Early Strength Agent Research Articles

Boosting MgO hydration through sodium bicarbonate: Enhancing the strength of MgO-calcined clay cementitious materials

The slow early strength development of MgO-calcined clay cementitious materials poses significant challenges in meeting industrial requirements, thereby limiting their large-scale application. To address this issue, this study employed 1 wt% of sodium bicarbonate to activate the MgO-calcined clay cementitious materials and compared its effectiveness with that of two commonly used early strength agents: calcium chloride and sodium sulfate. The results demonstrated that sodium bicarbonate promoted strength development more effectively than both calcium chloride and sodium sulfate. This enhanced performance was attributed to the superior ability of sodium bicarbonate to facilitate the dissolution of the Mg(OH)2 surface layer formed during the hydration of MgO. Consequently, the reactivity of MgO was improved, leading to the formation of more hydration products. This study provides a pathway for the activation and application of MgO-calcined clay cementitious materials.

Study on the anchoring properties of new water glass-modified alkali-activated portland cement materials

This study developed a novel water glass-modified alkali-activated portland cement (WGSC) anchoring agent, characterized by low cost, rapid setting, early strength, and slight expansion. It is suitable for fully bonded anchor rod anchoring in soft rock tunnels. Using orthogonal experiments and range analysis, the optimal composition of WGSC was determined: silicate cement as the matrix, water-cement ratio of 0.3, 20 % sodium silicate with a modulus of 3.3, 2.5 % early strength agent, and 9 % UEA. The initial/final setting times were 6.72/8.63 minutes, linear expansion rates at 3 days/7 days were 0.32 %/0.54 %, and compressive strengths at 1 hour/24 hours were 16.4/28.2 MPa. Pull-out model tests demonstrated a 47.95–92.85 % increase in 24 h tensile bond strength compared to conventional silicate cement anchoring agents. Corrosion resistance tests indicated WGSC maintained a stable appearance compared to traditional agents. SEM analysis revealed a dense structure and clear network in WGSC, while XRD analysis identified microcomponents mainly as CSH, active silica (SiO2), and CASH, contributing to its dense structure and ideal expansion properties, facilitating superior anchoring and durability. This research elucidates the mechanism of WGSC, confirming its rapid setting, compressive strength, and corrosion resistance advantages, making it economically feasible and advantageous for application and promotion, thereby providing insights for the development of anchoring materials in highly deformed soft rock tunnels.

Impact of calcium formate and anhydrous sodium sulfate on the flowability, mechanical properties, and hydration characteristics of UHPC

Ultra-high performance concrete (UHPC) is not an ideal material for rapid repairs of bridge expansion joints due to its slow early strength development. To mitigate this early strength limitation, this study investigated the use of calcium formate (CF) and anhydrous sodium sulfate (SS) as early strength agents to enhance the early strength of UHPC. The findings indicate that a composite addition of 0.25 % SS and 0.25 % CF significantly improves both strength and workability, resulting in a 28-day compressive strength of 141.3 MPa, a 28-day flexural strength of 12.6 MPa, and a flowability of 227 mm. SS facilitates the formation of ettringite (AFt) and calcium hydroxide (C-H), while CF promotes the generation of smaller gismondine particles. This substantial increase in hydration products reduces the porosity of UHPC, thereby densifying the matrix and enhancing its mechanical strength. The synergistic effect of both additives leads to a higher concentration of calcium ions (Ca2+) around the smaller gismondine particles, which generates additional hydration products, optimizes the pore structure of UHPC, and narrows the width of the interfacial transition zone (ITZ). Consequently, this combination significantly improves the early strength of UHPC, achieving a 45.6 % enhancement in the early hydration rate with the use of 0.25 % CF and 0.25 % SS.

