Cement Systems Research Articles

Understanding the utilization potential of lepidolite slag in cementitious materials: Composition characteristics, hydration properties, and high value-added cementitious system design

Lepidolite slag (LES) is a kind of solid waste produced by extracting lithium from lepidolite, but researches focused on it are still limited. This paper works on understanding the utilization potential of LES in cementitious materials from its composition characteristics, hydration properties, and high value-added cementitious system design. Results show that the reactivity of LES is notably low in both water and alkaline solution. As a supplementary cementitious material (SCM), LES exhibits several drawbacks, including prolonging setting time, reducing compressive strength, and deteriorating pore structure. With addition of 30 % LES, the compressive strength at 28 d decreased by 63.3 %. Considering the high gypsum content within LES, an all-solid waste cementitious system was designed using high-volume LES combined with blast furnace slag (BS) and a small amount of lime. System with 40 % LES presents similar compressive strength with system composed entirely of BS at both 7 d and 28 d. The mechanism is that the excellent compound activation effect of gypsum in LES and lime on BS promoted the transformation of hydrotalcite to ettringite in the system. Even when LES content reached 70 %, compressive strength at 28 d surpassed 15 MPa. This study proposes a novel high value-added utilization way for LES in cementitious materials.

Experimental study on the factors affecting cement bond strength at the second interface of oil-gas well

The second interface of cementing refers to the interface formed by the formation and the cement sheath after cementing. The bonding strength of the second cementing interface is mainly generated through the interaction between the cement sheath and the formation. This study employs sandstone and limestone as experimental materials to investigate the influencing factors on the bonding strength of the second cementing interface by altering the water-cement ratio of the cement slurry system, adding ultrafine materials, and incorporating latex materials. In a progressive experimental approach, various types of cement slurries are selected, including pure cement slurry systems under different water-cement ratios (L1, L2), cement slurry systems with added nano-silica on the basis of pure cement systems (L3), and cement slurry systems with latex added (L4). The influence factors of the bonding strength of the second cementing interface of cement sheaths are explored through self-developed bonding strength evaluation molds and micro-characterization of the bonding interface. The results indicate that increasing the water-cement ratio or adding nano-silica can promote the generation of calcium silicate hydrate (C-S-H), resulting in higher early bonding strength but lower later strength under sandstone interfaces; the reaction of latex particles themselves to form a three-dimensional network structure can enhance the bonding strength. This research provides significant insights into the bonding laws of the second cementing interface and is of great importance to the cementing construction process.

Investigation of the Effect of Indirect Ultrasonic Force and Different Cements on the Bond Strength of Glass Fiber Posts in Teeth with Different Post Space Preparation Methods

(1) Background: The main aim of this study was to investigate the effects of different cements (Panavia V5, RelyX U200) and placement techniques (indirect ultrasonic vibration and manual method) on the bond strength of glass fiber posts in teeth with different post space preparation methods and to evaluate the failure types. (2) Methods: A total of 120 human upper central teeth were collected for the study. One week after root canal treatment, 10 mm long post space preparation was performed using post drills. Post spaces were irrigated in three different ways, namely NaOCl, NaOCl + EDTA solutions, and passive ultrasonic irrigation of NaOCl and EDTA solutions. Glass fiber posts were divided into two groups for resin cement systems, namely RelyX U200 (3M ESPE) or Panavia V5 (Kuraray Medical). Two different methods, namely the indirect ultrasonic vibration and the manual method, were applied for post placement. Then, 1.2 mm sections were taken from the middle and coronal regions of the inserted fiber posts for a push-out test. (3) Results: the Irrigation × Cement (p = 0.002), Cement × Section (p = 0.043) and Placement × Section (p = 0.049) pairwise interactions were statistically significant. (4) Conclusions: in our study, the effects of different post cavity preparations, different cement types, and different placement techniques on the bond strength of the fiber post were observed.

Research on the connection performance and micro-hydration mechanism of super-performance grouting material for prefabricated component connections

Abstract The performance of grouting material determines the connection reliability of prefabricated components and the overall safety of the structure. In order to realize the rapid connection of prefabricated components, improve the construction efficiency and improve the quality of component connection. The research group has developed a grouting material with fast early strength improvement and high late strength. The formation mechanism was analyzed by scanning electron microscopy. The reliability of grouting material was verified by uniaxial tensile test of sleeve grouting connector. The results show that : CH, AFt and C-S-H are formed inside the grouting material, and the cementing system is dense inside. The skeleton formed by AFt inhibits the shrinkage of the system, which can provide support for the later compressive and flexural strength. Uniaxial tensile test results and theoretical analysis. The external reinforcement of the sleeve is broken, and the bonding performance between the grouting material and the reinforcement inside the sleeve is reliable. The internal bond stress is far less than the failure stress of the connector, which is about 50 % of the failure stress.

