Aluminate Hydrate Research Articles

The Influence of Hydrothermal Temperature on Alumina Hydrate and Ammonioalunite Synthesis by Reaction Crystallization

With the rapid development of the alumina industry and the shortage of bauxite, high-alumina coal fly ash (HACFA) has attracted more and more attention as a potential alternative alumina resource. In order to extract alumina from HACFA with newly developed technology, the investigation of the crucial step, the reaction between NH4Al(SO4)2·12H2O and NH3·H2O, is necessary and valuable. Thermodynamic analyses have shown that four kinds of alumina hydrate (boehmite, diaspore, gibbsite, and bayerite) might be formed at 120–200 °C, and ammonioalunite might be formed at temperatures over 180 °C. A hydrothermal reaction crystallization method was applied to this reaction. The experimental results showed that boehmite (AlOOH) could be formed at 150 °C and 200 °C after 12 h and NH4Al3(SO4)2(OH)6, an unstable intermediate, is formed during the initial stage and transformed into boehmite, eventually. The higher temperature (200 °C) was more energetically favorable for the formation of NH4Al3(SO4)2(OH)6, and the crystallinity of the products was better. More importantly, the sheet-like structure of boehmite (AlOOH) could be formed at 150 °C after 24 h of reaction time. The SEM results proved that the sheet-like structures evolutionary process of boehmite.

СИНТЕЗ И СВОЙСТВА ОКСИДОВ АЛЮМИНИЯ И ИТТРИЯ ДЛЯ КЕРАМООБРАЗУЮЩИХ МАТЕРИАЛОВ

Effect of hydroxide precursor nature on the morphology of alumina derived by heat treatment of ammonium aluminium carbonate hydroxide NH4AlCO3(OH)2 was investigated. It was found that the use of aluminum hydroxide as the precursor leads to the formation of needle-like particles of the product, while during synthesis using hydrated alumina, alumina particles of isometric shape were obtained. The effect of ammonia sulfate (NH4)2SO4 on the BET surface area of ammonium aluminium carbonate hydroxide powders was shown.

Controlling hydration of hydratable alumina via co‐grinding with Mg–Al hydrotalcite

AbstractThe on‐site industrial application of hydratable alumina (HA)‐bonded castables is inhibited by the high hydration rate of HA. In this study, the hydration behavior of HA co‐ground with sheetlike Mg–Al hydrotalcite (MAT) is investigated. The properties of castables bonded with MAT‐bearing HA are systematically assessed. The hydration rate of HA co‐ground MAT decreases as this allows MAT sheets to be effectively inserted into the microcracks of HA particles during grinding, thus decreasing the direct contact area between HA and water. The strength of MAT‐bearing castables (0.5 and 1 wt%) fired at 800°C improved slightly owning to the generation of magnesia–alumina spinel. The mechanical strength of castables fired at 1100 and 1550°C decreased as the MAT content increased owing to an increase in porosity. Based on an analysis of the hydration behavior of HA and the properties of HA‐bonded castables, the optimal MAT/HA weight ratio is approximately 1/10.

Mechanism of regulating the mechanical properties and paste structure of supersulfated cement through ultrafine iron tailings powder

To improve the defects of supersulfate cement (SSC), such as slow hydration rate and low strength, this research investigated the influence of ultra-fine iron tailings powder (UITP) on the strength of SSC. The mechanism of UITP's influence on the mechanical properties of SSC was studied by scanning electron microscope (SEM), X-ray diffraction (XRD), thermogravimetric–differential scanning calorimetry analysis (TG–DSC), and chemical titration analysis of ettringite (AFt). Results show that the compressive strength of SSC paste can be significantly improved when the UITP content was 30%–60%. When the UITP content was 40%, the compressive strength of the SSC paste was increased by 20.4% for 3 d and 44.1% for 28 d compared with that of the control sample. The hydration products of UITP and slag powder were consistent, which were calcium silicate aluminate hydrate (C-A-S-H) and ettringite (AFt); The addition of UITP can increase the amount of hydration products in SSC paste and regulate the ratio of AFt to C-A-S-H, thereby affecting the density of paste microstructure. Moreover, the pore structure of the SSC hardened paste can be improved by adding UITP, and the pore size distribution was optimal when the content of UITP was 40%. This study confirms that UITP and slag powder exhibit a synergistic hydration effect in the SSC system. This synergistic effect significantly improves mechanical properties and volume stability of the SSC hardened paste.

