The Conservation of Architectural Heritage Structures Built with Tuff and Coral Rock: A Systematic Review and Bibliometric Analysis of Geopolymer Formulation, Application, Compatibility and Durability

Abstract

The conservation of tuff- and coral rock-built architectural heritage structures (AHS) is challenging because access to original tuff and coral rock has become difficult and severely limited due to urbanization, land reclamation, the depletion of stone quarries, anti-mining and anti-quarrying legislation. An emerging approach to address this issue is to create compatible “replacement” rocks via geopolymerization, a process that is more sustainable and greener than the use of conventional cement and concrete. To explore the potential of geopolymers for AHS conservation strategies, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were implemented; 103 eligible articles were identified and classified into geopolymers for AHS (34 articles), tuff-built AHS (60 articles), and coral rock-built AHS (9 articles). Tuff substrates in AHSs appear in a variety of colors (yellowish-brown, grayish-cream, reddish-brown, pale greenish-gray and pink hues), densities (1.0–2.5 g/m³), and compressive strengths (3–100 MPa). Meanwhile, coral rock substrates in AHSs appear in whitish-cream color and are coarse-pored (1–5 MPa), fine-grained (8–15 MPa), and calcarenite (50–60 MPa). In terms of geopolymer formulation, metakaolin was reported as the most popular main precursor or admixture, while NaOH and Na₂SiO₃ were used simultaneously as alkaline activators. Aggregates used in geopolymer formulations depended on local availability, including quartz sand, river sand, crushed stones, carbonate stones, volcanic rock, volcanic sand, tuff, brick, ceramic tiles, and waste materials. Aesthetics, chemical composition, physical attributes, and mechanical properties have been identified as key criteria to ensure geopolymer compatibility for AHS conservation application. To date, geopolymers have been applied for AHS conservation as repair mortars, consolidants (i.e., grout and adhesives), and masonry strengthening (i.e., fiber-reinforced mortar). Finally, geopolymers formulated for AHS conservation have similar durability as the original substrate based on accelerated aging tests (i.e., salt mist, wet-dry, and freeze–thaw) and long-term outdoor exposure experiments.

1. Introduction

Architectural heritage structures (AHSs) are historical buildings, monuments, and sites constructed using natural stones or rocks which ae being preserved for their cultural importance and significance [1]. Many of these AHSs were built using relatively soft rocks, with effective porosities of 20%–60%, bulk densities of 1.0–2.5 g/m³, and compressive strengths of 3–30 MPa [2,3,4]. Two natural rocks commonly used in AHSs are tuffs—a type of volcanic rock—and coral rock—a type of sedimentary rock—because they are locally available, soft enough to be worked with ancient tools, more durable than wood, and aesthetically beautiful [5,6,7].

A serious, inevitable problem associated with AHSs built from tuff and coral rocks is their high susceptibility to weathering, resulting in visible material loss and degradation after prolonged exposure to the environment. For example, Seigesmund et al. [8] reported that scaling and flaking on the surface of tuffs used in Mexican and Armenian AHSs were caused by swelling clay minerals that expand and contract during wet–dry cycles, inducing rock breakage and spalling due to internal tensile stresses. Similarly, Çelik and Sert [9] attributed the deterioration of tuff used in Turkish AHSs to salt crystallization, which was promoted by the high volume of micropores (<0.1 µm in diameter) serving as nucleation sites for rapid salt crystal growth.

In comparison, coral rocks are susceptible to water erosion and acid-promoted dissolution attributed to dissolved CO₂ in the air that lowers the pH of rainwater to around 5.8 [10], which gradually dissolves CaCO₃ in limestones and coral rocks. Manohar et al. [11], for example, investigated the deterioration mechanism of coral rocks used in Ramanathaswamy Temple, Remeshwaram Island, India, and found insignificant mass loss even after 15 cycles of salt weathering tests, which was attributed to the large pore size of the rock (~70% are >0.5 µm). However, visible damage to coral rocks was observed due to erosion from rainwater seepage and wind. Similarly, Urbina Leonor et al. [12] assessed the deterioration pattern of coral rocks used in the Port City of Veracruz, Mexico, using three artificial rain solutions, with pH of 5.0 (acid), 6.2 (non-acid), and 5.2 (global), consisting of [H⁺], [Na⁺], [NH₄⁺], [K⁺], [Mg²⁺], [Ca²⁺], [F⁻], [Cl⁻], [NO₃⁻], [SO₄²⁻], [HCO₃⁻] or [CO₃²⁻] and found that a high precipitation volume promoted higher deterioration of the coral rock regardless of its acidity.

Strategies to conserve weathered and degraded AHSs built from tuff and coral rocks follow strict rules to limit damage to the original structure and substrate material. According to the International Council on Monuments and Sites (ICOMOS, 2003), materials for AHS conservation must have visual and mechanical properties similar to those of the original materials to maintain historical authenticity and prevent damage. Traditional restoration techniques for AHSs use stones exploited from original historical quarries [13,14,15]. For example, Siedel et al. [13] reported that Rochlitz tuff, obtained from active quarries in the Rochlitz Mountain, was still used to restore historical structures in Germany. As reported by Cárdenes et al. [15], three (3) active quarry areas in Arcuas have been exploiting stones for the restoration of heritage structures in the Canary Islands and Spain.

In recent years, however, urbanization, land reclamation, the depletion of stone quarries, and anti-mining and anti-quarrying legislations have limited access to original AHS materials. For example, the Cathedral Basilica of Aguascalientes, Mexico [16], was built using a light pink tuff called Barranca tuff, but because of the depletion of historical quarries, several restoration efforts have used Valladolid tuff. Similarly, the conservation of coral rock-built Spanish-era (16th to 18th century) churches and watchtowers in Bohol, Philippines, has become challenging because the entire island is a UNESCO Global Geopark, a designation that makes stone quarrying illegal.

An alternative approach to address this issue is to find replacement or substitute stones that have similar properties to the original rock substrate but are obtained from other quarry sites. Replacement stones have a similar visual appearance as the original substrate but their formation times differ, resulting in mismatched physico-mechanical properties and weathering behavior. For example, Blows et al. [17] reported that the currently unavailable Caen stone, a cream-colored, fine-grained French limestone formed during the Middle Jurassic period, used as a building material for AHS cathedrals in the United Kingdom, has often been replaced by Lepine stone, a whitish English limestone formed during the Upper Jurassic. Both stones were tested for salt weathering, and it was found that Lepine stone was less durable (14% mass loss) than the original Caen stone (4% mass loss) after 15 cycles of simulated salt weathering. These authors attributed the higher durability of Caen stone to its lower microcrystalline calcite content (50%) compared to Lepine stone (70%).

Similarly, Přikryl and Török [18] assessed the impacts of a new stone introduced to the Charles Bridge in Prague, Czech Republic, which was built using local sandstone that is now unavailable due to the closure of all sandstone quarries in Prague. The color, physical, mechanical, and weathering behaviors of the new stone differed from those of the original substrate, which likely accelerated the deterioration of the Charles Bridge. As countermeasures, both the original and replacement stones were investigated based on their petrophysical and mechanical properties to determine their compatibility, rather than solely relying on visual characteristics such as color, texture, and grain size. Rozenbaum et al. [19] also emphasized selecting replacement stone based on its aesthetic, physical, and mechanical properties. These authors noted that Saint-Maximin stone, a French limestone used in Le Mans Cathedral (11th–15th century), was incompatible with Romanian Limestone in terms of composition (SiO₂ of 12.5% and 0%; CaCO₃ of 87.4% and 97.1%), total porosity (38.0 and 27.0), and pore volume sizes (18 µm and 8 µm). In comparison to Garchy Stone, a limestone originating from south Paris, used in the cathedrals in Orleans, Nevers and Bourges (13th century) and its compatible replacement limestone in central Romania have a similar composition (SiO₂ of 0 and 1.7%, CaCO₃ of 99.6% and 97.5%), total porosity (19.7%, and 18.2%), and pore volume sizes of (1.3 µm and 1 µm). Meanwhile, Martínez-Martínez et al. [3] followed the recommendation of Rozenbaum et al. [19] and investigated other replacement tuff stones of the original rocks used in AHSs in Morelia from other quarries; they conducted compatibility tests based on petrophysical rather than solely aesthetic considerations.

In the absence of replacement stones or rocks, conservation activities for AHSs have historically relied on cementitious materials, such as cement- and concrete-based materials, due to their favorable mechanical properties and on-site applicability. In the 1950s, for example, concrete was used for the early restoration of Eger Castle in Hungary [20]. Concrete, however, promoted the formation of salt efflorescence, which led to severe deterioration of the original stone substrate. According to Török et al. [20], the lower permeability of the concrete sealing the tuff walls of the Eger Castle not only introduced more rainwater into the tuff substrate but also prevented pore water evaporation, thereby enhancing salt efflorescence formation.

To improve compatibility, “artificial stones”, a type of concrete designed to mimic the natural physical and mechanical properties, including color and texture, of the natural stone by mixing local materials (e.g., natural pozzolana, crushed stones, local soils, sand) with white cement, superplasticizer, and inorganic-colored pigments, have recently been developed. For example, Stefanidou et al. [21] formulated an artificial stone to restore three (3) AHSs in Greece, namely the Ancient Agora of Pella, (5th–4th B.C.), the Ancient Theatre of Maronia (4th–3rd B.C.), and the Fortress of Saint Nicolaos of the Medieval City of Rhodes (15th century) built from marl limestone, limestone/marble, and sandstone, respectively; the result showed compatibility to natural stones attributed to texture, color, pore size distribution and compressive strength. Similarly, Menningen et al. [22] developed an artificial stone to mimic the aesthetic appearance and petrophysical properties of the original Armenian tuff for the restoration of cultural heritage, using cement, crushed original stones, and colored pigment. However, the drawbacks of artificial stone include the use of ordinary Portland cement (OPC), a major contributor to carbon dioxide (CO₂) emissions.

An emerging, more sustainable and greener alternative for cement- and concrete-based materials is geopolymers [23,24]. Geopolymers have been widely used as a sustainable alternative to OPC for reducing carbon emissions, providing high chemical resistance and mechanical strength under optimal conditions [25,26,27], and repurposing waste streams [28,29,30,31]. For example, Amar et al. [32] obtained a higher 7-day unconfined compressive strength (UCS) of 44 MPa for geopolymer concrete in comparison to OPC concrete (36 MPa), while at later stages of hardening (28 days), both have comparable UCS values of 51 MPa for geopolymers and 54 MPa for OPC.

Geopolymers have also been recently formulated to mimic the properties of natural rocks, a key development in the successful application of these materials for AHS conservation. For example, Peltz et al. [33] designed a synthetic sandstone, using metakaolin as the precursor and quartz sand as the aggregate, achieving an effective porosity (22.8%) and compressive strength (19.6 MPa) lower than those of natural rocks such as Berea sandstone at 13.1%, and 91.2 MPa and Flechtingen Sandstone at 5.8% and 129 MPa, respectively. Meanwhile, Simão et al. [34] utilized stone cutting waste from marble (80 wt%) and volcanic rocks (20 wt%), at 50 wt% and 75 wt% replacement for metakaolin, resulting in open porosity of 49.5% and 53.6% and a compressive strength of 28 MPa and 14 MPa, respectively. Meanwhile, Khater et al. [35] prepared a slag–metakaolin–granite composite (12.5 wt%–12.5 wt%–75 wt%) via hydraulic pressing at 30 MPa, 50 MPa, and 70 MPa, resulting in a compressive strength of 20.9 MPa, 35.8 MPa, and 13.3 MPa, respectively.

To explore in detail the formulations and properties of geopolymers applied to AHS conservation strategies, we employed a systematic approach to review the literature. Although several review papers about stone heritage conservation have been published, the majority did not focus on the application of geopolymers [36,37]. Among the few reviews that included geopolymers, the content and discussion were limited to functionalizing geopolymers as coating mortars [23] and reinforcing historical structures [24]. These previous review papers did not include compatibility criteria or issues between geopolymers and the original substrate of AHSs in terms of aesthetics, chemical composition, physical attributes, and mechanical properties. Moreover, the durability of geopolymers applied to AHS conservation has not yet been explored in detail. These knowledge gaps on geopolymer application for AHS conservation are addressed in this work, which is guided by the following research questions:

• What are the physical and mechanical properties of original tuff and coral rock substrates in AHSs?
• What geopolymer formulations have been developed and applied for tuff-built and coral rock-built AHSs from 2013 to 2024?
• What are the compatibility criteria used and properties measured in selecting geopolymers for the conservation of AHSs built from tuff and coral rocks?
• How are geopolymers characterized, evaluated and applied in the conservation of AHSs built from tuff and coral rocks

2. Review Methodology

2.1. Search and Identification

This paper employed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [38] and the protocols recommended by Andrews [39] to review the conservation of heritage structures built with tuff and coral rock using geopolymers. Prior to the search, the protocol was pre-registered in the Open Science Framework (OSF) with registration DOI: 10.17605/OSF.IO/RQMJU. The search field remained within article title, abstract, and keywords and used the search string “geopolymer” OR (“coral” AND “rock”) OR “tuff” AND “heritage”, with publication dates restricted to 2013–2024, across the Scopus and Web of Science (WoS) databases and extracted during August 2025. A total of 487 items, 227 from Scopus and 260 from WoS, were retrieved, which were reduced to 368 after removing duplicates. Meanwhile, Google Scholar was used as a supplementary database with a similar search string, identifying 10 articles. The search, screening and article retrieval were conducted between June and July 2025.

2.2. Screening and Inclusion Criteria

Figure 1 illustrates the screening process, which excluded articles that did not focus on tuff, coral rocks, or geopolymers based on titles, abstracts, highlights, and keywords. These criteria removed 249 articles, resulting in 119 items for eligibility assessment. Based on the eligibility criteria, conference proceedings (13), inaccessible articles (2), review papers (1) and non-English studies (1) were removed. Full-text articles were also evaluated [40], and those not focusing on geopolymers for AHS conservation (9) were excluded. The PRISMA guidelines also allow for the inclusion of articles using other methods; thus, 10 articles were added using Google Scholar following the same screening criteria, and a total of 103 articles were classified for tuff-built heritage, coral rock-built heritage, and geopolymers applied to AHSs (Figure 2). Because this review focuses on geopolymers for AHS conservation, 34 papers containing experimental data sets were analyzed and summarized, while the remaining 69 articles about tuff-built and coral rock-built AHSs were included in this review because they had useful descriptive information about original substrate characterization and properties, assessment protocols and/or the extent of original substrate deterioration. The 34 geopolymer papers were further subdivided into different precursor materials used: metakaolin (12), fly ash (4), slag (4), volcanic ash (5), ceramic wastes (4), tuff waste (1), pumice (1), silica fume (1), local clay (1), and mine wastes (1).

Figure 1. A schematic diagram of the article selection process following the PRISMA guidelines. * Reported the number of records identified from each database. ** Excluded by human, no automation tools used.

Figure 2. Classification of eligible items related to AHS conservation analyzed in review.

