1 Introduction

Since antiquity, masonry constructions have been used for buildings due to their anisotropic ease of construction. Masonry elements such as walls are used in all forms of building construction around the world, even in modern buildings [1]. Masonry constructions are comparatively cost-effective, possess good sound, and heat insulation capabilities and are simple to use and can be found locally [2]. Masonry construction comprises of an arrangement of masonry units to form a structural element. It consists of masonry mortar that act as gluing agents for the masonry units. A masonry system is the comprehensive framework and approach employed in the construction of a masonry structure, defining the fundamental structure of the wall or building, including load-bearing ability, insulation, aesthetics, and overall stability [3]. It involves integrating materials, methodology and design principles in constructing masonry structures like walls and buildings, using units bound together with mortar. This system necessitates careful consideration of masonry unit selection, mortar composition, structural design and construction techniques, to bear the loads placed upon it, including the weight of the structure itself and any additional loads like wind, snow, or seismic forces [4]. Beyond fulfilling functional needs like load bearing capacity and weather resistance, this system also makes a substantial contribution to the architectural appeal of a construction project. In the realms of construction and architecture, masonry systems hold paramount importance, yielding strong and durable structures. Several scholars [5,6,7] experimented various methods to strengthen the bonds by insertion of one or several frogs, changing the roughness of the brick’s surface [8], and surface treatment with epoxy resin or fresh cement slurry are some notable ways.

The bond strength of masonry joints is a crucial factor in determining the overall strength and stability of masonry structures. A well-designed masonry system must consider the joint type, the quality of the mortar used, and the attention to joint detailing [9, 10]. Typically, masonry constructions comprised of clay bricks with lime mortar joints before cement was developed in the earlier investigations. Construction of a masonry wall needs a solid bond between the masonry unit and mortar. The bond strength becomes extremely essential when the masonry structure is subjected to both in-plane and out-of-plane loading during seismic disturbances [5]. There are several factors that affect the development of the masonry unit-mortar bond. These include the masonry unit’s surface roughness [11], initial rate of absorption of water or chemicals from the mortar, moisture content, sand grading, and mortar’s ability to retain water, consistency, and composition, as well as the bonding surfaces’ cleanliness. By considering factors such as masonry unit properties [12], mortar composition, joint geometry [13], and construction techniques, builders can optimize the bond strength and ensure the long-term performance of masonry assemblies [14]. The bond strength is influenced by factors such as the type of loading (compression, tension, shear), exposure conditions (moisture, temperature variations), and the overall structural design [15]. Numerous studies have investigated the bond strength of masonry. one of them investigated the impact of masonry compressive strength on the bond strength using local bricks and mortars [13] another study assessed the efficiency of strengthening by examining interfacial bonding and found that differential response of units and mortar lead to changes in the masonry properties [16]. Consequently, various strengthening techniques have been examined to enhance the performance of masonry structures [17].

Appropriate planning and construction of masonry bond can significantly boost the structure’s load-bearing capacity [18]. Masonry constructions are susceptible to a variety of movements [19, 20]. These movements are made possible by joints’ flexibility, which allows the structure to change without suffering major damage or cracking [21]. Effectively constructed joints, bonding the unit and mortar can significantly decrease the chances of excessive stress concentration, structural deformation, or collapse. The chances of structural failure is reduced by the capacity to transfer both vertical and horizontal loads through joints, which aids in the distribution of weight and forces acting on the structure [22, 23]. Therefore, masonry system must be properly designed and constructed to ensure longevity, durability, and overall performance of the buildings. It is crucial to ensure that the strength and performance of the joints match or exceed that of the masonry units to maintain the integrity and longevity of the structure.

A Masonry system involving units and mortar joints creates a cohesive, stable, and aesthetically beautiful masonry structure that can bear diverse loads and environmental variables while preserving its structural integrity. Masonry joints refer to the spaces between individual masonry units, such as bricks or blocks, that are filled with mortar or other bonding materials [24]. Masonry joints connect discrete masonry components, such as bricks or stones, to create a sturdy, unified structure. These joints distribute the stresses along the load path, thereby improving both its overall stability and strength. It forms an integral part of the masonry system because they keep the units together, provide structural stability, and guarantee the integrity of the wall, making them a crucial component of the masonry system. The joint becomes the weakest link in the system if the mortar is weaker than the units [25]. Weaker joints are unable to transfer the structure’s loads [26] and are susceptible to water infiltration and moisture-related issues due to their limited flexibility resulting in localized stress concentrations in the vicinity of masonry units [27]. If the joint is significantly stronger than the units, it can impede the necessary movement of the masonry units [28] preventing natural movements. This can result in increased stress levels on the units adjacent to the joint. Both types of joints potentially leads to cracks, deformations, or even the dislodgement It’s noted that the masonry mortars are typically designed to be weaker and more flexible than the individual masonry units to accommodate movement and stresses [29, 30]. An ideal situation that promotes structural integrity, load distribution, and overall stability is one where the strength of the masonry mortar and the strength of the individual masonry units are equal [31, 32].

