Research Topics

The effect of supplementary materials on structure and electrochemical property of passive film of carbon steel

Corrosion of rebar in concrete is initiated when the inert protective film on rebar surface is depassivated. Under chloride-induced corrosion, the depassivation is generally assumed to occur when chloride-to-hydroxyl ion ratio reaches a critical value, independent of the passive film properties. However, many studies have shown that matrix composition, pH level, pore composition, oxygen availability and presence of chloride can alter the physico-chemical properties of the passive film.  With electrochemical measurements and atomic characterization, this research aims to elucidate if corrosion initiation of rebar in concrete is affected by passive film properties. This study will provide a fundamental understanding of characteristics of passive film of carbon steel in concrete and would pave way for possible microstructural or atomic tailoring of passive film to extend lifespan of reinforced concrete structures against corrosion.

Figure 1. Corrosion process in reinforced concrete structure

Chloride binding and diffusivity of cement pastes with cenosphere

Under high pH environment of concrete pore solution, steel rebar is protected by a thin inert layer called passive film. This film can be depassivated and corrosion is initiated when chloride to hydroxyl ratio [Cl-]/[OH-] in the pore solution reaches a critical level. Chloride may exist in concrete in the form of bound and free chloride, and it is the latter that influences the depassivity of steel. The binding occurs through physical mechanism where chloride is absorbed onto C-S-H lattice or locked in the interlayer of C-S-H, and chemical process where chloride reacts with C3A and C4AF to form Friedel’s salt or its derivatives. Fly ash with relatively high content of C3A has been shown to reduce free chloride ion content in pore solution. In this study, chloride binding ability of cenosphere, also a by-product of coal-fired power plant with similar chemical compositions to fly ash, is investigated. The study will provide an alternative option to design concrete composite that is more resistance to chloride resistance and hence more durable against corrosion.  

Chloride binding enhancement for the use of seawater in concrete

Global demand for cement is predicted to rise to 5.2 billion tonnes in 2019 and Africa and the Middle East will be one of the fastest growing consumers. The rise in cement consumption is associated with an increased production of concrete and resulting demand for other concrete constituents including aggregates and water. This poses a great burden for water-stressed regions such as the United Arab Emirates whose arid climate offers very little rain to rely upon. Direct use of seawater is viewed as a possible solution. Typical seawater contains 3.5% of dissolved salts with cations Ca2+, Mg2+, Na+ and K+, and anions Cl-, CO32-, HCO3- and SO42- species. With the presence of Cl- and the associated risk of steel corrosion, seawater is not recommended for use in concrete. For this project, a systematic investigation of various methods to bind chloride and their effects on concrete properties, long-term stability of the binding products, and long-term durability of concrete regarding corrosion will be conducted. Supplementary materials are proposed to be the core materials used to immobilize chloride from seawater. Slag contains high alumina content and hence induces chloride binding through the formation of Friedel’s salt while silica fume with high surface is postulated to enhance the composite’s strength and permeability. The composites will also be doped with different types of smart polymers to enhance its chloride binding ability. The success of the study will lead to a potential use of seawater in concrete mixing without compromising concrete durability and thus will contribute the sustainability and ecology of construction industry.

Rheological control for printing of ultra-lightweight cement composite in warmer climates

Three-dimensional (3D) printing technique is a manufacturing process for the production of complex three-dimensional objects directly from a computer model by layered printing process. Over the past decades, the construction industry has been facing stagnant innovation and 3D printing serves as an emerging and exciting arena to rid the issue. 3D printing of cementitious materials has a potential to offer a myriad of benefits: designs tailored to specific tasks, faster and more accurate construction through controlled deposition of materials, reduction in waste, decrease in labor demand and subsequent labor cost, integration of services into 3D printed bodies, and reduction in interface detailing. A thorough consideration encompassing materials development (rheology, interlayer strength, and intrinsic support strength), predictive model for materials characterization, process improvement (step-wise, ultrasonic vibration, fiber reinforcement), validation through testing and simulation will be conducted. A focus will be on 3D printing of an ultra-lightweight cement composite (ULCC) whose mix proportion is based on a previously studied mix shown to exhibit high-strength, low density, low thermal conductivity and high solar reflectance. Our new formulation will contain cement, 60% of waste products as cement replacement, cenosphere as ultra-lightweight aggregate, polyethylene (PE) fibers and a new generation of admixture tailored to meet a set of design criteria to make the composite 3D printable. We will optimize the rheological properties of this formulation to obtain a printable material. Mechanical, physical and microstructural characterization will also carried be out to demonstrate the composite’s performance for 3D printing.

