The use of supplementary cementitious materials (SCMs) offers a viable solution to partially substitute Portland cement and reduce carbon emissions associated with cement production. However, concerns over the future availability of traditional SCM sources, such as fly ash and slag, have left the concrete industry in need of alternatives. Water treatment plants generate large volumes of waste alumina-rich sludge in the purification of water for potable supplies. In the UK, the commonly used long-term disposal method is landfilling but this is being actively discouraged due to limited landfill spaces and increasing landfill costs. To address this disposal challenge and the need for alternative sources of SCMs, this thesis assesses the suitability of alum sludge as an additive in cement. A clear understanding of the physical and chemical impact of binder materials is fundamental to predicting performance and durability. Consequently, this work investigates the physical, chemical, and mineralogical properties of alum sludge and its calcination products, and their impact on the hydration, phase assemblage, microstructural development and engineering performance of blended cements.
Alum sludge was calcined at 475-1100oC and then characterized to correlate thermal changes with cementitious activity and engineering performance. Alum sludge calcined at 825oC transforms to poorly crystalline η-alumina (eta) and has the best cementitious activity. It is found that the reactivity of the sludge leads to the massive precipitation of ettringite at very early age which impairs the ability of gypsum to control C3A hydration leading to an undersulfated cement which inhibits alite hydration. The poorly crystalline η-alumina is metastable, transforming to highly crystalline -alumina (alpha) at 1100oC, whereupon alite hydration is enhanced because undersulfation is avoided and the alumina provides nucleation sites, leading to improved performance. The results obtained highlight the main properties of calcined sludge, explaining their influence on calcined sludge reactivity, cement hydration and the microstructure of the matrix.
Synergistic interactions of calcined sludge in blended cements containing slag (amorphous silicon oxide source) and/or limestone (calcium carbonate source) were explored. Results show that coupled substitutions of slag and calcined sludge (at 825oC) also resulted in an undersulfated condition due to reduction of gypsum contained in PC and the increased Al3+ concentration contributed by both slag and sludge leading to significant inhibition of alite hydration. Consequently, slag hydration was also hindered, leading to further reduction in performance. However, a synergistic interaction between limestone and η-alumina from sludge calcined at 825oC was confirmed. Calcined sludge provides aluminates, increasing the aluminate-sulfate ratio and enhancing limestone reaction. This reaction leads to the formation of mono- or hemicarboaluminate hydrates instead of monosulfoaluminate hydrate and stabilizes ettringite, leading to enhanced mechanical strength. This synergistic effect produced comparable strength to neat PC within 7 days with Strength Activity Indices up to ~98%. The lower pH of calcined sludge favoured limestone dissolution which enhanced Al3+ reaction, which is limited by gypsum dissolution in the absence of limestone. Hence, with an increase in the calcined sludge content, the chemically reactive portion of limestone increased. However, the degree of alite hydration decreased and undersulfation was evident at higher doses of calcined sludge. At later ages, the hydration of clinker phases in the neat PC system progresses at a faster rate leading to slightly lower Strength Activity Indices in the ternary calcined sludge-limestone cements.
The findings of this study show that clinker hydration and mechanical strength are strongly influenced by Al2O3/SO3 ratio in the calcined sludge-blended cements. To avoid an undersulfated condition, it is proposed that optimal doses of gypsum and limestone powder can help control the accelerator effect of 825oC sludge on C3A hydration while exploiting reaction synergies with limestone. In this way, the higher reactivity of the 825oC sludge can be better utilized for improved performance. This study provides a fundamental base and a promising direction for further studies and the widespread utilisation of calcined alum sludge in the cement industry.
