High volume fly ash mixes supplemented with colloidal Nano Silica

Since cement manufacturing causes 7- 8% of total global CO₂ emissions, attempts have been made to minimize cement consumption. One method is to introduce pozzolans, commonly fly ash (FA), as supplementary materials to Ordinary Portland Cement (OPC). The main drawback of incorporating FA into cement is the reduction of early strength in concrete. This can be countered by incorporating Nano silica (NS) into the FA - OPC mix. Workability, compressive strength (3, 7, 28 & 100 day), sorptivity and thermogravimetric analysis (TGA) testing was carried out on cement paste specimens with FA replacements from 0% to 70% and NS percentages from 0 to 6%. All mixes were prepared with a 0.4 water binder ratio and a superplasticizer dosage to achieve equal workability for all mixes. Results showed that workability reduces with the increase of NS. Also, for a given %NS, compressive strength reduces with the increase of FA for early age strength, while 30% FA replacement gives optimum strength at 100 days. A 3% NS content for constant %FA gives optimum strength beyond 28 days. Sorptivity too is lowest at 30% FA replacement with 3% NS. Therefore 30% FA replacement with 3% NS gives optimum performance for both strength and durability. The mix with 50% FA replacement and 3% NS (a genuinely HVFA mix) is promising as a practically useful mix. The TGA enables us to obtain the Calcium Hydroxide (CH) consumed for pozzolanic reactions; this is used to validate a model that explains the strength variations in these mixes.