# Life Cycle Assessment of Substitutive Building Materials for Landfill Capping Systems in Vietnam
###### Abstract
Secondary waste materials which are often involved in landfill construction include slag, fly ash, brick waste, construction and demolition waste, glass waste, rice husk ash, mining waste, incineration residues etc., [7; 9; 11; 12]. Construction demolition waste (CDW) occurs due to road maintenance, construction, repairing, upgrading, renovation, excavation, and demolition activities. It is widely used in landfill construction operations [13; 14]. The construction waste is usually generated as a mixture of concrete, brick, ceramic, natural aggregates, glass, and other materials. The construction waste undergoes processing which generally involves sorting, crushing, milling, and grading according to the application and based on the materials such as concrete and brick, etc. as well [15]. The slag usually remains as a by-product of the mineral extraction, which involves quenching or cooling processing and further milling. The other large quantity of industrial waste is fly ash, which remains stored in ash ponds in large quantities, or is utilised in cement manufacturing [16]. Fly ash might need processing for pollutant reduction or pH neutralisation based on its physical and chemical properties [12; 17; 18]. Recent research developments focus on replacing river sand with mineral ore sand, which is a type of co-product or by-product of mineral ores that could be further processed to achieve sand properties [19]. In general, recycling the wastes requires energy inputs, which helps to convert the waste into aggregates for utilising in construction applications replacing energy-intensive natural aggregates. After reuse or recycling, municipal solid waste and other types of waste have their endpoint at the landfill. Waste landfilling over a longer period often triggers the biological, chemical, and physical processes in the waste body, resulting in the release of pollutants that may reach the aquifer system by leachate infiltration [20; 21]. The landfill design structure on top of the ground involves a special design of multilayers, which has a specific lifecycle. The multi-layers usually consists of a combination of geological barrier (hydrological), technical barriers (basal liner and capping), and the waste body [22]. It is proposed [23] that a landfill must be designed so that the protection barriers satisfy the requirements for a long duration, at least 30 years while in operation, and at least 30 years after closure. The landfill layer's design during its lifecycle should give unchanged quality protection. A landfill cover is a multi-layered construction system (Figure 1) that reduces water percolation to deposited waste and minimises leachate production and the uncontrolled release of landfill gas into the atmosphere [20]. One of the most important parameters considered for the performance of landfill capping layers is the hydraulic conductivity (k-value) of materials used in the construction of a hydraulic barrier in waste containment facilities [7]. A mineral sealing layer is an important section in the landfill layer covering the landfill base and sides at the bottom and top cover. The thickness of the layer needs to be designed based on a choice of material, such that the permeability of the mineral sealing layer has to be at least equivalent to k \(\leq\) 1.0 \(\times\) 10\({}^{-9}\) m/s for the effective protection of soil, groundwater, and surface water [24]. Therefore, a mineral sealing layer requires pre-laboratory testing to establish its permeability. The design and the layer thickness determination must be adapted to the location-dependent water balance and the physical properties of the utilised material. The commonly used mineral sealing layer materials are natural clay, compacted clay, and bentonite-based mixes [25]. The proper removal of leachate represents an important factor in the successful functioning of a landfill facility.