Compatibility in terms of material characteristics is ensured by having mortar and units with equivalent strengths. The mortar used in the joints should be made to complement the masonry units’ strength and features [33]. Professionally designed masonry system with equal strength in units and mortar play a vital role in enhancing the stability and performance of the masonry structure [34, 35]. The strength, expansion - contraction characteristics, and durability of the joint material should be compatible with those of the masonry units [36, 37]. In masonry construction, the overall bond strength is influenced by how well the brick and mortar work together as a system. The bond strength of the brickwork is dependent on the type of brick and the joint material [4]. Bricks’ capacity to withstand compression ensures the integrity of the masonry’s load-bearing capacity. Therefore, bricks with superior compressive strength establish a sturdy foundation for the entire masonry structure. Mortar with substantial strength ensures effective transmission of loads, ultimately enhancing the overall bond strength of the masonry assembly. An ideal bond between the brick and mortar ensures that they work together as a cohesive system, effectively sharing and redistributing the loads acting on it.

Alkali-Activated masonry systems are a new and evolving area of research. Alkali activation involves the use of alkaline solutions to activate a binder material, to form a cementitious material [17]. A significant number of by-products are generated from agriculture [38] and industrial activities [39], and these by-products are often disposed of in barren lands or the sea, adversely impacting the environment and society [40, 41]. Alkali activated system uses such wastes or byproducts for their production as investigated by several researchers [42]. The properties and performance of alkali-activated materials can vary depending on the specific formulation and mix design [43]. The brick manufacturing sector has recently shifted its emphasis to a more environmentally friendly method of manufacturing [44]. Alkali-activated bricks and mortar have attracted attention in the realm of search. This entails using industrial wastes in place of soil and kiln fire in the cementing process. Fly ash cement bricks are environmentally friendly than burnt clay bricks [39, 45]. In addition, they also use more cement which raises carbon dioxide emissions. 2–8% of global power consumption is attributed to the cement manufacturing process and the onset of the Industrial Revolution, atmospheric CO2 concentration has increased by 47% [46]. The possibility of using industrial byproducts in higher proportions as a base material to produce bricks, with no cement needed as a binding material, is provided by the alkali activation technology [47]. Hence the use of flyash in masonry significantly reduces the carbon footprint and energy consumption compared to traditional masonry materials. Also, it contributes to waste diversion by reusing an industrial byproduct contributing to resource conservation and economic sustainability [48]. From the previous literatures, it is noted that the alkali activated brick structure reduces the cost of structure by almost 7% and embodied energy and operational energy by 22 and 13% respectively [49]. However, numerous investigations on the mechanism, workability [39], mechanical characteristics, durability and sustainability of alkali activated flyash based pastes, mortars, and concrete [50] are carried out. All of this studies, encourages utilization of wastes in construction industry, thus promoting sustainable future. However, fewer studies have been conducted on other uses, such as masonry using alkali-activated units and mortar. Shear, flexural, axial tensile, and compressive strengths are some of masonry’s primary mechanical characteristics. The curing conditions, stress level, mortar type and strength and brick type all generally have an impact on these properties. The properties of the mortar are particularly important for the bond strength of masonry. Thus, the purpose of this current study was to assess how well different masonry units and mortar combinations adhere to one another. In this study, the bond strength of alkali activated masonry units and mortar is compared to that of traditional masonry constructions made of clay brick and cement mortar. The objective of this study was to investigate the influence of varying masonry unit and mortar combinations on bond strength, with the aim of providing insights for optimized masonry construction practices.

2 Materials and methodology

2.1 Materials

2.1.1 Cement

The local supplier supplied OPC grade 53, which was in accordance with American Society for Testing and Materials (ASTM) C150 [51]. The specific gravity of cement used for preparing the masonry mortar was 3.13. Tables 1 and 2 provide details on the chemical Composition and physical properties of the cement respectively. The Chemical composition was determined using X-Ray Fluorescence (XRF), a widely recognized analytical instrument for elemental analysis. XRF equipment operates by irradiating a sample inducing the emissions of secondary X-rays or fluorescent X-rays. The elemental composition of the sample is ascertained by measuring the energy and intensity of these emitted X-rays.

2.1.2 Flyash

Flyash captured in electrostatic precipitators as a byproduct of coal combustion are extracted and processed to its size by Jaycee Company and processed Class F flyash with brand name as Cemgaurd is procured from Maharashtra. Table 1 shows the chemical composition of the flyash. In accordance with ASTM C618 [52] Class-F the flyash is found to have alumina and silica rich elements where the calcium was comparatively less. Table 2 shows the physical properties of flyash.

Table 1 Chemical composition of the binders
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Table 2 Physical properties of the binders
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2.1.3 Fine aggregate

Manufactured sand (M- Sand) was used for the preparation of all the mortars used in the present study. Different types and size of aggregates also play an important role in Alkali activation [12]. Figure 1 depicts the granulometric curve. The analysis places it within zone II indicating its particle size distribution. Furthermore, it was determined that this sand has particles passing through a 4.75 mm sieve, and its specific gravity was measured at 2.63. This is in accordance with the specifications mentioned in ASTM C128 for M Sand [53]. The bulk density of M-sand was found to be 1693 kg/m3. The water absorption was measured by submerging the dry aggregate of the specified 24-hour period in water and was found to be 1.54%.