Efficient removal of refractory dyestuffs from textile wastewater by composite hydrated alumina derived from waste aluminum polishing solution

The present investigation is aimed at fabricating a cheap and efficient adsorbent for the removal of refractory dyestuffs from wastewater. By using waste aluminium polishing solution as the raw material, a composite hydrated alumina (CHA) was successfully prepared and its properties were characterized by the methods of FTIR, SEM, EDS, BET, TGA, and XPS. It was shown that the resulting two-layer adsorbent exhibits a flower-like shape, in which, alumina acts as the core and the surface is modified by phosphate groups and hydrogen bonds in the outer layer. Various functional groups on the surface provide multiple interactions with anionic dyestuffs such as Congo red (CR) and Reactive blue (RB). However, due to the contained N- double bond in the molecular structure in CR, the adsorbent exhibits an especially excellent selectivity and adsorption capacity for the removal of CR, which is caused by the formation of the hydrogen bonds between the hydroxyl groups of CHA and the azo and aromatic rings in CR molecule. The obtained results provide a novel approach to remove refractory organic pollutants from wastewater by using a cheap adsorbent derived from industrial waste.

Investigating the hydration of C3A in the combined presence of anhydrite and limestone using solid-state NMR, XRD and TG techniques

Diverse AFm phases are commonly found during the hydration of calcium sulfoaluminate (CSA), Portland (OPC), and calcium aluminate (CAC) cements. Due to the flexibility in their layered double hydroxide (LDH) structure, they are observed in variable single or multi-anion bearing forms, water contents and transient states. When multiple sulfate and carbonate anion sources, in variable compositions, are available during the hydration of cements, the identity, quantity, and stability of the formed AFm phases vary considerably. They may also precipitate as microcrystalline and/or amorphous phases, rendering their detection by X-ray powder diffraction (PXRD) difficult. In this study the AFm phases have been generated through the hydration of tricalcium aluminate (C3A) in the presence of sulfate (anhydrite) and carbonate ions (limestone) in various compositions. A multi-technique approach involving solid-state nuclear magnetic resonance spectroscopy (ssNMR), thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) and PXRD has been employed to govern the sequence of hydration reactions, the evolution, structure, and the stability of AFm phases at 1 and 28 days of C3A hydration. The results show that the identity and quantity of AFm phases formed vary considerably according to the nature of the anion sources available during the hydration. Hydration products such as crystalline C3AH6, ettringite and amorphous as well as crystalline/microcrystalline monoanionic AFm phases (monosulfate, monocarbonate, hydroxy-AFm) and bi-anionic phases (hemicarbonate and solid-solutions involving more anions) have been identified and quantified.

Use of Saline Water in Cemented Fine Tailings Backfill with One-Part Alkali-Activated Slag

