Performance of alkali-activated cementitious composite mortar used for insulating walls

https://doi.org/10.1016/j.jobe.2021.102867Get rights and content

Highlights

  • The fly ash, slag and silica fume were used in alkali-activated cementitious composite mortar for thermal insulating walls.

  • The effect of two-component alkali activator on the strength is better than that of single-component alkali activator.

  • Addition of polyvinyl alcohol and calcium formate improve the strength properties of insulation mortar.

  • Methyl cellulose ether can increase the consistency and porosity of the cementitious material.

Abstract

There are large volumes of fly ash, slag and silica fume produced in the world. The waste disposal to the landfill is problematic and causes many environmental issues. The objective of this work is to demonstrate how fly ash, slag and silica fume can be used to produce alkali-activated cementitious composite mortar (AACCM) used for insulating walls. The combination of both polymers and alkali-activated cementitious material (AACM) made from fly ash, slag powder and silica fume enables a full composite action. The polymers include Methyl Cellulose Ether (MCE), Polyvinyl Alcohol (PA), and Calcium Formate (CF). The thermal insulating aggregates were cemented into insulating mortar under the composite action. The ratio of fly ash, slag powder and silica fume in the AACM is 5:5:1. The experimental results show that the compressive strength, dry density and thermal conductivity of cement-based thermal insulation mortar (TIM) decrease significantly with the increase of MCE content. MCE has an obvious air-entraining effect on cement-based mortar. When the percentages of MCE, PA, and CF are 1%, 1.6% and 1%, the compressive strength, dry density and thermal conductivity of the TIM are 0.50 MPa, 398.7 kg/m3, and 0.0932 w/(m·k) respectively. MCE has a weak air-entraining effect on AACM. When the percentages of MCE, PA, and CF are 3%, 2% and 1%, the compressive strength, dry density and thermal conductivity of the TIM are 0.28 MPa, 328.7 kg/m3, and 0.0775 w/(m·k) respectively. The simulation results of heat transfer performance show AACCM can be used for insulating walls.

Introduction

Alkali-activated cementitious material (AACM) originated in Belgium. In the 1940s, Purdon used slag as a cementitious material and used NaOH and alkali metal ions as activators to prepare clinker-free cement [1]. In the 1950 s, the Chinese Academy of Building Materials Science and Technology studied lime-clay-cement, which can be categorized as an AACM [2]. It has long been a promising alternative to cement materials as it has outstanding advantages including relatively small carbon footprint, fast hardening time, high strength, high-thermal stability, and acid corrosion resistance [[3], [4], [5], [6]]. There are large volumes of fly ash, slag and silica fume produced in the world in the form of industrial byproducts. Waste disposal to landfill management is problematic and causes many environmental issues. More attention is increasingly paid to industrial wastes and approaches to upcycling them. The raw materials of AACM are widely available from metakaolin and other natural minerals to industrial by-products such as fly ash, slag and silica fume. The alkaline activation of these materials has been intensively studied. There is a range of suitable activator types and approaches to make these by-products work. By studying the microstructure of alkali-activated fly ash, it is mostly proposed that both aluminosilicate and the vitreous content of fly ash be dissolved and form a silica-alumina gel that will serve as a binder. This activation process is nonlinear. Some researchers have proposed that the alkali-induced hydration process of cementitious materials has multiple different phases. The gel material absorbs alkali ions during the initial phase, and then it dissolves during the induction phase. With the decrease of alkali metal ions and raw materials, it enters the deceleration phase and the more stable phase. It is also believed that combining calcium ions and magnesium ions (present on the surface of the slag) with hydroxide causes damage to the surface of the slag particles. Hydroxide ions enter the interior of slag, and the final slag hydration products such as Si(OH)4 and Ca(OH)2 react to form the C–S–H gel [[7], [8], [9]]. Many factors are affecting the performance of AACM. The type and quantity of activators, the volume and properties of each one of pozzolanic materials, the curing conditions, and the curing time can significantly impact material properties and performance [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]].

Polystyrene foam is another kind of thermal insulation material that has been widely used in the world. The external wall insulation board made of polystyrene foam is mostly an expanded polystyrene (EPS) board and extruded polystyrene (XPS) board. The most fundamental difference between EPS and XPS is in the manufacturing technique. Polystyrene materials are favored because of their low density, heat, cost and environmental impact. It is also known to have good sound insulation properties. However, the disadvantage with the use of the material is that it is a combustible material and has poor fire resistance. Moreover, the surface of polystyrene is hydrophobic, and the bonding with the wall and the hydrophilic mortar is not tight. Another disadvantage is that after a long period of freeze-thaw cycles and erosion, it easily swells, gets damaged and spalls. These are some of the difficulties associated with the application of polystyrene insulation board [23,24].

