Induction Period Of Hydration Research Articles

Preparation of reactive seedings by in-situ precarbonation under power ultrasound-assisted mixing: Enhancing the hydration and mechanical properties of OPC

In this study, a precarbonation process of cement paste was developed using a power ultrasound-assisted mixing method to prepare a highly reactive seedings admixture, which was subsequently added to the remaining cement paste for secondary mixing. The results revealed that power ultrasound greatly facilitated the release of ions from the cement into the pore solution, promoting the formation of calcium carbonate, aluminum hydroxide and C-S-H gels. After the 20-min ultrasound treatment, a substantial quantity of very fine seedings was observed, capturing 7.09 wt.% CO2 of the total cement used. These seedings also acted as nucleation sites to accelerate further cement hydration, which shortened the hydration induction period and ultimately formed a more homogenous, denser cement microstructure.

Effect of microscale C–S–H on the properties of Portland cement and hydration kinetics analysis at different curing temperatures

Microscale calcium–silicate–hydrate (C–S–H) is easily accessible and readily storable, enabling a cleaner production process of cement-waste grinding and simple chemical synthesis technology. However, the temperature effects of microscale C–S–H require further exploration. In this study, we evaluated the impact of microscale C–S–H admixture on the properties of Portland cement at different curing temperatures. Microscale C–S–H can not only increase the early strength of cement pastes, but also achieve high strength in the long term. Adding microscale C–S–H to cement can effectively reduce the apparent activation energy Ea of hydration products and accelerate the reaction of C3S and C2S with water and form more hydration products. At a curing temperature of 10 °C, the microscale C–S–H increased the 1 d compressive strength by 70.0% and shortened the hydration induction period by 1.2 h. Microscale C–S–H also increased the 12 h hydration degree by 38.1%. Microscale C–S–H promoted the formation of CH and xonotlite-like C–S–H and changed the composition of hydrates, although the effects diminished with increasing curing temperature. This paper offers theoretical and fundamental data on the usage of microscale C–S–H as an admixture in cement-based materials.

Effect of calcium nitrate on hydration properties and strength development of calcium sulfoaluminate cement

Global lithium salt price surges drive up costs for high-performance repair materials, creating demand for cost-effective alternatives. Calcium nitrate (CN), with its low production costs and capacity to enhance early cement strength, emerges as a preferred choice. Limited knowledge on combining calcium sulfoaluminate (CSA) cement and CN prompts further exploration for a potentially cost-effective solution in repair materials. This study investigated the impact of varied CN contents on fluidity, setting time, compressive strength, electrical resistivity, heat of hydration, hydration products, and pore structure of CSA cement. The results show that CN can enhanced fluidity, significantly reduced setting time, accelerated hydration process, shortened the hydration induction period, and promoted the nucleation and growth of ettringite. When CN content was 8%, the compressive strength of samples reached its peak at 40.8 MPa, 44.9 MPa, and 47.3 MPa at 3 d, 7 d, and 28 d, respectively. Exceeding 8% CN content will increase the cumulative heat release which reach the maximum of 314.5 J·g−1 at 72 h. The addition of CN will generate the NO3-AFm phase and improve the stability of ettringite, as well as refine the pore structure of the samples but increase their total porosity.

Roles of diatomite in hydration, microstructure and strength development of cement paste

The production of cement has a significant environmental impact, and the use of mineral admixtures can mitigate this impact by reducing the clinker rate. Diatomite powder (DP) possesses unique properties, such as micronized and sub-micronized porous structures, and a large specific surface area, making it an ideal cement replacement. To gain a better understanding of the mechanisms underlying its performance as a cement replacement, the reference used was quartz powder (QP) in this study, and zeta potential tests were conducted to characterize the surface chemical properties of DP. Subsequently, how it affects the cement paste hydration, microstructure and strength development was investigated. Results revealed that DP, owing to its sub-micronized porous structure and chemical composition, exhibited a higher affinity for Ca2+ ions compared to QP, which significantly affects the formation of C-S-H. This heightened affinity facilitated the nucleation and growth of C-S-H, leading to a shortened hydration induction period. Furthermore, the incorporation of DP into cement paste resulted in a refinement of the pore structure. This enhancement is due to the relatively small particle size of DP, its chemical reactivity, and its surface affinity for uniform distribution of hydration product. Notably, the DP-hydration interface exhibited significantly higher strength compared to the interface between QP and hydration products. As a result of improving pore structure and strengthening the DP-hydrate interface, the cement paste blended with DP demonstrated compressive strength comparable to that of pure cement paste and considerably higher than the cement paste incorporating QP.

