Production of a lightweight masonry block using alkaline activated natural pozzolana and natural fibers

Highlights

• Lightweight geopolymer blocks were produced for non-structural construction purposes.

• Compressive strength in the block was 8.1 MPa and bulk density was reduced to 1269 kg/m3.

• Hydrogen peroxide proved to be effective for reducing density in geopolymer matrices.

• Fine aggregate positively affected strength and dimensional stability of geopolymer mortars.

• Stabilization of geopolymer mortars with jute fibers was necessary to control cracking.

• The fibers were also effective for controlling the non-brittle failure of the blocks.

Abstract

Lightweight geopolymers are alternative low-carbon footprint building materials with high potential to produce value added products for the construction industry such as low-density masonry units. This work focuses on presenting the development of a new lightweight masonry block using natural pozzolana-based geopolymer mortar, hydrogen peroxide as a foaming agent and jute fibers as reinforcement additive. The experimental plan allowed the optimization of the production process and the definition of the optimum quantities of hydrogen peroxide content, pozzolana: fine aggregate ratios and the appropriate amount of fibers to be incorporated. The experimental results demonstrate the feasibility of the production of new masonry units, which can achieve up to 8.1 MPa of compressive strength after 28 days of fabrication with reduced bulk densities of 1269 kg/m³.

Introduction

Most lightweight building blocks are produced with lightweight ordinary Portland cement (OPC)-based concrete, a construction material with low density (300 – 1800 kg/m³), low thermal conductivity, high acoustic insulation capacity and fire-resistance [1]. Although lightweight concrete blocks are widely accepted in the construction sector, recent investigations aim to reduce the use of OPC due to the considerable high environmental impact and the consumption of a large amount of natural resources for its production [2], [3], [4], [5], [6]. For instance, Torkaman et al. [5] reports the use of wood fiber wastes, rice husk ash and limestone powder wastes as OPC replacement materials to fabricate lightweight building blocks. Their blocks reached compressive strengths from 1.4 MPa to 6.7 MPa with densities from 1400 kg/m³ to 2000 kg/m³ with replacements of up to 50% wt. of OPC. On the other hand, Khan et al. [7] investigated the use of recycled glass powder in the production of lightweight concrete. They found that the replacement of 20% of OPC with glass powder increased the compressive strength of lightweight concrete. In the same manner, Pelisser et al. [8] studied the use of recycled rubber from tires with added metakaolin for lightweight concrete production. In this study, lightweight concrete prepared with 40% rubber instead of sand and with the addition of 10% metakaolin achieved a compressive strength of 20 MPa. On the other hand, Munir et al. [9] explored the use of an industrial by-product such as palm oil fuel ash as partial replacement for OPC in the manufacture of lightweight concrete. Their results showed that palm oil fuel ash could replace up to 20% of OPC for the production of lightweight concrete to be used as nonstructural elements.

The use of geopolymers as a replacement of OPC for the manufacturing of lightweight mortars has also been investigated [10], [11]. Lightweight geopolymers have been produced using a variety of techniques, such as addition of pre-fabricated foams or foaming agents [10], [12]. Results have shown that both methods generate voids in the geopolymer mixture resulting in a decrease in the density of the matrix [10], [13]. The most common chemical foaming agents that have been reported for lightweight geopolymers production are aluminum powder and hydrogen peroxide (H₂O₂) [14], [15], [16]. Sodium hypochlorite (NaOCl) [13], sodium perborate (NaBO₃) [11] and silica fumes [16] have also been used as foaming agents. As shown in Table 1, recent literature reports the findings of lightweight geopolymers matrices [12], [13], [14], [15], [17], [18], [19] and mortars [11], [20], [21], [22], [23] made from fly ash (FA) and metakaolin (MK). The reports show lightweight geopolymer matrices with densities from 400 to 1150 kg/m³ and compressive strengths between 1 and 10 MPa. On the other hand, lightweight geopolymer mortars using fly-ash or metakaolin exhibited densities from 600 to 1500 kg/m³ and compressive strengths also between 1 and 10 MPa. The reports show studies about lightweight geopolymers at the material analysis level, which involved testing small cylindrical or cubic samples; the evaluation of real-size lightweight units or structural elements is still an open challenge.

Regarding the production of masonry units with geopolymers, Table 2 summarizes recent investigations in which geopolymer mortars from different sources (i.e. fly ash, mine tailings and red mud) were used. Abdullah et al. [24] prepared geopolymer masonry bricks from fly ash. These specimens were produced using a compacting pressure of 10 MPa, considered a fly ash : fine aggregate ratio of 1:3, and were cured in an oven at 60 °C for 24 h. The developed masonry brick had a 7th-day compressive strength and density of 13.2 MPa and 1828 kg/m³, respectively. Ahmari and Zhang [25], on the other hand, produced geopolymer masonry blocks from mine tailings. The blocks were also compacted using a forming pressure that varied from 0.5 to 15 MPa and were cured in an oven at 90 °C for 7 days. The resulting geopolymer blocks had a 7th-day compressive strength that varied from 5 MPa to 34 MPa, while the density was between 2004 kg/m³ and 2035 kg/m³. Finally, Kumar [26] produced geopolymer masonry blocks using red mud as the raw material. These blocks reached a compressive strength of 25 MPa, unfortunately, cracking issues were reported during the drying process. In this case, the author suggested that the origin of the cracks was the very fine particle size of the raw material and recommend the addition of fine aggregates as a filler to control the problem.

The development of sustainable materials with high strength is a growing demand in the construction industry. In this context, natural cellulosic fibers are gaining great attention as alternatives for replacing conventional steel or synthetic fibers as reinforcement in cementitious composites [27], [28], [29], [30]. In this regard, the natural fibers are highly promising as they can provide mechanical strength improvement with low environmental impact. They also have a high positive economic impact, low density, non-hazardous nature, and non-abrasive characteristics [29], [30], [31], [32], [33]. Natural cellulosic fibers are used in the form of pulp or filament fibers in cementitious composites resulting in a tension softening behavior with low tensile ultimate strength in non-structural applications [34]. Recent studies have reported about their influence to control cracking problems during the drying process of masonry units. For example, in the case of adobe blocks, the use of natural fibers such as straw, sisal and coconut was successful to control this issue [35]. Natural plant fibers have also been used as a reinforcement of geopolymer based construction materials [27], [30], [36], [37], [38]. In this field, fibers have been used to control micro-cracking and drying shrinkage caused by the expanding action caused by the foaming agent in the geopolymer mixtures. For instance, Abdollahnejad and Mastali [39] explored the possibility of using polypropylene fibers as reinforcement to FA-based lightweight geopolymer mortars and found that the drying shrinkage decreased in up to 60% with the addition of 1.4% (wt. %) of propylene fibers.

The present paper explores the use of natural pozzolana (NP) as a raw material for the production of lightweight geopolymer masonry blocks. NP is a non-metallic mineral with high silicate and aluminate content that is abundant in volcanic regions around the world [40]. For example, in Peru, the extraction of this mineral is approximately 1 million metric tons per year, being the southern region of the country the area where this material is most abundant [41], [42]. However, the industrial use of this material is limited; in the construction industry, its most common application is as an aggregate for the production of OPC-based concrete and as a substitute in blended cements [40]. To achieve the main objective of the study, the work was divided into two stages: Stage I - Development of a lightweight geopolymer mortar and Stage II - Development of a lightweight geopolymer masonry block. The following sections will provide a detailed discussion of the main findings at each stage.