Effect of supercritical carbonation on the strength and heavy metal retention of cement-solidified fly ash

This paper presents both experimental and multi-physics studies on the carbonation and heavy metal retention properties of cement-solidified fly ashes. Cement-solidified fly ash samples with 40% and 60% fly ash ratios were tested for carbonation depth after being supercritically carbonated. Tests were also carried out for compressive strength and retention capacity of heavy metals of the samples before and after supercritical carbonation. Using CO2 absorption instead of calcium carbonate to measure carbonation degree, a multi-physics model was developed and combined with a leaching model to study the impact of carbonation on Cu and Pb leaching from the cement-solidified fly ash. The results show that supercritical carbonation has both positive and negative impacts on the strength and retention capability of heavy metals of the cement-solidified fly ashes, which suggests that both the carbonation conditions and the amount of fly ash recycled in cementitious materials should be properly controlled to maximize potential positive effect

Synthesis and characterization of alkali-activated loess and its application as protective coating

Environmental friendly and sustainable repair materials with reduced carbon emission have been in high demand worldwide. Frequent deterioration of cement concrete structures is unpreventable and requires appropriate repair materials. Aside from the cementitious, polymeric or resinous materials used to remedy this problem, geopolymeric mortars are reported to be friendly and more sustainable, considering the lower energy required for its production and its intrinsic properties. When selecting geopolymer produced from aluminosilicate waste as repair materials, a measured dissolution in an adequate alkaline solution is required for geopolymerization. Accordingly, this paper examines the synthesis and properties of alkali-activated loess, followed by its effective application as concrete protective coating. The geopolymer mortars (GPMs) were made from loess and ground granulated blast slag (GGBS), and activated with either sodium hydroxide or a combination of sodium hydroxide and sodium silicate solutions. Experimental results showed that, both alkali activators have major influences on the apparent viscosity, roughness, compressibility and microstructural properties of loess GPMs. Results from nanoindentation also reveal good adhesion, higher bulk indent modulus and hardness of the applied mortar, a fact that makes loess geopolymer a great potential repair material to be used as barrier coating for cement concrete substrates

Phase dissolution and improving properties of completely decomposed granite through alkali fusion method

Low-reactive completely decomposed granite (CDG) was successfully synthesized by thermal activation with the addition of NaOH at low alkali/CDG mass ratio of 0.1/1. During alkali fusion, the degree of amorphicity of CDG rich in kaolinite (Al2Si2O5(OH)4) increased and a significant reduction of the peak intensities occurring between 20 and 45 (o2 theta) was observed. Reactivity analysis indicated that, initial CDG requires high molar NaOH to provide a proper dissolution, whereas fused CDG exhibits high reactivity (29Si = 555.57 ppm; 27Al = 223.73 ppm) at low NaOH concentration. Moreover, results from the setting time, varied between 15 and 45 min, indicating that alkali fusion is very effective for improving the dissolution of the fused CDG under Na2SiO3 solution. However, the setting time decreases as the reaction degree accelerates. FTIR analysis of the fused CDG presented lower wavenumber band of around 975 cm−1, confirming a decline of crystalline phases. In addition, SEM-EDS characterization and alkalinity analysis showed a compactness of the structure due to the liberation of enough sodium aluminosilicate gel. Finally, results from the mechanical test (4.75–39.55 MPa) and water solubility inferred that, by enhancing the reactivity of CDG by alkali fusion and by addition of up to 15% GGBS, CDG can be optimally recycled as an alternative source material to produce geopolymers