Abstract
Ceramic-added air lime mortars have been used since ancient times owing to the pozzolanic effect provided by crushed ceramic particles that impart hydraulic properties. This work reviews the historical use, composition, reaction mechanisms, characterization techniques, and performance properties of ceramic-added air lime mortars. The fine ceramic powder composed of silica and alumina phases reacts with calcium hydroxide released during lime hydration to form calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH) via pozzolanic reaction. This provides hydraulicity and reduces setting time compared to pure air lime mortars. The coarser ceramic particles also serve as aggregate and refine the microstructure as filler. The reactivity depends on the ceramic composition, amorphous phase content, particle size distribution, and firing temperature. Optimal proportioning of the fine ceramic powder and coarse ceramic aggregate is necessary to achieve desired properties. Ceramic addition enhances the durability of air lime mortars against weathering while maintaining compatibility with lime-based masonry structures. Key novelties of this review include: (i) in-depth analysis of the influence of ceramic characteristics (mineralogy, particle size, pozzolanicity) and processing on reaction kinetics and phase evolution; (ii) systematic assessment of mechanical, physical and durability properties in comparison to conventional air lime mortars and cement-based grouts; (iii) elucidation of microstructural mechanisms governing performance using advanced characterization techniques; (iv) critical appraisal of test methods and standards for evaluation; and (v) rigorous discussion on potential applications in construction, conservation and repair, with case studies.
Introduction
Air lime-based mortars have been used as construction materials since ancient times owing to the unique properties of air lime as a binder. 1 However, conventional air lime mortars suffer from some drawbacks such as slow setting time, low strength, and susceptibility to weathering which have limited their large-scale utilization in modern construction. 2 This has led to the widespread adoption of alternative binders like cement which, despite advantages in early strength development, suffer from serious compatibility issues when used in repair and renovation of air lime-based masonry structures. 3 In this context, the modification of air lime mortars through the incorporation of pozzolanic materials has received significant research attention. Pozzolanic additives react with air lime to provide hydraulic properties and overcome some of the deficiencies of pure air lime mortars. 4
Ceramic materials, in the form of crushed bricks, tiles, pottery or industrial wastes, have a long history of use as pozzolanic additives in air lime mortars.5,6 Structures like the Hagia Sophia Cathedral,7,8 the Medieval City of Rhodes, 8 as well as various churches, monasteries, and cathedrals in Kiev 9 and Israel, 10 which date back to the Byzantine period, serve as notable instances of the extensive application of ceramic fragments in lime mortars during that era. Archaeological evidence indicates the widespread use of ceramic-added air lime concretes and mortars in ancient Roman and Greek constructions.11,12 The crushed ceramic powder reacted with air lime to provide hydraulic properties while simultaneously acting as aggregate. This provided significant technical and economic benefits which led to the extensive utilization of ceramic-added air lime mortars in historic structures across Europe and the Mediterranean regions. 13 However, the emergence of early manufactured cements in the late 18th century, such as hydraulic lime and Roman cement, followed by the invention of Portland cement in the early 19th century, led to a gradual decline in the use of air lime-based concretes. 14 These new factory-made cements, with their faster setting times and higher strengths, began to displace traditional lime–pozzolan mixtures in construction applications during the industrial revolution. The compatibility issues and failures associated with the use of cementitious repair mortars have led to a renewed interest in air lime-based formulations, including ceramic-added systems, for conservation works. 15
The pozzolanic activity in ceramic powder is attributed to the presence of amorphous silica and alumina which react with calcium hydroxide released during the hydration of air lime. 16 The reactivity is influenced by the mineralogical composition, heating temperature, particle size distribution, and specific surface area of the ceramic material. While higher temperatures favor sintering and crystallization, reducing pozzolanicity, optimum activity is attained through controlled heating that promotes the formation of reactive amorphous phases. 17 The ceramic particles not only provide hydraulic properties but also refine the microstructure and reduce porosity as filler materials in the air lime matrix, enhancing mortar durability. 18 The use of crushed ceramics further adds technical advantages in terms of better cohesion and adhesion within the air lime–ceramic system compared to conventional aggregates like sand.
The properties of ceramic-added air lime mortars depend on the physical and chemical characteristics of the binder, ceramic powder, and aggregates. Hydraulic properties develop due to pozzolanic reactions between air lime and reactive silica from crushed ceramics, leading to the formation of calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH).19,20 The improved hydraulicity reduces setting time and enhances early strength development compared to pure air lime mortars. Ceramic particles refine the pore structure and limit total porosity, reducing capillary water absorption and permeability while maintaining adequate vapor permeability. The mortars also demonstrate higher bond strength and adhesion to masonry units, attributed to better compatibility between the ceramic particles and substrate. Enhanced durability has been noted against weathering actions like wetting–drying, freezing–thawing, and salt crystallization. 21 The mortar composition can be designed to achieve optimal balance between mechanical strength, deformation capacity, and vapor permeability depending on the application.
