Utilization of mswi bottom ash as a mineral resource for low-carbon construction materials: Quality-upgrade treatments, mix design method, and microstructure analysis

In recent years, considerable attention has been given to the utilization of municipal solid waste incineration (MSWI) bottom ash as a mineral resource for construction materials. MSWI bottom ash is the primary residue discharged after incinerating municipal solid waste. The generation of MSWI bottom ash is increasing dramatically with the wide application of waste incineration techniques. Different methods have been proposed to improve the quality of MSWI bottom ash and make it suitable as supplementary cementitious material (SCM) or precursor for alkali-activated materials (AAM). However, there is no systemic guidance on how to select quality-upgrade treatments for MSWI bottom ash. When using MSWI bottom ash to prepare blended cement pastes and alkali-activated pastes, the optimal mix design is usually found by trial and error. Very little information is available in the literature regarding the reaction of MSWI bottom ash as SCM and AAM precursor. The contribution of MSWI bottom ash to the microstructure formation and strength development of blended cement pastes and alkali-activated pastes is not very well understood.
The goal of this research is to develop knowledge that can be used to support the application of MSWI bottom ash as a mineral resource for construction materials. Based on this knowledge, a strategy for using MSWI bottom ash produced in the Netherlands (4-11 mm) as raw material to produce blended cement pastes and alkali-activated pastes is proposed. This research consists of the following parts:
1. Quality-upgrade treatments of as-received MSWI bottom ash
As-received MSWI bottom ash cannot be used directly as SCM and AAM precursor due to its large particle size and presence of metallic aluminum (Al). Mechanical treatments consisting of grinding and sieving were studied and selected to reduce the particle size and the metallic Al content of as-received MSWI bottom ash. The effectiveness of the mechanical treatments used to reduce the metallic Al content of MSWI bottom ash is strongly influenced by the distribution of metallic Al in bottom ash particles. Most metallic Al separated during mechanical treatment comes from the coarse particles. The metallic Al embedded in the particles smaller than 0.5 mm is difficult to be removed via mechanical treatments (see Chapter 3).
2. Development and microstructure analysis of blended cement pastes and alkali-activated pastes
The reactivity and leaching potential of mechanically treated MSWI bottom ash (MBA) are studied. This information is used in the development of blended cement pastes and alkali-activated pastes. A dissolution test is proposed to assess the reactivity of MBA as AAM precursor. The reactivity of MBA as SCM and AAM precursor is similar to that of Class F coal fly ash (FA), but much lower than that of blast furnace slag (BFS). The leaching of antimony (Sb) and sulfate from MBA is above the threshold value prescribed in Dutch Soil Quality Decree. The dosage of MBA in blended cement pasts and alkali-activated pastes should be controlled to prevent excessive leaching of contaminants into the environment (see Chapter 4).
The reactivity of MBA is determined by the content and the chemical composition of its amorphous phase. The amorphous phase of MBA has a chemical composition within the same range as that of the amorphous phase of FA. Given that the reactivity of MBA is close to that of FA, previous experience with the mix design of Class F coal fly ash-based pastes is used as a reference for the mix design of MBA-based AAM. Additionally, thermodynamic modeling is used to predict the assemblage of reaction products and the composition of pore solution in alkali-activated MBA paste when changing the Na2O content in the activator. The modeling results are also used to guide the mix design of MBA-based AAM (see Chapter 4).
When water treatment and NaOH solution treatment are part of the mixture preparation procedure, the compressive strength of the blended cement pastes and alkali-activated pastes made from MBA is close to that of the pastes prepared with the same amount of FA (Chapters 5 and 6). The metallic Al that cannot be removed during mechanical treatments can be oxidized by treating MBA in water or NaOH solution at room temperature. Apart from reducing metallic Al content, water treatment and NaOH solution treatment also slightly change the mineralogical composition of MBA.
Blending water-treated MBA (WMBA) with Portland cement paste leads to changes in the reaction products and microstructure. WMBA delays clinker hydration on the first day but enhances clinker hydration at later ages. The reaction products of WMBA contribute to the strength development of blended cement pastes (see Chapter 5).
NaOH solution-treated MBA (CMBA) is used together with BFS to prepare alkali-activated pastes. CMBA retards the reaction of BFS during the first seven days but promotes the reaction of BFS at later ages. Adding CMBA into alkali-activated pastes changes the reaction products and microstructure. The reaction products of CMBA contributes to the strength development of alkali-activated pastes (see Chapter 6).
3. Environmental impact assessment of blended cement pastes and alkali-activated pastes
Compared with Portland cement paste, blended cement pastes and alkali-activated pastes prepared using MSWI bottom ash SCM and AAM precursor have lower environmental impacts, especially in the impact category of global warming (see Chapter 7).
This research deepens the understanding of the reaction of MSWI bottom ash as SCM and AAM precursor. This study also demonstrates how to use MSWI bottom ash to prepare blended cement pastes and alkali-activated pastes by considering the chemical and physical properties of MSWI bottom ash. Since the MSWI bottom ash used in this research has chemical and mineralogical compositions within the same range as the MSWI bottom ash reported in the literature, the knowledge developed in this work stimulates the utilization of MSWI bottom ash produced in other regions as SCM and AAM precursor for construction materials.