Strength development is improved with the increases in sodium silicate content (sodium silicate/NaOH of 0.33–3.0) and curing temperature of 25–60 ☌ with the optimum NaOH concentration of 10 m. Also, the increase in the amount of fine aggregate results in an increase in concrete density due to the higher density of fine recycled lightweight aggregate. The fine aggregate with high surface area offers better bonding between alkali-activated matrix and fine aggregate than that of coarser aggregate, and thus the increase in fine aggregate from 0% to 70% results in concrete with increased strength. The strength slightly increases with the increase in the amount of fine recycled lightweight aggregate content (reduced recycled coarse aggregate) as shown in Figure 19.1. Crushed waste lightweight concrete block has been used successfully as recycled aggregates for making lightweight alkali-activated concrete using NaOH, sodium silicate and fly ash ( Posi et al., 2013) with acceptable 28-day compressive strength of 1.0–16.0 MPa, density of 860–1400 kg/m 3, water absorption of 10–31%, porosity of 12–34%, and modulus of elasticity of 2.9–9.9 GPa. Crushed lightweight aggregate concrete (LWAC) can also be used as an aggregate in concrete (recycled LWAC-aggregate) ( European Union – Brite EuRam III, 2000).
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