Climate change is the main global concern today, triggering a transformative shift in several sectors, especially the construction industry. High carbon emissions during Portland cement (PC) production necessitate a gradual shift toward more environmentally friendly alternatives, such as alkali-activated materials (AAMs). AAMs offer a promising, low-carbon solution for the production of alternative building materials. Introduction of these materials, however, require certification backed by well-documented test results. The widely used EuroCode 2 (EN 1995-1-1) (EC2) for PC concrete currently lacks guidelines for the use of AAC. Experimental data and reliable models for estimating long-term mechanical properties, especially creep and shrinkage phenomena, are still missing. The aim of this thesis is to evaluate creep behaviour of AAC, clarify underlying mechanisms and develop a creep model that takes into account specific microstructural and nanoscale properties of alkali-activated paste....

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.

Concrete made with Portland cement has good behaviour under compressive stress, but it is weak in tension. Cracking caused by tension is a typical problem in concrete practice. High autogenous shrinkage at early ages is one of the causes of cracking, especially in high/ultra-high performance concrete. In that stage shrinkage is strongly related to the decrease of relative humidity (RH) inside the concrete. To minimize the probability of cracking induced by autogenous shrinkage at early age, internal curing, which can provide moisture inside the concrete, has been proposed. Several internal curing agents, such as Super Absorbing Polymers (SAP) and saturated lightweight aggregate (LWA), are utilized in high performance concrete and have shown good results for mitigating autogenous shrinkage. The use of these agents, however, also have some drawbacks. Searching for an alternative internal curing methods for ultra-high performance concrete remains a challenge....

Alkali-activated Materials (AAMs), including those classified as geopolymer, are obtained through the reaction between a solid precursor and an alkaline solution. Compared with ordinary Portland cement (OPC) binders, these materials maintain comparable mechanical properties but have the advantage of reducing greenhouse gas emissions and utilization of industrial by-products and residuals which helps to meet sustainability goals. AAMs are thus considered an environment-friendly construction material with great potential for next-generation concrete.
AAMs are inherently brittle. The low ductility of AAMs makes them prone to cracking and corresponding performance degradation, which is detrimental to their durability. Based on the concept of strain-hardening cementitious composite (SHCC), one solution relates to a family of fiber-reinforced composites that have high tensile ductility and multiple-cracking characteristics, i.e., strain-hardening geopolymer composite (SHGC). While much effort has been taken to develop conventional SHCC, scientific and technical knowledge of SHGC is still in the very early stage of development. This PhD project deals with the development of a cement-free strain-hardening geopolymer composite (SHGC) as a high-performance construction material using industrial wastes and by-products through alkaline activation technology:
The fracture properties and other mechanical properties of the alkali-activated slag/fly ash (AASF) paste as the matrix for SHGC were experimentally tested. The microstructure and chemistry of the reaction products were investigated to understand the fracture mechanism. It was found that the fracture properties of pastes are strongly related to the chemical composition (Ca/Si ratio) of the main reaction product, i.e., C-(N-)A-S-H gel. The fracture properties were also found to be dominated by a cohesion/adhesion-based mechanism. Furthermore, the compressive strength of AASF paste is primarily determined by its capillary porosity.
The fiber/matrix properties, including chemical bonding energy, initial frictional bond, and slip-hardening behavior of fiber during the pullout process were also experimentally studied. The chemistry and microstructure of the reaction product in the fiber/matrix interfacial transition zone (ITZ) were characterized. Their influence on the interface bonding properties was also investigated. It is found that the chemical bonding between PVA fiber and AASF matrix increases with increasing Ca/Si and Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Hence, changing the slag content and the alkali activator Ms appears to be an effective way to modify chemical bonding. Unlike the formation of portlandite near the PVA fiber surface in conventional SHCC, a high-Ca C-(N-)A-S-H phase was formed in the fiber-matrix ITZ of SHGC. This explains the higher chemical bonding energy found in SHGC compared to that in conventional SHCCs. Furthermore, the adhesion mechanism of the PVA molecule in reaction products was studied using MD simulation. The study suggests that the adhesion between PVA fiber and C-(N-)A-S-H gel is primarily due to electrostatic interactions rather than van der Waals interactions.
Based on the result of fracture properties of the matrix and fiber/matrix interface properties, the SHGC is then systematically developed following a micromechanics-based design approach. The experimentally-attained matrix and interface properties served as input for the numerical micromechanics model to simulate the crack bridging behavior. Through the micromechanical modeling, the optimal fiber length and volume were selected and the behavior of mixtures with different fiber/matrix combinations was predicted. With this approach, researchers and materials engineers can design and tailor future SHGC more efficiently than by using the commonly used trial-and-error method.
Finally, the environmental impact of the SHGC with the most promising performance was also evaluated. This evaluation was conducted using a cradle-to-gate life-cycle assessment (LCA) of SHGC compared to that of conventional SHCC materials. The developed SHGC demonstrates a very promising environmental profile. It has a significant reduction of the global warming potential (GWP) and a lower or similar total environmental impact compared to conventional SHCC materials. In addition, the results also provide recommendations for further improvements in mixture design for the future development of SHGC.
This study successfully developed a sustainable slag/fly ash-based SHGC with a lower carbon footprint than conventional SHCC. It is considered a good example to utilize industrial by-products as secondary resources and at the same time contribute to a circular economy. Furthermore, this study helps to understand the fracture properties of AAMs. It also clarifies the adhesion mechanism of PVA fiber in AAMs. All of these give promising guidance for researchers and engineers to design fiber-reinforced AAMs with required fracture properties and interface bonding properties. In particular, it contributes to the design and tailoring strategies for high-performance composite, for instance, SHGC, through proper mixture design.