2.1.4 Water

Water plays a critical role in cement mortar, which is a mixture of cement, sand, and water used for various construction applications. Water is added to the dry mixture of cement and sand to form a workable mortar. In the present study, potable water with a pH of 7 was employed to mix cement and aggregate. A pH meter was used to measure the pH of water, providing a digital readout of the pH value. The pH scale typically ranges from 0 (highly acidic) to 14 (highly alkaline), with 7 being neutral.

Fig. 1
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Granulometric Curve of the M-sand

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2.1.5 Alkali activator

Alkali activators are typically alkaline solutions or compounds that provide the necessary alkali ions (such as sodium, potassium, or lithium) to initiate the reaction with the aluminosilicate based precursors. The alkali activators enable dissolution of the aluminosilicate precursor, resulting in the formation of a three-dimensional amorphous or semi-crystalline gel-like structure. Sodium silicate supplies both sodium ions and silicate ions, which participate in the formation of the gel. Sodium hydroxide and sodium silicate are used as alkali activators in the present study. Sodium hydroxide is a strong base and is commonly used as an alkali activator due to its availability and cost-effectiveness. It provides the necessary hydroxide ions (OH−) which aids the dissolution of precursors in the activation process. Sodium silicate is another widely used alkali activator. It is a combination of sodium oxide (Na2O) and silicon dioxide (SiO2). The selection of the alkali activator depends on factors such as the type of precursor material, desired properties of the final product, and environmental considerations [54]. The concentration and ratio of the alkali activator to the precursors also play a crucial role in determining the mechanism of its gel formation and its resulting properties [55]. In this study, activator solution with 3 Molar concentration of Sodium hydroxide was prepared and left for 24 h to stabilize and 1:0.5 ratio of sodium hydroxide to sodium silicate was mixed and kept for an hour before adding it to the precursor.

2.2 Masonry materials

2.2.1 Clay brick

Bricks are made with natural clay by moulding the clay into the appropriate shapes and sizes, and then dried for 24 h and burned at elevated temperatures in kiln for about 900 to 1200 °C to attain strength and durability. Burnt clay bricks for the present study having the dimension of 190 × 90 × 90 mm were collected from a local industry in Udupi, India. These blocks were tested for dry density, moisture content and compressive strength as per IS 3495 [56]. The Physical properties of the clay bricks are shown in Table 3.

Table 3 Physical properties of clay brick
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2.2.2 Alkali activated bricks

The Alkali activated bricks are manufactured of the same dimensions as a burnt clay brick. The required proportions of the materials based on the desired characteristics of the bricks were determined. Dry flyash and M-sand was mixed in the mixer in the ratio 1:2. The reason behind the mix 1:2 is that by using a lower ratio of aggregate to binder in mortar, also known as a rich mix, offers several advantages in certain construction applications. In cement mortar (CM), this higher binder content enhances adhesion to masonry units providing stronger bond, improved water resistance, reduced permeability and minimize shrinkage during curing. In Alkali activated mortar (AAM), a greater portion of flyash content in the binder phase increases the use of more waste material which facilitates more effective alkali activation. The reduced reliance on aggregates contributes to a reduction in the consumption of natural resources for construction purposes. Gradually, alkali activator solution was added while continuing to mix. The binder to activator solution ratio used for the study is 1:0.5. The mixing time is maintained at 3 min to attain uniform and homogeneous mixture. Brick mold was prepared by applying a greasing agent to prevent sticking. Mixture was transferred into the mould of 200 × 100 × 100 mm and compacted well to remove any air voids or gaps and surface of the brick is made smooth using a trowel. After demoulding the alkali activated bricks were cured in the air at ambient temperature.

2.2.3 Cement mortar

Cement mortar is a mixture of cement, sand, and water used in various construction applications. Building blocks, such as bricks or stones, together. It is commonly used for bricklaying, plastering, and masonry work. Cement mortar provides structural integrity, strength, and durability to the constructed elements. In this study, a binder comprising 32% water and a 1:2 ratio of cement to aggregate was used to reduce the amount of aggregate used, which can otherwise adversely affect the mortar’s properties.

2.2.4 Alkali activated mortar

Alkali activated mortar was used to bond the bricks together. In this study the alkali activated mortar is made of flyash aggregate and alkali activator. Flyash to aggregate ratio used was 1:2 and binder to solution ratio used was 1:0.5. In Alkali-Activated Mortar (AAM), increased flyash in the binder phase and decreased aggregates utilizes more waste materials, enhances alkali activation, and reduces dependence on aggregates, conserving natural resources in construction. A stiff mix was designed by trial mixes. Thus, the ratio of binder to activator used was 1:0.32 which provided stiff mix as per flow table test.

