CN105174768B - A kind of nano-cellulose fiber strengthens cement-based material - Google Patents

Abstract

The invention discloses a nano-cellulose fiber reinforced cement-based material, which contains nano-cellulose fiber components. The nanocellulose fiber reinforced cement-based material of the present invention, CNFs improves the hydration degree of cement paste, delays the cement solidification time, can prolong and maintain good fluidity, is conducive to the discharge of internal gas, and reduces the porosity of cement paste, Make the structure more compact and improve the mechanical properties of cement paste. The appropriate amount of CNFs is beneficial to the increase of hydration degree and the reduction of porosity. The test confirmed that the flexural and compressive strength of cement at 28d age increased by 15% and 20% respectively when the amount of CNFs was 0.15%.

Description

A kind of nanocellulose fiber reinforced cement-based material

technical field

The invention belongs to the technical field of cement, and in particular relates to a nano-cellulose fiber reinforced cement-based material.

Background technique

Cement-based materials have unique characteristics, such as low cost, high pressure resistance, and strong durability. In today's building materials industry, cement-based materials have always been one of the most widely used building materials. Nevertheless, cement is still a single-function, self-heavy brittle material, which is prone to a large number of cracks in structural engineering, and can no longer meet the sustainable and high-performance requirements of large bridges, dams, nuclear reactor peripherals, etc. . In recent decades, in the field of civil engineering science and application, people have been exploring research work on improving the performance of cement-based materials, such as using fiber-reinforced cement-based composite materials to realize intelligent control of the physical and chemical properties of cement-based composite materials, etc. . When glass fiber is used as a reinforcing material, although the strength is high, the fiber is not easy to bond with the matrix, and subsequent treatment is required to improve the adhesion with the matrix. In addition, glass fiber is brittle and can only bear tensile force, not bending. , shear and compressive stress; corrosion of steel fibers is the main reason affecting the durability of composite materials. For example, steel fiber reinforced concrete is prone to corrosion at cracks, which has become a thorny industry problem; carbon fiber and aramid fiber often have problems such as high energy consumption, high cost, easy pollution of the environment, and limited use of the environment as reinforcing materials. Natural plant fibers are rich in resources, green and environmentally friendly, and have excellent mechanical properties. Natural plant fiber-reinforced composite materials also have excellent properties, such as natural plant fiber used to reinforce cement, the tensile and compressive strength of cement-based composite materials are significantly improved, and cracks are reduced. The hydroxyl groups in natural plant fibers can form strong intermolecular hydrogen bonds and covalent bonds with cement matrix polymers, while unreacted hydroxyl groups make natural plant fibers exhibit hydrophilicity, with a moisture content of 8-12%, causing cement Poor adhesion to plant fibers, debonding over time during use, increased interfacial tension, material porosity, and environmental degradation. In addition, plant fiber bundles are prone to stress concentration during cement agglomeration. Carbon nanofibers are often used to reinforce composite materials due to their large specific surface area and small size effect. The larger aspect ratio of carbon nanofibers can form bridges in cement cracks to improve cement strength. For example, Li et al. found that adding 0.1wt% multi-walled carbon nanotubes (MWCNTs), the compressive strength of cement aged 7d and 28d increased by 22% and 15%, respectively, and the flexural strength was also improved. However, carbon nanofibers have high surface energy, are prone to entanglement and agglomeration, and have poor dispersion. In addition, the preparation of carbon nanofibers is complicated, and the cost of cement-based materials used in large quantities is high.

Contents of the invention

Purpose of the invention: In view of the deficiencies in the prior art, the purpose of the invention is to provide a nanocellulose fiber reinforced cement-based material, which reduces the porosity of the cement paste, makes the structure more compact, and improves the mechanical properties of the cement paste.

Technical scheme: in order to realize the above-mentioned purpose of the invention, the technical scheme adopted in the present invention is as follows:

A nano-cellulose fiber reinforced cement-based material, which contains nano-cellulose fiber components.

The mass of the nanocellulose fiber component does not exceed 0.40% of the mass of the cement-based material, preferably accounts for 0.10-0.25% of the mass of the cement-based material, and most preferably accounts for 0.15% of the mass of the cement-based material.

