An investigation of the mechanisms for strength gain or loss of geopolymer mortar after exposure to elevated temperature

When fly ash-based geopolymer mortars were exposed to a temperature of 800 °C, it was found that the strength after the exposure sometimes decreased, but at other times increased. This paper shows that ductility of the mortars has a major correlation to this strength gain/loss behaviour. Specimens prepared with two different fly ashes, with strengths ranging from 5 to 60 MPa, were investigated. Results indicate that the strength losses decrease with increasing ductility, with even strength gains at high levels of ductility. This correlation is attributed to the fact that mortars with high ductility have high capacity to accommodate thermal incompatibilities. It is believed that the two opposing processes occur in mortars: (1) further geopolymerisation and/or sintering at elevated temperatures leading to strength gain; (2) the damage to the mortar because of thermal incompatibility arising from non-uniform temperature distribution. The strength gain or loss occurs depending on the dominant process

Bond strength of reinforcing steel embedded in fly ash-based geopolymer concrete

Geopolymer concrete (GPC) is an emerging construction material that uses a by-product material such as fly ash as a complete substitute for cement. This paper evaluates the bond strength of fly ash based geopolymer concrete with reinforcing steel. Pull-out test in accordance with the ASTM A944 Standard was carried out on 24 geopolymer concrete and 24 ordinary Portland cement (OPC) concrete beam-end specimens, and the bond strengths of the two types of concrete were compared. The compressive strength of geopolymer concrete varied from 25 to 39 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 20 mm and 24 mm diameter 500 MPa steel deformed bars. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. Failure occurred with the splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This is because of the higher splitting tensile strength of geopolymer concrete than of OPC concrete of the same compressive strength. A comparison between the splitting tensile strengths of OPC and geopolymer concrete of compressive strengths ranging from 25 to 89 MPa shows that geopolymer concrete has higher splitting tensile strength than OPC concrete. This suggests that the existing analytical expressions for bond strength of OPC concrete can be conservatively used for calculation of bond strength of geopolymer concrete with reinforcing steel

Mechanical behavior of geopolymer concrete subjected to high strain rate compressive loadings

The effect of strain rate on the compressive behaviours of geopolymer concrete and mortar is reported. Split Hopkinson pressure bar was adopted for the high strain rate testings. The dynamic increase factors for compressive strength (DIFfc) and critical strain (DIFεc) were measured and compared with Concrete Comite Euro-international du Beton (CEB) recommendations. The results show that alkaline activators have significant influence on the quasi-static compressive strength of geopolymer concrete. With high strain rate loading, the DIFfc of geopolymer concrete and mortar mixes increase with respect to increasing strain rates and in agreement with CEB recommendations. In addition, the coarse aggregates in geopolymer concrete mixes play important role in the increase of compressive strength. However, CEB recommendations underestimate the DIFεc of critical strain for geopolymer concrete in the high strain rate loading. It is found that for the quasi-static loading and low strain rate loading, cracks propagate along interface transition zone (ITZ) and matrix of geopolymer concrete specimens whereas cracks occur at both the aggregates and ITZ under high strain rate loading