To evaluate the feasibility of mixing saline underground water (SUW) in the preparation of cemented fine tailings backfill (CFTB), the compressive strength, pore fluid pH, slag hydration degree, hydration phase composition/structure, and pore morphology of CFTB have been tested. The binder used was a one-part alkali-activated slag composed of ground-granulated blast furnace slag (67.8% by weight) as precursor and desulfurized gypsum (25.9% by weight) and hydrated lime (6.3% by weight) as composite activator. The results showed that using SUW instead of deionized water as mixing medium had both negative and positive effects on the strength development of CFTB, depending on the curing time. At the very early ages (1–3 days), mixing SUW resulted in a decrease of the pore solution pH, which is due to the reaction of positive ions in SUW with OH− ions and the decrease of slag hydration rate, leading to a lower strength of CFTB. The positive effects were observed at the later ages (7–28 days). The higher later-age strength of SUW-mixed CFTB is attributed to two factors: (1) a smaller amount of ettringite (AFt) was generated at the very early ages, which reduced the encapsulation-hindering effect on the slag decomposition; and (2) SUW increased the slag hydration rate, promoting the condensation of calcium silicate hydrates, which results in increased formation of a calcium silicate aluminate hydrate [C-(A)-S-H] phase with rigid three-dimensional structure. These findings can provide important information for the use of SUW in CFTB in coastal and arid regions.

Understanding the role of C–S–H seed/PCE nanocomposites, triethanolamine, sodium nitrate and PCE on hydration and performance at early age

Compared to thermal curing, the application of C–S–H/PCE nanocomposites (CPNs) and triethanolamine (TEOA) compound admixture is a green and highly efficient hydration acceleration strategy. Commercial CPNs usually contain polycarboxylate (PCE) and residual sodium nitrate (NN). It is still uncertain whether their presence has some influence on the hydration of cement with this admixture. The experimental results show that TEOA-NN promotes hydration, and then the setting times are shortened and the early compressive strength is increased; it advances the start of the acceleration period, slightly increases tricalcium silicate (C3S) hydration, or further improves tricalcium aluminate (C3A) hydration. TEOA-PCE significantly retards C3S and C3A hydration at 1 d. In contrast, TEOA-CPNs exhibit excellent mechanical properties and hydration heat owing to its strong promotion of the hydration of C3S and C3A. These results can provide guidance for the formulated design of CPNs-based admixtures.

Role of ionic diffusion in heat flow and chemical shrinkage of tricalcium aluminate hydration subjected to thermo-chemo-electrical coupled fields

The role of ion diffusion in the heat flow and chemical shrinkage of tricalcium aluminate (C3A) hydration was investigated using the proposed multi-ionic reactive transport model for C3A hydration. Ion diffusion, dissolution, and precipitation reactions were coupled in the governing equation of the chemical field during C3A hydration in the presence of gypsum. Ion diffusion and chemical reactions were also governed by the electric and temperature fields in the model. Theoretical computations were performed relating ion diffusion to heat flow and chemical shrinkage during all stages of C3A hydration. Excellent agreement was achieved by comparing the simulation results and the experimental data under various conditions. The effects of the gypsum content, specific surface area (SSA), and curing temperature on the ion diffusion, electrical potential, heat flow, and chemical shrinkage were numerically studied. The results indicated that (1) higher gypsum contents, SSAs, and curing temperatures increased the ion concentration gradient and improved the supersaturation concentration of sulfate near the C3A surface; (2) higher gypsum contents led to faster decreases in the Ca2+ and SO42− concentrations before the depletion of gypsum; and (3) the gypsum content affected the phase assemblage through spatial and temporal evolution of the ionic species. The proposed model may guide the design of the C3A-gypsum system and can be extended to optimize the fluidity and setting properties of the cement paste.

A quantitative approach to determining sulfate balance for LC3

Achieving the correct sulfate balance in limestone calcined clay cements (LC3) to control aluminate hydration is critical for early hydration and property development, but the role of the calcined kaolin (metakaolin) fraction relative to other compositional variables has not been previously well-explored. In addition, little published research has investigated the influence of water-to-solid ratio (w/s) and superplasticizers in this context. This study assesses the influence and quantifies the relative significance of compositional predictors on the sulfate balance and cumulative heat evolved by 24 h for LC3 through a stepwise regression model. Sulfate balance was defined as the time difference between the sulfate depletion point and the time of maximum of alite peak obtained from a time derivative of data obtained through isothermal calorimetry. A methodology based on Kernel smoothing was used to precisely identify these events. The first 24 h of hydration of some LC3 pastes was also monitored via in-situ X-ray diffraction to develop linkages between LC3 composition and hydrated phase assemblage. The statistical analysis identified the metakaolin fraction as particularly significant for the sulfate balance. The results suggest that the metakaolin fraction influences the sulfate balance of LC3 both directly and through its interactions with other constituent materials such as limestone.

