There is a growing interest in the development of new cementitious binders for building construction activities. In this study, biomass bottom ash (BBA) was used as aluminosilicate precursor and phosphogypsum (PG) was used as a calcium source. The mixtures of BBA and PG were activated with the sodium hydroxide solution or the mixture of sodium hydroxide solution and sodium silicate hydrate solution. Alkali activated binders were investigated using X-ray powder diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscopy (SEM) test methods. The compressive strength of hardened paste and fine-grained concrete was also evaluated. After 28 days, the highest compressive strength reached 30.0 MPa—when the BBA was substituted with 15% PG and activated with NaOH solution—which is 14 MPa more than control sample. In addition, BBA fine-grained concrete samples based on BBA with 15% PG substitute activated with NaOH/Na₂SiO₃ solution showed higher compressive strength compered to when NaOH activator was used −15.4 MPa and 12.9 MPa respectfully. The NaOH/Na₂SiO₃ activator solution resulted reduced open porosity, so potentially the fine-grained concrete resistance to freeze and thaw increased.

The utilization of waste generated from industrial production is a burden to overcome for society to reach a circular economy. Usually, production waste is associated with low-quality materials compared to its natural counterparts. In some cases, high-purity materials are generated, while different hazardous substances such as heavy metals, radioactive elements, or organic chemical substances are pollutants that often limit the materials’ further application. One such material that has accumulated for decades is phosphogypsum (PG). The extraction of fertilizers from metamorphous rocks results in large quantities of PG. Until now, PG has been deposited in large stockpiles near the production plant, causing problems for the environment in the surrounding area. However, the chemical composition of PG places it as a high-purity artificial gypsum material, which means that it could be used as a substitution or supplementary material in gypsum-based material production. The concerns, with respect to both legislation and prevailing prejudices in society, about its impurities strongly limit its application. This manuscript reviews current research practices for the effective use of PG and analyzes the importance of the circular economy. A life cycle assessment of current state-of-the-art technologies regarding PG application is proposed.

Phosphoanhydrite was obtained by processing dihydrate phosphogypsum generated during phosphoric acid production at Joint Stock Company (JSC) Lifosa (Lithuania). The influence on the properties of phosphoanhydrite of the treament process? parameters was determined, including the conditions of phosphogypsum neutralisation in the lime suspension; temperature and duration of calcination of neutralized phosphogypsum to phosphoanhydrite; fineness of ground anhydrite; selection of additives activating the hydration of anhydrite and their amounts. Upon adding activators of anhydrite hydration, K2SO4 or Na2SO4, the data of XRD and IR spectral analysis show that the specimens after 1 day of phosphoanhydrite hydration with additives contain complex salts - syngenite upon using K2SO4 additive and glauberite in the case of Na2SO4 additive. These additives act as crystallization centers and accelerate hydration. It has been determined that the specimens of phosphoanhydrite calcined at a temperature of 900 ? C with 2 % K2SO4 additive have the best physical and mechanical properties. Already after 1 day of hardening the specimens acquired a compressive strength of 21.18 MPa, and after 28 days of hardening they had a compressive strength of 58.5 MPa (dry specimens).

This study aims to investigate the effects of ammonium-bearing zeolitic waste (FCC) on alkali-activated biomass bottom ash (BBA). FCC was obtained from the oil-cracking process in petroleum plants. In this study, two types of production waste were used: biomass bottom ash and ammonium-bearing zeolitic waste. These binary alkali-activated FCC/BBA blends were investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) and scanning electron microscopy (SEM) methods. The compressive strength of the hardened samples was evaluated. The results show that the samples made from alkali-activated BBA biomass bottom ash had low (8.5 MPa) compressive strength, which could be explained with low reactive BBA and insufficient quantities of silicon and aluminum compounds. The reactivity of BBA was improved with incorporating zeolitic waste as an aluminosilicate material. This zeolitic waste was first used for ammonium sorption; then, it was incorporated in alkali-activated samples. Additional amounts of hydrated products formed, such as calcium silicate hydrate, calcium aluminum silicate hydrate and calcium sodium aluminum silicate hydrate. The silicon and aluminum compound, which varied in zeolitic waste, changed the mineral composition and microstructure of alkali-activated binder systems. NH₄Cl, which was incorporated in the zeolitic waste, did not negatively affect the compressive strength of the alkali-activated BBA samples. This investigation proved that waste materials can be reused by producing alkali-activated binders.

Phosphogypsum (PG) is a phosphate fertiliser by-product. This by-product has a low level of utilisation. Calcium sulphate is dominated in PG similar to gypsum and, therefore, has good binding properties (similar to natural gypsum). However, the presence of water-soluble phosphates and fluorides, an unwanted acidic impurity in PG, makes PG unsuitable for the manufacture of gypsum-based products. In this study, the binding material of PG (β-CaSO₄·0.5H₂O) was produced from β-CaSO₄·2H₂O by calcination. To neutralise the acidic PG impurities, 0.5 wt% quicklime was added to the PG. In the construction sector, 3D-printing technology is developing rapidly as this technology has many advantages. The current study is focused on creating a 3D-printable PG mixture. The 3D-printing paste was made using sand as the fine aggregate and a binder based on PG. The results obtained show that, despite the low degree of densification, 3D printing improves the mechanical properties of this material compared to cast samples. The 3D-printed specimens tested in [u] direction reached the highest compressive strength of 950 kPa. The cast specimens showed a 17% lower compressive strength of 810 kPa. The 3D-printed specimens tested in the [v] and [w] directions reached a compressive strength of 550 kPa and 710 kPa, respectively.