Recycled Conductive Mortar

Abstract

Due to the urgent need to care for the environment, the use of recycled materials is necessary. The creation of multifunctional materials with content of recycled materials presents an alternative to reduce the use of natural resources. This is through the addition of recycled fine aggregate, product of industrial waste in its manufacture, such as graphite powder (GP) and carbon fiber (CF), turning it into conductive recycled mortar (CRM). The sustainability of this new material brings great ecological benefits, such as the reduction in the use of fine aggregates, which are naturally present in rivers, and also, lower production of construction waste sent to landfills. In this research, an evaluation of the effect of the addition of carbon fiber and graphite powder on wet, dry and hardened electrical properties, electrical percolation in dry state, and flowability of the mixture of recycled conductive mortar in a wet state-based on cement―fine aggregate from waste blocks―graphite powder was carried out. The results obtained showed the effect of the addition of GP and CF to the mortar mix, mainly the reduction of its flowability, caused by the physical interaction between the recycled sand or recycled fine aggregate RFA and the carbon fiber CF, as well as the graphite powder GP.

1. Introduction

Cement-based mortars have a very low electrical conductivity (EC), in dry state, they are considered insulating materials, and in wet state, they are considered semiconductors; thanks to the ionic conductivity provided by the pore solution in its cementitious matrix. To improve the EC of cement-based mortars, the addition of conductive materials in the form of powder and/or fiber, such as CF, GP and carboxymethylcellulose (CMC). These materials are considered suitable conductive additions, since they have high EC and mechanical and physical properties that help to transform the cement-based mortar into a sustainable multifunctional material, by giving it mechanical and electrical properties of significant utility, not only for structural use but also for electrical and/or electrochemical use. In the case of GP and CF, their physical properties, such as length, particle size, average diameter, percentage of addition and dispersion in the cementitious matrix, determine the effect produced as an addition in mortars. The dispersion and concentration of CF considerably affect the air content of the mortar mixture, as it alters the mechanical and elastic properties of the material. The inhomogeneous dispersion of the conductive material in the cement matrix, either GP and/or CF, generates a negative effect on the mechanical and electrical properties of the material; compressive strength, tensile strength, flexural strength, conductivity and electrical percolation. A homogeneous dispersion of the CF in the cementitious matrix allows reaching electrical percolation limits with lower percentages than those of GP additions, given the high aspect ratio of the CF, obtaining multifunctional materials of high EC with low volumetric percentages of CF, a complex situation in the use of GP. For fine aggregate, it is possible to use fine recycled aggregates RFA obtained from concrete, produced by mechanical crushing, composed of natural fine aggregate coated with hardened mortar or paste, which results in lower mechanical strength, different setting times and higher water absorption. Its use is an action that contributes to the sustainability of construction materials [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].

2. Methodology

2.1 Materials

For the manufacture of conductive recycled mortars, composite Portland cement (CPC) 30R type I, RFA recycled fine aggregate, GP loresco SC-3 graphite powder, CF carbon fiber with a length of 10 ± 1 mm, carboxymethylcellulose and distilled water were used. The properties of the materials are shown in Table 1.

Material	Average Diameter (μm)	Superficial Area (m^2/gr)	Specific Density
Carbon Fiber CF	7.000	0.227	1.760
Graphite Powder GP	204.000	2.290	1.850

Table 1.

Properties of materials used in the fabrication of CRM.

The carbon fiber (CF) used for the manufacture of CRM specimens was obtained from industrial waste products, it was carefully selected and cut for use in the CRM mixtures. The diameter of the CF was constant, of continuous morphology, smooth and free of defects. A photograph of its cutting and selection is shown in Figure 1 item a). Figure 1 item b) shows a micrography of carbon fiber CF.

Figure 1.

a) Cut and selection of carbon fiber CF. b) Micrography of carbon fiber CF.

The concrete used to obtain the RFA recycled fine aggregate was waste material from the quality control laboratory; the concrete was crushed using a jaw crusher. Subsequently, the retained material was selected between mesh no. 4 and no. 50, to guarantee the absence of fines resulting from crushing, and cement dust, avoiding the production of a mortar mixture with high water demands. The particle size of the RFA was determined according to ASTM C 136 [20], ASTM C 33 [21], and ASTM C 125 [22] standards. Figure 2, item a) shows the crushing process and item b) the material after the crushing process, ready for the particle size test.

Figure 2.

a) Concrete crushing process. b) Recycled fine aggregate RFA after crushing process.

Carboxymethylcellulose CMC was also used as an emulsifier in the CRM mixtures, as this material has physical properties that allow it to act as an adherent between GP, CF, RFA and CPC particles.

The particle size distribution of the cement used is shown in Figure 3 item a. Regarding the coarse particles, the CPO has particles exceeding 100 μm, and the fine particles have sizes up to 1.0 μm. According to SEM shown in Figure 3 item b, the particles that make up the cement are irregular, but with smoother edges with sizes from 1.5 to 50.0 μm. The particle size distribution of the cementitious materials, and their physical and chemical properties define the microstructure of the hardened mortar, some of these properties are shown in Table 2.

