**2. Geopolymers**

Geopolymer cement (GP) has several beneficial properties as Portland cement (PC), but also it is much more ecological. It is known as green material [14],

*Incorporation of Phase Change Materials and Application of 3D Printing Technology... DOI: http://dx.doi.org/10.5772/intechopen.96886*

because of its low energy consumption, and low emission of polluting gases during manufacture, which makes it an alternative binder for cementitious materials. Geopolymer is produced from the activation of aluminosilicate materials such as metakaolin [15], blast furnace slag [16], fly ash [17] and so on, by a homogenous solution of alkali hydroxide and alkaline silicate, at ambient conditions [14, 18]. The mechanism consists of (1) the dissolution of the aluminosilicate structure under the effect of the alkali hydroxide to form oligomers of silicate [SiO(OH)3] − and aluminate [Al(OH)4] − , and (2) the condensation of these free oligomers under the effect of the alkaline silicate to form another amorphous three-dimensional network [14, 18, 19]. The role of Na+ and K+ ions consists in balancing the electrical charge of Al3+ in IV-fold coordination [20, 21]. Geopolymer materials can be used for encapsulating heavy metals. In fact, the charge-balancing-alkali ions in the geopolymer network can be partially replaced, by ion exchange, with radioactive elements, and reducing them migration into the environment.

Alkali silicate solutions, also called water glass, such as sodium silicate, potassium silicate, lithium silicate etc. are consolidating agents for the material, they increase the formation rate of tetrahedrally coordinated aluminum [22] and improve the mechanical and physical properties of the final materials. Alkali silicate solutions have many applications, such as the production of silica gel and formulation of refractory ceramics and cements. The use of a highly reactive alkaline silicate solution and a highly amorphous aluminosilicate material enhances the formation of the geopolymer network [22]. The reactivity of alkaline silicate solutions was found to be dependent on the SiO2/Al2O3 and SiO2/Na2O ratios. According to some literatures [15, 23–25], the highest mechanical properties of geopolymers is obtained when SiO2/Al2O3 molar ratio is between 3.0 and 3.8, Na2O/ Al2O3 molar ratio is about 1 and the NaOH solution concentration is approximately 10 M. The SiO2/Al2O3 ratio may change with the type of raw material used as source of alumino-silicate [17] and then impacts on geopolymers properties [26]. The physical properties of geopolymers are improved when SiO2 are added to the mixture [14, 27]. Kong and Sanjayan [28] stated that alkaline solution selection and SiO2/Al2O3 ratio are critical parameters necessary to optimize the performance of geopolymer (at ambient or at elevated temperature).

Recent studies show that use waste can offer an alternative to alkaline silicate with potential advantage of lower cost and lower environmental impact. Torres-Carrasco and Puertas [21] have studied the feasibility of using industrial waste glass as a source of silica to replace sodium silicate in the alkaline activation of fly ash. They found, according to the analysis of the mechanical properties, degree of reaction and microstructure of alkali-activated fly ash, that the dissolved waste glass silicate by the NaOH solution had a substantial impact on the composition of the geopolymerisation reaction. Other studies demonstrated that usage of waste glass in concrete enhances its acid resistance as well as its physical and mechanical properties [29, 30]. In contrast the addition of waste glass reduces the plasticity of the fresh paste, thereby reduce its workability, thus a super-plasticizer is needed [31].

Tong et al. [32] investigated the production of sodium silicate solution from Rice Husk Ash (RHA). A hydrothermal process for the dissolution of RHA in sodium hydroxide solution was developed. Optimized procedure parameters were found to be: NaOH concentration 3 M, heating temperature 80°C and heating duration 3 h. The obtained solution was used for the production of alkali-activated binder made with a blend of fly ash and ground granulated blast furnace slag. Obtained compressive strength of mortar was in the range of 60 MPa at 28 days, which confirmed the equivalence between the solution produced with the optimized method and commercially available options. Cost analysis indicated that the proposed method

could allow a reduction of almost 55% of the cost for the activation of alkaliactivated binder.

