Hydration Rate Research Articles

Effect of pre-hydrated CAC suspensions on the hydration behavior of CAC pastes

The relatively low hydration rate of calcium aluminate cement (CAC) at 15–35 °C leads to retarded setting time and reduced production efficiency of CAC-bonded castables. To accelerate the hydration rate of CAC within this temperature range, the present work proposes the fabrication of nano-hydrates acting as seeds by pre-hydration of CAC in suspensions. CAC powders are dispersed in water with water-to-solid ratio of 4.5 and stirred for varying durations at 25 °C to achieve different pre-hydration degrees. Then the pre-hydrated CAC suspensions replace a small part of cement in the pastes. The influence of the CAC suspensions addition on the hydration heat release and strength development of fresh CAC pastes at 25 °C is investigated. The results indicate that nano-scale hydrates C2AH8 and AH3 form on the surface of pre-hydrated CAC particles in the suspensions and grow rapidly with the extension of stirring duration. The presence of nano-scale hydrates in the pre-hydrated CAC suspensions is found to shorten the induction period of CAC hydration and thus be beneficial to the accelerated hardening of CAC pastes. The pre-hydrated CAC suspension stirred for 3.5 h has the most significant effect on enhancing early strength of the CAC pastes. At this stage, the pre-hydrated CAC within the suspension is in the accelerated period, and has just begun to precipitate large amounts of nano-scale C2AH8 and AH3 acting as seeds to catalyze the hydration and refine the pore structure of CAC pastes.

Experimental study on the influence of particle size and grain grading on the CO2 hydrate formation and storage process in porous media

CO2 sequestration in sediments as hydrate form is considered to be a promising way to achieve CO2 emission reduction in the atmosphere. The key problem with hydrate-based technology for CO2 geological storage is the evaluation of the formation process and gas storage capability of hydrate. In this study, the fundamental issue of different particle size and grain grading affected CO2 hydrate formation and storage characteristics in porous media was deeply analyzed. The effect of various particle sizes and grain grading on the gas consumption rate and gas storage capacity of CO2 hydrate were quantitatively illustrated. The results indicated that the hydrate formation is more difficult in porous media with tiny particle sizes. The gas storage capacity and gas consumption rate of CO2 hydrate increased as the particle size increased within a range of 10–63 μm. The maximum gas storage capacity attains 25.65 L/L when the particle size is 63 μm. Besides, compared with single particle size, porous media with grain gradation is adverse to hydrate formation. The results also show that, the obstruction effect on the gas consumption rate and gas storage capacity is more obvious with the increase of the differences in grain gradation. The cooperative impact of particle size and gradation on hydrate formation was studied. The closer the size of the mixed particles is, the more favorable to hydrate formation. The relevant results will aid in comprehending the hydrate formation properties in submarine sediments and will serve as an essential theoretical reference and guide for CO2 capture and sequestration hydrate-based method.

Effect of dispersion behavior of silica fume on the rheological properties and early hydration characteristics of ultra-high strength mortar

Silica fume plays a crucial role in ultra-high performance cementitious materials, but its dispersion, influenced by varying densification levels, leads to different degrees of agglomeration. However, the precise impact of this on rheological properties and early hydration characteristics of ultra-high performance cementitious materials remains unclear. This study assesses the impact of highly densified silica fume (HDSF), moderately densified silica fume (MDSF), and raw silica fume (RSF) on the rheological prperties of ultra-high strength mortar (UHSM). Additionally, the early hydration characteristics of UHSM influenced by these silica fumes are investigated through both heat of hydration and penetration resistance methods. The results showed that HDSF contained numerous large agglomerates, contrasting with MDSF and RSF, where the agglomerate sizes were comparable to those of cement particles. As the content of HDSF increased, the yield stress, viscosity, and thixotropy of UHSM gradually rose, accompanied by a notable decrease in hydration rate. Conversely, increasing MDSF and RSF content initially decreased and then increased these rheological properties, while the hydration rate remained relatively unchanged. The distribution behavior of the silica fumes influences the rheological properties of UHSM through water film thickness (particle spacing). Large agglomerates in HDSF reduce the activity of the entire cementitious system, significantly decreasing the hydration rate. In contrast, MDSF and RSF primarily influence the particle spacing within the cementitious system, subsequently impacting the hydration rate. This study provides theoretical insights and practical guidance for construction professionals in selecting appropriate silica fume for designing and regulating ultra-high performance cementitious materials performance.

Hydration characteristics and kinetic analysis of polyacrylamide (PAM) anti-dispersion cement-based materials

Abstract Through phase analysis and quantitative calculations of the hydration process, the inclusion of polyacrylamide (PAM) in Anti-dispersion Water Paste affects its hydration characteristics is discovered. Specifically, the presence of PAM reduces early reactivity while accelerating progress in the middle to late stages of hydration. By employing K-D hydration kinetic analysis, it is confirmed that the use of PAM results in the formation of a multi-nucleation homogeneous point system within the cement slurry. This system prolongs the NG (nucleation and growth) process time, increases the hydration degree during the transition from NG to I (induction) process, and inhibits the heterogeneous precipitation of hydration products. Consequently, the early hydration of cement is hindered. As the hydration reaction advances, the microstructure of the product under multi-nucleation points increases the specific surface area. This gradual breakthrough of the hydration threshold barrier shortens the duration of the phase boundary reaction process and reduces the hydration degree during the I and D (diffusion) processes. Consequently, the late-stage hydration rate is accelerated.

Study on the microscopic transmission characteristics and coupled erosion depth of tailings recycled concrete under the coupling effect of carbonization and salt-fog

In ocean and salt-lake district, especially in some industrial areas, concrete structures suffer from the coupling erosion of carbonation and salt-fog. As an eco-friendly material, tailings recycled concrete(TRC) has been used to its alleviate the deterioration of living environment. Based on series of physical and chemical equations of hydration, carbonization and salt-fog erosion, this paper introduces parameters such as hydration rate, carbonization rate and Friedel salt degradation coefficient. Then，a comprehensive analysis method that can reflect the erosion performance of TRC quantitatively is established, and the microscopic transmission characteristics and erosion depth calculation method under coupling effect are studied. The results indicate that aggregates have a “blocking effect” on the diffusion of corrosive substances, resulting in higher concentration around the front edge of aggregates, and the erosion of salt-fog plays a leading role under the coupling effect. Overall, the CO2 concentration decays faster than that of Cl−. The longer the erosion age, the higher the CaCO3 content, while the content of CH and CSH is approximately inversely proportional to that of CaCO3. In addition, the method for determining the depth of carbonation and salt-fog erosion under coupling effect is studied, and the calculated values are in good agreement with the experimental values. It shows that the coupling erosion depth is greater than that under single erosion, which leads to the salt-fog erosion depth increased by about 30 %, the longer the erosion age, the greater the erosion depth. The effect of iron ore tailings(IOT) on coupled erosion depth is similar to that of single erosion. The results also show that the coupled erosion performance of TRC can be analyzed by using the model proposed in the paper.

Structural build-up of cement pastes - Underlying mechanisms considering the particle packing, colloidal interactions and hydration kinetics

This study aims to investigate the decisive mechanisms of structural build-up of cement pastes. The structural build-up can be divided into three stages, with distinct developments in particle packing, colloidal interactions, and CSH bridges at each stage. Our model framework demonstrates that the first stage of structural build-up is influenced by heightened colloidal interactions and enhanced particle packing. The second stage is propelled by the strengthening of CSH bridges, while the third stage involves reinforced CSH bridges and increased particle packing. It should be noted that the impact of particle packing may be limited at low hydration levels. Upon examination, the physical-based structural build-up rates (Athix,f, ACA$H and ACSH) are identified as the results of enhanced colloidal interactions, increased particle packing and augmented CSH bridges, respectively. These parameters are validated as consequences of the initial particle packing parameter and rate factors, which denote the increase rate of van der Waals forces, the hydration rate at the beginning of hydration, and in the subsequent second and third stages.