2.3. Review Limitations

This review only focused on the formulation of geopolymers, including their compatibility and application for AHSs built from tuff and coral rocks. In addition, the discussion of geopolymer formulation was limited to the type of precursors, alkali activators, admixture, aggregate, and 28-day compressive strengths (unless specified). Meanwhile, material compatibility discussion focused on the use of aesthetic qualities as well as chemical, physical, and mechanical properties as criteria for selection. Aesthetic qualities were limited to color, texture, and the presence of effloresce while chemical properties focused on microstructure and the release of ions from the substrate. Physical properties were limited to open porosity, water absorption, capillarity, and bulk density. Finally, mechanical properties focused only on important factors like adhesion, surface hardness, compaction, and compressive, tensile, and flexural strengths. Geopolymer application in AHSs was limited to the most common type of interventions, including repair mortar, consolidants, and masonry strengthening. In addition, the long-term behavior of the geopolymers was also reviewed, but limited to accelerated and long-term stability tests.

3. Physical and Mechanical Properties of Original Tuff and Coral Rock Substrates of Architectural Heritage Structures

3.1. Tuff-Built Architectural Heritage Structures

Tuffs are rocks composed of pyroclastic materials (i.e., ash, rocks, and crystals) consolidated and naturally cemented with time due to extreme temperature and pressure generated during a volcanic eruption. Table 1 lists some tuff-built AHSs from different countries, along with their corresponding colors and physico-mechanical properties.

In general, tuffs exhibit a variety of colors, textures, and physico-mechanical properties depending on their location [2]. In Europe, tuff-built AHSs are found in Italy, Hungary, Spain, the Netherlands, Slovakia, and Germany (Figure 3). In Italy, for example, prominent AHSs such as the Ovo Castle, St. Elmo Castle, St. Domenico Maggiore Cathedral and St. Chiara were constructed during the 13th–16th century using Neopolitan Yellow tuff, a yellowish-brown, lightweight, and low-strength rock with a bulk density and compressive strength in the range of 1.0–1.3 g/m³ and 3–6 MPa, respectively [4]. Similarly in Hungary, a greyish-creamy to reddish-brown tuff with higher bulk density (1.6–1.8 g/cm³) and compressive strength (7–26 MPa) than Neopolitan Yellow tuff was used to construct the Sirok Castle, Eger Cathedral, and Ottoman Minaret during the 13th–19th century [5]. This tuff has a strength comparable to the greyish-beige tuffs used in cathedrals, churches, and historical buildings in the Netherlands, with compressive strengths ranging from 13 to 29 MPa [6,7]. In Rochlitz, Germany, a slightly higher strength tuff with a distinct deep reddish-brown color, with a similar appearance to a fired clay brick, was used to construct the Church of St. Maria in Mittweida, the Collegiate church in Wechselburg (Zchillen), and Portal of the church St. Kilian in Van Lausick, all of which were built around the 12th–15th centuries [13]. Two relatively high strength tuffs with compressive strengths ranging from 50 to 100 MPa have been reported in the literature. One was used in the Archaeological site of Mŏsnje, Slovakia (16th–19th century; pale green-grayish rock) [14], while the other is known as the Arucas stones—with a dominant grey and a hint of yellow, blue, and red—from Canary Island, Spain, which was used in constructing the Church of San Juan Bautista [15,41].

Figure 3. Examples of European tuff-built AHSs: (a) Ovo Castle, Naples, Italy (13th–16th century) (photo by Gmaiga from Pixabay), (b) Church of San Juan Bautista, Canary Islands, Spain (20th century) [15], (c) Church of St. Maria, Mittweida, Germany (15th century) [13], (d) Eger Cathedral, Eger, Hungary, (19th century) (photo by Molnár Tamás Photography from Pexels), and (e) Bergkerk, Deventer, Netherlands, (13th century) [7].

In North America, several AHSs were constructed in Mexico in the 16th century during the Spanish occupation and in Honduras during the Mayan civilization, dated 250–900 CE (Figure 4). In Mexico, there are a large variety of tuffs used in Spanish-influenced AHSs [42,43]; two of the most popular are the “Piera Vieja”, a pink-colored tuff with an effective porosity of 26.5%, bulk density of 1.8 g/cm³ and compressive strength of 30.8 MPa, and “Barranca Tuff”, with an effective porosity of 37.5% and bulk density of 1.5 g/cm³, which were used as building stones for Cathedrals in Morelia and Aguascalinetes, respectively. Similarly, other colored tuffs were used, including the green-reddish colored “Loseros tuff”, with an effective porosity of 6%–17%, bulk density of 2.1–2.3 g/cm³, and compressive strength of 21–74 MPa and the white-colored “Tezoantla tuff”, with an effective porosity of 14.9%, bulk density of 1.9 g/cm³, and compressive strength of 50–67 MPa used in several historical sites of Guanajuanto and Hidalgo, respectively, to construct historical churches, cathedrals, and government buildings in Mexico [3,16,44,45,46]. In Honduras, the famous Heritage Site, the Archeological Site of Copan, with a classic Mayan architecture, such as the Hieroglyphic Stairway, was built using Maya Tuff during 756 CE, a buff-greenish-colored stone with an effective porosity of 27%–28% [47,48].

Figure 4. Examples of representative tuff-built AHSs in North America: (a) Cathedral of Morelia, Mexico, (17th century) [42]; (b) Cathedral of Aguascalinetes, Mexico (16–20th century) [16]; (c) Façade of Santa Veracruz Chapel of Mineral del Monte, Mexico (16th century) [46]; (d) Façade of Legislative office, Loseros, Mexico, 19th century [45]; and (e) Maya tuff stones, in Archeological Site in Copan, Honduras, (763 CE) [47].

In West Asia, several AHSs were built in Turkey and Armenia using tuff (Figure 5). In Turkey, there are various types of tuffs but those quarried in the Cappadocia region [49] like the cream-beige colored Döğer Tuffs in Afyonkarahisar (effective porosity of 42%, bulk density of 2.5 g/cm³, and compressive strength of 26.5 MPa) [9,50,51,52,53] and whitish-gray tuff in Malatya (effective porosity of 14.8%, bulk density of 1.8–1.9 g/cm³, and compressive strength of 12.9 MPa) [54] are the most notable. These tuffs were used to construct caravanserai—a roadside inn alongside the Silk Road with a central courtyard used by travelers for trade and commerce—mosques, and churches such as the Kayseri Castle (527–565 B.C.) in Kayseri, Döğer Caravanserai (1434) in Afyonkarahisar, and the 12th-century Battalgazi Mosque in Malatya. In Armenia, the tuff had a variety of colors, including black, red, golden, and grey brown, and was used in the construction of religious monuments and buildings like the Cathedral of Etchmiadzin (constructed around 301–303 A.D.) and the All Savior Church in Gyumri, which was rebuilt using black tuff [55].

Figure 5. Examples of tuff-built AHSs: (a) Döğer Caravanserai, Afyonkarahisar, Turkey (1434) [51], (b) Kayseri Castle, Kayseri, Turkey, (527–565 B.C.) (Photo by Ahmed from Pexels.com), (c) All Savior Church, Gyumri, Armenia (1858–1872) [22], and (d) Cathedral of Etchmiadzin (301–303 A.D.), Armenia (photo by Areg Amirkhanian from worldhistory.org).

In East Asian countries such as China, Japan, and South Korea, tuff was used to construct platforms for temples, pagodas, shrines, and sculpted statues, which generally had a common grayish hue as a distinguishing characteristic (Figure 6). In China, for example, Guo et al. [56] characterized the stone platform used in Baoguosi Temple, located in Ningbo, dated from 880–1013 C.E., with a light purple to grey color, which was similar to the tuff used in the Sanlao Stone chamber in Hangzhou, Zhejiang Province, with a grey-green color built in 1921 [57]. Meanwhile, in South Korea, the stones used in the five-story pagoda at Geumgolsan Mountain (Korean Treasure No. 529), in Jingdo-gun are estimated to have been constructed in 918–1392 CE and originally had a yellow to light gray color but now appear black and brown from weathering [58]. Meanwhile, the Oya Stone, a soft tuff in Japan, can appear grayish, brownish/creamy, or greenish, and was used to build the famous Oya-ji temple in 810 CE [59,60].

Figure 6. Examples of tuff-built AHSs in East Asia: (a) Baoguosi Temple, Ningbo, China, (880–1013) [56], (b) Sanlao Stone House, Zhejiang, China (1921) [57], (c) Korean Treasure 529, Jingdo, S.Korea (918–1392 CE) (photo taken from Korean Heritage Service), and (d) Oya-ji Temple, Kanto, Japan (810 CE) [59].

Table 1. A list of notable AHSs around the world and the physical and mechanical properties of tuff used to build them.

Country	Heritage Structure, Region	Tuff Name	Color (Unweathered)	Effective Porosity (%)	Bulk Density (g/cm3)	Unconfined Compressive Strength (MPa)	Date Constructed	Ref.
Italy	Ovo Castle, Naples	Neopolitan Yellow Tuff	Yellow-brown	50–60	1.0–1.3	3–6	13th–16th	[4]
	St. Elmo Castle, Naples
	St. Domenico Maggiore Cathedral, Naples
	St. Chiara Cathedral, Naples
Hungary	Sirok Castle, Sirok Eger Cathedral, Eger Ottoman Minaret	Eger,	Greyish-cream	52.6	1.6	7.2	13th–19th	[5]
		Demjén,		41.8	1.5	10.4
		Sirok,		29.9	1.6	28.2
		Bogács,	Reddish-brown	25.6	1.8	26.5
Netherlands	St. Peter’s church, Leiden	Römer Tuff, Weiberner, Ettringer Tuff	Greyish-beige	−	−	13–29	11th−15th	[6,7]
	The Bergkerk, Deventer
	The Deanery, Deventer
Germany	Church of St. Maria in Mittweida	Rochlitz Porphyry Tuff	Red	27.1	1.7–2.2	13.5–56	12th–15th	[13]
	Church St Kilian in Bad Lausick
	Collegiate church in Wechselburg
Slovakia	Architectural site of Mŏsnje	Peračica Tuff	Green	19.7–20.6	−	72–85.8	16th−19th	[14]
Spain	Church of San Juan Bautista, Canary Islands,	Rosa Silva, Piedra Azul, Piedra Amarillia,	Grey-blue, ochre	0.4–19.6	2.1–2.4	74–109	20th	[15]
		Rosa Silva,	Grey	3.2	2.4	67–69		[40]
		Lomo Tomas	Grey	22.0	2.1	51–60
Mexico	Cathedral of Morelia	Piedra Vieja	Pink	26.5	1.8	30.8	17th	[3,42]
	Cathedral of Aguascalientes	Barranca Tuff	Pink	37.31	1.5	−	16th−20th	[16]
	Legislative Palace of Guanajuanto,	Loseros Tuff	Green-red	6.8–17.4	2.1–2.3	21–74	19th	[43]
	Reloj Monumental (Clock Tower), Hidalgo	Tezoantla Tuff	White	14.9	1.9	50–67	1910	[44]
	Santa Vera Cruz Chapel, Hidalgo						1563−1578
Honduras	Hieroglyphic Stairway, Copan	−	Buff-green	27–28	−	−	756 CE	[47,48]
Turkey	Kayseri Castle, Kayseri	Kuruköprü tuff	Greyish	22	1.9	52.5	527–565 BC	[49]
	Döğer Caravanserai, Afyonkarahisar,	Döğer Tuff	Cream-Beige	42	2.5	26.5	1434	[9,50,51,52,53]
	Battalgazi Heritage Mosque, Malatya	−	Greyish	14.8	1.8–1.9	12.9	12th	[54]
Armenia	All Savior’s Church in Gyumri,	Gyri Black	Black	14.7	2.1	78.2	19th	[22,55]
China	Baoguosi Temple, Ningbo	−	Light-purple to grey	−	−	−	880–1013	[56]
	Sanlao Stone Chamber, Hangzhou	−	Grey-green	26	2.1	−	1921	[57]
South Korea	Five Stone Pagoda, Geumgolsan Mountain, Jingdo-gun	−	Light gray	17.8	−	−	918–1392	[58]
Japan	Oya-ji Temple, and Stone Buddhas, Tochigi Prefecture, Kanto	Oya Stone	Gray, brownish/creamy, or greenish	40–45	1.4	10–15	810 CE	[59,60]

Table 1. A list of notable AHSs around the world and the physical and mechanical properties of tuff used to build them.

Country	Heritage Structure, Region	Tuff Name	Color (Unweathered)	Effective Porosity (%)	Bulk Density (g/cm3)	Unconfined Compressive Strength (MPa)	Date Constructed	Ref.
Italy	Ovo Castle, Naples	Neopolitan Yellow Tuff	Yellow-brown	50–60	1.0–1.3	3–6	13th–16th	[4]
	St. Elmo Castle, Naples
	St. Domenico Maggiore Cathedral, Naples
	St. Chiara Cathedral, Naples
Hungary	Sirok Castle, Sirok Eger Cathedral, Eger Ottoman Minaret	Eger,	Greyish-cream	52.6	1.6	7.2	13th–19th	[5]
		Demjén,		41.8	1.5	10.4
		Sirok,		29.9	1.6	28.2
		Bogács,	Reddish-brown	25.6	1.8	26.5
Netherlands	St. Peter’s church, Leiden	Römer Tuff, Weiberner, Ettringer Tuff	Greyish-beige	−	−	13–29	11th−15th	[6,7]
	The Bergkerk, Deventer
	The Deanery, Deventer
Germany	Church of St. Maria in Mittweida	Rochlitz Porphyry Tuff	Red	27.1	1.7–2.2	13.5–56	12th–15th	[13]
	Church St Kilian in Bad Lausick
	Collegiate church in Wechselburg
Slovakia	Architectural site of Mŏsnje	Peračica Tuff	Green	19.7–20.6	−	72–85.8	16th−19th	[14]
Spain	Church of San Juan Bautista, Canary Islands,	Rosa Silva, Piedra Azul, Piedra Amarillia,	Grey-blue, ochre	0.4–19.6	2.1–2.4	74–109	20th	[15]
		Rosa Silva,	Grey	3.2	2.4	67–69		[40]
		Lomo Tomas	Grey	22.0	2.1	51–60
Mexico	Cathedral of Morelia	Piedra Vieja	Pink	26.5	1.8	30.8	17th	[3,42]
	Cathedral of Aguascalientes	Barranca Tuff	Pink	37.31	1.5	−	16th−20th	[16]
	Legislative Palace of Guanajuanto,	Loseros Tuff	Green-red	6.8–17.4	2.1–2.3	21–74	19th	[43]
	Reloj Monumental (Clock Tower), Hidalgo	Tezoantla Tuff	White	14.9	1.9	50–67	1910	[44]
	Santa Vera Cruz Chapel, Hidalgo						1563−1578
Honduras	Hieroglyphic Stairway, Copan	−	Buff-green	27–28	−	−	756 CE	[47,48]
Turkey	Kayseri Castle, Kayseri	Kuruköprü tuff	Greyish	22	1.9	52.5	527–565 BC	[49]
	Döğer Caravanserai, Afyonkarahisar,	Döğer Tuff	Cream-Beige	42	2.5	26.5	1434	[9,50,51,52,53]
	Battalgazi Heritage Mosque, Malatya	−	Greyish	14.8	1.8–1.9	12.9	12th	[54]
Armenia	All Savior’s Church in Gyumri,	Gyri Black	Black	14.7	2.1	78.2	19th	[22,55]
China	Baoguosi Temple, Ningbo	−	Light-purple to grey	−	−	−	880–1013	[56]
	Sanlao Stone Chamber, Hangzhou	−	Grey-green	26	2.1	−	1921	[57]
South Korea	Five Stone Pagoda, Geumgolsan Mountain, Jingdo-gun	−	Light gray	17.8	−	−	918–1392	[58]
Japan	Oya-ji Temple, and Stone Buddhas, Tochigi Prefecture, Kanto	Oya Stone	Gray, brownish/creamy, or greenish	40–45	1.4	10–15	810 CE	[59,60]

3.2. Coral Rock-Built Architectural Heritage Structures

Another type of rock widely used for AHSs is coral rock, also known as “coral stone” or “coral reef limestone”. Coral rock is a type of limestone with a white to cream-white color formed from coral reefs in ancient oceans, with an appearance similar to that of corals. They are formed when corals—produced by a multicellular organism called polyps—slowly develop a thick calcareous exoskeleton from sorbing abundant calcium (Ca²⁺) and carbonate (CO₃²⁻) ions in marine ecosystems. As these polyps die, their hard exoskeletons, consisting of calcium carbonate (CaCO₃), remain and, over millions of years, are buried, cemented, and compacted, transforming into coral rocks [11]. Meng et al. [61] reported four (4) categories of non-porous coral rocks—boulder, framework, gravel, and calcarenite—depending on the grain sizes. This variation in coral rock grain sizes could be attributed to the extent of dolomitization, cementation, erosion, and dissolution of coral skeletons over millions of years. For example, the calcarenite coral rock used in constructing the Ramanathaswamy Temple (India) had a bulk density of 2.21 g/cm³ and a 50–60 MPa compressive strength [11].

