Use of calcium hydroxide in wood ash-based alkali-activated material

Abstract

The worldwide concern on sustainable development also proposes demands in the build ing and construction sectors, focusing on fabricating low-carbon footprint concrete. Alkali activated materials (AAMs) contribute to less environmental impact, in contrast with the traditional binder material Portland cement, and are regarded as eco-binders. Particularly, the possibility of the incorporation of industrial by-products as precursor materials provides a more sustainable approach for waste management than landfills. Wood ash (WA) is a waste produced in the biofuel energy industry during the procedures of combustion of wood saw dust and chips. With the promotion of renewable clean energy around the globe, the manage ment of WA waste becomes a significant challenge. Unlike other industrial wastes, such as slags and coal fly ash, which have been well researched regarding their application in AAMs, the utilization of WA in building materials is still limited. In this study, thus, WA, including wood fly ash and wood bottom ash, was used as a pre cursor to manufacture a waste-based AAM. Recycled sand valorized from biofuel energy plants substituted natural river sand as fine aggregates. Due to the consideration of clean pro duction, as well as to decrease alkali contents in the binder system, calcium hydroxide (CH) was added as a ternary alkaline activator at a ratio of 0, 5, 10, 15, and 20% by precursor mass, besides the conventional activators sodium hydroxide and sodium silicate at a low molarity. To understand the effects of CH content on the engineering properties and microstructure of the produced AAM, a series of tests were conducted, including compressive strength, water absorption, thermogravimetric-differential thermal analysis (TG-DTA), exothermic profiles, scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). The results show that the use of CH as a ternary activator greatly contributes to the im provement of the compressive strength within a range between 16.64% and 24.26%. The use of CH powder led to the densification of the AAM microstructure and was related to a reduction of water absorption. In the SEM graphs in Fig.1, the reference sample with 0% CH exhibited a mal-bond microstructure. With a 10% CH addition, a denser morphology was observed. The following increment of CH content to 20% led to less densification of the microstructure. This corresponds well to the strength development. Based on the thermal analysis, CH exerted a positive influence on the facilitation of the early hydration degree, with higher exothermic heat observed during 24 hrs. The TG-DTA examination on the 28-day sample also suggested an increment in the production of hydrates. This was also confirmed by the XRD analysis, where both crystalline and amorphous hydrates were identified.