Factors affecting the slump and strength development of geopolymer concrete

Cement production is estimated to be responsible for 5–8% of global total carbon dioxide (CO2) emissions. Geopolymer concrete (GC) is claimed to release up to 45% less CO2 for a comparable concrete, but is more difficult to manufacture. This study investigated the effect of factors other than mix design on the slump and strength development of GC produced from low-calcium fly ash (FA) and up to 50% ground granulated blastfurnace slag (GGBS) replacement; these were: curing methods and temperatures (at 10, 20 and 75 °C); FA fineness; superplasticiser type; water content; and GGBS/FA ratio. Methods are also presented for the volume-to-mass conversion of sodium hydroxide at a specific molarity and ambient temperature, and an effective combination of wax-based mixtures as mould agents to overcome the inherent manufacturing mould release difficulties. Steam-curing improved the compressive strength of the FA-based GC over oven drying by up to 20%, as did increasing the FA fineness (although this became negligible with 50% GGBS replacement). Both naphthalene and polycarboxylate superplasticizers improved the slump of GC (from 110 to 210 mm) without significantly reducing the compressive strength (less than 5 MPa). Water content of GC had a great effect on the slump, but less so on the compressive strength. Increasing the GGBS content gradually decreased the slump but rapidly increased the strength, regardless of the curing temperatures of 20 or 75 °C. The GC with the minimum of 20% GGBS replacement achieved 33 MPa after 28 days curing at 10 °C. Air-dry curing provided a greater strength development of FA-based GC than water curing, though the opposite was observed for the 50/50 GGBS/FA GC. Consideration of these factors can significantly ease the manufacture of GC, enhancing its potential application in real structures, and consequently helping reduce global (CO2) emissions.