Revolutionizing soil and waste treatment: MoS2 nanosheets turbocharge geopolymerization for eco-friendly remediation and self-cementation of arsenic-contaminated soils and municipal solid waste incineration fly ashes

In the face of escalating arsenic pollution and the mounting challenge of municipal solid waste incineration (MSWI) fly ashes, cutting-edge technology has emerged as a game-changer. By harnessing the power of MoS2 nanosheets, a revolutionary approach to double-self-cementation has been unlocked, addressing the limitations of previous methods. Through heterogeneous adsorption, MoS2 nanosheets bolstered the stability of the matrix, leading to a remarkable surge in compressive strengths and a substantial reduction in leaching toxicities of heavy metals. The compressive strengths of the cemented cubes increased as the dosage of nanosheets increased from 0.3 % to 2.4 %, with compressive strengths ranging from 24.34 to 34.21 MPa. The leaching toxicities of Zn, Cu, Cr, Cd, Pb, and As from the solidified matrix correspondingly decreased to 0.32, 0.21, 0.04, 0.01, 0.05, and 0.53 mg/L, respectively. The role played by nanosheets in inhibiting the release and transport of chloride species further underscored their pivotal contribution. Moreover, the study delved into the intricate mechanisms underlying the impact caused by nanoscale materials cementation and geopolymerization systems, shedding light on their dual function as solidification intensifiers and chemical stabilizers. The nanosheet-enhanced double-self process can be appropriately fitted by the JMAK model. The linear nuclei growth of gels controlled the solidification and geopolymerization of the precursor materials and the stabilization of HM contaminants. This groundbreaking research not only elucidates the transformative potential of MoS2 nanosheets but also paves the way for a paradigm shift in the treatment of contaminated soils and waste materials.

Bonding and Cleaning Effects of Irrigation Protocols Using Calcium Hypochlorite on the Post-space Radicular Dentin.

This study evaluated the effect of 2.5% sodium hypochlorite (SH) or calcium hypochlorite (CH) submitted to passive ultrasonic irrigation (PUI) or conventional irrigation (CI) on the incidence of residues and the bond strength of the cementation system to post-space dentin. Distilled water (DW) and 2.5% SH followed by 17% EDTA (SH-ED) were used as negative and positive control groups, respectively. The cervical, middle, and apical thirds of the post space were evaluated. One hundred and twenty bovine incisors were endodontically treated and post-space preparation was performed. The specimens were randomly assigned to six groups, according to the solution and irrigation method: DW-CI, SH-ED-CI-SH, SH-CI, SH-PUI, CH-CI, and CH-PUI. The incidence of residues (n=10) over the dentin was evaluated by scores using SEM images. Other specimens were irrigated as previously described and the post cementation was immediately performed using a conventional dual resin cement and a two-step etch-and-rinse adhesive system. Push-out and failure modes were performed for bonding evaluation. Kruskal-Wallis and Dunn test for incidence of residues data and one-way ANOVA and Tukey tests for bond strength data were used at a significance level of 5%. The protocols that showed a lower incidence of residues were: SH-ED-CI-SH, SH-PUI, and CH-PUI for the cervical third and SHED-CI-SH for the middle third (p<0.05). In the apical third, the protocols were similar to each other (p>0.05). Bond strength values were higher after irrigation with DW-CI for all thirds (p<0.05). 2.5% sodium or calcium hypochlorite negatively impacted the adhesion interface and exhibited a greater incidence of residues over the post-space radicular dentin.

Chlorellestadite-enriched waste-to-energy fly ashes in cementitious systems: Implications of ash treatment on end use

Waste-to-energy (WTE) plants generate fly ashes that are often destined for the landfill. These ashes are rich in chlorine, zinc, and various heavy metals. Here, we investigate the feasibility of their incorporation within cementitious systems in the context of a novel treatment process that chemically binds chlorines into chlorellestadite. Untreated ashes enhance the hydration kinetics and strength, likely due to excess Cl. However, the treated ashes retard the hydration because of the presence of ZnO (2–6 wt%). This retardation was successfully mitigated with up to 2 % calcium nitrate additive, resulting in cement-ash pastes that had mechanical strength comparable to 100% cement pastes. The untreated ashes leached excess Cl and led to the precipitation of hydrocalumite, whereas the treated ash–cement pastes were free from this phase. Finally, for ashes incorporated in hardened cement pastes, the leaching of heavy metals such as Pb and Cr was well under the toxicity limit, for both treated and untreated ashes. In summary, untreated ashes may find end use in unreinforced concrete applications, and treated ashes may be used within reinforced concrete, at least up to 10% cement replacement level.