Experimental study on application performance of foamed concrete prepared based on a colloidal NanoSiO2-stabilized foam

In this study, colloidal nanoSiO2 (CNS) was added into the foaming agent at 0, 0.5, 1, 1.6, and 2 wt% of the binder mass to enhance the stability of foaming agent and prefabricated foam, further the stability of foamed slurry and hardened foamed concrete. First, the stability of CNS on foaming agent and prefabricated foam was investigated by examining surface tension, surface viscoelastic modulus E, foam parameters, surface shear viscosity, and morphology. Prefabricated foam containing CNS was then used to prepare foamed concrete with a dry density of 700 kg/m3. The rheological characteristics, volume change, setting time, hydration properties, and gas content for fresh foamed slurry and the pore characteristics, compressive strength, and thermal conductivity of hardened foamed concrete were characterized. The microstructural crystalline structures of the foamed slurry and hardened foamed concrete were also investigated. Results of this study indicated that nanoSiO2 nanoparticles in colloidal state improved the stability of foam, especially when the CNS dosage was ≥ 1 wt%, because CNS caused increases in the surface viscoelasticity modulus E of the foaming agent and the surface shear viscosity of the prefabricated foam. Moreover, foamed slurry with a shorter hydration induction period, more stable volume, a shorter initial setting time, and higher viscosity was obtained when the CNS dosage was ≥ 1 wt%. Hardened foamed concrete with higher early compressive strength and superior thermal insulation properties was correspondingly obtained when fresh slurry with higher stability was used. However, the effect of CNS will decrease on the compressive strength of the hardened foamed concrete after 14 days of hydration. Finally, mechanisms were proposed regarding the stabilization effects of CNS on the prefabricated foams, foamed slurry, and hardened foamed concrete.

Study on Dispersion, Adsorption, and Hydration Effects of Polycarboxylate Superplasticizers with Different Side Chain Structures in Reference Cement and Belite Cement.

To investigate the effects of Reference cement (RC) and Belite cement (LC) systems, different molecular structures of polycarboxylate ether (PCE) were prepared through the free radical polymerization reaction and designated as PC-1 and PC-2. The PCE was characterized and tested using a particle charge detector, gel permeation chromatography, a rotational rheometer, a total organic carbon analyzer, and scanning electron microscopy. The results showed that compared to PC-2, PC-1 exhibited higher charge density and better molecular structure extension, with smaller side-chain molecular weight and molecular volume. PC-1 demonstrated enhanced adsorption capacity in cement, improved initial dispersibility of cement slurry, and a reduction in slurry yield stress of more than 27.8%. LC, with its higher C2S content and smaller specific surface area compared to RC, could decrease the formation of flocculated structures, resulting in a reduction in slurry yield stress of over 57.5% and displaying favorable fluidity in cement slurry. PC-1 had a greater retarding effect on the hydration induction period of cement compared to PC-2. RC, which had a higher C3S content, could adsorb more PCE, leading to a greater retarding effect on the hydration induction period compared to LC. LC and PC-2, on the other hand, exhibited inhibition during the hydration acceleration period. The addition of PCE with different structures did not significantly affect the morphology of hydration products in the later stage, which was consistent with the trend of KD variation. This indicates that the analysis of hydration kinetics can better reflect the final hydration morphology.

