Nature Of Hydration Products Research Articles

Alkanolamines-activated steel slag for stabilization/solidification of heavy metal contaminated soil

Steel slag (SS) is a byproduct discharged from steel-making industry with less than 25% utilization rate in China. The low utilisation rate of SS is associated with its low hydration activity in cement and concrete. In this study, four different alkanolamines (TEA, TIPA, EDIPA and DEIPA) were used to activate SS to improve its cementitious properties and metal binding performance, and hence its capacity on treating heavy metal-contaminated soils containing Cd, Cu, Ni, Pb and Zn. Compared with the reference SS without activators, concentrations of leached Cd, Cu, Ni, Pb, and Zn have reduced by 87.2%, 78.8%, 62.4%, 73.6% and 64.5% by using 0.1% TIPA-activated SS after 28 days, and they were all below their respective regulatory limits by Standard for Pollution Control on the Hazardous Waste Landfill (GB 18598–2019) in China, and the unconfined compressive strength (UCS) of the treated soil at 28 days was enhanced by 237.7% using 0.1% TIPA-activated SS. To elucidate the activation mechanism, the hydration process of SS was thoroughly followed via isothermal calorimetry (IC) and conductivity analysis, and the nature of hydration products was studied by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). It was concluded that alkanolamines facilitated the dissolution of minerals in SS and formation of hydration products (e.g., C-S-H, C-A-H, C-F-H and Mc), and hence significantly enhanced the microstructural development and engineering properties of SS. This work demonstrated a promising way of upcycling SS as an effective and sustainable S/S agent for handling complex heavy metal contaminated soil, with the potential of enhancing the SS utilization significantly.

On the early hydration of cubic ye’elimite: New insights from time-resolved in situ MAS NMR spectroscopy

Two polymorphs of ye’elimite are known to exist in calcium sulfoaluminate cements, namely, orthorhombic (iron free), C4A3$ and cubic/pseudo-cubic (iron incorporated), C4A3-xFx$ forms. The very early hydration behaviour of cubic ye’elimite (nominal composition: Ca3.8Na0.2Al5.6Fe0.2Si0.2O12SO4) has been investigated using a time-resolved in situ multi-nuclear (1H, 27Al, 23Na and 29Si) MAS NMR spectroscopic approach. The study shows that the reaction pathway and the intermediate species formed are substantially different from those that are observed during the hydration of orthorhombic ye’elimite, and the rate as well as the nature of hydration products are dictated by the presence or the absence of limestone (CaCO3). Furthermore, the detection of diverse transient species as well as the uncovering of different kinetic behaviour during the early hydration of cubic ye’elimite illustrate the advantages of time-resolved in situ MAS NMR experimental approach for fingerprinting hydrated phases.

Mechanochemical synthesis of one-part alkali aluminosilicate hydraulic cement

A novel approach was devised to synthesize a sustainable hydraulic cement based on alkali aluminosilicate chemistry via mechanochemical activation. This approach builds upon past work on activation of aluminosilicate precursors using alkaline solutions to produce inorganic binders for concrete construction. Recent efforts to develop one-part hydraulic cements based on the chemistry of alkali-activated aluminosilicates have resorted to high-temperature processing techniques which compromise the sustainability advantages of the system, and also require curing at elevated temperatures. The mechanochemical process developed here takes place at room temperature, and yields a hydraulic cement that does not require addition of caustic solutions to render binding effects via room-temperature curing. Processing of this hydraulic cement takes place at room temperature; this advantage together with extensive use of recycled raw materials yield significant sustainability benefits. The (dry) raw materials used for mechanochemical processing of hydraulic cement included coal fly ash, quick lime, sodium hydroxide and Magnesium oxide. The mechanochemically processed hydraulic cement was evaluated through performance of tests concerned with their pH, heat of hydration, chemical composition, crystallinity and microstructure. The hydraulic cement was used to produce concrete materials cured at room temperature. The resulting concrete materials were found to provide desired levels of workability in fresh state and compressive strength after curing, which were comparable to those of Portland cement concrete. Investigations were also conducted on the hydration kinetics of the hydraulic cement and the microstructure of its hydrates in order to gain insight into its hydration process and the nature of hydration products.

