Zuhua Zhang

Quantitative study of the reactivity of fly ash in geopolymerization by FTIR

Fourier transform infrared (FTIR) spectroscopy has been applied to analyse the environments of Al–O and Si–O bonds in fly ash, which are used as raw materials of geopolymer synthesis. It is noted that the relative intensities of the bands at around 1000, 910 and 700 cm−1 are much higher in fly ash with higher reactivity, as reflected by the compressive strength of geopolymer. Deconvolution analysis of the band from 400 to 1400 cm−1 shows that the cumulative area of these three resolved bands, together with the band at ∼1090 cm−1, which is assigned to the asymmetric stretching of Si(Al)–O–Si, is proportional to the reactivity of fly ash. If it is assumed that the area of the resolved bands is proportional to the concentration of the corresponding bonds, a general indication is therefore that fly ash containing more reactive bonds will exhibit higher reactivity in geopolymerisation. FTIR spectroscopy in combination with particle size analysis provides a fast approach to predict the reactivity of fly ash, from the perspective of aluminosilicate glass chemistry.

Mechanical property and structure of alkali-activated fly ash and slag blends

Mechanical property and structure of alkali-activated fly ash (FA)/ground granulated blast furnace slag (GGBFS) blends were investigated via compressive strength testing, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy/energy dispersive spectroscopy. It is shown that the incorporation of slag into solid precursors can improve compressive strength of the geopolymer binders and the optimal slag content ratio which will result in the highest strength is 0.8. GGBFS is relatively more reactive than FA in alkaline activation. The binder is predominantly a class of Al-substituted sodium calcium silicate hydrate (N-C-A-S-H) gel phase, which distributes around the solid particles homogeneously. Combining the results obtained from the glass diffraction maximum of XRD and the wavenumber of T-O-Si bands displayed in FTIR, it suggests that the degree of polymerization of geopolymer binders decreases and increases. This means that the microstructure of the binder is more complex. The microstructure and chemical composition of the binders are between those of aluminosilicate gel formed in silicate-activated FA and those of calcium silicate hydrate gel formed in silicate-activated GGBFS. The major reasons contributing to this structure are the numerous calcium cations which will lead to a depolymerisation influence on the network, and/or the majority of linear chains of Si-O-Si supplied by GGBFS.

The Pore characteristics of geopolymer Foam concrete and Their impact on the compressive strength and Modulus

The pore characteristics of GFCs manufactured in the laboratory with 0-16% foam additions were examined using image analysis (IA) and vacuum water saturation techniques. The pore size distribution, pore shape and porosity were obtained. The IA method provides a suitable approach to obtain the information of large pores, which are more important in affecting the compressive strength of GFC. By examining the applicability of the existing models of predicting compressive strength of foam concrete, a modified Ryshkevitch’s model is proposed for GFC, in which only the porosity that is contributed by the pores over a critical diameter (>100 μm) is considered. This “critical void model” is shown to have very satisfying prediction capability in the studied range of porosity. A compression-modulus model for Portland cement concrete is recommended for predicting the compression modulus elasticity of GFC. This study confirms that GFC have similar pore structures and mechanical behavior as those Portland cement foam concrete and can be used alternatively in the industry for the construction and insulation purposes.

Analysing the relation between pore structure and permeability of alkali-activated concrete binders

Alkali-activated binders are usually porous materials that consist of various scales of pores, gels and residual raw material particles. The importance of pore structure has been highlighted in recent studies as it impacts the permeability of fluids and further impacts the durability of alkali-activated binders. This chapter reviews the pore features of alkali-activated metakaolin (AAM), alkali-activated fly ash (AAFA) and alkali-activated slag (AAS). The water permeability, absorption and chloride iron diffusion are particularly discussed for AAM and AAFA binders, which are two relatively new materials. For comparison purposes, Portland cement binders are also investigated.

Numerical simulation of porosity on thermal properties and fire resistance of foamed concrete

The relationship between thermal insulation properties and porosity of fly ash based foam concrete was built, in which effective density, effective heat conductivity, and effective specific heat of fly ash based foam concrete were deduced as functions of porosity. Using the model, the effective heat conductivity of density of 580 kg/m3 fly ash based foam concrete was the theoretically calculated as 0.145 W/(m K) while the experimental measured value was 0.142 W/(m K). The relative error of heat conductivity was very low at 2.1%. The effective specific heat within the model was 967.05 J/kg K and the experimental value was 920 J/kg K with a relative error of 5.1%. Then, the effective heat conductivity and specific heat models were incorporated into heat transferring model to calculate the temperature field of fly ash based foam concrete wall during a fire incident. Finally, the temperature field of fly ash based foam concrete wall and traditional dense concrete wall during fire incident were calculated and compared. Comparing the temperature field of the fly ash based foam concrete wall with the traditional concrete wall, it was found that at close to fire-side surface, the temperature in the fly ash based foam concrete wall could reach 1039 °C, while the lowest temperature in the fly ash based foam concrete wall remained at 20 °C for a thickness of 7 mm. In contrast, at close to fire side of surface, the temperature of traditional concrete wall was 987.2 °C at 360 s and the lowest temperature in the traditional wall was 102.9 °C at the opposite side-wall surface far away from the fire direction. As expected, the data demonstrated that the use of fly ash based foam concrete in wall construction adds greatly to the time for people to leave in safety.

Alkali-activated cements for protective coating of OPC concrete

Recent innovations have led to the development of alkali-activated coating materials to replace organic coating and repairing materials for protecting and strengthening existing concrete structures, particularly for those under rigorous conditions. This chapter reports the laboratory and on-site works of examining the possibility of using alkali-activated metakaolin (AAM) as an inorganic coating material for marine concrete protection. As a new coating material, the basic engineering properties, including setting time, bond strength and durability, were investigated. The interface between AAM coating layer and OPC substrate under different curing conditions was examined, further providing valuable application guidance. The potential of developing fly ash and slag-based alkali-activated coatings are also discussed.

Inorganic polymer foams: transform from non-structural to structural upon fire

An inorganic polymer is formed by dissolution of aluminosilicate solid materials in a strong alkaline activator solution, polymerization, gelation and/or crystallization. This material possesses many superior properties to normal organic polymers, such as high temperature resistant and non-flammable. This study aims to explore the possiblity of using inorganic polymers as a fire resistant building material. Inorganic polymer foams containing ~8% of Na2O (mass ratio to solid materials) and 45-50% porosity (in volume) are synthesised from fly ash and slag and with sodium silicate solution as activator. The compressive strength, volumetric stability and phase features of the porous inorganic polymers before and after exposed to 100, 400 and 800oC temperatures are determined and analysed. After exposure to high temperature, the inorganic polymer foam without slag addition maintains the compressive strength at 100oC and 400oC and increases by 40% at 800oC. In contrast, the foam that contains 20% slag, although which has a much higher initial strength than the non-slag foam, can only maintain the strength at 100oC but lose strength dramatically at 400 and 800oC. The measurement of volumetric stability and XRD analysis indicate that the larger shrinkage of slag-containing foam and the decomposition of calcium silicate phases under high temperatures is accounting for the large strength loss. The current study shows a possibility to develop a kind of new building material with the function of transforming from nonstructural to structural upon fire.