Morphology and BET surface area of phosphoric acid stabilized tropical soils Amin Eisazadeh a, ⁎, Khairul Anuar Kassim a , Hadi Nur b a Geotechnic & Transportation Department, Faculty of Civil Engineering, Universiti Teknologi Malaysia, Malaysia b Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, Malaysia abstract article info Article history: Received 16 June 2011 Received in revised form 12 December 2012 Accepted 14 December 2012 Available online 4 January 2013 Keywords: Kaolinite Laterite clay Phosphoric acid Stabilization Morphology Surface area It has been well established that the treatment of natural soil with chemical additives will gradually affect the size, shape, and arrangement of soil particles. Furthermore, the degree of improvement is dependent on the quantity and the pattern of new formed products deposited on and around the clay particles. In this paper, the time-dependent changes induced in the morphology and the surface area of phosphoric acid treated ka- olinitic soils were studied using field emission scanning electron microscopy (FESEM) and Brunauer, Emmett and Teller (BET) surface area analysis method, respectively. Unconfined compressive strength test as an index of soil's improvement was also performed on cured samples. The results indicated that with the pro- gression of curing time, the vigorous action of acid had transformed the soil structure into a more weathered composition consisting of sharper edged particles. Furthermore, at 8 months curing period, the formations of white cementitious compounds in form of white lumps was evident. Based on the N 2 -BET results, it was found that the presence of iron oxides as part of laterite clay's secondary constituents (micro-aggregates), contributed to obtaining higher surface area values. These oxides were also believed to be the main cause of strength development at the early stages of curing. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The use of chemical agents other than lime to improve clay soils was introduced when Carroll and Starkey (1971), clay mineralogists, published their findings on reactivity of clay minerals with acids and alkalis. In recent years, there has been a proliferation of literature regard- ing the application of non-calcium based stabilizers for soil stabiliza- tion. As reported in previous studies, phosphoric acid stabilization is a potentially attractive alternative for treating lateritic soils (Lyons and McEwan, 1962; Sutton and McAlexander, 1987; Medina and Guida, 1995). This is due to the reaction of phosphoric acid with free iron and aluminum oxides present in the soil environment (Winterkorn, 1962). On the other hand, due to an extensive variability in the amount of impurities dictated by extreme weathering conditions and also the acidic nature of tropical kaolinitic soils, the success of alkaline stabilizers in improving the engineering properties of these soils has not been promising (Eisazadeh, 2010). Scanning electron microscopy (SEM) is a common technique used to determine the micro-structural properties of soil fabric, providing information on the size, shape, and the state of orientation and aggre- gation of soil particles. This method could be implemented in soil sta- bilization studies, in order to visualize the topographical features and moreover to observe the formation of new cementitious materials that are hard to detect using other analyzing techniques. Nonetheless, in contrast to lime treatment (Rajasekaran and Narasimha Rao, 1997), the researches carried out regarding the morphological changes asso- ciated with the use of non-traditional stabilizers such as phosphoric acid have been rather limited. The specific surface area is an important characteristic in deter- mining the chemical and physical interaction of the soil with its sur- roundings. This is due to the fact that most of the chemical reactions in soils take place at the surface of particles (Mitchell and Soga, 2005). One of the most common methods for determining the surface area of finely divided materials is that of Brunauer et al. (1938), known as the BET method. The method is based on collecting isotherm data for the physical adsorption of an inert gas and modeling the adsorp- tion data using the following BET isotherm equation (Rauch et al., 2003): ν=ν m ð Þ¼ cP=P 0 ð Þ= 1- P=P 0 ð Þ 1 þ c-1 ð Þ P=P 0 ð Þ ½  ð g f ð1Þ where: ν volume of gas adsorbed per unit weight of clay at a pressure P ν m volume of gas adsorbed for monolayer coverage (P/P 0 ) partial pressure of the gaseous adsorbate c constant. With regard to the importance of the research, the phosphoric acid treatment may be introduced as an alternative to the current lime Engineering Geology 154 (2013) 36–41 ⁎ Corresponding author. Tel.: +60 177080315; fax: +60 75566157. E-mail addresses: A.Eisazadeh@yahoo.com, aeisazadeh@utm.my (A. Eisazadeh). 0013-7952/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enggeo.2012.12.011 Contents lists available at SciVerse ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo