This study aims to investigate the time-dependent characteristics of sodium carbonate activated slag at the early stage and its compressive strength development. The reaction kinetics of Na2CO3 ...activated slag is analyzed at the curing ages of 1h, 6h, 1d, 2d, 3d, 4d and 7d, respectively, and the reaction products are characterized employing FTIR, XRD, TG-DTG and SEM. The results show that a weak reflection of gaylussite is observed after 1 d of curing, while intensive accumulation of gaylussite and formation of hydrotalcite are observed at 2 d/3 d. Meanwhile, the gelation of C-(A)SH after 3 d of curing is identified by FTIR. The role of pH, CO32− anion concentration and the formation of crystals such as gaylussite and hydrotalcite on the reaction process are discussed. In addition, a relationship between the initial alkali concentration and compressive strength at different curing ages of 7 d, 28 d and 180 d is derived.
•Nano-silica can decrease induction period and increase acceleration period.•Excessive nano-silica can inhibit its modification effect on cement hydration.•Nucleation effect of nano-silica in the ...early hydration is confirmed by BNG model.•Nucleation rate is more sensitive and growth rate is less sensitive to temperature.
This study investigated the influence of nano-silica dosages and hydration temperatures on the early-age hydration heat of cement using isothermal calorimetry, and a hydration kinetics model was used to analyze the nucleation and growth rates of hydrates. The calorimetry results show that the addition of nano-silica and the increase of hydration temperature can accelerate the process of hydration reaction. As the dosage of nano-silica increases, the induction period decreases, the acceleration period and exothermic rate peak value increase. The fast pozzolanic effect of nano-silica is responsible for accelerating the hydration reaction. However, the excessive addition (5%) of nano-silica may form many agglomerations, thereby inhibiting the modification effect of nano-silica on the cement hydration. The simulating results of kinetic parameters show that, at three hydration temperatures, the nucleation rate of hydrates increase with the increasing of nano-silica dosages, which indirectly confirms the nucleation effect of nano-silica in the early-age hydration. At 20 °C and 30 °C, the growth rate of hydrates also increases with the increasing of nano-silica dosages. At 40 °C, the growth rate of hydrates is decreased gradually, possibly due to the lack of sufficient Ca2+ and H2SiO42−. In addition, the nucleation rate is more sensitive and the growth rate is less sensitive to the hydration temperature.
The ASTM C311 strength activity index test is a potentially flawed test because inert materials are known to be able to pass the test. The early age of testing, low fly ash replacement levels, ...variable water-to-cementitious materials ratio, and low strength limits all contribute to the inability of the test to successfully distinguish inert and reactive materials. Test modifications utilizing higher replacement levels (50% by mass) and higher temperatures (50 °C) are evaluated in this study for three fly ashes and two inert materials. Bulk resistivity is also measured in addition to strength. Using higher replacement levels and higher temperatures provides improved differentiation of inert and reactive materials when using strength, however, the magnitude of differences is somewhat small, when considering variability in strength measurements. On the other hand, higher replacements and especially higher temperatures result in a very substantial increase in bulk resistivity for the fly ashes but not for inert materials. Therefore, a bulk resistivity index test carried out at higher temperatures provides for clear differentiation of inert and reactive materials and should considered for standardization/specification. The calcium hydroxide contents and heat release measured in corresponding cement pastes show that increased temperature enhances fly ash reaction but does not affect inert materials. At higher replacement levels, greater effects of fly ash reaction are manifested, resulting in better differentiation from inert materials than at lower replacement levels.
The influence of Fe2O3 on the hydration kinetics of tricalcium aluminate (C3A) was studied in order to clarify the mechanism of improving hydration resistance of CaO by in-situ synthesized tricalcium ...aluminate. The Krstulovic-Dabic model was used to investigate the hydration processes of Fe2O3-C3A solid solution and calculate the corresponding kinetic parameters. The hydration products were analyzed by the X-ray diffraction and scanning electron microscope. The results indicated that the Krstulovic-Dabic model simulated the hydration processes of Fe2O3-C3A solid solution at different stages effectively. The hydraulic activity of Fe2O3-C3A solid solution decreased with the addition of Fe2O3. Reasonable amount of Fe2O3 addition reduced the hydration rate in the initial stage of Fe2O3-C3A solid solution hydration, while the hydration rate of Fe2O3-C3A solid solution was increased with excessive amount Fe2O3. The hydration process was controlled by multiple mechanisms due to an incomplete layer of hydration products was formed on the surface.