Various methods have been employed to characterize the pozzolanic reactivity of ceramic powders for air lime modification, including chemical and physical tests as well as evaluations in lab-prepared mortars. Assessment techniques used for conventional cementitious systems can be applied to evaluate the key hardened properties of ceramic air lime mortars like strength, elastic modulus, porosity, pore structure, durability, and microstructure. 22 Trials are necessary to obtain the optimum mix design in terms of binder amount and type, ceramic powder content, and water/binder ratio to achieve the desired fresh, physical, and mechanical properties. 23 Pilot studies have demonstrated the capability to customize ceramic-added mortar properties for specific restoration requirements through proper proportioning and material selection. 24
The major drivers for the resurging interest in ceramic-added air lime mortars include their hydraulic properties, improved durability, and compatibility with masonry structures compared to conventional air lime or cement-based repair mortars. 25 The reduced setting time is advantageous for quicker finishing and opening to service compared to pure air lime mortars. 19 Long-term durability against weathering and environmental damage is enhanced while still maintaining moisture vapor permeability. 25 The similarity in composition to the original materials used provides high compatibility and prevents the damage associated with the use of cementitious mortars. 16 The incorporation of crushed ceramic waste also provides environmental benefits associated with recycling this otherwise landfill-bound material.26–28
Modified air lime mortars using crushed bricks, tiles, and other ceramic waste have demonstrated excellent potential for restoration and rehabilitation applications in air lime-based masonry structures. 29 Successful utilization has been reported in plasters, stuccos, grouting, and repair renders for monument conservation works. 30 Highly compatible repair mortars with adjustable properties can be designed to suit specific restoration requirements. Ceramic-added air lime concretes have also shown promise as sustainable, low-embodied energy alternatives to conventional concretes. However, despite the demonstrated potential, the technology has not yet been adopted into mainstream construction practice. Significant research is still required to standardize these materials through systematic composition–microstructure–property evaluations across a wider range of ceramic additives and air lime binders. Development of suitable testing methodology and specifications is also essential to facilitate the acceptance and large-scale utilization of ceramic-added air lime mortars. The incorporation of crushed ceramic wastes such as bricks, tiles, and pottery as pozzolanic additives and aggregates in air lime mortars has been found to enhance strength, durability, hydraulicity, and compatibility with historic substrates compared to plain air lime binders. The ceramic particles act as pozzolans, react with air lime to form hydraulic compounds, and also provide a filler effect to refine the microstructure. These favorable properties have led to renewed interest in utilizing ceramic-added air lime mortars for repair, restoration, and sustainable construction applications.
Historical use of ceramic-added air lime mortars
The addition of air lime mortars and concretes with crushed ceramic materials has been practiced since ancient times. Archaeological studies provide extensive evidence of the incorporation of pottery shreds, crushed bricks, and tile pieces in historic air lime-based binders across the world. The ceramic particles imparted hydraulic properties to the air lime mortars while also serving as aggregate. This enabled the production of durable and high-performance mortars using locally available materials.
One of the earliest known uses of ceramic-added air lime mortars dates back to around 3000 BC in structures found in Babylon. 31 The ancient Phoenicians who ruled between 1200 and 800 BC are also known to have developed air lime–ceramic concretes for construction purposes. 32 The first written records specifying the use of crushed bricks as pozzolanic additives are attributed to Cato and Vitruvius from the ancient Roman period. 32 With the expansive growth of the Roman empire, the technology spread across Europe and the Mediterranean regions. Large-scale evidence of ceramic-added air lime mortars has been extensively documented in architectural relics and structures from the ancient Roman, Greek, and Byzantine eras.
Several Roman archaeological sites in Italy, Portugal, Croatia, and Serbia have revealed plentiful remains of air lime–ceramic mortars.33–36 Analyses indicate the use of pottery and crushed brick particles in the preparation of strong and durable hydraulic mortars and concretes. A specific variety referred to as “cocciopesto” contained finely ground brick dust as the pozzolanic additive. Cocciopesto mixtures were employed in structural elements like arches, vaults, and foundations as well as in water-retaining structures such as aqueducts, cisterns, and baths. Zendri et al. 37 studied the chemical interactions between clay and air lime in historical “cocciopesto” mortars, which were commonly used in the Mediterranean region. The authors mimicked the composition of cocciopesto mortars by using clay samples containing 58% phyllosilicates, heating them to 500–700 °C, and then treating them with air lime putty. The samples were seasoned for 5 months either in air (with CO2) or under nitrogen (without CO2). Structural transformations were analyzed using 29Si MAS NMR spectroscopy. Heating clay produced an amorphous Q3 phase that could react with air lime. Air lime treatment in air (with CO2) yielded different products than without CO2, with more amorphous phases formed. The amorphous Q2 phase formed in air is likely responsible for the unique properties of historical cocciopesto mortars. This provides insight into the chemical interactions underlying these traditional building materials. Figure 1 shows the chemical interaction between air lime and clay. The excellent durability and water-resistance conferred by the ceramic powder addition justified its extensive use. The Byzantine era Hagia Sophia cathedral built in 537 AD extensively used brick dust-added mortars in its massive domes and walls which have withstood centuries of weathering. 8
Chemical interaction between lime and clay. 37
The technology continued to thrive during the Medieval ages as evidenced by relics across Cyprus, Crete, Malta, Spain, and several Mediterranean regions.38–42 Crushed pottery was a common mortar ingredient during the Moorish rule in Spain between 700 and 1400 AD. 43 Analyses of historical air lime mortars used in medieval structures in countries like Poland, Hungary, Czech Republic, Ireland, and UK between the 10th and 16th centuries provide extensive proofs of the widespread application of ceramic air lime technologies. 44 Brick and tile fragments have also been identified in historical mortars from the Ottoman period between the 14th and 18th centuries across Turkey and Middle Eastern regions.45,46 The mortars demonstrated significant hydraulicity attributed to pozzolanic reactions between binder and crushed ceramics.
Ceramic-added air lime concretes were also popular construction materials among ancient Mayan and Aztec civilizations of Mesoamerica. Archaeological studies reveal the extensive use of pottery shreds, crushed bricks, and pulverized potsherds as aggregates and pozzolanic additives in air lime concretes for monumental structures, plazas, pyramids, and other buildings.47–49 The fine ceramic powders added hydraulic properties while the coarse grains served as aggregates in these concretes. The technology was also common in historic constructions across Asia. Structures dating back 2000–3000 years in countries like China, Mongolia, Korea, Cambodia, and Indonesia provide evidence of crushed ceramic-added air lime mortars.50–54 The Great Wall of China extensively used ceramic-added air lime mortars which have endured centuries of harsh weathering. Lime–clay–tile compositions were popular as water-resistant mortars in traditional Chinese architecture. In India, ceramic particles called “surkhi” were used as pozzolanic additives in air lime mortars for historic temples, forts, and water structures across the country. 55
In Europe, the popularity of ceramic-added air lime mortars declined from the 18th century with the advent of industrialized binders like Portland cement. Cement-based formulations became the predominant construction materials in the 19th and 20th centuries given their ease of production, rapid hardening, and high strength development. 56 However, compatibility issues and failures associated with using cement mortars for repointing and repair works on historic air lime-based masonry structures led to a renewed interest in customized air lime mortars. There has been a resurgence of research focused on developing high-performance ceramic-added air lime formulations for restoration and conservation of heritage buildings without compromising compatibility. The environmental benefits associated with recycling ceramic wastes have also added to the interest in these sustainable materials.
