Effects of Water on Pore Structure and Thermal Conductivity of Fly Ash-Based Foam Geopolymers

Yong Cui,Dongmin Wang

doi:10.1155/2019/3202794

Abstract

The influence of the water-to-solid ratio (W/S) on the viscosity, pore characteristics, bulk density, compressive strength, and thermal conductivity of foamed fly ash-based geopolymers with thermal conductivity less than 0.065 W/(m·K) was investigated, and their properties and cost analysis were also compared with that of foamed ordinary Portland cement (OPC). When the W/S varied from 0.38 to 0.5, the apparent viscosity of geopolymer paste 15 min after the preparation decreased significantly from 168 Pa·s to 6 Pa·s. The increasing W/S ratio contributed to the rise of the number of microcapillaries (φ < 50 nm) and macrocapillaries (50 nm < φ < 50 μm) but contributed to the decline of artificial air pores (φ > 50 μm). The refinement of pore characteristics lowered the 28 d thermal conductivity of foamed geopolymers from 0.06 W/(m·K) to 0.048 W/(m·K). Although the slight increase of total porosity of foamed geopolymers from 89% to 92% with the increase of the W/S ratio weakened their 28 d compressive strength from 0.75 MPa to 0.45 MPa, this strength still meets the Ordinary Portland Cement (OPC) based Foam Insulation Board standard of JC/T2200-2013 (>0.4 MPa for 0.25 g/cm3). The production cost of foamed geopolymers was slightly higher by 1.1–1.5 times than that of foamed OPC. However, considering the more beneficial effect of environmental load reductions and better mechanical and thermal properties of foamed geopolymers than those of foamed OPC, slightly higher cost would be acceptable for practical application.

Highlights

Foamed geopolymer is a kind of alkali-activated aluminosilicate-based porous materials and can be manufactured by a chemical or mechanical foaming technology [1]
Due to the formation of amorphous geopolymerization products, an upward trend was observed in the viscosity of Fly ash (FA)-based geopolymer pastes with the time increasing to 30 min after preparation. e nonlinear decline of the viscosity showed that there exists a relationship between viscosity and water-to-solid ratio (W/S) as described by the following equation [28]: α αL
Where ki is the calculated thermal conductivity of foamed geopolymers with W/S i and i 0.41, 0.44, 0.47, or 0.50. k1 is the thermal conductivity of foamed geopolymers with the W/S 0.38 and 0.0594 W/(m·K). k2 is the thermal conductivity of air, 0.026 W/(m·K)

Summary

Introduction

Foamed geopolymer is a kind of alkali-activated aluminosilicate-based porous materials and can be manufactured by a chemical or mechanical foaming technology [1]. Foamed geopolymer is well known for its relatively low thermal conductivity, usually 10–50% of that normal concrete [2], but for its less energy consumption and less environmental loads compared to the typical ordinary Portland cement (OPC) foamed concrete [3, 4]. For thermal other than mechanical purpose, the thermal conductivity of the typically foamed geopolymer being used generally ranges from 0.072 W/(m·K) to 0.48 W/(m·K) with its corresponding density and compressive strength ranging from 300 kg/m3 to 1400 kg/m3 and 0.7 MPa to 48 MPa, respectively [6,7,8,9,10,11]. Few publications on geopolymer based foamed materials with thermal conductivity less than 0.065 W/(m·K) are available. With the improvement of energy efficiency standards from 50% to 65% and even to 75% in Beijing and Tianjin, China, the relatively high thermal conductivity (0.07–0.48 W/(m·K)) of these porous inorganic materials, compared with that of porous organic materials such as polyurethane (PU) board (0.026 W/(m·K)) and extruded polystyrene (XPS) (0.029 W/(m·K)) [12], has limited their use as the thermal insulator. e optimization of pore-size distribution is one of most effective paths to lower the thermal conductivity of foamed materials [13]

Methods

Results

Conclusion