When coral rocks have a reef-like appearance with visible pore structure, they can be classified into coarse-pore and fine-pore types, with annular and linear pore arrangements, respectively (Figure 7). Xu et al. [62], for example, employed a polarizing microscope and estimated pore sizes of 0.5–1 mm and 100–200 µm for coarse and fine coral rock, respectively. Wei et al. [63] also noted the lower compressive strengths (1.6–4.7 MPa) of coarse coral rock compared with fine coral rocks (7.6–15.1 MPa). Several examples of coarse coral rock are the Megalithic Temples of Malta, the Historical Houses of Jeddah in Saudi Arabia, the Historical Dome in Qeshm Island, Iran, and the Veracruz Port in Veracruz City, Mexico. In comparison, fine-pore coral rock was used in the Mosque in the Maldives, Historical Ruins in Kenya, Historical Houses in Tanzania, and Baroque Churches, government structures and Watchtowers during the Spanish Era in various islands and coastal regions in the Philippines. Table 2 summarizes some coral rock-built AHSs from the Maldives, Philippines, India, Mexico, Saudi Arabia, Iran, Kenya, and Tanzania. Like tuff, coral rocks come in several forms.

Figure 7. Different pore-structured coral rocks with a clear difference in pore structure: (a) coarse-pore coral rock and fine-pore coral rock [63], (b) coral rock used in the Port of Veracruz, Mexico, (c) a 5 cm cubic specimen of coarse-pore coral rock in Port of Veracruz, Mexico [12], (d) Maribojoc Watchtower, Bohol, Philippines, and (e) a 2 cm cubic specimen of fine-pore coral rock in Maribojoc Watchtower. Photos in (d,e) are from the authors.

Table 2. A list of notable AHSs built using coral rocks.

Country	Region	Coral Rock Type	Structure	Year	Ref.
Maldives	Several atolls	Fine-pored	Historical Mosque,	15th–17th	[64]
Philippines	Bohol	Fine-pored	Alburquerque Church, Dimiao Chapel, Maribojoc Church and Watchtower	17th	[65]
Kenya	Shanga	Fine-pored	Historical Ruins	10th–11th	[66]
Tanzania	Zanzibar	Fine-pored	Historical Residential Town	17th	[67]
India	Tamil Nadu,	Calcarenite	Ramanathaswamy Temple	13th	[11]
Mexico	Veracruz	Coarse-pored	Fortress of San Juan de Ulua and of the Portal de Miranda	16th–19th	[12]
Saudi Arabia	Jeddah	Coarse-pored; Calcarenites	Historical Residential Houses	646 AD	[68]
Malta	Malta Island	Coarse-pored	Megalithic Temples	4000–3000 BC	[69]
Iran	Qeshm Island	Coarse-pored	Historical Dome	800 AD	[70]

Figure 8 illustrates the locations of some AHSs constructed from tuff and coral rocks, indicating that most tuff-built AHSs are in areas with volcanoes, such as Japan, China, the Philippines, and Korea, all within the Pacific Ring of Fire. In addition, tuffs can be classified in terms of color depending on location, with tuffs found in South America (Mexico and Honduras) being light-colored, those found in Europe (e.g., Turkey) having grey to dark colorations, and those in Asian countries being characterized by intermediate (grey) to light colors. In comparison, AHSs made of coral rocks are situated in coastal regions (i.e., ports or islands) in tropical countries and can be further classified based on their physical property; that is, fine-pore coral rocks in AHSs are found in the Philippines, Kenya, Tanzania, and Maldives, coarse-pore coral rocks are common in AHSs in Mexico (South America), Jeddah, Iran (Middle East) and Malta (Europe), and non-pored coral rocks such as the calcarenite coral rock are common in India.

Figure 8. World map showing locations of some notable AHSs built from tuff or coral rocks.

4. Geopolymers in Architectural Heritage Structure Conservation

4.1. Publication Trends of Geopolymers in Architectural Heritage Structure Conservation

Figure 9 displays the trend of 34 articles on geopolymers for heritage conservation. The study of geopolymers for cultural heritage began in 2016, a year after the ratification of the United Nations Sustainable Development Goals (UN-SDGs), primarily focusing on developing sustainable repair mortars using metakaolin as a precursor material. In 2017, geopolymer composites with the addition of resins, mine waste, and fibers were reported, but the following year (2018), no publications on geopolymers for heritage conservation were found. In 2019, articles published focused on the repair of broken ancient tiles, while in 2020, there was a rapid increase in the use of pyroclastic deposits as geopolymer precursors and repurposing waste printed circuit boards. In 2021, waste bricks and stone waste were introduced as precursor materials and aggregate and mineral additives to improve the compressive strength of geopolymer mortar. The following year (2022), the addition of porcelain and tile wastes as a partial replacement for traditional metakaolin was evaluated, while silica fume was used as an alternative lime-based grout for historical masonry, and organic additives (polyvinyl acetate (PVA) and polydimethylsiloxane (PDMS)) were added to metakaolin-based geopolymers. In 2023, tuff and brick wastes were used as precursors and aggregates to develop geopolymers, ensuring characteristics similar to the original stone or materials, while in 2024, the apparent use of tile waste as repair mortar for suitability in historical brick masonry was attributed to its similar chromatic appearance.

Figure 9. Number of articles published from 2013 to 2024 on geopolymers for AHS conservation.

4.2. Active Countries, Journals, and Institutions

Table 3 lists the countries where research on geopolymers for heritage conservation is active. Italy has the highest frequency on the list at 56%, followed by Portugal (12%), the Czech Republic (12%), and Turkey (6%).

Table 3. Countries with active publications from 2013 to 2024 on geopolymers for AHS conservation.

Country	Frequency	% (N = 34)
Italy	19	56
Portugal	4	12
Czech Republic	4	12
Turkey	2	6
India	2	6
Morrocco	1	3
Philippines	1	3
China	1	3

Figure 10a shows the journals commonly publishing studies on geopolymer-based materials for heritage structures. Construction and Building Materials (21%), a Q1 journal based in the UK, has the highest publication rate, followed by Materials (11%), Ceramics International (8%), Journal of Building Engineering (8%), Crystals (5%), Polymers (5%), and Sustainability (5%).

Figure 10. Active (a) journals and (b) institutions publishing on geopolymers for heritage conservation from 2013 to 2024.

Figure 10b displays the affiliation of authors by the number of publications, highlighting that most institutions are located in countries such as Italy, the Czech Republic, Turkey, and Portugal. The most active institution is the University of Catania (24%) in Italy, followed by the Czech Academy of Sciences (11%) in Czech Republic, then the Consiglio Nazionale de Ricerche (8%), and Universita della Campania Vanvitelli (8%) both in Italy, Inonu University in Turkey (5%) and Univerista della Aviero in Portugal (5%).

4.3. Active Authors

Table 4 summarizes the most active authors from 2013 to 2024, with Roberta Occhipinti (10.5%) being the most active, with four articles on the development of geopolymers using pyroclastic deposits (i.e., pumice, volcanic ash, volcanic soil). She is followed by Fugazzotto, M. (7.9%), with three papers related to the valorization of ceramic tile waste for geopolymers, and Perná, I. (7.9%), who focused on developing metakaolin-based geopolymers. Other notable authors include Ricciotti, L. (utilized ceramic tile waste and the addition of organic additives), Finocchiaro, C. (utilized pyroclastic materials, including volcanic ash and soil, using potassium activators), and Montinho, S. (focused on metakaolin-based geopolymers for repairing fragmented ancient ceramic tile facades).

Table 4. List of active authors, number of published studies, and country.

Author	Frequency	% (N = 34)	Country
Occhipinti, R.	4	12	Italy
Fugazzotto, M.	3	9	Italy
Perná, I.	3	9	Czechia
Ricciotti, L.	3	9	Italy
Finocchiaro, C.	2	6	Italy
Moutinho, S.	2	6	Portugal
Tamburini, S.	1	3	Italy
Clausi, M.	1	3	Italy
Angelo, A.	1	3	Italy
Capasso, I.	1	3	Italy
Longo, F.	1	3	Italy
Vasanelli, E.	1	3	Italy
Rescic, S.	1	3	Italy
Maras, M.M.	1	3	Turkey
Kutlusoy, E.	1	3	Turkey
Baltazar, L.	1	3	Portugal
Beghoura, I.	1	3	Portugal
Allali, F.	1	3	Morrocco
Kakria, K.	1	3	India
Gupta, V.	1	3	India
Dollente, I	1	3	Philippines
Mácová, P.	1	3	Czechia
Jin, G.	1	3	China

4.4. Definition and Formation Mechanism

Geopolymers are widely considered green alternative cementitious materials because of their low carbon emissions, high chemical resistance, and good thermal properties and mechanical performance [71,72]. This alternative material is formed by mixing geological (e.g., metakaolin, clays or volcanic ash) or industrial by-products/wastes (e.g., coal fly ash, palm oil fuel ash, silica fume, mine tailings, and slags) rich in aluminosilicate-bearing phases as “precursors” with alkali hydroxides (e.g., NaOH, KOH, and lime) and silicate solutions (e.g., Na₂SiO₃ and K₂SiO₃) as “activators” to form an amorphous to semi-crystalline structure with tetrahedral coordination of silica and alumina covalently bonded to oxygen, similar to the 3D arrangement of zeolitic materials, but with the presence of interstitial water and a porous structure [28,29,30,31].

Figure 11 shows a simplified schematic illustration of geopolymerization, a process composed of three steps: (i) dissolution, (ii) condensation, and (iii) hardening. The dissolution step involves breaking the aluminosilicate bonds of precursor materials into aluminate and silicate species under alkaline conditions, while the alkali ions (Na⁺, K⁺, and Ca²⁺) neutralize the negative charge on alumina and silica monomers. This is then followed by condensation or coagulation–polymerization involving the formation of a geopolymeric matrix composed of monomers of sialate bonds (Si-O-Al) and siloxo bonds (Si-O-Si), and polymerization to form polysialate (-Si-O-Al-O-), poly(sialate-siloxo) (-Si-O-Al-O-Si-O-), poly(sialatedisiloxo) (-Si-O-Al-Si-O-Si-O), ferro-sialate (-Fe-Si-O-Al-O-), and aluminophospho (-Al-O-P-O) bonds [23,24]. Finally, the matrix hardens to form an amorphous to semi-crystalline 3D gel structure similar to zeolite and water [25,26,31].

Figure 11. A schematic process diagram of geopolymerization (figure from Mabroum et al. [31]).

4.5. Metakaolin-Based Geopolymers

Metakaolin (MK), produced by calcining kaolinite at temperatures between 700 and 800 °C for 2–4 h, is the most commonly used precursor for geopolymers, with chemical composition typically around 50%–67% SiO₂, 31%–42% Al₂O₃, 0.3%–2% Fe₂O₃, 0.3%–1.3% K₂O + Na₂O, 0.2%–1.0% CaO + MgO, and 0.2%–2.2% TiO₂ with an SiO₂/Al₂O₃ ratio of 1–2. Table 5 summarizes that 27 of the 34 studies used metakaolin as the precursor; that is, 12 papers used it as the main precursor, while the remaining 15 used it as an admixture. Allali et al. [73], for example, developed an MK mortar to substitute CaCO₃ and Ca(OH)₂ as alternatives to traditional lime- and cement-based mortar in Moroccan heritage masonry to prevent salt efflorescence and improve flexibility. The MK mortar achieved a 7-day compressive strength of 77 MPa using sodium silicate and sodium hydroxide as activators, with the addition of calcitic sand (CaCO₃) containing quartz, dolomite, and muscovite at a lime substitution rate of 47%. This approach is economically advantageous in Morocco because MK is cheaper and more widely available in comparison to slaked lime (Ca(OH)₂). Similarly, Clausi et al. [74] developed an MK mortar with dolostone and sandstone as aggregates using the <0.5 mm particle fraction for dolostone and sandstone substrates. These authors formulated dense geopolymers with bulk densities of 2.7 and 3.0 g/cm³ and compressive strengths of 18 and 21 MPa, using dolostone and sandstone as aggregates, respectively. These dolostone and sandstone geopolymers resembled the original stones at a binder/aggregate ratio of 1 and a liquid-to-solid (L/S) ratio of 0.4, which could serve as replacements for natural dolostone and sandstone for restoration.

Table 5. Metakaolin-based geopolymer formulations and compressive strengths.

Country	Precursors SiO2 = 50%–67%, Al2O3 = 31%–42%, Fe2O3 = 0.3%–2% K2O + Na2O = 0.3%–1.3%, CaO + MgO = 0.2%–1.0%, TiO2 = 0.2%–2.2%	Activators	Admixture	Aggregate	28-Day Compressive Strength (MPa)	Ref.
Morrocco	Metakaolin	KS, KOH	Calcareous sand (CaCO3)	–	16–41 (7 d)	[73]
		SS, NaOH			63–77 (7 d)
		KS, KOH	Ca(OH)2		7–10 (7 d)
		SS, NaOH			2–40 (7 d)
Italy	Metakaolin	SS, NaOH	–	Dolostone	18	[74]
				Sandstone	21
				Quartz Sand	75
Italy	Metakaolin	SS, NaOH	None	–	41	[75]
			Epoxy Resin		49
Portugal	Metakaolin	SS, NaOH	–	–	0.6–1.0	[76]
Italy	Metakaolin	SS, NaOH	Bromothymol Blue (Organic Dye)	–	–	[77]
			Phenolphthalein
			Cresol red
			Methyl orange
Portugal	Metakaolin	SS, NaOH	Zeolite	–	4.6	[78]
		NaOH	Ca (OH)2		1.0
Czech Republic	Metakaolin	SS, NaOH	Waste Foam Glass	–	2.7–8.3	[79]
Czech Republic	Metakaolin (LO5)	KS, KOH	None		86.6	[80]
			Dolostone	Quartz Sand	72.7–87.7
			Marble		73.8–75.7
			Marlstone		78–80
			Limestone		75–78
			Feldspar		94–96
Italy	Metakaolin	SS, NaOH	None	–	40	[81]
			Polyvinyl Acetate		38–42
			Polydimethylsiloxane		37–42
Czech Republic	Metakaolin (LO5)	KS, KOH	Slag, Colored Pigments	Quartz sand		[82]
Czech Republic	Metakaolin (LO5)	KS, KOH	None	Quartz sand	75.5	[83]
China	Metakaolin,	SS, KS	Silica Fume	–	40–70	[84]

Note: “LO5”: in house calcination of kaolinite; “SS”: sodium silicate; “NaOH”: sodium hydroxide; “KS”: potassium silicate; “KOH”: potassium hydroxide; “–”: no data reported.