Effects of mineral admixture on properties of cement-based foam material developed for preventing coal spontaneous combustion

This study investigated the cement-based foam material (CBFM) used in mine fire-extinguishing materials by using a self-made compound foaming agent, water, cement, mineral admixture included fly ash (FA), blast furnace slag (BFS) and silica fume (SF), etc. The effects of mineral admixture type, dosage, foam content and water-binder ratio on the properties of CBFM were analyzed using the orthogonal tests, further determining the optimal ratio of CBFM. Moreover, the influence of mineral admixture type on the performance of the CBFM was analyzed by comparing the scanning electronic microscope and X-ray diffraction test. As a result, when the BFS content was 20%, the foam content was 2 times that of the cementitious slurry, and the water-binder ratio is 0.5, the overall performance of CBFM could be optimized. The surface morphology and mineral component investigation exhibited that CBFM with BFS as a mineral admixture had the more uniform closed-cell structure than FA and SF. Moreover, the novel material had the inhibitory effect on the transformation of calcium aluminate hydrate and hydrated calcium sulfoaluminate, which in turn could maintain the strength of the composite. The experimental results showed that the fire extinguishing and cooling effect of CBFM was better than that of traditional fly ash foam, and it could quickly cool down the temperature burning coal pile within 30 min. In addition, the solidification products of CBFM would form a wrapping layer on the surface of the loose coal pile, inhibiting the contact between coal and air, further preventing coal spontaneous combustion effectively.

Harvesting of microcells from cyanobacteria-laden water by energy-efficient electro-flocculation-flotation with aluminum hydrates

Electrocoagulation-flotation (ECF) is able to effectively harvest microcells from eutrophicated drinking water reservoirs to reduce algogenic organic matter (AOM) during cyanobacteria blooming. However, ECF needs high current density (CD) and prolonged reaction time with inevitable high-energy consumption. This study aimed to investigate a novel electrocoagulation-flocculation-flotation (EFF) process, with 10 min EC reaction and 15 min flocculation followed by 10 min flotation, for Microcystis aeruginosa (MA) cell harvesting from water as well as AOM reduction with Al hydrates at various CD and pH conditions. The results have shown that significant MA cell separation and dissolved organic carbon (DOC) reduction by EFF occurred at pH 8 with 5 mA/cm2, accounting for 95 % and 56 %, respectively. At such a condition, the formation of dominant colloidal Al species (Alc) governed the EFF to promote cells and AOM destabilization by strong sweep flocculation along with weak charge neutralization by polymeric Al species (Alb). Moreover, the pronounced reduction in soluble microbial product-like (SMPL) substances could be further improved by up to 60 % with EFF. It was also found that EFF required only 2.07 × 10−2 kWh/kg in energy input and served energy saving ~63 % less than that by prolonged ECF at similar MA cell separations. It concluded that EFF was an energy-efficient technique to fast separate and harvest microcells from MA-laden water and AOM reduction with low energy input for practical application in drinking water treatment.

Scheffe's Simplex Optimization of Flexural Strength of Quarry Dust and Sawdust Ash Pervious Concrete for Sustainable Pavement Construction.

