Producing Green Concrete with Plastic Waste and Nano Silica Sand

Industrial and urban development has resulted in the spread of plastic waste and the increase in the emissions of carbon dioxide resulting from the cement manufacturing process. The current research aims to produce green (environmentally friendly) concrete by using plastic waste as coarse aggregates in different proportions (10% and 20%) and nano silica sand powder as an alternative to cement in different proportions (5% and 10% by weight). The results showed that compressive strength decreased by 12.10% and 19.23% for 10% and 20% plastic waste replacement and increased by 12.89% and 20.39% for 5% and 10% silica sand replacement respectively at 28 days. Flexural strength decreased by 12.95% and 19.64% for 10% and 20% plastic waste replacement and increased by 11.16% and 19.86% for 5% and 10% silica sand replacement. Splitting tensile strength decreased by 12.74% and 20.22% for 10% and 20% plastic waste replacement and increased by 10.86% and 19.66% for 5% and 10% silica sand replacement. Dry density decreased by 4.51% and 7.83% for 10% and 20% plastic waste replacement and increased by 2.78% and 4.10% for 5% and 10% silica sand replacement respectively at 28 days. Keywords-green concrete ; plastic waste; silica sand


INTRODUCTION
Green concrete was first produced in Denmark in 1998. The term green concrete refers to the environmental concept in which waste is utilized and thus recycled in its production. The production of green concrete is usually cheap because of the use of waste as a partial substitute for cement and aggregates, reducing energy consumption and causing less harm to the environment [1,12,13]. The emission of carbon dioxide (CO 2 ) is an environmental problem. The cement manufacturing process produces about 8 to 10% of the total CO 2 emissions globally, so researchers resorted to the production of green concrete, which utilized recycled materials, e.g. pozzolanic materials, which are used to reduce the amount of cement in the mixture, thus reducing cement production and CO 2 emissions from cement factories. Waste plastic, glass, ceramics, rubber, etc. have also been used as aggregates in the production of green concrete [2,17]. The process of disposing of plastic waste causes major environmental problems due to the large disposed quantities and their low biodegradability, so research has focused on the disposal of plastic waste through its use in concrete [3,14,16]. Plastic waste consisting of Low-Density Polyethylene (LDPE), such as plastic bags, has become a major environmental problem due to the difficulty of its eco-friendly disposal. Often plastic bags are burned, contributing to air pollution [4,15]. Green concrete developed with waste materials is an active area of research. Fly ash has been used as cement replacement in green concrete made with partial replacement of conventional coarse aggregates with coarse aggregates from demolishing waste [5]. Portland cement is considered a product most involved in environmental pollution. It is responsible for about 10% of global CO 2 emissions. Limestone dust is a by-product of limestone plants and it is produced in thousands of tons annually as waste [6]. For sustainable development construction, recycle or reuse of waste materials is utilized. Many researches tried to create an innovative green concrete, utilizing waste materials [7].

A. Cement
Ordinary Portland Cement (OPC) 42.5 R from the Al-mass company was used in all concrete mixes. The physical and chemical properties in accordance with the Iraqi specification No. 5/2019 as shown in Tables I and II.

B. Fine Aggregates
Using natural sand in Saturated Surface Dry (SSD) condition was used as fine aggregates. It is conformable with the Iraqi specification No.45/1984 zone three (Tables III and  IV).

D. Waste Plastic Aggregates
The plastic waste used as coarse aggregates had nominal maximum size of 14mm obtained from cutting vegetable boxes, garbage containers, shampoo, and detergent bottles. Sieve analysis, and the results of chemical and physical tests of the waste plastic aggregates are presented in Tables VII and VIII respectively.

E. Mixing and Curing
The water used for mixing and curing conforms to the Iraqi specification No. 1703/1992. The chemical properties of the water used are shown in Table IX.

G. Silica Sand Powder
Local silica sand was used in the form of powder. The particle size analysis test is presented in Figure 1. Chemical and physical tests for nano silica sand were carried out and were compared with ASTM C 618 as shown in Tables X and XI respectively.

III. EXPERIMENTAL PART
A. Mixtures ACI 211.1 code design was used to achieve compressive strength of 30MPa at 28 days for cylinder and 37.5MPa for cube specimens as the reference mix. The plastic waste was replaced in different proportions of the volume of the coarse aggregates (natural gravel), and the silica sand replaced cement in different proportions as shown in Table XII.

B. Molds Preparation
Cube steel molds having dimensions of 100×100×100mm were utilized for compressive strength tests, cylinder steel molds of 100×200mm for splitting tensile strength tests, and 100×100×400mm prisms for flexural strength tests.

A. Compressive Strength Measurements
Compressive strength test was conducted according to BS EN 12390-3:2019 (E) as shown in Figure 2. The compressive strength was tested for three cubes at the age of 7, 28, and 90 days. Compressive strength test machine.
The results for the reference mix and the concrete containing plastic aggregates are plotted in Figure 3. It was found that the compressive strength decreased by 10.84, 12.10, and 13.62% and by 18.18, 19.23, and 20.15% for 10% and 20% plastic waste replacement, respectively, at 7, 28, and 90 days. A possible reason for the decrease in the compressive strength is the weakness of the bonding strength between the smooth surface of the plastic aggregates and the cement paste, which leads to a weakening of the transition zone [21][22][23]. Plastic waste (high density polyethylene) in concrete mixtures as aggregate replacement was studied in [8]. The authors used plastic waste in the concrete mixture with proportions of 10, 20, and 30% of the volume of coarse aggregates. The compressive strength decreased with increasing plastic waste proportion [8]. The compressive strength test results for the reference mix and the concrete containing the silica sand powder are plotted in Figure 4. The compressive strength increased by 10.20, 12.89, and 12.64% and by 19.46, 20.39, and 19.11% for 5% and 10% silica sand replacement, respectively, at 7, 28, and 90 days. The reason for the increase in compressive strength is the reaction of calcium hydroxide (Ca(OH) 2 ), which is a product of the reaction of cement and water with silica sand, which leads to the formation of a secondary gel (C-S-H) [20]. The mechanical properties of concrete using silica sand as partial (5,10,15,20 and 25% by weight) replacement of cement were studied in [9]. It was found that the best replacement ratio was 15% [9]. Relationship between compressive strength and age for deferent percentages of silica sand powder.