Meanwhile, Ricciotti et al. [75] incorporated epoxy resin into MK slurry to improve its compressive strength, workability, and thixotropic behavior—a decrease in viscosity under constant stress/stirring and an increase when undisturbed—for application as fixing mortar on stone substrates for detached tuff-built AHSs. These authors achieved higher compressive strength (49 MPa) than the control, a plain MK geopolymer without epoxy resin (41 MPa), which can then be easily added to inorganic-colored pigments during restoration to match the original stone color. In addition, Moutinho et al. [76] developed a low compressive strength MK-based geopolymer (0.6 and 1 MPa), which was achieved at a high H₂O/Na₂O ratio (18 and 17) and formulated as an adhesive for the repair of broken ancient ceramic tiles. The advantages of using an MK-based geopolymer include (i) its good compressive strength even as the lone precursor material and (ii) its initial pale white color that can be easily modified by adding colored pigments to match the color of the original material to be repaired [77]. However, the main drawback of metakaolin is the need for thermal treatment via calcination before use to remove chemically bound water (OH groups) and to convert naturally occurring kaolinite clay to metakaolin. Thus, its use is limited by supply and sustainability issues, unlike other precursors that can be recycled and repurposed (e.g., fly ash, blast furnace slag, sludge, and rice husk ash).

4.6. Fly Ash-Based Geopolymers

Fly ash (FA) is an industrial waste from coal fired power plants and biomass (e.g., wood, sugar cane bagasse, rice husk, tea dust and coconut shells), power plants, palm fuel oil industry, municipal waste incineration plants, and thermal power plants [28,29,30,85,86]; it has been widely used as partial replacement for OPC for greener construction materials due to its pozzolanic properties. More recently, FA has become a popular precursor for geopolymers because of its high surface area and high alumina and amorphous silicate contents [87] (Table 6).

Table 6. Summary of chemical and mineralogical composition of fly ash used in geopolymers.

Plant	Region, Country	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	TiO2	Mineral Phases	Ref.
Neyveli thermal power plant	Tamil Nadu, India	52.2	25.8	3.6	3.0	2.2	0.6	1.3	Quartz (SiO2), Calcite (CaCO3), Hematite (Fe2O3)	[88]
Calaca coal-fired power plant	Batangas, Philippines	79.3	−	4.0	5.6	4.4	1.6	0.08	Anorthite (CaAl2Si2O8), Quartz (SiO2), Goethite (α-FeOOH), Mullite (3Al2O3·2SiO2)	[89]
CFBC, coal-fired power plant	Philippines	27.4	23.2	21.7	16.5	6.2	0.9	0.9	Quartz (SiO2), Calcite (CaCO3), Magnetite (Fe3O4), Hematite (Fe2O3), Maghemite (γ-Fe2O3), Illite, Biotite	[90,91]
−	Italy	37.3	16.1	30.8	6.13	−	5.28	−	−	[92]
−	Italy	59.5	27.3	2.9	1.8	−	2.7	0.6	−	[93]

Note: “−”: no data reported, “CFBC”: circulating fluidized bed combustion.

Table 7 lists five studies that employed FA, of which four used it as the main precursor material. Previous studies have shown that FA is generally blended with other precursors (e.g., MK and slag) to increase the compressive strength of the resulting geopolymer. For example, Kakria et al. [88] repurposed the non-metallic fraction (NMF), a by-product of separating metallic and non-metallic components of printed circuit boards (PCBs), from sorting and achieved a 28-day compressive strength of 27–29 MPa using a mixture of 85 wt% FA and 15 wt% NMF in comparison to 55 wt% FA, 30 wt% MK, and 15 wt% NMF achieivng 33–35 MPa with a binder/aggregate ratio of 0.33. Similarly, Dollente et al. [89] used FA mixed with silica fume, fiber, and fine aggregates as a restoration mortar for unreinforced historical masonry (URM), thereby increasing its tensile strength and improving its seismic competency. Out of the five (5) different types of fibers (i.e., polyvinyl alcohol (PVA), polypropylene (PP), stainless steel (STSL), chopped basalt (CB), and carbon fibers (CFs)) used in this previous work, PVA provided the highest increase in split tensile strength of 2.2 MPa in comparison to no fiber addition (1.6 MPa), while maintaining the compressive strength at around 12 MPa. Restoration of historical masonry was also reported by Longo et al. [92], who used FA with 5% MK and expanded glass aggregate as an inorganic matrix to create a fiber-reinforced cementitious material (FRCM), a composite composed of fiber mesh and mortar matrix, serving as reinforced plaster for historical masonry. This composite has a compressive strength of 6.0 MPa and a density of 1.03 g/cm³, with a lower thermal conductivity of 0.9 W/m⁻²K⁻¹ compared with the traditional natural hydraulic lime (NHL)-based matrix at 1.1 W/m⁻²K⁻¹ in tuff-built masonry. The advantages of using FA include its valorization, pozzolanic properties, and initially cement-gray color, which is quite helpful as a restoration material; thus, it is commonly used as a partial replacement for cement. To attain the desired strength, however, admixtures and careful adjustments of the concentrations of activators are required.

Table 7. Formulation of fly ash-based geopolymers and compressive strength.

Country	Precursors	Activators	Admixture	Aggregate	28-Day Compressive Strength (MPa)	Ref.
Italy	Fly ash	SS, NaOH	Metakaolin, Printed circuit board (non-metallic)	Sand	15.0–35.0	[88]
Philippines	Fly ash	SS, NaOH	Silica fume	Fine aggregate	12.6	[89]
Italy	Fly ash	SS, NaOH	Metakaolin	Expanded glass	6.0	[92]
Italy	Fly ash	SS, NaOH	Metakaolin	Expanded waste glass	7–10	[93]

Note: “SS”: sodium silicate; “NaOH”: sodium hydroxide.

4.7. Slag-Based Geopolymers

Slag, a by-product of the steelmaking industry, is generated when reducing and slagging agents (i.e., coke and limestone) are added to the iron ore to remove impurities. It is currently used as an alternative to Portland cement to develop sustainable construction materials, which are typically composed of 32.5%–35.7% SiO₂, 9.94%–11.6% Al₂O₃, 32.4%–42.1% CaO, and 7%–9.3% MgO. Table 8 summarizes four studies that used slag as the primary precursor and one article that used this material as an admixture. For example, Taburini et al. [94] developed a durable restoration mortar to strengthen ancient masonries by blending slag with other precursors (i.e., MK, wollastonite and sand) and achieved a 28-day compressive strength of 34.5 MPa, utilizing an MK/slag/aggregate ratio of 1/2/3.5 with 63% of the aggregate as sand and 37% as wollastonite fiber (composed of SiO₂ = 46.5%, Al₂O₃ = 0.96%, CaO = 42.1%, Na₂O = 0.15%, K₂O = 0.47%, MgO = 7.23%). Similarly, Maras et al. [95] achieved a comparable 28-day compressive strength of 33 MPa in a mixture of slag with the addition of slaked lime (Ca(OH)₂) at 5 wt% and brick powders (0–2 mm) with 8 M NaOH, used as restoration material for Turkish Heritage structures. Notable results were also reported by Gupta et al. [96], who produced colored geopolymers by utilizing colored stone processing waste and slag, mixed at a 1:1 ratio, and hydraulically pressed. These authors reported pink, yellow, and white geopolymers with compressive strengths ranging from 15 to 25 MPa. Meanwhile, Kutlusoy et al. [97] used similar precursor materials as Maras et al. [95], but with higher amounts of slaked lime at 48% of precursors (slag/brick powders/slaked lime) and the addition of sodium silicate and sand to be used as joint mortar for heritage masonry, showing a 28-day compressive strength of 30.6 MPa. These results suggest that slag-based geopolymers high in lime (CaO) exhibit better mechanical properties, reaching a compressive strength of at least 30 MPa after geopolymerization, which could be attributed to the dense formation of calcium–aluminosilicate–hydrate (CASH) and calcium–silicate–hydrate (CSH), similar to products found in hardened Portland cements. Slag is waste collected from blast furnaces in the steel industry and can be used for repair due to its cement-like properties: it hydrates readily and provides relatively high compressive strength.

Table 8. Formulations of slag-based geopolymers for heritage conservation and their compressive strength.

Country	Precursors SiO2 = 32.5%–35.7%, Al2O3 = 9.94%–11.6%, Fe2O3 = 0.32%–1.25% CaO = 32.5%–42.1%, MgO = 7%–9.3%	Activators	Admixture	Aggregate	28-Day Compressive (MPa)	Ref.
Italy	Slag	SS, NaOH	Metakaolin, Wollastonite	River Sand	34.5	[94]
Turkey	Slag	NaOH	Ca(OH)2, waste brick powder	−	25–33	[95]
India	Slag	SS, NaOH	Dholpur Stone	Sand	15	[96]
			Jaiselmer Stone		24
			Kota Stone		25
			Makarana Stone		25
Turkey	Slag	SS, NaOH	Slaked lime Brick powder	River Sand	18.1–30.6	[97]
		NaOH			11.5–26.2
		KOH			11.1–27.1

Note: “SS”: sodium silicate; “NaOH”: sodium hydroxide; “KOH”: potassium hydroxide; “−”: no data.

4.8. Pyroclastic Material-Based Geopolymers

Table 9 lists five studies that utilized volcanic ash (VA) as a precursor for geopolymerization, collected from a single location in a village on the eastern flank of Mt. Etna, Italy, during the volcanic eruption in 2013. This material is composed of amorphous phases attributed to the presence of volcanic glass (>60 wt%) and phases of plagioclase ((Ca,Na)Al₂Si₂O₈), augite (Ca(Mg,Fe)Si₂O₆), forsterite (Mg₂(SiO₄)), and Fe/Ti-oxides, with a lime (CaO) content of 10.5 and Si/Al > 2. For example, Finocchiaro et al. [98] achieved a 21-day compressive strength of 71 MPa by mixing VA with 20 wt% MK with potassium-based activators (potassium silicate and potassium hydroxide), while Na activators (sodium silicate and sodium hydroxide) achieved a lower compressive strength of 51 MPa. Meanwhile, Occhipinti et al. [99] only achieved a 28-day compressive strength of 45 MPa with VA and 20 wt% MK with Na activators, which decreased to 39 MPa after adding VA aggregates (0.075–2 mm). Similarly, Finocchiaro et al. [100] achieved a 28-day compressive strength of 25 MPa using Na activators containing 80 wt% VA and 20 wt% MK, which increased to 34 MPa upon incorporation of 2% slaked lime.

Another aluminosilicate-rich geological material promising for geopolymer applications is volcanic soil. Volcanic soils (VSs) are formed by the weathering of pyroclastic materials, such as lava, ash, and rock, and the accumulation of organic matter [101,102]. Table 9 lists a total of four studies that utilized of volcanic paleosoil (Ghiara) collected from the south slope Quarry of Mt. Etna, with a yellow-reddish color tone, amorphous with phases of anorthite, forsterite, hematite, augite, corundum and magnetite with Si/Al > 2, and repurposed to be used as restoration material in historical building in Mt. Etna areas. For example, Finocchiaro et al. [98] achieved a 21-day UCS of 89 MPa by blending 20 wt% MK into K solution, compared with 51 MPa using Na solution. Meanwhile, Occhipinti et al. [99,103] only achieved a 28-day UCS of 25 MPa using a Na solution, but the addition of volcanic paleosoil aggregates resulted in a lower strength value of 21 MPa.

Meanwhile, pumice—a silica-rich geological material formed when silica-rich magma rapidly cools, releasing gases that form visible vesicles—from Aeolian Island, Sicily, Italy, was used by Occhipinti et al. [104,105]. This volcanic rock had a pale gray tone, was highly amorphous, and contained phases of corundum and plagioclase, with a high SiO₂ content (70%) and SiO₂/Al₂O₃ > 5. It was used as a precursor to restore building materials in Baroque Architecture in Sicily, originally built from Sicilian limestone. The pumice-based mortar developed in this work achieved a 28-day compressive strength of 12.7 MPa with the addition of 30 wt% MK, an activated with sodium silicate (Na₂SiO₃)/sodium hydroxide (NaOH) ratio of 1.19, and a liquid-to-solid ratio (L/S) of 0.61, showing comparable strength to western Sicilian limestones, which have a compressive strength ranging from 0.5 to 20 MPa; the geopolymer has a lower modulus ratio according to the Deere–Miller diagram compared with Sicilian limestone, which according to the authors is an ideal restoration material as it is less rigid than the original material, avoiding induced stress.

Table 9. Formulations and mechanical properties of pyroclastic-based geopolymer mortars.

Country	Precursors In Molar Ratio: VA: Si/Al = 2.5 VS: Si/Al = 2.2 PM: Si/Al = 3.1	Activators	Admixture	Aggregate	28-Day Compressive Strength (MPa)	Ref.
Italy	Volcanic ash	SS, NaOH	Metakaolin		18–50 (21 d)	[98]
	Volcanic soil				17–51 (21 d)
	Volcanic ash	KOH, KS			29–71 (21 d)
	Volcanic soil				30–89 (21 d)
	Volcanic ash	SS, NaOH	Metakaolin	Volcanic rock	39	[99]
				None	45
	Volcanic soil			Volcanic soil	21
				None	25
	Volcanic ash	SS, NaOH	Metakaolin, Ca(OH)2	None	34	[100]
			Metakaolin		25
	Volcanic ash	SS, NaOH	Metakaolin	None	45	[103]
				Volcanic rock	39
	Volcanic soil			None	25
				Volcanic soil	21
	Volcanic ash	SS, NaOH	Metakaolin, Aluminum Powder	–	–	[104]
	Volcanic soil				–
	Pumice				–
	Pumice	SS, NaOH	Metakaolin		7.2–12.7	[105]

Note: “SS”: sodium silicate; “NaOH”: sodium hydroxide; “KS”: potassium silicate; “KOH”: potassium hydroxide; “VA”: volcanic ash; “VS”: volcanic soil; “–”: no data.

Table 9. Formulations and mechanical properties of pyroclastic-based geopolymer mortars.