The extensive historical use of crushed ceramics as pozzolanic additives and aggregates in air lime mortars and concretes across the world is a testament to the technical and economic benefits provided by these materials. The durability of structures built from ceramic-added air lime binders over centuries of weathering provides proof of their reliability. While cements became the mainstream binder from the 20th century onwards, the focus has now shifted back towards developing customized air lime-based solutions for compatible and sustainable construction and conservation. This offers significant potential for resurrecting and advancing the ancient technology of ceramic-added air lime mortars using modern scientific principles.
Composition and reaction mechanisms
Ceramic-added air lime mortars comprise an air lime-based binder, crushed ceramic particles, and sand or aggregate. The crushed ceramics provide pozzolanic properties in addition to filler and microstructural refinement effects. This section covers the composition of the air lime binder, characteristics of ceramic particles that influence reactivity, and the mechanisms involved in the pozzolanic reaction between air lime and ceramics.
Air lime binders
Air lime binders are prepared by calcining limestone to convert calcium carbonate to quicklime or calcium oxide which is then slaked with water to form air lime or calcium hydroxide. The calcium hydroxide reacts slowly with atmospheric carbon dioxide during curing to revert back to calcium carbonate, which provides the binding property. However, the hardening of conventional air lime mortars by carbonation is a slow process taking weeks or months or even years depend on the environmental conditions which restricts their use.
The curing rate can be accelerated by adding materials rich in silica and alumina which react with the calcium hydroxide through pozzolanic processes to form calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH). 57 Crushed ceramic particles composed predominantly of silica and alumina provide this pozzolanic effect in ceramic-added air lime mortars. Navrátilová and Rovnaníková 4 investigated the pozzolanic properties of six different brick dusts and their effect on the properties of modified air lime mortars. The brick dusts were characterized for their chemical and mineral composition, amorphous phase content, particle size distribution, and specific surface area. The pozzolanic activity was determined using the modified Chapelle test. 58 The brick dusts were used to prepare modified air lime mortars by replacing 50% of the air lime hydrate with brick dust. The results showed that the pozzolanic activity of the brick dusts depended mainly on the amorphous phase content, particle size, and surface area. Pozzolanic activity increased with higher amorphous phase content. The brick dust with the highest amorphous phase content (FL) showed the highest pozzolanic activity, while the dust with the lowest amorphous content (MOH) had the lowest activity. The strength of the modified air lime mortars also closely correlated with the pozzolanic activity value of the brick dust. The mortar containing the FL brick dust with the highest pozzolanic activity had the highest flexural and compressive strength at all testing ages. After 90 days, this mortar reached a compressive strength of 3.5 MPa, while the reference mortar without brick dust reached only 1.2 MPa. Figure 2 shows SEM images of two test mortars. Natural hydraulic limes (NHL) containing siliceous impurities which induce self-hydraulic behavior are also preferred binders for these mortars compared to pure air limes. 59 Artificial hydraulic limes produced by adding pozzolanic materials like volcanic ash, burnt clay or silica fume to improve the hydraulic properties are also suitable. The amount and type of air lime binder influences the quality and hydraulicity of the mortar.
SEM analysis of the test mortar. 4
Ceramic pozzolanic materials
Ceramic materials like crushed bricks, tiles, and pottery shards have been historically used as pozzolanic additives in air lime mortars. Modern applications utilize wastes from ceramic tile, brick, or pottery manufacturing as the source of reactive silica and alumina. The mineralogical composition, amorphous phase content, fineness, and specific surface area are key characteristics governing the pozzolanic reactivity of the ceramic powder with air lime. 60
The raw materials used in ceramic production include predominantly silica and alumina-rich clays, shales, feldspars, and other aluminosilicates. The ceramic particles are shaped and subjected to firing at high temperatures ranging from 600°C for pottery to over 1000°C for porcelain tiles and bricks. The heat treatment results in sintering, vitrification, crystal phase transitions, and creation of amorphous phases. If the heating is well-controlled, it promotes glassy phases high in reactive silica and alumina. However, excessive temperatures can lead to extensive sintering and crystallization which reduce amorphous content and pozzolanic potential. 61 The optimum firing range has been identified as 600–900 °C for clay-based ceramics. 62
The crushed ceramic particles provide the reactive silica and alumina for the pozzolanic reaction. A smaller particle size distribution and higher specific surface area improve reactivity by increasing the surface available for reaction with air lime. 63 Supplementary mechanical milling further activates the particles by creating defects and amorphous phases. The mineralogical composition, firing history, and particle characteristics of the ceramic powder govern its potential for pozzolanic reaction with lime binders. 64
Reaction mechanisms
The pozzolanic reaction involves the reaction between calcium hydroxide Ca(OH)2 released during air lime hydration and silica SiO2 from the ceramic powder to form CSH which provide strength and hydraulic properties. Alumina Al2O3 from the ceramics can also react with air lime to generate CAH. These pozzolanic reaction products are similar to those formed when cement hydrates in the presence of pozzolanic materials. 65
The generalized stoichiometric pozzolanic reaction between air lime and reactive silica can be represented as
66
:
Schematic of the CSH structure transformation with rising temperature. 67
Water plays a role in facilitating the pozzolanic reaction between calcium hydroxide from the air lime and the reactive silica and alumina from the ceramic powder. The pozzolanic reaction is a solution–precipitation process that requires an aqueous medium for ionic dissolution, diffusion, and product formation. When water is added to the air lime–ceramic mixture, it first hydrates the calcium oxide to form calcium hydroxide. The water then dissolves the calcium and hydroxide ions from the calcium hydroxide. It also dissolves the silicate and aluminate ions from the amorphous phases in the ceramic particles. These dissolved ionic species diffuse through the water medium and react to precipitate CSH and CAH phases. Barbero-Barrera et al. 68 provided insights into how the water affected the formation of C–S–H phases during the pozzolanic reaction using TGA and XRD. The high humidity promoted C–S–H formation in this sample. The SEM analysis revealed morphological differences in the C–S–H phases formed under different curing condition and demonstrated that the higher humidity and water content promoted the growth of C–S–H crystals.
