Aluminate Cement Research Articles

Thermodynamic analysis of the corrosion of high alumina cement by carbon dioxide

The integrity of the wellbore cement sheath plays a crucial role in the geological utilization and sequestration of carbon dioxide. Although high alumina cement (HAC) is a specialized cement with exceptional performance, additional research is required to assess its capability for long-term carbon dioxide sequestration. In this study, the effects of various corrosion conditions and pozzolanic materials on the corrosion of HAC were investigated by using the Gibbs energy minimization method. And the corrosion resistance of minerals was analyzed based on the multiphase chemical equilibrium theory. The results indicate that C3AH6 in HAC is the most susceptible to corrosion. As the temperature increases, more carbon dioxide is consumed for the complete reaction of the primary minerals in HAC. However, the corrosion of HAC is largely unaffected by system pressure. The high concentration of carbon dioxide in formation water accelerates mineral corrosion rates. In addition, the presence of Mg2+ and SO42− increases the formation of intermediate products during the corrosion of HAC, whereas the rate of mineral corrosion decreases as salinity increases. Fly ash can enhance the corrosion resistance of HAC during the initial stage of carbonization. Increasing the replacement ratio of fly ash to HAC reduces the generation of C3AH6 and increases the formation of C2ASH8. The corrosion resistance of each mineral is different, following the order: Al(OH)3 > C2ASH8 > C4AcH11 > C4Ac0.5H12 > C3AH6.

EVALUATION OF THE EFFECTIVENESS OF INFLUENCE CAUSED BY ULTRA AND NANO-DISPERSE ADDITIVES FOR MODIFICATION OF SULFATE PHASES AND SULFOALUMINATE PHASES

Problem statement. We offer stabilization of ettringite with the help of nanocomponents introduced. When conducting these studies, the problem of developing compositions of new construction materials based on gypsum and cement binder was solved by means of introducing nano-additives. Nanotechnology is a tool that allows you to reliably understand the processes that occur during hydration of composite materials, interaction of chemical and mineral additives with newly formed hydrated structures, formation and development of macro- and microstructure. The next stage in the effective application of nanotechnologies and techniques used in nanotechnologies for studying hydration processes and structure formation of gypsum- and cement-containing binding materials consists in the development of molecular models of hydration of Portland cement products. Creation of a high-strength cement frame is possible by adjusting the size of the solid phase and crystallization centers, as well as by modifying the gypsum- and cement-containing binder with ultra and nanodisperse additives. Purpose is to investigate effectiveness of ultra- and nanodisperse additives for modification of sulfate phases and sulfoaluminate phases. Conclusions. Modification of the compositions of radiation protection solutions of sulfate phases and sulfoaluminate phases with nanodisperse additives led to a decrease in the coefficient of linear expansion to 0.8 %. Herewith, an increase in solid indicators occurred by 8−12 %, which was caused by a change in the structure of neoplasms and the mineralogical composition. Thus, modification of the solution of sulfate and sulfoaluminate phases with carbon nanotubes (CNTs) leads to a decrease in the coefficient of linear expansion and an increase in the scattering coefficient of gamma rays by 30−40 % due to a high specific surface area of CNTs (80−120 m2/g). An optimal solution with the following composition has been experimentally developed: alumina cement, gypsum, BaSO4, nanotubes. It has been established that this solution has a 10−15 % higher water content, which is associated with the formation of ettringite during the process of setting and hardening. As a result, the arithmetic average amount of chemically bound moisture increased, which affected the change in the linear attenuation coefficient of ionizing radiation of the coating by 0.0088−0.009 cm-1. The total coefficient can reach 0,354 cm-1. Such results lead to reduced equivalent thickness (14,6 mm) of the radiation protective layer by 1−1,5 mm.

