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An alkali-free liquid accelerator was prepared with aluminium sulfate and aluminium formate as the main components. The effects of the accelerator on the setting of paste and the mechanical strength of mortar were studied, and the impact of the accelerator on the early hydration behaviour and hydration products of the paste were investigated by isothermal calorimeter analysis, X-ray diffraction analysis, scanning electron microscopy and thermogravimetric analysis. The results show that the accelerator is excellent for the rapid setting and hardening of the cement. When the content of the accelerator was 8 wt%, the initial setting time and final setting time of the paste were 85 s and 180 s, respectively, and the strengths of the mortar at 1 day and 28 days were 9.52 MPa and 44.42 MPa, respectively. The proposed accelerator can accelerate the dissolution and hydration of tricalcium aluminate and tricalcium silicate and promote the formation of ettringite, formate and portlandite. [ABSTRACT FROM AUTHOR]

The AFm phase found in hydrated Portland cements refers to a family of calcium aluminate phases. Their layer structure is derived from that of portlandite (calcium hydroxide (Ca(OH)2)), but with one third of the Ca2+ ions replaced by a trivalent ion, nominally Al3+ or Fe3+. The resulting charge imbalance gives the layers a positive charge, which is compensated by intercalated anions; the remaining interlayer space is filled with water (H2O). Hydrated cements will contain mixtures of AFm phases and their solid solutions. Portland cement is often modified by addition of soluble nitrate or nitrite salts to protect embedded steel or to shorten the setting time. These nitrate and nitrite ions are capable of entering and occupying AFm anion sites, hence impacting overall phase balances between cement hydrates. In this study AFm phases and solid solution formations were investigated between AFm phases containing SO42−, CO32−, OH−, Cl− and NO2− NO3− ions. Samples were synthesised and subsequently characterised using X-ray diffraction and through the measurement of aqueous solution compositions after 180 days of equilibration in water. [ABSTRACT FROM AUTHOR]

Calcium aluminate cement (CAC) is advantageous in marine engineering, and using materials directly from the sea when mixing CAC can reduce both construction costs and ecological pollution to the sea. However, using seawater as the mixing water in CAC is difficult due to the strength reduction. In this work, CACs were prepared with siliceous minerals (SMs) (diatomite and silica fume (SF)) and sodium chloride solution (SCS) as the mixing water. For comparison, other samples were immersed in SCS rather than mixed with SCS. The strength, mass and length changes of CAC mortars cured at 20°C and 40°C were determined. The phase assemblages and morphology were assessed using X-ray diffraction, thermogravimetric-differential scanning calorimetry and scanning electron microscopy. Using SCS as the mixing water was beneficial to the later strength development of CAC. It promoted the formation of C2ASH8 in CAC pastes and showed no strength loss even when cured at 40°C. The strengths of the SM-modified CAC mortars mixed with SCS were higher than those immersed in SCS. The combination of diatomite and SCS significantly increased the CAC strength, especially at 20°C, while SF contributed to better stability in mass and length. [ABSTRACT FROM AUTHOR]

The effects of replacing OPC with three types of calcium sulfoaluminate (CSA) cements (40 wt%, 20 wt%) were studied. The results were broadly consistent with previously published data, with a few notable exceptions. Because of the production of ettringite, setting times were decreased. The maximal hydration temperature increased in concretes containing 40 wt% CSA but dropped in concretes with 20 wt% replacement. The drop was caused by a lack of accessible sulfates, which limited the synthesis of ettringite. In comparison to OPC concretes, the inclusion of additional anhydrite and calcium oxide was related with a delayed formation of the second temperature peak. Their excess slowed the calcium aluminate and belite reactions and reactivated the synthesis of ettringite, C–S–H, and portlandite. The effects of aluminum hydroxide were also indicated as potentially significant, although this was not proven experimentally in this study. The slightly lower compressive strength reported for mixes containing 40 wt% CSA was associated with better-formed ettringite. The same was identified as the key to lowering total shrinkage in mixtures containing 40 wt% CSA and increasing for lower CSA replacement levels. In that case, the insufficient amount of ettringite generated resulted in expansion that was insufficient to alleviate or compensate for the developed shrinkage. [ABSTRACT FROM AUTHOR]

The main contribution of this research is the study of the effect of thermal inertia in the activation energy in calcium aluminate cement pastes. For this, several formulations of calcium aluminate cement pastes (CACPs) with 51 wt% and 71 wt% of alumina (Al2O3) were made at 0.4 water-to-cement ratio, and with additions of 0.0 wt% and 20 wt% of silica content. The characterisation was done using X-ray fluorescence and X-ray diffraction, and the behaviour of the phases with temperature was studied using thermal gravimetric analysis, including the corresponding derivative thermogravimetry (DTG). The influence of the thermal inertia on the activation energies and its dependence on the heating rate is established. The effect of thermal inertia on peaks of DTG curves and activation energies is analysed. The activation energies for dehydroxylation of the gibbsite were calculated based on the Kissenger–Akahira–Sunose method. The activation energy values obtained, in kJ/mol, for CACP71 – 0, 20 wt% and CACP51 – 0, 20 wt% silica content were 19.9, 6.37 and 19.8, 17.2, respectively. The influence of silica on phase formation, activation energies and the effect of thermal inertia is also analysed. [ABSTRACT FROM AUTHOR]

