Author: Chen, Zhen
Title: Feasibility of recycling sewage sludge ash as construction materials
Advisors: Poon, C. S. (CEE)
Degree: Ph.D.
Year: 2019
Subject: Hong Kong Polytechnic University -- Dissertations
Salvage (Waste, etc.)
Sewage sludge ash
Department: Department of Civil and Environmental Engineering
Pages: 205 pages : color illustrations
Language: English
Abstract: Sewage sludge ash (SSA) is generated from incinerating dewatered sewage sludge. As most SSA still need to be disposed of at landfills, the management of SSA becomes an environmental issue. Many places including Hong Kong face growing pressure in waste management due to limited landfilling capacity around cities and strong objections to their operation by the public. Dumping wastes in landfills is not a sustainable means of managing wastes. To relieve the burden on landfills, it is highly desirable to increase the recycling of wastes. One major approach is the use of wastes in construction products. The huge consumption of construction materials can effectively reduce some wastes by this means. The focus of the PhD study was on the recycling of SSA as construction materials. The study consists of three parts. First, use of SSA as partial cement replacements for making cement mortars. Second, use of SSA as partial cement replacements for making concrete blocks. Last, blending SSA with ground granulated blast-furnace slag (GGBS) to produce geopolymers. For producing cement mortars the effects of the SSA on the compressive and flexural strength of the cement mortars were studied. These effects were compared with those of pulverized fly ash (PFA) and fine sewage sludge ash (FSSA) which was prepared by grinding the SSA in a ball mill for 3 hours. Methods for characterization the properties included Frattini pozzolanic activity, compressive strength, flexural strength, drying shrinkage, Isothermal Conduction Calorimetry, mercury intrusion porosimetry, XRD, SEM and EDX tests. The results showed that the SSA and FSSA exhibited lower pozzolanic activities compared with PFA, but the cement mortars prepared with up to 20% SSA or FSSA yielded strength values comparable to those of the PFA cement mortars. The SSA and FSSA contributed to the strength through (i) acceleration of the rate of cement hydration at early stages while PFA did not produce this effect, (ii) the preservation of cumulative pore volume of the paste with low SSA or FSSA content up to 10%, and (iii) the formation of brushite, a kind of binding phase, in SSA or FSSA cement pastes at late ages. Unfortunately, SSA caused higher drying shrinkage in mortars due to increasing content of mesopores with sizes less than 0.025 μm. This undesirable effect was greater with the FSSA. Findings of the study filled the gap in knowledge about strength development in mortars containing a small amount of SSA despite its low pozzolanic activity.
Given the difficulty in achieving good workability in cement mortars incorporating SSA due to its hydrophilic nature, the second application of SSA in making precast concrete blocks was studied. This recycling method only require a small quantity of water to give a dry but cohesive mix which is then compressed into standard moulds. In this study, SSA was used as a partial substitution of cement together with recycled glass cullet for partial substitution of natural aggregates to produce concrete blocks. Through investigating the hardened density, water absorption, compressive strength, drying shrinkage, alkali silica reaction of the concrete blocks, it was found that the benefits and drawbacks of SSA and glass cullet aggregates were complimentary to each other. The porous nature of SSA absorbed more water in mixing resulting in high shrinkage. While glass cullet in the mix, being hydrophobic, substantially reduced shrinkage of the blocks. On the other hand, expansion due to alkali silica reaction caused by the reactive glass cullet aggregates could in turn be controlled by the SSA present in the blocks. Other findings in this study included use of SSA increased the water demand and more water was needed to attain suitable consistency in making concrete blocks while the FSSA with smoother particles and less pores required slightly less water and the blending of 20% SSA or FSSA in the binder increased the water absorption values of the concrete blocks but the effects on hardened densities were not obvious. Besides, leaching tests conducted on both the SSA and the concrete blocks showed full compliance with regulations. This combined use of the two types of wastes, i.e., SSA and glass cullet, is innovative and successful. Another complimentary use of two wastes is blending GGBS with SSA in equal proportion to form an aluminosilicate precursor for synthesizing geopolymer pastes. In studying this application, the effects of alkali dosage, expressed as weight percentage of Na₂O to the mixed solids, and modulus which represented the molar ratio of SiO₂ to Na₂O in the mixed sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) alkaline solution were assessed. Through various tests including compressive strength, XRD, QXRD, FTIR, SEM and EDX, insights into the microstructure of geopolymer and variations caused by different alkalinity was gained. SSA was found to be transformed in the geopolymerization and the quartz and hematite crystals in it were largely dissolved by the alkaline solution. Optimum alkali dosage of 4.0% and modulus of 0.95 could produce a geopolymer of maximum strength of 32.81 MPa. The main reaction product of the optimum mixture was a C-(N)-A-S-H gel with Fe substitutions. The drying shrinkage of all specimens were less than 0.06% at the age of 14 days. This study built up a theoretical basis for recycling SSA as aluminum silicate sources for producing geopolymer.
Rights: All rights reserved
Access: open access

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