The Development of Sustainable Internal Curing Agents Using Natural Hollow Fibres

Abstract

The incorporation of superabsorbent materials as internal curing agents has demonstrated effectiveness in mitigating self-desiccation and early-age cracking in cement mixes characterized by low water-to-binder ratios (w/b). However, concerns related to sustainability, availability, and cost have emerged, primarily due to the petroleum-based nature of common superabsorbent polymers (SAP). This PhD research aims to investigate the use of locally available milkweed (MW) fibres, which possess a unique hollow structure, as internal curing agents for low-w/b Portland cement mixes. The study focused on examining the impact of different pre-treatment methods on the morphology, chemical composition, and hygroscopic characteristics of MW fibres in the initial phase. Two pre-treatment methods, namely hydrothermal treatment (HT), and hybrid treatment (HY), a combination of hydrothermal treatment with alkaline treatment, were explored. Findings from the first phase indicated that both HT and HY treatments effectively enhanced the water absorption capacity of MW fibres. However, water retention was influenced by the morphology of the fibres. The HT treatment preserved the hollow structure of the fibres, leading to improved water retention, whereas the HY treatment caused significant morphological changes, resulting in a noticeable reduction in water retention. In the second phase of the study, the impact of pre-treated MW fibres on the effective w/b of cement mixes was assessed through rheological measurements. In addition, the influence of MW fibres on the heat of hydration and hydration products was investigated. The introduction of MW fibres to Portland cement paste induced a shift in their effective w/b to lower values, attributed to the absorption action of MW fibres during the pre-saturation process. Furthermore, using 0.1% wt. pre-treated MW fibres resulted in a 17% improvement in the degree of hydration compared to the reference sample. This was accomplished by ensuring an adequate amount of free water for the initial reaction of cement hydration and entrained water in the lumen for subsequent internal curing. In the third phase, the study investigated the kinetics of water transport from MW fibres to cementitious paste systems, using Nuclear Magnetic Resonance (NMR) and Differential Scanning Calorimetry (DSC). The observations revealed that both the morphological features and the release of extractives such as lignin impact the desorption kinetics of MW fibres' entrained water within cementitious matrices. The HT-MW fibres, with their preserved hollow morphology and enhanced hydrophilicity, exhibited the most effective performance, showing the highest amount of available internal curing water in the transverse relaxation time (T2) measurements. In contrast, the absence of lignin in the HY-MW fibres allowed hydration products to migrate into their lumen. This led to the early consumption of the entrained water, a process further exacerbated by the holes and defects in the fibre walls. The shrinkage, mechanical strength, and microstructural development of cementitious mixes after incorporating pre-treated MW fibres were examined in the final phase. The internal curing role of the pre-treated MW fibres proved effective in reducing autogenous shrinkage, with significant reductions of up to 28% for 7 days compared to the plain samples. In addition, hydration products were observed to penetrate the lumen of the HY-MW fibres to a depth of up to 150 micrometres. This penetration exacerbated drying shrinkage by facilitating moisture transport to the unsaturated environment. Consequently, this phenomenon led to a higher chance of cracking and also lower flexural strength in HY-MW fibres compared to HT- and N-MW fibres. The comprehensive analysis and findings of this PhD research supported the feasibility of using MW fibres as sustainable internal curing agents to effectively reduce autogenous shrinkage after applying straightforward, cost-effective, and low-carbon footprint pre-treatment methods.