Nanocellulose fiber is a renewable resource with high strength (tensile strength 7500MPa), high Young's modulus (100-140GPa) and high specific surface area (150-250m ² /g), and more importantly It is a natural nanocellulose fiber with a large number of free hydroxyl groups and chemically modified primary alcohol hydroxyl groups, as well as good interfacial compatibility. It can form a dense structure after being uniformly dispersed with the matrix, and exert its high strength, high stiffness, and light weight. Advantages, the present invention adopts the characteristics of large specific surface area and high chemical activity of CNFs, forms bridges and reacts with cement materials to form dense structures, and prepares nano-cellulose fiber reinforced cement-based materials.

Beneficial effects: the nanocellulose fiber reinforced cement-based material of the present invention, CNFs improves the hydration degree of cement paste, delays the cement solidification time, can prolong and maintain good fluidity, is conducive to the discharge of internal gas, and reduces the cost of cement paste. The porosity makes the structure more compact and improves the mechanical properties of cement paste. Appropriate amount of CNFs is beneficial to increase the degree of hydration and decrease the porosity. When the amount of CNFs is 0.15%, the flexural and compressive strengths of cement at 28d age are increased by 15% and 20%, respectively.

Description of drawings

Figure 1 is a size distribution diagram of CNFs;

Fig. 2 is the heat release rate curve diagram of cement slurry;

Figure 3 is a schematic diagram of the combination of CNFs and Ca ²⁺ to promote the hydration of Portland cement;

Fig. 4 is the change graph of the flexural strength of cement slurry with the CNFs content in different ages;

Fig. 5 is a graph showing the variation of compressive strength of cement slurry with the amount of CNFs at different ages;

Fig. 6 is the electric potential variation figure of cement paste at 100～800 ℃;

Fig. 7 is a mass change diagram of cement paste at 100-800°C;

Fig. 8 is the SEM figure of cement slurry hydration 28d;

Figure 9 is a pore size distribution diagram of Portland cement with different CNFs content;

Figure 10 is the porosity distribution diagram of Portland cement with different CNFs content.

detailed description

The present invention will be further described below in conjunction with specific embodiments. The present invention can be better understood from the following examples. However, those skilled in the art can easily understand that the specific material ratios, process conditions and results described in the examples are only used to illustrate the present invention, and should not and will not limit the protection scope of the present invention.

The main materials used in the following examples are as follows: Portland cement (Portland cement, PC): Jinning sheep brand PⅡ42.5R, density 3.1g/cm ³ , specific surface area 365m ² /kg, its chemical composition and main minerals The composition is shown in Table 1; absolute ethanol: analytically pure.

Table 1 Composition of cement

CaO MgO 54.6305 3.2725 2.9167 6.6034 24.061 0.5617 MnO LOI 0.1205 2.7257 0.0795 0.8466 0.4244 3.05

Example 1

Preparation of nanocellulose fibers (CNFs): oxidatively bleached hardwood pulp using TEMPO-NaBr-NaClO system. 10 g of dry bleached hardwood pulp was dispersed in 1 L of distilled water containing TEMPO (0.16 g) and NaBr (1.0 g), and NaClO (3.75 mmol/g) was added dropwise to the mixture at a rate of 2 Ml/min. The pH of the mixture was maintained at 10.5 by adding 0.5M NaOH and 0.5M HCl solution dropwise. After 6 hours of reaction, the reaction was terminated by adding 25ml of ethanol and adjusting the pH to 7. The above-treated cellulose fibers were filtered and washed to prepare a 1% suspension, and treated with a homogenizer for 10 minutes to prepare CNFs.

The sample ratio is 0.35 water-cement ratio, and the cement-based composite material specimens are equipped with different CNFs content (accounting for cement mass percentage): 0, 0.10%, 0.15%, 0.25% and 0.40%. Labeled as PC, 0.10% CNFs-PC, 0.15% CNFs-PC, 0.25% CNFs-PC, 0.40% CNFs-PC, respectively. The CNFs suspension was magnetically stirred for 15 min, and then ultrasonically treated for 45 min. Then add cement, CNFs suspension and water into the mixing pot in turn, stir slowly for 120s, stop stirring for 15s, then stop stirring quickly for 120s, then cast into a 40mm×40mm×160mm mold, vibrate 120 times, and disassemble after 24h Mold, placed in 20 ℃, 95% RH curing.

Determination of CNFs properties: The carboxyl content of CNFs was measured according to the conductometric titration method in TAPPI. The water retention value of CNFs was determined according to GB/T29286-2012. The size distribution of CNFs was determined using a BT-90 nanometer and a transmission electron microscope (TEM, JEM-1400, Japan).