Effect of interface properties between functionalized cellulose nanocrystals and tricalcium silicate on the early hydration mechanism of cement

The hydration process is a critical stage in the development of the properties of cement-based materials，however, the influence of cellulose nanocrystals (CNC) on cement during the early hydration is controversial. This study uses pure tricalcium silicate (C3S) as the object to analyze the influence of carboxyl CNC (CCNC) and sulfonic CNC (SCNC) on hydration parameters such as hydration rate, types, quality, and pore structure of hydration products during the early hydration period. The dissolution of C3S and the interaction between fluids and minerals at the C3S/CNC interface is the core mechanism of the early hydration. Ultraviolet and energy spectrum analyzed adsorption between CNC and C3S from a physical perspective, and molecular dynamics simulations deeply discussed the adsorption behavior between C3S/Water/CNC at nano-scale. The results show that CNC prolongs the hydration induction and acceleration period, delays the early hydration of C3S; CCNC exhibits a more significant effect than SCNC. Under the action of Ca-O and hydrogen bonds, CNC will adsorb on the surface of C3S and cover its surface, reducing the number of waters on the surface of C3S. Hydrophilic CNC molecules attract water molecules close to the CNC chain, and solidifying water molecules prevents it from entering the hydration surface. CNC also restricts the diffusion of Ca2+ on the surface of C3S to outside, improving the stability of Ca-O coordination, promoting the accumulation of hydration products at the C3S surface, and inhibiting the dissolution of C3S. Understanding the effect of functionalized CNCs on the diffusion and sedimentation mechanism of water molecules and Ca2+ during hydration can provide insights for guiding the design of nanomaterials with strong compatibility with cement systems and enlarging the application of functionalized nanomaterials in cement-based materials engineering fields.

Pozzolanic activity of secondary aluminum ash sintered and ground fine powder in Portland cement

To better facilitate the resource utilization of secondary aluminum ash and reduce the consumption of Portland cement, this study investigates the pozzolanic activity grinding fine powder of secondary aluminum ash sintered and ground fine powder (MP) in a Portland cement matrix with different fineness and content. The mechanism of volcanic ash activity was explored using microscopic testing techniques such as XRD, FTIR, TG, SEM, and MIP. The outcomes revealed that as the MP content increased, the fluidity of the cement-based matrix exhibited a declining trend. The addition of MP has a diluting effect on Portland cement, resulting in a decreasing trend of strength activity index with the increase of MP content. Furthermore, under identical content conditions, much finer MP showed a reduction in its pozzolanic activity within the Portland cement-based matrix. Hydration processes and microscopic tests indicated that the addition of the coarser MP particle resulted in higher pozzolanic activity, which is attributed to the faster hydration rate, lower porosity, and higher microstructural compactness. Simultaneously, with an increase in MP content, the porosity and detrimental pore content of the cement-based matrix notably increased, and the content of hydration products reduced.

Optimal development of the new type of ultra-fine fly ash semi flexible pavement grouting material: Rheological, mechanical, pore distribution, and hydration characteristics

In order to solve the problems that the fluidity of semi-flexible pavement grouting materials (SFPGM) is not up to standard, the early strength is not enough, or the fluidity meets the requirements but the early strength is not up to standard, the power plant by-Product -ultra-fine fly ash (UFA) was used to optimized the performance of grouting materials (GM) in this study. The microstructure of hardened slurry was analyzed by XRD, FTIR, MIP and SEM. The findings indicate that when the UFA content increases, the fluidity of the GM initially increases and then declines, whereas the compressive strength initially reduces and then increases. The sample exhibits optimal fluidity performance when the content of UFA is 9 %. The flow cone fluidity of the initial and 20 min is 12.33 % and 17.63 % lower than that of the reference sample. When the amount of UFA is 12 %, the compressive strength of sample is the highest at each age. Additionally, that the fluidity of sample S12 is sub-optimal in all samples, only worse than that of sample S9. In the early stage of GM hydration, UFA with slow hydration rate produces a significant ‘ball-bearings effect’ between irregular cement particles. This is beneficial to the relative sliding between the slurry particles, reduces the yield stress and plastic viscosity of the fresh slurry, and improves the flow performance of the GM. The pozzolanic effects of UFA in GM primarily manifests during the later stages of hydration. The primary hydration products of UFA modified GM include ettringite, C-S-H gel, C-A-S-H gel, calcium carbonate, and aluminium glue (AH3). During the progression of the hydration reaction, the quantity of ettringite (AFt) in the slurry steadily augments, causing the initial fine needle-like structure to evolve into a more fully formed needle-like structure. A complex three-dimensional network of interlocking spatial structures is formed by many hydration products that continually interlace and develop, improving strength. The higher the perfusion rate of the GM, the higher the compactness of the SFPGM, and the better the high temperature stability and low temperature crack resistance.

Dehydration behaviors and properties of anhydrite II prepared by phosphogypsum with low-temperature calcination

Certain concentration H+ can combine with the crystal water to form the H3O+, influencing the stability of the crystal water in dihydrate so that can be completely removed under lower temperature. Therefore, this paper introduces a method wherein sulfuric acid is employed as a medium, facilitating direct dry mixing and low calcination temperatures to produce anhydrite II. Initially, the low and high calcination temperatures were determined as 280°C and 500°C, respectively, through XRD analysis of the phase composition of calcination products under varying conditions. Subsequently, the influence of different sulfuric acid contents and concentrations on the calcination products at 280°C was investigated. Additionally, the impact of sulfuric acid on the dehydration behavior of PG was analyzed using TG-DSC and FTIR. The present work also assessed the physical-mechanical properties of anhydrite II at 280°C and 500°C, along with their hydration activity and microscopic morphology. The results indicated that the optimal sulfuric acid content and concentration were determined as 2 % and 40 wt%, respectively, yielding dehydration temperatures of 86.08°C and 146.95°C. The water requirement (WR) for normal consistency for anhydrite II at 280°C was 0.68, exhibiting higher flexural and compressive strength compared to calcination at 500°C (WR=0.58), and the hydration rate at 28 days reached 78.5 %. At 280°C, the dihydrate crystals formed by anhydrite II at 28 days exhibited a compact microstructure characterized by relatively small crystals with well overlapping.

EVALUATION OF THE EFFECT OF AIR ON THE HYDRATION SYSTEM OF THE AXIAL PISTONS

This study investigates the effect of air exposure on the hydration system of axial pistons, critical components in various hydraulic systems. The hydration process, which is integral to the functionality and longevity of axial pistons, is potentially sensitive to environmental factors such as air. This research aims to elucidate the effect of air on the hydration kinetics, surface properties and performance of axial pistons. Experimental studies were conducted using controlled environments to analyze the hydration behavior of piston samples under varying degrees of air exposure. Techniques such as surface microscopy, chemical analysis and mechanical testing were used to comprehensively evaluate the effects. Preliminary findings indicate that exposure to air significantly affects the hydration kinetics of axial pistons, affecting the rate and extent of hydration. Surface analysis reveals changes in surface morphology and chemical composition indicating changes in the air-induced hydration process. Moreover, mechanical testing reveals notable differences in the performance characteristics, including wear resistance and friction properties, of pistons exposed to varying weather environments. Overall, this study underlines the importance of considering air exposure in understanding and optimizing the hydration system of axial pistons and provides valuable information for improving hydraulic system efficiency and reliability. Keywords: axial pistons, hydration system, piston, hydraulic machine, rotor.

A low-carbon cement based on silicomanganese slag and granulated blast furnace slag: Hydration process, microstructure, mechanical properties and leaching risks

The calcium content of silicomanganese slag is only about 50 % of that in granulated blast furnace slag but the manganese content is high (>10 wt%), which limits the wide application of silicomanganese slag in cementitious materials. The present study provides a low-carbon cementitious material with an above-average silicomanganese slag content, which provides a new method for the recycling of silicomanganese slag and other solid wastes. The mechanical properties, hydration mechanisms, and pore structures of low-carbon cementitious materials with different silicomanganese slag and granulated blast furnace slag ratios were investigated using carbide slag as alkali-activator and flue gas desulfurization gypsum as sulfate-activator. The feasibility of silicomanganese slag as a significant replacement for granulated blast furnace slag in cementitious materials was explored, and the risk of manganese leaching was assessed. Silicomanganese slag has slower hydration rate in the early stage, but the reaction accelerates in a later stage and the gap with granulated blast furnace slag decreases. The 3, 7, and 28-d compressive strengths of mortars consisting of 40 % silicomanganese slag, 40 % granulated blast furnace slag, 12 % flue gas desulfurization gypsum, and 8 % carbide slag reached 28.7, 41.2, and 49.9 MPa, respectively. When the silicomanganese slag content is 80 %, the leaching toxicity test results showed ≤0.1 mg/L, and jouravskite was present in the hydration products, which indicated a low risk of manganese leaching in normal service environments.