•Micro-silica has slight influence on hydration kinetics of cement at 20 °C and 40 °C.•The effects of micro- and nano-silica on hydration kinetics are very similar under 50 °C and 60 °C.•Nano-silica ...improves the phase boundary reaction at 60 °C.
Exothermic characteristics of Portland cement incorporating 1%wt micro- and nano-silica were researched by measuring the heat releases at different testing conditions. The presence of nano-SiO2 reduces the induction period, and this effect changes with temperature. The rising temperature makes silica fume generate more significant influence on the induction period. The maximal heat release rate is influenced slightly at normal temperature and decreased to some extent at elevated temperatures through replacing 1%wt cement by micro- and nano-silica. The effects of nano-SiO2 and silica fume at 60 °C are almost completely consistent with each other within 7 h of hydration including induction state, acceleration state and deceleration state. Silica fume presents the property of nano-SiO2 at high temperatures, which can also be observed by analyzing their cementitious indexes. The reaction kinetics of cement with nano-SiO2 and silica fume was analyzed by the classical Krstulović-Dabić model. The analysis results indicate that nano-SiO2 and silica fume can improve the hydration degrees (α1 and α2) of the transformation from nucleation and crystal growth (NG) process to phase boundary reaction (I) process or I process to diffusion (D) process only at 60 °C. Nano-SiO2 can improve the phase boundary reaction at 50 °C and 60 °C judging by the difference between α1 and α2. The possible mechanism of nano-SiO2 and silica fume influencing the hydration kinetics was also proposed.
This work intends to contribute to the understanding about the effect of sugar cane bagasse ash (SCBA) on the hydration of cement-based pastes by focusing on the influence of different particle size ...(or specific surface area). Isothermal calorimetry, thermogravimetry, chemical shrinkage, and strength pozzolanic index tests were performed to compare the hydration and chemical evolution of pastes containing SCBA with different fineness and pastes with inert (quartz) and pozzolanic (rice husk ash) materials. The results showed that a clear change in the kinetics of hydration and portlandite content by comparing the SCBA with different particle sizes. In addition, the different SCBAs had a marked effect on the chemical shrinkage and portlandite, with an intermediate behavior between the pastes with quartz and rice husk ash. Finally, with increasing in specific surface area and soluble fraction of SCBA, its pozzolanic activity was progressively enhanced due to both portlandite consumption and physical effects.
In order to study the abnormal viscosity evolution behavior of oil well cement slurries containing tartaric acid, the influence of tartaric acid on the early-age hydration of oil well cement at high ...temperatures was investigated by isothermal calorimetry (IC), thermogravimetric analysis (TGA) and quantitative XRD (QXRD) analysis in the temperature range of 60 °C to 89 °C. Results show that, a high dosage of tartaric acid can fundamentally change the typical shape of the cement hydration heat flow curve - two additional exothermic peaks associated with aluminate phase hydration (C3A and C4AF) appear before the main hydration peak. The unsynchronized hydration of the aluminate phases and the silicate phases was found to be the cause of premature setting. Additionally, long-term curing study (up to 28 d) showed that increasing the dosage of tartaric acid decreased the ultimate hydration extent of C2S while it increased the ultimate hydration extent of C4AF.
Geopolymers are considered alternative materials to ordinary Portland cement (OPC) with similar or superior properties. SiC imparts interesting thermal and mechanical properties to cementitious ...materials. However, its effects on geopolymeric matrices remain poorly understood, although it can improve the matrix. In this work, the effect of incorporating 0.10%, 0.20%, and 0.50% (wt) of silicon carbide microwhiskers (SCMW) into a metakaolin-based geopolymer matrix was studied. The rotational rheology, mechanical properties, and thermal behavior were evaluated using isothermal calorimetry and thermogravimetric analysis. The results showed that the SCMW modified the rheological parameters of the geopolymer, increasing the viscosity at higher content and slightly modifying the velocity of the reaction. Meanwhile, the composite with 0.20% SCMW content showed better mechanical performance after 28 days, although the composite with 0.50% SCMW content had a similar or higher increase in mechanical performance after 24 h, comparing compressive and flexural strength respectively. However, these results are because of the SCMW properties rather than any new phases formed.