Silica fume has been used as activator in metakaolin-geopolymer preparation [24], the Mk-based geopolymer with a silica fume content of 6 wt% (compared with those with 2% and 10%), corresponding to a SiO2/Al2O3 molar ratio of 3.84, resulted in the highest compressive strength, which was explained based on its high compactness with the smallest porosity. Silica fume improved the compressive strength by filling interstitial voids of the microstructure because of its fine particle size.

Geopolymer materials are differentiated by their SiO2/Al2O3 ratio that affects their structure and application. Aluminosilicate-based GPs are designated by the term "poly (sialate)," which is an abbreviation of poly (silico-oxo-aluminate) or (-Si-O-Al-O)n. The various types of poly (sialate), according to Davidovits [14], are polysialate or (-Si-O-Al-O-) or (PS) (with SiO2/Al2O3 = 2), poly (sialate-siloxo) or (-Si-O-Al-O-Si-O-) or (PSS) (with SiO2/Al2O3 = 4) and poly (sialate-disiloxo) (-Si-O-Al-O-Si-O-Si-O-) or (PSDS) (with SiO2/Al2O3 = 6). Each type of GP cement possesses special features: good thermal insulation for PS, high strength and good solidification in presence of toxic waste for PSS, excellent fire resistance and high adhesion for PSDS [14, 33].

Silva et al. [15] revealed that the properties of GP systems can be drastically affected by minor changes in the available SiO2 and Al2O3 concentrations during synthesis. The best resistance of metakaolin-based GPs was obtained when the SiO2/Al2O3 molar ratio is between 3.0 and 3.8. These ratios could however change according to the type of raw material used as a source of aluminosilicate. Yunsheng et al. [34] found for example the highest compressive strength (34.9 MPa) for metakaolin-based GPs with a higher SiO2/Al2O3 ratio (equal to 5.5).

Burciaga-Diaz et al. [35] investigated the compressive strength evolution as a function of the curing temperature at 20 and 80°C, and using the molar ratios of SiO2/Al2O3 (2.64–4.04) and Na2O/Al2O3 (0.62–1.54). The results revealed that the optimal ratios that yielded the greatest compressive strength were SiO2/ Al2O3 = 2.96 and Na2O/Al2O3 = 0.62, 0.93. For greater SiO2/Al2O3 ratios, good final compressive strengths were registered, but the setting time was very long. Curing at 80°C for 24 h was favorable for a rapid strength gain only at early ages, however, at later ages, the highest compressive strengths were obtained after curing at 20°C (this temperature makes it possible to avoid any thermal degradation of the geopolymer). Catauro et al. [36] investigated the structure and the mechanical behavior of the organic–inorganic hybrid materials consisting of an inorganic matrix of a metakaolin-based GP in which the polyethylene glycol (PEG) was added as a plasticizer. The results revealed that the elastic strain increased for a fixed value of stress with the percentage of PEG increases but a decrease of flexural and compressive strengths due to the increasing of porosity. PEG-free samples can reach final mechanical resistance faster than hybrid systems.

### **3. PCM and construction materials**

Numerous studies have investigated the integration of PCM to improve the building energy efficiency. Shadnia et al. [37] studied the mechanical and thermal properties of fly ash-based GP mortar containing different amount of PCM in the form of micro-encapsulated powder. They found that the unit weight and compressive strength of the GP mortar decreased when more PCM was incorporated due to lightweight, small shear strength and stiffness of PCM. In the meantime, the effect of the melting of PCM on the strength of GP mortar

#### *Incorporation of Phase Change Materials and Application of 3D Printing Technology... DOI: http://dx.doi.org/10.5772/intechopen.96886*

is negligible. Cao et al. [7] investigated the effect of microencapsulated PCM (paraffin Rubitherm) on the thermal performance and compressive strength of PC concrete and GP concrete. The results revealed that the increasing of the amount of micro-encapsulated PCM (MPCM) improved the latent heat, lowered the thermal conductivity, which could improve the thermal performance of building materials, and decreased the compressive strength in GP concrete compared to PC concrete due to the enhancement of porosity. The results also revealed that GP concrete exhibited better energy saving properties than PC concrete at the same conditions.