Performance study of modified aluminate cement under in-situ conversion conditions of shale oil

The resource potential of in-situ conversion of shale oil is enormous, and it is a strategic alternative resource for China's oil and gas industry. However, extreme high temperatures under in-situ conversion conditions of low maturity shale oil can lead to a decline in the mechanical strength of cement. In this paper, the influence of modified sodium hexametaphosphate on the hydration behavior of aluminate cement and its application performance at 650 °C was systematically studied, and its macro performance and microstructure were deeply explored. As a result, it was found that the sodium hexametaphosphate can significantly reduce the permeability of aluminate mudstone, but not significantly improve the strength, at the same time, it was found that the permeability of aluminate mudstone modified by micro-silica composite sodium hexametaphosphate is significantly reduced, and the compressive strength is increased. After treated at 650 °C, the compressive strength of 5.0% sodium hexametaphosphate modified aluminate mudstone is the highest (47.19 MPa), while the compressive strength of micro-silica composite sodium hexametaphosphate modified aluminate mudstone shows a downward trend. The aluminate brine debris has obvious transformation of hydration products before and after treatment at 650 °C, including mainly C3AH6 and AH3 into C12A7 and CA. Among them, C3AH6 and AH3 mainly undergo thermal decomposition at 180–400 °C, and the pores of aluminate mudstone increase attribute to the transformation of crystal form. The C2ASH8 generated by aluminate mudstone at 50 °C due to micro-silica is helpful to improve the microstructure. But, the decomposition of C2ASH8 after 650 °C treatment will also lead to the increase of cement paste pores.

Metakaolin-based geopolymer as an alternative binder in semi-insulating refractories

Semi-insulating refractories are recognized for their capability to minimize thermal losses and potentially save energy. Encouraging the advancement of cement-free compositions, cold-setting binders like geopolymers have emerged as captivating alternatives. This study focused on evaluating the incorporation of a metakaolin-based geopolymer binder in semi-insulating fused silica-containing castables. As reference materials, calcium aluminate cement (CAC)-bonded compositions with and without polymeric microspheres were also investigated. Three refractory castables were processed, and the prepared samples underwent characterization, including mechanical strength, apparent density and porosity, thermal shock resistance, thermal conductivity measurements, and SEM analysis. The geopolymer-bonded castable presented improved performance, exhibiting a thermal conductivity of 0.735 W/m.K at 800 °C, flexural strength of 6.24 MPa and apparent density of 1.68 g/cm3 after firing at 815 °C. The results underscore that utilizing geopolymer as a replacement for CAC enabled the design of a semi-insulating castable with enhanced properties and without the need for using porogenic agents.

Application of titanium gypsum as raw materials in cement-based self-leveling mortars

There are studies on titanium gypsum (TG) as a cement retarder, cementing material, or the production of lightweight wall materials and roadbed materials, but the application of TG is still limited, the utilization of TG is in the stage of exploration. This work studied the possibility of recycling TG as raw materials for the preparation of cement-based (Sulphate Aluminate Cement and Ordinary Portland Cement) self-leveling mortars. The effects of TG on mechanical strength, fluidity, shrinkage, and wear resistance were carried out. With the help of XRD, TG-DTG, and SEM, the microstructural characteristics were investigated to determine the mechanism of TG. It was concluded that the 1 d compressive and flexural strength reached 9.6 MPa and 3.5 MPa, respectively, and the 28 d compressive and flexural strength reached 4.3 MPa and 17.9 MPa, respectively, when the mass ratio of Sulphate Aluminate Cement (SAC) and Ordinary Portland Cement (OPC) was 7:3, the TG content was 45 %. TG could significantly improve the shrinkage of the mortars to 0.10 %, which was far better than the standard requirements of 0.15 %, but reduce the wear resistance to some extent. The primary component of calcium sulfate dihydrate (CaSO4·2H2O) in TG can further react with the aluminum phase in cement to grow more ettringite, which can make the microstructure denser, thereby improving the strength and the deformation of the cement-based self-leveling mortars.

