RESEARCH ARTICLES CURRENT SCIENCE, VOL. 119, NO. 8, 25 OCTOBER 2020 1300 *For correspondence. (e-mail: swathymanohar@yahoo.com) Characterization of historic bricks and binder at Vat Phou World Heritage Site in Lao PDR and selection of compatible replacement units for restoration Swathy Manohar*, Shivangi Shukla, Arun Menon and Manu Santhanam Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600 036, India This article presents results of material characteriza- tion studies conducted on historical clay bricks and binder sampled from the brick masonry gallery of the southern quadrangle of the World Heritage Site of Vat Phou in Champasak, Province of Lao PDR. The primary objective of the sampling was to conduct micro-analytical laboratory studies on the binder in the brick masonry to establish the nature of the bind- ing mortar used and to verify the presence of lime in it, if any. In addition, micro-analytical and physico- mechanical tests conducted on the historic brick sam- ples were used to establish compatibility with the newly manufactured replacement bricks for use in the restoration of the dilapidated brick gallery. X-ray diffraction technique was used to study the mineral composition in the system, and scanning electron microscopy provided images indicative of the material and binder characteristics. The study revealed that lime was not present in the binding mortar and it could possibly be a natural wood resin. The use of natural resins as a binder in masonry has been a popular theory in Southeast Asia, and these results provide first empirical proof of their use. This study also demonstrates how compatibility checks can be carried out between historic and new replacement bricks through analysis of their pore structure and physical properties. In addition, the test results estab- lished that lime was not a component of the ancient binder, which together with the evidence of the resin- based binder provides an important direction for res- toration efforts of historical sites in the Mekong River Basin region covering countries such as Lao PDR, Cambodia and Vietnam. Keywords: Binder, heritage conservation, historic bricks, restoration efforts. VAT PHOU is a UNESCO World Heritage Site, a Khmer Hindu temple complex located in southern Laos, at the base of Mount Phou Khao. Though the original temple at the site was built in the early 5th century, current elements in the vast complex are reported to be con- structed during the 11th–13th century, and are now in a partially dilapidated condition 1 . The temple complex has different structures made of various materials like later- ite, sandstone, clay bricks, etc. major among them being the sanctuary, northern quadrangle and southern quadran- gle. Figure 1 shows the plan of monuments in the Vat Phou site 2 and a panoramic view of the site. Various parts of the heritage site are undergoing extensive restoration for the past two decades 3–5 . The southern quadrangle marked with a rectangle at the bottom right corner in Figure 1 a is the focus of the present study. Ensuring efficient and optimized restoration protocols from a scientific perspective of understanding the mate- rials are necessary in the field of built heritage conserva- tion 6,7 . The International Council of Monuments and Sites (ICOMOS) has specified in its guidelines on conservation practices that information on the structure in its earlier and current state, construction techniques used, repairs in the past and the phenomena (deterioration mechanisms) that the material underwent shall be inferred in detail 8 . Compatibility between new and old materials is a key factor to consider during restoration works in line with internationally accepted principles of guaranteeing that the values of authenticity and historicity are not compro- mised. In this regard, ICOMOS suggests ‘the choice between traditional and innovative techniques should be weighed up on a case-by-case basis and preference given to those that are least invasive and most compatible with heritage values, bearing in mind safety and durability requirements’ 8 . The International Scientific Committee on the Analysis and Restoration of Structures of Archi- tectural Heritage (ISCARSAH), which is an international scientific committee founded by ICOMOS, has also spe- cified the significance of compatibility between tradition- al/original materials and new materials in conservation. Compatibility shall be ensured with respect to masonry units and binders. It is mentioned that the choice of fluid mortars, which include resins, lime, cement, etc. for injec- tion into the existing masonry for repair works, should depend upon the characteristics of existing materials in