Abstract: A simple and green method for the preparation of nanofibrillated cellulose (NFC) by heating surgical cotton in glycerol is demonstrated as an alternative to the existing mechanical degradation method. The heat treatment of cotton in the presence of 9% w/w sulphuric acid in glycerol (1 M), under relatively milder conditions than those reported in the literature in the absence of glycerol, resulted in the formation of nanocrystalline cellulose (NCC) due to extensive hydrolysis of the amorphous segments. The method reported offers certain unique advantages in the preparation of NFC such as high yield (71%) and much easier post-processing compared to the mechanical degradation method of preparation of NFC. It also offers certain unique advantages in the preparation of NCC such as relatively high yield (56%), the use of lesser quantity of sulphuric acid as well as elimination of the quenching of the reaction through the addition of excess water to the reaction mixture. The residual ‘green solvent’, separated by decantation or centrifugal separation, post-reaction, could be reused for several cycles after filtration with activated carbon. A simple utility of the NCC prepared as reinforcing additive to cement is demonstrated. The addition of 1% (w/w of cement) of NCC and tetraethylorthosilicate modified NCC enhanced the workability of cement mortar and the compressive strength of cured cement composite in sharp contrast to the use of microcrystalline cellulose that required 10% (w/w) for the same enhancement in strength but with poorer workability. Graphical abstract: A sustainable route for preparing NFC through heat treatment in glycerol is reported. In the presence of 1 M (9% w/w) sulphuric acid in glycerol, similar heat treatment resulted in the formation of both NFC and NCC. The residual ‘green solvent’ could be reused for several cycles. The addition of 1% (w/w) of nanocellulose prepared via this method enhanced the workability of cement mortar and the compressive strength of cured cement composite.[Figure not available: see fulltext.]. © 2019, Springer Nature B.V.