Pervious concrete provides a tailored surface course with high permeability properties which permit the easy flow of water through a larger interconnected porous structure to prevent flooding hazards. This paper reports the modeling of the flexural properties of quarry dust (QD) and sawdust ash (SDA) blended green pervious concrete for sustainable road pavement construction using Scheffe's (5,2) optimization approach. The simplex mixture design method was adapted to formulate the mixture proportion to eliminate the set-backs encountered in empirical or trials and the error design approach, which consume more time and resources to design with experimental runs required to evaluate the response function. For the laboratory evaluation exercise, a maximum flexural strength of 3.703 N/mm2 was obtained with a mix proportion of 0.435:0.95:0.1:1.55:0.05 for water, cement, QD, coarse aggregate and SDA, respectively. Moreover, the minimal flexural strength response of 2.504 N/mm2 was obtained with a mix ratio of 0.6:0.75:0.3:4.1:0.25 for water, cement, QD, coarse aggregate and SDA, respectively. The test of the appropriateness of the developed model was statistically verified using the Student' t-test and an analysis of variance (ANOVA), and was confirmed to be acceptable based on computational outcomes at the 95% confidence interval. Furthermore, the scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) were used to evaluate the morphological and mineralogical behavior of green prior concrete samples with various additive mixture compositions. The addition of QD and SDA, on the other hand, aided the creation of porous microstructures in the concrete matrix due to fabric changes in the concrete mixture, potentially aided by the formation of cementitious compounds such as calcium aluminate hydrate and calcium silicate hydrate.

Design optimization and statistical modeling of recycled waste-based additive for a variety of construction scenarios on heaving ground.

To minimize the environmental burdens and to promote natural resource conservation and sustainability, a composite additive (CA) is proposed using paper and wood industry waste, i.e., lignosulphonate (LS) and lime (LM) as a replacement for conventional stabilizers. However, the implication of this proposed stabilizer for real construction scenarios requires a multi-objective optimization for a thorough guideline for practitioners. In this regard, the response surface methodology is used for the mix design optimization of the proposed CA for various construction scenarios (i.e., buildings, roadways, and slopes). An extensive testing program is designed and conducted to obtain different geotechnical parameters related to the mechanical, volumetric change, and hydraulic behavior of the soil with special attention to the stabilization mechanism. The interplay between variables (LS and LM) and responses is examined using the effective 3D surface diagrams, and mathematical models are derived for which the difference between R2, Adj R2, and Pred R2 is found to be less than 0.2. In addition, LM is found to be more sensitive in terms of mechanical and hydraulic parameters than LS whereas LS moderately contributes to altering the parameters related to the volumetric change and hydraulic behavior. The optimized mix design of CA (i.e., LS:LM) is determined against the expansive soil stabilization for foundation, subgrade, and slope stability cases which are found to be 1.03:3.57, 0.84:2.90, and 0.9:2.75 as best suitable for these cases, respectively. Predicted responses for the optimal solution for slope stability cases are found to have an error of 0-9.6%. The stabilization mechanism shows that LS and LM work well in conjunction and lead to a more stable soil structure with better interlocking and cementation between soil particles along with the formation of new cementing materials, i.e., calcium aluminate hydrate (CAH) and calcium silicate gel (CSH). The LS in CA is observed to reduce the LM concentration in soil stabilization by up to 45% with improved geotechnical performance. Thus, the proposed CA is vital for natural resource conservation and paper and wood industry-related waste management.

Disordered interfaces of alkaline aluminate salt hydrates provide glimpses of Al3+ coordination changes

