Intelligently designed fly-ash based hybrid composites with very high hardness and Young’s modulus Dipak Kr Chanda a , Subhro Roy Chowdhury b , Manjima Bhattacharya a,c , Ashok Kumar Mandal a , Nitai Dey a , Anoop Kumar Mukhopadhyay a,⇑ a Advanced Mechanical and Material Characterization Division, CSIR-Central Glass and Ceramic Research Institute, Kolkata 700 032, India b Department of Metallurgical and Materials Engineering, National Institute of Technology, Durgapur 713209, India c Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata 741246, India 1 highlights  Alkali activated samples prepared from mixture of 80 wt% fly ash (FA), 18 wt% WCC and 2 wt% chopped glass fibre.  Synthesized alkali activated samples showed a characteristic load independence of nanohardness and Young’s modulus.  Unique strain tolerance behaviour. article info Article history: Received 24 June 2017 Received in revised form 4 October 2017 Accepted 10 October 2017 Keywords: Alkali activated fly ash Nanoindentation Nanohardness Young’s modulus abstract Currently, India generates annually about 112 million tones of fly ash (FA), as an industrial waste from thermal power plants. As part of the global journey to convert waste to wealth here we report the intel- ligent design based synthesis of FA based hybrid composites with spectacular improvement in Young’s modulus and nanohardness. The novel design approach utilized alkali activation as well as simultaneous reinforcements of the porous FA matrix with a layered white china clay (WCC) and chopped E glass fiber. The developed materials were subsequently characterized by nanoindentation technique, pH measure- ment, alkali dissolution, FESEM, etc. techniques to evolve the structure-property correlation. The opti- mized design and optimal alkali activation lead to achievements of about 233% and 545% enhancements in Young’s modulus and hardness, respectively. These results are rationalized in terms of chemical analysis, Si:Al ratio, presence of silicate network modifiers e.g., Na 2 O and CaO, microstruc- ture, density, extent of polymerization due to alkali activation, processing condition and elastic recovery as well as the ratio of energy spent in elastic and plastic deformations during the nanoindentation pro- cesses. Finally, a schematic model is proposed to explain the experimental observations. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The global research on alkali activated materials (AAMs) e.g., geopolymers is the call of the day to reduce the carbon foot prints due to green house gas emissions from the cement industry [1,2]. The quest for better alkali activated materials encompass low cost fly ash, clay, rice husk ash etc. as raw materials [3–9]. Even very recent efforts [10–14] tried to focus on the basic aspects of alkali activation of a wide variety of starting raw materials, geopolymer formation and its strengthening and/or toughening through use of various reinforcements. For instance recent efforts were directed to develop metakaolin based geopolymers using various alumi- nosilicate species [10]. During the process of geopolymerization, the nanostructural evolution of the aluminosilicates and the corre- sponding ability of dissolution in different aqueous silicate species were studied. In the process, the dissolution to release aluminum was categorized by the gradual conversion of 4- to 6-coordinated aluminum, which was further followed by the formation of alumi- nosilicate oligomers near the metakaolin particle surface with sub- sequent condensation [10]. On the other hand, Ca and Al reinforcements were also used in a blast furnace slag and fly ash based geopolymer system [11]. Alu- minosilicate glasses were used during the process and were exposed to different activator solutions in a continuously stirred closed system reactor for a period of 3 h. The presence of Ca https://doi.org/10.1016/j.conbuildmat.2017.10.049 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Central Glass and Ceramic Research Institute, 196, Raja S.C. Mullick Road, Kolkata 32, India. E-mail address: anoopmukherjee@cgcri.res.in (A.K. Mukhopadhyay). 1 Current address. Construction and Building Materials 158 (2018) 516–534 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat