The objective of the work is to examine the performance of tetraethyl orthosilicate (TEOS) modified microcrystalline cellulose (MCC) fiber, derived from cotton, as a mineral admixture that could be compatible in cement mortar composites. The effectiveness of surface modification of MCC is characterized by powder X-ray diffraction, FTIR, TGA and SEM techniques. The present silane based surface modifier (TEOS) minimizes the water uptake and acts as a pozzolan, that could result in additional calcium silicate hydrates (C-S-H) linkages. This is reflected by the enhancement in the mechanical properties of the cement mortar composite. A dramatic two fold enhancement of flexural strength and almost 45% increase of compressive strength are observed in the case of TEOS-MCC when compared with the cement mortar composites without any mineral admixture there by validating the method chosen. The enhancement of compressive and flexural strength could be due to proper dispersion of smaller size MCC fibers within the pores of the cement mortar composite. When an optimized amount of chemical admixture (polycarboxylate ether (PCE) superplasticizer) is used along with TEOS- MCC a greater enhancement in flexural strength and compressive strength is observed with good workability, at a lower water/cement ratio. © 2016 Elsevier Ltd