HypothesisThe precipitation and dissolution of aluminum-bearing mineral phases in aqueous systems often proceed via changes in both aluminum coordination number and connectivity, complicating molecular-scale interpretation of the transformation mechanism. Here, the thermally induced transformation of crystalline sodium aluminum salt hydrate, a phase comprised of monomeric octahedrally coordinated aluminate which is of relevance to industrial aluminum processing, has been studied. Because intermediate aluminum coordination states during melting have not previously been detected, it is hypothesized that the transition to lower coordinated aluminum ions occurs within ahighly disordered quasi-two-dimensional phase at the solid-solution interface. Experiments and simulationsIn situ X-ray diffraction (XRD), Raman and27Al nuclear magnetic resonance (NMR) spectroscopy were used to monitor the melting transition of nonasodium aluminate hydrate (NSA, Na9[Al(OH)6]2·3(OH)·6H2O). A mechanistic interpretation was developed based on complementary classical molecular dynamics (CMD) simulations including enhanced sampling. A reactive forcefield was developed to bridge speciation in the solution and in the solid phase. FindingsIn contrast to classical dissolution, aluminum coordination change proceeds through a dynamically stabilized ensemble of intermediate states in a disordered layer at the solid-solution interface. In both melting and dissolution of NSA, octahedral, monomeric aluminum transition through an intermediate of pentahedral coordination. The intermediate dehydroxylates to form tetrahedral aluminate (Al(OH)4−) in the liquid phase. This coordination change is concomitant with a breaking of the ionic aluminate-sodium ionlinkages. The solution phase Al(OH)4− ions subsequently polymerize into polynuclear aluminate ions. However, there are some differences between bulk melting and interfacial dissolution, with the onset of the surface-controlled process occurring at a lower temperature (∼30 °C) and the coordination change taking place more gradually as a function of temperature. This work to determine the local structure and dynamics of aluminum in the disordered layer provides a new basis to understand mechanisms controlling aluminum phase transformations in highly alkaline solutions.

Chloride binding behaviors and early age hydration of tricalcium aluminate in chloride-containing solutions

Understanding the hydration kinetics and interactions of hydration products of cement components with chlorides is crucial for designing durable cement-based materials. The most reactive component of ordinary Portland cement (OPC), tricalcium aluminate (C3A), was investigated on its hydration kinetics, chloride binding behaviors and hydrated phase assemblages through multiple ex-situ and in-situ methods. The hydrated phases of C3A demonstrated different chloride binding isotherms in NaCl and CaCl2 solutions at different ages. Chloride ions can suppress the formation of calcium aluminate hydrates (OH-AFm and C3AH6) while calcium ions were favorable to increase the content of hydrocalumite (Cl-AFm) and regulate its crystal structure. The carbonation process deteriorated the chloride binding capacity of C3A pastes through producing more calcium mono/hemi-carboaluminate phases (Mc/Hc) or forming a solid solution between Mc and Cl-AFm. These findings can be used as a guide for corrosion protection design of cement-based materials.

Phase Transformations and Properties Evolution of Alumina-Based Refractory Castables Containing ZnO and SiO2

Although most of the studies presented in the literature are focused on MgAl2O4 formation and its role on alumina-based refractories performance, ZnO has been reported as a promising spinel inducer. Aiming to investigate the influence of ZnAl2O4 (ZA) and MgAl2O4 (MA) generation on the properties of alumina-based castables, three vibratable compositions containing calcium aluminate cement or hydratable alumina as binders and 1 wt% of silica fume, were evaluated in this work. Flexural strength, apparent porosity, hot elastic modulus, corrosion cup-tests, thermodynamic simulations, were carried out to analyze the performance of such ceramics. The results indicated that ZnAl2O4 was mainly formed above 800 °C, favoring an earlier sintering of the samples. Besides that, the softening of the castables was observed above 1200 °C, which resulted in the elastic modulus decay of the samples during their first heating cycle due to the formation of SiO2-rich liquid phase in the resulting microstructure. Cement-free samples obtained after calcination (600 °C for 5h) presented enhanced corrosion resistance when placed in contact with molten slag at 1500°C. Although, silica fume addition to the castables negatively affected their corrosion performance, it helped to counterbalance the expansion associated with the spinel and calcium aluminates formation.