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•Surface chemistry controls rheology in alkali-activated slag cements.•Particle–particle interactions and gelation influenced by activator nature.•Yield stress depends on square of ...zeta potential.•Silicate and hydroxide activators differ notably in behavior.•Influence of pH on fresh paste chemistry is indirect.
Ground granulated blast furnace slag can react with an alkaline activating solution to form a cement-like binder based on a calcium–sodium aluminosilicate gel, which is a potential alternative to Portland cement in many applications. This study provides new information regarding the effect of activator type and dosage on rheology by monitoring changes in pH, particle surface charge (zeta potential), and heat evolution in the early stages of the reaction process. Sodium and potassium hydroxide silicate solutions, at two different M2O (M: Na, K) dosages, are used here as activators. Alkali hydroxide activators cause a significant increase in the yield stress of an activated slag paste, especially at higher dosages as reactions take place rapidly, while within the same timeframe, the yield stress of the silicate activated slag remains unchanged. The results imply a direct relationship between a higher reaction rate with the formation of solid products (causing both spatial blockage effects and consumption of free water), and a rapid yield stress increase. However, the dependence of reaction rate on pH for different alkali-activated pastes is, at most, indirect. All activators induce a highly alkaline pH and a concentrated electrolyte solution environment in the fluid paste. As a result of complexation of poorly-hydrated ions on the surfaces of the particles, the magnitude of the zeta potential increases. A direct relationship is observed between the dosage of the activators and zeta potential. A zeta potential further from neutrality generally reduces yield stress by increasing the magnitude of double layer repulsive forces, with the exception of a higher dosage of silicate activator, which shows an indication of some attractive double layer forces.
•The hydration reaction kinetics of PS activated by a combination of water glass and sodium hydroxide was studied.•The hydration reaction of alkali activated phosphorus slag (AAPS) was facilitated by ...higher temperature and lower silicate Modulus (Ms).•The differences in the hydration reaction kinetics of AAPS as varying the Ms and curing temperatures were quantified by the kinetic models.•The rate controlling processes of AAPS have shifted with changes in the Ms and curing temperatures discussed.
The kinetics of alkali activation of phosphorus slag (PS) is highly related to silicate modulus of the activator and reaction temperature. In present study, it was probed into the hydration reaction kinetics of PS activated by a combination of water glass and sodium hydroxide. The influence of reaction temperatures (from 25 ℃ to 60 ℃) and the activator silicate modulus (Ms, SiO2/Na2O ratio by mass, from 1 to 2) on the hydration reaction kinetics of AAPS were explored by isothermal calorimetric test. The three kinetic models (the exponential mode, the Knudsen linear dispersion mode and the Jander mode) were selected to extract the correlation parameters and quantitatively analyze the acting mechanism on the hydration reaction kinetics of AAPS. The results showed that the effect of different reaction temperatures and Ms on the hydration reaction kinetics of AAPS were prominent. The exponential method was able to satisfactorily model the hydration reaction kinetics of AAPS pastes. However, higher reaction temperatures and lower Ms were required to abstract the desired kinetic parameters when the Knudsen linear dispersion model was used. With the varied curing temperatures and Ms, the variation trends of the time parameters τ and τo (from the exponential mode and the Knudsen linear dispersion mode, respectively) were in line with those trends that the time to acceleration peak and the ending time of induction stage, respectively. According to the Jander model, the rate controlling processes of AAPS shifted from phase boundary reaction controlling process (the N values less than 1) to diffusion controlling process (the N values more than 1) with decreasing Ms and increasing curing temperatures in the case of higher temperature and lower Ms, respectively. On the contrary, phase boundary reaction controlling process kept unchanged at lower temperature (25 °C) and higher Ms (2). Therefore, the conversion of the rate controlling processes of AAPS was responsible for the differences in the hydration reaction kinetics of AAPS.
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