The objective of this study was to explore the influence of an alkali liquid accelerator based on sodium aluminate (ASa) on the early hydration behaviour of cement paste, tricalcium aluminate (C3A) and tricalcium silicate (C3S) and identify its accelerating mechanism. The setting times and mechanical properties of cement paste and mortar mixed with different amounts of ASa were determined. Isothermal calorimetry showed that the addition of ASa increased the first exothermic peak of cement hydration and advanced the second exothermic peak. The morphology of hydration products was analysed by scanning electron microscopy (SEM) and a model for the early hydration of cement paste was established. X-ray diffraction and SEM were also used to analyse the phase composition and morphology changes of C3A and C3S with or without ASa and early hydration models were established. Analysis of the experimental results revealed that the sodium hydroxide produced by ASa hydrolysis had the function of consuming gypsum, breaking calcium silicate hydrate gel film and increasing the calcium hydroxide concentration in the cement paste. The ettringite in the hydration products interweaved together to form a skeleton structure, which was conducive to cement paste hardening. [ABSTRACT FROM AUTHOR]

Strict environmental norms and increasing awareness of the need to recycle solid wastes generated during ironmaking and steelmaking processes are the key driving force for the development of various new materials and manufacturing technologies. In this study, a calcium aluminate clinker was prepared from steel ladle slag by modifying its mineral composition. The slag paste, prepared by mixing slag with water, exhibited flash setting behaviour due to the presence of C12A7 and C3A phases. In contrast, the slag clinker, developed by sintering a mixture of a pre-determined quantity of slag and aluminium oxide at 1400°C for 2 h and 4 h, contained CA, CA2, gehlenite and 'Q' phases. Hydrated slag clinker contained stable C3AH6, AH3 and stratlingite with preferential growth of calcium aluminate hydrate prisms along the c-axis, which provided a well-defined raceme-like morphology with an interlinked structure. This improved the setting time and crushing strength of the clinkers after 6 h and 24 h of curing at room temperature. Additionally, the presence of the Q phase, with lamellar prismatic crystals, also helped to enhance strength. The developed clinker also exhibited superior crushing strength as compared with commercially available calcium aluminate cement of medium purity. The slag, used as a source of calcium oxide, could replace calcium carbonate completely and thus contribute to reductions in carbon dioxide emissions during the clinker-making process. [ABSTRACT FROM AUTHOR]

The early hydration process of calcium aluminate cement (CAC) was characterised by low-field nuclear magnetic resonance (LF-NMR), isothermal calorimetry (IC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was revealed that LF-NMR characterisation of the hydration process gives results very similar to those of IC; both show that the hydration reaction is faster in about 10–17 h. In addition, the hydration process of CAC could be quantitatively divided into four stages according to the second derivative of LF-NMR signal amplitude. LF-NMR is also used in the study of CAC microstructure. The results of the T2 distribution curve prove that there are mainly two kinds of pores of different sizes in the slurry, and they gradually become smaller, especially in more than 10 h. The weighted average T2 and specific surface area decreased and increased, respectively, due to the gradual densification of the slurry. Owing to the conversion of CAH10 and C2AH8 to C3AH6, the solid specific surface area decreased, resulting in the LF-NMR specific surface area curve slowing down or even decreasing in about 20–50 h of hydration. XRD and SEM results further validate the characterisation of hydrated CAC microstructure by LF-NMR. [ABSTRACT FROM AUTHOR]

Replacement of Portland cement (PC) by fly ash (FA) is currently limited to 15–30% by mass, mainly due to low early age strength development of concrete. This research uses calcium formate (Ca(HCO2)2; CF) as an admixture to high-volume FA (HVFA) composites to improve its strength properties. HVFA represents 60–70% of cement replaced by FA and dosage of CF varies from 0.5% up to high dosage of 9% of cement content. Compressive strength, isothermal calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy were conducted to investigate the effects of CF on hydration and microstructural aspects. The results show that both HVFA pastes with 60% and 70% FA achieved the highest strength at the CF dosage of 3%. At the age of 28 days, adding 3% CF to HVFA mixes led to higher consumption of FA as well as higher formation of calcium hydroxide (Ca(OH)2), calcium silicate hydrates, calcium carbonate (CaCO3) and ettringite, which contribute to the increase of strength. The addition of very high dosages of CF at 9% increased the hydration of tricalcium aluminate but could hinder the hydration of tricalcium silicate in both PC and HVFA pastes with 60% and 70% FA. [ABSTRACT FROM AUTHOR]

Chloride-based accelerators are widely used to accelerate cement hydration; however, there is no focus on the effect of the interaction between Cl− and SO42− on cement hydration. Hence, the interaction effect between initial gypsum and sodium chloride (NaCl) in the cement hydration process was investigated. Isothermal calorimetry, scanning electron microscopy, thermogravimetric analysis, X-ray diffraction and inductively coupled plasma spectroscopy were used to evaluate the hydration process of cement paste. Moreover, a thermodynamic model and boundary nucleation and a growth model were applied to calculate the hydration products and describe the hydration process, respectively. It is clear that sodium chloride can accelerate cement hydration, but as the incorporated sodium chloride increases, the acceleration effect is weakened. The incorporation of chloride salts will promote the dissolution of minerals and increase the growth rate. In the early stage of hydration, the formation of ettringite precedes formation of Friedel's salt, owing to the fact that tricalcium aluminate (C3A) favours binding with SO42−, which can be demonstrated by the change in ion concentrations and the evolution of hydration products. The formation of Kuzel's salt depends not only on the chloride concentration but also on the hydration time. [ABSTRACT FROM AUTHOR]

