Synthesis and Performance Evaluation of Hydrated Calcium Silicate-Based Early-Strength Admixture for Concrete ^†

Abstract

Calcium silicate hydrate (C-S-H) serves as the primary hydration product of cement and constitutes the fundamental strength contributor in concrete matrices. This study presents a novel approach for synthesizing an early-strength admixture for hydrated calcium silicate through the sol–gel methodology. A comprehensive experimental investigation was conducted to evaluate its performance characteristics. The research findings demonstrate that the synthesized admixture, prepared using high-speed mechanical agitation in conjunction with a proprietary dispersant solution, exhibits optimal particle size distribution and dispersion stability at ambient temperature conditions (25 °C). When incorporated into cementitious systems, this innovative additive significantly reduces the energy barrier for nucleation processes, thereby accelerating cement hydration kinetics. The resulting concrete specimens show marked reductions in initial setting times and substantial improvements in early-age compressive strength development.

1. Introduction

Urban infrastructure projects like prefabricated housing and subways require high-strength concrete components. Enhancing early strength is crucial [1,2,3,4]. Calcium silicate hydrate (C-S-H), a key cement hydration product, boosts strength when used as an additive. It accelerates hydration, shortens setting time, and improves early strength [5]. Current C-S-H production methods have drawbacks—hydrothermal, which needs high pressure/temperature, consuming high energy, and precipitation, which is time-consuming and inefficient [6,7,8]. Existing methods also cause uneven particle mixing, clumping, and poor dispersion, reducing effectiveness.

The sol–gel method for preparing calcium silicate hydrate (C-S-H) offers advantages such as simple equipment, stable reaction, and small particle size with high stability [9,10]. Using silicate esters as the silicon source, highly dispersed SiO₂ particles are generated under alkaline conditions, with Igepal CO-520 as a dispersant to enhance solution stability. The resulting C-S-H exhibits small particle size, high purity, and no precipitation during long-term storage [11,12,13].

Furthermore, papermaking white mud (a high-calcium industrial waste) can be used as a calcium source after calcination and acid treatment, enabling waste recycling and cost reduction. This early-strength agent lowers the nucleation energy barrier of cement, advances hydration exothermic peaks, and significantly shortens setting time while improving early strength.

2. Experiment

2.1. Experimental Raw Materials

Papermaking waste lime: A paper factory in Nanping, Fujian Province, with a pH value of 11. The main components are shown in

Table 1. Components of papermaking waste lime.

Components	CaO	MgO	Fe2O3	SiO2	Al2O3	Loss on Ignition
Ratio/%	51.7	1.87	0.68	5.79	2.64	35.42

; Nitric acid: concentration 0.5 mol/L, analytical purity, Xiling Chemical Co., Ltd.; Tetraethyl orthosilicate: TEOS, analytical purity, National Pharmaceutical Group Chemical Reagents Co., Ltd.; Polyoxyethylene (5) nonylphenyl ether: Igepal CO-520, Mn = 441, Sigma-Aldrich Company; Cyclohexane: analytical purity, National Pharmaceutical Group Chemical Reagents Co., Ltd.; Sodium hydroxide: concentration 0.5 mol/L, analytical purity, Guangdong Guanghua Technology Co., Ltd.; Anhydrous ethanol: analytical purity, Shantou Daohao Fine Chemicals Co., Ltd.; Hexadecyltrimethylammonium bromide: CTAB, content ≥ 99%, Sigma-Aldrich Company; Sodium hexametaphosphate: SHMP, content ≥ 96%, Sigma-Aldrich Company.

Cement: Minfu P·O 42.5 (performance indicators in

Table 2. Performance indicators of cement.

Compressive Strength/MPa		Flexural Strength/MPa		Requirement of Normal Consistency/%	Setting Time/min		Soundness/%	Fineness/%
3 d	28 d	3 d	28 d		Initial	Final
30.3	54.0	7.1	10.3	28.8	141	194	Pass	1.0

). Sand: Fineness modulus 2.6–2.9, silt content < 1%. Aggregates: 6–20 mm nominal size, 10–20 mm nominal size, continuously graded. Mineral powder: S95 grade. Fly ash: Grade II. Superplasticizer: Point-S polycarboxylate: 30% water reduction, 50% solid content, Prominent Science New Materials Group Co., Ltd. (Xiamen, China).

2.2. Performance Testing and Characterization

2.2.1. Particle Size Analysis

It was analyzed using a laser diffraction particle size analyzer (Winner 2308C, Jinan Nanoparticle Technology Co., Ltd., Shandong, China) equipped with a high-stability He-Ne laser operating at 632.8 nm wavelength. Before measurement, the specimens were dispersed in deionized water and subjected to ultrasonic treatment to achieve a homogeneous suspension.