Determination of standard consistency water consumption, setting time and clean paste strength: according to the relevant regulations of GB/T 1346-2011 "Cement Standard Consistency Water Consumption, Setting Time, Stability Test Method". Determination of standard consistency water demand and coagulation time at different CNF content. After the specimens containing different CNFs content were cured to different ages (3d, 7d, 28d), the flexural and compressive strengths were tested on a microcomputer-controlled compressive and flexural integrated testing machine according to GB/T 17671-1999.

Heat of hydration test: According to GB/T 12959-2008, TAM Air8 channel microcalorimeter was used to test the heat of hydration of the blank group and the cement slurry mixed with 0.15% and 0.40% CNFs respectively within 3 days, and the water-cement ratio is 0.50.

Hydration degree test: Under the same temperature and humidity curing conditions, the amount of chemically bound water in hardened cement paste increases with the increase of hydration degree. The ratio of the chemically bound water w _t of the hardened cement paste to the chemically bound water w _∞ of the fully hydrated cement paste at time t is the hydration degree α t of the hardened cement paste at time _t . This application tests the hydration degree of cement paste by DTA-TG. Under the protection of _N2 , CH2DTG-60AH was used to raise the temperature at a rate of 10°C/min, and the hydration degree of cement paste was evaluated according to the change of calcium hydroxide content and the change of chemically bound water in the cement specimen at 20°C to 800°C.

Microscopic analysis and test: crush the cement specimens of the specified age, soak them in absolute ethanol, stop hydration for 48 hours, dry them in a vacuum oven at 40°C to constant weight, and place them under a field emission scanning electron microscope (SEM, JSM -7600F, Japan) for morphology observation and mercury porosimetry (MIP, Micromeritics AutoPore 9500, America) to analyze the pore structure of cement paste.

According to the conductometric titration method, the carboxyl content of CNFs was 1.85mmol/g. The size distribution of CNFs is shown in Figure 1. The diameter is mainly distributed in the range of 20-100nm, and the length is distributed in the range of 0.6-1.7um. The aspect ratio is large, and it is easy to form a cross-linked network structure. The water retention capacity of CNFs is high, and the mass of adsorbed water is 37 times of its own mass. With small size effect and large specific surface area, CNFs have many exposed hydroxyl groups, have strong hydrophilicity, and exhibit adsorption and cohesion. As shown in Table 2, with the increase of CNFs content, the water consumption of standard consistency increases; the setting time also increases with the increase of CNFs content, but the initial setting time of engineering cement should not be earlier than 45min, and the final The coagulation time is not greater than the requirement of 600min. CNFs are polysaccharides connected by D-glucopyranose with β(1-4) glycosidic bonds, which have the properties of polyols. The oxygen atom of each alcoholic hydroxyl group in the molecular structure of CNFs has two pairs of lone electrons. And Ca ²⁺ can provide empty orbitals, and can form complexes with free Ca ²⁺ to adsorb on the surface of cement particles to form a hydrophilic adsorption stable layer, thereby reducing some active points of calcium silicate cement and water reaction, increasing The distance between Portland cement particles. In addition, the high water retention of CNFs increases the solid-to-liquid ratio, thereby prolonging the hydration time of Portland cement and causing retardation.

Table 2 The change of water requirement of standard consistency with the content of CNFs

Dosage (%) Standard Consistency Water Requirement (%) Initial setting time (min) Final setting time (min)

0 29.78 170 219 0.05 30.22 178 228 0.10 30.89 190 236 0.15 31.33 222 273 0.25 32.67 250 288 0.40 34.44 272 310

The heat of hydration is an important index to reflect the hydration process of cement. Water-cement ratio W/C=0.50, the heat release rate curve of the cement slurry of control group (PC), 0.15%CNFs-PC and 0.40%CNFs-PC dosage is shown in Figure 2, as can be seen, the nanocellulose fiber Adding it to cement hydration for about 20 hours will prolong the cement induction period and the exothermic peak of hydration will shift backward. When 0.15% nanocellulose fibers are added, the exothermic peak of hydration will be delayed for 8 hours. The incorporation of nanocellulose fibers has a significant retarding effect on cement hydration, prolonging the induction period of cement hydration and shifting the exothermic peak of hydration. The reason for retardation is that nanocellulose has a small size and a large specific surface area, and the exposed hydroxyl and carboxyl groups can combine with Ca ²⁺ in cement to form a hydrophilic complex, as shown in Figure 3. When cement and water are mixed, they can be adsorbed on the surface of cement particles to form a hydrophilic adsorption stable layer, and change the network structure of cement hydration products, reduce the activity of hydration products, and delay Ca(OH) ₂ and hydration. The formation of calcium silicate (CSH) produces retardation.

Figure 4 and Figure 5 show the static mechanical strength of cement-based composite materials in various groups at different curing ages. At the 3d age, the incorporation of CNFs has no obvious effect on the flexural strength and compressive strength of the cement paste. During the initial hydration, the hydroxyl groups of CNFs combine with Ca ²⁺ in cement and then adsorb on the surface of cement particles, which hinders the further hydration of cement particles and causes retardation. However, as the hydration of Portland cement proceeds, the surrounding water gradually decreases, CNFs release adsorbed water, Portland cement continues to hydrate, and the flexural strength and compressive strength of the cement paste with 0.15% CNFs at 28 days respectively reach 24.3MPa (15.2% increase) and 99.8MPa (18.8% increase). The addition of CNFs delays the hydration rate of cement, makes the distribution of hydration products such as calcium silicate hydrate (CSH) in the solution around the cement particles more uniform, and makes the grid structure more compact, reducing the unhydration of cement particles due to agglomeration. Structural defects make the internal structure of cement dense and improve the bonding strength of hydration products.

In addition, the added CNFs are 20-100nm long, 0.6-1.7um wide, have a high aspect ratio, and are in the form of filaments. When the cement paste resists stress damage, the CNFs with high strength can pass through the bridging effect. The closing stress is applied on the crack surface, thereby reducing the force on the crack tip, inhibiting the crack from continuing to expand and increasing the fracture toughness of the cement paste. The CNFs content continued to increase, and the mechanical strength of the cement slurry showed a downward trend.

The water in the hardened cement paste can be divided into two categories: chemically bound water (also known as non-evaporating water content) and non-chemically bound water. Chemically bound water exists in the form of ^OH- or neutral water molecules, which are connected to other elements by chemical bonds or hydrogen bonds. Under the same temperature and humidity oxidation conditions, the amount of chemically bound water in hardened cement paste increases with the increase of hydrates, and increases with the increase of hydration degree. This application uses DTA-TG to measure the influence of CNFs on the chemically bound water of cement slurry.

The thermal analysis method can use the heat or weight change that occurs during the heat treatment of the substance to evaluate the phase composition of the sample or understand the characteristics of the thermal change of the sample. According to the temperature range of various peaks or valleys on the DTA curve after cement hydration and the weight change reflected on the TG curve, the type of hydration product is determined. According to the DTA-TG curve of cement hydration reaction, the chemically bound water w _b and the generation of hydration product Ca(OH) ₂ can be used to evaluate the degree of cement hydration reaction.

Figure 6 shows the thermal reaction of cement paste at 100-800°C. It can be seen from Figure 7 that endothermic peaks appear at 103°C, 110°C, 140°C, 450°C, 710°C and 725°C on the DTA curve. The endothermic peak at 103°C is accompanied by a weight loss of 1.31%, which is the process of removing free water from the hydrated sample. Continue heating, and the endothermic peaks at 110°C, 140°C, 450°C, and 710°C are the endothermic peaks of C-S-H gel dehydration, calcium sulfoaluminate hydrate (AFt) dehydration, calcium hydroxide dehydration, and calcium carbonate decomposition. , these processes are accompanied by varying degrees of weightlessness. Among these endothermic peaks, the most obvious endothermic peaks are dehydration of calcium silicate hydrate, dehydration of calcium hydroxide and decomposition of calcium carbonate. It can be seen from Figures 6, 7, and 8 that the incorporation of CNFs delays the cement hydration reaction during the 3d hydration age. But at the hydration age of 28d, with the increase of CNFs content, the degree of hydration increased. The weight loss ratio of the control group at 100-800°C was 19.3%, and the weight loss ratio of the cement paste with 0.40% CNFs was 21.0%.

Table 3. Mass change of cement hydration product Ca(OH) ₂ at 100-800°C

w _CH: Ca(OH) ₂ per gram of cement slurry; w _b: chemically bound water per gram of cement slurry.