Alumina from ceramics reacts with calcium hydroxide to form crystalline and non-crystalline calcium aluminate hydrates as per the reaction
69
:
In addition to providing pozzolanic silica and alumina, the crushed ceramic particles influence the properties of air lime mortars via filler and microstructural effects. The fine particles fill pores within the air lime matrix, enhancing compactness and lowering permeability similar to cementitious supplementary cementitious materials (SCMs) like fly ash and slag. 72 This filler effect depends on the particle size distribution and specific surface area, with finer particles providing greater densification. The ceramic particles also act as nucleation sites for precipitation and growth of CSH and CAH reaction products, providing a refinement of the binder matrix. 73 These combined effects of pozzolanic reactions and microstructural modifications enable ceramic particles to overcome some of the deficiencies of plain air lime mortars.
Role of ceramic particles as aggregates
In addition to the pozzolanic and filler effects of fine ceramic powders, crushed ceramic particles also serve as an aggregate in the mortar mixture. Ceramic aggregates display better performance compared to conventional aggregates like sand and gravel in terms of durability and bond with the binder matrix owing to their composition and morphology. 74 The rough texture and angular shape of crushed ceramics provide improved mechanical interlock and friction with the binder matrix compared to smooth, rounded river sand particles.
Certain studies have indicated the occurrence of pozzolanic activity between air lime binder and crushed brick aggregates, attributed to penetration of air lime into pores and reaction with internal surfaces. 20 The enhanced bonding is manifested in the improved mechanical strengths of air lime mortars with crushed ceramic aggregate compared to natural sand. Air lime–ceramic aggregate combinations also display minimal moisture expansion and superior resistance to weathering actions. 75 The crushed ceramic serves as a durable, reactive aggregate that complements the pozzolanic and filler effects of the fine ceramic powder. The synergistic effect of the components is critical to developing high performance mortars.
The combination of air lime binder, fine ceramic powder, and crushed ceramic aggregate in suitable proportions results in a composite ceramic-added air lime mortar that displays hydraulic behavior, improved microstructure, and resistance to environmental degradation. 76 For example, Moropoulou et al. 76 examined the properties of the mortars used in the construction of the Hagia Sophia in Istanbul (Figure 4). The mortars were found to have high tensile strength and elastic modulus compared to typical medieval air lime mortars. Testing showed tensile strengths from 0.4 to 1.2 MPa and elastic moduli around 0.66 GPa. The mortars contain crushed brick as an aggregate which provides pozzolanic properties, as well as air lime. Analysis showed the mortars have a lower calcite content and higher hydraulic compound content compared to other traditional mortars. The microstructure was examined and showed a compact matrix with strong adhesion between the crushed brick fragments and binding matrix. The binding matrix was found to contain amorphous CSH and CAH gels, which provide strength and durability. The CSH in particular allows energy absorption without fracture initiation. The improved technical properties compared to conventional air lime mortars justified the extensive historical utilization of this sustainable material. Ongoing research focused on characterizing the relationship between composition, structure, and properties is critical for advancing the technology to enable the widespread contemporary use of ceramic-added air lime mortars.
Hagia Sophia mortar sample M-26 from the base of rib 7 at the west face of the main dome—dated by Van Nice to ad 558–563. 76
Properties of ceramic-added air lime mortars
The characteristics of ceramic-added air lime mortars depend on the properties of the individual components and their interactions in the composite system. The crushed ceramic particles influence both the fresh state behavior and the hardened properties of the air lime-based binder. The key properties of interest include rheological properties, setting time, mechanical strength, deformation capacity, durability against environmental actions, porosity, pore structure, permeability, and microstructure. These govern the performance and suitability of the mortars for specific applications.
Fresh state properties
The addition of fine ceramic particles has been found to improve the water retention of air lime mortars, reducing bleeding and segregation. 77 However, higher amounts can increase water demand due to the high specific surface area of crushed particles. 78 The shape, texture, and intra-particle porosity of ceramic particles also contribute to water absorption. Rheological measurements indicate that ceramic powders act as thixotropic agents, increasing yield stress and plastic viscosity which improves workability. 79 The higher surface area and angular shape of crushed ceramics provide greater resistance to flow compared to smoother, rounded sands. However, the influence on flow behavior is dependent on ceramic powder dosage and particle size distribution.
The partial replacement of air lime binder with ceramic particles slows down setting compared to plain air lime mortars according to studies. 6 This retardation effect is attributed to the ceramic powder absorbing mixing water and delaying air lime hydration. The delayed setting provides adequate open time for mixing, transport, placing, and finishing the mortar. However, very high ceramic replacement levels can prolong setting beyond practical limits. These fresh state properties have implications for the mixing process, application, and hardening behavior of the mortars.
Mechanical strength
A major motivation for the incorporation of crushed ceramics in air lime mortars is to enhance the relatively low mechanical strength of plain air lime binders. The pozzolanic hydration products, pore refinement, and improved air lime–ceramic matrix bonding all contribute to higher strength. Studies have demonstrated significant improvements in compressive and flexural strength with the addition of ceramic powders to both air lime and hydraulic lime compared to plain binders. 80
The factors influencing strength include ceramic powder content, air lime type and quantity, and curing conditions. While higher ceramic addition increases pozzolanicity, excess amounts can have a dilutive effect reducing strength. 81 Optimal hydraulic behavior and strength are attained at around 10–30% replacement levels. 82 The strength enhancement also depends on air lime characteristics, with hydraulic limes (EN 459-1) generally showing greater response compared to pure air limes. 83 Strength development is reliant on sufficient moisture for pozzolanic hydration reactions to progress, favoring moist curing conditions compared to dry ambient exposure. 84 The mortar composition can be optimized based on the specific binder and ceramic powder to provide the desired strength for an application.
