the pastes before starting their analysis, which randomly decreases the W/C ratio at which were prepared, which indicates that, when results are compared on respective paste initial mass basis, assuming that the ratio W/C has not changed, possible calculation errors may be done. Type I, II and G analyzed cements have shown similar hydration characteristics as a function of time, while the analyzed type III cement has shown a different hydration behavior, mainly due to its highest Al₂O₃ and lowest SO₃ contents, promoting the formation of hydrated calcium aluminates, by the pozzolanic action of the excess of alumina, consuming Ca(OH)₂, which final content at 28 days was the lowest one, among the hydrated cements.     

Hydration heat evolution of high-belite cement–phosphate slag binder

The influence of phosphate slag with different finenesses and activators on the hydration of high-belite cement has been studied by using the hydration heat of binders, the DTA curves, the SEM images, and the specific strength. Results indicated that doped phosphorus slag in the cement will reduce heat of hydration. The activity of phosphate slag was low at early stage, but pozzolanic activity of phosphorus slag is higher than that of fly ash. Increasing the specific surface area and curing time and using Ca(OH)₂ combined with gypsum can clearly promote the hydration degree of phosphorus slag. The findings in this paper show that since phosphorus slag can promote the hydration of high-belite cement, the strength contribution of cement is increased. Moreover, the greater the specific surface area is, the more significant the promotion effect at 90 d is.

     

Thermal analysis and microstructure of Portland cement-fly ash-silica fume pastes

Thermal analysis (thermogravimetry and differential thermal analysis) was used with scanning electron microscopy technique to investigate the hydration mechanisms and the microstructure of Portland cement-Fly ash-silica fume mixes. Calcium silicate hydrate (C–S–H), ettringite, gehlenite hydrate (C₂ASH₈), calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃) phases were detected in all mixes. In the mixes with the use of silica fume addition, there is a decrease in Ca(OH)₂ with increasing silica fume content at 5 and 10% compared to that of the reference Portland-fly ash cement paste and a corresponding increase in calcium silicate hydrate (C–S–H).     

Fast physics-chemical and calorimetric characterization of natural pozzolans and other aspects

This research reports on the effects of including natural pozzolans in two Portland cements with different mineralogical compositions, with and without excess gypsum at amounts equivalent to 7.0% SO₃. The main analytical techniques used to study these effects were: the amount of water needed to make a paste of normal consistency, the 2-day Frattini pozzolanicity test and conduction calorimetry. The results obtained showed that these natural pozzolans caused contradictory (accelerating and retarding) effects on the rheology of the resulting cements, depending on the mineralogical composition of the respective Portland clinkers as well as the reactive chemical composition of the pozzolans, in particular their reactive alumina content (Al₂O₃ ^r−). The addition of gypsum also caused acceleration and delays in the calorimetric evolution of the resulting pastes, which proved to be heavily dependent upon the more or less aluminic chemical character of the natural pozzolans studied. This, in turn, was conditioned by the higher or lower Al₂O₃ ^r− content (for the SiO₂ ^r− content was of a very similar order of magnitude in all three pozzolans analyzed). The Al₂O₃ ^r− content was likewise responsible for paste behaviour in the afore-mentioned trials and analyses, and the pozzolanic activity exhibited by the compound was found to be more specific than generic, indirectly stimulating C₃A hydration more intensely and rapidly than C₃S hydration in PC1, one of the two Portland cements used. Indeed, when these natural pozzolans exhibited such prior pozzolanic activity in the second cement studied, PC2, the hydration of its 79.5% of C₃S was not indirectly stimulated to the same degree; rather, the contrary effect was observed, i.e., this cement was physically diluted by the three pozzolans. Pozzolan O stimulated hydration directly and non-directly more than indirectly, while pozzolan C acted conversely, and A exhibited varying combinations of the two patterns. The physical state of the reactive alumina, Al₂O₃ ^r−, in these three natural pozzolans, must be more amorphous than vitreous, i.e., resembling metakaolin more than fly ash in this regard. That notwithstanding, the reactive alumina content in each pozzolan must have conditioned the water/cementitious material ratio obtained for the respective blends with both types of Portland cement (a finding that could be used in future for speedy, simple, reliable and economical characterization), as well as their specific pozzolanicity developed and the rate and total heat of hydration generated by such blended cements.     

Investigation regarding the effect of viscosity modifying admixtures upon the Portland cement hydration using thermal analysis

The effect of 13 viscosity modifying admixtures (VMA) on the Portland cement hydration was studied in this paper. In this purpose, thermal analyses (DTA and TG) were performed after 1, 7 and 28 days of hydration on cement pastes containing 0.01–0.5 % from the following VMA: diutan gum, welan gum, polygalactomannane ether, natural cellulose fibres, modified polysaccharide, polyacrylamide, high-molecular mass synthetic copolymer, hydroxypropyl starch and a chemically modified starch. It was noticed that the proportion of Ca(OH)₂ from the samples containing polygalactomannane ether and modified polysaccharide was smaller than in the reference sample, which proved their effect of cement hydration delay. For the other VMA, this effect was not detected, on the contrary, the amount of Ca(OH)₂ was higher than in the reference sample.