Calcium Aluminate Research Articles

Sustainable Stabilization of Cadmium (Cd)-Contaminated Soil with Biochar-Cement: Evaluation of Strength, Leachability, Microstructural Characteristics, and Stabilization Mechanism

ABSTRACT To facilitate the solid waste reuse of cadmium (Cd)-contaminated soil and biochar and reduce the high emission of cement materials, we employed biochar-cement composite stabilizer to remedy Cd-contaminated soil. We assessed the effects of the biochar-cement composite stabilizer dosage and curing time on stabilized soil strength by using unconfined compressive strength (UCS) tests, leaching concentration and stabilization rate in Cd-contaminated soil stabilized by biochar-cement by using toxicity characteristic leaching procedure, and mechanism of change in strength and leaching characteristics by using scanning electron microscopy and X-ray diffraction. The research results demonstrate that biochar addition to cement-stabilized soil promoted cement hydration reaction and increased particle agglomeration. A low dosage of biochar improved stabilization effects. The optimal biochar dosage was 5%, which enhanced the early growth speed of the UCS. More cement improved the stabilized soil’s UCS. At a 5% cement dosage, adding biochar significantly reduced Cd leaching concentration, and improved the stabilization rate. At a 10% cement dosage, the biochar-cement composite stabilized soil’s stabilization rate was >99.8%. Calcium silicate hydrate, calcium aluminate hydrate, and ettringite were the main products of biochar-cement stabilized soil that filled soil and biochar pores; these substances encapsulated, adsorbed, or exchanged ions of Cd2+, achieving stabilization effects.

Aluminate 2K systems in digital concrete: Process, design, chemistry, and outlook

Digital concrete is advancing due to growing economic incentives for construction automation. Achieving more sustainable concrete construction requires carbon reduction, and digital concrete technologies enable material-saving designs. By decoupling production strength from design strength, two-component (2K) systems utilizing aluminate precipitation offer the most flexibility, allowing more sustainable mixes with higher substitution levels. However, 2K aluminate systems are complex and demand a deeper understanding of their chemistry and strength buildup. This article reviews the basics of 2K aluminate systems, specifically aluminum sulfate-based and calcium aluminate cement/calcium sulfate-based systems, and their use in an inline active mixing reactor. An example reaction engineering analysis predicts the degree of reaction in a given reactor design, relating it to yield stress. The two chemical systems are compared, and future research recommendations are provided.

Photon and thermal neutron shielding behaviors of aluminum calcium fluoroborate glass modified with barium oxide: FLUKA Monte Carlo, XCOM and experimental investigations

The borate glass system was prepared using the melt-quenching method. The XRD was used to confirm the amorphous structure of present samples, the FTIR was utilized to observe vibration modes, and the UV–Vis-NIR spectrophotometer was employed to measure transmittance spectra. The radiation shielding properties were examined by FLUKA Monte Carlo simulation, XCOM programs, and experimental methods. The density increased, while the molar volume decreased as the BaO content increased. For the X-ray and gamma-ray radiation, indicate that the addition of BaO instead of B2O3 improve the shielding properties, while the Σ is decreased for thermal neutron radiation. According to the results, adding BaO enhanced the X-ray and gamma-ray shielding properties. However, adding BaO led to the reduces its ability of shielding the thermal neutrons of the glass samples. These data are expected to be accommodate to considering for any applications which requires high efficiency of photon or thermal neutrons shielding.

Improvement of strength and microstructure of low-carbon cementitious materials with high volume of calcined coal gangue