Exploring the viability of fly ash bricks as an alternative material for building construction

Fly ash, which is recovered from the gases produced when coal is burned to produce power, is essentially a fine glass powder. Fly ash is a siliceous substance that has an amorphous or glassy bulk associated with it. These tiny earth elements are mainly composed of silica, alumina, and iron. When flyash is combined with lime and water, a cementitious substance is created. Under conditions of high steam pressure and temperature, the components of flyash combine with lime to form calcium silicate-hydrate and calcium aluminate hydrate. Portland cement and this newly produced solvent have characteristics that are very comparable. Due to their similarity, fly ash and cement can be substituted as a fundamental element in concrete, with fly ash having certain clear quality advantages. The surfaces produced by this concrete's formation are smoother, tighter, and denser, improving the characteristics of fly ash. In developing countries, flyash bricks are an economical and efficient means of low and medium-rise building construction, using very less resources. This paper examines the case of Flyash bricks for load-bearing wall construction as well as filler material and also discusses the engineering viability and properties of such Flyash bricks. The paper comes to the conclusion that Flyash bricks are a practical substitute for traditional bricks with more ancillary advantages.

Reactivity of raw, pyroprocessed and GGBS-blended alum sludge (AS) waste for sustainable cement production

This paper investigates AS waste to produce a construction binder of low environmental impact. It studies the reactivity of AS so that it can be used instead of traditional (higher impact) binders. Currently, most construction binders are Portland cement (PC) based, hence carrying a major environmental impact. Significant impact can be offset by using wastes as binders. Alum sludge (AS) waste is massively produced in drinking water plants, and its primary component is aluminium hydroxide (Al(OH)3). Both the raw and the calcined AS exhibit pozzolanic activity measured by the Chapelle test. The organic matter in raw AS inhibits pozzolanic reaction and hydrate formation, hence the raw AS specimens display low strength. Calcination removed the organic matter and induced the dehydroxylation of Al tetrahedra thereby increasing the pozzolanic activity of AS. The AS burned at 450 and 600 °C shows excessive reactivity which results in a flash set. However, at 800 °C, some amorphous Al(OH)3 transforms into semi-amorphous γ-Al2O3 avoiding flash set and allowing handling while maintaining significant reactivity that yields substantial strength. At 1000 °C, the conversion of amorphous Al(OH)3 into inert corundum significantly lowers reactivity. Therefore, pyroprocessing AS at 800 °C produces material of high reactivity: abundant calcium aluminate hydrates were evident in the pastes due to pozzolanic reaction and secondary hydration involving CO32− ions. Replacing 50% 800°C-AS with GGBS enhances strength and avoids strength reduction (caused by the conversion of carbonate AFm phases to Si-rich AFm phases, possibly stratlingite). A blend of 800°C-calcined AS and GGBS, in a lime or PC system, renders workable setting times and a high ultimate strength. Calcination is the key parameter affecting AS as binder replacement. When incorporated into lime or PC systems, improper calcination may cause problems such as flash setting and a strength reduction.

Effects of bauxite tailings and sodium silicate on mechanical properties and hydration mechanism of magnesium phosphate cement

Aiming at the problem of poor water resistance of magnesium phosphate cement (MPC) and large amount of bauxite tailings (BTs) accumulation, the effects of BTs and sodium silicate (NA) on mechanical properties and the hydration mechanism of MPC are investigated. The experimental results show that adding 15.0 wt% BTs can decrease the production cost without decreasing the strength of MPC, while NA can enhance the strength and water resistance of MPC with BTs, delay the setting time, and decrease the cumulative heat release during hydration. The compressive strength reaches 42.6 ± 0.4 MPa after curing in air for 28 d. Microanalysis shows that the main hydration product is struvite-K (MgKPO4·6H2O). NA can activate BTs, release silicon and aluminum components from BTs, and produce the zeolite-like hydration product sodium silicate aluminate hydrate (N-ASH) gel. Adding BTs and NA to MPC reduces the generation of microcracks and makes matrix more compact. Therefore, this work not only improves MPC performance, but also promotes the resource utilization of BTs.