Performance Evaluation and Mechanism Study of Solid Waste-Based Cementitious Materials for Solidifying Marine Soft Soil under Seawater Mixing and Erosion Action

The main purpose of this research is to develop a solid waste-based cementitious material (SWC) instead of cement for solidifying a large amount of marine soft soil with high water content and low bearing capacity in coastal areas. This aims to solve the problems encountered in the practical application of cement soil, such as slow strength growth and poor durability. The SWC includes ground granulated blast furnace slag (GGBS), dust ash (DA), and activated cinder powder (ACP), with admixtures of naphthalene sulfonate formaldehyde condensate (NS) and compound salt early strength agent (SA). Both the 7 d and 28 d compressive strength values of the SWC formulations G4 and G7 are about twice as strong as those of cement soil (GC), even when mixed with seawater. Immersion tests revealed that stabilized soil had superior resistance to seawater corrosion compared to cement soil. X-ray diffraction, scanning electron microscopy, infrared spectroscopy, and thermogravimetric analysis explained that the main hydration products in cement soil are C-S-H and CH, while in stabilized soil, SWC generates a large amount of C-A-S-H with gelling properties and AFt with filling properties. These hydration products have better effects on strength and seawater erosion resistance.

The Research Effects of Variable Temperature and Early Strength Agent on the Mechanical Properties of Cement-Stabilized Macadam.

In cold regions with high daily temperature gradients (>20 °C), the durability of cement-stabilized macadam (CSM) base materials is poor and prone to cracking. To effectively reduce the cracking of semi-rigid base layers in cold regions with high daily temperature gradients and extend fatigue life, this study focused on cracking and fatigue characteristics of CSM with a 10% commercial early strength agent (ESA) added by the external mixing method under different curing conditions. The ESA was manufactured by Jiangsu Subote New Materials Co., Ltd. (Nanjing, China). The curing conditions were divided into variable temperature (0-20 °C) and standard temperature (20 °C). CSM curing was carried out through a programmable curing box. The research results indicated that the variable temperature curing conditions reduced the strength and fatigue resistance of CSM and accelerated the modulus attenuation rate of CSM. At the same time, the drying shrinkage of CSM was greater. The temperature shrinkage coefficient and strain of CSM under variable temperature conditions were smaller than those under standard temperature conditions. The effect of variable temperature conditions on the cracking and durability of CSM could not be ignored in cold regions. Compared to standard temperature curing conditions, the indirect tensile strength of CSM reduced by 31.04% under variable temperature conditions, the coefficient of variation increased by 2.97 times, and the discrete type significantly increased. Compared with CSM without ESA, the dry and temperature shrinkage strains of CSM with 10% ESA were reduced by 24.65% and 26.10%, respectively. At a stress level of 0.6, compared to standard temperature curing conditions, the fatigue life of CSM decreased by 97.19% under variable temperature conditions. Under variable temperature conditions, the fatigue life of CSM with 10% ESA increased by 196 times compared to 0% ESA. Adding ESA enhanced the anti-shrinkage cracking, strength, and durability of CSM under variable temperatures. ESA incorporation effectively compensated for the weakened characteristics of CSM under variable temperature conditions. The study proposed a practical approach for boosting the durability of CSM in cold environments.

Synergistic Effect of Early-Strength Agents on the Mechanical Strength of Alkali-Free Liquid Accelerator Mortar

Synergistic Effect of Early-Strength Agents on the Mechanical Strength of Alkali-Free Liquid Accelerator Mortar

Analysis of Different Early Strength Agents on the Performance of Prefabricated UHPC.

Precast ultra-high-performance concrete (UHPC) has emerged as indispensable in the engineering sector due to its cost-effectiveness and superior performance. Currently, precast UHPC grapples with challenges pertaining to slow setting times and insufficient early strength, largely attributed to its high water-reducing agent content. Effective utilization of early strength agents to augment UHPC's early strength is pivotal in addressing this issue. This study investigates the efficacy of two distinct concrete early strength agents, namely calcium formate (Ca(HCO2)2) and aluminum sulfate (Al2(SO4)3). A UHPC system with a water/cement ratio of 0.17 was used; both single and compound doping experiments were conducted using varied dosages of the aforementioned early strength agents. Our results show that both early strength agents significantly reduce setting time and enhance early strength at appropriate dosages. Specifically, the addition of 0.3% Ca(HCO2)2 led to a 33.07% decrease in setting time for UHPC. Moreover, the incorporation of 0.3% Ca(HCO2)2 and 0.5% Al2(SO4)3 resulted in a strength of 81.9 MPa at 1.5 days, representing a remarkable increase of 118.4%. It is noteworthy that excessive use of Ca(HCO2)2 inhibits the hydration process, whereas an abundance of Al2(SO4)3 diminishes the early strength effect. Simultaneously, this article provides recommendations regarding the dosage of two distinct early strength agents, offering a novel solution for expediting the production of prefabricated UHPC with a low water/cement ratio and high water-reducing agent content.