High-volume recycled glass cementitious and geopolymer composites incorporating graphene oxide

This study presents the development of high-volume recycled glass construction materials using recycled glass aggregate (RGA) as a 100 % sand replacement, recycled glass powder (RGP) as 40 wt% of the binder, and 0.1 wt% graphene oxide (GO) as nano-reinforcement. The research investigates the individual and their combined effects on the engineering performance, reaction kinetics (using calorimetry, X-ray diffraction-XRD, Fourier Transform Infrared-FTIR and Thermogravimetric analysis-TGA), and microstructure (Scanning Electron Microscopy/ Energy Dispersive X-ray Spectroscopy-SEM/EDS) for both cementitious and geopolymer systems. The results indicate that while RGA reduces strength and exacerbates alkali-silica reaction (ASR), the presence of 40 wt% RGP significantly mitigates ASR despite a lower strength gain in both systems. Geopolymers exhibit significantly lower ASR expansion compared to cementitious matrices owing to their superior alkali-binding capacity and denser microstructure, which restricts water and alkali ion mobility. 0.1 wt% GO was found to accelerate geopolymerisation, enhancing the strength of geopolymer mixes, but it decreased compressive strength in cement-based mixes due to self-desiccation-induced micro-cracking under ambient curing condition. The study also highlights that cement and metasilicate, as well as slag, are the main contributors to cost and CO2 emissions in cementitious and geopolymer systems, respectively. Overall, geopolymers exhibit a significantly lower global warming potential (< 180 kg CO2-eq/m3) compared to cement-based mixes (> 365 kg CO2-eq/m3) with a comparable cost (190–230 AUD/m3). The results indicate the potential for sustainable construction applications using high volumes of recycled glass, incorporating over 70 % of the mixture by weight.

Influence of chloride acceleration methods on the electrochemical behaviour of superplasticiser admixed reinforced cementitious systems

Influence of chloride acceleration methods on the electrochemical behaviour of superplasticiser admixed reinforced cementitious systems

Evaluation on Preparation and Performance of a Low-Carbon Alkali-Activated Recycled Concrete under Different Cementitious Material Systems.

A green, low-carbon concrete is a top way to recycle waste in construction. This study uses industrial solid waste slag powder (S95) and fly ash (FA) as binders to completely replace cement. This study used recycled coarse aggregate (RCA) instead of natural coarse aggregate (NCA). This is to prepare alkali-activated recycled concrete (AARC) with different cementitious material systems. Alkali-activated concrete (AAC) mixtures are modified for strength and performance based on the mechanical qualities and durability of AARC. Also, the time-varying effects of the environment on AARC properties are explored. The results show that with the performance enhancement of RCA, the mechanical performance of AARC is significantly improved. As RCA's quality improves, so does AARC's compressive strength. At a cementitious material content of 550 kg/m3, AARC's 28d compressive strengths using I-, II-, and III-class RCA were reduced by 2.2%, 12.7%, and 21.8%, respectively. I-class AARC has characteristics similar to natural aggregate concrete (NAC) in terms of shrinkage, resistance to chloride penetration, carbonization, and frost resistance. AARC is a new type of green building material that uses industrial solid waste to prepare alkali-activated cementitious materials. It can effectively reduce the amount of cement and alleviate energy consumption. This is conducive to the reuse of resources, environmental protection, and sustainable development.

Enhanced pozzolanic reactivity in hydrogen-form zeolites as supplementary cementitious materials