B. Flexural Strength
Flexural strength was determined according to ASTM C293-16 as shown in Figure 5. The flexural strength average of three models per mixture was examined at the age of 7, 28, and 90 days. Flexural strength test machine.
The results of the flexural strength measurements for the reference mix and the concrete containing plastic aggregates are plotted in Figure 6. Flexural strength decreased by 12.26, 12.95, and 10.81% and by 21.63, 19.64 and 19.10% for 10% and 20% plastic waste replacement, respectively, at 7, 28, and 90 days. The reason is the same with the one mentioned in compressive strength analysis [24]. Some properties of sustainable concrete with mixed plastic waste aggregates can be seen in [10]. When replacing aggregates with plastic waste at proportions of 5, 25, and 45% of volume, a decrease in flexural strength was observed with an increase in the replacement ratio. The flexural strength test results for the reference mix and the concrete containing the silica sand powder are plotted in Figure 7. Flexural strength increased by 11.1, 11.16, and 11.6% and by 17.78, 19.86, and 18.64% for 5% and 10% of silica sand replacement, respectively, at 7, 28, and 90 days. The reason is the one mentioned in compressive strength analysis [9,19]. The use of silica sand in partial replacement of the fine material (sand) in concrete was studied in [20]. The best replacement ratio was found to be 15% [20]. Relationship between flexural strength and age for deferent percentages of plastic waste as coarse aggregates. Relationship between flexural strength and age for deferent percentages of silica sand powder.

C. Splitting Tensile Strength
Splitting tensile strength test was determined according to ASTM C496/C496M-17 at the age of 7, 28, and 90 days ( Figure 8). The average of three measurements was considered the final value of each mixture. Splitting tensile strength test.
The results of the splitting tensile strength tests for the reference mix and the mixtures containing plastic aggregates are plotted in Figure 9. Splitting tensile strength decreased by 11.77, 12.74, and 13.76% and 19.34, 20.22, and 20.63% for 10% and 20% of plastic waste replacement, respectively, at 7, 28, and 90 days. The reason is the same mentioned in compressive strength analysis [10,25]. The use of plastic waste (high density polyethylene) in concrete mixture as aggregate 200  replacement was studied in [8]. Plastic waste was utilized in concrete mixtures with proportions of 10, 20, and 30% of the volume of coarse aggregates. It was found that splitting tensile strength decreased by increasing percentage of plastic waste [8]. Splitting tensile strength for the reference mix and the concrete containing silica sand powder is plotted in Figure 10. Splitting tensile strength increased by 10.71, 10.86, and 11.37% and by 17.60, 19.66, and 18.50% for 5% and 10% of silica sand replacement, respectively, at 7, 28, and 90 days. The reason mentioned in compressive strength analysis [9,18] also applies here. The use of silica sand as partial replacement of fine materials in concrete was studied in [20]. The best replacement ratio was proved to be 15% of cement weight [20]. Relationship between splitting tensile strength and age for deferent percentages of plastic waste as coarse aggregates.

D. Dry Density
Dry density test was found by taking the average density of three cubs of 100×100×*100 dimensions at the age of 28 days according to BS 1881-114: 1983. The cubes were drying in the oven at 105 ± 5 ºC until constant mass. Dry density decreased by 4.51% and 7.83% for 10% and 20% plastic waste replacement, respectively, at 28 days. This decrease in the dry density of concrete is caused by the decrease in the plastic density compared to the density of the natural coarse aggregate [26][27][28]. Experimental investigation on recycled plastics as aggregates in concrete was conducted in [11]. It was found that dry density decreased with increasing plastic waste percentage [11].
Silica sand powder was utilized at proportions of 5% and 10%. It was found that the density of dry concrete increases by 2.78% and 4.10% respectively at 28 days. The reaction of calcium hydroxide Ca(OH) 2 with silica sand powder leads to the formation of a secondary gel (C-S-H) that fills the voids in concrete, making the mixture more denser [9]. The effect of nanomaterials on the properties of limestone dust green concrete was studied in [6]. It was found that dry density increased with the increase in nano Al 2 O 3 percentage [6]. The dry density for the reference mix and the concrete containing plastic and silica sand powder are plotted in Figure 11

V. CONCLUSIONS
Based on the obtained experimental results, the most notable outcomes of the current study are: • Compressive strength decreased by 10 • Dry density decreased by 4.51% and 7.83% for 10% and 20% plastic waste replacement, respectively, at 28 days. It increased by 2.78% and 4.10% for 5% and 10% silica sand replacement, respectively, at 28 days.
• The cost of concrete was reduced by using local and recycled materials.
• The concrete production had reduced pollution by using plastic waste as coarse aggregates.