Role of polyferric sulphate in hydration regulation of phosphogypsum-based excess-sulphate slag cement: A multiscale investigation

Current demand for waste recycling, phosphogypsum-based excess-sulphate slag cement (PESSC) as a sustainable cement prepared by solid wastes, urges enhancing its performance development based on microstructure optimisation. For the purpose of improving the performance and durability of PESSC used in normal or corrosive environments, it is deemed an efficient technique to produce iron-doped compounds with high thermodynamic stability. This paper presents a systematic study of the effect of iron modification on PESSC binders by introducing 0%–2% polyferric sulphate (PFS) from a multiscale viewpoint. XPS, 29Si and 27Al NMR, and TEM were used to characterise the nanostructure of solid particles firstly at Level I. Then, the chemical composition and phase assemblage of PESSC binders were revealed at Level II in terms of ICC, ICP, DTG-DSC, FTIR, BSE-EDS and XRD. Finally, setting time and strength development were determined at Level III. Results indicated that the soluble FeOH4− supplied by the hydrolysis of PFS promotes the generation of iron-doped ettringite with a greater length-to-diameter ratio and thermodynamic stability. Seeding effect of iron doping also promotes the production of spherical and retiform gels with a slight influence on the chemical components and polymerisation. Despite the fact that iron doping weakens the early strength of PESSC mortars, it promotes the persistent hydration rate by retarding precipitation and encapsulation of hydrates on the surface of the slag, showing excellent strength in the later stages. In view of microstructure evolution and performance development during each stage, PFS supplementation within 1.0% is considered a feasible modification of PESSC relying on the formation control of iron-doped hydrates.

Effect of Water to Cement Ratio on Properties of Calcium Sulfoaluminate Cement Mortars

Calcium sulfoaluminate (CSA) cements are a promising alternative to Portland clinker, however, a thorough understanding of their properties is needed for their broader use in the industry. One of the topics that requires a good understanding is the effect of the w/c ratio on the properties of CSA cements. To this end, the aim of this paper was to provide research into the effects of a w/c ratio in the range of 0.45–0.6 on the properties of fresh and hardened CSA pastes and mortars. For fresh mortars, consistency and setting time, as well as plastic shrinkage tests, were conducted, and were complemented by hydration heat tests, carried out on pastes. For hardened mortars, tests of compressive and flexural strength and dry shrinkage, as well as SEM photography, were conducted. It was found that, regardless of a higher hydration rate, the increase in w/c ratio decreased flexural and compressive strength, as well as shrinkage, while increasing consistency, setting time, and hydration heat. Also observed was a significant decrease in strength between 3 and 7 days of curing in mortars with a high w/c ratio. It can be concluded that, regardless of the hydration rate, low w/c ratios in CSA mortars provide better properties than high w/c ratios.

Hydration kinetics and apparent activation energy of cement pastes containing high silica fume content at lower curing temperature

To promote the application of silica fume (SF) in civil engineering in cold regions, relative influences from SF content and low temperature environment on the early hydration kinetics of composite cement pastes were evaluated based on the isothermal calorimetric method. The effect of high SF content on the apparent activation energy (Ea) of Portland cement and the filler effect (via the index of area multiplier) of SF on the hydration parameters was also analyzed. The results showed that the intensity of the shoulder peak (Al peak) of the hydration kinetic curve (heat flow curve) gradually grew as the SF content increased, and the double-peak phenomenon was gradually clear. The heat flow peak is delayed and lowered with the decreasing curing temperature, while the final time of the induction stage and the duration of the acceleration stage are delayed. Moreover, the duration of the acceleration stage was slightly extended with the increase of SF content. The full-width at half-maximum (FWHM) value of the heat flow curves dropped as the SF content and curing temperature increased. The low temperature environment and the inclusion of SF inhibited the growth in early hydration degree. The higher SF content mitigated the impact of low temperature on the hydration time parameter τ. The apparent Ea results also indicated that the SF reduced the temperature sensitivity of the hydration process of the composite cement pastes, which might contribute to the development of the hydration reaction at low temperatures. The AM (Area Multiplier) factor increased approximately linearly with increasing SF replacement levels. Apparent Ea decreased linearly with increasing AM, which may be attributed to a further reduction in the diffusion-controlled hydration rate of composite cement pastes with larger AM.