Thermal insulating material (TIM) emerges as an alternative to polystyrene foam board. TIM is typically made of a mixture of pozzolanic and cement-based materials, thermal insulation aggregate, and additives. Typically, TIM has low density, high strength and low thermal conductivity properties. The rate of deterioration, cracking and damage is less frequent than that of polystyrene materials. This kind of mortar can be made with inorganic and organic materials. Waste glass powder and expanded perlite are widely used as inorganic TIM, while polyphenylene particle is widely used as aggregates in organic TIM. It is noted that inorganic materials generally have better fire resistance while organic materials have better thermal insulation properties.

Some research progress has been made in the recent studies of TIM. Contrafatto et al. used pyroclastic products generated by volcano eruption as aggregates to make TIM. The content of the resin added is 0.1% of the cementitious material, which can compensate for the decrease of strength due to the increase of porosity [25]. Tittarelli et al. used recycled expanded polystyrene (EPS) particles to prepare TIM, and the performance differences between recycled EPS mortar and new EPS mortar were compared. The recycled EPS does not effect on the workability but will increase the density and water absorption capacity of the mortar. The use of recycled EPS particles can save about 25% of the cost, compared with the new EPS particles mortar [26]. Leyton-Vergara et al. analyzed the influence of the granulometry of expanded perlite as a light aggregate on the thermal and mechanical properties of cement mortar. It is found that the larger the fineness modulus of aggregates, the lower the compressive strength of the end-product. The regression analysis shows that the linear relationship between the fineness modulus of the expanded perlite and the compressive strength of the mortar [27]. Sukontasukkul et al. added phase change materials (paraffin wax, polyethylene glycol) to the plaster mortar. It is found that paraffin wax has the characteristics of reducing fluidity and water retention [28]. Kazmierczak et al. developed a way to reuse rubber crumbs and studied the impact of adding rubber crumbs on the mechanical, thermal insulation and durability of the mortar. The replacement amount of rubber crumbs was up to 6% and the thermal conductivity can be reduced by 57% [29]. Advantages of using AACM include high-strength, durability and lower environmental impact. Some issues related to workability such as efflorescence and rapid coagulation are being overcome. At present, there are many studies on the preparation of concrete using AACM [[30], [31], [32], [33]]. However, it is difficult to find the research on making TIM with AACM [[34], [35], [36], [37]]. The present research investigates whether AACM and lightweight thermal insulation aggregates mix well combined with additives. The objective of this work is to demonstrate how fly ash, slag and silica fume can be used to produce alkali-activated cementitious composite mortar (AACCM) used for insulating walls.

Section snippets

Materials and methods

In this study, four groups of experiments were designed to test the effect of the alkaline activator, the effect of supplementary cementitious material (SCM), and the effect of additive on the mechanical and thermal behaviors of the TIM. Six tests of group A were designed to investigate the effect of activators with different equivalent alkalies and with different silica moduli. The alkaline activators used in this work included sodium hydroxide solution and a mixed solution of sodium silicate

Test results of group A

The compressive strength of group A results tested at 3, 7, and 28 days of six groups of test specimens are shown in Table 7 and Fig. 4. The sample size is 100 mm × 100 mm × 100 mm. The analysis of test results shows that the compressive strength of TIM made by cementitious materials activated by a two-component activator is higher. Both the hydrolysis of sodium silicate and sodium hydroxide can provide OH ions. The difference is that sodium silicate (Na2O·nSiO2) can also provide SiO2, which

Conclusions

Alkali-activated slag powder, fly ash and silica fume were used to prepare thermal insulation mortar, and the performance was compared with cement-based mortar. The thermal insulation performance of mortar is improved by adding additives. The following outcomes are derived from the investigation:

  • 1)

    The effect of a two-component alkali activator on the compressive strength is better than that of single-component alkali activator. The alkali content in the compound activator is 4%, modulus of sodium

CRediT authorship contribution statement

Chongyang Wang: Data curation, Writing – original draft. Hongtao Peng: Writing – review & editing, Conceptualization, Methodology. Libo Bian: Investigation. Hao Yin: Supervision. Massoud Sofi: Writing – review & editing. Zihao Song: Software, Writing – review & editing. Zhiyuan Zhou: Reviewing and polishing up.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was financially supported by the General Project of Beijing Municipal Education Commission Scientific Research Program (KM201710016016) and the Beijing University of Civil Engineering and Architecture QN Youth Scientific Research and Innovation Project (x18260). Funding from Australian Research Council's Industrial Transformation Research Hubs (PROJECT ID: IH200100010) is duly acknowledged for related research activities by author Massoud Sofi.

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