2.2.2. Performance Testing of Cement Mortar

The workability of cement paste was assessed following GB/T 8077-2012 standard procedures for admixture uniformity testing. The test formulation comprised 300 g of Portland cement, 1.0% by weight of Point-S superplasticizer, 0.05% of the synthesized accelerator, and 87 g of mixing water [14].

2.2.3. Concrete Performance Testing

It included slump, slump loss over time, setting time, compressive strength, and flexural strength, and was evaluated following GB/T 8076-2008 “Concrete Admixtures.” And according to the provisions of GB/T 50080-2016, the slump of concrete is controlled at (200 ± 10) mm, and the expansion of concrete is controlled at (500 ± 20) mm [15,16].

2.3. Synthesis of Calcium Silicate Hydrate Early-Strength Agent (CSH-A)

Calcium source was prepared from papermaking waste lime. The lime was crushed, heated to 150 °C at 5 °C/min, held for 30 min, then heated to 600 °C at 10 °C/min and held for 1 h, and finally heated to 800 °C at 5 °C/min and held for 2 h. The cooled product was sieved (<80 μm) and washed 3–4 times with distilled water. It was then soaked in 0.5 mol/L HNO₃ for 12 h, filtered, and adjusted to a Ca²⁺ concentration of 3 mol/L with distilled water. After aging for 10 h, the solution was used as the calcium source.

Tetraethyl orthosilicate (TEOS) was used as the silicon source, diluted with distilled water to a Si⁴⁺ concentration of 1 mol/L. A dispersant solution was prepared by mixing Igepal CO-520 with cyclohexane in a 1:10 volume ratio.

In a high-speed stirring apparatus at 25 °C, 150 mL of the dispersant solution was stirred at 500 rpm. A total of 25 mL of each of the calcium and silicon solutions was slowly added over 2 h (Ca/Si molar ratio = 3:1) via a peristaltic pump. Simultaneously, 0.5 mol/L NaOH solution was added to adjust the pH to 12. The stirring speed was increased to 1000 rpm to shear and disperse the mixture uniformly. The reaction continued for 3 h to form a white C-S-H gel, which was centrifugally washed 4 times with a 1:1 ethanol/water solution and vacuum-dried at 70 °C for 2 days to obtain the final product.

3. Results and Discussion

3.1. Study on the Influence of Different Dosages on the Properties of Early-Strength Materials

Effect of Reaction Temperature on CSH-A

CSH-A were synthesized via the sol–gel method, where the reaction temperature was controlled to adjust particle size and dispersion performance. The reference specimens were prepared as per Section 2.3, with Reaction Specimen 1# synthesized at 50 °C and Specimen 2# at 100 °C.

As shown in

Table 3. Effect of reaction temperature on early strength of CSH-A.

Project	Temperature/°C	Particle Size/nm	Paste/mm
Reference specimen	25	193.5	225
Specimen 1#	50	227.4	220
Specimen 2#	100	411.8	200

, C-S-H particles at 25 °C exhibited the smallest size (193.5 nm) due to controlled gel growth under high-speed stirring and dispersant action, minimizing agglomeration. At 50 °C, particle size slightly increased to 34 nm, while a significant rise to 411.8 nm occurred at 100 °C, attributed to temperature-induced agglomeration and poor monodispersity.

Cement paste fluidity decreased from 225 mm (25 °C) to 220 mm (50 °C) and 200 mm (100 °C). The 25 °C specimen, with its finer particles, enhanced dispersion by filling micro-pores, maintaining fluidity without bleeding.

3.2. Effect of Stirring Speed on CSH-A

The particle size and dispersibility of CSH-A can be adjusted by varying the stirring speed [17,18]. The preparation method for Specimen 3# (control) remained largely unchanged, except for a reduced stirring speed during the synthesis of CSH-A gel. Specifically, calcium and silicate solutions were added dropwise to a dispersant solution at 200 r/min via a peristaltic pump over 2 h, followed by continued stirring at 400 r/min for 3 h (in

Table 4. Effect of stirring speed on early strength of C-S-H.

Project	Particle Size/nm	Paste/mm
Reference specimen	193.5	225
Specimen 3#	356.8	205

).

A significant reduction in stirring speed led to a noticeable increase in particle size, while the cement paste fluidity decreased from 225 mm to 205 mm, indicating weakened dispersion. High-speed stirring shears the reaction materials, cutting, compressing, and folding them to achieve rapid and uniform mixing. This prevents agglomeration, yielding CSH-A with a smaller particle size and better dispersibility. When incorporated into cement, it significantly promotes hydration, acts as a water-reducing agent, and enhances fluidity.

3.3. Effect of Dispersant Solutions on CSH-A

The choice of dispersant solution significantly impacts the particle size and dispersion performance of CSH-A. By varying the dispersant solution, its effect on dispersion can be evaluated. In the Control Specimen 4#, no dispersant was added during preparation, using distilled water instead; Specimen 5# replaced the dispersant with a CTAB and SHMP in a 4:6 ratio.

The data from

Table 5. Effect of dispersant solution on early strength of C-S-H.

Project	Particle Size/nm	Paste/mm
Reference specimen	193.5	225
Specimen 4#	418.4	200
Specimen 5#	276.2	215

show that when Igepal CO-520 and cyclohexane were used as the dispersant, the particle size was smallest and the mortar fluidity was optimal. This is because the dispersant forms hydrophilic groups on sol particle surfaces, stabilizing the dispersion and preventing aggregation, thereby enhancing product performance. Specimen 4# exhibited aggregation and larger particle sizes due to the absence of a dispersant, while Specimen 5#’s CTAB-SHMP combination performed poorly, further confirming the superiority of the Igepal CO-520-based dispersant.

3.4. Concrete Application Test

To validate the efficacy of the CSH-A developed in this study, concrete application assessments were systematically executed following the guidelines stipulated in GB/T 8076-2008. The designated concrete mix proportions are delineated in

Table 6. Concrete mix proportions. Kg/m³.

C	S	Gsmall	Glarge	F	M	W
330	700	452	678	70	40	148.5

, while the comprehensive performance evaluation outcomes are documented in

Table 7. Concrete performance.

Specimen	Slump (mm)	Spread (mm)	Setting Time (min)		Compressive Strength (MPa)		Flexural Strength (MPa)
			Initial	Final	12 h	1 d	12 h	1 d
Blank specimen	190	450	198	326	5.2	8.9	0.7	1.8
Reference specimen	220	525	76	131	16.2	24.4	4.3	7.9
Specimen 1#	220	520	82	138	15.9	24.2	4.2	7.7
Specimen 2#	190	500	144	241	10.1	17.8	1.7	3.6
Specimen 3#	220	500	150	235	12.5	20.7	2.1	5.5
Specimen 4#	210	500	135	223	13.4	21.9	2.9	6.1
Specimen 5#	210	510	120	210	12.8	21.5	2.6	5.7

.

As listed in the table, the blank specimen without C-S-H accelerator exhibited the worst concrete performance. CSH-A addition significantly increased slump, shortened initial and final setting times, and improved compressive and flexural strengths, outperforming the blank and other control specimens. This indicates that CSH-A reduces the nucleation barrier of hydration products and accelerates cement hydration through adsorption-nucleation effects. It promotes the formation of primary C-S-H gel and ettringite, whose network structure fills pores and voids in the cement matrix and enhances early-age strength.

Compared with the 25 °C sample, 50 °C reaction temperature induced slight performance degradation, showing high-shear stirring largely offsets thermal disturbance during sol-gel synthesis. At 100 °C (Control Specimen 2#), obvious slump reduction, prolonged setting time and strength loss occurred, originating from severe thermally triggered particle agglomeration. Reduced stirring speed also deteriorated performance. High-shear stirring reconstructs raw materials to generate fresh interfaces and realize rapid uniform dispersion, producing CSH-A with finer particle size, higher stability and better performance.

Without dispersant (Control Specimen 4#), concrete performance improved but remained inferior to dispersant-containing groups. The dispersant inhibits particle aggregation and maintains fine particle size for optimized performance. Replacing the Igepal CO-520/cyclohexane dispersant system with CTAB-SHMP (Control Specimen 5#) degraded concrete performance. Therefore, the original dispersant system achieves fast and homogeneous raw-material dispersion, yielding high-performance accelerators with small particle size and good dispersibility.

4. Conclusions

The synthesis of CSH-A gel was meticulously controlled by employing a high-shear mechanical mixing apparatus in conjunction with the sol–gel methodology at 25 °C. Through the synergistic action of high-intensity agitation and a proprietary dispersant solution comprising Igepal CO-520 and cyclohexane, the growth kinetics of CSH-A particles were effectively regulated. This innovation successfully mitigated particle aggregation issues stemming from non-uniform mixing, ultimately yielding CSH-A products characterized by nanoscale particle dimensions, superior colloidal stability, and exceptional dispersion properties.

When integrated into cementitious systems, this CSH-A-based accelerator demonstrated remarkable efficacy in promoting cement hydration reactions. The additive significantly reduced the initial setting time of concrete while concurrently enhancing its early-age compressive strength development.

Notably, the production process utilized papermaking waste lime as the calcium source, achieving both cost reduction and sustainable utilization of industrial byproducts. The method’s inherent advantages include simplified equipment requirements and economical operational parameters, positioning it as a highly viable solution for precast concrete manufacturing applications.