Durability against Wetting–Drying Cycles of Water Treatment Sludge–Fly Ash Geopolymer and Water Treatment Sludge–Cement and Silty Clay–Cement Systems

Abstract

The viability of using two waste materials, water treatment sludge (WTS) and fly ash (FA), for developing sustainable masonry units has been previously investigated in terms of strength but the important aspect of durability against wetting–drying (w–d) cycles has yet to be studied. A study on durability against w–d cycles, an important parameter for service life design of the sustainable masonry units, is investigated in this paper. The liquid alkaline activator (L) was a mixture of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH), and a high calcium fly ash (FA) was used as a precursor. The results of cyclic w–d test indicate that the WTS–FA geopolymer manufactured with an optimum ingredient (L:FA=1.6, Na2SiO3:NaOH=90∶10) and at an optimum heat condition of 85°C for 72 h can be used as durable bearing masonry units; i.e., the compressive strength is greater than 12 MPa after 12 w–d cycles. For this optimum ingredient, the w–d cycle strength, qu(w−d) at heat temperatures between 65 and 95°C and durations between 24 and 120 h was found to be mainly dependent upon the initial soaked (without w–d cycle) strength qu0, and the normalized strength qu(w−d)/qu0 versus number of w–d cycles relationship expresses as a logarithm function. This relationship facilitates a mix design to attain the required strength at a target service life, which is very useful for civil engineering practitioners and researchers alike. It is evident from this research that portland cement is not a suitable cementing agent to manufacture WTS masonry units because alum in WTS retards the cement hydration, unlike a geopolymer binder, which was proven to be suitable. Compared with a traditional clay–cement sample at the same initial soaked strength, the WTS–FA geopolymer sample exhibits higher durability. This indicates that the WTS–FA geopolymer masonry units have a longer service life than clay–cement masonry units, which is typically used in many countries. This research enables WTS traditionally destined for landfill to be used in a sustainable manner as an aggregate in geopolymer masonry units, which is significant from engineering, economical, and environmental perspectives.