Effect of mechanical properties and microscopic mechanism of cement-stabilized macadam under variable temperature environment with early strength agent

The early strength formation of Cement-Stabilized Macadam (CSM) base materials in cold regions is affected by environmental changes. At the same time, the curing temperature of CSM in the actual service environment did not meet the laboratory standard temperature curing environment (ST-CE). This study explores the evolution law of CSM performance and the influence of an early strength agent (ESA-I) on the early performance of CSM under variable temperature curing environment (VT-CE) in cold regions. The actual service environment for CSM curing was simulated in the laboratory, and CSM specimens with five ESA-I dosages of 0%, 5%, 10%, 15%, and 20% were prepared. The effect of ESA-I on the early properties of CSM under VT-CE was analyzed. The micro mechanism of CSM under different curing environments and ESA-I dosages was investigated based on X-ray diffraction, Fourier transform infrared spectroscopy, and Scanning electron microscope. The experimental results indicated that the early strength formation of CSM was slow in VT-CE, and the 7-day unconfined compressive strength did not meet the construction quality control indicators of CSM material under heavy traffic conditions. It was necessary to extend the curing period to meet the construction requirements. Adding ESA-I could enhance the strength, modulus, frost resistance, and fatigue performance of CSM under different curing environments. Compared with the ST-CE, the VT-CE would weaken the improvement effect of ESA-I on the performance of CSM. The recommended range for ESA-I in VT-CE was 10% −15% based on comprehensive compressive strength and resilient modulus indicators. Microscopic experiments show that under a VT-CE, the hydration rate of cement decreases, the Ettringite content in the hydration products is higher, and the microstructure is loose. Adding ESA-I improves the hydration rate of cement under VT-CE, and the microstructure contains more C-S-H hydration products.

The influence and mechanism analysis of aluminum sulfate as an environmentally friendly early strength agent on the properties of cement-based materials

Environmentally friendly chemical admixtures are becoming important in the cement industry. The research objective of this paper is to use aluminum sulfate as an early-strengthening agent to achieve the goals of a large increase in early strength and a reduction in CO2 emissions. The experimental design was as follows: The macroscopic properties and microscopic characteristics of aluminum sulfate were experimentally investigated on five different replacement ratios of 0%, 1%, 2%, 3%, and 4% paste specimens of cement at different ages. The results show that (1) AS4 demonstrated a notable enhancement in early strength, while AS2 and AS3 contributed to increased late-stage strength. Over time, the Ultrasonic Pulse Velocity (UPV) of all samples exhibited a steady rise. Additionally, aluminum sulfate was found to reduce the electrical resistivity of the samples. (2) aluminum sulfate increased the rate of exothermic heat release of samples at induction and acceleration phases, advanced the accumulated heat was high in the samples with high aluminum sulfate content, and the accumulated heat was strongly correlated with the compressive strength. FT-IR and XRD results show that aluminum sulfate promoted the formation of AFt while depleting the CH content, and the TGA results show the content of bound water in the samples increased from Day 1 to Day 28, which had a positive effect on the compressive strength. UHR-SEM results show that increasing aluminum sulfate substitution results in the formation of a denser microstructure. (3) for all the various mixtures, AS2 has the lowest CO2 emissions per unit strength.

