Gypsum binder is widely used in construction due to its ease of production, availability, and low price [
1]. Physically, gypsum is infinitely recyclable; however, the recycling process requires additional energy [
2]. Despite these benefits, the disadvantage of gypsum binder is its brittleness, poor resistance to cracking, and unsuitability for damp conditions. Traditional gypsum binder use has been defined in EN 12859, where the main gypsum application is associated with the production of plasters, blocks, tiles, and boards [
3]. Besides natural gypsum, synthetic gypsum, produced as chemical by-product, is used widely for the production of gypsum products. There are more than 50 different types of gypsum waste [
2]. However, the most common by-product is phosphogypsum (PG), flue gas desulphurization gypsum, and borogypsum [
4,
5]. Phosphogypsum (PG) is produced as a by-product from phosphate fertilizer production and the annual PG production reaches 280 mlj.t worldwide and only about 15% of PG is used as secondary raw material, but rest is disposed in open-type stacks [
5]. Partially or completely, synthetic gypsum can be a substitute for natural gypsum as cement admixtures, gypsum-based plasters, drywalls. Researches on production of traditional gypsum binders based on PG are widely published, but they have rather limited practical application due to specific nature of PG [
6]. Moreover, there are legislation limits and prejudice coming from society regarding PG, so the direct use of PG as substitution of natural gypsum is problematic. More complicated and effective way of the utilization of PG is to create an advanced and new type of binder, which has a much lower carbon footprint comparing to Portland cement, while remaining strength properties similar to Portland cement. It was reported that the addition of blast furnace slag and cement could effectively improve the mechanical strength of PG. However, fly ash played a negative role on the compressive strength of PG [
7]. S. Kumar described properties of fly ash–lime–phosphogypsum ternary binder [
8] or recently anhydrous gypsum was used to develop lime-pozzolan green binder [
9]. Two waste-stream materials were used and only lime was defined as a primary resource, the calcination temperature (900–1100 °C) of latter is lower compared to Portland cement. In this case, the amount of PG in the binder was in the range from 10–40%. However, the disadvantage comparing to Portland cement is low compressive strength—which could be in range from 2–4 MPa [
8]; nevertheless it is reasonable if it is compared to the lime binder and hydraulic lime binder. Higher strength results were obtained in ternary binder system phosphogypsum–steel slag–granulated blast-furnace slag (GGBS)–limestone cement, where the content of PG was from 25–65%, while the amount of slag was from 22–48%. The obtained strength at the age of 28 days was up to 45 MPa while the obtained binder is characterized with fast setting time (initial setting time 6–9 min, final 10–12 min) [
10]. These results are more comparable to traditional binders, while the problem could be a fast setting time. The fast setting may not benefit the engineering application because there is not enough time for casting before the cement sets. In some case it was reported that the citric acid in amount from 0.03–0.15 wt.% of cement could retard setting time significantly. The use of citric acid could increase the open time from 25 to 47 min, while this admixture tended to slightly reduce compressive strength of the binder [
11]. In ternary systems where Portland cement is present, superplasticizer can be used and low water-binder ratio can be achieved. Traditionally, supplementary materials that are utilized to replace ordinary Portland cement decrease the workability of the cementitious mixtures and superplasticizers such as polycarboxylate based are usually added to cement to control their fluidity [
12]. The use of polycarboxylate acid-based superplasticizer could be used from 0.75–1.75 wt.%; however, reports say that it could slightly reduce early compressive strength of the material while final strength tended to increase [
11]. These aspects regarding to the utilization of PG in new types of binders were considered in present research by choosing mixture composition, including use of chemical admixtures.
To continue the development of alternative waste stream binders in a production of new materials and enhance its valorization possibilities, novel lightweight foam material based on developed ternary binder was elaborated. In construction industry, there is growing interest in lightweight concretes. It combines positive properties of constructive and insulation materials and is characterized by moderate strength, low density, and improved thermal properties. Cellular concrete is composed on mortar matrix and specially created system of air cells, which occupies up to 85% of material volume [
13]. High porosity limits potential of mechanical strength, but high volume of open pores is the main reason for increased water absorption and drying shrinkage. The density of traditional gypsum ranges from 600 to 1500 kg/m
3, as given in Clause 4.8.1. of EN 12859 [
3]. This well-known standard covers the gypsum application range, and beyond this range, research is being conducted to make gypsum material more sustainable. Attempts to produce lightweight gypsum with foaming admixtures have yielded a material with density ranging from 300 to 600 kg/m
3 [
14]. Such material has low density, superior sound, and thermal insulation and can be considered a sustainable high-performance material. High-efficiency sound-absorbing material was made also with PG, which composite structure was described by Baoguo Ma et al. [
15]. Thus, aim of the work was to produce in laboratory conditions lightweight ternary system based material with density less than 600 kg/m
3, which is outside the traditional boundaries to bring the novelty of the research. Bulk density and thermal conductivity were set as target values, which should be determined together with technological properties so that gypsum material could be easily produced and handled (workability, strength). Here, research on development of highly porous ternary system gypsum-based binder material was evaluated and compared.