27 Al NMR MAS Spectral Studies Inferring the Initiation of Geopolymerization Reaction on Together Mechanochemical Grinding of Raw Materials Pooja Bhardwaj, a,b Rainy Gupta, a,b Deepti Mishra , b * Manish Mudgal b and Sudhir Sitaram Amritphale b a Academy of Scientific and Innovative Research-AcSIR-AMPRI, Bhopal, India b Materials for Radiation Shielding and Cement-Free Concrete Division, CSIR-AMPRI (Advanced Materials and Processes Research Institute), Hoshangabad Road, Bhopal, M.P., 462064, India (Received: April 29, 2017; Accepted: November 4, 2017; DOI: 10.1002/jccs.201700149) Chemical shifts and intensities of the 27 Al NMR signals provide structural information about the environment of Al nuclei in presence of an external magnetic field. This paper analyzes the struc- tural information of the aluminum nuclei present in the precursor material after mechanochemical co-grinding of the raw materials, namely fly ash, NaOH, and amorphous tricalcium phosphate [Ca 3 (PO 4 ) 2 ], with the help of 27 Al MAS NMR spectral studies. The results indicate transformation of the sixfold coordination Al ions with oxygen AlQ6 present in aluminosilicate material fly ash to fourfold AlQ4 and fivefold AlQ5 in the precursor material. The variation in chemical shift is between δ 64 and 65 ppm. This indicates that, in addition to direct bonding to the oxygen atom, the Al tetrahedron is also bonded to Si as [AlQ4(4Si)]. Thus, the mechanochemical co-grinding of the raw materials initiates a solid-state chemical reaction among them. The addition of water alone to this precursor material results in the formation of the geopolymeric material unlike the conventional geopolymeric system which requires the addition of a highly alkaline aqueous solu- tion to fly ash. This study helps in the determination of the reaction mechanism during the mecha- nochemical transformation of raw materials into the geopolymeric product by a novel process. Keywords: Chemical shifts; Grinding; Coordination; Conventional geopolymeric materials; Shielding effects. INTRODUCTION 27 Al magic angle spinning NMR (MAS-NMR) has proven to be a very informative technique for struc- tural studies of silicates and aluminosilicates. It pro- vides a deeper understanding of their structural properties as well as the process of formation of alumi- nosilicate materials such as geopolymers, 1,2 natural abundance of 100%, and the half-integral value of the nuclear spin moment 5/2 because 27 Al nuclei are NMR- sensitive quadrupole nuclei. The quadrupolar moment of 27 Al nuclei is 0.15 b and the gyromagnetic ratio Υ/2π is 11.10 MHz/T. Short relaxation time makes the 27 Al NMR very sensitive, which can detect even traces of alu- minum. 27 Al NMR correlates the chemical shift to the coordination of the aluminum atoms present in the mate- rial. The MAS-NMR spectrum of solid samples reduces the dipolar interactions and chemical shift anisotropy. The high field reduces the second-order effects and maxi- mizes the chemical shift. The nonequivalent chemical environment increases the width of the NMR signal. Fly ash, a waste product of coal combustion, minerologically consists of quartz (SiO 2 ), mullite (3Al 2 O 3 Á2SiO 2 ), and hematite as the major crystalline phases and aluminosilicates as the amorphous part. The aluminosilicates present in fly ash actively participate in the geopolymerization process, and 27 Al NMR spectro- scopic studies have proven to be an informative tool for the structural elucidation of the aluminosilicate species in fly ash and fly ash-based geopolymers. 3,4 Aluminosili- cates are ionic compounds with aluminum in the form Al 3+ and oxygen in the form of O 2- ions. Two aluminum atoms donate six electrons to satisfy the three oxygen atoms and to form an ionic bond. Thus ratio of Al 3+ to O 2- is 2:3, giving rise to the empirical formula Al 2 O 3 . *Corresponding author. Email: deeptimishrampri@gmail.com J. Chin. Chem. Soc. 2017 1 © 2017 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim JOURNAL OF THE CHINESE CHEMICAL SOCIETY Article