From waste to wealth: Converting ferronickel slag to MgO–Al2O3–SiO2 castables

The feasibility of direct conversion of ferronickel slag (FNS) to MgO–Al2O3–SiO2 castables was assessed using large FNS particles as aggregate and fine FNS powders and calcium aluminate cement (CAC) as matrix. The phase compositions, microstructural features and properties of castables with the values of mass ratio of matrix to aggregate (M/A) between 1/4 and 2/3, corresponding to castables 1–5, were investigated based on both thermodynamic and experimental analyses. It was shown that with the increase of M/A, the content of fine FNS powders increased, promoting the interfacial reactions between FNS and CAC which suppressed the generation of clinoenstatite but promoted the formation of anorthite. During the firing process, the presence of liquid phase caused the conversion of binding mode from hydraulic binding to ceramic binding, which could optimize the structure and mechanical properties of the castables. When the value of M/A was 7/13, the castable after firing at 1250 °C for 3 h had the main crystalline phases of clinoenstatite and anorthite. It obtained the optimal comprehensive properties, i.e., bulk density of 2.45 g/cm3, apparent porosity of 20.68%, water absorption of 8.46%, linear shrinkage of −0.37%, bending strength of 9.30 MPa, and thermal conductivity of 1.69 W/(m·K) at 1000 °C, which were better than those of commercial counterparts. The findings confirmed the viability of using mainly FNS for preparing high-quality castables, achieving efficient value-added utilization of the waste.

Fluorides immobilization through calcium aluminate cement-based backfill: Accessing the detailed leaching characterization under torrential rainfall

Urbanization and economic development have increased the demand for fertilizers to sustain food crop yields. Huge amounts of by-products, especially phosphogypsum (PG), are generated during the wet processing of rock phosphate to produce fertilizers. Chronic exposure to fluoride in phosphogypsum in groundwater as a result of the weathering of fluoride-containing waste poses a significant health risk to millions of people. We propose a method for using calcium aluminate cement (CAC) to remediate high fluoride contents in solid waste. Column leaching tests under harsh rainfall conditions confirmed the efficient fluoride immobilization capacity of a CAC binder. Although the fluoride concentrations in leachates during the first 1–2 days (1.25 mg/L) slightly exceeded the threshold of 1.00 mg/L, the concentrations over 3–28 days (ranging from 0.98 to 0.83 mg/L) consistently remained well within the acceptable range. Furthermore, our characterization and geochemical modeling revealed the fluoride retention mechanisms of CAC-stabilized PG under laboratory-simulated conditions of torrential rainfall. During leaching, physical encapsulation prevents fluoride from contacting leachate. However, an unfavorable pH value can cause the release of fluoride from the cement matrix, which is subsequently captured by aluminate hydrate through adsorption or co-precipitation. We quantified the carbon footprint of CAC for immobilizing 1 mg of fluoride in PG, obtaining a remarkably low value of 4.4 kg of CO2, in contrast to the emissions associated with the use of ordinary Portland cement (OPC). The findings suggest a unique opportunity for extensive PG remediation. This opportunity extends the horizons of achieving zero-waste emissions in the phosphorus industry and has practical significance in the context of reducing carbon emissions.

Effects of Fly Ash Composition to Mitigate Conversion of Calcium Aluminate Cement Composites

Calcium aluminate cement (CAC) is one of the alternative cements that is widely used for special applications. However, during the hydration process degradation of CAC microstructure, the so-called hydrate conversion process, hexagonal calcium aluminate hydrate (CAH10) transforms into a cubic (C3AH6) phase, resulting in increased porosity and reduced strengths. It is known that alternative means for stabilizing the CAC conversion are conducted by introducing fly ash (FA) in CAC, where its microstructure is attributed to aluminosilicates. However, no study has yet been conducted on different FA compositions influencing CAC performance. This study aims to evaluate the effects of different compositions of FA on CACs’ fresh and hardened characteristics. Results revealed that the microstructure was denser when CAC was mixed with FA. Regarding reactivity, CAC with calcium-rich FA systems is 13% faster than the silica-rich one. The higher the density and the lower the porosity of calcium-rich FA mixtures were found compared with silica-rich FA in both micro- and macro-structures. As seen in the microscopic structure, this is due to the calcium-rich phase formation.

Advanced Self-Healing Ceramics with Controlled Degradation and Repair by Chemical Reaction.

Controlling the chemical reaction rate concerning degradation and repair is found to be important to design advanced self-healing ceramics. The recovery and degradation behaviors of strength and stiffness were investigated by exposing aqueous solutions of different pH and calcium ion concentrations to the introduced crack on typical self-healing ceramics dispersed with alumina cement as a self-healing agent. The chemical reaction of cement undergoes the following three stages: dissolution of components such as calcium ions, formation of a gel, and formation of final products. Experimental and thermodynamic assessments revealed that even under conditions where the final products are identical (thermodynamic equilibrium), kinetic effects (excessive dissolution of components or insufficient crystal formation) result in strength degradation rather than repair. It was also suggested that the repair function could be enhanced by controlling the nucleation site of the crystals.

Performance optimization design of high ductility cement-based alkali-activated municipal solid waste incineration fly ash composite for rapid repair material

As a residue that results from the high-temperature incineration of urban household waste, municipal Solid Waste Incineration Fly Ash (MSWIFA) contains various harmful organic pollutants like dioxins and heavy metals. These contaminants pose significant health risks to humans, including deformities and carcinogenic effects. The focus of this study is the development of a High Ductility SAC Rapid Repair Material (HD-SACRRM) through MSWIFA as a partial replacement for Sulphate Aluminate Cement (SAC), so as to achieve high ductility, rapid hardening, and the strength required for pavement layers. The reactivity of MSWIFA is investigated by adjusting its content and the type and dosage of alkali-activators. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) tests are conducted to analyze the variety and microstructure of alkali-activated MSWIFA-SAC hydration products. Also, the efficiency of immobilizing heavy metal ions within the alkali-activated system is examined. Based on these findings, the proportions of HD-SACRRM are optimized. The experimental results demonstrate that the optimal conditions for MSWIFA reactivity are met when the MSWIFA content is 20% and the 30% soluble silicate with a modulus of 1.8 is present. The optimized conditions of HD-SACRRM are as follows: a 20% MSWIFA content, 30% soluble silicate with a modulus of 1.8, a water-cement ratio of 0.30, a 2.0% fiber volume fraction, a sand-cement ratio of 0.30, and 5% Li2CO3 as an early strength agent. After 48 h of curing as standard, the material exhibits a compressive strength of 45.7 MPa, an ultimate elongation of 2.70%, and an average crack width of 127.59 μm. The alkali-activated MSWIFA-SAC system generated more gels such as C-S-H and C-A-H, thus resulting in a dense microstructure that enhances both the capacity of heavy metal immobilization and mechanical strength. These findings shed some light on the recycling of MSWIFA solid waste and the repair of asphalt concrete road surfaces.

Inhibiting effect of styrene-butadiene copolymer on the whitening of calcium aluminate cement mortar

This paper was based on the results of the research group's previous research on early whitening of mortars. The inhibition of whitening and changes of white deposit on the surface of calcium aluminate cement (CAC) mortar with styrene-butadiene copolymer (SB) were studied using a long-term accelerated whitening method under conditions of (20 ± 2) ℃ and (60 ± 5)% RH. The whitening phenomenon and white deposits of CAC mortars were studied using digital processing of images, optical microscopy (OM) and scanning electron microscopy (SEM). The results indicate that the white deposit on the surface of CAC mortar primarily consists of newly formed carbonation products, calcium carboaluminate hydrate (C4AC¯H11) and calcium carbonate (CaCO3), and with the extension of this exposure time, C4AC¯H11 will be carbonized to generate CaCO3 and Al(OH)3 (AH3). It is interesting to note that SB reduces the production of C4AC¯H11 and CaCO3 and that at SB dosing levels above 12% the C4AC¯H11 disappears and there is hardly whitening of mortar at this time. The inhibition mechanism of SB on the whitening of CAC mortar was investigated using X-ray diffraction (XRD), capillary water absorption and water contact angle tests. The analysis reveals that hydration product of CAC is mainly CAH10 after curing at 20 ℃ for 1 d, and the addition of SB did not change the type of CAC hydration products. Furthermore, incorporating SB enhances the hydrophobicity and reduces the capillary water absorption in CAC mortar, and forms a polymer film in CAC mortar. They prevent the transfer of ions from the interior of CAC mortar to the exterior and the contact of CO2 from the air with the cement hydrate on the surface of CAC mortar, thus reduces the production of white deposit on the mortar surface and inhibits the whitening of CAC mortar.

Effect of Variable Synthesis Conditions on the Formation of Ye'elimite-Aluminate-Calcium (YAC) Cement and Its Hydration in the Presence of Portland Cement (OPC) and Several Accessory Additives.

In the presented study, ye'elimite-aluminate-calcium (YAC) cement was synthesized. Complete synthesis of crystalline phases was achieved at a temperature of 1300 °C, which is 150 °C lower than the temperature standardly used in the processes of obtaining calcium aluminate cements (CAC). The greatest amount of ye'elimite phase (Klein complex), roughly 87% by mass, was acquired utilizing a sulphur ion transporter derived from artificial dihydrate gypsum obtained in the flue gas desulphurization process (variation I). In the case of anhydrite, the amount of synthesized crystalline ye'elimite in the clinker was 67% by weight (variant II). Depending on the synthesis conditions in the clinkers, the quantity of obtained calcium aluminates (C12A7, CA, and CA2) ranged from 20 to 40% by weight. Studies on the hydration process of YAC cement samples showed that the main products are hydrated calcium aluminates and dodecahydrate calcium alumino-sulphate. In sinters of YAC and OPC, no crystalline ettringite was observed. Hydration analysis of Chinese cement revealed the presence of crystalline ettringite and dodecahydrate calcium alumino-sulphate, as well as hydrated calcium silicates of the CSH type accompanied with pseudo-crystalline Al(OH)3. The obtained clinkers from variants I and II constitute a special binder, which, due to its phase composition after hydration, can be used in the construction of reactors for biogas production in eco-energy applications.

Investigating the synergistic effect of graphene nanoplatelets and fly ash on the mechanical properties and microstructure of calcium aluminate cement composites

Nanomaterials like graphene nanoplatelets (GNP) have been extensively applied in various fields as reinforcing materials offering excellent mechanical properties. However, for calcium aluminate cement (CAC) composites, the investigation of the GNP reinforcing effect is lacking. The synergistic effects of GNP and fly ash (FA) in CAC composites were evaluated in this study. The GNP was initially dispersed in mixing water using a dispersant combined with ultrasonication. The CAC composites with different GNP contents were prepared and their mechanical performance was investigated. In addition, partial replacement of CAC with FA was investigated for increasing sustainability and material cost of GNP-CAC composites. Their fresh properties, mechanical properties, and microstructural performance were analyzed using different techniques involving flowability, bulk density, volume of permeable void, water absorption, compressive strength, splitting tensile strength, flexural strength, nitrogen adsorption, TGA, and SEM-EDS tests. Using FA and GNP in CAC composites offers a positive impact by significantly increasing their strengths. Optimized mix was achieved by replacing 20% of FA and adding 0.3% of GNP in CAC composites. Results revealed that the 20% FA replacement of cement with 0.3% GNP addition remarkably increased the compressive, splitting tensile and flexural strengths at 28 days by about 57.2%, 35.5%, and 21.7%, respectively. The results suggest that using FA and GNP contributes to synergistic effects to promote the formation of silica tetrahedron in calcium aluminate silicate hydrate (CASH) gel, to refine pore characteristics, and to reduce conversion effects of metastable phases of CAC such that it can ultimately produce a green rapid-set high-performance concrete system.

Changes in the physicochemical properties of calcium aluminate cement paste with high alumina content under deep seas

Changes in the physicochemical properties of calcium aluminate cement paste with high alumina content under deep seas

Development of a novel ultra-high performance concrete (UHPC) suitable for underwater operation: Design and performance evaluation

This study has developed a new type of ultra-high performance concrete (UHPC) with excellent performance that can be used for underwater operations. Firstly, the sulfur aluminate cement(C S‾ A)-gypsum(CS‾)-silicate cement(OPC) ternary system is used as the cementitious material, and the optimal mixture ratio of underwater non-dispersive ultra-high performance concrete (UN-UHPC) is designed by the D-optimal design method. Then, the macroscopic properties, hydration process, and microscopic structure of the developed UHPC are evaluated. The experimental results show that UN-UHPC has excellent performance and good anti-dispersion properties, with a cement loss rate of 4.8% and a total porosity of 3.39%. Besides, due to the incremental effect of water, UN-UHPC has better mechanical properties and lower porosity than the one formed on land. In general, the developed UN-UHPC in this study further demonstrates the excellence and feasibility of D-optimal design in advanced building materials area and provides technical support for rapid underwater repair projects and the development of ocean resources.

From dolomite waste to katoite-based binder: Synthesis, performance and characterization

Calcium aluminate cement, in which katoite (KT) is the main binder, has garnered significant attention for its resistivity against aggressive environments. Despite its impressive durability, the production of this cement is costly and generates a substantial CO2 emission. Therefore, the objective of this study was to develop an eco-friendly KT-based binder using exchange reaction between sodium aluminate (NaAlO2) solution and dolomite waste powder. The performance of KT-binder was evaluated by measuring compressive strength, drying shrinkage, bulk density, and porosity. The phase composition and microstructural changes were also investigated by XRD, TG/DTG, FT-IR, and SEM/EDS techniques. The resulting hardened materials exhibited 28-day compressive strengths ranging from 5 MPa to 50 MPa and contained primarily katoite Ca3Al2(OH)12, along with pirssonite Na2CO3·CaCO3·2H2O, thermonatrite Na2CO3·H2O, and aluminum hydroxide Al(OH)3 phases. EDS mapping indicated that only a small amount of Mg2+ was present in the katoite phase, suggesting that the alkalinity and atomic radius difference between Ca2+ and Mg2+ were the main factors. Altering the content of concrete waste (CoW) in the cementitious system within the range from 10 to 50 wt% leads to an improvement in compressive strength and a reduction in both pore volume and drying shrinkage of NaAlO2-DW mixture. Conversely, perlite waste (PW) shows an opposite trend despite its high amorphicity. The presence of highly water-soluble thermonatrite, which is a byproduct of the cationic exchange reaction, is responsible for the low water resistivity of the prepared binder. As a result, more research is needed to tackle this problem.

Self-healing capability of engineered cementitious composites with calcium aluminate cement

The reduced heating temperatures required for producing calcium aluminate cement (CAC) make it highly suitable for reducing CO2 emissions in engineered cementitious composites (ECCs) known with their significant amount of cement. However, it is not clear to what extent this can influence the advanced mechanical and self-healing ability of control ECC, especially with the risk of conversion in CAC reaction products. This study investigates the self-healing capability of ECCs produced using CAC instead of conventional ordinary Portland cement (OPC). It also assesses the impact of incorporating fly ash (FA) into CAC-ECC blends at different FA/CAC ratios. In addition to the mechanical characterisation of sound specimens, flexural properties, cracking behavior, ultra-sonic pulse velocity (UPV) and rapid chloride permeability test (RCPT) were performed on preloaded OPC- and CAC-ECCs. The study also analyzed the microstructural changes of self-healing products associated with the high alumina content of CAC. The results show that CAC can be used to produce high mechanical strengths ECCs, with more than 34% and 7% higher compressive and flexural strengths respectively than those of ECC-CTL at early age, though the addition of FA was important to reach improved patterns of mechanical properties at advanced ages. In addition, significant improvements were recorded for the recovery rates of CAC-ECCs, reaching more than 29% for flexural strengths, 11% for UPV and 75% for RCPT than the ECC-CTL. The self-healing products characterized with SEM-EDS confirmed the occurrence of conversion when FA was not included in CAC-ECCs, nevertheless with limited effect on the self-healing efficiency of these mixtures.