Country	Precursors In Molar Ratio: VA: Si/Al = 2.5 VS: Si/Al = 2.2 PM: Si/Al = 3.1	Activators	Admixture	Aggregate	28-Day Compressive Strength (MPa)	Ref.
Italy	Volcanic ash	SS, NaOH	Metakaolin		18–50 (21 d)	[98]
	Volcanic soil				17–51 (21 d)
	Volcanic ash	KOH, KS			29–71 (21 d)
	Volcanic soil				30–89 (21 d)
	Volcanic ash	SS, NaOH	Metakaolin	Volcanic rock	39	[99]
				None	45
	Volcanic soil			Volcanic soil	21
				None	25
	Volcanic ash	SS, NaOH	Metakaolin, Ca(OH)2	None	34	[100]
			Metakaolin		25
	Volcanic ash	SS, NaOH	Metakaolin	None	45	[103]
				Volcanic rock	39
	Volcanic soil			None	25
				Volcanic soil	21
	Volcanic ash	SS, NaOH	Metakaolin, Aluminum Powder	–	–	[104]
	Volcanic soil				–
	Pumice				–
	Pumice	SS, NaOH	Metakaolin		7.2–12.7	[105]

Note: “SS”: sodium silicate; “NaOH”: sodium hydroxide; “KS”: potassium silicate; “KOH”: potassium hydroxide; “VA”: volcanic ash; “VS”: volcanic soil; “–”: no data.

4.9. Ceramic Waste-Based Geopolymers

Table 10 summarizes five studies that used ceramic waste as precursors for geopolymer production. This waste has a similar chemical and mineralogical composition to MK, thus showing potential as an MK substitute. Ceramic wastes are typically composed of 57% SiO₂, 33% Al₂O₃, 3.5% Fe₂O₃, 1.4% K₂O, 0.8% Na₂O, and 0.8% CaO, with quartz (SiO₂), mica (KAl₂(AlSi₃O₁₀)(F,OH)₂), feldspar (K,N,Ca)(Al,Si)₄O₈, and hematite (Fe₂O₃) as their major mineral components [106]. Several ceramic wastes, such as porcelain stone wares, raw pressed tiles, and bricks, or mixed ceramic waste are incorporated with MK for geopolymer application. For example, Ricciotti et al. [107] utilized wastes from porcelain stoneware and ceramic tiles manufactured in Vicenza, Italy, and achieved promising results by using calcined raw pressed ceramic waste at 750 °C for 5 h and blending it with MK at 11.6 wt%, achieving a compressive strength of 41 MPa; porcelain waste showed a compressive strength of 30 MPa, and mixed ceramic waste achieved 38 MPa. These authors conducted a preliminary investigation for conservation and restoration wherein the slurry had good workability, making it easy to spread and model the slurry in tuff substrate as a fixing paste. The ease in producing a colored product with the addition of a water-based pigment should be noted, resulting in various types of colored MK geopolymers. Fugazzotto et al. [108,109,110] used tile waste to develop a brownish-red geopolymer product. For example, Fugazzotto et al. [108] blended tile waste with MK and hydraulic lime in small amounts to promote binding, resulting in a compressive strength of 14.3 MPa when cured at room temp in comparison to curing for 24 h at 65 °C, which resulted in 9.79 MPa, attributed to the crystallization of crypto-efflorescence which inhibits gel formation at higher temperature, lowering its compactness and resistance, coinciding with the results showing an increase in the effective porosity; this serves as a promising result in terms of ambient application. Therefore, in their recent case studies, these authors developed a reintegration mortar to apply on site to the deteriorated brick masonry of the Roman Odéon at Catania, due to its similar color tones to the brick elements. The test mortar was formulated using (i) tile waste or (ii) brick waste with MK and lime, and blended with aggregates such as carbonate sand, volcanic aggregates, and marble powder, depending on in situ availability, color, and fineness. After application, the test mortar was monitored for 7 days and 28 days and had good adhesion to the substrate, negligible efflorescence, low shrinkage and less visible cracks; the brick waste formulation showed higher similarity to the substrate in comparison to the tile waste formulation. Thus, it was not considered ethical due to intervention recognizability, since during restoration the intervention material must still be recognizable. In another formulation by Fugazzotto et al. [110], various aggregates, including carbonate sand, silica sand, and tile waste, were evaluated, and the highest compressive strength of 32 MPa was achieved with carbonate sand as the aggregate and mixing waste tiles with 20 wt% MK. In addition, Capasso et al. [111] used brick waste blended with 20 wt% fly ash (FA) and achieved a compressive strength of 13.4 MPa, which is suitable as an alternative restoration mortar to traditional restoration materials such as lime mortar, OPC mortar, crushed bricks, sand, magnesian mortar, and natural limestone.

Table 10. Formulations and properties of ceramic waste-based geopolymers.

Country	Precursors SiO2 = 57.3, Al2O3 = 32.7, Fe2O3 = 3.5, K2O = 1.4, Na2O = 0.78, CaO = 0.83, with Quartz, Mica, Feldspar, and Hematite	Activators	Admixture	Aggregate	28-Day Compressive Strength (MPa)	Ref.
Italy	Porcelain and stoneware waste,	SS, NaOH	Metakaolin	–	30	[107]
	Raw pressed ceramic waste,				25
	Calcined Raw pressed ceramic,				41
	Gypsum				4
	Mixed				38
	Ceramic tile waste	SS, NaOH	Metakaolin, lime		3.5–14.3	[108]
	Tile waste or Brick Wate	SS, NaOH	Metakaolin, lime	Carbonate sand,	–	[109]
				Volcanic aggregate	–
				marble powder	–
	Ceramic tile waste	SS, NaOH	Metakaolin	None	14–27	[110]
				Carbonate sand	18–32
				Silica sand	6–7
				Tile waste	6–16
	Brick waste	SS, NaOH	Fly Ash	Brick waste	13.4	[111]
			None		5.3

Note: “SS”: sodium silicate; “NaOH”: sodium hydroxide; “–”: no data.

Table 10. Formulations and properties of ceramic waste-based geopolymers.

Country	Precursors SiO2 = 57.3, Al2O3 = 32.7, Fe2O3 = 3.5, K2O = 1.4, Na2O = 0.78, CaO = 0.83, with Quartz, Mica, Feldspar, and Hematite	Activators	Admixture	Aggregate	28-Day Compressive Strength (MPa)	Ref.
Italy	Porcelain and stoneware waste,	SS, NaOH	Metakaolin	–	30	[107]
	Raw pressed ceramic waste,				25
	Calcined Raw pressed ceramic,				41
	Gypsum				4
	Mixed				38
	Ceramic tile waste	SS, NaOH	Metakaolin, lime		3.5–14.3	[108]
	Tile waste or Brick Wate	SS, NaOH	Metakaolin, lime	Carbonate sand,	–	[109]
				Volcanic aggregate	–
				marble powder	–
	Ceramic tile waste	SS, NaOH	Metakaolin	None	14–27	[110]
				Carbonate sand	18–32
				Silica sand	6–7
				Tile waste	6–16
	Brick waste	SS, NaOH	Fly Ash	Brick waste	13.4	[111]
			None		5.3

Note: “SS”: sodium silicate; “NaOH”: sodium hydroxide; “–”: no data.

4.10. Geopolymers Formulated from Other Materials

Table 11 lists one study that utilized tuff waste obtained from the natural stone processing industry. Capasso et al. [111] repurposed tuff waste consisting of a mixture of Neopolitan yellow tuff and Viterbo red tuff as restoration material for Italian heritage structures, and it was mixed with FA at 10 and 20 wt%, resulting in compressive strengths of 1.48 and 1.34 MPa.

Table 11. Geopolymers formulated from wastes and local clays.

Country	Precursors	Activators	Admixture	Aggregate	28-Day Compressive Strength (MPa)	Ref.
Italy	Tuff waste (SiO2 = 58.8%, Al2O3 = 31.82% Fe2O3 = 2.99% CaO = 3.10% Na2O = 3.44% MgO = 1.11% K2O = 9.34% TiO2 = 0.53%)	SS, NaOH	Fly ash	Tuff waste	1.3–1.5	[111]
			None		0.8
Portugal	Silica fume waste (SiO2 = 97% Al2O3 = 0.15% Fe2O3 = 0.03% CaO = 0.2% Na2O = 0.05% MgO = 0.30% K2O = 0.80%)	SS, NaOH	Polycarboxylate-based superplasticizer	−	10–12	[112]
			None		14–15
Portugal	Tungsten mine mud waste (SiO2 = 46.7% Al2O3 = 17.0% Fe2O3 = 15.7% CaO = 0.69% Na2O = 0.85% MgO = 4.83%)	SS, NaOH	Portland cement, glass, metakaolin	Expanded Cork	10–19.5	[113]
Italy	Santena clay (Smectite- Montmorillonite, Kaolinite, Illite, quartz, feldspars)	KOH, KS	−	−	Not conducted	[114]

Note: “SS”: sodium silicate; “NaOH”: sodium hydroxide; “KS”: potassium silicate; “KOH”: potassium hydroxide; “−”: no data.

A silica fume (SF)-based geopolymer was formulated by Baltazar et al. [112] as an alternative for traditional natural hydraulic lime (NHL) by mixing SF with superplasticizer to improve its fluidity, resulting in a compressive strength of 10–12 MPa, which was slightly lower in comparison to that without the addition at 15 MPa.

Table 9 also summarizes a notable study by Beghoura et al. [113] that proposes repurposing tungsten mine tailings as a potential historical restoration material in Portugal. Mortar was formulated by blending mine waste, glass, MK, and Portland cement with expanded cork and was activated with sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH), resulting in a maximum compressive strength of 19.5 after 28 days. The application of aluminum powder and hydrogen peroxide as foaming agents also resulted in volume increases of 358% and 141%, respectively, providing a lightweight material for restoration.

Meanwhile, Rescic et al. [114] utilized Santena clay mixed with alkaline activators potassium hydroxide and potassium silicate that stabilize plasters for use as earthen-based plaster in architectural heritage; the mixture was then subjected to several durability tests like thermo-hygrometric cycling (an accelerated aging test for earthen material to assess durability caused by swelling) and shrinking testing in a Climatic Chamber (heating first at 50 °C and 30% relative humidity for 6 h and then cooling at 20 °C at 90% relative humidity for 6 h, which is repeated 50 times). The results showed negligible weight loss of 0.30% for the stabilized plaster, identical to the original unstabilized plaster at 0.29%, indicating that the durability of the stabilized material was not significantly affected.

5. Compatibility Criteria in Using Geopolymers for Architectural Heritage Structure Conservation

According to the International Council on Monuments and Sites (ICOMOS, 2003), compatibility is critical for the structural conservation of AHSs; that is, the restoration (new) material and existing (old) material should have negligible negative impacts on the historical materials, especially in technical (chemical and mechanical), aesthetic, and historical (physical) aspects. In addition, Legislative Decree no. 42 of 22, 2004, of the Ministry of Cultural Heritage and Activities (Ministero per i beni e le attività culturali, Rome) known as the “Code of Cultural Heritage and Landscape” defines restoration (in Art. 29, paragraph 4) as “the direct intervention on the property through a complex of operations aimed at the material integrity and recovery of the property itself at the protection and transmission of its cultural values” which also include structural upgrading in properties at risk of earthquakes [115]. Meanwhile, in the Philippines, Republic Act No. 10,066 (National Cultural Heritage Act of 2009) defines restoration as “the action taken or the technical intervention to correct deterioration and alterations” [116]. This section provides an overview of the compatibility criteria used for geopolymers applied in the restoration and conservation of heritage structures, which are categorized in terms of (i) aesthetic, (ii) chemical, (iii) physical, and (iv) mechanical compatibility.

5.1. Aesthetic Compatibility

The aesthetic compatibility criteria govern the visual appearance of structures, which is primarily perceived by the naked eye, to determine the similarity between the intervention and the original material in terms of color, efflorescence formation, and texture.

5.1.1. Color

To maintain the aesthetic appearance of the original material when using a geopolymer, colorimetric analysis is conducted to assess the similarity of geopolymers with original materials. Color analysis is conducted using a spectrophotometer with a spot diameter of around 6 mm, measured at least six times on different areas of the material, and reported in terms of the CIELab (a color coordinate space developed by Commission Internationale de l’Éclairage in 1976 to describe the color visible to the naked eye). The CIELab provides each chromatic coordinate—L*, a*, and b—to determine the color difference (ΔE), following the formula in Equation (1), where L* denotes the lightness and ranges from 0 (total absorption or black) to +100 (white), while a* and b* denote the green/red and blue/yellow values, respectively, both ranging between −60 and 60. The value of ΔE indicates the color similarity between two objects, with ΔE = 3 defined as the perceptibility limit; that is, values of ΔE > 3 indicate color differences readily perceptible to the naked eye [105,112].

$$∆\mathrm{E}=\sqrt{{\left({\mathrm{L}}_{\mathrm{n}\mathrm{e}\mathrm{w}}-{\mathrm{L}}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}}\right)}^2+{\left({\mathrm{a}}_{\mathrm{n}\mathrm{e}\mathrm{w}}-{\mathrm{a}}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}}\right)}^2{+\left({\mathrm{b}}_{\mathrm{n}\mathrm{e}\mathrm{w}}-{\mathrm{b}}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}}\right)}^2}$$

(1)

Table 12 lists the colorimetric analytical results reported in recent studies to assess color differences. For example, Clausi et al. [74] determined the chromatic difference between original stones (i.e., sandstone and dolostone) and the metakaolin-based geopolymer binder with ornamental stone aggregates (i.e., sandstone and dolostone), yielding ΔE values of 5 and 11 for dolostone and sandstone, respectively. These authors attributed the high value to changes in the degree of lightness of sandstone at ΔL = −10, resulting in a paler tone compared to natural sandstones, allowing for similar but recognizable color differences after restoration. Similarly, Fugazzotto et al. [110] formulated ceramic tile-based geopolymers with the addition of either carbonate sand, siliceous sand, or tile waste as aggregates. The ceramic tile-based geopolymer for Messana bricks, which were used during 2nd–3rd B.C. in the ancient urban city of Zancle-Messana, had the lowest observed ΔE of 6 using tile aggregates and 10 wt% MK, while the highest ΔE of 21 was obtained using silica sand and 20 wt% MK. Meanwhile, the ceramic tile-based geopolymer for Gela Bricks, a reproduction of ancient bricks, achieved the lowest ΔE of 6.5 (10 wt% MK + silica sand) and 13 (10 wt% MK + tile waste), attributed to the different types of aggregate. In addition, Fugazzotto et al. [109] conducted a case study for on-site application using tile waste-based and brick waste-based geopolymers as restoration mortars for the brick walls of Roman Odéon of Catania (2nd century A.D.), and after 3 months of monitoring, although no color difference (ΔE) was measured, a general visual comparison was conducted, indicating an indistinguishable color difference using the brick waste geopolymer compared to the tile waste geopolymer as restoration mortar for ancient brick walls.

Table 12. Colorimetric analytical results of geopolymer mortars.