Effect of 2-acrylamide-2-methylpropane sulfonic acid on the early strength enhancement of calcium silicate hydrate seed

Nanomaterials are becoming widely used in cement to enhance the properties of cement-based materials. Artificially synthesized nano-C-S-H serves as a nucleation site for the hydration product C–S–H gel, facilitating its formation. This is deemed as an early strength agent to effectively promote cement hydration. Introducing comb-like polycarboxylate ether (PCE) during the synthesis of C–S–H can effectively reduce the size of C–S–H, resulting in better early strength effects. In this study, the acid-ether ratio of PCE is set at 6:1, and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) substitutes acrylic acid (AA) at varying molar ratios to obtain modified PCE. Calcium silicate hydrate seed-polycarboxylate ether (C-S-Hs-PCE) is synthesized using co-precipitation technique. This study investigated impacts of AMPS on the particle size of C-S-Hs-PCE and the early strength of cement-based materials using dynamic light scattering analysis, total organic carbon analyzer, X-ray diffraction analysis, low-field nuclear magnetic resonance, scanning electronic microscopy, isothermal calorimeter, and compressive strength testing. The results show that the C-S-Hs-PCE, synthesized at an AMPS to AA molar ratio of 5:95, turns out to possess a reduced particle size compared to use of PCE without AMPS. This is pivotal in promoting the hydration of the silicate phase and the formation of calcium hydroxide and C–S–H gel in the paste, and thereby reducing the total porosity while improving the early mechanical strength of the mortar, especially within first 24 h.

Research on High Efficiency Charging and Mechanized Sealing Technology of Deep Hole Presplitting Blasting in Roof

In order to solve the problem of long sealing time and poor effect in the deep-hole presplitter blasting technology, the inorganic two-component blasting sealing material with a fixed water-cement ratio of 0.8:1, with "cement, gypsum and lime" as the main, "quick-setting agent, water-reducing agent and early-strength agent" as the auxiliary, and the matching double-bag sealing device were developed, and a set of rapid sealing process suitable for deep-hole blasting was designed. The field test was carried out in 43153 multipurpose roadway of Chengzhuang Mine. The application results show that the blasting sealing material with fast setting speed and high strength can meet the needs of sealing holes, and the perforation rate is far less than 4-8% of the blasting mud sealing hole. The fast sealing process designed and developed to complete a hole sealing operation only takes 14min, which not only improves the sealing efficiency, but also reduces the labor intensity of workers and realizes the mechanization of the sealing operation.

Research on the preparation and properties of GBFS-based mud solidification materials

Various problems are often encountered during the backfilling process of deep foundation pits. The development of low-cost and efficient solidified materials for the preparation of fluidized solidified soil is currently an ideal solution. This article used industrial solid waste (granulated blast furnace slag, fly ash, carbide slag, etc.) as the main raw material to study the hydration hardening properties of solidified materials and the construction feasibility of fluidized solidified soil prepared from solid waste materials. The results are as follows: Compared with cement-based materials, solid waste-based solidified materials had lower early activity. The cumulative heat release within 72 h was less than 200 J/g. Different solid wastes, such as fly ash and carbide slag, had different effects on the properties of solidified materials. Overall, they had the potential to prepare fluidized solidified soil. The prepared fluidized solidified soil had a fluidity greater than 350 mm, a 28d compressive strength greater than 3 MPa, and exhibited good workability and excellent mechanical properties. Hydration products such as C-S-H and AFt were filled in the soil structure. The 28d compressive strength well above the design requirements of general engineering projects. Meanwhile, the prepared fluidized solidified soil had good adaptability to conventional water reducers (fluidity could be increased by more than 40%) and early strength agents (1d compressive strength could be increased by more than 60%).

