The mechanism of supplementary cementitious materials enhancing the water resistance of magnesium oxychloride cement (MOC): A comparison between pulverized fuel ash and incinerated sewage sludge ash

doi:10.1016/j.cemconcomp.2020.103562

Cement and Concrete Composites

Volume 109, May 2020, 103562

https://doi.org/10.1016/j.cemconcomp.2020.103562 Get rights and content

Abstract

Magnesium oxychloride cement (MOC) pastes incorporating supplementary cementitious materials (SCMs) including pulverized fuel ash (PFA) and incinerated sewage sludge ash (ISSA) were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with energy dispersive X-ray spectrometry (EDX). The result showed that the mechanism of PFA and ISSA in improving the water resistance of MOC paste is similar, even though the molar ratios of the hydration product in the ISSA-incorporated paste and the PFA-incorporated paste were different. The active phases in PFA or ISSA could react with MgO and produce an amorphous phase (amorphous magnesium aluminosilicate gel), which was interspersed with Phase 5 and changed the morphology of Phase 5 to fibroid or lath-like phases. These fibroid or lath-like phases interlocked with each other and also connected with the amorphous phase in the matrix to form a stable compact structure. Therefore, the water resistance of MOC was improved. The ISSA-blended MOC paste had higher water resistance compared to the PFA-blended MOC, which may be due to the different chemical composition of amorphous phase and the dissolved phosphorus from ISSA.

Introduction

It is known that MOC exhibits many advantageous properties such as high early strength [1,2], high fire resistance [3], high abrasion resistance [4], etc. It can be used for industrial flooring [5], ship decks [4], fire protection [3], lightweight panel [2], etc. However, the application of MOC is not widespread due to the poor water resistance. It was reported that the compressive strength of MOC decreased by about 90% when it was immersed in water for 28 days [6]. It was concluded that Mg(OH)₂ would be formed as a result of the decomposition of the MOC hydration products-Phase 5(5 Mg(OH)₂•MgCl₂•8H₂O) and Phase 3(3 Mg(OH)₂•MgCl₂•8H₂O) [7] which were the main strength sources of MOC. And the Mg(OH)₂ has a much lower strength than that of Phase 5 and Phase 3. Therefore, the compressive strength of MOC decreased when immersed in water. Besides, the length change of MOC mortar was 1.8% after 28 days of water immersion [6], as the hydration of excess MgO resulting the expansion of MOC mortar. The expansion may also lead to crack of specimens and then the reduction of strength.

According to a previous study, the use of SCMs - class F pulverized fuel ash (PFA) [6], glass powder [8], and incinerated sewage sludge ash (ISSA) [9] could improve the water resistance of MOC. The PFA was a pozzolanic waste material generated during the combustion of coal sourced from a local power plant in Hong Kong. GP was obtained by milling recycled glass cullet collected from a local glass recycler (Laputa Eco-Construction Material Co. Ltd.). ISSA was obtained from the sewage sludge incineration center-T park, Hong Kong. The quantitative X-ray diffraction (Q-XRD) result showed that the amorphous content in the MOC paste increased after adding PFA and ISSA (higher than 50% after water immersion for 14 days) and was much higher than the amorphous content in the raw materials (around 10% of the whole mixture). These amorphous phases may be the reaction products between the MOC and Si or Al in SCMs as shown in Table 1. While the Phase 5 in the pure MOC paste decomposed completely during water immersion, the sample prepared with the incorporation of PFA or ISSA still contained certain amount of Phase 5 which was intermixed with some new amorphous phases. This indicated that the new amorphous phases helped to improve the stability of MOC in water. Therefore, it is important to understand the microstructure the amorphous phase in SCMs-blended MOC.

SEM/EDX has been used previously to characterize the morphology of MOC incorporating PFA and ISSA [6,9]. By using SEM-EDX, it was found that the morphologies of the hydration products were changed after adding ISSA and PFA. The main product in a pure MOC was the needle-like Phase 5, while the hydration products were present as flat and wide plates containing Si and Al in the PFA/ISSA-blended cement. However, due to nature of SEM and the limitation of the sample preparation, it is difficult to characterize the elemental compositions and the morphology of the new amorphous phase using SEM-EDX. TEM is an effective technique to investigate the microstructure of intermixed phases [10,11] and to distinguish the amorphous phases from the crystalline phases. Therefore, the objective of this study is to investigate the morphology and distribution of the amorphous phases formed in MOC incorporating PFA and ISSA by using TEM. EDX was employed to further investigate the elemental compositions of the products formed in the PFA/ISSA-blended MOC pastes.

Section snippets

Experimental program

The materials used were as follows: Light-burned magnesia powder (MgO), Bischofite (MgCl₂• 6H₂O), Class F PFA, and ISSA. The details of these materials have been given in our previous study [6,9]. The bischofite was mixed with water to ensure complete dissolution and then poured into the mixing bowl before MgO was added. The molar ratio of MgO/MgCl₂ and H₂O/MgCl₂ were 9 and 10 respectively. The PFA and ISSA was respectively used to replace MgO at a dosage level of 30% by mass of MgO. The MgCl₂•

Compressive strength

The compressive strengths of the MOC pastes are shown in Fig. 1(A14 means 14 days of air curing, W28 means 14 days of air curing followed by 28 days of water immersion). It can be seen that MOC pastes showed high compressive strength before water immersion. The compressive strength of control MOC paste was about 160 MPa after 14 days of air curing. The incorporation of PFA and ISSA slightly decreased the strength, but the strength of PFA-blended MOC was still higher than 100 MPa. It can be

Discussion

As can be seen from Table 2, the MOC paste had high compressive strength and the strength was 165 MPa after 14 days of air curing. However, the compressive strength decreased significantly when immersed in water due to the decomposition of hydration products (Phase 3 and Phase 5) and the strength retention coefficient was only 10% after 28 days of water immersion. When adding PFA or ISSA, the compressive strength decreased slightly during air curing (112 MPa and 150 MPa respectively). However,

Conclusion

MOC showed poor water resistance due to the instability of the hydration products in water. Using PFA or ISSA could significantly improve the water resistance. New amorphous phases were observed in PFA or ISSA blended MOC, which may be the reason why they improved the water resistance.

The morphologies of magnesium oxychloride cement incorporating PFA and ISSA were examined using SEM and TEM/EDX. The SEM result showed that the main hydration product in MOC paste was needle-like crystalline-

Acknowledgement

The authors would like to acknowledge the financial support from the Construction Industry Council.

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The mechanism of supplementary cementitious materials enhancing the water resistance of magnesium oxychloride cement (MOC): A comparison between pulverized fuel ash and incinerated sewage sludge ash

Abstract

Introduction

Section snippets

Experimental program

Compressive strength

Discussion

Conclusion

Acknowledgement

Construct. Build. Mater.

Cement Concr. Res.

Cement Concr. Res.

Cement Concr. Res.

Cement Concr. Compos.

Construct. Build. Mater.

Cement Concr. Compos.

Cement Concr. Res.