The aim of this study was to improve the acid resistance of a metakaolin-based geopolymer (MKG) by substituting metakaolin with calcium aluminate cement (CAC). The CAC was added at weight ratios of 5% and 10%. The raw materials were activated with a mixture of sodium hydroxide solution and sodium silicate solution. The cured MKG mortars were immersed in a sulfuric acid solution with a pH of 2 for 75 days. Tests for the volume of permeable voids, unconfined compressive strength (UCS) tests, thermogravimetric analysis and Fourier transform infrared spectroscopy were conducted before sulfuric acid immersion. It was found that the addition of CAC facilitated geopolymerisation and reduced the volume of permeable voids of the MKGs. Changes in UCS, ultrasonic pulse velocity, mass and length were monitored to evaluate acid resistance. It was found that the addition of CAC dramatically improved the acid resistance of the MKG due to the reduced volume of permeable voids and enhanced neutralisation capacity. The mixture with 10% CAC showed the lowest deterioration. [ABSTRACT FROM AUTHOR]

During recent decades, alkali-free accelerators have appeared on the market as setting accelerator additives for shotcrete. In this work, copolymers poly(AANa-co-DMAA) with a variation of their charge densities (monomeric molar ratios) were synthesised by aqueous solution polymerisation and characterised by Fourier transform infrared spectroscopy. Then the copolymers were used to optimise the setting behaviour of cement paste in the presence of alkali-free accelerator. The setting times of cement pastes with a combination of alkali-free accelerators and copolymers were tested. Isothermal calorimetry, X-ray diffraction, thermal gravimetric analysis and total organic carbon were also performed to obtain the comprehensive optimisation effects of poly(AANa-co-DMAA). The results clearly show that the capacity of the alkali-free accelerator for shortening the setting time of cement paste was enhanced significantly by introducing poly(AANa-co-DMAA). More AA units in the copolymers have better adsorption capacity, leading to a faster dissolution rate of tricalcium aluminate and the formation of massive ettringite crystals, thus finally the setting time of the cement paste with a combination of alkali-free accelerators and copolymers was shortened markedly compared with that containing alkali-free accelerators alone. The good synergy effect of the copolymers and alkali-free accelerator indicates great potential for practical applications. [ABSTRACT FROM AUTHOR]

Different liquid accelerators including sodium aluminate liquid accelerator (SAa), fluoaluminate liquid accelerator (Fa) and aluminium sulfate liquid accelerator (ASa) were selected to investigate their influences on the carbonation properties and morphology of tricalcium aluminate (C3A) and tricalcium silicate (C3S) hydration products. After an accelerated carbonation test, the relative contents of ettringite (AFt), calcium hydroxide (CH) and calcium carbonate (CaCO3) in the hydration products of C3A and C3S samples were analysed by thermogravimetric analysis. The results indicated that the hydration products of C3A and C3S with liquid accelerators were easier to carbonate, and the promotion effect of SAa was the most obvious. The morphology of hydrated products of C3A and C3S was observed by scanning electron microscopy analysis. It was observed that different liquid accelerators had a different influence on the morphology of C3A and C3S hydration products. Among them, ASa had the greatest influence on the morphology of C3A and C3S hydration products, and experiments proved that ASa could promote the transformation of AFt into AFm and make the CH crystal structure become thicker and larger. [ABSTRACT FROM AUTHOR]

Concretes, depending on their application, may be exposed to elevated temperatures during their lifetime, especially when placed near or in a furnace or during an accidental fire. The thermal stability of alkali-activated mortars and hydrated mortars prepared using ladle slag and two different aggregates was studied. A commercial calcium aluminate cement was used as a comparison for the properties after exposure to temperatures up to 1000°C. All samples were characterised and analysed through a strength test, dilatometric analysis, thermogravimetric analysis, ultrasonic pulse velocity and scanning electron microscopy. The hydrated ladle slag mortars exhibited properties similar to or better than the commercial calcium aluminate cement, while the alkali-activated ladle slag's thermal properties were inferior to the hydrated ladle slag mortars. The tests also showed that the use of ferrochrome slag aggregates in hydrated ladle slag mortars exhibited similar properties to those using sand aggregate mortars. The damage behaviours of the mortars were dependent on the temperature-induced dehydration, dehydroxylation and thermal incompatibility between the paste and aggregates. The results from this study specify the possibilities of using 100% industrial by-product-based mortar and near-zero carbon dioxide emissions as a fire-resistant material in furnaces, buildings and fireplaces. [ABSTRACT FROM AUTHOR]

The influence of sodium chloride (NaCl) on the hydration evolution of calcium aluminate cement (CAC) cured at 5°C, 20°C and 40°C was investigated by calorimetry, X-ray diffraction, differential scanning calorimetry–thermogravimetry, scanning electron microscopy and mercury intrusion porosimetry. The chloride binding capacity and compressive strength of the CAC were also determined. The results showed that the chloride binding capacities of all the CAC samples increased with an increasing sodium chloride content, but the trend of growth gradually slowed. The influence of sodium chloride on the hydration of CAC was found to be highly dependent on its dosage and the curing temperature. At 5°C and 20°C, the early hydration of CAC was accelerated by a small amount of sodium chloride, leading to greater amounts of CAH10. Additionally, the pore structure of hardened cement paste was optimised for a higher compressive strength. However, excessive sodium chloride was found to delay the early hydration to some extent. When the curing temperature was increased to 40°C, sodium chloride accelerated the phase conversion, accompanied with an increase of harmful pores and a sharp decrease in compressive strength. The findings indicate a potential application of CAC-based constructions made with sea sand, especially at temperatures below 20°C. [ABSTRACT FROM AUTHOR]