Effect of additive compounding on early properties of belite sulfoaluminate cement

Belite sulfoaluminate cement is attracting increasing attention and being considered as a potential alternative to ordinary Portland cement owing to lower preparation temperature and carbon dioxide (CO2) emission. However, it still has disadvantages of poor workability and low early strength. This study investigated the effects of compound polycarboxylate superplasticiser (PCE) and early strength agent, calcium nitrite (Ca(NO2)2), on flowability and early strength of belite calcium sulfoaluminate (BCSA) cement. Variation patterns of setting time, fluidity, strength and polycarboxylate superplasticiser (PCE) adsorption of the paste were measured; the hydration products and microscopic morphology were also analysed by the heat of hydration, X-ray diffraction and scanning electron microscopy. Results showed that calcium nitrite effectively enhanced adsorption of PCE onto the surface of BCSA cement particles, leading to notable improvement in the fluidity of the paste (reaching up to 275 mm). In the initial hydration stage, 0.7% PCE compounded with 1.2% calcium nitrite inhibited the formation of ettringite (AFt), resulting in prolonged setting time. However, it deepened the degree of hydration of BCSA for 3 days and refined the hydration product, AFt crystals. Consequently, the compressive strength was increased to 95.75 MPa and 107.13 MPa for BCSA cement at 3 days and 28 days, respectively.

Effects of early vibration and resistant strategies on bond performances between high-strength concrete and rebar

The local bond performances between rebar and high-strength concrete (HSC) exposed to early vibration were experimentally investigated in this study. In addition, three vibration-resistant strategies, including additions of special rapid-hardening cement(calcium sulphoaluminate cement, CSA), shrinkage compensation admixture (expansion agent, EA), and toughening metallic fibers (steel fiber), were verified through penetration resistance, compressive, and central pull-out tests, respectively. Finally, two-way ANOVA tests were conducted to understand the significance of each strategy from a statistical perspective. The results demonstrated that early disturbances compromise bond performances by weakening chemical adherence and mechanical interlocking effects, which considerably degrade bond strength and stiffness. The presence of EA delayed the setting and hydration of concrete and prolonged vibration-sensitive duration. However, the later expansive hydrate products effectively strengthen the bond performances and compensate for early-stage vibration-induced damage. In comparison, the synergetic effects between CSA and Portland markedly enhance the hydration reactivity of blends at early stages, which can achieve threshold maturity and vibration resistance within hours. However, excessive CSA (10%) severely compromises the construction time of mixtures and amplifies the cracking potentials of HSC under the constraints of steel bars. The synergetic effects of steel fibers and aggregates formed disordered networks to enhance the vibration resistance and rigid skeletons for bridging cracks of HSC. The two-way ANOVA results indicate that EA significantly improves the mechanical performances of HSC, followed by steel fibers. The additions of expansive agents and steel fibers are highly recommended in engineering with vibration scenarios. In contrast, the dosages of CSA (or similar early-strength agents) should be carefully controlled unless with a robust shrinkage compensation strategy.

An orthogonal experimental study on the influence of steam-curing on mechanical properties of foam concrete with fly ash

Foam concrete can be widely utilized as precast walls for its low density, excellent heat resistance and sound insulation. The challenge of its application is its low strength due to the porous matrix, which prolongs the demolding time. Steam curing at an early time is a common strategy to improve its early-age strength. Here, an orthogonal experiment (OA16) was adopted to investigate the influence of curing parameters on the properties of foam concrete. Specific compressive strengths (SCS) given as the ratio of the strength and the density at 12 h (12 h) and 28 days (28d) were calculated to evaluate the early-age and long-age strength. The range analysis and the analysis of variance (ANOVA) showed that the optimum steam-curing system that can yield the highest 12 h SCS (1.9 MPa, which meets the demand for demolding) is that preheating temperature of 25 ℃, preheating duration of 4 h, curing temperature of 50 ℃, together with 1.5% early-strength agent Na2SO4. Compared to the normal cured concrete, the addition of mineral power (fly ash by 20%) will successfully increase the early-age strength under steam curing, while XRD, SEM and FTIR analysis all show there are more hydrated products formed after steam curing and the activity of fly ash is activated to form C-S-H gels. However, improving early-age strength too much will lead to a deficiency of later-age strength growth. MIP analysis also exhibited high pore probability in the sample cured under higher steam-curing temperatures. These results can provide experiences for the wide application of foam concrete in precast walls and help improve the efficiency of their template turnover.

