Clay as a Sustainable Binder for Concrete—A Review

The negative environmental impacts of Portland cement as a binder in the construction industry have created a growing impetus to develop sustainable alternative binders. Various types of clay have been considered as potential cement replacements. The impact of clays as cement replacement depends on the dosage and treatment methods. This paper presents a comprehensive review to determine the effects of different types of clay on the fresh and hardened properties of concrete mixtures by analyzing the experimental database reported by the literature, including raw, calcined, modified, nano, and organo. This study intends to show the process of optimizing the use of clay in concrete, the reason behind converting raw clay to modified types, and research gaps through a comparison study between different types of clays. The present review study shows that clay-based concrete mixtures have higher thixotropy and yield stress values, improving shape stability. This results in lower early-age shrinkage of the concrete. However, the high floc strength of clay-based concrete causes a reduction in flowability. Treatment methods of raw clay, such as calcination and nano-sized clay particles, improve concrete compressive strength. General results of the previous studies highlight that all types of clay investigated positively affect the resistance of concrete to environmental attack.

Understanding the coagulation mechanism and floc properties induced by Fe(VI) and FeCl3: population balance modeling

Coagulation kinetics and floc properties are of great fundamental and practical importance in the field of water treatment. To investigate the performance of Fe(VI) and Fe(III) salt on particle coagulation, Malvern Mastersizer 2000 was employed to continuously and simultaneously monitor the kaolin floc size and structure change, and population balance modeling was used to investigate the coagulation mechanism. The results show dosage increase had positive effect on collision efficiency and floc strength and negative effect on restructure rate. Low shear rate resulted in higher collision efficiency and stronger floc. Low water temperature had a pronounced detrimental effect on coagulation kinetics. Temperature increase showed the most significant positive effect on collision efficiency, floc strength and restructure rate. The optimum pH zone for the coagulation was found to be between 6 and 8. Further pH increase lowered the collision efficiency and floc strength and increased the restructure rate. FeCl3 resulted in a larger ratio of the mass to volume of kaolin flocs (compactness) than those induced by ferrate.

Enhancing paper sludge dewatering by waste polyester fiber and FeCl3 for preparation of Fe-rich biochar

The paper sludge conditioning effects of waste polyester textile fibers as skeleton builders combined with ferric chloride (FeCl3) were evaluated and the sludge dewatering mechanism was explored. The catalytic effect of Fe-rich sludge biochar (Fe-SB) on enhancing sludge dewaterability was evaluated. Results showed the combined fiber-FeCl3 conditioning significantly promoted the sludge dewaterability compared with conditioner alone, leading to a 77.5% decrease in specific filtration resistance and a 68.9% increase in net yield. The decrease of extracellular polymeric substances (EPS) contents showed that the EPS were difficult to extract and sludge floc strength was enhanced as a result of chemical reactions such as complexation processes and charge neutralization. Hence, the enhancement of sludge dewaterability was primarily due to the sludge cake with a porous and incompressible structure formed by fiber and FeCl3, and the rigidity structure of fiber. Moreover, the Fe-SB prepared by fiber-FeCl3 conditioning sludge could effectively activate persulfate to enhance the sludge dewaterability, with water content of dewatered sludge decreasing by 14.6%. The Fe-SB had dual functions of the heterogeneous catalyst of persulfate and skeleton builder. This study presents a sludge recycling method that combined physicochemical conditioning and sludge biochar materials prepared by pyrolysis.

Shear-Dependent Yield Stress of Iron Ore Fine Tailings in Two-Step Flocculation Process

Both shear and flocculation have a significant influence on the rheological behavior of tailings, especially the yield stress. In the two-step flocculation process, the above two kinds of actions exist at the same time, and they influence each other. In order to explore the change rule of the yield stress and its internal causes, a two-step flocculation process of the iron ore fine tailings with different shear conditions in the four different phases was designed. In the primary flocculation phase, tailing particles combined with the primary flocculant and formed a primary floc network structure. In the primary broken phase, shear destroyed the primary floc network structure and decreased the average floc size, so the shear-dependent yield stress, the floc strength factor, and the fractal dimension decreased. In the secondary flocculation phase, broken floc combined with the secondary flocculant and produced a more compact floc network structure which had a better shear resistance. Therefore, in the secondary broken phase, with the increase of shear, the decrease of yield stress, the floc strength factor, and fractal dimension were less obvious than that in the primary broken phase. In both two broken phases, the yield stress of the secondary flocculating slurry was always higher than that of the primary flocculating slurry, but with the increase of shear, the difference became smaller. The floc strength and fractal dimension also showed the same rule. The internal reasons for the stronger shear resistance of the secondary flocculating slurry were the increase of the number of binding sites, the electric neutralization between the two flocculants, and the steric hindrance effect of the flocculants.

