www.ijird.com June, 2022 Vol 11 Issue 6 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH & DEVELOPMENT DOI No. : 10.24940/ ijird/ 2022/ v11/ i6/ MAR22055 Page 76 The Compressive Strength Indices of Calcined Clay-Cement Blended Concrete as a Function of Calcination and Cement Replacement Level Using Locally Sourced Clay 1. Introduction With the largest manufacturing volume, Portland cement (PC) is one of the most commonly utilized materials on the planet. However, its manufacture is associated with high energy consumption and significant CO 2 emissions globally, mainly during clinker production (Martirena, 2009), all representing severe sustainability concerns. One of the most comprehensive solutions to this problem is the use of Supplementary Cementitious Materials (SCM), which allows for the maintenance or even improvement of the mechanical qualities and durability of finished products, as well as improving environmental sustainability by reducing CO 2 emissions associated with clinker production (Martirena, 2009, Alujas, 2010, Escobar 2020). Between 1995 and 2014, there was a significant increase in worldwide cement output, which was seen as a standard response to fulfill the ever-increasing demand for cement-related structures to keep up with the growing population. However, worldwide cement production data shows no substantial variations in the amount of cement produced between 2015 and 2020. This is thought to be the outcome of the construction industry's integration of several supplementary cementitious materials (Statista, 2022). ISSN 2278 – 0211 (Online) Happiness D. Mac-Eteli Lecturer, Department of Civil Engineering, Niger Delta University, Nigeria Somina Sopakirite Lecturer, Department of Civil Engineering, Federal University Otuoke, Nigeria Abstract: Cement is highly valued primarily for its enhanced rate of hydration when compared to its predecessors. Its production process, however, is of adverse effects economically and mostly environmentally. Amidst the existing research routes for solving the problems associated with cement production, partial cement replaceability using sustainable supplementary cementitious materials appears to be gaining the most attention. However, the present research gap exists in the limited potential of the available materials in terms of availability or structural efficiency. As such, the abundance of local clay as well as its siliceous nature, in addition to the established structural properties of kaolinite (calcined Kaolin), suggests to the researchers that the index properties of clay, when properly optimized, could result in more efficient material with enhanced performance in concrete as a partial supplementary cementitious material. In the research, locally-sourced clay calcined at 450, 600, 750, and 900 o C were pulverized to pass through 150μm and used to replace Portland cement at 15%, 30%, 45%, and 60%. Results obtained indicated optimum calcination temperature for local clay to fall between 600 and 700 o C, yielding a compressive strength range of 15 – 22 MPA and 19 – 28MPA for 7 and 28 days, respectively. However, a more comprehensive range of cement replaceability was observed for clays calcined between 750 - 900 o C at a reduced mechanical index. As such, it is recommended that local clays be calcined between 600 and 700 o C, pulverized and sieved to pass through 150μm, and used to replace a maximum of 30% cement (by mass) for optimum efficiency and structural performance of calcined clay-cement blended concrete. Keywords: Calcination, locally sourced clay, siliceous materials, cement hydration, compressive strength