# Strength and Compressibility of Ammonia-Soda Residue from the Solvay Sodium Plant
## 1 Introduction  
Ammonia-soda residue (ASR), also known as ammonia-soda white mud or ammonia-soda ash waste, is an alkaline calcareous sludge formed during the production of industrial soda ash (SA) (Solvay processes). ASR is a white-coloured fine-grained material commonly stored in landfills or settling ponds and may be potentially hazardous and negatively impacting natural and human systems. SA (sodium carbonate, Na\({}_{2}\)CO\({}_{3}\)) is an essential inorganic product that has several diversified applications in the chemical industry [1, 2, 3], metallurgy [4, 5], textile [6, 7], printing and dyeing [8], glass [9, 10, 11], medicine [12], and food industries [13, 14]. SA is also exploited in geotechnics to stabilise the subgrade soft soil, improve the geotechnical properties of soil, and accelerate the hardening of soil-binder mixtures. In 2020, the total global production of soda ash was estimated to be about 52 million metric tons [15], with a considerable amount of ASR as a by-product material from manufacturing. The ASR is usually slurry-deposited and exhibits a unique structure with high moisture content, high compressibility, and unusual geotechnical properties [16, 17], which presented several challenges to the capping landfill design [18, 19]. Another feature is the fact that the ASR consists of extremely fine particles with a large specific surface area that exhibits a strong adsorption capacity [20, 21]. [PERSON] et al. [22] considered SA as an additive to soil-cement mixtures to determine its effect on soil-cement and soil-cement-fly ash strength properties. It was found that the beneficial effects of the addition of0.5 wt.% sodium carbonate were most observed after short curing periods. Over a longer time, SA can be detrimental to mixtures containing low cement contents. However, the authors reported various effects of addition of SA on the strengths of the soil-cement and soil-cement-fly ash mixtures. [PERSON] and [PERSON] [23] reported that sodium hydroxide and carbonate were effectively used as chemical additives with Iraqi soils stabilised by cement to increase shear strength. [PERSON] et al. [24] studied the engineering properties of the solidified soil with 20% non-clinker incorporating soda residue curing agent. The strength properties of this sample were almost the same as that of the solidified soil with about 10% composite cement. [PERSON] et al. [25] examined alkaline residue as a new binder to treat heavy metal-contaminated soil because of its strong adsorption capacity for heavy metal ions. The increase of the treated soil's unconfined compressive strength (UCS) was associated with the drop of the leached Pb\({}^{2+}\) concentration and the rise of the alkaline residue content in the specimen. [PERSON] et al. [26] investigated the shear strength properties of stabilised soft soil composed of ASR and ground granulated blast furnace slag and compared it to cement-stabilised soil in deep mixing and soil-cement columns. [PERSON] et al. [27] studied the effect of SA on pH and the compaction characteristics of Malaysian laterite soil. Based on the experimental investigation, the addition of soda ash led to the increase of pH of the soil and improvement of the compaction properties. [PERSON] et al. [28] used calcium chloride and SA solutions to decrease the soil expansion potential. The reduction in the expansion parameters was primarily attributed to the strong short-term reactions between clay and stabilisers. [PERSON] et al. [29] considered the improvement of ASR properties by using liquid soda residue and liquid fly ash. Mixing ASR and fly ash resulted in the elevation of the water content, liquid limit, and plastic limit, and lowering of permeability and compressibility in the pretreated soil. Furthermore, potential benefits of the exploitation of SA for the preparation of ASR were assessed through laboratory and field tests. The field tests on ASR were also carried out by [PERSON] et al. [30] to study the impact of gravel, sand, and lime on the mechanical properties of ASR.