The accumulation of coal gangue poses significant environmental challenges due to its massive volume and low reactivity. This study investigates the influence of calcium hydroxide (Ca(OH)₂) and gypsum on the mechanical properties, hydration, and pore structure of low-carbon cementitious materials incorporating high volumes of calcined coal gangue (CCG) and limestone. The results showed that the addition of Ca(OH)₂ significantly increased compressive strength, with a 19 % improvement at 28 days in LCC-40-P mix, which contained 40 % cement and 2.73 % Ca(OH)₂ by weight. The incorporation of 10 % gypsum in the LCC-40-P mix and 20 % gypsum in the LCC-70 mix (containing 70 % cement without Ca(OH)₂) effectively refined the pore structure, reduced overall porosity, and enhanced the material's density and mechanical properties, resulting in compressive strengths of 38.4 MPa (72 % of the strength of Portland cement) and 50.2 MPa (94 % of the strength of Portland cement) at 28 days, respectively. Microstructure and hydration heat tests showed that the addition of Ca(OH)₂ accelerates the hydration reaction and significantly enhanced compressive strength by ensuring sufficient Ca(OH)₂ for pozzolanic reactions, leading to the formation of additional calcium silicate hydrate, calcium aluminate silicate hydrate and carboaluminate phases. Gypsum refined the pore structure by shifting the pore size distribution towards smaller sizes, reducing overall porosity, and increasing the proportion of gel and small capillary pores. These findings highlight the potential of using CCG with optimized additives to develop sustainable, low-carbon construction materials with enhanced performance characteristics.

Enhancing marl soil stability: nanosilica’s role in mitigating ettringite formation

Marl soil is highly prone to erosion when exposed to water flow, posing a potential threat to structural stability. The common practice of stabilizing soil involves the addition of cement and lime. However, persistent reports of severe ruptures in many stabilized soils, even after extended periods, have raised concerns. In stabilized marls, unexpected ruptures primarily result from the formation of ettringite, which gradually damages the soil structure. This article aims to assess the impact of nanosilica on the formation of ettringite and the nanostructure of calcium silicate hydrate (C-S-H) during the marl soil stabilization process with lime. To achieve this, marl soil was stabilized with varying percentages of lime and nanosilica. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images were collected to observe changes in mineralogy and microstructural properties. Various geotechnical parameters, including granularity, Atterberg limits, compressive strength, and pH, were measured. The results indicate that the uniform distribution of nanosilica in marl-lime soils enhances pozzolanic activities, calcium aluminate hydrate growth (C-A-H), and the nanostructure of calcium silicate hydrate (C-S-H). According to XRD and SEM experiments, the presence of nanosilica reduces the formation of ettringite. Moreover, the compressive strength of modified samples exhibited an upward trend. In the experimental sample manipulated with 1% nanosilica combined with 6% lime, the compressive strength increased by 1.84 MPa during the initial 7 days, representing an approximately 18-fold improvement compared to the control sample.

Resource utilization of sulfidic copper tailings in calcium aluminate cement: Mechanical property, hydration process and environmental impact

Sulfidic copper tailings (STs) is a byproduct generated from mineral processing activities. To address this environmental burden, calcium aluminate cement (CAC) was used to utilize STs as cementitious materials. The influence of STs on the hydration process, mechanical properties and life-cycle assessment of STs blended CAC were investigated. Results show that the initial setting time of the samples is significantly shortened with the presence of STs. Moreover, the addition of STs greatly accelerates cement hydration and promotes the formation of ettringite, which is caused by the dissolution of sulfidic-rich STs. Hence, the hardened cement paste containing 5 wt.% STs with dense microstructure and high elastic modulus is achieved, which enhances the compressive strength for STs blended CAC. From an environmental point of view, the utilization of STs can save energy and reduce CO2 emissions. Such work may provide new insights into the application of sulfidic copper tailings in low-carbon cementitious materials.

Performance of high-strength green strain-hardening cementitious composites incorporating CSA/CAC cements and GGBS

Strain-hardening cementitious composites (SHCCs) possess crack control and durability properties. In the pursuit of reducing CO2 emissions and addressing potential issues of slow strength development for SHCCs with pozzolans, the objective of this study was to develop high-strength green SHCCs (HSG-SHCCs) that exhibit improved early strength, drying shrinkage, and environmental friendliness. A ternary binder system was proposed, comprising of ground granulated blast furnace slag (GGBS), ordinary Portland cement (OPC), and either calcium sulfoaluminate cement (CSA) or calcium aluminate cement (CAC). The fresh, mechanical, and durability properties of the proposed HSG-SHCCs were experimentally investigated. Additionally, the microstructures and chemical phases of the developed CSA/CAC-GGBS-OPC ternary binder system were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results revealed that the developed HSG-SHCCs demonstrated 6-hour and 28-day strengths of up to 11 MPa and 110 MPa, respectively. The CSA/CAC-based ternary binder system also exhibited enhanced durability and improved drying shrinkage. However, the inclusion of CSA and CAC in HSG-SHCCs could lead to a reduction in tensile performance, specifically in strain capacity. The main hydration products of the CSA/CAC-GGBS-OPC ternary binder system were C-S-H, ettringite, gibbsite, and calcium hydrate. In addition to the fresh, mechanical, and durability properties, the environmental impact of the developed HSG-SHCCs was also assessed. The results indicated that a 30 % replacement of OPC with CSA cement in the HSG-SHCC mix notably improved the trade-off among CO2 emissions, compressive strength, and tensile cracking resistance.