Binder Composition	Alkali Activator Ratio (NS/NaOH)	Aggregates	Reference Color (Lref)	Color Difference (ΔE)	Change in Color Coordinate			Ref.
					ΔL	Δa	Δb
MK	In molar ratio	Dolostone	Dolostone	4.7	−1.1	1.1	4.4	[74]
		Sandstone	Sandstone	10.9	−10.8	−0.52	1.6
Tile + 10 wt% MK	1	SS	Messina Bricks (Ancient Brick)	19	–			[110]
		CS		9
		CER		6
Tile + 20 wt% MK	1	SS		21
		CS		12
		CER		14
Tile + 20 wt% MK	2.3	SS		–
		CS		12
		CER		18
Tile + 10 wt% MK	1	SS	Gela Bricks (Modern Brick)	6.5
		CS		12
		CER		13
Tile + 20 wt% MK	1	SS		8.5
		CS		8.3
		CER		10
Tile + 20 wt% MK	2.3	SS		–
		CS		7
		CER		8
Tile + 10 wt% MK	1	CS	Odéon Bricks (Ancient Brick)	Recognizable	–			[109]
Brick + 10 wt% MK				Very Similar
Pumice + 20 wt% MK	1.33	None	Raw Pumice	1.3	0.5	0.25	1.16	[105]
Pumice + 30 wt% MK	1.19			3.2	−2.9	0.24	1.2
Santena Clay	1 (KS/KOH)	None	Raw Clay	11.09	−9.49	−2.16	–5.32	[112]
Tile + 10 wt% MK	1	SS	Raw Ceramic Tile	12	–			[110]
		CS		4
		CER		6
Tile + 20 wt% MK	1	SS		3.2	–
		CS		9.2
		CER		9.5
Tile + 20 wt% MK	2.3	SS		–
		CS		5
		CER		1.5
VA + 20 wt% MK	n.v.	None	Prior to Weathering	1.87	–			[99]
		VA		2.49
VS + 20 wt% MK		None		7.10
		VS		6.19
Santena Clay	1	None	Prior to Weathering	2.09	2.05	0.36	0.23	[112]

Note: “−”: no data; “MK”: metakaolin; “SS”: silica sand; “CS”: carbonate sand; “CER”: tile waste; “VA”: volcanic ash; “VS”: volcanic soil; “NS”: sodium silicate; “NaOH”: sodium hydroxide; “KS”: potassium silicate; “KOH”: potassium hydroxide.

Meanwhile, some studies conducted colorimetric analysis to obtain different pseudo-colors for each formulation, compared with raw pumice, due to its original whitish color, which can be easily adjusted to match the color of Sicilian limestones in UNESCO-listed Sicilian baroque cities. For example, Occhipinti et al. [105] evaluated the pseudo-colors of each pumice-based geopolymer mortar formulation and achieved the best ΔE values of 1.3 (20% MK) and 3.2 (30% MK), compared with the original pumice, highlighting the significant effects of precursors on the resulting color. Similarly, Fugazzotto et al. [110] evaluated the pseudo-colors of formulations containing different types of aggregates (i.e., carbonate sand and ceramic tiles) compared with the original ceramic tile waste. These authors reported ΔE of 1.5 using tile aggregates with 20 wt% MK and a high Na₂SiO₃/NaOH ratio of 2.3, suggesting that high amounts of Na₂SiO₃ in the mixture had a strong effect on the final color. The results of Fugazzotto et al. [110] were consistent with those of Occhipinti et al. [105], who reported that increasing Na₂SiO₃/NaOH from 0.33 to 2.67 decreased the ΔE values from 6.4 to 2.3, respectively, leading to a higher yellow component. Rescic et al. [112] evaluated the color difference between the original raw clay and the geopolymer plaster for stabilizing earthen cultural heritage, resulting in a ΔE of 11.1 (prominent darkening, negative ΔL), but after subjecting it to thermo-hygrometric cycles, the ΔE decreased to 2.09.

Furthermore, colorimetric analysis has been conducted to determine color changes in geopolymers after exposure to environmental factors. For example, Occhipinti et al. [99] evaluated the color changes of VA- and VS-based geopolymers with or without 6 months of outdoor exposure, resulting in insignificant color changes (ΔE = 1.9–2.5) for VA-based geopolymers but with noticeable color differences for VS-based geopolymers (ΔE = 6.2–7.1). Similarly, Rescic et al. [112] mixed earthen Santena clay material with K₂SiO₃ and KOH for stabilization, resulting in ΔE of 2.09 after subjecting the stabilized earthen materials to accelerated weathering via thermohydrometric cycle tests.

Several authors also developed colored geopolymers to achieve the desired product color using various methods. For example, Perná et al. [82] applied an on-site application protocol using inorganic pigments to match the original color of ceramic tiles during the renovation of churches in the Czech Republic. Similarly, Angelo et al. [77] developed a colored MK-based geopolymer using organic dyes (e.g., Bromothymol blue, phenolphthalein, cresol red, methyl orange), resulting in a blue, purple, violet, and brown-colored geopolymer. Meanwhile, Gupta et al. [96] developed a colored brick by adding colored waste stone powder from stone processing industries, resulting in pink, light yellow, and white colored geopolymers.

5.1.2. Presence of Efflorescence

The formation of efflorescence on AHS substrates not only deteriorates the durability of historic materials but also their aesthetic properties. Figure 12 illustrates a schematic diagram of how efflorescence forms in geopolymers, typically manifesting as white salt-like deposits on the surface or subsurface, gradually damaging the material. Recently, several authors monitored the presence of efflorescence; that is, the surface precipitation of white salt-like deposits composed of soluble sodium carbonates such as trona (Na₃HCO₃∙2H₂O), thermonatrite (Na₂CO₃∙H₂O), natrite (Na₂CO₃), and natron (Na2CO₃∙10H₂O) on the surface of geopolymers, a process promoted by the movement of excess alkali activators through the porous network into the surface and then reacting with atmospheric CO₂ [23]. For example, Occhipinti et al. [99] evaluated the effects of atmospheric exposure for 6 months on VA- and VS-based geopolymers in Catania and found the presence of trona (Na₃H(CO₃)₂∙2H₂O), natrite (Na₂CO₃), and natron (Na₂CO₃∙10H₂O) on unexposed mortars, with Trona and Natron present on the surface and in bulk. Moreover, these authors reported the formation of natrite (Na₂CO₃) covering two-thirds of the external surfaces of both VA-based mortar and binder, whereas for the VS-based geopolymers, only one-quarter of the external surface was covered in mortar and one-fifth in binder. The presence of natrite was attributed in this previous work to the leaching of natron and trona by rainwater, while natrite precipitated during dryer conditions. Similarly, Fugazzotto et al. [108] characterized the salt-like powdered deposits formed by scratching the surface of a ceramic-based geopolymer with a scalpel; they were characterized by X-ray diffraction (XRD) and identified as thermonatrite (Na₂CO₃∙H₂O) and trona (Na₃(CO₃)(HCO₃)∙2H₂O) in cured samples (65 °C for 24 h) but not in samples cured under ambient conditions. Meanwhile, Clausi et al. [74] prevented the formation of efflorescence using aggregates rich in aluminum (sandstone) and calcium (dolostone). This was confirmed by the absence of new crystalline phases in the XRD results after 90 days, including a reduction in FTIR bands around 1400 cm⁻¹, related to the carbonate stretching vibration of natron.

Figure 12. A schematic diagram of salt efflorescence formation [23].

Other researchers have prevented the formation of efflorescence by subjecting the geopolymer paste to a desalination process involving a series of washes or baths to leach excess alkalis. For example, Geraldes et al. [117] investigated controlling salt efflorescence formation by monitoring the electrical conductivity (EC) of the baths using either NaOH or KOH as alkaline activators and drying at 21 °C and 40 °C. These authors reported that the lowest EC was found when NaOH was used as the alkaline activator and dried at 40 °C, resulting in electrical conductivity (σ) of 5000 µS/cm in the initial water bath consisting of 3.5 g of geopolymer paste in 50 g of distilled water; the other formulation reached approximately 8300 (NaOH at 21 °C), 9000 (NaOH at 21 °C) and 13,000 (KOH at 40 °C attributed to the increased curing temperature and smaller cation size of Na⁺, which easily leached in the surface. After the second and third baths, the EC reached approximately 500 and 0 µS/cm (NaOH at 40 °C) and 1200 and 100 µS/cm (NaOH at 21 °C), respectively, indicating that soluble salts can be easily removed by immersion.

5.1.3. Texture

It is important to maintain a surface texture similar to that of the original stone. This property is evaluated through macroscopic investigation depending on the type, distribution, and sizes of aggregates used in the restoration mortar to mimic the original stone. For example, Clausi et al. [74] utilized natural stones as aggregates with a particle size distribution of less than 0.5 mm and a binder/aggregate ratio of 1:1. Similarly, Occhipinti et al. [105] incorporated natural volcanic rocks as aggregate at 30 wt% of binders as restoration mortar with varied size distribution at 32.5% (2< Ø < 1 mm), 31.5% (1.0 < Ø < 0.5 mm), 29% (0.5 < Ø < 0.125 mm), 6% (0.125 < Ø < 0.075 mm), and 1% (Ø <0.075 mm), while Capasso et al. [111] incorporated tuff waste with an aggregate size of 0.3 to 4 mm at an aggregate/binder ratio of 0.5; however, they did not conduct a macroscopic investigation, focusing on repurposing materials, which is common for geopolymers due to their infancy in AHS applications.

Unlike geopolymers used as restoration mortars, other studies developed artificial stone mortars (ASMs), with the distribution, type, and size of aggregates carefully designed to maintain similarity to stone; however, the drawback is their sustainability due to the use of OPC as the binder. Several studies have analyzed the difference in texture through microscopic [19,55] and macroscopic analysis [21] between original and new stone, assessing textural properties (i.e., grains, phenocrysts, and matrix). An on-site approach to achieve an antique appearance involves scraping the mortar to expose the aggregates [109]. For example, Stefanidou et al. [21] designed an artificial stone to replicate an old marl limestone in the archeological site of Pella, Greece; they recycled the old stone by crushing into 0–2 mm, with the addition of 0–2 mm and 2–4 mm sand and dry soil, binding with cement, pre-casting, curing for 14 days, and grooving with a chisel to resemble the surface of original stone. Similarly, Menningen et al. [22] crushed the original tuff into fine and coarse aggregate ranging from 0 to 8 mm, bound it with cement, colored it with an inorganic pigment to match the matrix color, and macroscopically compared it with the original tuffs. The results showed satisfactory results as the addition of natural stone maintains the fine-grained glassy matrix, mimicking the original volcanic tuff. Another notable challenge in synthesizing black tuffs is the difficulty of replicating the original color of the natural rock, as cement pastes tend to lighten during setting and hardening.

5.2. Chemical Compatibility

5.2.1. Microstructure

Figure 13 shows scanning electron microscopy (SEM) photomicrographs collected to evaluate the microstructural differences between the original substrate and the restoration mortar. Ricciotti et al. [75,81,107], for example, observed through microstructural analysis a homogenous and continuous phase in the interface between the geopolymer and substrate (i.e., ceramic, tuff, and concrete) with no presence of microfractures and voids, indicating high chemical compatibility and good adhesion. Similarly, Fugazzotto et al. [105] conducted SEM analysis and found that the interface between the geopolymer and ancient brick substrates was well established, with no visible cracks or microcracks.

Figure 13. Interface transition zone (marked by blue arrows or red lines) between geopolymer and (a) ceramic, (b) tuff, (c) concrete, and (d) ancient bricks (figures from Ricciotti [75,81,107], MDPI, 2017, 2022, and Fugazzotto [105], reprinted with permission from Taylor & Francis (2024)).

5.2.2. Ionic Release

Ionic release is evaluated to measure the quantity of ionic species released from restoration mortars, enabling us to assess the potential formation of soluble salts that can further degrade ancient stones. For example, Ricciotti et al. [75] evaluated the chemical compatibility by measuring the release of ionic species when in contact with water to determine the soluble salt content with respect to thickness; their results showed trace concentrations of sodium ions (Na⁺) after the initial wash using ultrapure water at around 6.7 and 5.5 mg/g sample for 3 mm and 6 mm thicknesses, respectively. Then, following a second wash, the sodium ions (Na⁺) decreased to 1.7 mg/g sample (3 mm) and 1.6 mg/g sample (6 mm), while the third wash resulted in 1.3 mg/g (3 mm) and 1.1 mg/g (6 mm) and the final wash resulted in comparable ionic release of 0.7 mg/g (3 mm) and 0.7 mg/g sample (6 mm). The pH was maintained at 10–11, thus indicating a comparable difference regardless of the thickness of the sample.

5.3. Physical Compatibility

Table 13 summarizes the physical characteristics of restoration material, specifically porosity, water absorption, capillary absorption, and density compared with the properties of the original substrates, enabling us to determine the similarity based on the percent change. For example, Clausi et al. [74] produced a mortar with porosity and density (2.7–2.9 g/cm³) similar to those of natural stone, providing an alternative for substitution or as a repair mortar compatible with the original material. Similarly, Capasso et al. [111] reported a high similarity of the open porosity capacity of tuff waste (39%–42%) and brick waste (27%–34%) geopolymers compared with traditional mortars (30%–40%) and bioclastic limestone (24%–30%); they also maintained similar water absorption through capillarity between tuff waste (20–33 mg/cm² s^−1/2) and brick waste (19–26 mg/cm² s^−1/2) geopolymers in comparison to traditional mortars (28.5 mg/cm² s^−1/2) and natural bioclastic limestone (31.3 mg/cm² s^−1/2) used in stone heritage and found high similarities in these properties between the geopolymers and original materials. Meanwhile, Occhipinti et al. [105] reported the importance of a lightweight pumice-based geopolymer, with an average density of 1.4 g/cm³ and an open porosity of 35%–37%, employed to prevent additional stress on Western Sicilian limestone used in Sicilian AHSs, with compressive strength ranging from 0.5 to 20 MPa during restoration. Meanwhile, Baltazar et al. [112] compared the open porosity and density of traditional lime-based grouts with those of geopolymer grouts and reported a lower density of approximately 1.35 g/m³ for the former and 1.2 g/cm³ for the latter. In contrast, open porosity was around 50% for lime-based grouts and 20%–40% for geopolymer grouts.

Table 13. Physical properties of geopolymers in comparison with reference material.

Property	Geopolymer Type	Geopolymer Value	Reference Material	Reference Material Value	Percent Change (%)	Similarity	Ref.
Open Porosity (%)	Pumice + MK	36.5	Sicilian Limestone	−	−	−	[105]
	MK + dolostone	17.8	Dolostone	−	−	Similar (from author)	[74]
	MK + sandstone	14.1	Sandstone	2–6	57–85	High
	FA	20–40	Natural Hydraulic Lime	50	20–60	Very High to High	[112]
	Tuff	39.4–42.3	Aerial Lime Mortar	25.9	52–63	High	[111]
			Natural Hydraulic Lime	23.5	67–80	High
			OPC	18.8	113–128	Medium
			Repair Mortars	26.2	50–61	High
			Historical Magnesian Mortar	30–40	6–31	Very High
			Natural Limestone	24–30	39–40	Very High
	Brick	26.2–34.2	Aerial Lime Mortar	25.9	1.1–23	Very High
			Natural Hydraulic Lime	23.5	11.5–45.5	Very High
			OPC	18.8	39.4–81	Very High
			Repair Mortars	26.2	0–31	Very High
			Historical Magnesian Mortar	30–40	12.7–15	Very High
			Natural Limestone	24–30	8.3–13	Very High
Water Absorption (%)	Tuff	26.7–31.0	n.d.	–	−	−	[111]
	Brick	15.7–19.8	n.d.	−	−	−
Capillarity Absorption (mg/cm2 s−1/2)	Tuff	20.9–33.4	Aerial Lime Mortar	13.9	50–140	Medium to High	[111]
			Natural Hydraulic Lime	9.40	122–225	Medium
			OPC	3.4	514–882	Very Low
			Repair Mortars	24.2	13.6–38.01	Very High
			Historical Magnesian Mortar	28.5	17–27	Very High
			Natural Limestone	31.3	6.7–33	Very high
	Brick Based	19.5–26.6	Aerial Lime Mortar	13.9	40–91	High
			Natural Hydraulic Lime	9.40	107–183	Medium
			OPC	3.4	473–682.3	Low
			Repair Mortars	24.2	10–19	Very High
			Historical Magnesian Mortar	28.5	6.6–31	Very High
			Natural Limestone	31.3	15–37	Very High
Bulk Density (g/cm3)	MK + dolostone	2.8	Dolostone	2.7	3.7	Very High	[74]
	MK + sandstone	2.9	Sandstone	2.9	0	Very High
	Pumice + MK	1.43–1.46	Sicilian Limestone	−	−		[105]
	Silica Fume	1.0–1.2	Natural Hydraulic Lime	1.35	11–25	Very High	[112]
	Tuff	1.35–1.4	-	−	−		[111]
	Brick	1.73–1.76	-	−	−

Note: “−”: no data. Similarity = Very High (0–50), High (50–100), Medium (100–300), Low (300–500), and Very Low (>500).