Effect of aluminum inclusion on morphology and mechanical properties of calcium alumino silicate hydrate (C-A-S-H)

Calcium aluminate silicate hydrate (C-A-S-H) is one of the main binding phases in aluminium-rich blended cement. Existing literature shows that Al-incorporated crosslinking site increases the bulk modulus of calcium alumino silicate hydrate (C-A-S-H). However, structural, physical and mechanical properties of C-A-S-H are still a subject of research. Here, calcium (alumino)silicate hydrate (C-(A-)S-H) with bulk molar ratio Ca/(Al+Si) = 1 and Al/(Al+Si) = 0-0.33 were synthesized and equilibrated at 80°C for 4 months by mixing stoichiometric amounts of SiO2, CaO and CaO·Al2O3 at a water-to-solid mass ratio of 45 in a N2-filled glove box. X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermogravimetry were used as methods of analysis. XRD results showed a comparable interlayer spacing of the C-A-S-H samples. Thermogravimetric analysis revealed that minor or trace of third aluminate hydrate and/or katoite present in the C-A-S-H products at Al/(Al+Si)=0.05-0.2. TEM images showed that all C-(A-)S-H samples at these conditions are foil-like although the inclusion of aluminium led to a coarser morphology (Fig.2), indicating a more polymerization and cross-linking of C-A-S-H. 27Al and 29Si nuclear magnetic resonance spectroscopy will be performed to study Al incorporation on the main dreierketten C-S-H chain. Nano-indentation work will be conducted to validate the effect of Al-incorporation on the mechanical properties of C-A-S-H with limited variation in the interlayer spacing.

Al/(Al+Si) = 0
Al/(Al+Si) = 0.1
Al/(Al+Si) = 0.33

Figure 2: Bright-field TEM images showing morphology of C-(A-)S-H at Al/(Al+Si) = 0, 0.1 and 0.33

Carbon-neutral MgO-based cement production

The world population is projected to grow nearly 30% by 2050, which will be accompanied with urbanization at an accelerated pace. This will increase the consumption of key building materials, such as Portland cement (PC). Carbon dioxide (CO2) emissions associated with the production of PC is a major sustainability issue. To alleviate the environmental impact of increasing PC consumption within the construction industry, alternative binder systems, such as reactive magnesium oxide (MgO) cements are proposed as one of the emerging solutions.

MgO is not mined directly, instead it is primarily produced through either by a dry route from the calcination of mined magnesite (MgCO3), availability of which geographically restricted within the handful of countries, or by a wet route from the calcination of magnesium hydroxide (Mg(OH)2) that is obtained from magnesium-rich sources such as sea water and natural brine. In this study, the reactive MgO will be obtained from the reject brine of United Arab Emirates (UAE) desalination plants. This is realised in two steps, first by precipitation of Mg(OH)2 after adding a base, second by calcination of the produced precipitate.

Reactive MgO will then be used to produce cements that will be carbonated. Carbonation is a crucial process for reactive MgO cement binder as it hardens and gain strength through the formations carbonates and hydroxycarbonates.

This project will provide a sustainable source of magnesium oxide by valorization of seawater desalination reject brine valorization. Not only the reaction process is chemically carbon neutral, the source of magnesium from the sea is virtually unlimited and worldwide available but it would also reduce the quantity of salts rejected to sea and its environmental impact. Carbonation of the cements obtained has the potential to lead to a carbon negative source of cement.