Multi-technique investigation of metakaolin and slag blended portland cement pastes

Increasing incorporation of metakaolin (MK) into field concrete mixtures necessitates careful study of the portland cement/MK hydrating systems. While a number of studies have been conducted on MK blends with cement, the current knowledge on its effect on hydration products and paste microstructure remains incomplete. This study evaluated the effect of MK on the nature of hydration products through X-ray diffraction, while the effect on microstructure was assessed by measuring porosity with nitrogen adsorption and determining nanoindentation modulus as well as volume fractions of CSH with varying packing densities. The 10MK paste hydrated for 7days was compared to the plain ordinary portland cement (OPC) paste as well as to paste containing 52% slag (52SL). No significant effect was observed on the nature of hydration products with MK or SL addition. However, nitrogen-accessible porosity increased with MK and SL addition, the increase being larger with SL. The average indentation modulus for hydration products decreased with addition of MK and SL which corresponded to increasing nitrogen accessible pores.

The Role of Temperature on Hydration of Binary System of Metakaolin/Portland Cement

The role of temperature of metakaolin/Portland cement binary system was studied by isothermal calorimetry. Sample with 50 % of metakaolin replacement were monitored at 30 °C, 40 °C, 50 °C and 60 °C. Structural and chemical characterization of hardened pastes was obtained by scanning electron and Raman microscopy. Paper deals with kinetics of main exothermal reactions during early hydration of the system. Activation energies were calculated for processes related to each exothermic peak. The nature of hydration products at different temperatures was revealed by microstructural studies.

Chemical composition and microstructure of hydration products of hardened white portland cement pastes containing admixtures

This study investigated the nature of hydration products of white portland cement (WPC) containing 20 mM malic acid or 1 M calcium chloride hydrated for 11 years. The study identified the hydration products and characterized the chemical composition, morphology, micro/nano structure of C-S-H and the main binding phase in cementitious materials. Calcium hydroxide (CH), ettringite and C-S-H were identified in WPC with 20 mM malic acid paste hydrated for 11 years. WPC with 1 M calcium chloride paste hydrated for 11 years contained the same phases, but with less CH, and the presence of Friedel’s salt (Ca2Al(OH)6Cl·2H2O). There were still small amount of anhydrous cement particles remaining in both pastes after 11 years hydration according to the SEM and 29Si MAS NMR results. The hydration products of paste containing malic acid had a lower porosity than those prepared with calcium chloride upon visual inspection under SEM. The morphology of the outer product (Op) C-S-H was coarse fibrillar and the inner product (Ip) C-S-H had a very fine microstructure in both pastes under TEM. Both Ip and Op C-S-H formed in paste containing malic acid had lower Ca/Si and higher Al/Si than those in paste containing calcium chloride. C-S-H in paste containing calcium chloride had longer MCL and less percentage of bridging tetrahedra occupied by aluminum in silicon/aluminum chains due to relatively less Q 1 and more Q 2. A new type of silicon tetrahedra, Q 2B, was introduced during deconvolution of 29Si MAS NMR results. Ip and Op C-S-H in both pastes had aluminum substituted tobermorite-type and jennite-type structure, and all the charges caused by aluminum substituting silicon bridging tetrahedra were balanced by Ca2+.

Lightweight bricks made of diatomaceous earth, lime and gypsum

Abstract Diatomaceous earth from Lampang Province in the north of Thailand composes of diatom, kaolinite, montmorillonite and illite, and has porous cellular structure. In this work, the diatomite, hydrated lime and gypsum are the main ingredients in making autoclaved lightweight bricks. Water content, pre-curing period, lime content, gypsum content and calcined temperature are the factors investigated. Mechanical and thermal properties are used to indicate their quality. The nature of hydration products and morphological characteristics of the lightweight bricks are also investigated. The results show that the diatomaceous earth possesses pozzolanic property and can be used for making lightweight bricks. The bricks made with untreated diatomite with 15% lime and 5% gypsum shows reasonably high strength of 14.5 MPa and low density of 0.88 g/cm 3 . Higher strength (17.5 MPa) and lower density (0.73 g/cm 3 ) are obtained with the use of diatomite calcined at 500 °C. The incorporation of lime and gypsum with a Ca/Si ratio of 0.55, 50% water content and 6 days pre-curing results in formations of calcium silicate hydrate and gismondine which enhance strength development. Calcination of diatomaceous earth results in dehydroxylation of the clay minerals and contributes to additional pozzolanic reaction and better mechanical and thermal properties of the lightweight brick.