2.3 Test methods

Alkali activated mortars were developed for the manufacturing of bricks and preparation of joining material. The developed mortar was used for both the components of masonry. Excess water can weaken the mortar, reduce its bonding strength, increase shrinkage, and lead to other undesirable effects. It is crucial to follow the recommended water-to-cement ratio and mix the mortar thoroughly to ensure a consistent and binding material for construction purposes. Thus, amount of alkali activator solution to be used in the preparation was set by conducting flow table test. In any case the mortar was not allowed to flow after 25 blows. This test is in accordance with ASTM C30 [57] as shown in Fig. 2. Manufactured alkali activated bricks are in Fig. 3 and burnt clay bricks are tested for its physical and mechanical properties before joining masonry prisms. Thus the compression strength test, flexural strength test, density and water absorption were tested according to ASTM C67 [58]. The alkali activated mortar which is used to lay the masonry specimens is evaluated for its compressive strength by three specimens moulded in 100 mm cube, compacted and cured in the air. The compressive strength of the mortar was assessed using an electrohydraulic compressive testing machine with a loading rate of 1.0 kN/s. The compressive strength test of alkali activated mortar was carried out according to ASTM C 109 [59].

Fig. 2
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Flow Table Test

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Fig. 3
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Alkali Activated Bricks

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2.3.1 Production of masonry prisms

An assembly of masonry units and mortar is called a “masonry prism,“ and it is made to serve as a test sample for identifying the characteristics of masonry assemblages. Masonry prisms are produced with bricks stacked one over the other placed with mortar joints [30]. Masonry prisms of alkali activated brick and clay bricks are used for conducting tests on bond between masonry units. 2 brick prisms are used in the testing of compressive bond strength. 2 brick cross prisms are used in testing tension bond strength. 3 brick prism setup is used to test shear bond strength and 5 brick prisms are used to test flexural bond strength. All the specimen used to test various bond strength are shown in Fig. 4.

Fig. 4
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a Compression Bond Strength Specimen b Flexural Bond Strength Specimen c Tension Bond Strength Specimen d Shear Bond Strength Specimen

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2.3.2 Compression bond strength

Masonry prisms were prepared for compression testing as per the specifications mentioned in ASTM C 1314 [62]. The respective mortars were applied to the top and bottom rough surfaces of the masonry prisms to level them out and guarantee a consistent axial load distribution during compression testing. As seen in Fig. 5a, the compressive strength of all the masonry prisms was tested using 2000kN universal compression machine at loading rates of 1.0kN/s. For each combination of mortar and masonry elements three specimen were tested and average is reported.

2.3.3 Flexural bond strength

5 brick prisms are used to test flexural bond strength. As indicated in Fig. 5b, the flexural strength of the masonry prisms was tested using a 2000kN universal compression machine at loading rates of 50 N/s in order to determine the flexural bond strength of masonry. Prisms were built to measure the flexural bond strength of masonry using ASTM E 518 [60] Standard Test Methods for Flexural Bond Strength of Masonry. The prism was subjected to 2 point loading in a universal testing machine. The specimen was supported horizontally by 2 roller supports. The load was applied on the specimen gradually from the top using 2 roller setups. This load induces moment in the prism which further causes the failure of the masonry unit and the mortar.

Fig. 5
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a Compression Bond Strength Test setup b Flexural bond strength test setup c Tension bond strength test setup d Shear bond strength test setup

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2.3.4 Tension bond strength

Bond strength of brick masonry was calculated by using brick couplet specimen. Compressive load was applied to create a tensile force on the mortar joint. Bond strength was calculated by determining the ratio of ultimate load and mortar area. The masonry prism setup for measuring tension bond strength is shown in Figs. 2 and 5c bricks cross bond setup is used for this test. The tensile bond strength of the masonry prisms was tested and built according to the specifications based on ASTM C 1072 [61] Standard Test Method for Measurement of Masonry Flexural Bond Strength with minor modifications. The specimen was placed on top of one plate of universal testing machine with support of wooden blocks split apart. The load at which the splitting or separation takes place is noted.

2.3.5 Shear bond strength

To assess the “strength of the brick-mortar joint under shear stress,“ masonry triplets are subjected to direct shear loading tests. The masonry triplet samples that were used in this study were constructed and tested in accordance with the guidelines stipulated in ASTM C1823 [62]. Shear strength was determined by calculating the ratio of force to the area parallel to the mortar joint. A Universal testing machine was used to test each samples. Among the three bonded bricks, central brick is loaded from the top. Two bricks on either side are supported at the bottom. On application of the vertically downward load in a universal testing machine, the adhering mortar on either side of the central bricks’ experiences shear stress. The setup is loaded until the mortar loses its stability, resulting in separation of adhered side bricks. The load at which the separation occurs is noted. The strength of a brick prisms can be calculated by dividing the load by twice the surface area of the brick. Figure 5d depicts the test setup for the triplet specimen shear bond strength test.

2.4 Methodology

The bonding properties of 2 different masonry units bonded with 2 different masonry mortars are evaluated in the present study. Here, Clay brick and alkali activated brick were bonded with cement mortar and alkali activated mortar in different combinations. In accordance with Table 4 the brick mortar combinations are termed as A1 (Clay brick - Cement mortar), A2 (Clay brick - Alkali activated mortar), B1 (Alkali activated brick- Cement mortar) and B2 (Alkali activated brick- Alkali activated mortar). All the bonded prisms were subjected to testing under the loads of compression, flexure, shear, and tension, following the testing methods as described.