In another investigation, Duy Cao et al. [38] conducted a more detailed study on the effect of the polymer shell on the microstructure, thermal, and mechanical properties of GP concrete by using different kinds of MPCMs. They have found that the integration of PCM with a polymer shell containing polar functional groups into GP concrete at 5.2% by weight shows the best thermal performance but the lowest compressive strength due to the largest increase of GP concrete porosity and better interface bonds between microcapsules and the concrete matrix.

In order to know and examine the cause of low compressive strength after adding PCM, Pilehvar et al. [39] investigated the effect of incorporation of MPCM in solid and liquid states on the mechanical properties and microstructure of both GP and PC concretes. The results revealed that the compressive strength of both GP concrete and PC concrete decreased with increasing amount of MPCM. This decrease might be caused by the lower stiffness and strength of MPCM compared to sand, causing MPCM to be deformed or broken during the compression test [37]. It is also possible that air gaps, low adhesion and weak bonds between MPCM and the surrounding matrix may contribute to the strength reduction. The results also revealed that whether the PCM was in solid (Temperature below melting point) or liquid (temperature above melting point) state did not significantly affect the mechanical properties of GPC, while melting the PCM was found to reduce the strength of PC concrete. This was due to the fact that GP concrete has higher compression strength than PC concrete.

GP has been considered as an alternative carrier matrix to hold PCM in macro or micro encapsulated form as well as their being of more environmentally friendly material. Jacob et al. [33] fabricated encapsulated phase change materials EPCM (10 mm) consisting of molten chloride salt as a core encapsulated by fly ash geopolymer-based shells. They employed tow methods (dip-coating and preformed shells) to fabricate the geopolymer capsules. They found that the dip-coated capsules resulted in non-uniform shell thickness and shape, which promotes uneven heat transfer and high stress areas during phase change. However, the preformed capsules method allowed a greater control over size, shape and shell thickness.

Frattini et al. [40] synthesized GP mortar containing an expanded clay aggregate (supporting-shape) incorporated paraffine PCM. The results showed that with the paraffin content up to about 30–40% by weight, the matrix exhibited good mechanical properties and very high fire resistance.

Sukontasukkul et al. [41] studied the mechanical properties and heat insulation of wall panels made of fly ash-GPC containing porous lightweight aerated block (9.5–19 mm) impregnated with paraffin PCM. The results showed that the incorporation of PCM aggregate improved both thermal storage and heat insulation of GP panel. The results also showed that the density of GP increased with the increase in paraffin content in aggregate. Nonetheless, since lightweight aggregate was much weaker than GP paste, the increase in lightweight aggregate content caused the strength reduction.

Wang et al. [42] studied the influence of PCM on mechanical and thermal properties of clay GP mortar, PCM has been prepared by vacuum method using paraffin as the heat-absorbing material, expanded perlite as the supportive

material, and two encapsulation methods were developed by using CaSiO3 and Na2SiO3 as the capsules, and the corresponding PCMs were incorporated with clay GP mortar. They found that the incorporated PCM in clay GP mortar effectively reduced the transport heat. In addition to this, expanded perlite plays good package effect on paraffin during the process of solid–liquid phase change, which can guarantee the composite PCM in solid conditions during the solid–liquid phase change in the macro. By using CaSiO3 and Na2SiO3 as the capsules, the PCM building materials can effectively avoid the leakage of paraffin.

In order to obtain effective composite PCMs for thermal energy storage purpose in buildings, Sarı et al. [3] developed a form-stable composite PCM composed of the cement impregnated with the eutectic mixture of capric acid(CA)-myristic acid(MA) as PCM through vacuum impregnation method. And then they investigated its chemical structure. The FTIR spectra/XRD patterns not showed any new band/peak. This result confirms the fact that any chemical reaction is not carried out between the cement matrix and form-stable composite PCMs.

