In monument preservation, the importance of restoring historic buildings is increasingly recognized, with scientific analysis of original materials forming the basis for conservation strategies. Pakistan, particularly Sindh, a region steeped in history, is home to various ancient sites and architectural wonders, many of which are several centuries old [1]. Sindh is home to the highest density of historical monuments and cities, typically built of stone, brick, limestone and mud; However, these cultural monuments are experiencing significant deterioration and require immediate conservation measures. The deterioration is primarily due to the decomposition of binders such as mortar and grout [2]. These materials are among the most vulnerable components of historic buildings and are often the first to suffer damage from natural weathering, corrosive environments and natural disasters. Therefore, they need to be replaced regularly [3], [4]. Nevertheless, most researchers point to the frequent use of incompatible restoration materials, which exacerbate this deterioration and contribute to further damage to historical structures [5]. An incompatible material is a material whose chemical composition, physical properties and mechanical behavior do not match well with the original materials. This can cause damage or accelerate the deterioration of the historic masonry [4]. This incompatibility often occurs when renovation materials have higher compressive strength and lower porosity than the original mortar or brick, resulting in internal stresses and accelerated decay [6]. Therefore, the selection of restoration materials that are as close as possible to the original in terms of their chemical, physical and mechanical properties is crucial to ensure the long-term preservation of historical buildings.
In the past, masonry construction relied heavily on lime mortar, which was typically made directly on the construction site. However, the inherent limitations of lime mortars, particularly their long setting and curing times at high relative humidity, presented significant challenges in application [7]. As a result, lime mortar was gradually replaced by cement-based material. Although cement mortars are widely used, they are criticized for their high coefficient of thermal expansion, brittleness, and excessive strength, all of which contribute to their incompatibility with historic masonry structures [6], [8]. These limitations highlight the need to develop an alternative repair material that achieves the technical performance of traditional mortar while providing improved durability and compatibility with traditional materials [9].
The development of compatible repair mortars remains a complex and challenging endeavor, primarily due to the limited availability of scientific data to effectively inform restoration practices [10]. Ongoing research aims to develop repair materials that replicate the physical and mechanical behavior of original mortars. However, their successful implementation depends critically on a detailed understanding of the existing materials within the specific historical structure. This initial diagnostic phase is important for understanding the intrinsic properties of the current materials [11]. Nevertheless, the high cultural and historical value of such buildings often precludes the use of destructive testing methods for evaluation. Therefore, minimally destructive and non-destructive testing (NDT) techniques are increasingly preferred for material evaluation and condition assessment [12]. Effective selection of restoration materials must therefore be based on a rigorous, science-based methodology that integrates in situ NDT with comprehensive laboratory analyzes of historic and proposed repair materials. This combined approach increases diagnostic accuracy and ensures compatibility, performance and long-term durability of interventions within the historic structure [13].
Recent advances in destructive and non-destructive techniques have significantly improved the ability to carry out detailed analysis of existing construction materials, enabling comprehensive assessment of mineralogical, morphological, petrographic, chemical and physical properties. Numerous researchers [14], [15], [16] have adopted these advanced methods to support the restoration of historical structures through systematic material characterization. Therefore, techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) are often used to assess the composition and uniformity of historical materials. For example, Moropoulou et al. [2] investigated the causes of decay of historical structures using XRD, optical microscopy and mercury intrusion porosimetry. Similarly, Casadio et al. [17] characterized mortars containing carbonate aggregates using wet chemical methods, manual disaggregation and digital image analysis of thin sections. Gleize et al. [18] extended this line of investigation by examining mortars from nine historic buildings dating from 1750 to 1922 using XRD and TGA. In another study, Sandrolini et al. [19] conducted detailed evaluations of ancient mortars and plasters through physical separation of components, moisture analysis and characterization using XRD and SEM. Furthermore, Moropoulou et al. [20] applied a multitechnical approach including optical microscopy, mercury intrusion porosimetry, calcimetry, SEM and XRD to analyze materials from one of the oldest monastic complexes in South America.