Table 4 Masonry unit and Mortar Combination
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3 Results and discussions

Experimental results of bond strength evaluation in terms of bond compression, bond flexure, bond tension and bond shear along with water absorption provide an idea about mechanical behavior of masonry.

3.1 Density of the masonry units

Masonry units have varying densities based on the materials they are made of and their manufacturing processes. Figure 7a shows the results of densities of burnt clay brick and alkali activated bricks to be 1570 and 1825 kg/m3 respectively. The alkali activated masonry bricks were made by activating flyash, thus the resulting binder creates a dense matrix within the bricks [63]. The chemical reaction between the precursors and an alkaline activator reaction forms a network of chains [64], resulting in a more compact structure with reduced porosity. The elimination of excess porosity leads to higher density compared to clay bricks. Clay bricks are made from natural clay or shale, which contains small particles and organic matter. During the firing process, the clay undergoes physical and chemical changes, including the release of water and the decomposition of organic materials [65]. These processes create voids or pores within the brick, resulting in increased porosity. The presence of pores reduces the overall density of the clay bricks [66]. The density of flyash is much lower than that of clay or cement due to differences in their composition and manufacturing processes, yet alkali activated bricks (AAB) still have a higher density. This is because, the flyash used for alkali activation are smaller and more uniform shape which allows them to pack more densely, resulting in a higher overall material density compared to the irregularly shaped clay and shale particles found in red bricks. This tighter particle packing, coupled with the smaller and more uniform particle size, contributes to the increased density of AAB. The higher density of alkali-activated bricks contributes to their improved mechanical properties [67], including compressive strength, durability, thermal properties and fire resistance. These bricks can withstand higher loads and exhibit better structural integrity compared to clay bricks, which are more porous and have lower density. This makes alkali-activated bricks suitable for structural applications where strength and load-bearing capacity are crucial.

3.2 Compressive strength of individual masonry units

Figure 6a shows the failure of bricks in compression and Fig. 7b represents the graphical results of both the bricks. On comparisons of compressive strength of the burnt clay brick and alkali activated brick it was found to be was found to be 11.46 and 12.14 MPa respectively, similar to the previous findings [68]. The results reveal that the compressive strength of the alkali activated bricks and compressive strength of the burnt clay bricks were near with about 0.6 MPa difference. Bricks are manufactured using different materials and processes, which may contribute to their higher strength properties [44], [69]. Alkali-activated bricks form a different type of chemical bonding compared to clay bricks. This chemical bonding mechanism can result in superior strength properties. This strength advantage can be attributed to the reaction, which forms a strong and rigid structure with improved load-bearing capacity [70]. Alkali-activated bricks typically have lower porosity compared to clay bricks because it results in a denser microstructure with fewer voids or pores. Reduced porosity enhances the strength and durability of the material [71]. The strength of a brick can also be influenced by its density. Bricks with higher density tend to exhibit reduced porosity and fewer pores or voids because they possess a denser microstructure within the matrix. This results in a more uniform and solid structure, which can withstand greater compressive forces. The results also satisfy the standard codal provision which says the ultimate compressive strength of the masonry units used for the load bearing wall should have minimum 3.5 MPa strength. Therefore both the bricks could be considered for building masonry structures [72].

Fig. 6
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a Failure of Bricks in Compression b Water Absorption of the bricks c Cracks formed due to flexural load

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3.3 Water absorption of the masonry units

The 28 days air cured bricks were dipped in water for 24 h for assessing the percentage of water absorption of the bricks. According to the standard codal provision [62] [73], the maximum available water absorption for a clay brick is 20%. Results showed that the percentage of average water absorption of three different samples were 3.3 and 2.47% for burnt clay bricks and alkali activated bricks respectively. 0.83% variation in the water absorption was seen when two bricks were compared. Figure 6b shows the sample kept for water absorption and Fig. 7c shows the percentage of water absorption by the specimens. In construction practice, both water absorption and compressive strength are considered essential properties for determining the suitability of bricks for specific applications. Bricks with lower water absorption percentages generally have better compressive strength because lower porosity indicates fewer voids or weaknesses within the brick’s structure. Burnt clay bricks tend to have more pores as a result of the expulsion of water during the firing process in their manufacturing. Meanwhile, Alkali-activated bricks generally exhibit reduced porosity compared to clay bricks [74]. This is attributed to the formation of denser structure during alkali activation. This decreased porosity effectively restricts water penetration into the material, resulting in reduced water absorption [75].