Kastiukas et al. [43] performed an investigation to determine the most efficient coating method and material regarding its ability at retaining the PCM. They produced macro-encapsulated aggregates using expanded clay lightweight aggregates (in the 2–10 mm size range + numerous small and large pores) as supportive shape impregnated with paraffin PCM. The different coating materials used were: Sika Latex, Weber dry-lastic and polyester resin adhesive (Palatal). The macro-encapsulated aggregates were then used as aggregates in GP binders made from a combination of aluminosilicate rich mud and waste glass. The results revealed that PCM vacuum impregnation was very successful for expanded clay lightweight aggregate. The polyester resin was determined to be the most suitable choice of coating material for the PCM impregnated lightweight aggregates. Polyester resin coating was also chemically stable and neutral, and improving thermal conductivity.

As conclusion several methods have been proposed to incorporate PCM in buildings:


*Incorporation of Phase Change Materials and Application of 3D Printing Technology... DOI: http://dx.doi.org/10.5772/intechopen.96886*

the porosity in buildings, and crack the concrete matrix, thereby causing the concrete matrix to crack and reduce mechanical strength.

4.Another method is the impregnation of PCMs in porous supportive shape materials through vacuum impregnation to obtain a macro-encapsulated PCMs and then mixing them with fresh concrete [33]. This method controls the volume expansion during phase change process. However, the solidification of PCM only around the edges during the time of heat regaining from liquid state prevents effective heat transfer. Overall, encapsulated PCM may be spherical, rectangular, cylindrical or irregular. It was found that spherical capsules resulted in the highest heat transfer rate. Also, the small size of encapsulated PCM leads to an increase in the heat transfer rate [33]. Moreover, the small encapsulated PCM can fill the cavities between aggregates and sand, thereby reducing the concrete porosity [7].

Therefore, there is strongly needed to develop geopolymer material with thermal energy storage TES ability besides without including these difficulties. In order to make this product more efficient, cheaper, and more environmentally friendly, the 3D printing technology could be used [11, 13].

## **4. Additive manufacturing**

Additive manufacturing technology, also called 3D printing technology, is also one of the promising methods used to reduce the energy demand and to reduce greenhouse gas emissions, in fact the application of this method takes less time than conventional manufacturing, and it uses the precise amount of material needed for the construction, which reduces the material costs, waste and negative impact on the environment [47, 48]. The digitally controlled construction process is done by a 3D printer. It consists in placing the fresh paste inside the loading container. Then, this last one is installed on the construction robot to start the printing process. The movable extrusion head is controlled by software to move in x, y, and z directions. The printer nozzle, on the extrusion head, traces the desired shape, layer by layer, until the final structure is achieved. The printing conditions could be optimized by modifying the nozzle diameter, the extrusion speed, layer heights, and the time gap between each layer [47–49].

The development of building materials adapted to the construction-based 3D printing technology has become of great importance in the world since the beginning of the second decade of the 21st century [11, 50].

3D printing process depends on two main factors, namely flowability and buildability [51]. The flowability is the fresh concrete extrusion capacity through the nozzle of the printer. The buildability is the ability of fresh concrete to retain the desired shape and hold layer overlays without collapsing. The buildability depends on the setting time and the mechanical property of the fresh material. Flowability and buildability can be considered as against each other, indeed if flow is increased, buildability decreases, and vice versa. Which is the main obstacle to applying the 3D printing method in the building construction [11, 52].

The mechanical activation of raw materials is one of many solutions used to overcome these obstacles. The mechanical activation reduces the setting time and improves the mechanical performance of the resulting material [53]. For example, the increasing of grinding time of the raw materials breaks the crystalline structure of the inert material and increases its reactivity which improves the mechanical

properties of the hardened product. However, overgrinding leads to a decrease in fresh material flowability, which causes further demand of water to reincrease the flowability [54]. Then the evaporation of water during curing process increases shrinkage and cracks in the resulting material which causes a decrease in its mechanical properties. If water does not added, the interaction between activators and aluminosilicate material remains low, thereby the unreacted materials causes cracks and affects the mechanical properties of the resulting material [55]. To increase the flowability and workability of fresh past while maintaining its mechanical performance, plasticizers should be used [56].

To reinforce the geopolymer material, Ma et al. [57] entrained a continuous micro steel cable during filaments deposition process, which demonstrated significant improvement of mechanical strength, toughness and post-cracking deformation of geopolymer composite. Shakor et al. [58] added lithium carbonate to reduce the setting time for the cement mixture.
