*4.1. Geopolymer Production Using CBW*

Considering the various studies conducted on the methods of using CBW to produce new building materials, as well as the limited volume of reviews, the use of CBW is analyzed only for geopolymers, without mentioning other building materials.

The most extensively investigated precursor materials are slag, fly ash, silica fume, and metakaolin [2,28,29]. However, recent studies [22,30] have confirmed the feasibility of using low-amorphous aluminosilicates for the synthesis of geopolymers. Waste clay bricks, on the other hand, are excellent low-amorphous aluminosilicates for geopolymer production due to their chemical and mineralogical properties, allowing one to obtain samples with sufficient strength [8]. Clay, obtained from CBW, has the natural advantage of being already calcined at up to a high temperature of 950 ◦C [31] during the manufacturing process. The combined water in clay minerals evaporates, creating disordered amorphous phases of alumina and silica, which, in turn, allows us to look at CBW as an environmentally friendly and low-cost raw material for the production of geopolymers [8]. In addition, CBW can be used to produce geopolymers with or without widely used precursor materials such as fly ash and slag. More information on CBW as a precursor material and relevant studies is summarized in Table 1. It also provides information about components, the curing conditions, activator type, and compressive strength.

**Table 1.** CBW-based geopolymer material properties and curing conditions.


Upon analyzing the information available in Table 1, it can be seen that more strength is obtained for geopolymers that use CBW in combination with slag. High-strength samples cannot be obtained using only CBW at ambient conditions, as the compressive strength of the samples, in this case, does not exceed 7 MPa, even after 28 days. The activator type for geopolymers, which are produced using CBW, does not differ from activators which are used in other geopolymers. In most cases, it is NaOH together with Na2SiO3; however, in some studies, they have been used alone. These conclusions are also confirmed by J. Migunthanna, who states in her study [2] that CBW-only one-part geopolymers and CBW-only two-part geopolymers are not capable of achieving high compressive strength in ambient curing conditions. She evidences this with the low degree of reaction of CBW, suggesting that elevated curing temperatures are more suitable for 100% CBW-based geopolymers [2,22]. It should be noted that Table 1 includes information on only a few studies wherein CBW has been used to produce geopolymers. Due to the limited volume of the review, its purpose was to show the vast possibilities of how this kind of C&D waste, alone or in combination with other raw materials, can be used in the production of geopolymers. Secondly, Table 1 deliberately includes studies with different curing temperatures to show that the creation of geopolymers is possible not only at temperatures of 60 ◦C and higher, but also in ambient conditions.

Many studies have determined the best production conditions of geopolymers, thus providing good geopolymerization [35]. Nonetheless, this analysis is complicated by the fact that, not only must the aforementioned curing conditions be observed, but also different precursor materials and activators, alkaline solution concentration [39], and particle size. Therefore, more information on the curing temperature and activator type is provided in Sections 4.2 and 4.3.

### *4.2. Curing Temperature*

Curing conditions have a significant impact on the process of geopolymerization, which is a process of forming an amorphous or semi-crystalline polymeric structure consisting of sialate (Si-O-Al) and siloxo (Si-O-Si) bonds as a result of heterogeneous reactions of powder aluminosilicate oxides dissolved in a high-alkaline solution medium. It is reported that the curing temperature from 25 ◦C to 145 ◦C and curing time from 2 h to 24 h highly affects the dissolution of the precursor material [35]. According to Udawattha et al. [40], the recommended curing temperature is 50–80 ◦C. It is also confirmed by Chen K. et al. in their study [32], showing that the dissolution rate and geopolymerization increase with an increase in temperature. This is explained by the fact that an increase in temperature also increases the average kinetic energy of reactant molecules. Chen K. with his colleagues further emphasized that the control of the temperature may not only significantly affect the geopolymerization process, but might also affect the hardened characterizations of geopolymers [39,40]. Moreover, the results of Yener's and Karaaslan's research on the curing time and temperature effect on the properties of pumice-based geopolymers has shown that a curing treatment at 60 ◦C and 75 ◦C up to 168 h increased the strength of the geopolymer samples by almost two-fold compared to the 24-h heat-curing time and approximately 13 times compared to ambient curing [35].

However, one of the biggest disadvantages of geopolymers is the fact that, in the case of fly ash-based and other geopolymers, curing must be conducted at a relatively high temperature due to the poor hydration reactivity. The heat-curing process leads to high costs and energy consumption. It creates a barrier for the broad application of fly ash-based geopolymers in building processes due to the formulation of in situ cast concrete [41]; therefore, during the development of the review, special attention was paid to the possibility of using ambient conditions during the curing time. Despite the apparent advantages of using a relatively high temperature, several studies have confirmed that the curing of a fly ash-based geopolymer at an ambient temperature could be significantly accelerated after adding a small proportion of slag [42] or OPC with high CaO content [41]. Moreover, J. Migunthanna et al. have proven in their research [2] that geopolymers from binary blends of CBW with other aluminosilicate precursors such as slag and fly ash show good compressive strength also at ambient curing conditions. Of course, a more thorough analysis of the existing research is needed. However, it is already clear that geopolymers containing CBW can be successfully manufactured even if curing temperatures are not elevated.

### *4.3. Activator Type*

Such alkaline activators as sodium hydroxide, sodium silicate, KOH, potassium silicate, and sodium metasilicate are widely used to produce geopolymers [43,44]. There are two types of activators, namely, solid and liquid. Liquid activators are usually used in twopart geopolymers, whereas solid activators are usually used in one-part geopolymers. The use of solid activators lowers the cost of materials and environmental footprint, and transportation becomes easier [45].

Unfortunately, the aqueous activators, which are used in two-part geopolymer production, are highly corrosive and hazardous, therefore it is difficult to use them on an industrial scale [2].

The information gathered in Table 1 confirms a general trend that NaOH, sodium silicate Na2SiO3, and their combination are mainly used as activators for geopolymers in the production in which CBW is used. It should be noted that Na2SiO3 is produced via the direct fusion of pure silica with soda ash in a furnace at a temperature of approximately 1400 ◦C [2]. This process is highly energy consuming, and CO2 is emitted not only when furnaces are fired using oil and gas, but also during the chemical reaction. Of course, this activator is not used in large quantities, therefore the environmental impact is small; however, this is a factor to consider when choosing a suitable activator.