5.4. Mechanical Compatibility

5.4.1. Adhesion

Baltazar et al. [112] developed a silica fume (SF)-based geopolymer mortar as grouting for historical masonry. To assess the adhesion strength—defined as the bond strength between the restoration material and the substrate—of silica fume mortar compared to traditional lime-based mortar, an adhesion test is typically conducted. The adhesion test is a quantitative method for measuring the bond strength between the restoration material (e.g., mortar, grout, adhesive) and the original substrate (e.g., stone, tiles, ceramics, bricks) by developing several hybrid specimens. Based on the study of Baltazar et al. [112], silica fume-based geopolymers had higher flexural strength (0.6–0.9 MPa) compared with traditional natural hydraulic lime grout (0.1 MPa), an increase of around 90% (Figure 14a). Meanwhile, Tamburini et al. [94] conducted a pull-off test on fiber-reinforced slag–MK geopolymer mortar applied to ancient and modern construction clay bricks and reported good adhesion (0.9–1.7 MPa) to the ancient bricks, with failures observed in the original substrate (Figure 14b). In comparison, failure in the modern geopolymer bricks occurred at the reinforcement layer, with slightly higher adhesion of around 1.2–2.81 MPa. Similarly, Fugazzotto et al. [110] conducted an adhesion test with a “sandwich” specimen (Figure 14c) using two ancient bricks and reported poor adhesion with sand aggregates and a ceramic-based geopolymer, attributed to fracture at the interface, but no quantitative values were presented.

Figure 14. Different adhesion tests conducted for (a) grout from Baltazar et al. [112], (b) plaster from Taburini et al. [94], and (c) adhesive from Fugazzotto et al. [110].

5.4.2. Surface Hardness

Maras [95] employed an in situ non-destructive technique (NDT) to evaluate the surface hardness of historical mortars and geopolymer mortars and reported a medium level of similarity based on the rebound hardness test. Also, Capasso et al. [111] conducted a surface hardness test using the Shore-D hardness test but did not compare the results to the original materials.

5.4.3. Compactness

Compactness is measured using ultrasonic pulse velocity (UPV), another in situ NDT method used to determine the degree of damage in the material; it depends on the material’s density and modulus of elasticity, which can result in a degree of deterioration. For example, Maras [95] evaluated the compactness of geopolymers and original mortars and found a higher value for geopolymers, with an increase of 50% compared to the in situ joint historical mortars.

5.4.4. Compressive Strength

Occhipinti et al. [105] compared the mechanical properties of a pumice-based geopolymer and traditional limestone using Miller diagrams and showed similarities to low-strength limestone (Figure 15). Meanwhile, Capasso et al. [111] compared tuff and brick waste geopolymers with conventional materials in cultural heritage, showing similarities to the original materials, with the tuff waste geopolymer (0.81–1.34 MPa) and brick waste geopolymer (5.34–13.4 MPa) matching natural limestone (10.1 MPa). Similarly, Baltazar et al. [112] also reported a higher compressive strength for a silica fume-based geopolymer (10–15 MPa) than for traditional lime-based mortar (8 MPa). However, Dollente et al. [89] compared the compressive strength without (12.6 MPa) and with the addition of fibers (1.81–12.1 MPa), finding a similar compressive strength depending on the type of fiber added to the mixture.

Figure 15. Redrawn and modified Deer–Miller diagram from Occhipinti et al. [105]. Comparing mechanical properties of geopolymers and limestones in Sicily, Italy, and coral rocks in Zanzibar, Tanzania (data from Ali et al. [67]).

5.4.5. Tensile Strength

Dollente et al. [89] incorporated five types of fibers (polyvinyl alcohol, chopped basalt, copper-coated stainless steel, polypropylene, and carbon fiber) into a fly ash–silica fume geopolymer mortar to enhance tensile strength while maintaining competent compressive strength, potentially improving the earthquake resistance of historical masonry. The results showed that the addition of PVA, stainless steel, and chopped basalt fibers achieved a comparable compressive strength of 12.0, 12.1 MPa, and 9.9 MPa, respectively, from a 12.6 MPa control mortar, while the increase in tensile strength was 2.2, 1.64, and 1.8 MPa in comparison to 1.58 MPa of control mortar. Meanwhile, Tamburini et al. [94] developed a slag–MK geopolymer with river sand as aggregate and fibrous wollastonite as filler, resulting in 28-day compressive and tensile strengths of 34.5 MPa and 4.8 MPa, respectively; the geopolymer was used as a mortar matrix for fiber/mesh-reinforced cementitious composites.

5.4.6. Flexural Strength

Flexural strength is defined as the ability of a material to resist bending under load, as measured by the stress prior to fracturing. It is important to determine the susceptibility of a reinforcement material to resist deformation and cracking, with a similar resistance compared to the original material (i.e., traditional mortars or rocks). For example, Capasso et al. [111] compared the flexural strength of traditional tuff and brick waste stone-based geopolymers and found a high degree of similarity for the tuff waste-based geopolymer, while the brick waste-based geopolymer showed low to high similarity to traditional mortars (Table 14). Meanwhile, the silica fume-based grout in Baltazar et al. [112] had a higher strength at 1.0–1.5 MPa than traditional natural hydraulic lime (NHL) mortars at 0.2 MPa, attributed to the reaction of silica fume in alkali activators forming an aluminosilicate gel in the microstructure. Similarly, Clausi et al. [74] obtained MK geopolymers that had the highest flexural strength of 3.2–3.6, but did not conduct a comparison with the original stones (i.e., dolostone and sandstone).

Table 14. Mechanical properties of geopolymer applied to AHSs in comparison with reference materials.

Property	Geopolymer Type	Geopolymer	Reference	Ref. Material Value	Percent Change%	Similarity	Ref.
Adhesion (MPa)	Silica fume	0.6–0.9	NHL	0.1	500–800	Very Low	[112]
	Slag + MK	0.89–1.65	SM Bricks (substrate 1)	−	−	−	[94]
	Slag + MK	1.21–2.81	EM Bricks (substrate 2)	−	−	−
Surface Hardness	Slag	31–39	Historical Mortars	10–20	95–210	Medium	[95]
	Tuff	26.5–53	In situ Mortar	–	−	−	[111]
	Brick	31.2–75.8	In situ Mortar	−	−	−
Compactness (m/s)	Slag	3000–4000	In situ Mortar	2000–3000 m/s	50	Very High	[95]
Compressive (MPa)	Pumice	3–6 MPa	Type A WSL	8–20 MPa	62–70	High	[105]
			Type B1 WSL	3–10 MPa	0–40	Very High
			Type B2 WSL	1–3 MPa	0–200	Very High to Medium
			Type C WSL	0.5–1.5 MPa	100–1100	Medium to Very Low
	Silica Fume	10–16	NHL	9 MPa	11–77	High to Very High	[112]
	Tuff	0.81–1.48	Aerial Lime Mortar	0.5	62–196	Medium to High	[111]
			NHL	3.7	78–60	High
			OPC	24.8	94–96	High
			Repair Mortars	3.5	57–77	High
			Historical Magnesian Mortar	−	−	−
			Natural Limestone	10.1	85–92	High
	Brick	5.3–13.4	Aerial Lime Mortar	0.5	960–2580	Very Low
			NHL	3.7	43–262	Medium to High
			OPC	24.8	46–75	High to Very High
			Repair Mortars	3.5	51–282	Medium to High
			Historical Magnesian Mortar	−	−	−
			Natural Limestone	10.1	47.5–32.7	Very High
	MK	18	Dolostone	−		−	[74]
	MK	21	Sandstone	−		−
	FA	1.8–12.1	Plain Mortar	12.5	0–86	High to Very High	[89]
Tensile (MPa)	FA	0.6–2.2	Plain Mortar	1.6	37.5–62.5	High to Very High	[89]
	Slag + MK	4.8	None	n.d.	−	−	[94]
Flexural (MPa)	Silica Fume	0.2	NHL	1.0–1.5	80–86	High	[112]
	Tuff	0.4–1.23	Aerial Lime Mortar	0.24	66–412	Medium to High	[111]
			NHL	1.00	23–60	Very High
			OPC	4.70	74–91	High
			Repair Mortars	−	−
			Historical Magnesian Mortar	−	−
			Natural Limestone	−	−
	Brick	2.85–4.58	Aerial Lime Mortar	0.24	1087–1808	Very Low
			NHL	1.00	185–358	Low to Medium
			OPC	4.70	2–39	Very High
			Repair Mortars	−	−	−
			Historical Magnesian Mortar	−	−	−
			Natural Limestone	−	−	−
	MK	3.2	Dolostone	−	−	−	[74]
	MK	3.6	Sandstone	−	−	−

Note: “WSL”: Western Sicilian limestone; “NHL”: natural hydraulic lime; “OPC”: ordinary Portland cement; “−”: no data. Similarity = Very High (0–50), High (50–100), Medium (100–300), Low (300–500), and Very Low (>500).

6. Recent Applications of Geopolymers in Architectural Heritage Structure Conservation

Recent AHS conservation research has commonly formulated geopolymer-based repair mortars to restore or patch degraded parts of the original stone substrate, followed by the application of consolidants to fill cracks and fix loosened material. In addition, masonry strengthening has been studied as a retrofitting technique to improve its resistance against seismic activity; however, the majority of these studies only presented potential for restoration and a preliminary on-site evaluation. A few studies have also reported the successful on-site application of the material, prioritizing ceramic tiles and bricks, while studies related to on-site application on natural rocks have been well reported. Table 15 summarizes recent applications of geopolymers in cultural heritage, indicating the type of geopolymer, purpose, and original substrate.

6.1. Repair Mortars

Repair mortars are applied to the original material as an alternative to restoring large patches of degraded original material. For example, Allali et al. [73] developed an MK-based geopolymer with Ca-rich components (lime and calcareous sand) as additives to maintain chemical compatibility with traditional materials for Moroccan AHSs instead of traditional lime-based mortars and concrete, which easily form salts that are dense and have low flexibility; the developed geopolymer showed a 7-day compressive strength ranging from 16 to 77 MPa (CaCO₃) and 2 to 40 MPa (Ca(OH)₂). Similarly, Maras [95] developed a slag-based geopolymer as repair mortar for Turkish AHSs because the use of traditional low-strength mortars resulted in visible cracks post earthquake episodes; the geopolymer had a 7-day compressive strength of 10–20 MPa. Meanwhile, the 28-day compressive strength of repair mortars for Italian AHSs using metakaolin and pumice were 18–21 MPa and 7 to 12 MPa, respectively [74,105]. In addition, Kutlusoy et al. [97] aimed for structurally competent masonry by utilizing a high-strength slag-based geopolymer with a 7-day compressive strength ranging from 20.7 MPa to 36 MPa.

6.2. Consolidants

Consolidants are adhesives or grouts used to fill small gaps and cracks and fix broken original ceramics and natural stones. For example, Moutinho et al. [76,78] used an MK-based geopolymer as consolidants to fix broken ceramic tiles, resulting in a 7-day compressive strength of 0.6 MPa to 3 MPa, whereas Baltazar et al. [112] utilized it as grout for old stones in historical masonry and achieved a 28-day compressive strength of 10–12 MPa (with superplasticizer) and 14–15 MPa (control). Meanwhile, Ricciotti et al. [75] applied an MK-based geopolymer with epoxy resin as a fixing paste for tuff and concrete substrates, resulting in a 28-day compressive strength of 40–50 MPa.

6.3. Masonry Strengthening

Furthermore, strengthening historical masonry with high-strength geopolymer mortars reinforced with fibers to withstand seismic vibration has been explored. For example, Dollente et al. [89] applied an FA-based geopolymer mortar with added fiber to unreinforced historical masonry, resulting in tensile strengths of 0.6 MPa to 2.2 MPa, compared to the control value of 1.6 MPa. Meanwhile, other studies aimed to provide seismic reinforcement for historical masonry. For example, Tamburini et al. [94] focused on fiber mesh-reinforced mortar, for which the adhesion strength was measured by conducting pull-out tests on the original substrate (clay bricks), resulting in 0.9 to 1.65 MPa in historic bricks and 0.9 MPa in the control, and 1.6 to 2.8 MPa in modern bricks and 2.7 MPa in the control. Similarly, Longo et al. [92] found a tensile strength of 868 MPa.

6.4. Others

Finally, the remaining studies did not discuss applications for AHSs but showed potential for restoration through the development of colored geopolymers [77,96] without the addition of commercial colored pigments. For example, Gupta et al. [96] formulated slag-based geopolymer bricks with the addition of colored-stone processing waste, resulting in pink, yellow, greyish, and white-colored bricks with a compressive strength of 15–25 MPa. Meanwhile, Angelo et al. [77] developed colored MK-based mortar using the adsorption Bromothymol blue, methyl orange, cresol red, and phenolphthalein, resulting in blue, yellow, violet, and purple geopolymers, respectively.

Another notable result is the effects of maturation in different environments on the potential for restoration of MK-based geopolymers. Perná et al. [83], for example, reported different 28-day compressive strengths of geopolymers allowed to mature in air (86.5 MPa), demineralized water (79.3 MPa), and seawater (87.2 MPa). Meanwhile, MK–silica fume combined with sodium silicate (Na₂SiO₃) and potassium silicate (K₂SiO₃) resulted in a 7-day compressive strength of 30 to 58 MPa [84], while an MK–fly ash geopolymer with expanded waste glass aggregates achieved a 48-day compressive strength of 10 MPa [93].

Recently, on-site applications in restoration have been slowly rising. For example, Fugazzotto et al. [109] applied a ceramic-based geopolymer mortar to ancient bricks as a substrate, resulting in a similar chromatic difference. Similar to Perná et al. [82], the application of geopolymers for fixing historical ceramic tiles and sandstones has been shown. However, further developments are needed to enable application to highly weathered ancient stones and to develop a sustainable material that enhances the long-term preservation of stone architectural heritage structures.

Table 15. Current application of geopolymers for AHS conservation.