Pozzolans rich in silica and alumina react with lime to form cementing compounds and are incorporated into portland cement as supplementary cementitious materials (SCMs). However, pozzolanic reactions progress slower than portland cement hydration, limiting their use in modern construction due to insufficient early-age strength. Hence, alternative SCMs that enable faster pozzolanic reactions are necessary including synthetic zeolites, which have high surface areas and compositional purity that indicate the possibility of rapid pozzolanic reactivity. Synthetic zeolites with varying cation composition (Na-zeolite, H-zeolite), SiO2/Al2O3 ratio, and framework type were evaluated for pozzolanic reactivity via Ca(OH)2 consumption using ion exchange and in-situ X-ray diffraction experiments. Na-zeolites exhibited limited exchange reactions with KOH and Ca(OH)2 due to the occupancy of acid sites by Na+ and hydroxyl groups. Meanwhile, H-zeolites readily adsorbed K+ and Ca2+ from a hydroxide solution by exchanging cations with H+ at Brønsted acid sites or cation adsorption at vacant acid sites. By adsorbing cations, the H-zeolite reduced the pH and increased Ca2+ solubility to promote pozzolanic reactions in a system where Ca(OH)2 dissolution/diffusion was a rate limiting factor. High H-zeolite reactivity resulted in 0.8 g of Ca(OH)2 consumed per 1 g of zeolites after 16 h of reaction versus 0.4 g of Ca(OH)2 consumed per 1 g of Na-zeolite. The H-zeolite modulated the pore fluid alkalinity and created a low-density amorphous silicate phase via mechanisms analogous to two-step C-S-H nucleation experiments. Controlling these reaction mechanisms is key to developing next generation pozzolanic cementitious systems with comparable hydration rates to portland cement.

Bond and cracking behavior of tailored limestone calcined clay cement-based composites including bicomponent polypropylene fibers with enhanced mechanical interlocking

This study examines the potential of combining tailored binder formulations with engineered polypropylene (PP) fibers to develop a range of Fiber-Reinforced Cementitious (FRC) systems with enhanced ductility and strain-hardening properties, while encompassing sustainability and economic viability. The experimental investigation compares the surface microstructure of novel bicomponent PP fibers, produced using a pilot fiber spinning device, with that of standard PP fibers. Micro-scale single-fiber pull-out tests are conducted to ascertain the extent to which this surface modification contributes to enhanced energy absorption. The effectiveness of these novel fibers at the composite scale is assessed when embedded into two limestone calcined clay cement (LC3) binder systems, in terms of the fresh and hardened properties of the resulting FRLC3, with low cement content (35% of the total binder). The effect of incorporating super absorbent polymer (SAP) on tailoring the internal porosity of the matrix, thereby promoting the potential for stress transfer via multiple crack pathways, is assessed. A Finite Element Method (FEM) analysis, calibrated with the materials and bond laws retrieved experimentally, is conducted to simulate the tensile and cracking behaviour of the optimal material combination investigated in this study, demonstrating a high degree of correlation with the tensile tests.

Comparative Analysis of Coronal Sealing Materials in Endodontics: Exploring Non-Eugenol Zinc Oxide-Based versus Glass-Ionomer Cement Systems.

The proper closure of the access cavity between appointments during endodontic treatment is paramount and relies on temporary fillings. This systematic review evaluates the effectiveness of zinc oxide-based materials and glass-ionomer cement (GIC) as temporary coronal sealers after root canal treatment in extracted human teeth. Three databases were searched to identify randomized clinical trials that examined the sealing properties of various temporary sealing materials using dyes or stains as indicators. A total of seven in vitro studies that fulfilled the eligibility criteria were critically analyzed. These indicated significant variations in the relative sealing ability of the coronal breach of endodontically treated teeth, either by zinc oxide or GIC-based materials. While GIC-based material (e.g., Fuji IX and Fuji II) exhibited superior sealing of single-rooted teeth, zinc oxide-based material (e.g., Cavit, Coltosol, Caviton) also showed promising attributes. Resin-modified GIC formulations displayed enhanced physical properties, yet challenges related to adhesive failure and shrinkage during polymerization were observed. Zinc oxide-based materials have demonstrated superior coronal sealing effectiveness over certain GIC in controlled settings. Their premixed nature ensures consistent application and hygroscopic properties improve cavity sealing. However, the focus on dye penetration tests for microleakage in vitro may not fully represent the risk of bacterial infiltration. Thus, in vivo studies are crucial for validating these findings in clinical contexts.

