Study of the degradation of non-conventional MgO-SiO₂ cement reinforced with lignocellulosic fibers

Abstract

This paper assesses the use of low alkaline composites based on magnesium oxide and silica (MgO-SiO₂) cement and reinforced with cellulose fibers for the production of thin elements to resist bending loads. The strategy adopted was to study the durability of lignocellulosic fibers in a lower pH environment than the ordinary Portland cement (OPC), by comparing the flexural performance of samples at 28 days and after 200 accelerated ageing cycles. Two types of vegetable fibers were used: eucalyptus and pine pulps. For both types of fibers, composites made out of MgO-SiO₂ cement after ageing treatment show a better mechanical performance than OPC samples (modulus of rupture of ∼10.5 and 9 MPa respectively). When used in MgO-SiO₂ cement matrices, eucalyptus fibers offer excellent specific energy (SE) values (∼5 kJ/m²) compared to OPC samples in which SE drastically decreases after ageing from 4.97 kJ/m² to 0.14 kJ/m². The preservation of the reinforcing capacity of the composite materials after ageing was also proved by SEM techniques. In the light of the results, the use of MgO-SiO₂ cements is an effective way to apply cellulosic fibers as reinforcement in fiber-cement products since no signs of degradation were found, even improving flexural properties over time.

Introduction

The production of asbestos-free fiber-cement is still a key challenge from a technical point of view [1]. At present, the necessity for new fiber cement technologies is increasing to attend the huge worldwide demand. Synthetic polymeric fibers have become the most common alternative as reinforcing elements for asbestos-free fiber-cement products in more advanced societies. In spite of their suitable properties, these fibers introduce an extra cost in the production of fiber-cement and are not the best option from an environmental point of view [2]. Cellulose fibers have become a possible substitute as reinforcement material in air cured cement composites [3], because of their technical features, i.e. tensile strength, modulus of elasticity and elongation at break [4]. These fibers are also non-hazardous, renewable and readily available [5]. Vegetable fibers present very reduced cost of manufacture when compared to synthetic fibers and also requires a lower energy demand for its production [4]. The development of this type of materials may introduce new chances for innovative farming business in rural areas around the world where good quality cellulosic fibers species are available. Vegetable fibers can be also obtained from agricultural by-products, generating an additional value to agro-industrial products.

However, lignocellulosic fibers in matrices made out of Portland cement undergo a process of degradation when exposed to humid environments, resulting in a loss of mechanical properties. The degradation process in fiber-cement composites is due to the alkaline decomposition and mineralization of the reinforcing fibers, generating a decrease of tensile strength of the whole composite and a reduction of fiber-matrix adhesion in a post-cracking state. The process of mineralization comes from the migration of the hydration products of Portland cement, mainly Ca(OH)₂, to the fiber structure. Various studies [5], [6], [7], [8] have addressed the problem of cellulosic fibers durability in cement matrices and several measures have been proposed to avoid their mineralization process and the alkaline degradation. All the actions are directed ultimately to reduce the amount of calcium hydroxide present in the matrices. Among these measures could be highlighted: 1) the partial replacement of Portland cement by pozzolanic materials [9], 2) the use of low alkalinity matrices [10] and 3) carbonation of Portland cement matrices [11].

According to Toledo Filho et al. [12], when fibers are submitted to alkaline solutions of different hydroxyl compounds, the worst conditions are those where Ca is present, suggesting that the fiber embrittlement occurs due to their mineralization by the filling of the lumen, walls and voids with calcium hydroxide. Despite the improvement on durability of the vegetable fibers in cementitious matrices achieved by the treatments mentioned above, they are still partially successful since it is not possible to protect fibers from a calcium rich environment during the early hydration stages.

New types of cement as magnesium oxysulfate (MOS) cements have been successfully used to prevent cellulosic fibers degradation over time [13]. However, the mechanical results of this new kind of cement are far lower than those obtained with Portland cement products. Besides MOS cement presents leaching problems with time [14]. To solve these inconveniencies, this work introduces a new type of magnesia cement for the production of fiber-cement elements from the combination of low-burnt MgO powder and reactive SiO₂ (MgO-SiO₂ cement). Over the last decade, many authors [15], [16], [17], [18], [19] have studied this MgO-SiO₂ cement, which hydrates to form magnesium silicate hydrated (M-S-H) gel and magnesium hydroxide (MH), bringing as result a binding material with low-alkalinity [20] and excellent mechanical properties [21]. This cement also has the advantage of offering a chemical composition with irrelevant amounts of calcium, thus avoiding fiber mineralization.

In order to analyze the effect of low-alkaline calcium-free cement on cellulosic fibers integrity over time, this work evaluates the flexural properties of the composites before and after ageing. As the flexural performance of the composite after the first cracking event is mainly related to fiber reinforcements capacity, this research focuses on this aspect to assess fiber long-term integrity. Thereby, in order to simulate long-term conditions in a faster manner, accelerated ageing is applied, by an adaptation of the BS EN 494:2012 Standard [22], exposing the samples to an elevated number of soaking-drying cycles. Based on results of Almeida et al. [11], 200 cycles of accelerated ageing are enough to produce a drastic change on mechanical behavior for fiber-cement composites, thus expecting to simulate fibers performance after long period under weathering.