In addition to enhancing peak strength, the ceramic particles also improve post-cracking ductility and fracture energy of the air lime mortars by countering their inherently brittle behavior. 85 The mechanisms include crack impedance and fiber bridging effects of elongated ceramic particles as well as improved air lime–ceramic bond characteristics. The higher fracture toughness suits these mortars for applications requiring deformation capacity like masonry bedding or renders which are subject to structural movements. The appropriate balance between strength and deformability can be achieved based on the mix design and curing.
The rough, angular morphology and porous microstructure of the crushed ceramic particles may enhance the adhesion between the binder matrix and masonry substrates. 6 Moropoulou and co-workers 86 investigated the physico-chemical adhesion and cohesion bonds formed when adding ceramic particles into air lime mortar. The ceramic particles provide improved mechanical interlock and frictional resistance at the interfacial transition zone. Microscale studies have revealed a denser and more compact interface between ceramic inclusions and the air lime paste compared to quartz aggregates. 87 The increased adhesion is attributed to the pozzolanic activity and filler effects of the ceramics which refine the microstructure and improve binder–aggregate interactions.
Physical properties
A major benefit of ceramic addition is the densification of the air lime binder matrix and refinement of pore structure. The inclusion of fine ceramic particles reduces both total porosity and pore sizes compared to plain air lime mortars as confirmed from mercury intrusion porosimetry studies. 88 The pore blocking effect depends on the particle size distribution and specific surface area, with greater refinement achieved using finer ceramic powders.75,89 The decrease in porosity and permeability improves durability against water ingress while still retaining sufficient vapor permeability.
The incorporation of crushed ceramics also reduces drying shrinkage cracking which is a major issue with pure air lime binders. The restraining effect exerted by the rigid ceramic particles limits shrinkage, although excessive amounts can increase cracking. 90 Ceramic-added mortars also display minimal moisture movement and superior dimensional stability compared to conventional air lime–sand mortars as per investigations. 91 The reduction in drying shrinkage and moisture-induced deformation enhances service life and durability. The ceramic particles also improve the thermal stability of air lime mortars by minimizing deterioration from temperature fluctuations.
Microstructure and morphology
Microstructural studies reveal changes in the air lime binder matrix with the addition of crushed ceramics. The unreacted ceramic particles appear as rigid inclusions within the air lime binding phase in SEM imaging. 92 EDX analysis indicates higher silica and alumina contents in regions adjacent to the embedded particles compared to the bulk air lime matrix, attributed to pozzolanic hydrate formation. 93 The interfacial transition zone between ceramic inclusions and air lime binder assumes a more compact morphology with lesser pores compared to the air lime–sand matrix. Nežerka et al. 93 studied the interface between crushed brick fragments and the surrounding air lime-based matrix in ancient mortar samples using microscopy and nanoindentation. The mortar samples were taken from a late Byzantine church and contained a high proportion of crushed brick aggregate. Microscopy analysis revealed the presence of a distinct reaction rim around the crushed brick fragments that was richer in silica and alumina compared to the surrounding matrix, indicating the formation of hydraulic products (Figure 5). Charola et al. 94 studied the composition and microstructure of air lime mortars containing various pozzolanic materials, including natural pozzolan, fired clays, and fly ash. The results demonstrated a correlation between the microstructure and the mechanical performance of these mortars. Humid curing conditions generally led to a higher proliferation of CSH compared to dry curing. In mortars containing fly ash, humid curing resulted in the growth of typical fibrous CSH gel, while dry curing showed CSH growth starting on the surface of the fly-ash particles. Mortars with fired ceramic material showed some large fibrous crystal clumps of CSH gel and smaller growth on the surface of the clay particles. Thermal analysis confirmed the presence of hydraulic components in the mortars, evidenced by a shoulder on the DTG curve in the 600°C range, corresponding to the dehydration of the CSH formed through the pozzolanic reaction.
BSE image and SEM–EDX elemental maps, magnified 362×. 93
Filler effects of the fine ceramic powder result in a reduced Ca(OH)2 crystal size and more homogeneous microstructure compared to plain air lime pastes as seen in XRD studies. 5 The modified matrix also demonstrates reduced micro-cracks and improved inter-particle contact. The changes in binder morphology indicate the pore-filling and crack-mitigating effects of the secondary pozzolanic hydrates formed by the air lime–ceramic reaction. The microstructural refinement mechanisms contribute to the improved strength and durability of the composite mortars.
Durability
The reduced permeability and enhanced resistance to moisture movement imparted by the ceramic particles translate into improved durability of air lime mortars against environmental deterioration. Ceramic-added mortars display superior resistance to actions like wetting–drying, freezing–thawing, and salt crystallization compared to conventional air lime–sand mortars. 95 The ceramic inclusions restrain binder matrix deformation and damage from moisture fluctuations and crystallization pressures.
Researchers have also demonstrated the greater resistance of ceramic-added air lime mortars to salt weathering from sodium chloride, magnesium sulfate, and sodium sulfate exposures.75,96 The reduced capillary uptake and improved binder–aggregate interaction minimizes deterioration. Enhanced resistance to acid erosion and alkali–silica reaction has also been noted. The lack of reactive constituents additionally provides excellent chemical stability. 97 The selection of durable crushed ceramic particles as aggregate and pozzolanic reinforcement enables wider environmental stability of the air lime–ceramic mortars compared to pure air lime binders.
The composite effect of the components in ceramic-added air lime mortars enables a combination of beneficial properties ideal for specialized applications like restoration and conservation where compatibility and reliability are critical. Ongoing research is focused on extending the understanding of structure–property relationships to facilitate customized design and enhanced utilization.
Advantages over conventional air lime mortars
The incorporation of crushed ceramic particles as pozzolanic additives and aggregates imparts significant enhancements in the properties of air lime mortars compared to plain non-hydraulic lime formulations. The major advantages include hydraulic properties, reduced setting time, improved strength and durability, compatibility with masonry substrates, and sustainable use of ceramic waste. These benefits have driven the resurgence of interest in ceramic-added air lime mortars.