Preparation of Nano-Sized C-S-H and Its Acceleration Mechanism on Portland Cement Hydration at Different Temperatures.

Nano-sized C-S-H, a promising early strength agent, can accelerate the hydration rate of Portland cement and increase the early compressive strength of cement-based composites effectively. Nano-sized C-S-H suspensions with different contents of effective constituent and size distributions were prepared by a convenient coprecipitation method and the microstructures were analyzed by Zeta potential, XRD and FT-IR. The exothermic heat, early mechanical properties, hydration degree and hydration products of cement with/without nano-sized C-S-H cured at different temperatures were studied by hydration exothermic, XRD, SEM and TG analysis. Nano-sized C-S-H with semi-crystalline structures was prepared, and the size of the nano-sized C-S-H seeds showed an obvious increase with an increase in theoretical concentration, and slight precipitation in the suspension was observed when the theoretical concentration was 2%. The XRD, TG and SEM analyses showed that nano-sized C-S-H expedites the reaction of C3S in the first 24 h; therefore, the hydration induction period is obviously shortened. The 8 h, 16 h and 24 h compressive strength of mortars containing nano-sized C-S-H increased by 176.0%, 145.6% and 43.9%, respectively, compared with the reference mortar. The enhancement effects of nano-sized C-S-H at 10 °C were lower than that at 20 °C.

Study on the characteristics of alkali-activated fly ash-slag improved by cenosphere: Hydration and drying shrinkage

Cenosphere is coal-based solid waste material, which has similar chemical compositions to fly ash. In this paper, the effect of the cenosphere on hydration kinetics and drying shrinkage characteristics of AAFS pastes were explored, and the action mechanism of the cenosphere was deeply revealed. The results showed that the hydration induction period was prolonged, the hydration exothermic rate was reduced or the peak value was delayed during the acceleration period, and the 72 h hydration degree of the sample was increased when mixed with the cenosphere. The incorporation of more than 20% cenosphere significantly reduced the drying shrinkage of AAFS pastes. Comparing the results of compressive strength, drying shrinkage, and pore structure, the most suitable content of the cenosphere is 20%. To significantly improve the drying shrinkage of alkali-activated materials, the content can also be increased. The action mechanisms of the cenosphere are shown as the filling effect, pozzolanic activity, and surface effect. The addition of cenosphere reduces the amount of C-S-H, C-A-S-H and N-A-S-H gel phases, but increases the formation of silicates (calcium hydrosilitates and calcium hydroaluminates).

ПОЛУЧЕНИЕ СМЕШАННЫХ КОМПОЗИЦИОННЫХ ЦЕМЕНТОВ НА ОСНОВЕ ФЕРРИТНОГО КЛИНКЕРА

The possibility of using ferritic clinker as a component of binders with special performance properties is considered. Such types of cements can solve the problems of providing the construction industry with high-strength, resistant to external aggressive effects and relatively cheap binders. However, the mechanism of hardening and curing of cement stone obtained by using ferritic clinkers is complicated and depends on a number of parameters.
 It was found that the greatest thermal effect (more than 70%) during the hydration of the resulting ferritic clinker with the introduction of various additives (lime and gypsum stone) is achieved up to 4 hours from the moment the binder compositions are mixed by means of a calorimetric analysis carried out during the hydration of the developed special binder compositions. That corresponds to the course of the induction period of hydration of the used Portland cement, at the time of the occurrence of primary contacts between crystalline hydrates and sufficient mobility of the cement mass. In this time interval, the strength of the cement stone is not high enough and the produced iron-containing hydration products are able to fill the space of the cement matrix, making it more dense and less defective. 
 The formation of iron-containing crystalline hydrates from ferritic clinker can proceed in several stages: hydrolysis of dicalcium ferrite at the initial stage of the reaction and further interaction of its products with portlandite in the period from 7 to 14 days from the moment of mixing the binder compositions. The main crystalline hydrates formed from ferritic clinker are iron-bearing ettringite and hydrogarnet.
 The study of the physical and mechanical characteristics of cement stone showed that the optimal content of ferritic clinker in the composition of the binder compositions is 5%. With a given quantity, the strength of the cement stone in compression and bending exceeds that of the control sample by 11.5%. The input of 15 and 30% ferritic clinkers into the composition of special binders reduces the strength characteristics by 15-20%.

