# Soil Stabilization Using Waste Paper Fly Ash: Precautions for Its Correct Use
## 1 Introduction  
A sector that has been criticized over the past decade is pulp and paper manufacturing [1]. This sector, other than being an intense consumer of resources, produces a large amount of waste [2]. Europe is the second largest pulp and paper producer in the world, holding 25% of world production. Producing pulp and paper generates 11 million tons of waste yearly [3; 4]. It has been estimated that 25-40% of the municipal solid waste generated each year is related to paper [5]. To diminish the environmental impact of paper manufacturing, paper recycling had been growing, and some companies use only recycled paper as their raw material source. During paper manufacturing, waste sludge is produced that has been shown to be a valuable source of energy if being used in waste-to-energy plants. The resultant ash, which is called waste paper sludge ash (PSA or WSA) or waste paper ash (WPA), has been ending up in landfill sites. However, nowadays, Europe is facing the challenge of waste management, particularly industrial waste, due to some traditional disposals like landfills being restricted or banned. In recent years, researchers have conducted many studies to find a solution for environmental problems by the utilization of waste paper ash (WPA) in the civil engineering field, like using it in mortars and concretes [6; 7], supplementary cementing material [8], and bricks [9], as well as using it as a binder to stabilize claye soils [4; 10] or as a potential raw material for mesoporous silica synthesis [11]. The WPA and other similar pulp and paper waste ashes have shown to have few cementitious properties. The composition of WPA can be variant depending on the type, grade, and quality of recycled paper [1]. The major elements present in its chemical composition normally consist of calcium, silicon, and aluminum, as well as some iron and magnesium [4; 6; 7; 12; 13]. The compounds present in its mineralogical composition consist of quartz, gehlenite, free lime, calcite, anorthite, and merwinite [4; 12]. [PERSON] and [PERSON] [4; 12] found that gehlenite together with free lime are the major minerals and give the strongest diffraction pattern. The WSA has negative and a positive effects in its use as a binder. The first negative effect is that when WSA mixes with water, it leads to expansion, due to hydration of CaO to form Ca(OH)\({}_{2}\)[12]. The second negative effect is that WSA has a high porosity that leads to a greater water/binder ratio, penalizing the compressive strength of WSA mortars [14]. Despite its negative effects, WSA has a positive effect when mixing with water, due to it providing a highly alkaline solution. The high alkaline solution leads to the release of more reactive phases like Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\), which belong to the WSA or from other cementitious or pozzolanic materials into the system [13; 14]. Some studies have shown the possibility of combining WSA with ground, granulated blast-furnace slag (GGBS) in mortars and concretes without incorporating Portland cement (PC) [6; 12; 13; 15]. A concrete mix of 50:50 of WSA/GGBS gained the most strength at the age of 90 days among other ratios of WSA/GGBS blends with a water/binder ratio of 0.5, showing a good match between WSA and GGBS [15]. It was determined that the combination of WSA and GGBS gained better strength because the amount of GGBS diluted the into the system, reducing the amount of expansive material per unit of available pore space. Likewise, by adding GGBS, the water-to-WSA ratio improves, which provides more surface for lime to be adsorbed and interact, therefore providing a better pH environment for a slow hydration material like GGBS [13]. [PERSON] [4] studied WSA as the main binder for stabilizing a clayey soil. The study found that a mixture of WSA and gypsum improves the compressive strength of treated soil samples. It was found that mixing WSA and gypsum in mortars leads to a high amount of ettringite.