Physico-chemical characterization of cupola slag: Enhancing its utility in construction

Cupola slag is a waste material of the steel and iron industries. Its composition is determined by the cupola furnace and other elements used in steel and iron manufacturing. This paper investigates the characterization behavior of various cupola slag materials. As a result, x-ray fluorescence (XRF), x-ray diffraction (XRD), thermogravimetry differential thermal analysis, and scanning electron microscopy (SEM) methods were used to characterize three cupola slag samples from distinct origins. In addition, various physical properties were used to compare different cupola slags. The specific gravity values of CS-1 (cupola slag-1 sample), CS-2 (cupola slag-2 sample), and CS-3 (cupola slag-3 sample) are 1.36, 2.5, and 2.917, respectively. The density and water absorption for CS-1, CS-2, and CS-3 are 1414.86, 1477.71, and 1796 kg/m3, and 0.37%, 0.32%, and 0.26%, respectively. Cupola slag also includes a larger percentage of lime, according to XRF data, which contributes to its improved binding characteristics. A higher calcium oxide content in CS-3 could facilitate the pozzolanic process. The presence of angular particles that aid in material binding is seen in the SEM image. Compounds with a nanostructure are then flawlessly blended into the mixture and grouped with calcium alumina silicates formed by cement hydration. The XRD pattern of cupola slag exhibits high peaks, indicating that the material is crystalline in character and can be utilized as sand. It also shows the presence of other chemical compounds, such as silica, which ranges from 30% to 45%. CS-1 and CS-2 have comparable XRD patterns. However, CS-3 has a somewhat different pattern because of the greater CaO content. Weight loss begins at higher temperatures, which shows that the material is stable at higher temperatures, according to a thermo-gravimetric study. The differential thermal analysis curve of CS-3 indicates that the material remains stable up to a temperature of 600 °C. The physical characteristics of all cupola slag samples show that cupola slag may be utilized to make sustainable building materials because of its lower specific gravity, density, and water absorption.

Preparation of calcium aluminate and spinel by hydrolysis and calcination from secondary aluminum dross

AbstractThe direct extraction of alumina from secondary aluminum dross (SAD), which is a dangerous solid waste, is difficult. Moreover, this process easily produces a large amount of solid waste residue, which is not easily utilized. In this paper, a new green process was developed to prepare calcium aluminate and Mg‐Al spinel from SAD by hydrolysis–calcification roasting. The effects of calcium oxide (CaO) content, sintering temperature, and holding time on the properties of calcium aluminate were investigated by single‐factor experiments. The phase transformation mechanism of calcium aluminate was studied by thermodynamic analysis, X‐ray diffraction analysis, X‐ray fluorescence spectroscopy, and scanning electron microscopy. Under the optimal conditions (Ca/Al molar ratio of 0.8, sintering temperature of 1300°C, and holding time of 2 h), the main calcium aluminate phases are CaAl2O4 and Ca2Al2SiO7, the soluble alumina content of the calcium aluminate sample is 49.71 wt.%, and the main phases of the acid‐insoluble residue are Mg‐Al spinel and a very small amount of CaTiO3. The Ca/Al ratio is the key factor affecting the calcium aluminate phase—with increasing Ca/Al ratio, the calcium aluminate phase is transformed from CaAl4O7 to CaAl2O4 and eventually to Ca12Al14O33, and the Si‐containing phase changes from Ca2Al2SiO7 to CaSiO4.