Fig. 7
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Properties of Masonry Bricks: a Density b Compressive Strength c Water Absorption d Flexural Strength

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3.4 Flexural strength of the individual masonry units

The flexural strength of both burnt clay bricks and alkali activated bricks are determined using a compression testing machine with one point loading method. With increment of loads during testing, the bricks failed by initiation of cracks at the center followed by rapid vertical propagation of the cracks. All three bricks were seen getting cracked at the center on application of flexural loads. Figure 6c illustrates the formation of cracks at failure loads, while Fig. 7d presents the flexural strengths of two types of bricks at the point of failure. The results revealed that the average flexural strength of the burnt clay brick was and 2.56 MPa, while that of alkali activated bricks was notably higher at 3.63 MPa. This disparity in flexural strength can be attributed to the unique composition of alkali-activated bricks, often formulated with materials that inherently possess properties conducive to increased flexural strength [76]. The flexural strength of a material is also closely linked to its density. There is a direct and proportional relationship between these two parameters. As the density of a material increases, its flexural strength tends to increase as well. This is because denser materials typically have more particles or structural elements packed closely together, resulting in stronger interparticle bonds, improved load-bearing capacity, enhanced structural integrity, reduced porosity, a more homogeneous microstructure, higher stiffness, and greater resistance to deformation when subjected to bending forces. When flexural forces are applied to the burnt clay bricks (BCB), exhibit characteristics that can significantly impact their performance. The presence of distributed pores within BCB acts as stress concentrators providing pathway for the propagation of cracks. Additionally, the pores reduces the available material and density of the brick for resisting these forces, weakening the bonds between particles. Consequently, these effects result in the initiation and propagation of cracks, diminishing cohesion within the material. This, in turn, renders the burnt clay bricks more susceptible to fracture and leads to lower flexural strength compared to alkali activated bricks. As a result, the BCB’s ability to withstand bending and flexural stresses is compromised. The presence of a denser microstructure plays a pivotal role in augmenting resistance to cracks and elevating the overall flexural strength of these bricks [71]. These findings underscore the superior flexural behavior exhibited by alkali-activated bricks when compared to their traditional burnt clay bricks.

3.5 Compressive strength of the joining material/mortar

The compressive strength of masonry mortar refers to its ability to resist compressive forces or loads without failure. It is an important property that influences the overall structural integrity and performance of masonry construction. The compressive strength of cement masonry mortar and alkali activated masonry mortar is typically determined through laboratory testing, where mortar samples moulded and cured for 28 days. After curing the samples are subjected to compressive loading until failure occurs. The test results provide an indication of the maximum load or stress that the mortar can withstand before it breaks or experiences significant deformation [77]. The results on compressive strength of cement masonry mortar and alkali activated masonry mortar shows that the compressive strength of cement masonry mortar is 4.94 MPa and alkali activated masonry mortar is 6.65 MPa. The compressive strength of cement masonry mortar was lower than alkali activated masonry mortar as similar to the previous results [78, 79].

3.6 Compression bond strength

Enhancing compressive bond strength carries several positive implications for the performance of walls in construction. It improves structural integrity, load-bearing capacity, durability, resistance to settlement, lateral forces, and overall construction efficiency. Overall, this contributes to the safety, longevity, and functionality of the building as a whole. The compression bond strength of the brickwork is dependent on the type of brick and the type of mortar [5]. Bricks capacity to withstand compression ensures the integrity of the masonry’s load-bearing capacity. Hence, bricks with superior compressive strength establish a sturdy foundation for the entire masonry structure. Additionally, mortar with substantial compressive strength facilitates the effective transmission of compressive loads, ultimately enhancing the overall bond strength of the masonry assembly. Previous studies affirm that an increase in mortar strength positively impacts masonry strength [80, 81]. In this study, significant differences in the strength was noted between cement mortars and alkali-activated mortars (Sect. 3.5). On the 28th day of air curing, bond strength was tested for two brick prism configurations, one composed of Alkali activated bricks and the other of burnt clay bricks, each paired with alkali activated mortar and cement mortar. Three masonry prisms from each setup underwent compression tests. As shown in Fig. 10, the compressive bond strengths of these masonry prisms were as follows: 6.45 MPa for combination A2, 4.9 MPa for A1, 10.23 MPa for B2, and 7.86 MPa for B1. Observations in Figs. 8 and 9 revealed identical patterns of cracks under compression loading of the prism setup for both A1 and A2. Additionally, B1 exhibited diagonal cracks, while B2 prism displayed a vertical crack spreading from the left face to the right face under compression loading. These findings highlight the masonry behavior through cracks due to compression load.

Fig. 8
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Failure of Alkali Activated Brick Bonded Prism in Compression a Prism - B2 b Prism - B1

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Fig. 9
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Failure of Burnt Clay Brick Prism in Compression a Prism - A2 b Prism - A1

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Fig. 10
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Bond Strength of Different Combinations of Unit and Mortar under Compression Load

