This paper presents experimental investigations and theoretical modeling of the hydration reaction of nanosilica blended concrete with different water-to-binder ratios and different nanosilica replacement ratios. The developments of chemically bound water contents, calcium hydroxide contents, and compressive strength of Portland cement control specimens and nanosilica blended specimens were measured at different ages: 1 day, 3 days, 7 days, 14 days, and 28 days. Due to the pozzolanic reaction of nanosilica, the contents of calcium hydroxide in nanosilica blended pastes are considerably lower than those in the control specimens. Compared with the control specimens, the extent of compressive strength enhancement in the nanosilica blended specimens is much higher at early ages. Additionally, a blended cement hydration model that considers both the hydration reaction of cement and the pozzolanic reaction of nanosilica is proposed. The properties of nanosilica blended concrete during hardening were evaluated using the degree of hydration of cement and the reaction degree of nanosilica. The calculated chemically bound water contents, calcium hydroxide contents, and compressive strength were generally consistent with the experimental results.
Scientists have considered nanomaterials to be the most promising materials of the 21st century. In recent years, considerable attention has been focused on civil engineering applications for nanomaterials because nanoparticles possess many unique properties due to their small size, such as large specific surface areas and high activity [
Many experimental studies have investigated the influence of adding nanosilica on the properties of concrete. Li et al. [
Although numerous experimental studies have investigated the physical and chemical properties of nanosilica blended concrete, research on the modeling of hydration of nanosilica blended concrete is very limited. Some existing hydration models [
Tomosawa et al. [
The reaction coefficient
The effective diffusion coefficient of water is affected by the tortuosity of the gel pores and by the radii of the gel pores in the hydrate. This phenomenon can be described as a function of the degree of hydration and is expressed as follows:
In addition, free water in the capillary pores is depleted as the hydration of cement minerals progresses. Some water is bound in the gel pores, and this water is not available for further hydration, which is an effect that must be taken into consideration during every step of the hydration process. Therefore, the amount of water in the capillary pores
The effect of temperature on these reaction coefficients is assumed to follow Arrhenius’s law, as shown in
Using the proposed Portland cement hydration model, Tomosawa et al. [
During the hydration of Portland cement paste, the amount of calcium hydroxide is directly proportional to the degree of hydration of cement [
Similar to the hydration of cement, as the pozzolanic reaction proceeds, water will be physically adsorbed in the nanosilica hydration products. Lura et al. [
Because of the high specific surface area of nanosilica and its considerable pozzolanic activity, in this paper, it is assumed that the hydration of nanosilica includes two processes: phase-boundary reaction process and diffusion process. Considering these points, based on the method proposed by Saeki and Monteiro [
The influence of temperature on hydration is considered using the Arrhenius law, as in the following equations:
When nanosilica is incorporated in concrete, two possible reasons may be used to explain the change in the hydration process. The first is the pozzolanic reaction of amorphous phases in nanosilica, and the second is the influence of nanosilica on the hydration of cement. In the current paper, a new model is proposed that can describe the pozzolanic reaction between calcium hydroxide and nanosilica. In addition, the influence of nanosilica on the hydration of cement is considered through the amount of capillary water (
In this study, (
Note that Tomosawa’s model is only valid for Portland cement; this model is not valid for nanosilica blended cement because of the coexistence of Portland cement hydration and the pozzolanic reaction of nanosilica. To model the hydration of nanosilica blended concrete, a new reaction model of nanosilica is constructed, and the mutual interactions between cement hydration and the nanosilica reaction are clarified.
An outline of the modeling process is shown in Figure
The outline of modeling.
To verify the proposed model, a series of experimental investigations were conducted to investigate the properties of nanosilica blended paste. The cementitious materials used were ordinary Portland cement (OPC) and nano-SiO2. The chemical composition of the Portland cement is shown in Table
Mineral compositions of Portland cement.
Mineral composition (% by mass) | Chemical composition (% by mass) | ||||||||||
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C3S | C2S | C3A | C4AF | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | SO3 | K2O | Na2O |
58.48 | 11.16 | 8.83 | 8.74 | 19.29 | 5.16 | 2.87 | 61.68 | 4.17 | 2.53 | 0.92 | 0.21 |
Properties of nanosilica.
Particle size (nm) | Specific surface area (m2/g) | Density (g/cm3) | SiO2 purity (%) |
---|---|---|---|
15 | 250 | 0.05 | 99.9 |
Mixing proportions of cement-nanosilica paste specimens.
Water-to-binder ratio (%) | Mixing proportions (kg/m3) | ||||
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Water | Cement | Nanosilica | AE water reducing agent | ||
OPC | 30 | 486 | 1619 | — | |
50 | 612 | 1224 | — | ||
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OPC + NS 3 | 30 | 486 | 1570 | 49 | Binder * 0.8% |
50 | 612 | 1187 | 37 | Binder * 0.5% | |
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OPC + NS 6 | 30 | 486 | 1522 | 97 | Binder * 1.6% |
50 | 612 | 1151 | 73 | Binder * 1.0% |
The specimens were prepared according to the Korean Agency for Technology and Standard KSL 5109 (testing method for mechanical mixing of hydraulic cement paste and mortars of plastic consistency). A rotary mortar mixer was used to prepare specimens. To uniformly disperse nanosilica particles, after dissolving the AE agent in water, nanosilica was added to water and mixed at a high speed for approximately 2 minutes. Then, the cement was added into the mixer and mixed for approximately 1 minute. After preparing the paste specimens (10 cm × 10 cm × 40 cm), the specimens were sealed and cured in a chamber at 20°C until they reached the testing age.
The compressive strength, chemically bound water content, and calcium hydroxide content were measured at different ages: 1 day and 3, 7, 14, and 28 days.
The compressive strength tests were performed according to the Korean Agency for Technology and Standard KSL 2426 (standard test method for compressive strength of mortar grout). The dimensions of the test specimens were 10 cm × 10 cm × 40 cm. The compressive strength was measured using a universal testing machine with a loading rate of 0.4 N/mm2/s. For each mixing proportion, three specimens were tested, and the compressive strength was determined from the average value of three specimens.
The fractured pieces after the compression test were preserved for the calcium hydroxide tests and chemically bound water tests. To stop the hydration reactions, the samples were soaked in acetone for 7 days and were then placed in a vacuum desiccator overnight to remove the acetone. These samples were further dried at 60°C in an oven for 24 hours and were ground to pass through a 150
To determine the content of chemically bound water, 1 g of the hydrated sample was dried in an oven at 105°C for 3 hours and was then ignited at 1050°C in an electric furnace for 1 hour. For pastes, the chemically bound water content was calculated as the difference between the weight on ignition at 1050°C and the weight after oven drying at 105°C.
The amount of calcium hydroxide (CH) in the samples was determined by TG/DTA (thermogravimetry/differential thermal analysis). Analyses were conducted at a heating rate of 10°C/min from 20°C to 1100°C under flowing nitrogen. The mass loss corresponding to the decomposition of Ca(OH)2 occurs between 440°C and 520°C.
During the hydration of ordinary Portland cement, the amount of calcium hydroxide will increase until it reaches a steady state. During the hydration of cement-nanosilica blends, the evolution of the amount of CH depends on two factors: the Portland cement hydration that produces CH and the pozzolanic reaction that consumes CH. In the initial period, the production of CH is the dominant process, and then the consumption of CH becomes the dominant process. In the experimental range, the amount of CH initially increases, reaches a maximum value, and then decreases. Using the experimentally determined calcium hydroxide content and chemically bound water content of Portland cement paste, the value of
A total of 12 series of experimental results regarding the calcium hydroxide contents and chemically bound water contents are presented in Figure
Evaluation of calcium hydroxide contents.
OPC paste with water-to-cement ratio 0.5
OPC paste with water-to-cement ratio 0.3
OPC-nanosilica paste with water-to-binder ratio 0.5 and 3% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.3 and 3% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.5 and 6% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.3 and 6% nanosilica
Evaluation of chemically bound water contents.
OPC paste with water-to-cement ratio 0.5
OPC paste with water-to-cement ratio 0.3
OPC-nanosilica paste with water-to-binder ratio 0.5 and 3% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.3 and 3% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.5 and 6% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.3 and 6% nanosilica
Calibration process: in the calibration process, using calcium hydroxide contents for
Reaction coefficients of the proposed hydration model.
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4.310 * 10−7 | 0.035 | 1.073 * 10−5 | 5.474 * 10−8 | 1.00 * 10−6 | 2.50 * 10−13 |
Validation process: in the validation process, the remaining experimental results, such as the calcium hydroxide contents and chemically bound water contents shown in Figures
In summary, to use the proposed theoretical model, a small number of experimental results are required to calibrate the reaction coefficients. Furthermore, a large number of experimental results can be predicted. The time-consuming and expensive experimental investigations can be significantly reduced.
The evolution of the CH amount is shown as a function of the hydration time in Figure
As proposed in the hydration model and reference [
As shown in Figures
When calculating the calcium hydroxide contents in pastes with higher nanosilica replacement levels (Figures
When calculating the chemically bound water contents in pastes with lower water-to-binder ratios (Figures
Hence, at the current stage, the model is not perfect because some variables are not accounted for; thus, the model will be subjected to further improvements in the future.
It is well known that the compressive strength of concrete depends on the gel/space ratio determined from the degree of cement hydration and the
Furthermore, the development of compressive strength in blended concrete can be evaluated through Powers’ strength theory as
Based on the compressive strength of nanosilica blended paste, the values of the coefficients of
Evaluation of compressive strength developments.
OPC paste with water-to-cement ratio 0.5
OPC paste with water-to-cement ratio 0.3
OPC-nanosilica paste with water-to-binder ratio 0.5 and 3% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.3 and 3% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.5 and 6% nanosilica
OPC-nanosilica paste with water-to-binder ratio 0.3 and 6% nanosilica
This paper presents the results from experimental investigations and theoretical modeling of the hydration reaction of nanosilica blended concrete. The contents of chemically bound water in nanosilica blended paste are similar to those in the control specimens, which means that the pozzolanic reaction of nanosilica does not produce chemically bound water. Due to the pozzolanic reaction of nanosilica, the contents of calcium hydroxide in nanosilica blended paste are much lower than those in the control specimens. Compared with the control specimens, the extent of compressive strength enhancement in nanosilica blended specimens is considerably higher at early ages.
A numerical model is proposed to simulate the hydration of concrete containing nanosilica. The reaction coefficients of nanosilica are obtained from the experimentally determined calcium hydroxide contents in nanosilica blended concrete. The degree of hydration of cement and the degree of reaction of nanosilica are calculated to accompany the results from the proposed hydration model. The development of compressive strength in nanosilica blended paste is evaluated using the gel-space ratio, which considers the contributions of cement hydration and the nanosilica reaction. The calculated results regarding chemically bound water, calcium hydroxide, and compressive strength generally agree with the experimental results.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study was financially supported by the National Research Foundation of Korea (Grant no. NRF-2013R1A1A2060231; Project name: An Integrated Program for Predicting Chloride Penetration into Reinforced Concrete Structures by Using a Cement Hydration Model).