Influence of silica fume addition and content on the early hydration of calcium aluminate cement – The role of soluble silicon

Hydration of a commercial white calcium aluminate cement (CAC) at 23 °C was modified by silica fume (SF) addition in varying amounts. The process was followed by heat flow calorimetry, quantitative in-situ XRD analysis and Gillmore needle experiments supplemented by pore solution analysis, thermodynamic modelling, and 1H-TD-NMR measurements. Lower SF/cement ratios accelerate the hydration of CAC. Higher ratios trigger an intermediate heat flow event, which is correlated to increased setting rates. This intermediate event (IE) initiates an induction period of constant duration, which appears later with increasing SF/cement ratios. Results show the IE is caused by an initially hindered CA dissolution, in which dissolved silicon provided by SF plays a crucial role. Increasing the [Si] concentration in the pore solution leads to a further retardation of the IE and eventually prevents the entire hydration reaction if a critical amount is reached. A detailed model explaining the observed behavior is proposed.

The influence of low calcium sulfate contents on early calcium aluminate cement–calcite hydration kinetics and pore solution chemistry

A binder mix consisting of CAC, calcite and H2O was investigated to clarify the influence of the addition of low amounts of calcium sulfate in the form of gypsum, hemihydrate or anhydrite on the hydration at 23 °C. Based on experimental data, a model for the hydration of CAC + calcite in the presence of sulfate ions could be developed. Using heat flow calorimetry at 23 °C, it was shown that hydration is accelerated depending on the solubility rates of the different sulfates. In addition, early hydration is characterized by a sequential reaction in which the calcium sulfate must be consumed completely from the pore solution by the precipitation of ettringite before the already known course of CAC + Cc hydration can occur. Monocarbonate and AH3 precipitate as the stable dominant hydrate phases during the main reaction, but conversion-sensitive hydrate phases such as CAH10 are not stable.

Mechanical, microstructural, and thermal characterization of geopolymer composites with nano‐alumina particles and micro steel fibers

AbstractCement production is responsible for 5%–7% of global CO2 emissions, highlighting the need for sustainable alternatives like geopolymer composite (GCOMP) to meet the growing demand for concrete. This study investigates the mechanical, microstructural, and thermal properties of GCOMP by incorporating nano‐alumina (n‐alumina) and MSF (MSF). The n‐alumina content was varied at 1%, 2%, and 3% by weight of the mix, while the MSF content remained fixed at 0.5% by weight. Thermal characterization was conducted up to 800°C. The performance of GCOMP blends with n‐alumina was compared to a control blend consisting of only 0.5% MSF. Various mechanical properties were evaluated for all GCOMP blends. Microstructural and mineralogical characteristics were analyzed using scanning electron microscopy (SEM), X‐ray diffraction (XRD), and Fourier‐transform infrared spectroscopy (FTIR). Thermogravimetric and differential thermal analysis were performed up to 800°C for the thermal analysis of the GCOMP mix. The results indicate that the optimal mechanical properties were achieved with 2% n‐alumina (compressive and flexural strength increased by 35.65% and 77.7%, respectively). Additionally, the incorporation of n‐alumina improves the interfacial zones and results in a denser structure. GCOMP mortars portrayed a mass loss between 25°C and 250°C, with a marginal mass loss occurring between 250°C and 715°C. No mass loss was observed between 715°C and 800°C. The MSF‐reinforced GCOMP mortars experienced an ultimate mass loss of approximately 12%, with the MSF showing negligible influence. The addition of n‐alumina particles to MSF‐reinforced GCOMP resulted in the development of stronger samples characterized by the presence of C–S–H, calcium aluminate oxide hydroxide, and quartz.

Synthesis and reaction mechanism of high injected electron concentration and low work function [Ca24Al28O64]4+:(4e-) by Sol-gel method combined with spark plasma sintering

The high injected electron concentration and low work function C12A7:e- was successfully prepared by the sol-gel method combined with spark plasma sintering(SPS) using calcium nitrate, aluminum nitrate, citric acid, and ethylene glycol as raw materials. By analyzing the morphology of carbon and its role in the reactive synthesis, we reveal the reaction synthesis mechanism of C12A7:e- under the SPS. The analysis shows that C12A7:e- started to be generated at 800 °C. During the SPS reaction sintering process, carbon atoms from citric acid and ethylene glycol were excited by a pulsed current at high temperature, forming the carbon ions C22- and C (gas). Both act as reducing agents and react with the free oxygen ions O2- in the C12A7 cage cavity to form gaseous CO or CO2, enabling electrons to be injected into the cage cavity, thus generating C12A7:e-. Meanwhile, the graphene oxide (GO) formed on the surface of C12A7:e- grain is produced by the precursor of sol-gel synthesis in the high temperature oxidation environment, which will be conducive to improving the stability of C12A7: e- in air and high temperature oxidation resistance.