Country	Geopolymer	Purpose/Application	Original Materials	Architectural Heritage Structure	Ref
Morrocco	Metakaolin	Repair mortar	Lime-based mortar	Moroccan Historical Monument	[73]
Italy	Metakaolin	Repair mortar	Sandstone and dolostone	Not specified	[74]
Turkey	Slag	Repair mortars	Low strength mortars	Turkish Historical Masonry	[95]
Italy	Volcanic ash	Repair mortars/binders	Not specified	Catania Architectures	[98]
	Volcanic soil
Italy	Volcanic ash	Repair mortars/binders	Not specified	Catania Architectures	[99]
	Volcanic soil
Italy	Volcanic ash	Repair mortars/binders	Not specified	Catania Architectures	[103]
	Volcanic soil
Italy	Volcanic ash,	Repair mortars/binders	Not specified	Not specified	[104]
	Volcanic soil,
	Pumice
Italy	Volcanic ash,	Repair mortars	Mosaics	Cathedral of Monreale, Sicily	[100]
Italy	Pumice	Repair mortars	Sicilian limestones	Sicilian Baroque Cities	[105]
Italy	Tile waste or Brick waste	Repair mortars	Ancient bricks	Cathedral of Messina, Sicily, and Archeological Ruins in Gela, Caltanissetta	[110]
Italy	Ceramic tile waste	Repair mortar	Ancient bricks	Roman Odéon of Catania	[109]
Italy	Brick waste	Repair mortar	Limestone, lime mortars	Not specified	[111]
Italy	Tuff waste	Repair mortar	Limestone, lime-based mortars	Not specified	[111]
Portugal	Tungsten mine waste	Repair mortar	None	Not specified	[113]
Italy	Metakaolin	Consolidant	Tuff, concrete, ceramic	None	[75]
Portugal	Metakaolin	Consolidant	Ceramic tiles	None	[78]
Portugal	Metakaolin	Consolidant	Ceramic tiles	None	[76]
Italy	Metakaolin	Consolidant	Ceramic artworks	None	[85]
Czech Republic	Metakaolin (LO5)	Consolidant	Ceramic tiles	None	[82]
Italy	Porcelain stoneware Waste	Consolidant	Ceramic artworks	None	[107]
	Raw pressed ceramic
	Calcined raw pressed ceramic
	Gypsum
	Mixed
Portugal	Silica fume	Consolidant/Grout	Hydraulic lime	Non specified	[112]
Italy	Local clay	Consolidant/Plaster	Local clay	Earthen Architectures	[114]
Turkey	Slag	Strengthening historical masonry	Ceramic brick masonry	Turkish Historical Architectures	[97]
Italy	Fly ash	Strengthening historical masonry	Tuff and brick masonry	Not specified	[92]
Philippines	Fly Ash	Strengthening historical masonry	Unreinforced stone masonry	Not specified	[89]
Italy	Slag	Strengthening historical masonry	Ancient and modern brick masonry	Not specified	[94]
Czech Republic	Metakaolin	Valorization	None	None	[79]
Czech Republic	Metakaolin (LO5)	Valorization	None	None	[80]
Italy	Ceramic tile waste	Valorization	None	None	[108]
Italy	Fly ash	Valorization	None	None	[88]
Italy	Metakaolin	Colored geopolymer	None	None	[77]
India	Slag	Colored geopolymer	None	None	[96]
Czech Republic	Metakaolin (LO5)	Investigation of maturation environment	None	None	[83]
China	Metakaolin,	Investigation of modified activators (Na2SiO3 and K2SiO3)	None	None	[84]
Italy	Fly ash	Investigation of lightweight Aggregates	None	None	[93]

Note: “LO5” means in-house calcination of clay at 750 °C for 4–5 h.

Table 15. Current application of geopolymers for AHS conservation.

Country	Geopolymer	Purpose/Application	Original Materials	Architectural Heritage Structure	Ref
Morrocco	Metakaolin	Repair mortar	Lime-based mortar	Moroccan Historical Monument	[73]
Italy	Metakaolin	Repair mortar	Sandstone and dolostone	Not specified	[74]
Turkey	Slag	Repair mortars	Low strength mortars	Turkish Historical Masonry	[95]
Italy	Volcanic ash	Repair mortars/binders	Not specified	Catania Architectures	[98]
	Volcanic soil
Italy	Volcanic ash	Repair mortars/binders	Not specified	Catania Architectures	[99]
	Volcanic soil
Italy	Volcanic ash	Repair mortars/binders	Not specified	Catania Architectures	[103]
	Volcanic soil
Italy	Volcanic ash,	Repair mortars/binders	Not specified	Not specified	[104]
	Volcanic soil,
	Pumice
Italy	Volcanic ash,	Repair mortars	Mosaics	Cathedral of Monreale, Sicily	[100]
Italy	Pumice	Repair mortars	Sicilian limestones	Sicilian Baroque Cities	[105]
Italy	Tile waste or Brick waste	Repair mortars	Ancient bricks	Cathedral of Messina, Sicily, and Archeological Ruins in Gela, Caltanissetta	[110]
Italy	Ceramic tile waste	Repair mortar	Ancient bricks	Roman Odéon of Catania	[109]
Italy	Brick waste	Repair mortar	Limestone, lime mortars	Not specified	[111]
Italy	Tuff waste	Repair mortar	Limestone, lime-based mortars	Not specified	[111]
Portugal	Tungsten mine waste	Repair mortar	None	Not specified	[113]
Italy	Metakaolin	Consolidant	Tuff, concrete, ceramic	None	[75]
Portugal	Metakaolin	Consolidant	Ceramic tiles	None	[78]
Portugal	Metakaolin	Consolidant	Ceramic tiles	None	[76]
Italy	Metakaolin	Consolidant	Ceramic artworks	None	[85]
Czech Republic	Metakaolin (LO5)	Consolidant	Ceramic tiles	None	[82]
Italy	Porcelain stoneware Waste	Consolidant	Ceramic artworks	None	[107]
	Raw pressed ceramic
	Calcined raw pressed ceramic
	Gypsum
	Mixed
Portugal	Silica fume	Consolidant/Grout	Hydraulic lime	Non specified	[112]
Italy	Local clay	Consolidant/Plaster	Local clay	Earthen Architectures	[114]
Turkey	Slag	Strengthening historical masonry	Ceramic brick masonry	Turkish Historical Architectures	[97]
Italy	Fly ash	Strengthening historical masonry	Tuff and brick masonry	Not specified	[92]
Philippines	Fly Ash	Strengthening historical masonry	Unreinforced stone masonry	Not specified	[89]
Italy	Slag	Strengthening historical masonry	Ancient and modern brick masonry	Not specified	[94]
Czech Republic	Metakaolin	Valorization	None	None	[79]
Czech Republic	Metakaolin (LO5)	Valorization	None	None	[80]
Italy	Ceramic tile waste	Valorization	None	None	[108]
Italy	Fly ash	Valorization	None	None	[88]
Italy	Metakaolin	Colored geopolymer	None	None	[77]
India	Slag	Colored geopolymer	None	None	[96]
Czech Republic	Metakaolin (LO5)	Investigation of maturation environment	None	None	[83]
China	Metakaolin,	Investigation of modified activators (Na2SiO3 and K2SiO3)	None	None	[84]
Italy	Fly ash	Investigation of lightweight Aggregates	None	None	[93]

Note: “LO5” means in-house calcination of clay at 750 °C for 4–5 h.

7. Long-Term Durability of Geopolymers for Architectural Heritage Structures

Another crucial property of geopolymer-based replacement materials for tuff- and coral rock-built AHS conservation is their durability. In this section, the durability and performance of geopolymers under (i) accelerated aging and (ii) long-term outdoor exposure are discussed.

7.1. Accelerated Aging

This type of durability or weathering test is conducted under cyclic conditions to assess the durability of geopolymers by subjecting the material to simulated long-term degradation under extreme environmental conditions within a relatively short time. Some tests categorized under accelerated aging applied to geopolymers are (i) salt mist environment [76], (ii) wet–dry cycles [114], and (iii) freeze–thaw cycles [76,78,94].

7.1.1. Salt Mist Environment

Moutinho et al. [76] conducted a salt mist environment test to evaluate the behavior of an MK-based geopolymer by subjecting it to an artificial environment in a climatic chamber with salt spray (fog), following ASTM B117-03, including a 3.5% salt solution, for 14, 28, and 60 days at 30 °C. The results showed no visible presence of salts, but XRD analysis identified the formation of halite (NaCl) in the specimen with higher water content (H₂O/Na₂O = 18) and finer precursor particle size (D₅₀ = 1.5 µm).

7.1.2. Wet–Dry Cycle

Rescic et al. [114] conducted thermo-hygrometric cycles on earthen plasters, formulated using local Santena clay, a common material used in the construction of earthen architecture in Italy. The plaster was subjected to 50 °C and 30% RH for 5 h in a climatic chamber, followed by 20 °C and 90% RH for 5 h as one cycle and repeated 50 times. The results showed a change in color (∆E) from 11.1 to 2.09, while the weight loss was low at 0.33 compared with the untreated material (0.29), inferring that a loss of cohesion of the conservation material to the substrate was negligible.

7.1.3. Freeze–Thaw Cycle

Moutinho et al. [76] conducted freeze–thaw cycles of MK-based geopolymers used as consolidants for ceramic tiles in a climatic chamber with 29 summer/winter cycles. Summer conditions consisted of 10 °C at 40% RH, then 60 °C at 95% RH. Winter conditions included −10 °C at 95% RH and 30 °C at 100% RH, with each cycle divided into four series, each lasting 7 h, with 8 h of drying, giving each cycle a total of 36 h. The results were visually inspected to identify the formation of cracks or a loss of adhesion on the substrate; after the first cycle in winter, visible cracks on the specimen with a low water content (H₂O/Na₂O = 17) and precursor particle size (D₅₀ = 6.0 µm) were observed. Meanwhile, the tenth summer cycle showed a loss of adhesion of both geopolymers, and after nineteen cycles, no new cracks or material loss was observed. Meanwhile, mass loss at the second cycle was 16% for geopolymers with a high water content and 10% for geopolymers with a low water content. After the fourth cycle, mass loss was 18% and 14%, respectively. Additionally, Moutinho et al. [78] formulated metakaolin with the addition of zeolite, observing a mass loss of 10% and loss of adhesion to the substrate after 10 cycles. Meanwhile, Taburini et al. [94] conducted 50 freeze–thaw cycles over 12 h between 20 and −20 °C, with specimens placed under water at 20 °C; after the geopolymer had aged for one year, no cracks or surface loss was observed, weight loss was 0.67%, and residual compressive strength ranged from 40.2 MPa to 45 MPa.

7.2. Long-Term Outdoor Exposure

In contrast to accelerated aging tests that require special “climate” chambers, long-term outdoor exposure, as the name suggests, takes advantage of the natural environment. Although simple and cheap, this approach is tedious because constant monitoring of environmental parameters like temperature, relative humidity, and rainfall is required. For example, Occhipinti et al. [105] formulated volcanic ash- and volcanic soil-based geopolymers obtained from pyroclastic residues from Mt. Etna, Catania, Italy. After the mortars were cured for 28 days, they were exposed on a rooftop in Catania city center for six months (July 2020–January 2021) with a mean temperature of 19.6 °C and mean relative humidity of 75%. The results showed good stability of the geopolymers under outdoor conditions with no cracks, no visible efflorescence, no color difference and higher compressive strengths.

Meanwhile, Fugazzoto et al. [109] formulated ceramic brick waste geopolymers and applied them as ancient brick substrates in the walls of Roman Odéon monuments in Catania, Italy, with a mean temperature of 26.5 °C and 60% relative humidity. Post-monitoring was conducted after three months and visual inspection showed good adhesion, no visible efflorescence, cracks and shrinkage, while maintaining high chromatic similarity to the original brick substrate.

8. Summary and Future Perspectives

The conservation of AHSs built with tuff and coral rock is challenging because it requires restoration materials that are environmentally acceptable and sustainable while maintaining their compatibility with the original substrate. Recently, geopolymers have become a popular alternative restoration material to conventional OPC in AHS conservation due to their low carbon footprint, reduced energy consumption, and favorable physical, mechanical, and aesthetic properties. Although several geopolymers have shown potential application for AHS conservation, their compatibility with the original materials remains unclear.

This work provided a systematic and comprehensive review of the different geopolymers applied to the conservation of tuff- and coral rock-built AHSs, including the precursor materials, types of aggregates, composition of the admixture, and resulting physical, mechanical, and aesthetic properties of the material. In addition, the compatibility criteria for natural stones used in AHS conservation was reviewed, focusing on aesthetic, physical, chemical, and mechanical compatibilities with the original substrate. Furthermore, long-term durability test protocols and the performance of geopolymers under accelerated aging or long-term outdoor exposure were outlined. Specifically, the findings of this work are outlined below.

• Original tuff substrates of AHSs have variable properties, including color (yellowish-brown and grayish-cream to reddish-brown, with some portions showing pale greenish-gray and pink hues), density (1.0–2.5 g/m³), and compressive strength (3–100 MPa).
• Original coral rock substrates of AHSs typically have coarse–fine pores and compressive strengths of 1–60 MPa.
• Metakaolin was prominently identified as important in geopolymer formulations for AHS conservation, either as the main precursor or as an additive/admixture, due to its high reactivity and aluminosilicate content.
• Other materials used for geopolymer formulations include fly ash, slags, local clays, volcanic ash, volcanic soil, pumice, and wastes (ceramics, mining, and silica fume), all having high aluminosilicate contents.
• Sodium silicate and sodium hydroxide were the most commonly reported alkaline activators for geopolymers.
• Aggregates in geopolymers for AHS conservation depend largely on availability, so readily and locally available materials like quartz sand, river sand, crushed stones, carbonate stones, volcanic rock, volcanic sand, tuff, brick, and ceramic aggregates are commonly used.
• Compatibility criteria for AHS conservation include aesthetics (i.e., color and presence of efflorescence), physical attributes (i.e., porosity, water absorption, capillary absorption, and density), chemical composition (i.e., microstructure and ionic release), and mechanical properties (i.e., adhesion, surface hardness, compactness, compressive strength, flexural strength, and tensile strength).
• Geopolymer applications in AHS conservation vary from repair mortars for patching deteriorated stones, consolidants (e.g., adhesives or grouts), and strengthening mortars to replacement stones as substitutes for the original substrate.
• Key considerations for geopolymers when applied as repair mortars include breathability, adhesion, and surface finish identical to original substrate.
• Geopolymers as consolidants require appropriate viscosity to penetrate voids or cracks while maintaining cohesion between fragments to strengthen and reattach deteriorated/broken fragments of the original substrate.
• Key properties for geopolymers when used as strengthening mortars include high tensile and flexural strengths while maintaining considerable compressive strength to enhance resistance during seismic disturbances like earthquakes.
• When geopolymers as applied as replacement stones, they must have color, texture, compressive strength and weathering durability similar to the original substrate.
• Geopolymer durability for AHS conservation is tested using accelerated aging (i.e., salt mist, wet–dry cycle, and freeze–thaw cycle) and long-term outdoor exposure.
• Tuff- and coral rock-built AHSs are high susceptible to damage from earthquakes, so future works will likely focus on mortar strengthening strategies and seismic masonry resilience like fiber-reinforced geopolymer development.
• Modern-day pollution is rapidly accelerating the deterioration of tuff- and coral built-built AHSs, so developing compatible stone consolidants to retard weathering while maintaining the aesthetic appearance of the original substrate is crucial.