Insight into the flexural characteristics of rapid hardening ultra-high performance concrete (RH-UHPC) incorporating C-S-H seeds using acoustic emission technology

With the increasing requirements for emergency repair projects, there is a pressing need for high-quality repair materials. In this work, a series of rapid hardening ultra-high performance concrete (RH-UHPC) were prepared by using C-S-H seeds and SAC as early-strength agents. The effects of these early-strength agents on RH-UHPC’s flexural behaviors, including failure pattern, strength, energy absorption capacity, and toughness index, were investigated. Also, the variations of acoustic emission signals and microstructure were studied to provide a thorough insight into the influence of early-strength agents on RH-UHPC. The results show that C-S-H seeds and SAC exhibited a good effect on improving the 1-day flexural strength of RH-UHPC, but increased the risk of shear failure at 28 days. The flexural strength, energy absorption capacity, and toughness of RH-UHPC at 28 days were reduced by adding C-S-H seeds. However, combining C-S-H seeds with SAC alleviated this reduction. Moreover, incorporating early-strength agents enhanced the ringing counts, AE energy, damage rate, and the proportion of shear cracks. The noticeable increase in r value (i.e., RA/AF) judged the damage stage of RH-UHPC more reliably. Micro-analysis revealed that introducing C-S-H seeds increased the internal cracks and the Ca/Si ratio of C-S-H gel, but the hybrid of C-S-H seeds and SAC partially mitigated above problem. The incorporation of SAC notably increased the AFt content and decreased the CH content. C-S-H seeds presented little effect on the hydration degree of RH-UHPC, but roughened the pore size and enlarged the average pore size.

Study on Early Hydration Mechanism of Double-Liquid Grouting Material Modified by Composite Early Strength Agent.

To achieve an adjustable setting time and significantly improved early strength of a new type of sulphoaluminate cement-based double-liquid grouting material (SACDL), the effects of calcium formate, sodium sulfate, lithium carbonate, and a composite early strength agent on the setting hardening and early hydration behavior of SACDL paste were studied by means of setting time, fluidity, compressive strength, and viscosity tests. The results showed that the adsorption and osmosis of calcium formate, the complex decomposition of sodium sulfate, the precipitation polarization of lithium carbonate and the synergistic action of the composite early strength agent could accelerate the early hydration rate of SACDL, shorten the coagulation time, and improve the early strength of SACDL. The composite effect of 0.8% calcium formate and 0.5% sodium sulfate is the most significant in promoting coagulation and early strength; the initial setting time and final setting time of the slurry were shortened to 5 min and 10 min, respectively; and the 3 h compressive strength was capable of reaching 16.7 MPa, 31% higher than that of the blank group. In addition, X-ray diffraction and SEM morphology observation were used to study the composition of the hydration products and the evolution of the microstructure, which revealed the early hydration mechanism of SACDL under the synergistic effect of the composite early strength agent: (1) The solubility of tricalcium aluminate (C3A) and dihydrate gypsum (CaSO4·2H2O) increased under the low content composite early strength agent condition, which increased the ettringite (AFt) formation rate. HCOO- was able to penetrate the hydration layers of tricalcium silicate (C3S) and dicalcium silicate (C2S), accelerating the dissolution of C3S and C2S and promoting the early hydration of SACDL. (2) Under the condition of a high dosage of the composite early strength agent, the further increase in Ca2+ concentration promoted the crystallization nodules and precipitation of CH and accelerated the formation of calcium silicate hydrate (C-S-H) gel. C-S-H was filled between a large number of rod-like AFt crystals, thus making the structure more dense.

Performance optimization design of high ductility cement-based alkali-activated municipal solid waste incineration fly ash composite for rapid repair material