Study on the Mechanical Performance, Durability, and Microscopic Mechanism of Cement Mortar Modified by a Composite of Graphene Oxide and Nano-Calcium Carbonate

Nano-calcium carbonate (NC) is a novel ultrafine solid powder material that possesses quantum size effects, small size effects, surface effects, and macroscopic quantum effects that ordinary calcium carbonate lacks. As a nanomaterial with superior properties, graphene oxide (GO) has been studied extensively in the field of construction. In microscopic characterization, the reaction between NC and tricalcium aluminate (C3A) formed a new hydration product, hydrated calcium aluminum carbonate (C3A·CaCO3·11H2O), which enhanced the arrangement of hydration products and optimized the distribution of pore size in the mortar. Regarding the mechanical properties, the addition of GO and NC significantly enhanced the early-age mechanical performance of the mortar. In terms of durability, the incorporation of GO and NC significantly improved the water permeability, chloride ion permeability, and resistance to sulfate attack of the cement mortar. In this study, it was found that adding 1 wt% NC and 0.02 wt% GO not only improves the mechanical and durability properties but also promotes the hydration reaction according to the microstructure analysis. With the help of NC, compared with other studies, the amount of GO is reduced, while the cost is reduced, and the application of GO in the field of cement-based materials is promoted.

High-temperature heat treatment of hydrogen-reduced bauxite residue-calcite pellets for iron particles growth and separationTraitement thermique à haute température de boulettes de résidus de bauxite et calcite réduites à l’hydrogène pour la croissance et la séparation des particules de fer

ABSTRACT Bauxite residue characterised as a polymetallic source containing iron, alumina, titanium, calcium, silicon and rare earth elements offers an opportunity for reduced waste volume and the recovery of valuable metals. This study focuses on the agglomeration of bauxite residue with calcite, followed by hydrogen reduction at 1000°C. The resulting iron particles in the reduced pellets were initially very small distributed in a porous oxide matrix, posing challenges for physical separation methods. To address this issue, further heat treatments were conducted on the reduced pellets in an argon atmosphere, ranging from 1100°C to 1550°C. Phase analysis and microstructural examinations were performed using X-ray powder diffraction and scanning electron microscopy, respectively. Experimental findings indicate that the size of iron particles in the reduced pellets increases with higher heat treatment temperatures. Up to 1200°C, the oxide matrix remains a solid state, transitioning to a semi-molten and molten state above this heat treatment temperature and complete separation of iron from the oxide matrix above 1500°C. An alkali-leachable phase is present in the reduced pellets, initially in the form of calcium aluminate (mayenite), while above 1400°C, this phase dissociates to form monocalcium aluminate, and complete conversion at 1500°C. As observed in the microstructural analysis, there is an increase in the particle size of iron in both the solid and semi-molten states with an increasing in temperature from 1100°C up to 1300°C. This comprehensive study sheds light on the transformative effects of varying heat treatment temperatures on the composition and microstructure of bauxite residue-based materials.

Link between the Reactivity of Slag and the Strength Development of Calcium Aluminate Cement.

The problem of loss of strength caused by the conversion reaction with calcium aluminate cements (CAC) is well known. It has been shown that the addition of ground granulated blast furnace slag (GGBS) to CAC inhibits the conversion process. Different slags can have a different chemical and mineralogical composition depending on their origin and production process, which can significantly influence their reactivity. This work investigated the extent to which the R3 test, developed for Portland cement and based on isothermal calorimetry and/or bound water, was used to predict the reactivity of ground granulated blast furnace slag in a CAC. Mortars and cement pastes with a 30% replacement of slag were tested to evaluate their compressive strength and microstructure. The results show that slags with higher reactivity due to their hydraulic properties lead to a lower compressive strength loss within the first 6 h, a higher strength loss after 24 h due to stratlingite formation and a lower strength loss after 28 days due to pozzolanic reaction and stratlingite formation. The results also confirm that the R3 test was used as a rapid method to predict the effects of slag on the compressive strength of CAC.

