Production of modified concrete using molecular sieve and PET waste simultaneously

The increase in the production of industrial and plastic waste, especially molecular sieve waste and polyethylene terephthalate (PET), has created a serious challenge in the field of environmental waste management. On the other hand, the current management method for some of this waste is to bury it, which clearly shows that this approach is not appropriate for environmental standards. Molecular sieves, which mainly have a zeolitic and porous structure and are used in the oil, gas, and petrochemical industries for the absorption and separation of molecules, are classified as industrial waste after their efficiency decreases due to saturation of the pores. This research aimed to investigate the feasibility of simultaneously using molecular sieve waste and PET in the production of sustainable concrete and to evaluate their impact on the mechanical, physical, and environmental properties of concrete. The simultaneous use of molecular sieve and PET waste in concrete is an innovative approach in civil engineering. Due to its porous crystalline structure and high ability to absorb moisture, molecular sieve can increase density, adhesion, and reduce water absorption in concrete; however, its excessive use may lead to a decrease in compressive strength. On the other hand, using recycled PET waste as a partial replacement for natural aggregates helps reduce the density of concrete, reduce the weight of the structure, and optimize material consumption and transportation costs. Molecular sieve waste (synthesized HYG03C) after mechanical crushing and sieving was used as a partial replacement for cement in the range of 5 to 30% by weight, and PET after washing, grinding, and sieving was used as a partial replacement for sand in the range of 2 to 10% by weight. The experiments were designed using response surface methodology (RSM) and central composite design (CCD) in Design Expert software. In this design, four variable parameters were defined including cement content (200 to 400 g), molecular sieve to cement ratio, water to cement ratio (0.40 to 0.60), and PET to sand ratio. It should be noted that the amount of sand and gravel in concrete mixtures is determined to be 42 and 33 percent by weight of the total concrete, respectively. Cubic specimens with dimensions of 10×10×10 cm were made in accordance with the requirements of the Iranian Concrete Code and were subjected to porosity and compressive strength tests (Standard 1608-3). Considering the research objectives of producing sustainable concrete, the optimal composition included 393 grams of cement, 21.56% molecular sieve, 2.85% PET, and a water-to-cement ratio of 0.515, which resulted in a porosity of 4.3% and a compressive strength of 34 MPa. The results of the slump test showed that the concrete flowability in the optimal sample was 75 mm and in all samples it was within the allowable range of 50 to 100 mm. Increasing the water-cement ratio increased the slump and increasing the amount of cement, molecular sieve waste, and PET decreased the concrete flowability. Porosity and compressive strength prediction models had high accuracy and generalizability with high coefficients of determination (R² equal to 0.9978 and 0.9970, respectively) and significance level (P-value less than 0.05). This research demonstrated the effectiveness of simultaneously utilizing molecular sieve and PET waste as a sustainable method for producing concrete with improved properties and reduced environmental impacts, which has widespread application in construction projects.