Microstructure and durability of rapid repair mortar with self-emulsifying waterborne epoxy polymer

Due to the low hydration rate of ordinary Portland cement (OPC), it cannot meet the requirements of rapid repair, especially in negative temperature environments. Simultaneously, facing the harsh marine environment, repair materials urgently need high durability. In this paper, a self-emulsifying waterborne epoxy polymer (WEP) suitable for cement systems was combined with sulfoaluminate cement (SAC) to prepare repair materials. The effect of the WEP on the development of microstructure, auto-drying shrinkage and durability was investigated by nuclear magnetic resonance, XRD and SEM methods. Results showed that a continuous interpenetration polymer network (IPN) was intricately woven within the multiphase composites, using the macromolecular network structure formed by —OH groups in WEP and Ca2+ and Al3+ in cement as a bridge. The resistance to chloride migration and water absorption exhibited an increase followed by a decrease. In particular, the resistance to chloride migration was improved by 45 %. Similar trends were observed for the carbonation resistance. Due to the lower porosity and capillary pressure caused by IPN, leading to a decrease in auto-drying shrinkage. Nevertheless, the larger percentage of capillary pores in a mortar with a P/C of 15 % increased the auto-drying shrinkage. The WEP decreased the availability of pores of critical diameter (14 nm) of mortar exposed to water freezing, leading to an increase in the freeze-thaw resistance. The change in P/C hardly affected the resistance to sulfate attack. Under −10°C, at 2 hours, the compressive strength of CSA-based mortar satisfied the load requirements for vehicular operations. This research provides theoretical and practical implications for the repair work of large-scale projects serving in harsh environments.

Recycling of coal gasification fine ash as a cement cleaning substitute: Performance, microstructure, and sustainability assessment of blended cement

Coal gasification fine ash (CGFA) is a large amount of solid waste produced by the coal chemical industry and has a broad prospect of recycling in building materials. This paper aims to investigate the effect of CGFA on the flowability, rheological behavior, hydration properties, and sustainability of blended cement. CGFA contains mineral components similar to fly ash (FA) and has morphological and physical filling effects that promote the flow of cement pastes. However, water absorption by the residual carbon component in CGFA resulted in severe deterioration of the rheological properties and flowability of the paste. As levels of CGFA replacement increase, the blended cement shows a pattern of decreasing early hydration rate, reducing flowability, and a significant increase in dynamic yield stress. The characteristic hydration reaction peak indicating the transformation of ettringite (AFt) to kuzelite (AFm) was little observed in the mixed samples of CGFA and cement. This phenomenon is attributed to the low content of reactive minerals (mainly aluminates) in CGFA. The residual carbon in CGFA almost did not participate in the hydration reaction of the blended cement system. It also limited the accumulation of hydration products, significantly reducing the strength of the mortar. Considering multiple factors such as cost, carbon emission, embodied energy, and cement consumption, the combined sustainability assessment of the (15%CGFA + 15%FA) combination replacing cement is 14.5% higher than that of pure cement mortar. This study can provide theoretical and technical support for the sustainable development of the coal chemical industry and the construction industry.

Strength evolution and microstructure of alkali-activated ultra-fine slag in low-temperature environments

At low temperatures, the hydration rate of Portland cement significantly slows down, resulting in a decrease in its strength. Additionally, the pore solution freezes easily. As a kind of low-carbon cementitious material, alkali-activated slag (AAS) material has a short setting time at room temperature and a high early strength, meanwhile the alkali solution does not freeze at low temperatures. This study investigates the evolution mechanism of the strength and microstructure of AAS pastes cured at 0℃. Slag with different fineness was activated by a sodium silicate solution with different moduli, whose strength was compared with that of an AAS paste cured at 20 °C. The setting time, compressive strength, hydration process, pore structure, freezing point, and microstructure of the AAS paste were studied, whose results showed that the setting time of the AAS paste was obviously prolonged, and the compressive strength decreased under low-temperature curing. After the activation of slag ground, the early compressive strength of the AAS paste was significantly improved, whose reason was that the hydration degree of the paste was improved after slag ground, and that a dense pore structure was formed. The freezing point of the AAS paste was approximately −26 °C, which, remarkably, even at −40 °C, was still capable of undergoing a weak chemical reaction, producing an exothermic enthalpy. This characteristic makes alkali-activated ultrafine slag a highiy-promising cementitious material for curing at low temperatures.