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There are also several factors contributing to the reduction in bond strength, including interface irregularities, material disparities causing stress concentrations, localized loads at the interface and the complexity of mortar joints. The results also suggested that bond strength of B1 and B2 was superior to A1 and A2 combinations due to better adhesion of alkali activated materials to masonry units compared to clay or cement based materials [17]. The difference in the material composition of the unit phase material and motor phase material impact the strength of the masonry prisms. A2 exhibits higher compressive strength than A1 because of improved bonding with alkali-activated mortar. Also the change in the properties of mortar influence the compression bond strength. Thus the higher compressive strength of the alkali activated mortar compared to cement mortar helps to increase the bond strength of the masonry prisms. It’s worth noting that the individual elements of a masonry structure, such as bricks and mortar, typically exhibit higher compressive strengths than the bond strength between them. The compressive strength was found to be higher in B2 than B1 due to the alkali activated brick’s superior bonding with alkali activated mortar. This bond allows the alkali activator in the mortars to easily penetrate the unit surfaces of the units, leading to a stronger chemical bond at the interface. In contrast, burnt clay bricks with larger pores and lower density absorb more liquid from mortar, reducing joint surface area and compressive strength. This enhanced adhesion contributes to the overall compressive bond strength of the alkali-activated masonry system [82]. Furthermore, the microstructure of alkali-activated materials contributes to their improved bond strength [83]. It results in dense compact structure with smaller pores, improving interlocking and bonding between masonry units by reducing crack propagation ultimately strengthening the masonry system.

3.7 Flexural bond strength

Enhancing the flexural bond strength in masonry walls proves advantageous, particularly in scenarios where the walls face bending or lateral forces like wind, seismic pressures, and structural movements. Elevating the flexural bond strength equips the wall to effectively withstand these real-world circumstance of forces, ultimately enhancing its stability and structural integrity. Higher flexural strength equips walls to resist external forces without cracking or failing, ensuring the safety and longevity of the constructed building. This characteristic is crucial for determining the effectiveness of a masonry assembly’s ability to withstand loads that force it to bend. This investigation employs a five-brick prism to compare the ability of various masonry bond made of different unit type and mortar type to withstand flexural forces following prior researcher [84]. In the present study, four different masonry prisms are used to evaluate the flexural bond capabilities. It is observed from Fig. 11 that, A1 and A2 masonry unit mortar combinations masonry prisms showed mortar failure and cracking of brick and bond respectively [85]. While in Fig. 12, B1 showed solely bond failure, and the prism with B2 combination showed both brick and bond cracking. From Fig. 13, the average flexural bond strength for B1 was 0.09 MPa as compared to B2 that is 0.16 MPa. The average flexural bond strength for A2 masonry prism was determined to be 0.34 MPa, compared to 0.21 MPa of A1. This comparison shows that cement mortars are significantly less able to handle flexural loads than alkali activated mortars. It is worth noting that, flexural strength of the individual masonry bricks is found to be higher than the flexural bond strength of the prisms due to the uneven load distribution through the brick and mortar and other factors. In this study, the flexural strength of the alkali activated brick prism bonded with alkali activated mortar (B2) exhibited twice the flexural strength than the clay brick prism bonded with alkali activated mortar (A1).

Fig. 11
figure 11

Failure of Burnt Clay Brick Prism in Flexure a Prism - A2 b Prism - A1

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Fig. 12
figure 12

Failure of Alkali Activated Brick Bonded Prism in flexure a Prism - B2 b Prism - B1

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Fig. 13
figure 13

Bond Strength of Different Combinations of Unit and Mortar under Flexural Load

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The results indicate that the bond strength exhibited by combinations B1 and B2 surpassed that of A1 and A2. A2 demonstrates higher flexural strength compared to A1. This is due to the use of alkali-activated mortar. The bonding of different material composition in unit phase and mortar phase doesn’t allow proper bonding of materials with each other losing the flexural strength of the prism. Where, in combination A2, the superior properties of alkali activated mortar with burnt clay brick enhances the flexural stability of the prism by the individual properties of the mortar. The superior flexural bond strength observed in B1 and B2 combinations can be linked to the unique properties of alkali-activated materials. These materials have shown an exceptional ability to adhere effectively to the surfaces of masonry units. This enhanced adhesion arises from the chemical interactions and denser microstructure between alkali activators in the mortar and the masonry unit’s surface. Thus increasing the resistance to flexural load. Here B2 combination masonry prisms have superior strength due to the enhanced bonding between alkali activated brick and alkali activated mortar. This enhanced flexural strength is due to the reduced porosity and strong chemical bonds improving the integrity of the system.

3.8 Tension bond strength

Strength of the walls under tension loads are crucial for withstanding tensile stresses that arise from causes like differential settlement, temperature changes, or external forces. Increased tensile strength increases resistance against tensile loads and crack development, contributing to the durability of the wall. Tension in masonry walls can arise due the applied loads acting on the structure and connected other sections of the building. These loads can include vertical loads from the weight of the structure, horizontal loads from wind or seismic forces, or any other external forces acting on the masonry wall. Tensile stresses develop within the mortar or grout to resist applied loads. Tension can also occur in masonry joints when there is differential movement between adjacent masonry units or structural elements [86]. Differential movement can be caused by factors like settlement, thermal expansion and contraction, or moisture-related changes. These movements can induce tensile stresses within the mortar or grout, potentially leading to cracking or joint failure if not properly accommodated. In the present study, two types of bricks are bonded with two types of mortar. In the case of compression bond area, the tension bond prism exhibits a lower bond area between the bricks bonded with mortar. In Fig. 14 it is observed that the both brick and mortar failure was observed in AAB-AAM (B2) prisms and bond failure in AAB-CM (B1) prism while Fig. 15 shows bond failure in both A1 and A2 combinations. Figure 16 presents the average tension bond strength values for all the type of prisms made of all the combinations of brick and mortar. As a result, lower strength in tension will be withstood compared to compression and flexural bond strength [87]. The test was performed according to Sect. 5 of this paper. Three masonry prisms were tested for each combination of prism.