Study on the Physical and Chemical Properties of Cement-Based Grout Containing Coal-Fly Ash.

To study the physical and chemical properties of grout containing fly ash, Class II fly ash was used as a mineral admixture and mixed with silicate cement to produce grout, and the rheological properties, strength properties, hydration properties, and microscopic mechanism were studied. The results of the study showed the following. The incorporation of fly ash reduced the thixotropic area of the composite cement slurry, which facilitated pumping in the pipeline conveying process. The inclusion of fly ash reduced the yield stress and plastic viscosity of the cement paste, but the rheological index increased and then decreased with the increase in fly ash, and the composite paste had the lowest degree of shear thinning at 30% fly ash inclusion. The incorporation of fly ash reduced the hydration exothermic rate and total hydration exothermic amount of the composite slurry and prolonged the hydration induction period, but the promotion effect of fly ash on the hydration rate of cement was obvious at 10% fly ash admixture. The admixture of fly ash increased the empty volume of the composite slurry, but the effect on the most probable aperture was not significant, and the porosity of the system increased, resulting in a decrease in compressive strength. The effect of adding fly ash on the hydration products was reflected mainly by the C-S-H gel produced by cement hydration and the change in calcium alumina and Ca(OH)2. Fly ash does not directly participate in the hydration reaction of cement, but it can promote cement hydration and increase the reaction rate of cement. By analyzing the rheological properties, mechanical properties, and hydration properties of fly ash composite cement paste, the comprehensive analysis found that the rheological properties are excellent when the fly ash admixture is 20-30%, and the water-cement ratio can be reduced to improve the strength without affecting the pumping demand.

Recycled concrete fine powder (RFP) as cement partial replacement: Influences on the physical properties, hydration characteristics, and microstructure of blended cement

Construction and demolition (C&D) waste has caused various economic, environmental, and social problems. This paper investigates the potential of recycled concrete fine powder (RFP) as a supplementary cementitious material. The effect of RFP with different replacement levels on the properties of cement and the mechanism of action were explored from the physical characteristics, mechanical properties, hydration kinetics, hydration products, and pore structure. In this study, scanning electron microscopy, Energy Dispersive X-Ray Spectroscopy, X-ray diffraction, X-ray fluorescence, Thermogravimetric analysis, and Isothermal calorimeter were applied. The results showed that RFP (less than 30%) shortened the setting time and improved the fluidity of cement. The early compressive strength increased by 17.62% of the cement mortar with 10% RFP. When the RFP content was 10%, the flowability of the blended slurry (175.5 mm) increased by 8.0% compared to the pure cement (162.5 mm). With the RFP content increase of the blended cement, the hydration induction period was prolonged, and the peak hydration exothermic rate and the accumulated heat release showed an apparent decreasing trend. The reaction rate of the nucleation and crystal growth (NG) process was much higher than that of the interface (I) and diffusion (D) processes. The low field nuclear magnetic resonance (NMR) technique revealed that RFP filled the large and medium pores of the mortar. The non-destructive layered scanning found that the pores were distributed mainly in the central zone and had a gradient distribution law of gradually decreasing from the middle area to the edge area. Visual analysis of the dynamic distribution of water molecules in the pores and the non-homogeneous characterization also demonstrated the good filling effect of RFP on the mortar. Therefore, RFP can be used as an economical and environmentally friendly cement mixing material to replace partial cement and contribute to reducing carbon emissions in the construction industry.