Hydraulic properties
Conventional air lime mortars and concretes rely solely on the slow process of carbonation for hardening and strength development. The absence of hydraulic constituents limits their use where higher early strength is required. The addition of crushed ceramics provides reactive silica which reacts with air lime to form calcium silicate hydrates that impart hydraulicity to the mortar. Even small replacement levels of 5–10% ceramic powder can significantly accelerate curing and hardening. 98
The improved hydraulicity leads to higher strength within 7–28 days compared to the weak carbonation-hardened strength of plain air lime mortars. The hydraulic reaction products also enhance durability against water damage. Artificial pozzolans like metakaolin are sometimes added to further supplement the hydraulicity provided by ceramics. 99 The hydraulic properties expand the setting and curing versatility of air lime mortars suited for wider construction applications.
Reduced setting time
An important drawback of conventional air lime mortars is the slow setting and prolonged maturing times often extending to several weeks or months before full hardening is achieved. The delayed setting hinders finishing and timely completion of application processes. The pozzolanic reaction between air lime and ceramic powders generates hydraulic products which accelerate initial and final set times of the fresh mortar. 100
Air lime–ceramic combinations can achieve sufficient early strength within 3–7 days rather than the 28 days typical for air lime carbonation. The faster setting enables quicker finishing and opening to service. Excessively high ceramic content and fineness can however induce undesired quick stiffening detrimental for workability and application. 101 Optimal balance between fast curing and adequate open times is essential. Nevertheless, the accelerated strength development compared to pure air limes is advantageous.
Improved strength
The low tensile and compressive strength of conventional pure air lime binders poses limitations for load-bearing structural applications. The strength arises solely from weak air lime–carbonate bonds. The pozzolanic hydrates and pore refinement induced by fine ceramic particles significantly enhance the strength of air lime mortars. 102 Compressive strength improvements of over 2–3 times have been demonstrated with ceramic replacement levels of 20–30% in some studies. 84
The morphology and composition of the pozzolanic CSH/CAH phases impart greater strength compared to air lime carbonate. The filler effect of fine particles also reduces porosity. Factors like ceramic powder fineness, air lime characteristics, and curing moisture influence strength. 103 Service loads can be sustained by designing compositions to achieve sufficient strength for intended applications. The higher strengths expand the structural possibilities for air lime-based construction.
Enhanced durability
Plain air lime mortars and concretes demonstrate poor resistance to water and environmental damage which limits their service life. The higher permeability allows ingress of water, aggressive ions, and gases facilitating decay mechanisms like leaching, freeze–thaw damage, and acid–base reactions. 104 The crushed ceramics refine pores, reduce capillarity, and improve microstructure, thereby enhancing durability.
The pozzolanic hydrates foster chemical stability in aggressive conditions compared to air lime carbonates. Ceramic–air lime combinations exhibit superior resistance to water erosion, frost damage, salt weathering, alkali–silica reaction, and chemical attack by acids, sulfates, and chlorides. 105 The durability improvement enables use in external applications subject to weathering which are challenging for conventional air lime mortars. Appropriate balance of permeability for moisture dissipation and densification for durability is however necessary.
Compatibility with masonry
An important advantage of ceramic-added air lime mortars is their high compatibility with masonry units in historic structures compared to standard Portland cementitious repairs. The similarity in composition with traditional air lime-based materials provides good adhesion and vapor transport ability without damage. 106 The rough, porous ceramic particles also improve bond to aged masonry surfaces compared to smooth natural sand aggregates.
The relatively lower strength and elastic modulus compared to cement-based mortars minimizes stresses at the repair–substrate interface. The improved deformation capacity and ductility accommodate movements arising from moisture, temperature, and foundation settlement changes. Absorption of excess free air lime by pozzolanic ceramics minimizes harmful recrystallization. These aspects minimize cracking and debonding failures leading to superior service life and compatibility. 107 The compatible characteristics justify the preference for ceramic air lime repairs over cementitious alternatives.
Sustainable use of ceramic wastes
The incorporation of crushed brick, tile, and ceramic waste as air lime pozzolans provides an eco-friendly solution to handle these otherwise landfill-bound materials. The recycling addresses issues of waste disposal and saves natural resources associated with quarrying sand and limestone. Recovery of construction/demolition debris as crushed ceramic aggregate offers similar sustainability benefits. 108 The pozzolanic activity of waste fired ceramics is not significantly affected by their end-of-life demolition origin, as long as they are appropriately sorted, cleaned of contaminants, and processed to suitable fineness. In fact, the heterogeneous nature and weathering of construction-sourced ceramics may even enhance their reactivity compared to industrial rejects. Pilot studies and field applications of air lime mortars and concretes incorporating recycled ceramic aggregates from C&D waste have demonstrated comparable or even superior mechanical and durability properties compared to mixtures with natural sand. Ioannou et al. 109 studied the use of crushed fired clay ceramics as a replacement for cement in the production of mortars. They found that mortars produced using hydrated lime and ceramic powder in a 1:1 ratio, with a binder to aggregate ratio of 1:3, developed satisfactory physico-mechanical properties even in the absence of cement. The most important factors determining the strength and workability of these mortars were the water/binder ratio and the type and particle size of the ceramic powder. Mortars with ceramic powder particle sizes below 63 μm achieved 28-day compressive strengths exceeding 3.5 MPa. The experimental mortars had porosities of 35–40%, making them compatible with traditional building materials in Cyprus such as fired clay bricks and natural building stones.
Applications in building materials
The favorable technical characteristics and sustainability benefits have generated significant interest in the adoption of ceramic-added air lime mortars for various applications in modern construction as well as restoration and conservation of historic structures. The suitability has been demonstrated for specialized uses including repointing and repair renders, plasters, stuccos, grouts, and composites.
Repointing and rendering mortars
A major application area is the development of compatible repointing and repair rendering mortars for conservation interventions on aged masonry structures constructed from limestone, brick, earthen blocks stabilized with air lime, or air lime–sand concretes. The similarity in composition to the original materials provides good adhesion and vapor permeability while still imparting sufficient strength and durability.
