###### Abstract  
Low-grade calcined clay, due to its low cost, availability and low temperature calcination, has been gaining attention in recent times as a supplementary cementitious material (SCM) in the manufacture of revolutionary building materials to improve the fresh and hardened properties of concrete. Silica fume, on the other hand, has been used, over the years, to improve the performance of concrete due to its reduced porosity and improved transition zone quality. In spite of the individual contribution of these two pozzolans to the strength and durability of concrete, there is a knowledge gap in the properties of ternary blended mixes utilizing calcined clay and silica fume. In this study, the synergistic effect of calcined clay and silica fume on the fresh and hardened properties of cementitious mortar have been investigated. The two pozzolans were used to partially substitute Portland cement to form a ternary blended composite binder having, at a maximum, a replacement of 30% by weight and a varying content of calcined clay and silica fume. The influence of the binary and ternary blended mixes on hydration, pozzolanic reactivity and the mechanical and durability properties of mortar was studied. From the results, partial replacement of cement with 30% calcined clay and silica fume caused significant reductions in the portlandite content of the two hydrated pastes at all curing ages. Drying shrinkage was found to be less severe in the control mortar than the blended cement mixes. Compared to the blended cement specimens, the control suffered the most weight (13.3%) and strength (10%) losses, as indicated by the sulphate resistance test. aclined clay; drying shrinkage; sulphate resistance; pozzolanic reactivity; silica fume; ternary blends; freeze and thaw; durability +
Footnote †: journal: _Sciences_  
does not entail the emission of carbon dioxide. So, compared to the manufacturing of Portland clinkers, the production of calcined clays requires less energy and emits significantly less carbon dioxide. To improve the performance of concrete, silica fume (SF) has historically been utilised more frequently [12; 13; 14]. Because of its reduced porosity and improved transition zone quality, SF, which must have a silica concentration of at least 85%, has been known to improve both strength and durability [15; 16]. However, since silica fume is a waste product from the silicon industry, its cost and availability are greatly influenced by its proximity to raw material source. Additionally, because the particles are ultrafine, they have a large specific surface area, necessitating the use of water-reducing admixtures to improve workability [1]. Calcined clay is mainly aluminosiliceous, whereas silica fume is only a siliceous material. When calcined clay and silica fume react with the Ca(OH)\({}_{2}\) that is released during cement hydration, a gel is formed that is used to improve the binding properties of the solid components of the concrete mix. Calcium aluminate hydrate (C-A-H) and calcium silicate hydrate (C-S-H) are both components of the gel generated in calcined clay-cement reactions, whereas C-S-H is the major component of the gel produced in silica fume-cement reactions [17]. The effectiveness of calcined clay and silica fume when used alone in concrete have been thoroughly researched and reported [18; 19; 20; 21]. Calcined clay typically has a higher workability than silica fume, but the impacts of calcined clay and silica fume on strength and durability depend on their pozzolanic reactivity and fineness. When used separately, calcined clay and silica fume are both quite effective in improving compressive and tensile strengths as well as lowering porosity, chloride ingress and drying shrinkage [16; 21]. Because calcined clay and silica fume both have a higher surface area than cement, they would fill the voids and eventually increase packing density, water demand and setting times.