Influence of different additives on the creep property of oil well cement

As the exploitation of oil and gas field becomes more difficulty nowadays, severe casing deformation may occur if anti-extrusion strength of casing decreases to a certain degree and this brings large economic loss and severe safety problems. As an annular sheath between casing and formation, creep of oil well cement paste is one of the most important factors in deciding abnormal casing deformation and wellbore integrity. To figure out the mechanism on how to reduce the creep value of oil well cement paste, effect of different additives, e.g., retarder, glass fibre reinforcement and CSA expansive agent on the creep value of oil well cement are investigated in this study. It can be found out that retarder plays a negative role in reducing the creep deformation and creep value of cement with 1, 2, 3% retarder is 2.39%, 7.85%, and 15.02% higher than that without retarder at 12 h. Results from heat flow indicates that retarder may play an important role in prolonging the early induction period of hydration and reducing the hydration rate in the middle and later stages. On the contrary, Glass fibre shows a positive effect in reducing the creep value of oil well cement paste and creep value reaches with 0.2%, 0.4%, 0.6%, and 0.8% is 1.27%, 6.61%, 9.82%, and 12.60% lower than that without fibre addition at 72 h. This is because high porosity between and around fibers based on a microstructure observation may lead to a more continuous and uniform stress field between cement paste, and consequently reduce the creep value of cement paste. It can also be found out that CSA cement is considerably effective in limiting creep deformation of oil well cement paste at early ages. There is also an acceleration effect on the heat flow of oil well cement in both induction and acceleration periods and the corresponding maximum heat flow is 7.3%, 9.0%, and 11.2% higher than the reference cement with no CSA addition. The results obtained in this study are aimed to provide theoretical basis for the wellbore integrity and improvement of oil and gas recovery.

Application of molten iron desulfurization slag to replace steel slag as an alkaline component in solid waste-based cementitious materials

Desulfurization slag (DS), generated from molten iron pre-desulfurization process, has a high alkalinity that may cause seriously environmental issues. To solve the DS-disposal problem, this research proposes a new type of cementitious material composited of DS, ground granulated blast furnace slag (GGBS), steel slag (SS), and flue gas desulfurization gypsum (FGDG). The effects of DS on cementitious material were studied by testing the working performance indexes such as mortar fluidity and compressive strength, and the microscopic analysis such as XRD, TG and SEM. The results show that DS reduces the fluidity of the mortar, and significantly improves compressive strength of 3 days. The optimal ratio of DS/SS ratio in the GGBS–SS–DS–FGDG cementitious material is 1.0, and its compressive strength of 3 days and 28 days reached 24.6 and 52 MPa, respectively. The 3 days’ compressive strength of GGBS-SS-DS-FGDG is 748% higher than that of GGBS–SS–FGDG. The high alkalinity of DS promotes the dissociation of GGBS, and enhances the early hydration rate, which shortened the hydration induction period, and enhanced the cumulative released heat. Also, DS promotes the formation of ettringite (AFt) and increases the polymerization degree of the C–(A)–S–H gel at early stage. However, cementitious material containing DS has high porosity, which can be offset by the gel pores generated by SS. In summary, the GGBS–SS–DS–FGDG as a cementitious material with low carbon emission may be used to replace cement. The results of this paper can guide the large-scale and comprehensive utilization of DS with other solid waste.

Study of the Hydration Temperature Field in the Hinge Seal Layer for a Large-Volume UHPC Arch Base

For large arch bridges, the sealing hinge layer at the arch footing is critical to the long-term performance and durability of the structure. This study investigated the temperature distribution of the sealing hinge layer inside a large UHPC arch seat during the hydration process in a natural environment, based on a design of a 600-m-span reinforced concrete rigid skeleton arch bridge. A certain suggestion for the design of a large-volume UHPC layer was proposed through the consideration of temperature and age effects of the thermal field solid finite element analysis of early-age UHPC hydration heat. The results show that UHPC has a hydration induction period of 10 h and reaches the peak temperature of 69°C in 20 h. Then, the hydration heat is gradually released in 70 h after pouring. The maximum temperature difference between the core and surface reaches 40°C. The results also show that the early temperature field and heat release rate of UHPC are affected by initial temperature. The actual heat release of some materials of UHPC components at 75 h is equivalent to that at 300 h, and the minimum effective age was found close to 100 h, where the heat release reaches 0.25 of the total heat release. A simplified algorithm for the spatially time-varying non-linear temperature field of the UHPC sealing hinge layer under the influence of temperature and age is proposed, which provides a simple calculation method for the temperature control of the UHPC arch seat during the hydration process in real projects.

