Contents lists available at ScienceDirect Journal of Building Engineering journal homepage: www.elsevier.com/locate/jobe Effects of rice straw ash and micro silica on mechanical properties of pavement quality concrete Arunabh Pandey * , Brind Kumar Department of Civil Engineering, IIT (BHU), Varanasi, 221005, India ARTICLE INFO Keywords: Rice straw ash Microsilica Compressive strength Flexural strength Split tensile strength XRD SEM ABSTRACT This research deals with improvement in the mechanical strength of Pavement Quality Concrete (PQC) when admixed with Rice Straw Ash (RSA) and Microsilica (MS). Nine mixes were prepared by partially substituting Ordinary Portland Cement (OPC) by MS (2.5%, 5%, 7.5%, and 10%), RSA (10%) and RSA-MS composite (5%–5%, 5%–7.5%, 10%–5% and 10%–7.5%). Maximum improvement was found when OPC was partially re- placed by 7.5% in the case of MS and 5%–7.5% in case of RSA-MS composite. All the mix showed increased strength, w.r.t the control mix. X-Ray powder diffraction (XRD) and Scanning Electron Microscope (SEM) techniques were employed for characterization of the selected samples. Power regression equations were es- tablished to predict split tensile and flexural strength from compressive strength. They were compared with universally accepted equations and were found to be more accurate for RSA and MS admixed concrete. Mix R1M3 (5% RSA, 7.5% MS) is recommended based on the findings. 1. Introduction The construction sector of India is growing at a rapid rate and ce- ment is the most important material required in the construction sector. In fact, India is the largest producer of cement in the world behind China. The capacity of cement plants in India was around 502 million tonnes in 2018 [1]. The production of cement raises serious environ- mental concerns like emission of carbon dioxide gas (CO 2 )[2]. Emis- sion of carbon dioxide gas causes the greenhouse effect. In fact, cement plants account for 5% of the global emission of CO 2 . Around 900 kg of CO 2 is liberated into the atmosphere in production of 1000 kg of cement [3]. As the consumption of cement is increasing day by day, a reduction in CO 2 emissions can be done by partially substituting OPC with mi- neral admixtures. Mineral admixtures such as rice straw ash [4–8] and micro silica [9–12] can be a feasible solution for partial replacement of OPC in PQC, thus reducing the emission of CO 2 . The resulting ce- mentitious material will be cheaper, resulting in more affordable con- crete for pavement construction. Rice straw is an agricultural by-product of rice. It is mainly pro- duced in Asia where its yearly production is 95% of world production [13]. Rice straw production is highest among the agro-residues pro- duction in India [14]. Traditionally rice straw has numerous competing uses such as animal feed, fodder, fuel, roof thatching, packaging and composting. These uses, however, will soon diminish because advanced technologies are making them unprofitable. Rice straw can be con- verted into ash without using enhanced burning techniques. When rice straw is burnt, it produces ash which is highly pozzolanic and fulfils the necessities of ASTM C618-19 [15] Class N, F, and C pozzolan. The specific gravity and specific surface area of rice straw ash are 2.25 and 1.846 m 2 /g, respectively [5]. Micro silica is an indistinguishable (non-crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder gathered as a by-product of the silicon and ferrosilicon composite production and comprises spherical particles with an average particle diameter of 150 nm [12]. There has been less amount of work done to explore the possible results of using rice straw ash in PQC and even less number of these explorations tends to the use of natural RSA in PQC (RSA that is either not ground to fine particle sizes and/or that isn't produced by an en- hanced burning technique). The present study is the continuation of the work in which analysis of rice straw ash was done by its chemical, mineralogical, thermal and structural properties [6]. Also, a pre- liminary study was done on the cement paste containing rice straw ash, microsilica, and their composite by finding out its normal consistency and setting times [7]. The compressive strength of the mortar cubes of various proportions was determined after 3, 7, 28, 60, 90 and 365 days of curing in water [16]. It was found out that the mortar cubes of mix M3 (7.5% microsilica) give the maximum strength. The principal purpose of this analysis is to develop the use of rice https://doi.org/10.1016/j.jobe.2019.100889 Received 23 February 2019; Received in revised form 1 June 2019; Accepted 19 July 2019 * Corresponding author. E-mail address: arunabh.rs.civ13@iitbhu.ac.in (A. Pandey). Journal of Building Engineering 26 (2019) 100889 Available online 20 July 2019 2352-7102/ © 2019 Elsevier Ltd. All rights reserved. T