Strength of Flocs Formed by the Complexation of Lysozyme with Leonardite Humic Acid

Aggregation and aggregates properties of natural organic and nanosized macromolecules such as humic substances and proteins are crucial to explore so-called colloid-mediated transport and the fate of substances in soil and water environments. Therefore, the aggregation and dispersion, charging, and floc strength of lysozyme (LSZ)–leonardite humic acid (LHA) flocs were experimentally investigated. The experiments were performed in different salt concentrations and LSZ to LHA mass ratios as a function of pH. We obtained the stronger flocs at pH 4.4, where the isoelectric point (IEP) of the complex with the mass ratio 2.5 was confirmed. Thus, the aggregation of LSZ–LHA flocs is mainly caused by charge neutralization. We obtained the floc strength of 4.7 nN around IEP at low salt concentration of 3 mM, which was stronger than 2.8 nN in high salt concentration of 50 mM. The effect of salt concentration can be rationalized by charge-patch attraction at low salt concentration. With increasing mass ratio, the IEP shifted to higher pH. This is due to the increase in positive charge from LSZ in the mixture. The effect of the LSZ to LHA mass ratio on the maximum strength was weak in the range studied.

INFLUENCE OF FLOCCULANT TYPE ON STRENGTH CHARACTERISTICS OF FLOCS IN CLAY-SALT SUSPENSIONS

The results of studies on the effect of the flocculant type on the strength characteristics of floccules in clay-salt suspensions, obtained using a laser analyzer of particle size Lasentec D600L FBRM systems were presented in the article. The coefficients of strength and reduction of aggregates formed by polyacrylamide and its anionic and cationic copolymers were determined. Increasing the speed of mixing by using of polymers various types leads to the destruction of the formed aggregates in the flocculated suspension. The size of flocs induced by a cationic flocculant is reduced from 250 to 110 mm, for anionic and non-ionic flocculants - from 500 to 250 mm. The dependence of floc strength on the macromolecules adsorption mechanism on a solid surface was established. The decrease in hydrodynamic effects entails the partial restoration of floccules. It was shown that the recovery of flocs for all studied polymers was limited, which indicates a significant irreversibility of the aggregates destruction process. When using cationic flocculant, the ability to restore the aggregates is 2.5 times lower compared to anionic and non-ionic polymers. For nonionic and anionic polymers, the dependence of flocs coefficients of strength and recovery on the flocculant dosage was established. Dosage of cationic polymer does not affect these indicators. The dynamics of flocculation, destruction and reflocculation processes using various flocculants was considered. The influence of the charge type and polymer consumption on the mechanism of particles interaction was established. The potential possibility of using the obtained results to improve the technology of potassium chloride production at the stages of thickening and dehydrating clay-salt sludge was shown.

Polyamidine as a New-Style Coagulant Aid for Dye Wastewater Treatment and its Floc Characteristics

Polyamidine (PA) is a novel high cation coagulant aid for water treatment. In the present study the structure, coagulation ability and floc characteristics (including coagulation kinetics, floc dimension, floc strength and floc recover ability) were discussed. As a dual-coagulation, polyamidine was combined with AlCl3and PAC to remove dispersed yellow from wastewater. The results showed that the polyamidine exhibited higher intrinsic viscosity and higher charge neutralization ability than cationic polyacrylamide (PAM). Atomic force microscopic (AFM) and transmission electronic microscopic (TEM) images showed a dendritic, stretched structure of PA. The dosage of PAC/PA and AlCl3/PA and pH were investigated and optimized. Under the optimum condition, the color removal efficiency reached 96% as the concentration of 14/0.2 mg/L for AlCl3/PA (PA was combined with AlCl3) at pH 6. Meanwhile, the highest decoloring efficiency (97%) was achieved in the process when PAC/PA (PA was combined with PAC) dosage was 18/0.6 mg/L at pH 6.0-8.0. The addition of polyamidine could improve the color removal efficiency, including increasing the zeta potential evidently and reducing the aluminum coagulants dosage as well. Compared with AlCl3/PA, PAC/PA which contained higher polymeric aluminum, exhibited a better coagulation ability to adapt a wide range of pH. However, when PAC/PA and AlCl3/PA expressed the same removal efficiency, AlCl3/PA showed a less coagulant dosage in raw dye wastewater pH. With PA dosages increased, the floc of AlCl3/PA showed a gradually adding strength and recovery ability, but in PAC/PA system the tendency was unclear. In PAC/PA system, PA dosage increase only improved floc density.