Activation of fly ash cementitious systems in the presence of quicklime. Part II: Nature of hydration products, porosity and microstructure development

Following the first paper of a two-part series study on the activating effect of industrial quicklime upon different fly ashes, the current paper concludes on its beneficial role by focusing on the nature of hydration products, pore size and microstructure evolution of each system that was investigated. By looking into the development of these properties, the exact effect of the chemical activator could be elucidated, but furthermore a first approximation of the durability efficiency of the examined systems could be obtained. In the frame of this second part, it was demonstrated that, apart from portlandite, calcium sulphoaluminates (ettringite and monosulfoaluminate) along with some gismondine and gehlenite hydrate (C 2ASH 8) crystals are formed in activated fly ash-cement pastes from the first week of hydration. Mercury intrusion porosimetry data confirmed the beneficial action of quicklime, towards decreasing the total pore volume (and concurrent increase in the volume occupied by fine pores) in high-lime ash blends. Gel/space ratios were estimated for each blended paste, and fine, almost linear, correlation was established with total porosity values. Microscopical observations revealed that the critical contribution of quicklime in the production of secondary C–S–H nearby the ash particles. The flocculent-like gel collaborates smoothly with needle-like products and finely-dispersed crystals towards the gradual impletion of the pores and strengthening of the paste.

Use of DTA-TG in the evaluation of autoclaved cement-based systems

Fired-clay products such as bricks, tiles and pavers, are made in large volumes for use in a variety of construction applications throughout the world. A significant proportion of them ends up being a waste product either during their production process or the demolition of buildings. High pressure steam curing or autoclaving has proven extremely versatile for the manufacture of cement-based building products incorporating waste materials such as fly-ash and blast furnace slag. The nature of hydration products in an autoclaved cement based system incorporating different amounts of finely ground brick waste was investigated by means of thermal analysis and XRD, and is the subject of this paper.

Effect of quartz content on the nature of Al-substituted 11 Å tobermorite in hydrothermally treated CaO—Al2O3—SiO2—H2O systems

Lime-metakaolin-quartz mixtures containing varying amounts of quartz were prepared and hydrothermally cured at 180°C. The nature of hydration products with reaction time was studied using a combination of solid-state27 Al MAS NMR, XRD, DTA-TGA, SEM and wet chemical techniques. Hydrogarnet always preceded Al-substituted 11Å tobermorite irrespective of the amount of quartz used and supplied most, if not all, of the Al to form Al-substituted 11Å tobermorite. The availability of silicate anions from quartz was an important factor in providing a sink for the Al species to be in tetrahedral coordination. It also offered a means for controlling the extent of Al substitution into the 11Å tobermorite crystal lattice with reaction time and the extent of breakdown of the hydrogarnet. The growth of hydrogarnet was inhibited with increasing amounts of quartz, while the formation of Al-substituted 11Å tobermorite was accelerated. With increasing amounts of quartz, the crystallite size in the c-direction of Al-substituted 11Å tobermorite decreased and a change in its morphology was evident from platelets to aggregated petal-like, fine crystals.

Effects of quartz particle size on hydrogarnet formation during autoclaving at 180°c in the cao-al2o3-sio2-h2o system

The effects of quartz particle size on the nature of hydration products in unstirred, autoclaved metakaolin-lime-quartz slurries with Ca/(Al + Si) and Al/(Al + Si) ratios of 0.80 and 0.13 were investigated. While hydrogarnet invariably appeared before 11 Å tobermorite when using either fine or coarse quartz, its formation was more prominent in the presence of coarse quartz. The breakdown of hydrogarnet and the formation of 11 Å tobermorite were delayed in the presence of coarse quartz, suggesting that when using aluminous additives under autoclaving conditions, quartz fineness is a deciding factor. The amount of hydrogarnet formed during autoclaving followed by its breakdown is significant in relation to the formation of 11 Å tobermorite.

Detection of Failures around Main Cracks and Their Relation to the Fracture Toughness of Cement Pastes

Fluorescence microscopy examinations revealed the characteristics of crack path and the presence of local failures around the main cracks in hardened cement pastes. Reduction in the water: cement ratio and the addition of silica fume led to a decrease in locally damaged regions around the main cracks. However, the R‐ curve of the silica fume‐free cement paste with the water: cement ratio of 0.55 was much different from that of the silica fume‐bearing paste. The silica fume‐bearing cement paste exhibited a relatively flat R‐curve, whereas reduction in the water: cement ratio in the silica fume‐free paste led to a rising R‐curve. These results were related to the toughening mechanisms of cement pastes derived from the characteristics of the crack path and the areas of local failures around main cracks that were revealed by fluoresence microscopy. Presumably, these differences in fracture toughness resulted from the nature of hydration products and/or their inherent microstructure.