Page 75 - New Trends in Eco efficient and Recycled Concrete

50 New Trends in Eco-efficient and Recycled Concrete

Table 2.7 introduces the main physical and mechanical properties of geopolymer
concrete manufactured with both biomass ashes, as found in the literature. In gen-
eral, it can be seen that POFA has turned out to be the best partial substitute of pre-
cursors used in alkali-activated concrete, due to its capacity to contribute to
achieving faster and higher physic and mechanical requirements.
Particularly, in the fresh state, the addition of both biomass ashes in geopolymer
concrete plays an important role in the workability as they have a high loss of igni-
tion and an irregular shape (Islam et al., 2015). The increase in mixing water
needed is due to the ashes having a high amount of micro-porosity and pore fluid
(Suksiripattanapong et al., 2017). At the hardened state, geopolymer concrete with
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densities less than 2000 kg/m could be categorised as lightweight concrete (BS EN
206-1, 2008). On the other hand, in comparison with OPC concrete, the compres-
sive strength developed, much higher for POFA geopolymer concrete than RHA
geopolymer concrete, has been mainly achieved by the pozzolanic activity of the
mix binder (Lim et al., 2018). Compressive strength also depends on the quality
and ratio of alkaline activators, curing time, curing temperature, quantity and
finesse of solid compounds and relative ratio of the reactive base materials. (Islam
et al., 2015; Kabir et al., 2017; Lim et al., 2018). For its part, flexural behaviour
exhibit strengths lower than traditional OPC concrete, that usually range between
8% and 11% of compressive strength (Islam et al., 2015). This result is attributed
by Lim et al (2018) to the lubricant effect of the spherical particles of the biomass
ash and to the high proportion of precursor in the mixing design. Finally, Yusuf
et al. (2014) concluded that higher compressive strength is directly related to the
higher modulus value, which is also directly allied to the higher Si/Al ratio of the
reaction products.

Table 2.7 Characterisation of geopolymer concrete
RHA POFA
Temperature 21 50 (Suksiripattanapong 25 90 (Islam et al., 2015;

curing ( C) et al., 2017) Yusuf et al., 2014)
Time curing (h) 7 90 days (Suksiripattanapong 6 48 (Yusuf et al., 2014;
et al., 2017) Liu et al., 2016)
Density oven 1784 1935 (Bashar et al.,
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dry (kg/m ) 2016; Islam et al., 2015)
Compressive 0.29 3.00 (Suksiripattanapong 28.0 80.0 (Islam et al.,
strength (MPa) et al., 2017) 2015; Lim et al., 2018)
Flexural strength 1.23 4.50 (Islam et al.,
(MPa) 2015; Bashar et al.,
2016; Lim et al., 2018)
Modulus of 6.25 11.12 (Yusuf et al.,
elasticity (GPa) 2014; Islam et al., 2015)
RHA, rice husk ash; POFA, palm oil fuel ash.