This study examines the mechanism of adsorption–dispersion by superplasticisers with varying side chain density in cement paste. Acrylic acid was used to copolymerise with 2-methylprop-2-ene polyethylene glycol, using chain transfer agent to synthesise polycarboxylate ether superplasticisers (PCEs) with comparable main chain lengths and different side chain densities. Gel permeation chromatography, zeta potential, total organic carbon and fluidity tests were used to investigate the adsorption–dispersion of PCE samples in cement pastes with different amounts of tricalcium aluminate (C3A) additive. The results indicate: products with higher side chain densities also have correspondingly shorter main chains, which allows them to form conformations with comparable initial adsorption performance; with C3A added, samples with denser side chains performed better, and their resistance to adsorption became more prominent at higher C3A content. [ABSTRACT FROM AUTHOR]

A repair material for use in the marine environment is developed. The ternary cementitious system (TCS), which contains ordinary Portland cement, calcium aluminate cement and gypsum, causes rapid hardening and displays high strength, and hence can be used as a building repair material. However, the hydration products of the TCS contain a large amount of ettringite, calcium hydroxide and calcium aluminate hydrate, which can be easily corroded by the inorganic ions in seawater. Owing to its pozzolanic activity and ability to bind chloride, slag is added to the TCS. The content of slag in the TCS is a critical factor; hence, the effects of 0–35 wt% slag on the compressive strength, mass change, apparent porosity, and free and total chloride content of TCS immersed in seawater are investigated. The systems with slag show a higher compressive strength than the TCS in seawater, and the reduction in compressive strength of the system with 25 wt% slag is only 14·14% after 180 d. Also, the addition of slag significantly reduces the free and total chloride content of the TCS immersed in seawater. The optimum amount of slag in the TCS is determined to be about 25 wt%. [ABSTRACT FROM AUTHOR]

The main goal of the investigation reported in this paper was the preparation of a novel aromatic admixture and its use for improving the physicochemical and rheological properties of fresh and hardened ordinary Portland cement (OPC) pastes. The effects of the addition of 0·25, 0·50, 0·75 and 1·00% (by mass of OPC) of the novel aromatic organic admixture (sodium 5,5′-diformyl-2,2′-dioxido-1,1′-binaphthyl-4,4′-disulfonate (SDD)) on the hydration characteristics, rheological and mechanical properties of OPC pastes were studied. The admixture was characterised using elemental analysis, infrared spectra and hydrogen magnetic resonance spectra techniques. The results revealed that the addition of SDD to OPC pastes caused a notable improvement in both rheological and mechanical properties of various hardened cement pastes at almost all ages of hydration. The results of X-ray diffraction analysis, differential scanning calorimetry and thermogravimetric analysis obtained for the neat and admixed OPC hardened pastes indicated that the main hydration products formed were nearly amorphous and microcrystalline calcium silicate hydrates, gehlenite-like calcium alumino-silicate hydrate and/or calcium aluminate hydrate and portlandite. [ABSTRACT FROM AUTHOR]

Cementitious fire-retardant coatings are widely used for fire protection of structures. Recently, blended pastes of ordinary Portland cement (OPC) and calcium aluminate cement (CAC) have started to be used as binders in cementitious fire-retardant coatings, but the fire resistance and chemical composition of various OPC/CAC pastes are still not clear. The residual compressive strengths of binary cement pastes with various OPC/CAC ratios after fire exposure were studied in this work. The mineral composition and microstructure of the pastes were characterised by X-ray diffraction and environmental scanning electron microscopy/energy dispersive spectrometry, respectively. It was found that the paste with an OPC/CAC ratio of 60/40 was the most fire-resistant. [ABSTRACT FROM AUTHOR]

Ye'elimite (C4A3$) is the dominant phase in calcium sulfoaluminate cement. Through substitutions of Sr2+ ions for Ca2+ ions in C4A3$, Sr-bearing ye'elimite can be synthesised. In this paper, a kind of Sr-bearing ye'elimite was synthesised with the formula 3CaO·SrO·3Al2O3·SO3 (abbreviated as C3ŠA3$); the characterisation and hydration process of C3ŠA3$ are reported here. Comparing to C4A3$, mineralogical analysis of C3ŠA3$ was conducted, and Rietveld quantitative phase analysis was employed to quantify the hydration products of C3ŠA3$ and correlate to calorimetric data. Results show that C3ŠA3$ accounts for over 93% in the clinker, and the ACn contents are insignificant in terms of amounts. Without gypsum, the hydration process of C3ŠA3$ is more retarded and prolonged than C4A3$ at an early stage. In the hydration products of C3ŠA3$ at 28 d, AFt accounts for 28·2% and AFm occupies 13·4%; CAH10 is quantified to be 5·5%. Compared to C4A3$, C3ŠA3$ tends to yield more AFt. As regards C3ŠA3$ hydrating with a certain amount of gypsum, the hydration rate of C3ŠA3$ is accelerated to approach C4A3$ blended with gypsum. In the paste of C3ŠA3$ blended with gypsum at 28 d, AFt occupies the majority of crystalline phases to take up 42·1%; minor phases AFm and strontium sulfate account for 3·1% and 7·4%, respectively. [ABSTRACT FROM AUTHOR]