It can be seen from Figure 7 that there is an obvious peak shape in the range of 400°C-500°C on the DTA curve, which is the process of calcium hydroxide decomposition. With the addition of CNFs, although there will be retardation in the early stage of hydration, the amount of calcium hydroxide finally produced will increase. When hydrating for 28 days, w _b added 0.40% nanocellulose increased from 0.193 to 0.210. The non-evaporating water content w _b of ordinary silicate is generally 0.23 to 0.25. According to the research of Ivindra Pane et al., w _b,∞ = 0.23 when the cement is fully hydrated. According to the cement hydration degree α _t = w _b,t /w _{b ,∞} , then the cement hydration degree can be calculated, see Table 3 and Table 4. It can be seen that the content of CNFs increases, and the degree of hydration of cement increases accordingly.

Table 4 Hydration degree of cement at different ages

Figure 8 shows the SEM morphology of the cement slurry of the control group and the cement slurry with a content of 0.15% CNFs at the age of 28d. The comparison shows that the cement paste with 0.15% CNFs has a tighter microstructure, less pores than the control paste, and a dense interface. The pores of the cement paste are reduced and the structure is tighter, which reduces the occurrence of stress concentration when bearing a load, and is beneficial to the improvement of the mechanical strength of the cement paste.

The impact of table 5CNFs on the pore structure of cement paste;

sample Average pore size (nm) Porosity(%) Critical pore size (nm) Ref 14.0 13.9 21.9 0.15% 13.1 13.5 21.1 0.40% 13.5 16.2 12.3

Under room temperature curing conditions, the pore size distribution of the cement slurry with different CNFs content at an age of 28 days is shown in Figure 9, Figure 10 and Table 5. The results show that as the content of CNFs increases, the porosity first decreases and then increases, but the pore size distribution tends to be small pores, with capillary pores of 10-20nm being the main ones, and the critical pore size also becomes smaller. The reason is that an appropriate amount of CNFs can be evenly dispersed among the cement particles, which is helpful for the complete hydration of the cement particles and reduces the pores produced by the unhydration of the cement particles; in addition, the incorporation of CNFs prolongs the solidification time of the cement slurry, that is, keeps The extended time of good fluidity helps to discharge the internal gas, making the structure more compact, thereby reducing the porosity of the cement paste. However, when 0.40% CNFs are added, the total pore volume of the cement slurry increases, mainly because too much CNFs will increase the interface inside the material, and some pores will inevitably be generated in these transition zones, which is the main reason for the increase in porosity. The reason, and too much CNFs will inevitably lead to agglomeration, which will also lead to an increase in pores. A dense structure with small pores is formed between cement hydration products, which helps to improve the strength of cement paste; too much CNFs will increase the porosity, cause serious stress concentration in the interface transition zone, and reduce the cement paste. The mechanical strength of pulp.

The above examples confirm that nanocellulose fibers (CNFs) with good mechanical properties have a greater impact on the flexural and compressive strength of Portland cement (PC). The results show that: CNFs can significantly improve the resistance of Portland cement. Compared with the control group, the flexural and compressive strength of Portland cement mixed with 0.15% CNFs increased by 15% and 20% respectively; the setting time of Portland cement increased with the increase of CNFs content. Extended, but in line with engineering requirements; DTA-TG and IC results showed that non-evaporating water content, calcium hydroxide (CH) and cumulative heat of hydration all increased with the increase of CNFs content, and the degree of hydration also increased accordingly; SEM The results of MIP and MIP show that the addition of appropriate amount of CNFs makes the internal structure of Portland cement more dense, and the pore size distribution tends to be small pores, mainly with capillary pores of 10-20nm.

Claims (4)

1. A nano-cellulose fiber reinforced cement-based material, characterized in that: in the cement-based material, containing nano-cellulose fiber components; described nano-cellulose fiber by using TEMPO-NaBr-NaClO system oxidation bleaching Prepared from broad-leaf pulp, the carboxyl content is 1.85mmol/g, the diameter distribution is 20-100nm, and the length distribution is 0.6-1.7um. 2. The nanocellulose fiber reinforced cement-based material according to claim 1, wherein the quality of the nanocellulose fiber component is not more than 0.40% of the cement-based material. 3. The nanocellulose fiber reinforced cement-based material according to claim 1 or 2, characterized in that the mass of the nanocellulose fiber component accounts for 0.10-0.25% of the mass of the cement-based material. 4. The nanocellulose fiber reinforced cement-based material according to claim 1 or 2, characterized in that the mass of the nanocellulose fiber component accounts for 0.15% of the mass of the cement-based material.