https://www.um.edu.mt/library/oar/handle/123456789/16068 2026-09-26T02:38:31Z https://www.um.edu.mt/library/oar/handle/123456789/149388 Title: Machine learning-based prediction of compressive strength and CO2 uptake in CO2-cured cementitious materials Authors: Han, Seongho; Kim, Jayon; Farrugia, Kay; Jun, Heejung; Borg, Ruben Paul; Hong Kim, Jae; Yoon, Jinyoung Abstract: Predicting the compressive strength and CO2 uptake of CO2-cured cementitious materials is challenging because these properties are influenced by complex interactions among mixture composition, curing conditions, and carbonation behavior. Accurate prediction and optimization of CO2 curing efficiency are therefore essential for developing sustainable low-carbon cementitious materials. This study developed a Least Squares Boosting (LSBoost)-based machine-learning framework for predicting and optimizing compressive strength and CO2 Uptake using a literature-derived database consisting of 423 compressive strength datasets and 131 CO2 uptake datasets. The input parameters incorporated binder chemistry through the Steinour formula, mixture proportions, pre-conditioning conditions, and CO2 curing variables. Hyperparameter tuning identified an optimal LSBoost configuration consisting of 10 splits, a minimum leaf size of 3, a minimum parent size of 10, 500 learning cycles, and a learning rate of 0.5. The developed models demonstrated strong predictive performance, with testing correlation coefficients exceeding 0.95 for both compressive strength and CO2 uptake predictions. The compressive strength model achieved a testing mean absolute percentage error of approximately 10.4%, while the CO2 uptake model yielded a testing RMSE of 3.76 despite the presence of zero-uptake datasets. Feature importance analysis revealed that compressive strength was mainly governed by the Steinour formula, water-to-binder ratio, curing duration, and curing age, whereas CO2 uptake was strongly influenced by CO2 concentration, relative humidity, and exposure duration during CO2 curing. Monte Carlo-simulation-based optimization further suggested that CO2 concentrations above approximately 15%, relative humidity above 60%, and curing durations longer than 48 h promoted higher predicted CO2 uptake under the investigated conditions. 2026-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149376 Title: Effect of Lithium Sulfate on the Hydration Mechanisms and Performance of Ferroaluminate Cement Authors: Zhang, Yuanzhi; Shi, Penghang; Fu, Lifeng; Yang, Yi; Li, Lichen; Tang, Shengwen; Borg, Ruben Paul; Al Qunaynah, Siraj; Liao, Yishun Abstract: The effect of lithium sulfate (LS) admixture on the performance of ferroaluminate cement (FAC) remains poorly understood, and no systematic investigation of the regulatory effects of LS hydration in FAC systems has been conducted. By revealing how LS modulates the macroscopic properties and hydration progression of FAC, this work addresses an important knowledge deficit in the existing literature. This study investigated the performance evolution of FAC pastes with varying amounts of LS addition. It examined the setting time, fluidity, compressive strength, pH and electrical conductivity of the pore solution, as well as X-ray diffraction (XRD) and electrical resistivity. The findings indicate that LS significantly accelerates the early hydration rate of FAC, shortening setting time and reducing paste fluidity as its content increases. Notably, LS inhibits compressive strength development at 12 h, yet it enhances the rate of strength development after 1 day. The analysis of electrical conductivity and pH demonstrated that LS increases ion concentration and alkalinity within the first day, and these parameters stabilize after 28 days. Resistivity measurements indicate that LS raises paste resistivity within 3 h and enhances the peak hydration rate. Quantitative XRD demonstrates that LS induces complex lithium-sulfate coupling effects on ettringite (AFt): LS20 hits the critical $\text{Li}^+$ inhibition threshold, while the sulfate from the higher LS dosage mitigates such negative influences. These findings establish a theoretical foundation for the targeted application of LS in materials based on FAC. 2026-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149375 Title: Evaluating the printability and mechanical performance of low impact concrete with slag and waste limestone for 3D printing Authors: Al Mawed, Loai; Borg, Ruben Paul Abstract: Additive manufacturing of concrete has addressed several challenges associated with traditional casting methods by reducing the need for formwork, reducing labour costs and enabling complex architectural designs. The rheological properties of 3D printed concrete particularly flowability, extrudability and buildability are important to ensure quality. These printability requirements are often achieved by significantly increasing the cement content in the mix. However, such an approach raises critical concerns regarding the environmental sustainability of 3D concrete printing (3DCP). This research explores the feasibility of using ground granulated blast furnace slag and globigerina limestone waste as partial replacement of ordinary Portland cement to develop low-carbon concrete mixes for 3DCP. Five different mix compositions were developed: a control mix containing 100% Portland cement (CEM I 52.5R), and mixes with partial cement replacement; 20% slag and three additional mixes incorporating 20% slag combined with 10%, 15%, and 20% limestone. Experimental investigations were conducted to evaluate the fresh and hardened properties of the developed mixes. Fresh-state tests covered flowability, extrudability, buildability and open time, while hardened-state tests examined compressive strength, and flexural strength with emphasis on anisotropic behaviour under different loading orientations. Comparisons between cast and printed specimens were carried out to assess anisotropy for the printed specimens. The results show that GGBFS enhances the mix's flowability while the inclusion of limestone waste reduces it. Therefore, an appropriate balance between slag and limestone is essential to achieve optimal flowability and ensure printability. Incorporation of a high proportion of limestone negatively influences the mechanical performance and reduces the printability of the mix. Cast specimens exhibited higher compressive strength than 3D printed concrete, the latter showing anisotropy with lower strength in the Z-direction. GGBFS and limestone waste were effectively used to produce low carbon 3D printed concrete with sufficient rheological and mechanical properties, thereby reducing the environmental impact of 3D Printing of concrete. 2026-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/149373 Title: Assessment of recycled low-carbon ultra-high performance concrete based on recycled UHPC Authors: Borg, Ruben Paul; Semenov, Igor Abstract: Ultra-high-performance concrete (UHPC) is widely used in construction for its outstanding strength and durability. However, its high cement and steel fiber content leads to increased carbon emissions and environmental concerns. This study explores the use of sustainable materials in UHPC to reduce its overall environmental impact. The objective of the research is to develop a low-carbon Ultra High Performance Concrete (UHPC) and to assess the performance of recycled low-carbon ultra-high performance concrete (R-UHPC), produced from different fractions of recycled aggregate crushed from low-carbon UHPC. Low carbon UHPC based on a cement with low environmental impact (CEM IV) was first produced and optimised to ensure an improved performance in terms of fresh properties and mechanical properties including strength and durability. In the development process, UHPC with different types of fibres were assessed, including amorphous alloy and steel fibres. The Low Carbon UHPC developed, was then used as source material for the production of the Recycled Low-Carbon UHPC, based on recycled UHPC aggregate, as a substitute of the natural aggregates as is typical of recycled aggregate concrete. Different recycled UHPC mixes were designed and produced with a reference mix based on natural aggregate and three mixes with the natural aggregate replaced using recycled UHPC according to different percentage replacement values (50 and 100%). This supports the cradle-to-cradle approach in life cycle engineering applications. The research confirmed the effective regeneration of new UHPC based on the recycled UHPC, attaining the required rheological characteristics, mechanical properties, durability performance. This work supports the sustainability assessment of the end of life of Low Carbon UHPC materials and structures and the potential of recycled UHPC for new structures and retrofit structural applications. 2026-01-01T00:00:00Z