**1. Introduction**

The construction of structures involving concrete underwater placement usually require additional considerations due to its unique circumstances. Typically, the effective placement of conventional concrete mixture underwater depends on two main factors: the mix design of concrete itself and placement method during concreting [1,2]. For the mix design of concrete, additions of anti-washout admixtures (AWA) and viscosity-modifying admixtures (VMAs) are necessary for conventional concrete to minimise the washout effect and the ability to self-consolidate during the underwater placement [3–5]. Concrete resistance against washout can also be improved using mineral admixture with high fineness. The most used mineral admixture includes silica fume, ground granulated blast furnace slag (GGBS) and fly ash (FA) [2,6]. Heniegal [7] confirmed that the inclusion of

**Citation:** Yahya, Z.; Abdullah, M.M.A.B.; Li, L.-y.; Burduhos Nergis, D.D.; Hakimi, M.A.A.Z.; Sandu, A.V.; Vizureanu, P.; Razak, R.A. Behavior of Alkali-Activated Fly Ash through Underwater Placement. *Materials* **2021**, *14*, 6865. https://doi.org/ 10.3390/ma14226865

Academic Editor: Francisca Puertas

Received: 15 October 2021 Accepted: 12 November 2021 Published: 14 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

fly ash and silica fume with the addition of limestone or bentonite powder improved the flowing properties of conventional concrete and minimised the washout effect.

Past researchers had also investigated the use of seawater as replacement for water in ordinary Portland cement (OPC) concrete. The justification for using seawater in concrete is for offshore structures where it involves underwater concreting. According to Wang et al. [8] it is possible to mix seawater with cement where the early strength increased due to the existence of Cl<sup>−</sup> and Na+ ions. Specimens with low water/cement (w/c) ratio showed more significant early strength development. For 28 days strength, it also showed an increment about 10% compared to specimen produced with fresh water but it induced corrosion on the rebars [9].

Meanwhile for the placement method, the concreting process can be made using Tremie pipe for mass concrete. This method required steel pipe with a hopper attached to the upper end and injectable plug on the bottom of pipe. The pipe is immersed in water and when the pipe is full of concrete, its bottom is opened for the concreting process. Using this technique, a hydro crane is required to lift the Tremie pipe after finishing concrete placement and the bottom of the pipe need to be kept in the fresh concrete during the process to avoid washout effect [2]. For a small concrete placement underwater, the skip and toggle bag methods are most suitable [1]. The concrete is filled up into different sized buckets, where the top covers are sealed to prevent water infiltration during the lowering process of the concrete placement. The bottom door of the bucket is slowly opened during concreting to allow free flowing of the concrete.

Moreover, for underwater concrete mostly refer to the Standard by Japan Society of Civil Engineers (JSCE). This standard stated that the w/c ratio should be in range 0.50 to 0.55 when placing reinforced concrete in seawater and fresh water [10]. The w/c ratio can be increased up to 0.60 and 0.65 when concreting for non-reinforced concrete. For the strength of hardened concrete through underwater placement, JSCE standards required the compressive strength of specimens attain a minimum of 80% strength with respect to specimen cast in dry conditions [2,10].

The manufacturing of ordinary Portland cement (OPC) consumes a lot of natural resources, is energy intensive and contributed to carbon dioxide (CO2) emission to the atmosphere [11]. It was estimated that 7% of CO2 emission comes from the OPC industry [12], which is about 1.35 billion tons per annum. This is a serious environmental concern, and research endeavours involve finding a suitable alternative binder to replace OPC in concrete. The literature [13] refers to the work of Davidovits that found geopolymer in 1978 which also known as amorphous alkali aluminosilicate, and are sometimes referred to as inorganic polymers, geocements, or alkali-activated cements. This new alternative binder is produced by activating source materials with alkaline activators, and its classification is dictated by the content of silica, aluminium, and calcium. If the source materials are made up of mainly silica and aluminium (Class F fly ash, metakaolin, or some natural pozzolan), its final product is the sodium aluminosilicate hydrate (N-A-S-H) backbone of the geopolymer [14]. If the source materials are made up of calcium, aluminium, and silica (Class C fly ash and slag), then the main product after hardening is calcium silicate hydrate (C-S-H) or calcium alumino silicate hydrate (C-A-S-H), which also can be described as alkali-activated materials (AAM) [15].

Fly ash is an industrial waste material that is ubiquitous due to the increasing demand for energy, which is met by increasing coal-fired power plant's usage. The world coal production is expected to rise between 2006 and 2030 by almost 60%, with volumes output to 7011 Mtce by 2030 [16–19]. The management of fly ash disposal is always concerned by environmentalist since only 20–30% of the generated fly ash is reused whereas the rest was disposed either in landfills or ponds [17,18]. Therefore, the use of fly ash as aluminosilicates sources in AAM production is a waste-to-health approach that could also mitigate environmental concerns.

The parameters that influence the properties of AAM have been intensively investigated [19–25], and AAM are known to be resistant against aggressive ions, freeze-thaw

resistance, have high early and long-term strength, and excellent fire resistance [13,26–30]. The main issue of using conventional concrete for underwater structure is its resistance to washout. Concrete resistance to washout depends on the content of fine fraction in the binder, water cement ratio, and cement content. Concrete resistance to washout depends on the content of fine fractions in the binder, water cement ratio, and cement. Previous studies are mainly focused on underwater concrete placement using OPC as a binder and addition of special admixture for construction offshore structures such as bridge piers, but studies involving the application of AAM for underwater concreting remain scarce. The current study investigated the performance of alkali-activated fly ash (AAFA) paste through underwater placement in seawater and freshwater (river water and lake water). The compressive strength, changes in pH, X-ray fluorescence (XRF) and Field Emission Scanning Electron Microscope coupled with Energy Dispersive X-ray spectroscopy (FESEM-EDS) are analysed, respectively.