Flammability, Toxicity, and Microbiological Properties of Polyurethane Flexible Foams.

The aim of the research was to investigate the influence of calcium phosphinate (HPCA) and aluminum phosphinate (HPAL) in synergistic systems with organophosphorus compounds, i.e., diphenylcresyl phosphate (CDP) and trichloropropyl phosphate (TCPP), on the thermal stability, flammability, smoke density, and emission of toxic gases during the thermal decomposition of polyurethane (PUR) foams. Thermogravimetric analysis (TGA), along with cone calorimetry and microcalorimetry, were used to assess the influence of fillers on the thermal stability and flammability of PUR foams. The analysis of toxic gas products was performed with the use of a coupled TG-gas analyzer system. The optical density of gases was measured with the use of a smoke density chamber (SDC). The obtained results showed an increase in thermal stability and a decrease in the flammability of the PUR composites. However, the results regarding smoke and gas emissions, as well as toxic combustion by-products, present ambiguity. On one hand, the applied flame retardant systems in the form of PUR-HPCA-CDP and PUR-HPCA-TCPP led to a reduction in the concentration of CO and HCN in the gas by-products. On the other hand, they clearly increased the concentration of CO2, NOx, and smoke emissions. Microbiological studies indicated that the obtained foam material is completely safe for use and does not exhibit biocidal properties.

Utilization of industrial waste sodium sulfate for calcined clay-based sustainable binder

This study explored the utilization of industrial waste Na2SO4 to develop a calcined clay-based sustainable cementitious composite inspired by ancient Roman concrete. Na2SO4 with or without the addition of NaCl was utilized, keeping the total Na2O quantity constant, and their performances at the macro and micro scale were compared. A separate set of samples was prepared using seawater. A medium-grade calcined clay, along with lime, was utilized as the binder. The salts were first dissolved in water and then used as the mixing water for sample preparation. A diverse array of characterization methods was utilized, encompassing thermogravimetric analysis, chemical extraction, X-ray diffraction (XRD), and Backscattered Scanning electron microscopy (SEM). It was observed that the salt-containing batches achieved compressive strength faster than the seawater samples. However, the strength remained in a comparable range (∼30 MPa) for all the samples after 28 days of curing. The initial strength of salt-containing samples was attributed to the formation of gypsum, ettringite, and hydrocalumite. Additionally, the geopolymer gel and calcium aluminum silicate hydrate (C-A-S-H) gel phases were present in all samples. Finally, this study also showed that lime-clay mortars can achieve a reduction in carbon footprint of up to 50 % compared to ordinary portland cement (OPC) mortars.

Influence of CaCO3 on Density and Compressive Strength of Calcium Aluminate Cement-Based Cementitious Materials in Binder Jetting.

We investigated the impact of CaCO3 addition on the density and compressive strength of calcium aluminate cement (CAC)-based cementitious materials in binder jetting additive manufacturing (BJAM). To confirm the formation of a uniform powder bed, we examined the powder flowability and powder bed density for CaCO3 contents ranging from 0 to 20 wt.%. Specifically, powders with avalanche angles between 40.1-45.6° formed a uniform powder bed density with a standard deviation within 1%. Thus, a 3D printing specimen (green body) fabricated via BJAM exhibited dimensional accuracy of less than 1% across the entire plane. Additionally, we measured the hydration characteristics of CAC and the changes in compressive strength over 30 days with the addition of CaCO3. The results indicate that the addition of CaCO3 to CAC-based cementitious materials forms multimodal powders that enhance the density of both the powder bed and the green body. Furthermore, CaCO3 promotes the formation of highly crystalline monocarbonate (C4AcH11) and stable hydrate (C3AH6), effectively inhibiting the conversion of CAC and showing compressive strengths of up to 5.2 MPa. These findings suggest a strong potential for expanding the use of BJAM across various applications, including complex casting molds, cores, catalyst supports, and functional architectural interiors.

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.