Heated cementitious pastes could be used as thermal energy storage materials. The effects of nano-silica, nano-magnesia and nano-zirconia on the evolution of the mechanical and thermal properties of calcium aluminate cement (CAC) pastes at three temperatures (105, 350 and 900°C) were investigated. At 105°C, the addition of nano-zirconia resulted in better residual compressive strength than those of the pastes with the other nanoparticles. The improvement in thermal properties of the pastes with added nanoparticles was mainly at temperatures of 350°C and 900°C. The main reason for the difference of properties is that the incorporation of nanoparticles accelerates the forming process of the solid phase according to calorimetric analysis. The microstructure of the pastes was also found to be affected by the nanoparticles. X-ray diffraction and thermogravimetry analyses were conducted to investigate the phases and the mass changes, respectively. The experimental results show such nanoparticle optimisation would enable the development of novel forms of functional CAC materials for use at high temperature. [ABSTRACT FROM AUTHOR]

Calcium aluminate cements (CACs) are used in combination with calcium sulfates as binders in self-levelling mortars. The influence of commercial CAC/anhydrite ratio on the solid phase assemblage and pore solution at water to binder (w/b) ratios of 0·40 and 0·72 was investigated between 1·5 h and 91 d of hydration using a multi-method approach including x-ray diffraction, thermogravimetric analysis and thermodynamic calculations. The reduction of the extent of hydration at the lower w/b ratio can be explained by a lack of water for complete hydration and lack of space. In the system with a CAC/anhydrite weight ratio of 1, ettringite and AH3 were the main hydration products, whereas in the case of a CAC/anhydrite ratio of 2·3, in addition to ettringite and AH3, monosulfate was present along with strätlingite and at a later age. In contrast to the thermodynamic modelling results, anhydrite persisted even at low calcium sulfate contents as the precipitation of AH3 around the anhydrite grains seemed to lead to a chemical blocking. [ABSTRACT FROM AUTHOR]

The use of styrene-butadiene rubber (SBR) polymeric latexes is common in cementitious materials intended for repair and precast works to improve bond to existing substrates and durability of structures. Limited information exists regarding the feasibility of structural grade lightweight self-consolidating concrete (LWSCC) containing such additions, including the altered stability and bond properties. Four series of LWSCC mixtures prepared with various lightweight aggregate (LWA) and SBR concentrations were considered in this project; the free water was adjusted to secure compressive strength of 40 ± 3·5 MPa. The resulting unit weight varied from 1790 to 2280 kg/m3. Test results have shown that SBR additions reduced concrete flowability and passing ability; however, they led to improved static stability, for example in terms of bleeding, segregation and floating of LWA. Special emphasis was placed on highlighting the altered stability responses with respect to the European guidelines for SCC. Remarkable improvement in splitting tensile strength and bond to existing concrete substrates was noticed with SBR additions. A Ψ-factor was developed to predict the combined effect of free water, viscosity modifier, LWA and SBR on the stability and hardened properties of polymer-modified LWSCC. [ABSTRACT FROM AUTHOR]

The formation of calcium aluminate hydrates in Portland cement pastes modified with two organic admixtures, hydroxyethyl methyl cellulose (HEMC) and ethylene-vinyl acetate redispersible polymer powder (EVA), was evaluated by using X-ray diffraction analysis. The results show that both the HEMC and the EVA retard the formation of ettringite and postpone the time to reach maximum ettringite content. However, the stability of ettringite is enhanced and the conversion of ettringite to monosulfate (AFm) is delayed by these two organic admixtures to retain a higher ettringite content in cement pastes at later hydration time. With calcite powders contained in EVA as the anti-blocking agents, the formation of two AFm phases, hemicarboaluminate and monocarboaluminate, is promoted by EVA after 3 d of hydration. Accordingly, more types of calcium aluminate hydrates coexist in the cement paste modified with EVA than in the control paste or in cement paste modified with HEMC. Also, having been synchronously mixed in the cement paste, these two organic admixtures have obvious superposition effects on the formation and evolution of calcium aluminate hydrates. [ABSTRACT FROM AUTHOR]

Approaches to improve the early strength of geopolymer mortar were studied in order to apply it to a fast-tack concrete pavement repair. The effects of modifying the molar ratio of silicon dioxide/sodium oxide (SiO2/Na2O) of the alkaline solution in a conventional geopolymer mixture and the addition of calcium aluminate cement and slag were studied. The early strength measured in the experiments was at curing times of 8 and 24 h, which are typically construction controlling times for pavement repair. It was found that a molar ratio of silicon dioxide/sodium oxide of 1·0 gave the shortest curing time and the highest early strength after 24 h. Adding calcium aluminate cement was found to reduce the curing time, but decrease the early strength. Slag substitution helped workability, but reduced the early strength at 8 and 24 h. Durability was also tested for the purpose of finding an economical pavement repair solution. A freeze-thaw durability experiment of the suggested metakaolin-based geopolymer concrete was conducted. It was found that a mix with a silicon dioxide/sodium oxide molar ratio of 1·0 produced higher durability than a mix with a silicon dioxide/sodium oxide molar ratio of 1·4. The geopolymer concrete mix with slag showed less weight loss than the other mixes. [ABSTRACT FROM AUTHOR]