A novel shrinkage-reducing polycarboxylate superplasticizer for cement-based materials: Synthesis, performance and mechanisms

The incorporation of polycarboxylate superplasticizers may increase the risk of shrinkage cracking of cement-based materials. Co-adding shrinkage-reducing admixtures (SRAs) may decrease the compressive strength or arise the incompatibility problem. Designing a novel superplasticizer with excellent shrinkage-reducing function by modifying molecular structures of polycarboxylate superplasticizers can be a promising approach.This study aims to synthesize a novel shrinkage-reducing polycarboxylate superplasticizer (SR-PCA) possessing shrinkage-reducing and water-reducing functions and to investigate its performance and working mechanisms in cementitious systems. The results indicate that SR-PCA improves the fluidity of cement paste and has a distinguished fluidity retention capability. SR-PCA prolongs the cement hydration induction period, hinders the nucleation and growth of C-S-H in the acceleration period, while enhances hydration degree and refines the pore structure at later ages. Moreover, it can also be deduced that most PCA adsorbs on the cement surfaces, however, SR-PCA is mainly in the pore solution. The shrinkage-reducing mechanism of SR-PCA in cement-based materials is attributed to lowering the surface tension of the pore solution, due to the presence of the unreacted esterification macromonomers (AA-C4-PE-600) and the unadsorbed SR-PCA molecular.

Pore connectivity of oil well cement in the early hydration stage by in situ electrical resistivity measurements and low-field nuclear magnetic resonance

The pore connectivity of cement slurry is an important parameter to be considered when investigating short-term gas migration in the cementing engineering of natural gas wells. This study combined electrical resistivity measurements (ERM), low-field nuclear magnetic resonance, and multi-phase phenomenological models to create an in situ evaluation method for the pore connectivity of cement slurry. This method was effective for investigating the porosity, pore size, and pore connectivity of the cement slurry in the early hydration stage. The results showed that the initial formation factor of the cement slurry was determined by the water-to-cement ratio (w/c). In the initial hydration stage, the porosity of the cement slurry (w/c = 0.44) was approximately 58%. Additionally, the pore size ranged from 11 to 75 nm, and the pore connectivity was 0.65–0.70. The small amount of Q1 calcium silicate hydrate (C-S-H) generated during the hydration induction period did not affect the dispersed-particle microstructure of the cement slurry and had little effect on its pore structure. During the hydration acceleration period, numerous Q1 and Q2 SiO4 species were detected, which converted the microstructure of the cement slurry to a cross-linked structure, thereby reducing the pore size and pore connectivity of the cement slurry. The porosity of the cement slurry reduced to 52% after hydration for 12 h at 30 °C. However, the pore size distribution decreased to 4.5–35 nm, and the pore connectivity varied from 0.5 to 0.58.

Mechanical Properties and Shrinkage of Ultrahigh-Performance Concrete Containing Lithium Carbonate and Nano-Calcium Carbonate

Early strength generation is essential for the successful application and usage of Ultrahigh-Performance Concrete (UHPC) in reinforcing concrete structures. The work contained in this paper focused on evaluating the effects of lithium carbonate (Li2CO3, denoted as LC) and nano-calcium carbonate (NC) on the early mechanical properties and autogenous shrinkage of UHPC under normal temperature curing conditions. In the study, scanning electron microscope (SEM) was utilized to investigate and quantify the morphology of the early hydration products. The corresponding results indicated that the 1-day comprehensive strength of the UHPC increased significantly with the addition of Li2CO3 and NC. Likewise, the addition of NC mitigated the loss of the 28-day compressive strength. For the materials evaluated, the 1-day compressive and flexural strengths reached peak values of 72.1 and 13.9 MPa, respectively, for optimum dosages of 0.075%∼0.1% Li2CO3 and 3%∼4% NC, respectively. The results also indicated that the combined LC-NC dosage had profound effects on the early autogenous shrinkage of UHPC, which could, however, be minimized by adjusting the Li2CO3 and NC dosages. That is, the combined dosage of the two early strength enhancers shortens the hydration induction period of cement whilst concurrently accelerating the hydration rate of the cement. The early strength agent increases the number of crystals in the hydration product and the crystal grain size becomes larger. These make the microstructure of the slurry more compact after hardening and therefore improve the overall performance of UHPC.

Unground granulated slag as a substitute for natural aggregate during concrete production

Styrene-butadiene copolymer [SB] modified cement-based materials are widely used, but the addition of SB can delay the setting and hardening of cement, which limits its application in some projects. In this paper, nanosilica was selected as the modifying component to study its influence on the early hydration, setting and hardening of SB/cement composite material. By measuring the setting time and early strength of nanosilica modified SB/cement composite material, the influence of nanosilica on the setting and hardening process of composite cementitious material was analyzed. The hydration heat of nanosilica modified SB/cement composite material was determined by isothermal calorimetry, and its hydration products were examined by X-ray diffraction, so as to analyze the influence of nanosilica on the early hydration process of composite cementitious material. The results show that the addition of nanosilica can effectively promote the setting and hardening process of composite cementitious material, and the higher the dosage is, the more significant the effect is. It also indicates that addition of nanosilica accelerates the formation of ettringite and calcium hydroxide, by promoting the hydration of tricalcium aluminate and tricalcium silicate. Shortens the hydration induction period and acceleration period of the composite cementitious material and accelerates the hydration process, thereby shortening the setting time and increasing the early strength.

Preliminary Interpretation of the Induction Period in Hydration of Sodium Hydroxide/Silicate Activated Slag.

Many calorimetric studies have been carried out to investigate the reaction process of alkali-activated slag paste. However, the origin of the induction period and action mechanism of soluble Si in the dissolution of slag are still not clear. Moreover, the mechanisms behind different reaction periods are not well described. In this study, the reaction kinetics of alkali-activated slag paste was monitored by isothermal calorimetry and the effect of soluble Si was investigated through a dissolution test. The results showed that occurrence of the induction period in hydration of alkali-activated slag paste depended on the presence of soluble Si in alkaline activator and the soluble Si slowed down the dissolution of slag. A dissolution theory-based mechanism was introduced and applied to the dissolution of slag, showing good interpretation of the action mechanism of soluble Si. With this dissolution theory-based mechanism, origin of the induction period in hydration of alkali-activated slag was explicitly interpreted.

Hydration Kinetics of Portland Cement-Silica Fume Binary System at Low Temperature.

Portland cement–silica fume binary cementitious materials are widely used in engineering construction and have been investigated from micro- to macroscopic aspects. However, the theoretical background on the hydration kinetics of the binary system has not been sufficiently covered in the literature. In this study, the hydration dynamic characteristics of the Portland cement–silica fume binary system curing at low temperature were investigated. Hydration kinetics equations were optimized and a hydration model followed by a computer program was developed to calculate the reaction rate constant K and the reaction order n. This model presented that the hydration process of the binary system at low temperature could be divided into three stages, namely, nucleation and growth (NG), interactions at phase boundaries (I), and diffusion (D). The n values for the binary system varied in the range of 1.2 to 1.6, indicating that the hydration of the binary system at low temperature was a complex elementary reaction. Silica fume can reduce the total heat at the later stage of the hydration and the reaction order n, but increase the heat flow at the accelerating stage and the hydration rate constant K. Low temperature prolonged the hydration induction period, decreased and delayed the secondary exothermic peak, as well as reduced the n and K value.