Figure 3.

a) Cement particle size distribution. b) Cement SEM.

Property	CPO
Mass density (kg/m^3)	2975
Superficial Area (m^2/g)	19.26
Average particle size (Фm)	27.61

Table 2.

Particle size distribution of cementitious materials.

2.2 Conductive mortar specimens

The electrical conductivity (EC) was determined in specimens of CRM recycled conductive mortar with 1, 3, 7, and 28 days of curing in distilled water, specimens were manufactured in the shape of a quadrangular prism of 40 mm x 40 mm x 160 mm. The material ratios for the mixtures were: sand/cement = 1.00, water/cement = 0.60, and CF = 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, and 1.5% with respect to the weight of cement. Two types of specimens were manufactured, with and without GP, in both cases, using the CF percentages mentioned above. Table 3 shows the dosages used in the mixtures.

Mortar	CF percentage in relation to cement weight							Graphite/Cement Ratio
M-CF	0.10	0.20	0.30	0.40	0.50	1.00	1.50	0.00
M-CF-GP	0.10	0.20	0.30	0.40	0.50	1.00	1.50	1.00

Table 3.

Dosages for CRM mixtures.

The process for the fabrication of CRM with CF and GP established by Table 3 was carried out using the procedure described in ASTM C 305–14 [22] with the following variants:

1. For the case of CRM type M-GP-CF, the RFA was manually mixed with the GP until a homogeneous material in visual appearance was achieved, prior to the start of the mixture manufacturing.

2. The CF was dispersed with the total mixing water and the CMC in ultrasound for 30 minutes.

3. The CF was placed with the total water in a mixing vessel, adding the total cement and mixing at a slow speed of 140 ± 5 r/min for 30 s.

4. The total amount of recycled sand and GP, if any, was added slowly for 30 s while mixing at a slow speed of 140 ± 5 r/min.

5. Mixed for 30 s at an average speed of 285 ± 10 r/min.

6. The mixer was stopped, remaining covered at rest for 90 sec. In the first 15 s the walls of the container were scraped quickly, using a stainless-steel spatula.

7. It was mixed for 60 s at an average speed of 285 ± 10 r/min.

Figure 4 shows the industrial mixer used during the process.

Figure 4.

Industrial mixer used during the process.

2.3 Electrical conductivity determination

After 24 hours had elapsed since the mixtures were placed in the molds, the electrical conductivity (EC) was determined for the specimens made with the mixtures according to the dosages in Table 3 for the ages of 1, 3, 7, and 28 days. The electrical resistivity was measured using 2 methods: four-point method with miller 400A resistivity meter (4 PM-RM) and four-point direct current method (4 PM-DC) according to [23, 24]. The experimental arrangement is shown in Figure 5, the resistivity was determined from Eq. (1)) and the electrical conductivity (EC) with Eq. (2)).

Figure 5.

Experimental arrangement for determination of EC electrical conductivity.

where:

$$\rho =Fm*2*\pi *a*R$$E1

$$\sigma =\frac{1}{\rho }$$E2

• ρ = Resistivity (Ω.cm).

• σ = Conductivity (S/cm)

• Fm = Geometric factor involving the length of the specimen (L)

• and the separation between the electrodes

• a = Separation between the electrodes (cm)

• R = Electrical resistance (Ω)

Fm was determined based on the L/a ratio and methodology proposed by Morris et al., 1996 [25] and Garzón et al. 2014 [23]. Fm corresponds to 0.1547 for the used dimensions of the CRM specimens and the electrode spacing.

3. Environmental problems

It is recognized that the use of natural fine aggregate, commonly extracted from rivers, has a negative impact on the environment since the demand for this material grows over time and its presence in the environment becomes scarce. In addition, the concrete waste deposited in landfills, a product of construction waste, is highly polluting and affects aquatic ecosystems.

The question is to define the potential that exists in replacing natural fine aggregate with recycled fine aggregate, obtained directly from the crushing of concrete-based construction waste. It is important to determine the possible benefits of using RFA recycled fine aggregate, since this, if proven as a viable alternative to the use of natural fine aggregate, implies a significant reduction in the exploitation of ecosystems and natural resources, as well as the preservation of the environment and, of course, sustainability.

The research “Percolation of a conductive recycled mortar”, considers the sustainable development goals (SDGs) presented by the United Nations (UN). In particular, they are the following:

Objective 9. Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation.

Objective 12. Ensure sustainable consumption and production patterns.

Objective 13. Take urgent action on climate change through education and public awareness.

Objective 17. Strengthen and revitalize the global partnership for sustainable development.

4. Theoretical basis of the research

• ASTM C 136-06. Sieve particle size analysis of fine and coarse aggregates.

• ASTM C-33. Establishes limits on harmful or noxious substances in fine concrete aggregate.

• ASTM C 125-00. Verification of mixing equipment, information required for mix design.

• ASTM C 305-14. For mechanical mixing of hydraulic concrete, cement pastes, and mortars of plastic consistency.

• ASTM C1437-13. Standard test method for flowability of hydraulic cement-based mortar.

5. Analysis and results

5.1 Dispersion of the carbonaceous material

The PCC with weight ratio CT/C = 0.50 and MS with a ratio of 0.10 with respect to the weight of the cement, which was manufactured with a water/cement ratio of 0.60, was checked for dispersion of the carbonaceous material by scanning electron microscopy (Figure 5). With the mechanical mixing procedure, uniform dispersion of the carbonaceous material was achieved.

Figure 6 shows the PCC, it can be seen that the crushed coke particles are uniformly dispersed in the cement matrix, presenting a spherical geometry and with a particle size less than or equal to 0.25 mm.

Figure 6.

Micrography of conductive cement paste weight ratio of CT/C = 0.50, 50X.

The addition of the DM in the mixture favors the dispersion of the carbonaceous material since, in mixtures made without this addition, the material is segregated at the ends of the specimens.

The segregation of the material without the addition of CMC does not allow a uniform contact between the grains of the material and impairs the electrical conductivity.

The dispersion of the carbonaceous material favors the electrical conductivity of the PCC, since the conductivity is due to the contact of the carbonaceous material grains, even in its dry state.

5.2 Granulometry of recycled fine aggregate

The granulometry of the crushed aggregates to be used in the preparation of the mixtures was verified by taking representative samples to determine their granulometry, according to Table 4. The fineness modulus is 2.77 for fine aggregate.

Mesh	Aperture (mm)	Retained weight (g)	Partially retained %	Accumulated retained %	Passing %
No. 4	4.76	0.00	0.00	0.00	100.00
No. 8	3	140.00	30.16	30.16	69.84
No. 16	1	151.20	32.57	62.73	37.27
No. 30	0.63	103.70	22.34	85.07	14.93
No. 50	0.5	64.80	13.96	99.03	0.97
Suma		459.70		276.99
Fineness modulus = 2.77

Table 4.

Granulometry of fine aggregates.

The particle sizes are not within the particle size limits specified by the ASTM C33 standard, as shown in Figure 6.

5.3 Electrical conductivity

The simultaneous use of GP and CF produces a synergistic effect on the properties of EC when incorporated in pastes and mortars [23]. Figure 4 shows the behavior of the EC in the wet state for different % CF with respect to the weight of cement; increasing the % CF produces increases in EC for all curing ages, in approximately the same proportions. Therefore, it is confirmed that the EC does not depend on the age of the RCMs, which makes it impossible to define an electrical percolation threshold, due to the contribution of the ionic conductivity of the pore solution to the EC of the RCMs, as in the case of M-CF mixtures. Similarly, Figure 5 shows the EC in dry state for different % of CF for 28 days of age. As the % CF increases, there are increases in the EC, the most notorious being the one presented when using 30% of CF, so it was determined that this value is the percolation threshold, since the values for subsequent EC did not change much with respect to the 30% of EC (Figures 7–9).

Figure 7.

Granulometry of fine aggregates according to ASTM C33.

Figure 8.

Conductivity in CRM type M-GP-CF in wet state.

Figure 9.

Conductivity in CRM type M-GP-CF in dry state, 28 days of age.

6. Conclusions

Cement-based CRMs—recycled fine aggregate—graphite powder with added CF are an alternative as a multifunctional material, as well as sustainable, because they promote the reuse of recycled materials, which is beneficial for the environment and the construction market. These CRMs present a rapid decrease in flowability when the percentage of CF increases due to the physical interaction between the CF and the RFA in the wet state. For percentages above 1.0% of CF, the mixture with GP is no longer workable, with a tendency to inhomogeneous dispersion of CF and high air contents; in the case of CRM with the absence of GP, this situation occurs after 2.0% of CF. The addition of CF in CRM reduces the fluidity of the mixtures due to the opposition generated by its interaction with RFA and GP, in addition to the viscosity contributed by the Carboxymethylcellulose CMC, in its case. The electrical percolation threshold for CRM with GP content was estimated at 0.30% CF, below the case of no GP with 0.45% CF. This is because the increases in EC without GP are governed by the contact between the CF and the conductive pathways they form, while with GP the EC is defined by the contact between the CF and GP simultaneously, forming conductive pathways with higher EC performance.

Abbreviations

ASTM	American society for testing and materials
CF	carbon fiber
CMC	carboxymethylcellulose
CPC	composite portland cement
CRM	conductive recycled mortar
EC	electrical conductivity
GP	graphite powder
SEM	scanning electron microscope
4 PM-DC	four-point direct current method
4 PM-RM	four-point method with miller 400A resistivity meter