Structure Of Hydration Products Research Articles

Effect of amorphous aluminum hydroxide on the phosphogypsum-based cementitious materials in prefabricated building field

Gypsum-based lightweight panels are increasingly used in prefabricated buildings due to their unique advantages, such as low cost and energy saving, good moisture absorption, good habitability, and fire resistance. At present, how to optimize performance, accelerate curing speed and improve production efficiency in the production process are the problems that need to be studied. In this research, the influence of amorphous aluminum hydroxide (A-AH) on the phosphogypsum-based solid waste cementitious material, including the mechanical properties, hydration products, and volume stability, were investigated. Pastes with 0–4% content of A-AH in the ternary system consisting of β-hemihydrate phosphogypsum (β-HPG), ground blast-furnace slag (BFS) and steel slag (SS) were made. Results showed that the addition of A-AH could promote the rapid and stable curing of the β-HPG-BFS-SS (HBS) system, increase the early strength by accelerating the early generation of AFt, and promote the formation of C-A-S-H gel. Specially, 1% content of A-AH could enhance both the early and late mechanical property, improve the volume stability, promote the hydration degree of silicates and increase the generation of AFt and C-(A)-S-H gels, and increase the polymerization degree of C-(A)-S-H gel in HBS. Analyses of hydration products were based on XRD, TG-DTG, 27Al-NMR, FTIR and 29Si-NMR. Hydration process was evaluated by hydration heat and pH value analysis. The morphology and structure of hydration products were characterized by SEM-EDS and pore structure, respectively. The mechanism of A-AH acting on the system was analyzed in detail. The conclusions paved a new way to provide reference and insight for the better application of ternary system HBS in prefabricated building field.

The toxic leaching behavior of MSWI fly ash made green and non-sintered lightweight aggregates

With the rapid development of urbanization and the economy, the amount of municipal solid waste incineration fly ash (MSWI fly ash) dramatically increases and stacks up. Declining the leaching concentration of hazardous heavy metals in municipal solid waste incineration fly ash and enlarging the application range in wastewater and biosorption of non-sintered LWAs have always got more attention from more and more researchers. Green and non-sintered LWAs were prepared by using the waste solids (MSWI fly ash and coal fly ash) as raw materials through autoclave technology. In the meantime, the effect of severe leaching environment (pH = 1, 3, 5 and 7) on the stabilization of heavy metals in the LWAs with MSWI fly ash and extra heavy metals were systematically investigated by means of Inductively Coupled Plasma Optical Emission Spectrometer, X-ray diffraction, Fourier Transform Infrared Spectrometer, X-ray photoelectron spectra and Scanning electron microscope. The results revealed that the heavy metals are well immobilized in the LWA matrix through physical encapsulation and adsorption by hydration products (C-S-H). The leaching rate (LR) and cumulative leaching rate (CLR) of heavy metals, phase compositions and chemical structures in LWAs with heavy metals at pH of 1 are significantly changed, but the cumulative leaching rate of Pb2+ is lower than that of Zn2+ and Cu2+. The structure of hydration products in the matrix will be broken and reformed to gypsum under an acid environment (pH = 1). The leaching rate and cumulative leaching rate of heavy metals in LWAs with heavy metals under various leaching environments are much lower when the pH is above 1, which could meet the leaching requirement. This research could provide theoretical support for the application of non-sintered and municipal solid waste incineration fly ash based LWAs in concrete.

Effect of Hydration Process on Properties and Microstructure of Coal Gangue Admixture Concrete

The mineral of coal gangue is mainly kaolinite, which can be used as mineral admixture after activation. In order to study the effect of coal gangue on the hydration process of concrete, 20% coal gangue powder was used to replace cement to prepare concrete. The phase composition, microstructure morphology and pore structure of concrete hydration products at different ages were observed by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). At the same time, the experimental study on the compressive strength, splitting tensile strength and flexural strength of coal gangue admixture concrete with time and relationship was carried out. The results indicate that the addition of coal gangue powder significantly affected the hydration process of cement. In the cement-coal gangue powder-water system, coal gangue particles adsorbed a large amount of CH generated by cement hydration after curing for 14 d. The (AlO4)4− group in the coal gangue power replaced the (SiO4)4− on the calcium silicate hydrate (C-S-H) structure of the hydration product, thereby generating a new flocculated polymer calcium aluminosilicate hydrate (C-A-S-H). Until the curing age exceeded 56 d, the CH content in the slurry was less, and the hydration reaction rate of coal gangue became slowe. The unhydrated coal gangue particles played the “micro-aggregate effect”, filled in the pores and reduced the number of macropores and pores. The density of concrete microstructure increased, which promoted the continuous increase of concrete strength.

Pozzolanic reactivity of aluminum-rich sewage sludge ash: Influence of calcination process and effect of calcination products on cement hydration

The application of aluminum-based flocculant in wastewater treatment results in a large amount of aluminum-rich sewage sludge. This work investigated the influence of calcination process on physicochemical characteristics and pozzolanic activity of aluminum (Al)-rich sludge ash and studied the effect of sludge ash on cement hydration. The results showed that higher calcination temperature from 600 ℃ to 900 ℃ increased the amorphous content in sludge ash. The pozzolanic activity of sludge ash calcined at 800 ℃ and 900 ℃ was confirmed by Frattini test. In view of strength activity index of blended mortar and energy conservation, the optimal calcination condition of sewage sludge ash was calcined at 800 ℃ with air-cooling. The addition of sludge ash promoted the transformation of ettringite to monosulfate phase in cement paste. However, the high Al concentration dissolved from S6 and S7 ash inhibited significantly the cement hydration and resulted in low compressive strength values of the blended mortars. The pozzolanic reaction of S8 and S9 ash produced more hydration heat and additional Al-bearing products such as katoite and monosulfate which contributed to the strength development of mortars. Furthermore, the heavy metals in sewage sludge can be immobilized in ash structure during calcination process and the structure of hydration products, which ensures the environmental security of sludge ash utilization in construction materials.

