495 Unsaturated Soils: Theory and Pracice 2011 Joisankasa, Sawangsuriya, Soralump and Mairaing (Editors) Kasetsart University, Thailand, ISBN 978-616-7522-77-7 ABSTRACT: In this paper, the time-dependent changes induced in thermal characteristic of phosphoric acid stabilized tropical soils comprised mainly of kaolinite mineral were investigated. From thermal gravimetric analysis it was evident that evaporation of the adsorbed surface water was the main reason responsible for the weight losses observed in the lower temperature regions. In addition, a loss at around 300ºC in 8 months cured samples was evident. The latter was tentatively assigned to the evaporation of moisture encapsulated into the reaction products. Also, it was found that in laterite clay samples, the phosphoric acid treatment had a noticeable impact on the dehydroxylation process of kaolinite minerals. KEYWORDS: Tropical soil; Phosphoric acid stabilization; TGA 1 INTRODUCTION Thermal analysis involves a dynamic phenomenological approach to the study of soils by observing its response to a change in temperature. The results for this type of analysis can be obtained in three different ways, i.e., thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and derivative thermal gravimetric (DTG) analysis. For clays, endothermic reactions involve desorption of surface H 2 O, dehydration at low temperatures, dehydroxylation and eventually melting at more elevated temperatures. Exothermic reactions are related to recrystallization at high temperatures that may be nearly concurrent with or after dehydroxylation and melting (Guggenheim and Koster van Groos, 2001). Throughout these years considerable research has been carried out in studying the effect of traditional stabilizers such as lime on various properties of the soil (Locat et al., 1990; Bell, 1996; Narasimha Rao and Rajasekaran, 1996). However, the use of phosphoric acid as a stabilizer has been limited. In the present paper an attempt was made to study the changes induced in thermal properties of the phosphoric acid stabilized tropical soils at different time intervals. 2 MATERIALS & EXPERIMENTAL PROGRAM 2.1 Materials Two soil types formed under extreme weathering conditions were chosen for this study. The first type was the slightly acidic White Kaolin (WK) sample. The soil was dominated by kaolinite and illite minerals along with minor constituents of quartz (Eisazadeh, 2010). Also, a reddish laterite clay (LC) rich in iron oxide excavated from a hillside located in Southern part of Malaysian Peninsular was used in this investigation. The morphology of natural soils is shown in Fig. 1. The physical properties and chemical composition of the untreated samples are also presented in Table 1. It should be noted that the phosphoric acid used in this research was a Merck analysed, 85% H 3 PO 4 , of specific gravity 1.71. 2.2 Preparation of specimens According to the previous studies conducted on phosphoric acid stabilization of clayey soils (Demirel et al., 1962; Medina and Guida, 1995), different amounts of phosphoric acid were selected for each individual soil. Samples were then compacted into a thin wall PVC tubes (50 mm diameter × 100 mm length) under constant compactive effort as specified in clause 4.1.5 of BS 1924: Part2 (BSI, 1990). They were sealed to the Thermal characteristics of phosphoric acid treated tropical soils A. Eisazadeh & K.A. Kassim H. Nur Geotechnic & Transportation Department, Faculty of Civil Engineering, Universiti Teknologi Malaysia, Johor, Malaysia, aeisazadeh@utm.my, kanuar@utm.my Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, Johor, Malaysia, hadi@kimia.fs.utm.my