Potential of sodium chloride solution as mixing water in calcium aluminate cement modified with siliceous minerals

Calcium aluminate cement (CAC) is advantageous in marine engineering, and using materials directly from the sea when mixing CAC can reduce both construction costs and ecological pollution to the sea. However, using seawater as the mixing water in CAC is difficult due to the strength reduction. In this work, CACs were prepared with siliceous minerals (SMs) (diatomite and silica fume (SF)) and sodium chloride solution (SCS) as the mixing water. For comparison, other samples were immersed in SCS rather than mixed with SCS. The strength, mass and length changes of CAC mortars cured at 20°C and 40°C were determined. The phase assemblages and morphology were assessed using X-ray diffraction, thermogravimetric-differential scanning calorimetry and scanning electron microscopy. Using SCS as the mixing water was beneficial to the later strength development of CAC. It promoted the formation of C2ASH8 in CAC pastes and showed no strength loss even when cured at 40°C. The strengths of the SM-modified CAC mortars mixed with SCS were higher than those immersed in SCS. The combination of diatomite and SCS significantly increased the CAC strength, especially at 20°C, while SF contributed to better stability in mass and length.

Development and performance characterization of a composite grouting material suitable for sealing and reinforcement of microcracked mudstone

Micro-pore grouting consolidation constitutes a pivotal technical solution for addressing the issue of water-related weakening in mudstone. Given the engineering challenges such as the sub-optimal injectability of cement slurry materials into mudstone, and the deficient reinforcement strength of silica sol in the same, this study provisionally developed a composite grouting material with a multi-particle size distribution, which is suited for the closure and reinforcement of microcracks in mudstone. We characterized the fundamental solidification and grouting properties of the modified composite grouting material, and unveiled the reinforcement mechanism of aluminate cement (AC) modified silica sol. The research findings indicate that the composite grouting material exhibits a two-tiered advantageous particle size distribution at the nanometer and micrometer levels, resulting in a marked enhancement in injectability. When the quantity of AC is 50% of the silica sol mass, and the water-cement ratio of cement slurry is 0.6, the solidification time of the composite grouting material can be adjusted between 21 and 114.5 mins. The initial viscosity is less than 5 mPa·s, and the stone rate exceeds 97%. The 28-day cemented structure exhibits a volume shrinkage rate of merely 0.57%, and the compressive strength can attain 14.5 MPa. In terms of the modification mechanism, SiO2 nanoparticles effectively expedite the hydration process of AC within the composite grouting material, engendering chemical bonds in the hydration products. These fill the pre-existing pores of the cement cemented structure, thereby enhancing the mechanical strength and compactness of the internal structure of the composite grouting material cemented structure.

Is the diffuse ultrasound method reliable for evaluating autonomous self-healing in cementitious materials with expansive agent pellets?

This study applied the diffuse ultrasonic method as a technique to assess the autonomous self-healing of Portland cement mortars with the incorporation of pellets with different sizes (0.6–1.18 mm, 1.18–2.34 mm, and 2.34–4.75 mm) composed of expansive agents, namely as calcium aluminate cement (CAC) and flue gas desulfurization gypsum (FGD). Surface cracks of 200–300 µm were induced in mortar specimens through a three-point bending test. The self-healing of cracks after 28 days was evaluated from the observed variations on three diffuse ultrasonic parameters: diffusivity, the arrival time of the maximum energy (ATME), and dissipation rate. The results indicated that the incorporation of pellets produced self-healing properties, which could be identified by the observed increased diffusivity (almost comparable to the reference non-cracked mortar), corroborating with the strength regain results. Therefore, the diffuse ultrasonic technique (especially the diffusivity parameter) is a promising alternative for evaluating the self-healing of cementitious materials by incorporating pellets with expansive agents.