The short-term resistance to sulfuric acid at 90°C of four calcium aluminate cement (CAC)-fly ash class F (FAF) blends activated with sodium metasilicate (thermal shock resistant cements (TSRCs)), cured at 300°C, was compared to that of a calcium phosphate cement (CPC) (CAC-FAF blend activated with sodium hexametaphosphate) and a Portland cement class G/silica blend. The mechanical properties and compositions of the acid-exposed samples were evaluated by measuring their compressive strength and by means of x-ray diffraction, μEDX (energy-dispersive x-ray spectrometry), thermogravimetric and Fourier transform infrared analyses. All calcium-containing hydrates were sensitive to the conditions of acid exposure. In the TSRC blends, these hydrates included hydrogrossular, feldspar family minerals and zeolites; in CPC, feldspar minerals and phosphate phases; and in the class G/silica blend, portlandite and tobermorite. Crystalline calcium sulfates formed in the acid-exposed surfaces with the exception of the most aluminium-rich TSRC samples where only potassium(sodium) aluminium sulfate, alunite, was detected. This sample underwent the least changes in weight, compressive strength and had the lowest sulfur permeation into the sample core. Calcium sulfates precipitated on sample surfaces limited sulfur penetration into the core of calcium-rich TSRC, CPC and G/silica blends. [ABSTRACT FROM AUTHOR]

The effects of three commonly used superplasticisers (SPs) - sulfonated naphthalene formaldehyde (SNF), sulfonated acetone formaldehyde (SAF) and polycarboxylates (PCB) - on the hydration process, products and microstructure of tricalcium aluminate (C3A) pastes in the presence of gypsum were systematically investigated using isothermal calorimetry, x-ray diffraction, environmental scanning electron microscopy, infrared spectroscopy and magic angle spinning-nuclear magnetic resonance spectroscopy. The results show that the hydration reaction rate of C3A paste at the initial stage (0-2 h) was promoted by the incorporation of SPs, especially in the C3A-SNF paste. Hydration was rapidly reduced at the early stage (2-12 h), but an acceleration in C3A-SNF/PCB pastes occurred again at the late stage. The morphology of the C3A-SNF/SAF paste changed from thin needle-shaped crystals at the initial stages to thick and coarse rod-like crystals at late stages. In the C3A-PCB paste, very few crystals occurred at an early age, while a large amount of prismatic crystals were observed at the later age. Chemical environment studies revealed that the SPs did not visibly change the position or shape of the characteristic bands of aluminates, but only influenced their relative intensities. A predominant resonance, assigned to six-coordinated aluminium as [AlO6] type, was observed for all the pastes with and without SP. The interaction mechanism between SP and C3A was also explored. [ABSTRACT FROM AUTHOR]

Stratlingite, Ca4Al2(OH)12[AlSi(OH)8]2·2H2O, is an AFm phase which appears as a hydration product of aluminium-rich cements. These binders may be calcium aluminate cements, calcium sulfoaluminate cements and also belite calcium sulfo-aluminate (BCSA) cements. The structure of stratlingite is known from single-crystal studies of tiny minerals but their bulk formation, crystal structure and microstructure of powders is poorly understood. This paper reports on the synthesis of stratlingite and a complete structural and microstructural characterisation by synchrotron X-ray powder diffraction, nuclear magnetic resonance, scanning electron microscopy and thermal analyses. The structural and microstructural models have important implications for a correct quantitative phase analysis of stratlingite in cement pastes (for instance, in pastes of BCSA cements). The microstructure of stratlingite formed in cement pastes is highly dependent on the hydration conditions. In BCSA pastes, the (003) line position of stratlingite appears slightly shifted towards higher diffracting angles (lower inter-layered distance) after stopping hydration compared to that of a similar phase present in a paste analysed without stopping hydration. This is related to dehydration and disorder. This shift and peak broadening is even larger when the paste has suffered partial dehydration during curing (apart from stopping hydration). A microstructural study is presented. [ABSTRACT FROM AUTHOR]

For refractory application inert alumina fillers frequently are mixed with calcium aluminate cements to improve particle packaging and to reduce the demand of water for low-cement castables. This finally results in decreased porosity and higher strength. Investigations by isothermal heat flow calorimetry show that the presence of alumina filler material with high specific surface area leads to a shortening of the induction period of calcium aluminate cement hydration. The study also demonstrates that the specific surface area of the chosen alumina filler changes the starting point of the main hydration reaction and in consequence the setting time of the calcium aluminate cement mixture is strongly influenced. During the dormant period the heat flow of calcium aluminate cement hydration with coarse alumina filler material of lower specific surface area is characterised by negative values. By addition of filler with higher specific surface area, the surface available for heterogeneous nucleation can be increased. Owing to more available surfaces the critical energy barrier for stable nuclei formation can be reduced. This results in a positive heat flow during the dormant period and a shift of the main reaction to earlier points in time. [ABSTRACT FROM AUTHOR]

