**1. Introduction**

Porous materials have numerous applications which depend on the composition of material, as well as number of pores, distribution, shape, size, connectivity, etc. [1]. Among the numerous applications of porous materials are thermal and acoustic insulations, filtration membranes, heavy metal absorption, catalysis, electro-magnetic interference shielding, energy and health (scaffolds, substrates for controlled drug release, wound healing, etc.) [1–3].

Porous materials used for thermal insulations are numerous and possess a wide range of properties. Nowadays, the most common thermal insulation materials are those based on organic compounds such as polyurethane and polystyrene; these organic materials have low densities and low values of thermal conductivity, but their main disadvantage is reduced fire resistance. Therefore, part of the research currently performed in this area focusses on the development of inorganic, fire-resistant thermal insulation materials [4–6]. Another topic related to the development of highly efficient and resistant thermal insulation materials is the incorporation of nanomaterials such as graphene oxide, nanocellulose or aerogels [2,7–9]. The high manufacturing cost of these nanomaterials is the main drawback which limits their current application on a large scale in the construction industry [9].

Another material which can be used for the development of cost-effective light weight porous materials with good thermal and acoustical insulation properties is gypsum [4,10].

Gypsum binders are obtained by the thermal treatment of gypsum rock (CaSO4·2H2O–CsD) at temperatures above 105 ◦C, when it can be transformed in hemi-hydrate (CaSO4·0.5H2O–CsH) or in anhydrite (CaSO4)—at higher temperatures [11]. Due to the relatively low thermal treatment temperatures and the possibility to recycle it in close loops, the gypsum binder is considered environmentally friendly as compared with other types of inorganic cements [12]. Nowadays, one of the main utilizations of gypsum binder is the production of gypsum plasterboards (dry-walls) [11,12]; another utilization of gypsum binder is to produce thermal and sound insulation materials [4,11,13–18]. According to Dolezelova et al. [10], gypsum-based light materials can be used in construction to replace autoclaved aerated concrete (AAC) and the energy consumption for the manufacture of gypsum materials is lower as compared with the one used for the manufacture of AAC.

The thermal insulation materials based on gypsum can be obtained either by its mixing with lightweight aggregates, including various waste such as expanded polystyrene [18], rubber, polyurethane foam and chopped electric cables waste [15,16], or direct foaming using various types of gas generators (aluminum sulphate with citric acid, calcium carbonate or sodium bicarbonate [4,13,14]). The thermal conductivity of this type of materials can vary from 0.085 up to 0.416 W/(m.K) depending on the nature and dosage of the additions and the assessment method [4,10,14,15].

In this paper gypsum-based materials were prepared using two foaming additions: hydroxyethyl methyl cellulose (HEMC) and sodium bicarbonate. In contact with water, sodium bicarbonate generates CO2, which can be trapped in the binding matrix, thus generating supplementary porosity. The presence of sodium bicarbonate in gypsum paste delays the binders setting, and so therefore in the materials presented in this paper, flue gas desulfurization gypsum (FGD gypsum) was used as a setting accelerator.

FGD gypsum is a waste produced in the desulfurization process of combustion gases generated in power plants. The desulphurization process of exhaust gasses from combustion plants is imposed by the EU legislative framework regarding emissions [19]. This process consists of SO2 (from flue gas) reaction with an alkaline substance (such as dolomite, limestone, lime, or hydrated lime) to produce sulphite or sulphate [20]. The presence of CaSO4·2H2O (CsD) as a main component in FGD gypsum justifies its use as a setting accelerator in gypsum binders. The CsD particles from FGD gypsum can act as nucleation sites for newly formed CsD crystals (by CaSO4·0.5H2O hydration), therefore accelerating the setting process [14,16].

Hydroxyethyl methyl cellulose addition was used to produce supplementary porosity, due to its ability to stabilize the air entrained during the mixing of the components (high affinity for the water-air interface) [21,22].

This paper presents the influence of these three additions (sodium bicarbonate, FGD gypsum and hydroxyethyl methyl cellulose) on the main properties of gypsum binders i.e., setting time, geometrical density, open porosity, compressive strength, and thermal conductivity.