Alkali-activated materials (AAMs), as eco-friendly alternatives to Ordinary Portland cement (OPC), have attracted increasing attention of researchers in the past decades. Unlike cement, which requires calcination of limestone, AAMs can be made from industrial by-products, or even wastes, with the use of alkali-activator. The production of AAMs consumes 40% less energy and emits 25-50% less CO2 compared to the production of OPC.
Despite the eco-friendly nature of AAMs, doubts about these materials as an essential ingredient of concrete exist, regarding, for example, their volume stability. One possible volume change concerns autogenous shrinkage. Autogenous shrinkage is the reduction in volume caused by the material itself without substance or heat exchange with the environment. If the autogenous shrinkage of a binder material is too large, cracking might happen, which will seriously impair the durability of concrete. According to the literature, AAMs can show higher autogenous shrinkage than OPC-based materials. However, the mechanism behind the high autogenous shrinkage of AAMs is still unclear. Existing shrinkage-mitigating strategies for OPC are not necessarily applicable for AAMs. There is also a lack of new strategies particularly designed for AAMs. Moreover, the cracking sensitivity of AAMs-based concrete induced by restrained autogenous shrinkage has not been investigated yet.
The aim of this study is, therefore, set to understand and mitigate the autogenous shrinkage and the cracking tendency of AAMs.

Concrete is a brittle composite material that easily fractures under tension. Due to the fact that the early-age deformation of the concrete member is restrained by adjoining structures, cracking can occur throughout the concrete prior to application of any load. The cracks would provide preferential access for aggressive agents penetrating in the concrete and then cause corrosion of reinforcement and degradation of concrete. As a result, the service life of concrete structure would be decreased. There are many different types of early-age deformation of concrete, e.g. temperature induced strain, drying shrinkage and autogenous shrinkage. Among these types of early-age deformation, autogenous shrinkage is a consequence of the self-desiccation during the cement hydration process. For a long time autogenous shrinkage was considered negligible compared with drying shrinkage. In recent years, autogenous shrinkage has drawn more and more attention due to the increasing use of concretes with low water-binder ratios. Despite the fact that phenomenon of autogenous shrinkage has been recognized for several decades, the mechanism behind it is still not fully understood and no consensus has yet been reached. Three is a general agreement about the existence of a relationship between autogenous deformation and relative humidity change in the capillary pores of the hardening cement paste. Many simulation models were built based on this relationship to predict the development of autogenous shrinkage. The reliability of these predictions, however, is not always satisfactory. The discrepancy between the measured and calculated autogenous deformation becomes very pronounced at later ages. In those simulation models, cement paste was considered as an elastic material and only the elastic part of autogenous shrinkage was predicted. In fact, cement paste is not ideal elastic material. When a cement paste is subjected to a sustained load, it will deform elastically and continue to deform further with time, which process is known as creep. Creep plays an important role in autogenous shrinkage of hydrating cement paste. The ignorance of creep would lead to an underestimation of the autogenous shrinkage. The aim of this project is to study the autogenous shrinkage of Portland cement pastes and blended pastes with supplementary materials. The autogenous shrinkage is supposed to consist of two parts, elastic part and time-dependent part (creep), which are simulated separately. Based on the autogenous shrinkage of cement pastes, autogenous shrinkage of cement mortars and concretes were simulated by taking the restraining effect of rigid sand/aggregate particles into consideration.

Nature displays a multitude of fascinating materials, from beautiful colors of butterfly wings to the toughness of mullosc shells, which are formed in mild enviornmental conditions with commonly occuring materials, such as chitosan or calcium carbonate. These composite materials display an intricate interplay of biopolymers and minerals forming highly ordered structure. The function of these materials is primarily determined by the selective pressure of the enviornment that certain organisms are placed in. It varies from the “delicate” signaling colors to a robust impact resistence.

Bacteria-based self-healing concrete is an innovative self-healing materials approach, whereby bacteria embedded in concrete can form a crack healing mineral precipitate. Structures made from self-healing concrete promise longer service lives, with associated economic benefits [1]. Despite concretes susceptibility to marine-based degradation phenomena [2], and much of the world’s marine infrastructure being located in cool with freezing climatic zones (annual average temperature < 10°C and average summer temperature generally < 20 °C) [3], research on the development of bacteria-based self-healing concrete has been largely restricted to room temperature freshwater studies [4-14]. The objective of the current project was, therefore, to develop a cost-effective bacteria-based self-healing cementitious composite for application in low-temperature marine environments. The current thesis charts the development of this composite. In Chapter 2 the autogenous healing capacities of ordinary Portland cement (OPC) and blast-furnace slag (BFS) cement mortar specimens submerged in fresh and seawater, are visually quantified and characterised. The BFS cement specimens healed all crack widths up to 104 µm, and OPC specimens healed all crack widths up to 592 µm, after 56 days in seawater. BFS cement specimens healed all crack widths up to 408 µm, and OPC specimens healed all crack widths up to 168 µm, after 56 days in freshwater. OPC specimens in seawater displaying the higher crack healing capacity also demonstrated considerable losses in compressive strength. Differences in performance are attributable to the amount of calcium hydroxide in these mortars and specific ions present in seawater. Chapter 3 reports on the crack healing capacity of seawater submerged mortar specimens with the aid of a crack permeability test. Cracks of defined widths were created in BFS cement specimens allowing reference crack permeability values to be generated for unhealed-specimens against which healed-specimens were quantified. Specimens with 0.2 mm wide cracks demonstrated no water flow after 28 days submersion. Specimens with 0.4 mm cracks demonstrated decreases in water flow of 66% after 28 days submersion and 50 to 53% after 56 days submersion. Chapter 4 presents a modified permeability test for generating crack permeability data for cementitious materials. To gauge for any improvement both the modified and unmodified tests were tested and compared. Cracks were generated in mortar specimens using both tests, the accuracy of these cracks was analysed through stereomicroscopy and computer tomography (CT), and the water flow through the cracks determined. Reduction factors and crack flow models were generated, and the accuracy and reliability of the predictions assessed. All of the models had high predictive accuracies (r2 = 0.97-0.98), while the reliability of these predictions was higher for the models generated with the crack width analysis through stereomicroscopy. The cracks generated by the modified test were more accurate (within 20 µm of the desired crack widths) than those of the unmodified test. The modified test was 30% quicker (10 hours for twenty-one specimens) than the unmodified test at generating the crack permeability data. Further, crack width analysis through stereomicroscopy is currently/generally quicker than analysis through CT. Chapter 5 presents a bacterial isolate and organic mineral precursor compound, as part of a cost-effective healing agent for low-temperature marine concrete applications. Organic compounds were screened based on their cost and concrete compatibility, and bacterial isolates based on their ability to metabolise concrete compatible organic compound and to function in a low-temperature marine concrete crack. Magnesium acetate was the cheapest organic compound screened, and when incorporated (1% of cement weight) in mortar specimens had one of the lowest impacts on compressive strength. Bacterial isolate designated psychrophile (PSY) 5 demonstrated very good growth under saline (3%), high pH (9.2), low-temperature (8ºC) conditions, with sodium lactate as an organic carbon source; and good growth at room temperature using magnesium acetate as