Reinforced geopolymer concrete for marine applications

Jack Moore

Geopolymer concrete is an emerging alternative for ordinary Portland cement (OPC) which does not require large carbon dioxide emissions in its manufacture. With sea level rise and changing ocean currents, there has been some interest in coastal protection with concrete structures. Existing fly ash-based geopolymer concrete is not resistant enough to chloride penetration to replace Portland cement in steel reinforced structures. Blast furnace slag, which is not produced in Australia, is required. This study investigated whether the addition of calcium hydroxide to fly ash geopolymer concrete could produce similar chloride resistance to alkali-activated slag. Geopolymer cement was synthesised from Collie and Muja fly ash, sodium silicate solution, and sodium hydroxide solution. Cements were tested for compressive strength, VPV, and non-steady-state chloride migration coefficient according to the accelerated chloride test NT BUILD 492 [1]. Collie fly ash was tested with 5% Ca(OH)2 and other admixtures in a fractional factorial trial involving high levels of SiO2. Most of these mixtures failed due to too much silica or too little water so the effects of the admixtures could not be determined. The highest compressive strength achieved was 21.8 MPa. Chloride migration coefficients ranged from 148–263 ´ 10–12 m2/s, compared to 19 ´ 10–12 m2/s (OPC) and 3 ´ 10–12 m2/s (alkali-activated slag cements); chloride will rapidly penetrate purely fly-ash geopolymer concrete and corrode reinforcements. Calcium was found to prohibit the formation of silica gel and reduced VPV but had no effect on chloride migration rate. A range of compositions were then tested using a mixture experiment design approach with Muja fly ash. None had a compressive strength above 20 MPa and had similarly high chloride migration coefficients (81–263 ´ 10–12 m2/s), far higher than typical values for OPC. A well-fitting (R2 = 0.9776) empirical mathematical model to predict strength based on composition (fly ash, SiO2, Na2O, water) was developed for Muja fly ash. Ultimately, no replacement for blast furnace slag was found. Calcium is not an effective substitute for blast furnace slag, and it is likely that no mixture of purely fly ash geopolymer exists that has comparable chloride resistance to OPC. Purely fly ash geopolymer is not suitable for reinforced structures in saline environments.

Reinforced geopolymer concrete for marine applications

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