Corinaldesi 110 studied using environmentally-friendly mortars made with crushed bricks instead of sand as bedding mortars for repairing historical buildings. The authors studied two different crushed brick aggregates with different particle size distributions to replace sand. They also tested a blended cement and a hydraulic lime binder that were proven to be insensitive to sulfates, which can cause deterioration in historical masonry. Mortars were tested for mechanical properties and bond strength to bricks. Finely ground bricks greatly improved bond strength to bricks compared to sand mortars. Coarsely ground bricks provided a good balance of vapor permeability and capillary absorption. Microstructure, vapor permeability, and capillary absorption were also examined. Overall, recycled brick aggregates were found to be a viable replacement for sand in bedding mortars for historical building restoration, providing good mechanical performance, bond strength, and physical behavior. The choice of crushing method impacted mortar properties, with fine crushing improving bond strength and coarse crushing optimizing vapor permeability and absorption. The binders resisted sulfate attack. Environmentally-friendly recycled brick mortars show promise for restoration work compatibility and performance. Pachta et al. 111 studied the development and testing of repair mortars to conserve the substrates of ancient floor mosaics (Figure 6). The substrate layers (statumen, rudus, nucleus, supra nucleus) played a critical role in the durability of floor mosaics by providing stability and resistance to loading and environmental factors. The study aimed to understand the impact of substrate properties on performance, define parameters influencing characteristics, and propose an approach to designing and testing multilayer repair mortars. Four air lime–pozzolan mortar compositions and three double-layered mortar series were produced based on ancient substrate characteristics like the binder/aggregate ratio, aggregate gradation, brick dust addition, and thickness. The mortars were compacted to minimize pores and voids, enhancing strength and interlayer adhesion. Tests were conducted at 28, 90, and 180 days on physical properties like porosity and water absorption, mechanical properties like strength and modulus of elasticity, and adhesion properties like bond strength. Key results showed the importance of compaction and low water/binder ratio for attaining 5–10 MPa compressive strength. The addition of brick dust and crushed brick also improved performance. The study concluded that considering the composite substrate system and interrelating properties of layers is critical in designing effective repair mortars for mosaic conservation. The systematic methodology and tests on factors like layer thickness, aggregate type, and curing duration provided insights on producing durable, compatible repair mortars for ancient mosaic substrates.
Cracking of repaired ancient floor mosaic edges and tesserae loosening. 111
Plasters and renders
Ceramic-added air lime mortars are well-suited to site-manufactured and pre-mixed plasters for wall rendering applications in new construction as well as overlay plasters for renovating existing surfaces based on their workability, permeability, and cracking resistance. Air lime-crushed brick/tile combinations as total or partial replacements for sand have demonstrated suitable technical performance and application versatility. 112
Anti-cracking properties can be enhanced by incorporating natural reinforcing fibers like sisal, jute, or straw which provide restraint against shrinkage stresses induced during curing. 113 Researchers have shown the ability to customize composition for specific requirements of adhesion, strength, flexibility, water retention, and vapor permeability. Surface finishes with improved abrasion and scrub resistance compared to plain air lime renders are also possible by incorporating hard, durable crushed ceramics.
The plasters demonstrate good working compatibility with diverse masonry substrates. Torres and Matias 114 studied the viability of incorporating ceramic waste materials into air lime mortars for building rehabilitation. Mortars were prepared with an NHL binder and two types of river sand aggregates. Ceramic waste residues from red ceramics (bricks, roof tiles, pottery) and white ceramics (porcelain, glazed, and non-glazed stoneware) were milled to a particle size distribution similar to the river sands and used to partially replace the sand aggregates at 20% and 40% by volume. The mortars were tested after 28 days of curing to determine their mechanical properties. The results showed that mortars made with 20% substitution of red ceramic residues had comparable or slightly improved compressive and flexural strength compared to the reference mortars without ceramic waste. Increasing the red ceramic residue content to 40% caused a more significant decrease in strength. The mortars with white ceramic residues showed a slight decrease in strength at 20% substitution, which was more pronounced at 40%. However, even at 40% substitution the white ceramic mortars still achieved reasonable strength values, around 4–5 MPa in compression and 1–2 MPa in flexion. In terms of workability, the red ceramic mortars had similar or slightly reduced consistency compared to the reference. The white ceramic mortars showed improved workability, attributed to the lower water absorption and smoother surface of these residues. Overall, the results demonstrate the feasibility of incorporating up to 40% ceramic waste residues in air lime mortars without excessively compromising the mechanical performance. The use of ceramic waste provides environmental benefits and can produce sustainable and compatible mortars for rehabilitation of old buildings. Kočí et al. 115 studied using waste ceramic dust as a substitute for cement in air lime–cement plasters. The researchers tested three air lime–pozzolan plasters containing different proportions of ceramic waste dust, referred to as LPCD1, LPCD2, and LPCD3. They compared the properties of these plasters to a reference air lime–cement plaster (LCP) without ceramic dust. The results showed that replacing cement with ceramic dust significantly increased the compressive strength of the plasters, with LPCD2 and LPCD3 having over three times higher strength than the LCP reference. However, the ceramic dust also increased thermal conductivity. The researchers then used computer modeling to analyze how these plasters would impact the hygrothermal performance of a typical building envelope when used as a surface layer. The modeling showed that, despite the higher thermal conductivity, the LPCD plasters did not negatively affect the temperature or moisture performance of the envelope compared to the LCP reference. Overall, the experimental and modeling results demonstrated that waste ceramic dust can effectively replace cement in air lime–cement plasters, enhancing strength while maintaining good hygrothermal performance. Given the high energy use and CO2 emissions associated with cement production, the authors conclude that using waste ceramic dust is an environmentally-friendly and energy-efficient solution for sustainable construction. This research provides strong evidence to support replacing cement with waste ceramic dust in plasters for building envelope applications.
