**2. Review Methodology**

Data from peer-reviewed journal articles were mainly used to analyze the existing information on the production of geopolymers using various construction wastes, including their mechanical and thermal properties and the porosity of samples. The related literature was searched for in Scopus and Web of Science databases using the search keywords "autoclaved aerated concrete waste", "clay brick waste", and "construction waste". Once the search results were obtained, only the articles from the last 6 years were selected for use. The articles, which were used in the introduction section, had no restrictions related to their publication date.

#### **3. Autoclaved Aerated Concrete Waste**

Autoclaved aerated concrete (AAC) is a combination of silica sand, lime, cement, water, and an expansion agent. It is relatively lightweight and has lower thermal conductivity and lower shrinkage in comparison to traditional concrete [9]. Disposal of AAC waste to landfills may cause contaminating leaching and pH changes in the water and surrounding soil [10]; therefore, instead of hiding AAC waste from our sight and pretending that the problem is fixed, an approach where AAC waste is reused and recycled should be chosen.

It is important to understand that AAC waste is produced not only during the demolition and construction works, but also during AAC production and transportation, as it is easily damaged [9,11]. In their article, He X. et al. reported that 3–5% of the entire AAC production is waste [12], while Zou D., using the situation in China as an example, wrote that AAC waste will account for about 40% of the various types of building material waste by 2025 [13].

In terms of production volumes, according to the European Autoclaved Aerated Concrete Association, there are more than 100 AAC manufacturing plants in 18 countries, which are producing 15 million cubic meters of AAC every year, while the global AAC market was valued at more than 4445 million EUR in 2021 and is expected to reach more than 8255 million EUR by 2030 [13]. This means that the wider production will be followed by an increasingly large amount of waste, and that the conventional disposal of AAC waste, including backfilling and stacking, will not meet the increasing demand for AAC waste disposal [12]. Additionally, landfill capacities are limited, therefore it is expected that landfill fees will increase, which could lead to the development of more complex and cost-intensive recycling processes [14].

### *Recycling of AAC Waste*

In order to eliminate the above-mentioned environmental problems, several scientists have studied AAC waste-recycling methods. Extensive research has been conducted on AAC waste as an adsorbent material that can solidify harmful chemical components due to its porous structure [13,15] and its incorporation into the mortar, which is one of the most widely used building materials [16]. Because AAC waste is rich in silicate, it may partially replace sand, which is necessary for making mortar; thus, not only does it reduce AAC waste, but it also saves the natural river sand resources. According to Lam's study [17], AAC waste can replace up to 25% of natural sand in new AAC fabrication, yielding materials whose main properties (compressive strength, bulk density volume, and drying shrinkage) meet the technical requirements of ASTM C 1693—standard specification for autoclaved aerated concrete.

Several researchers have been focusing on the chemical properties of AAC waste. They have reported that this type of waste could produce sodium aluminosilicate, aluminosilicate zeolites, or replace cementitious materials. However, the preparation procedures require not only a specific environment, but also a special chemical treatment, which significantly limits this type of AAC waste utilization [11,12].

Scientists' interest in using AAC waste as a supplementary cementitious material is significant. This can be explained by the low strength and reactivity of hydrated AAC particles, though there may be some residual unhydrated phases inside the AAC waste [18]. Within the framework of the study, He X. et al. attempted to find a possible approach to using AAC waste as a cement substitute in building materials. He used a mechanical grinding of the AAC waste in the water environment to promote particle refinement and ion dissolution promotion. This experiment showed that AAC waste could be efficiently used as an alternative cementitious material in cement and concrete after the wet-milling treatment, which notably improved the particle fineness, distribution homogeneity, pH value, and other properties of the AAC waste slurry [12].

Although there are studies on the use of AAC waste for the replacement of sand, which is also a component of geopolymers, or on the development of alternative cementitious materials, during the preparation of this review, no articles were found on the possibilities of using AAC waste in the production of geopolymers.

High amorphous precursors, such as fly ash, silica fume, and ground granulated blastfurnace slag, are unavailable locally in Latvia; therefore, it was essential to evaluate the possibilities of using locally available construction and demolition waste for the production of geopolymers. The literature analysis revealed that clay brick waste (CBW) is much more promising for this purpose.

### **4. Clay Brick Waste**

CBW constitutes a major part of the solid waste generated by construction and demolition activities worldwide. Its disposal results in both the pollution of the environment and the occupation of large areas of land. However, the actuality of the problem can be entirely understood when considering the following data: firstly, construction and demolition (hereafter C&D) activities in the Europe Union are responsible for generating more than 850 million tons of C&D waste per year [19]. In China, this amount exceeds 1.5 billion tons of C&D waste per year, which has led to severe environmental and social problems [20]. In the United States of America, 600 million tons of C&D debris was generated in 2018, which is more than twice the amount, and is generated as municipal solid waste [21].

Secondly, research data indicate that CBW accounts for an average of 30% of total C&D waste in the EU [22]. According to Zhu L. and Zhu Z. [23], CBW from demolished brick walls accounted for approximately 54% of C&D waste in Spain; however, it must be understood that CBW is obtained not only as a result of the demolition, as a large amount of broken clay bricks is obtained after the firing activities, transportation, and also construction with this material.

In their article [24], L. M. Beleuk a Moungam et al. mentioned a brick factory in Cameroon where the annual volume of bricks produced is almost 4000 tons; however, 17% of the production is broken. Thirdly, a significant part of CBW is deposited in landfills or reclamation sites, which are expensive and inefficient. The distances between demolition sites and disposal areas are increasing, negatively affecting transportation costs. In addition, landfills and reclamation sites are limited; however, CBW occupies significant areas, damaging the soil structure [21,23]. Therefore, many scientists are looking for alternatives, both in terms of providing effective waste management practices, thus ensuring a cleaner and greener environment, and also searching for different ways to successfully reuse and recycle the already existing CBW [25–27].