On the non-destructive side, widely used techniques include the surface hardening method using a rebound hammer to estimate near-surface strength [21] and penetration resistance tests, such as the Windsor probe to assess compressive strength [22]. Accordingly, ultrasonic pulse velocity (UPV) is used to evaluate material homogeneity, discontinuities and strength [23], [24]while thermal imaging is used to detect problems related to consolidation and moisture distribution [25], [26]. Additionally, combined approaches, such as integrating Schmidt hammer testing with penetration resistance methods, offer efficient alternatives to core drilling for on-site strength assessment [27], [28].
Although the South Asian region is home to vast historic architecture, it faces significant and persistent conservation challenges. Most World Heritage sites are falling into disrepair due to inadequate maintenance and lack of expertise in the area [29]. This decline occurs despite the proven effectiveness of both non-destructive and destructive testing methods to evaluate such historic structures [30], [31]. These techniques are particularly important in this region, where damage assessments often do not have the depth required for sustainable conservation [32] and where the integration of scientific analysis into practical conservation efforts remains inconsistent [33], [34].
Systematic assessment of historical materials prior to the restoration of ancient sites is essential to ensure long-term compatibility and durability. However, there are still few such systematic evaluations in Pakistan, particularly for sites such as the Makli Necropolis, which is of immense historical importance worldwide. As one of the largest burial complexes in the world and a UNESCO World Heritage Site, Makli represents a compelling but under-researched case where the lack of scientific data has often led to the use of incompatible repair materials. To fill this critical gap, the present study provides a comprehensive characterization of mortar, plaster and brick samples from three representative monuments of the Makli Necropolis, namely Ameer Khani Qabristan, Meeran Bai and the M-24 monument. According to current knowledge, this is the first systematic characterization of historical mortar, plaster and bricks from the Makli necropolis. Through integrated physical, mechanical, chemical, mineralogical and microstructural analyses, this work establishes a region-specific baseline of the original materials and provides new insights into their composition and current condition. This data is critical for guiding restoration strategies and developing compatible repair materials. This fills important gaps in the existing literature and supports evidence-based conservation practices.
Makli, also called Makli Necropolis, is one of the largest and most important burial sites in the world and is located in Thatta, Pakistan. It features historical structures that are 500 to 600 years old and cover an area of approximately 10 square kilometers. It houses between 500,000 and 1 million graves, mausoleums and tombs of saints, poets, writers, nobles, governors, ministers, princes, kings and queens from the 14th to 18th centuries [35]. The site embodies Sindh's rich cultural, architectural and historical heritage with monuments displaying a mix of Islamic, Persian, Hindu and Mughal influences, indicating a diverse range of historical building materials and construction techniques that require in-depth scientific study [29]. Declared a UNESCO World Heritage Site in 1981, Makli is a testament to the region's historical significance and enduring legacy [36]. The intricate stone carvings, large domes and ornate tile work on these tombs highlight the artistic sophistication of the period and make Makli not just a burial site but a monumental museum of architecture and history [37].
The monuments at Makli are arranged in reverse chronological order and generally extend from the southern to the northern area of the necropolis. These structures are constructed primarily of brick and stone and are often decorated with glazed tile decoration. Nevertheless, significant deterioration occurred due to environmental influences, material degradation and human intervention. These historic buildings show clear signs of weathering, biological growth and structural instability, exacerbated by improper restoration practices. In particular, the use of cement-based materials and traditional restoration techniques has exacerbated this deterioration by introducing compatibility issues. Like all UNESCO World Heritage Sites, Makli has an officially monitored and updated management plan; However, the specific conservation challenges observed at the site, such as material degradation, weathering and the incompatibility of previous restoration materials, are only partially addressed within this framework. The aim of this study is to provide detailed material characterization and propose tailored repair solutions that ensure both material stability and long-term preservation of the necropolis, contributing to informed conservation planning for Makli and similar cultural monuments.