Influence of Biofuel Bottom Ash and Chemical Additives on Hydration and Physical, Mechanical Properties of Cementitious Materials

This research supports the United Nations Sustainable Development Goals by promoting innovation in construction and fostering more sustainable, resilient cities. Using waste and additives in cementitious materials aligns with circular economy principles, reducing the demand for primary raw materials, encouraging resource efficiency, and lowering CO₂ emissions. Although waste often negatively affects cement properties, this study aims to address that challenge. The research explores the use of biofuel bottom ash combined with two innovative chemical admixtures: Master X-Seed 100, a hardening accelerator based on active crystal-seed nanoparticles, and MasterCO2re, a next-generation superplasticizer formulated using an Intelligent Cluster System (ICS). These additives were selected to counteract the delayed hydration and reduced workability typically caused by ash incorporation. Given the growing construction sector and the energy-intensive nature of cement production, finding sustainable alternatives is crucial. Therefore, this study investigates the partial replacement of cement with 5% and 10% biofuel bottom ash by mass. The primary goal is to analyse the effects of biofuel bottom ash and chemical additives on the hydration, as well as the physical and mechanical properties, of cementitious materials, and to determine their suitability for partial cement replacement. The experimental programme included calorimetry tests, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and compressive strength testing of the cement stone. Mortar mixes containing fine aggregates were also prepared and tested for consistency, hardened mortar density, ultrasound pulse velocity (UPV), and compressive strength after 7 and 28 days. The results showed that biofuel bottom ash retards cement hydration, and the superplasticizer further prolongs this process. However, introducing the accelerator offsets this delay, enabling hydration to occur similar as pure cement. When both the superplasticizer and accelerator are used, hydration is still somewhat retarded, but SEM analysis revealed that over time, biofuel bottom ash particles dissolve and react more actively, especially when chemical admixtures are present. Compositions with the accelerator achieved the highest compressive strength at both 7 and 28 days. Compared to the reference sample, compressive strength increased by about 10% (from 84.7 MPa to 93.5 MPa). Using only superplasticizer, the compressive strength remained similar to that of the reference sample without biofuel bottom ash. The improved strength is attributed to the enhanced reactivity of biofuel bottom ash, stimulated by the chemical additives, which led to increased formation of calcium silicate hydrate (C-S-H) and a denser mortar microstructure. The chemical admixtures notably improved the workability of the mortar and boosted compressive strength. Overall, this research highlights that through the strategic use of chemical additives, the negative effects of waste incorporation in cementitious materials can be overcome, offering a more sustainable solution for the construction industry.