As a residue that results from the high-temperature incineration of urban household waste, municipal Solid Waste Incineration Fly Ash (MSWIFA) contains various harmful organic pollutants like dioxins and heavy metals. These contaminants pose significant health risks to humans, including deformities and carcinogenic effects. The focus of this study is the development of a High Ductility SAC Rapid Repair Material (HD-SACRRM) through MSWIFA as a partial replacement for Sulphate Aluminate Cement (SAC), so as to achieve high ductility, rapid hardening, and the strength required for pavement layers. The reactivity of MSWIFA is investigated by adjusting its content and the type and dosage of alkali-activators. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) tests are conducted to analyze the variety and microstructure of alkali-activated MSWIFA-SAC hydration products. Also, the efficiency of immobilizing heavy metal ions within the alkali-activated system is examined. Based on these findings, the proportions of HD-SACRRM are optimized. The experimental results demonstrate that the optimal conditions for MSWIFA reactivity are met when the MSWIFA content is 20% and the 30% soluble silicate with a modulus of 1.8 is present. The optimized conditions of HD-SACRRM are as follows: a 20% MSWIFA content, 30% soluble silicate with a modulus of 1.8, a water-cement ratio of 0.30, a 2.0% fiber volume fraction, a sand-cement ratio of 0.30, and 5% Li2CO3 as an early strength agent. After 48 h of curing as standard, the material exhibits a compressive strength of 45.7 MPa, an ultimate elongation of 2.70%, and an average crack width of 127.59 μm. The alkali-activated MSWIFA-SAC system generated more gels such as C-S-H and C-A-H, thus resulting in a dense microstructure that enhances both the capacity of heavy metal immobilization and mechanical strength. These findings shed some light on the recycling of MSWIFA solid waste and the repair of asphalt concrete road surfaces.

Effect and mechanism of high Li2CO3 concentrations on the early hydration of sulfoaluminate cement-based materials

Lithium carbonate (Li2CO3) is a commonly used early strength agent for sulfoaluminate cement based grouting materials. Low concentrations of Li2CO3 (≤30 μmol Li/g in sulfoaluminate cement) can significantly accelerate the early hydration of sulfoaluminate cement. However, the effect of high Li2CO3 concentrations (>30 μmol Li/g in sulfoaluminate cement) on the early hydration of sulfoaluminate-cement-based materials (SCBMs) is unclear. In this study, the effects of high concentrations of lithium ion (Li+) and carbonate ion (CO32) on the early hydration of SCBMs (C4A3S¯–CaSO4–CaO) with different mineral compositions (systems T1 and T2) were investigated by hydration calorimetry, X-ray diffraction, and ion concentration measurements. The results revealed that both Li+ and CO32− promoted the hydration of C4A3S¯. Li+ and CO32− each reacted with aluminum and calcium in solution, and the formation of ettringite was suppressed when they were present. At low aluminum and high calcium concentrations in solution (system T1), ettringite formation was controlled mainly by the low concentration of aluminum. Less ettringite was generated, and the inhibition of ettringite formation by both Li+ and CO32− was weaker. At high concentrations of both aluminum and calcium in solution (system T2), ettringite formed more rapidly than in system T1, and Li+ and CO32− inhibited its formation to a greater extent. Li2CO3 affected the amount of ettringite by changing aluminum ion and calcium ion in slurry, thus leading to the change of hydration heat release. These new insights into the interaction between Li2CO3 and SCBMs contribute to the theoretical foundation for cement hydration control.

Development and Promotion of Concrete Strength at Initial 24 Hours.

Knowing and promoting the strength development of concrete at an earlier age is essential for accelerating formwork circulation of the on-site construction and precast product manufacture. The strength development rate at earlier ages of less than the initial 24 h was investigated. The effect of measures of adding silica fume, calcium sulfoaluminate cement, and early strength agent on the strength development of earlier concrete at ambient temperatures of 10, 15, 20, 25, and 30 °C was studied. The microstructure and long-term properties were further tested. It is shown that the strength increases exponentially first and then logarithmically, different from what is commonly recognized. Increasing cement content exhibited a certain effect only above 25 °C. When the cement content increased from 420 to 460 kg/m3, the strength only increased from 6.2 to 6.7 MPa after 12 h at 25 °C. The early strength agent could increase the strength significantly, the strength could be increased from 6.4 to 10.8 MPa after 20 h at 10 °C and from 7.2 to 20.6 MPa after 14 h at 20 °C. All measures for promoting earlier strength did not have an evident negative effect. The results could be potentially referred for the formwork removal at a suitable moment.