Influence of Colloidal Nanosilica on the Early Performance of Wood Ash Cement Paste, Mortar, and Concrete

Abstract

To reduce carbon dioxide emissions from cement and concrete production, in accordance with Sustainable Development Goals 3, 9, and 11, colloidal nanosilica has been investigated as a tool to advance alternative and sustainable cementitious material production by evaluating its effect on the early performance of mechanical and microstructural properties of wood ash cement paste, mortar, and concrete. In this study, the control group with Portland Cement without nanosilica and wood ash (WA) in mortar will be denoted as CM and CC in concrete, respectively. The experimental groups are labeled Zawa 5–25, indicating the use of South African WA as a partial replacement of cement at various proportions (5, 10, 15, 20, and 25). The nanosilica solution (NSS) of 0.265 mol/L was used throughout; NSS was made by dissolving 3.5 g of nanosilica (SiO2) in 13.201 L of distilled water. Eighteen combinations having different mixtures of mortar samples were made with a standard sand and water/binder ratio of 0.5. Similarly, 144 concrete mixtures having a constant 0.5 water/binder ratio were prepared, and they were evaluated for both compressive and splitting tensile strength. Compressive and flexural tests were performed on the mortar samples at a 28-day curing period and the concrete samples at 3, 7, 14, and 28-day curing periods for splitting tests. The investigation involves examination of chemical composition, specific gravity, and size distribution of the wood ash and fine and coarse aggregates used in the mix. The paper assesses the uniformity of the fresh cement paste characteristics using flowability and setting time. The hardened properties, like compressive strength and flexural strength of the mortar, were examined. The CC concrete samples were also examined in terms of the mechanical properties (compressive and splitting tensile strength) and microstructure. A mixing formula of 5% WA showed an improved workability with an initial setting time of 250.09 s and a final setting time of 348.13 s, showing a favorable ratio between setting time and workability, as the mixture of 5% WA gives the best spread. The time of initial setting of the WA cement paste with 5% is greater than that of the control sample by 10.4%. In addition, the optimal strength development was realized on 28 days with the mixture of 5% WA with mortar and corresponding strengths of 59.2 MPa of mortar, which is an improvement of 8% over the control mix. At 28 days, the compressive strength of the concrete samples with 5 and 10% levels of WA is increased by 8 and 14%, respectively, compared to the reference concrete samples. On the other hand, irrespective of the percentage of replacement of WA at 28 days of curing, the results showed that the splitting tensile strength decreased with time. This reduction in the tensile strength of splitting could be explained by the higher porosity and microstructure alteration related to the higher wood ash content of the concrete mix. The filling effect of NSS is very important in making the concrete denser and stronger by physically filling the openings left by cementitious materials, capillary holes, and calcium silicate hydrate crystals, resulting in denser and more uniform hydration products. The novelty of the present research is that wood ash and nanosilica addition may result in the formation of a high-performance, cost-effective green concrete. The commercial implications of the energy sector are enormous as the world evolves to greener technologies. The research findings may lead to the development of innovative solutions not only to reduce carbon discharge but also to improve the qualities of construction materials.