Chloride-induced corrosion of reinforcement is one of the main causes of concrete deterioration in marine environments. The service life of reinforced concrete (RC) structures exposed to chloride ions is closely related to the rate of chloride ion diffusion through the concrete. Chloride binding has a remarkable influence on chloride penetration and hence on the time to corrosion initiation in RC structures. Determination of the apparent chloride diffusion coefficient of concrete takes into account the chloride-binding capacity but not the effect of temperature on the chloride-binding capacity. This paper examines the effect of temperature, from −4°C to 70°C, on the chloride-binding capacity of paste mixes containing supplementary cementitious materials (SCMs) as cement replacements. The results of the current study suggest that Friedel's salt formation is the major cause of the reaction between chloride ions and tricalcium aluminate (CA) and hence the chloride-binding capacity decreases with increasing temperature; the order of chloride-binding capacity as a function of exposure temperature is 22°C > −4°C > 3°C > 35°C > 50°C > 70°C. Both the experimental and modelled results indicate a non-linear relationship between the exposure temperature and the chloride-binding capacity for cement paste containing SCMs as cement replacement. [ABSTRACT FROM AUTHOR]

The effect of polycarboxylate ether (PCE)-based superplasticiser on the fresh and hardened properties of calcium aluminate cement (CAC)-based materials was investigated. Cohesion of the fresh mortar was improved by addition of methyl cellulose, thus avoiding segregation induced by the superplasticiser action. As PCE significantly retarded the setting time, lithium carbonate (LiCO) was investigated as an accelerator by calorimetric measurements. Compressive and flexural strengths, porosity and dynamic modulus were tested after 1 d and 9 d cured at 23°C and after transformation of metastable hydration products to stable ones at 60°C. The combined usage of PCE with methyl cellulose and lithium carbonate showed promising superplasticising properties for CAC that improved properties of the material. [ABSTRACT FROM AUTHOR]

This work assessed the performance of rice husk ash (RHA)-based ASTM type I and type V (low tricalcium aluminate (CA)) blended cement concrete mixtures subjected to a mixed magnesium-sodium sulfate environment for an immersion period up to 48 months. The concrete mixtures comprised a combination of two Portland cements (type I and type V) and two RHA-blended cements with two water-to-binder ratios of 0·35 and 0·45. In addition to fresh and strength properties, X-ray diffraction, differential scanning calorimetry, mercury intrusion porosimetry and rapid chloride permeability tests were conducted on all concrete mixtures to determine phase composition, pozzolanic activity, porosity and chloride ion resistance. Deterioration of concrete due to mixed sulfate attack and corrosion of reinforcing steel were evaluated by assessing concrete weight loss and measuring corrosion potentials and polarisation resistance at periodic intervals throughout the immersion period of 48 months. Plain (type I/V) cement concretes performed better in terms of deterioration and corrosion resistance than RHA-based type I/V blended cement concrete mixtures in a mixed sulfate environment. [ABSTRACT FROM AUTHOR]

Reactive magnesia (MgO) and carbide slag (CS) were used to activate ground granulated blastfurnace slag (GGBS) to stabilise a marine soft clay, and the stabilised clays were subjected to accelerated sodium sulfate attack. The results indicated MgO-GGBS stabilised clay was nearly inert to this sodium sulfate solution. The resistance of CS-GGBS stabilised clay to sodium sulfate attack was lower than that of MgO-GGBS stabilised clay, but higher than that of Portland cement stabilised clay. Unlike CS-GGBS stabilised clay, there was no calcium aluminate hydrate or alumino-ferrite monosulfate produced in MgO-GGBS stabilised clay, and hence no ettringite, which would cause deterioration, formed when exposed to sodium sulfate. [ABSTRACT FROM AUTHOR]

Calcium sulfoaluminate (3CaO·3AlO·CaSO, CAS¯) was synthesised with pure chemical reagents. The adsorption amounts of polynaphthalene sulfonate superplasticiser (NSF) on the surface of CAS¯ mineral and the mineral mixture (CAS¯+Ca(OH)+CaSO·2HO) were detected by ultraviolet-visible absorption spectrometry. The adsorptive behaviours of NSF on the surface of mineral particles in the hydration system were studied. The results showed that the adsorption amount of NSF on CAS¯ mineral and the mineral mixture increased with higher initial concentration, the adsorption amount and the maximum adsorption amount increased with prolonged hydration time, and the adsorption amount and maximum adsorption amount of NSF on the mineral mixture were more than those on CAS¯ mineral in the hydration system within the same hydration time. [ABSTRACT FROM AUTHOR]

The early-age hydration of a number of calcium aluminate cement pastes and mortar materials was investigated using compressive strength and ultrasonic through-transmission measurements. Ultrasonic signals obtained were successfully described in the temporal domain by a single Gaussian wave packet equation giving five characteristic parameters - amplitude , angular frequency , phase shift, wave centre arrival time and signal width. The group velocity of ultrasonic wave propagation ( ) is suggested to be a more sound parameter characterising the material under study than the generally employed ultrasound velocity determined from the 'time of flight' parameter. The combination of parameters showed a linear correlation with the measured compressive strengths for all the samples tested. The physical meaning of the group of parameters is given and its application is further discussed in relation to other testing procedures. [ABSTRACT FROM AUTHOR]