an organic carbon source. Further, PSY 5 also demonstrated good spore production when grown on monosodium glutamate at room temperature. Chapter 6 presents a bacteria-based bead for realising self-healing concrete in low-temperature marine environments. The bead, consisting of calcium alginate encapsulating bacterial spores and mineral precursor compounds, was assessed for: oxygen consumption, swelling, and its ability to form an organic-inorganic composite in a simulative marine concrete crack solution (SMCCS) at 8ºC. After six days in the SMCCS, the bacteria-based beads formed a calcite crust on their surface and calcite inclusions in their network, resulting in a calcite-alginate organic-inorganic composite. The beads swell by 300% to a maximum diameter of 3 mm, while theoretical calculations estimate that 0.1 g of the beads are able to produce ~1 mm3 of calcite after 14 days submersion. Swelling and the formation of bacteria induced mineral precipitation providing the bead with considerable crack healing potential. It is estimated, based on the bacteria-based beads costing roughly 0.7 €.kg-1, that bacteria-based self-healing concrete made using these beads would cost 135 €.m-3. Chapter 7 presents a bacteria-based self-healing cementitious composite for application in low-temperature marine environments. The composite was tested for its crack healing capacity with the water permeability test presented in Chapter 4, and for its strength development through compression testing. The composite displayed an excellent crack healing capacity, reducing the permeability of cracks 0.4 mm wide by 95%, and cracks 0.6 mm wide by 93%, following 56 days submersion in artificial seawater at 8ºC. Some conclusions were drawn based on the results obtained during the development of the bacteria-based self-healing cementitious composite: • Visual crack closure is not a measurement for the regain of functional properties such as strength. Visual crack closure, therefore, should only be conducted as a complementary method when measuring the regain of such a property. • The capacity of a cementitious material to heal a crack depends on the width of the crack, thermodynamic considerations, the presence of water and the amount of ions available in the crack. Autogenous crack healing for seawater submerged cementitious materials is principally attributable to the precipitation of aragonite and brucite in the cracks. • The crack healing capacity of a bacteria-based cementitious composite is directly related to the amount of organic carbon available to the bacteria, and so the cheaper the organic mineral precursor compound, the cheaper the bacteria-based self-healing technology in general. Further, the compound must not have an adverse effect on concrete properties when included and must be readily metabolised by the bacteria as part of the healing agent. Magnesium acetate, in the current study, best balanced these criteria making it a good candidate as the organic mineral precursor compound for the healing agent. • A large number of specimen replicates (≥ 7) are required to generate reliable crack permeability data, and hence to quantify the crack healing capacity of cementitious materials through their functional water tightness. • The bacteria-based self-healing cementitious composite displayed an excellent crack healing capacity, reducing the permeability of cracks 0.4 mm wide by 95% and cracks 0.6 mm wide by 93%, following 56 days submersion in artificial seawater at 8ºC. This crack healing capacity was attributable to: mineral precipitation as a result of chemical interactions between the cement paste and seawater; bead swelling; magnesium-based precipitates as a result of chemical interactions between the magnesium of the beads and hydroxide ions of the cement paste; and bacteria-induced mineral precipitation. • The 28-day compressive strength of mortar specimens incorporated with beads was 55% of plain mortar specimens. Reducing the amount of bacteria-based beads will likely increase the compressive strength of the bacteria-based self-healing cementitious composite. Such a reduction, given the swellability of the beads, may have relatively little impact on the healing capacity of the composite. • The bacteria-based self-healing cementitious composite shows great potential for realising self-healing concrete in low-temperature marine environments, while the organic-inorganic healing material formed by the composite represents an exciting avenue for self-healing concrete research. I hope that the work presented herein provide a valuable reference for those interested in bacteria-based self-healing concrete, particularly for application in marine environments, and more generally for those interested in the wider field of self-healing materials research.