Fig. 14
figure 14

Failure of Alkali Activated Brick Bonded Prism in Tension a Prism - B2 b Prism - B1

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Fig. 15
figure 15

Failure of Burnt Clay Brick prism in tension a Prism - A2 b Prism - A1

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Fig. 16
figure 16

Bond Strength of Different Combinations of Unit and Mortar under Tension Load

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Comparing the tensile strengths between cement mortar and alkali activated mortar bonds, it was observed that the B2 combination specimen outperformed B1. B2 combination could withstand higher tensile strengths with 0.09 MPa as compared to B1 with tensile strength of 0.03 MPa. While A2 had an average tensile bond strength of 0.041 MPa and A1 as 0.024 MPa strength. Alkali-activated materials have shown potential in various applications, the specific performance in terms of tension bond strength may vary depending on multiple factors such as material composition, curing conditions, and construction practices. A2 combination specimen exhibited double the tension strength of A1, this is due to the fact of improved adhesion property of alkali activated blocks in A2 masonry prism. This bonding of the materials enhances the tension bond strength of the masonry prism. Similarly, B2 combination specimen with alkali activated brick–alkali activated mortar showed superior tension strength than B1. The tension strength difference between A2 and A1 masonry is remarkable, threefold, primarily stemming from variations in material composition. The use of similar materials in both unit and mortar phases contributes to the substantial increase in tensile bond strength.

3.9 Shear bond strength

Walls must have strong shear points, especially in seismically active areas. It ensures that the wall can withstand lateral or shear forces, as those produced during an earthquake. A lower likelihood of structural failure or collapse during seismic events results from higher shear bond strength, which translates to stronger resistance against shear stresses. In the present study, to test the shear bond strength capabilities of two different bricks with two different mortar, a three-brick prism arrangement was employed. From Fig. 17, we observe B2 prism and B1 mortar prism, the bricks got detached from both the bond in mortar failure. And from Fig. 18, masonry prisms made of burnt clay brick and two different mortar shows that A2 failed both in bond and brick while the A1 bond prism failed in bond due to shear stress.

Fig. 17
figure 17

Failure of Alkali Activated Brick Prism in Shear a Prism - B2 b Prism - B1

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Fig. 18
figure 18

Failure of Burnt Clay Brick Prism in Shear a Prism - A2 b Prism - A1

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Fig. 19
figure 19

Bond Strength of Different Combinations of Unit and Mortar under Shear Load

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Figure 19 reveals the average shear strength of A2 masonry was 0.56 MPa while that of A1 bond masonry was 0.45 MPa. Similarly, B2 and B1 showed 1.11 and 0.73 MPa respectively. The increase in the shear bond strength is due to good adhesion between the mortar and masonry units ensures a strong bond at the interface, allowing the transfer of shear stresses. B2 has the higher shear strength among all the prism combinations. This is because, the adequate adhesion prevents slippage or separation between the mortar and units, contributing to higher shear bond strength [88]. The mechanical interlocking between the mortar and masonry units enhances shear bond strength [89]. Interlocking occurs when the mortar fills the voids and irregularities on the surface of the units, creating a strong physical connection. This interlocking mechanism resists shear forces and improves the overall shear bond strength. As shear forces are applied, the friction between the contacting surfaces resists the sliding or movement of the units relative to the mortar. Higher frictional forces lead to increased shear bond strength. This increased contact area improves the load transfer capacity and enhances shear bond strength.

4 Conclusions

  • Alkali activated bricks developed exhibits enhanced characteristics like high density, compressive strength, flexural strength, and water absorption as compared to clay bricks.

  • Alkali activated mortar exhibited greater bonding with both clay as well as alkali activated bricks when compared to cement mortar.

  • Alkali activated brick with alkali activated mortar demonstrated best performance amongst the combination tested for bond strength qualities such as compressive strength, shear strength, flexural strength, and tensile strength.

  • Alkali activated brick with alkali activated mortar had the highest bond strength, followed by Alkali activated brick with cement mortar, clay bricks with AAFA mortar, and clay bricks with cement mortar had the lowest bond strength .

  • Building a masonry structure using similar materials, both in masonry unit phase and the mortar phase helps the construction look more uniform and perform better by increasing the characteristic strength.

  • Masonry structures made by bonding Alkali activated brick with alkali activated mortar technology promotes sustainable buildings exhibiting improved structural performance and lower carbon emissions.

  • Present study focuses on short-term mechanical properties. Hence leaves room for further investigation into the long-term durability and performance of these materials under varying environmental conditions.

  • A comprehensive environmental impact assessment, including a life cycle analysis, can be done for further advancing sustainable construction practices.