Two commercial comb-like copolymer dispersants (also called polycarboxylate-based superplasticisers, hereinafter called PCs) were employed to investigate their interaction with tricalcium silicate (CS). It is determined that when tricalcium silicate dissolves in deionised water, it releases a large amount of cations (mainly Ca). Those cations adsorb the surface of initially negatively charged tricalcium silicate and finally reach saturation. Through this mechanism, tricalcium silicate attains a slightly positive zeta potential. Thus, it is emphasised that with a larger amount of cations present in the synthetic pore solution, higher zeta potential values are measured. Owing to the positively charged layer of adsorbed cations, anionic PC adsorption occurs by chelating with calcium ions. Ultimately, the adsorbed PCs impart dispersity owing to its steric hindrance from hydrophilic graft side chains. Moreover, the introduced PCs lead to a strong retardation of tricalcium silicate hydration, and consequently have a significant effect on the initial dispersity of tricalcium silicate paste. Results are presented for a better understanding of the interaction between PCs and tricalcium silicate. [ABSTRACT FROM AUTHOR]

Tricalcium silicate (CS) doped with strontium dioxide was sintered at high temperature of 1600°C by solid reaction. The crystal structure and properties of CS doped with strontium dioxide were investigated by means of chemical analysis, X-ray diffraction analysis, infrared analysis and hydration heat analysis. The experimental results show that pure CS crystal has a triclinic crystalline form at room temperature. When doped strontium dioxide concentration is no more than 0·025 M, alite crystal is the same as the pure CS crystalline form, whereas when doped strontium dioxide concentration is in the range 0·025-0·075 M, alite crystal is in monoclinic crystalline form. A small amount of strontium dioxide has the effect of decreasing the free lime content and promoting the formation of alite. Infrared analysis indicates that the addition of strontium dioxide can improve the crystal symmetry of alite. The hydration heat analysis shows that the addition of strontium dioxide can enhance hydration activity of alite and accelerate the hydration process. [ABSTRACT FROM AUTHOR]

In most kiln systems the coal ash is incorporated into the kiln feed, thus affecting the final compound composition of clinker. This work examines the effects of waste ash on clinker phase composition and crystalline structure (microstructure). Clinkers were prepared from mixtures of industrial raw mix with coal ash and waste ash, respectively. The coal ash was substituted on a calorific basis by 50% and 100% of waste ash originating from refuse-derived fuel (RDF). The thermal treatment of the mixtures was monitored by differential thermal analysis (DTA). The phase composition and microstructure of the produced clinkers were investigated by X-ray diffraction and optical microscopy and the level of trace element content was measured. According to the results of DTA, the addition of waste ash shifts clinkering reactions to lower temperatures. The unit cell volume calculations of clinker phases suggest only limited alterations in the crystal lattice of alite and cubic tricalcium aluminate compared to the reference clinker. Free lime tends to increase when moving from coal to RDF ash but decreases with increasing RDF addition. The microscopic study of clinkers did not reveal significant differences. Finally, clinkers produced with waste ash addition meet environmental criteria relating to trace element content. [ABSTRACT FROM AUTHOR]

In this paper the influence of alkali, alkaline earth and transition metals in the form of chlorides salts on the hydration of commercial iron-rich calcium aluminate cement (CAC) is investigated. The effect on setting time and mechanical properties is studied. The results of the setting time obtained from the measured temperature evolution of cement pastes are compared to standard Vicat needle measurements. Addition of alkali metal salts both accelerates the setting time of CAC and deteriorates long-term strengths in the order of decreasing ionic radius of the metals. The effect of alkaline earth metal salts on setting behaviour depends on the amount of addition: low concentrations retard, but high concentrations accelerate setting behaviour. Calcium is found to have the strongest acceleration effect with the lowest deterioration in strength. An interesting regularity is observed when plotting the results of both retarded setting times and strengths against atomic number of added transition metal cation. [ABSTRACT FROM AUTHOR]

The article presents a research that studies the hydration properties of a mixture of tricalcium aluminate and gypsum in a mole ration of 1:1, the reaction was conducted at 30 and 50 degree celsius, both with and without sodium salt of naphthalene formaldehyde sulfonic acid polycondensate (NFS). Also, the techniques of combined water content, X-ray diffraction and scanning electron microscopy were used and it is observed that NFS slows the development of ettringite.

The article presents a study that analyzes the changes in the flexural strength and microstructures of calcium aluminate-phenol resin composite, when added with alkali resistant glass fibers. It is informed that calcium aluminate-phenol resin composites are macro-defect-free cements that presents better water resistant properties. The electron microscopy examination showed that too high fiber content present problems in dispersing of fibers.

The hydration characteristics of a tricalcium aluminate and gypsum mixture with a mole ratio of 1 : 1 in the presence and absence of calcium lignosulfonate at 30 and 50°C were investigated. Different ratios of calcium lignosulfonate were used namely 0, 1 and 3% by weight. The water-to-solid ratio used was 10. Hydration was carried out for various time intervals from 0·5 h up to 90 days. Ettringite and monosulfate hydrate were the only hydration products formed as indicated by X-ray diffraction analysis and scanning electron microscopy. The results of combined water content, X-ray diffraction and scanning electron microscopy indicate that calcium lignosulfonate retarded the formation of ettringite at early hydration ages. The extent of hydration was inversely proportional to the ratio of calcium lignosulfonate. [ABSTRACT FROM AUTHOR]

The article investigates the link of hydration of the clinker minerals tricalcium silicate, tricalcium aluminate and the ordinary Portland cement in early stage of process. It mentions that the Small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) patterns caused by microstructural objects and crystalline phases were observed at 40 degree Celsius room temperature. The result reveals that two types of internal surfaces simultaneously occurred. .