Mortars and grouts
Structural masonry applications require mortars with sufficient early strength, workability, and bond with masonry units to effectively transfer stresses while also accommodating movements. Ceramic-added air lime mortars offer suitable technical and functional characteristics to replace conventional cement–air lime or cement-only masonry mortars. The good adhesion with masonry units, deformability, and crack resistance help minimize stresses. 111
Grouting applications for filling voids and cracks in existing masonry also benefit from the flowability, penetration ability, and stability of ceramic air lime binders. Nepomuceno et al. 116 studied the influence of incorporating high volumes (50% by weight) of mineral additions on the properties of cement-based grouts for masonry consolidation. The mineral additions studied were limestone filler, metakaolin, glass powder, and ceramic waste. The results showed that replacing 50% of cement with these mineral additions produced grouts that met the requirements for masonry consolidation in terms of rheological behavior, injectability, and physical and mechanical properties. Specifically, the ceramic powder addition led to the best overall performance, followed by limestone filler, glass powder, and metakaolin. The mineral additions increased yield stress and viscosity compared to plain cement grouts, which improves injectability. Bleeding was reduced with the mineral additions. The methodology previously used for interpreting mortar flow behavior could be applied to analyze grout flow. The authors conclude that high volumes of mineral additions can replace cement in grouts for masonry consolidation to reduce cement content while still meeting performance requirements. This complies with principles of sustainability by reducing waste and consumption of natural resources. Azeiteiro et al. 117 studied the development of air lime-based grouts to consolidate and restore adhesion of old, detached wall renders. Grouts were formulated with air lime binder, fine silica sand aggregate, and varying amounts of metakaolin pozzolan and chemical admixtures (Figure 7). The rheological properties of the grouts were characterized using a mortar rheometer and a specialized testing method to determine yield stress and plastic viscosity over time. Results showed that grouts with 10–20% metakaolin addition had suitable rheological properties for injectability, including moderate yield stress for stability and low viscosity for flow through cracks. Hardened properties were also tested. Compressive strength increased with higher metakaolin content, up to 2.5 MPa with 30% addition. Capillarity and adhesion loss were lowest with 20% metakaolin. Overall, the study concluded that grouts with 10–20% metakaolin, cellulose ether water retainer, sodium gluconate plasticizer, and acrylic resin adhesive provided the best balance of fresh state rheology for injectability and hardened state properties for compatibility and performance in consolidating old wall renders. The rheometer testing method allowed precise characterization of the time-dependent rheological properties.
Adhesion on grout and render in the samples of simulation of detachment. 117
Conclusions and future outlook
This review has encompassed the significant research performed over the past few decades on ceramic-added air lime mortars. The prominent historical use of crushed ceramics as pozzolanic additives and aggregates in air lime binders has been highlighted along with associated reaction mechanisms. The influence of ceramic incorporation in enhancing the properties of air lime mortars has been discussed based on investigations into characteristics like strength, permeability, pore structure, durability, and microstructure. Test methods for evaluation of properties and performance have been covered. The advantages over conventional air lime mortars like hydraulicity, strength, and masonry compatibility have been enumerated along with sustainability benefits. Specific application areas including repair mortars, plasters, and composites have been identified as potential adopters.
In summary, the ability of crushed ceramic particles to impart hydraulic properties and durability improvements to air lime mortars has been successfully demonstrated through both laboratory studies as well as field performance validations. Optimized compositions can provide a beneficial balance of strength, deformation capacity, permeability, and vapor transport suited for specialized building materials applications. The similarity to traditional formulations also imparts compatibility advantages for repair and restoration of historic structures compared to conventional cement-based alternatives. These favorable technical attributes coupled with ecological benefits of reusing mineral wastes provide a compelling incentive for broader adoption in sustainable construction.
However, despite the demonstrated potential and advantages, ceramic-added air lime mortars have yet to penetrate mainstream building materials technology and construction practice. Significant scope exists for advancing the materials science and technology underpinning these eco-efficient composites. Systematic research across a wider spectrum of raw material sources and compositions is necessary to develop generalized understanding of structure–property relationships, performance capabilities, and limitations which can aid customized design and optimization. Development of suitable testing techniques tailored to the unique characteristics of air lime-based composites is also an essential requirement for reliable product specifications.
The long-term durability under diverse service environments needs to be established through laboratory simulation studies and extensive field validation programs across a range of climatic conditions. Life cycle assessment and cost–benefit studies are imperative to quantify sustainability credentials and economic viability. Standardization of manufacturing and application protocols will further aid adoption by addressing variability and quality assurance challenges. Demonstration projects to validate field performance and constructive engagement with stakeholders across academia, R&D agencies, industry, and regulatory bodies will be crucial to promote wider acceptance.
While significant advances have been made, there is tremendous potential for progress through interdisciplinary approaches encompassing chemistry, materials science, construction engineering, and conservation science. Developments in advanced characterization tools like electron microscopy, spectroscopy, and tomographic imaging can shed further light on reaction mechanisms, microstructure evolution, and degradation processes. Materials informatics leveraging artificial intelligence methods can uncover new knowledge from scientific findings to identify optimal compositions and processing pathways. Performance enhancements are possible through nanotechnology, biotechnology, and additive manufacturing. Adoption of Industry 4.0 and digitalization concepts can aid commercial translation.
The growing recognition of sustainability imperatives in the built environment provides a renewed window of opportunity to elevate ceramic-added air lime technologies from their niche status to a competitive green construction material of the future. Collaborative networks between researchers and industry practitioners will be the crucial driver to accelerate promising lab-scale concepts to field-validated and market-ready products. The rich ancient heritage coupled with modern materials science approaches provide a solid foundation to rediscover and reinvent ceramic-added air lime mortars as sustainable mainstream building materials in the 21st century context.
Footnotes
Author contributions
W.Y. and B.Y. conceived of the presented idea. W.Y. and B.Y. did literature research. W.Y. wrote the manuscript draft. B.Y. proofread the manuscript. B.Y. supervised the project.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by 2021 Construction Scientific Research Program of Zhejiang Province, grant number 2021K014, Basic Scientific Research Program of Wenzhou in 2021, grant number S20210027, Zhejiang College of Security Technology horizontal research project, grant number hx202118, and Zhejiang College of Security Technology horizontal research project, grant number hx202215.