Experimental study on Portland cement/calcium sulfoaluminate binder of paste filling

The curing time and compressive strength of ordinary Portland cement have its limitations, which limits its application range. In this article, the ratio of ordinary Portland cement/CSA ratio is optimized by laboratory test, and the composition and structure of hydration products are changed to achieve the performance adjustment. Then, the microstructures of the optimized cementing materials at different stages were observed and compared by the analysis of hydration process and scanning electron microscopy. The experimental results show that the setting time decreases with the increase of sulphoaluminate cement content in the system and prolongs with the increase of gypsum content, and the change trend of initial setting time and final setting time is consistent. When the content of sulphoaluminate cement exceeds 14% or the content of gypsum exceeds 16%, the compressive strength of cementing material decreases greatly in the early and late stages, but the strength of cementing material does not increase beyond this range. Composite admixtures H3BO3 (0.3%) + Na2SO4 (0.1%) and H3BO3 (0.3%) + NaNO3 (0.1%) can not only improve the setting time of composite cementing materials, but also improve the strength of composite cement.

Effects of Pumice-Based Porous Material on Hydration Characteristics and Persistent Shrinkage of Ultra-High Performance Concrete (UHPC).

In this paper, two kinds of pumice particles with different diameters and water absorption rates are employed to substitute the corresponding size of river sands by volume fraction, and their effects on the hydration characteristics and persistent shrinkage of Ultra-High Performance Concrete (UHPC) are investigated. The obtained experimental results show that adopting a low dosage of 0.6–1.25 mm saturated pumice as the internal curing agent in UHPC can effectively retract the persistent shrinkage deformation of concrete without a decrease of strength. Heat flow calorimetry results demonstrate that the additional water has a retarding effect and promotes the hydration process. X-ray Diffraction (XRD) and Differential Thermal Gravimetry (DTG) are utilized to quantify the Ca(OH)2 content in the hardened paste, which can confirm that the external moisture could accelerate the early cement hydration and secondary hydration of active mineral admixtures. The Ca/Si ratio of C–S–H calculated by the Energy Dispersive Spectrometer (EDS) reveals that the incorporation of wet pumice can transform the composition and structure of hydration products in its effective area.

Carbon dioxide use in beneficiation of landfilled coal ash for hazardous waste immobilization

Landfilled coal fly ash and supplementary minerals were processed into a hydraulic cement binder via input of mechanical energy at ambient temperature in the presence of carbon dioxide (CO2) (considered as a gaseous raw material). An experimental program was conducted with the purpose of optimizing the raw materials formulation for achieving a desired balance of cement chemistry for CO2 capture, compressive strength and heavy metals immobilization qualities of the cement hydration products. The optimum formulation was found to complement high levels of CO2 capture with desired material properties offered by the hydration products. Insight into the structure of hydration products was gained by evaluating their chemical bond structure and thermogravimetry attributes.

Hydration products of cement-silica fume-quartz powder mixture under different curing regimes

Composition, morphology, and structure of hydration products in hardened pastes of three kinds of blended cement (cement-silica fume, cement-quartz powder and cement-silica fume-quartz powder) hydrated under different curing regimes (standard curing, 90 °C steam curing, 200 °C and 250 °C autoclave curing) were investigated by X-ray diffraction and field emission scanning electron microscope equipped with EDAX system. Results showed that the main hydration products in three kinds of hardened pastes under standard curing condition are all C-S-H gels, CH, and AFt. Under 90 °C steam curing condition, the main hydration products of cement-silica fume and cement-silica fume-quartz powder are C-S-H gels, whereas those of cement- quartz powder are C-S-H and CH. Under 200 or 250 °C autoclave curing condition, no obvious crystallized CH phase is found in hardened pastes of three kinds of blended cement, and C-S-H gels are transformed into one or more crystalline phases such as tobermorite, jennite, and xonotlite. The chemical composition and morphology of these crystalline phases depend on the composition of mixture and autoclave temperature.

Investigation on Nano-Scale Structure of Plain and FA Cement Paste

The shape, size and packing structure of the hydration products in the plain and fly ash (FA) cement pastes were conducted by the atomic force microscope (AFM). The water-binder ratio was 0.35, two cement pastes was plain cement as control and FA replacing 30% cement by mass, and the curing ages were adopted the 7 days and 90 days respectively. The results show that there are many round particles about 80nm. However, the size of the round particles has been tended to become smaller with the curing time increase. But the packing structure became tight because of the rearranged particle with the curing time increase. Moreover, fly ash would affect markedly the shape, size and packing structure of hydration products. The results indicate that C-S-H clusters in FA cement paste appear the flat shape and looser packing structure due to its low calcium and silicate ratio.

Freezing as a method of study of early cement paste hydration

A method for studying early stage hydration of cement paste is proposed. The method consists in freezing the specimens at specific times in liquid nitrogen at −195°C. These are then vacuum dried and studied under scanning electron microscope. By this method the hydration can be stopped at any time and exactly the same structure of hydration products, which was present at the freezing time, can be studied under microscope. It is though that this method can be useful specially to study structure of the fresh cement paste. Chemical reactions (phase identification) in different ceramic materials at various stages may also be studied by this method.