Effects of hydrotalcites with different laminate element composition on the properties of sulfoaluminate cement-based materials

Sulfoaluminate cement-based materials (SCBMs) are widely used for grout reinforcement. It is common to use a large water-to-binder ratio in the construction of sulfoaluminate cement-based materials (SAGMs) to obtain good workability and permeability, which results in poor early strength. Nanotechnology has been increasingly employed to enhance the mechanical properties of cementitious materials. Nano Zn-Mg-Al hydrotalcite-like (LDHs) and nano Mg-Al LDHs increased the early compressive strength, while nano Zn-Al LDHs had the opposite effect. However, the reason for this observation remains unclear. In this study, the influence of LDHs structures with different laminar element compositions on the hydration and hardening processes of SCBMs was analysed. The results showed that nano-ZnAl-LDHs, ZnAl-LDHs, and MgAl-LDHs could provide nucleation sites for ettringites. The three hydrotalcite-like materials slowly released different concentrations of zinc and/or magnesium ions and carbonate ions in the cement paste. Zinc ions entered the hydration product of ettringite and aluminum gel and promoted the transformation of amorphous aluminum gel to bayerite, which inhibited the hydration reaction of SCBMs. Magnesium ions could also enter the ettringite, and carbonate ions promoted the formation of calcium carbonate, which facilitated the hydration reaction of SCBMs. The nucleation effect and the release of zinc and/or magnesium ions and carbonate ions acted synergically to influence the hydration rate, pore size distribution, and compressive strength of SCBMs.

Effect of organic phosphonate types on performance of alkali-activated slag-based materials and its mechanism

The strong early hydration reaction and short setting time of alkali-activated slag composites limit its practical engineering application. Compared with traditional chemical admixture, organic phosphates show higher efficiency in delaying the hydration rate of alkali-activated slag system. However, there is limited research on the performance of alkali-activated materials modified by different organic phosphonate type, and their modification mechanism is also unclear. In this study, three typical organic phosphonates, hydroxyethylidene diphosphonic acid Tetrasodium (HEDP-4Na), aminotris (methylene phosphonic acid) tetrasodium (ATMP-4Na), and diethylenetriamine penta (methylene phosphonic acid) pentasodium (DTPMP-5Na), were selected to clarify the effect of organic phosphonate type on the properties of alkali-activated slag materials (AASM) and its mechanism under different dosage and alkali content. The results indicate that DTPMP-5Na has the most significant effect on the improvement of fresh properties of AASM, compared to other two types of organic phosphonates. Using 0.3 % DTPMP-5Na can delay the initial and final setting time of AASM by 240 % and 190 % respectively, increase the slump flow by 20 %, and reduce yield stress and plastic viscosity by 57 % and 65 % respectively. This was because DTPMP-5Na has the largest number of monomolecular phosphonic acid groups, which can chelate Ca2+ in AASM, form stable complex and cover the surface of slag particles to prevent further hydration reaction. The phosphonate reduced the hydration rate and increased the surface negative potential of slag particles. The use of phosphonates and the increase in alkali content can significantly enhance the mechanical properties of AASM. Using 0.3 % of ATMP-4Na increased the 28-day compressive and flexural strengths of AASM made with 5 % alkali content by 155 % and 53 %, respectively, compared to the reference AASM with 3 % alkali content. Additionally, the 28-day compressive and flexural strengths of reference at 7 % alkali content increased by 115 % and 12 %, respectively, compared to that at 3 % alkali content. This improvement was mainly attributed to the consumption of organic phosphonates at the early stage, which provides more activation channels for slag later, resulting in a denser structure and improved pore structure of AASM, thus enhancing its long-term mechanical properties. However, with the increase of alkali content, the effect of organic phosphonates in strengthening fresh and hardened properties of AASM decreased. The use of 0.3 % HEDP-4Na increased 28-day compressive and flexural strengths of AASM with 3 % alkali contents increased by 25 % and 27 %, respectively, compared to the reference AASM. Such improvement was limited to 4 % and 19 % for AASM made with 5 % alkali content. This can be attributed to the significant promotion of OH− concentration in the system, increasing the yield of hydration products at early age on the surface of slag, and reduced activation channels for slag over the long term. In summary, the use of ATMP-4Na is recommended in AASM, as it can significantly improve the fresh properties of AASM and provide excellent mechanical properties.