Development and property optimization of a sustainable phosphogypsum-based cementitious system with ground-granulated blast furnace slag and carbide slag

β-hemihydrate phosphogypsum (β-HPG) is a promising renewable building material with the potential to reduce phosphogypsum (PG) stockpiles significantly. However, its application is hindered by the low mechanical strength, subpar water resistance, and presence of hazardous components. In this study, ground-granulated blast furnace slag (GGBS) and carbide slag (CS) were together with β-HPG into a high-performance and sustainable phosphogypsum-based cementitious material system (PCMS). Various PCMS specimens were prepared by mixing β-HPG (60 %–100 %), GGBS (0–40 %), and CS (0–40 %). To optimize the PCMS properties, the mechanical, water resistance, and toxicity stabilization investigations of PCMS specimens were performed. Also, the mineralogical and microstructure characteristics of PCMSs were analyzed. The results highlighted that ternary PCMS presented excellent compressive strength and softening coefficient, with low water absorption and P, F leaching concentrations. The formulation with 60 wt% β-HPG and a GGBS/CS ratio of 3:1 showed the best overall property. The primary hydration products included ettringite (AFT), C-(A)-S-H gel, and dihydrate gypsum, contributing to the strength development, water resistance, and toxicity stabilization of PCMS. Furthermore, The increased proportion of GGBS promoted the formation of hydration products, improving the gypsum crystal structure and thus optimizing the properties of PCMS. This study provides valuable insights for further development of green building materials and environmental protection.

Early hydration and mechanical performance of composited cementitious system prepared from high temperature calcined molybdenum tailings

To improve the recycling of solid waste and achieve the secondary utilization of waste resources, this study investigates the preparation of molybdenum tailings composited binders by high temperature calcination using molybdenum tailings, limestone, bauxite, and iron concentrate powder. Besides, to explore the mechanical properties of the new molybdenum tailings composited binders, a molybdenum tailings composited mortar was prepared. The mechanical properties, micro performance and hydration mechanism of the new molybdenum tailings composited binders were studied. Finally, the carbon dioxide emission of green molybdenum tailings composited binders/molybdenum tailings composited mortar was evaluated by the life cycle assessment method. The results show that the compressive strength of molybdenum tailings composited binders gradually increases with the increase of calcination temperature. When the calcination temperature is 1350 °C, the compressive strength and flexural strength at 28 days are 44.17 MPa and 7.56 MPa, respectively, which are 3.65 % and 4.63 % higher than control group made by P·O 42.5 cement. The early strength of molybdenum tailings composited binders increases slowly, and the later strength increases rapidly. In addition, there is more C2S in the matrix, resulting in a slow hydration heat rate and low cumulative hydration heat. Compared with other groups, the hydration kinetic parameters and hydration rate of molybdenum tailings composited binders under 1350 °C were the lowest. The main hydration prodcuts of molybdenum tailings composited binders are C-S-H, calcium hydroxide, and Aft gels. Compared with P·O 42.5 cement paste and mortar, the carbon dioxide emission of molybdenum tailings composited binders/molybdenum tailings composited mortar is reduced by 42.82 % and 29.08 %, respectively, which can indicate that the new binders can produce less carbon dioxide. Overall, this study can provide new ways for the development of the new binder mixed with plenty of molybdenum tailings, and offer a novel way for recycling the molybdenum tailings, and also present some valuable experimental results for the application of molybdenum tailings in infrastructures.

Development and Evaluation of Insulation Materials for Deepwater Cementing.

Marine oil and gas resources are abundant in deepwater regions, where the shallow seabed harbors natural gas hydrate layers due to the cold temperature and high-pressure environment. Hydrates are prone to thermal decomposition, which can compromise the integrity of cement sealing and even lead to accidents like blowouts. While current low-hydrated heat cement systems mitigate hydrate decomposition from cement hydration heat during the waiting period for cementing, they do not address heat transfer from deep strata to shallow hydrate layers through fluid circulation in the tubing during deep oil and gas development. Rather than utilizing costly pipe insulation technology, adjusting the thermal conductivity of well cement emerges as a cost-effective approach to prevent heat loss in the wellbore to the hydrate layer and thus inhibit hydrate decomposition. The critical aspect lies in utilizing insulation functional materials. This study delves into the functional and structural design of insulation materials suitable for cement slurry systems in well cementing, outlining the preparation methods and processes for two insulation materials (SDBW and SDBW-II) tailored for deepwater well cementing. SDBW and SDBW-II are both nuclear shell structural materials. The core is made of high-strength hollow microspheres, produced by using a reverse suspension polymerization method to form spheres followed by high-temperature sintering. The shell consists of a wear-resistant BPA epoxy resin layer, with the surface of SDBW-II also containing highly reflective glass microspheres. Incorporating 20% of material SDBW into the cement reduces the thermal conductivity of the cement stone from 0.8 to 0.31 W·(m·K)-1 while achieving a compressive strength of 6 MPa after 24 h at 20 °C. Material SDBW-II offers both thermal resistance and reflection functions, increasing reflectivity (R) from 0.3 to 0.5. By adding 20% of this material to the cement, under the same conditions, although the compressive strength decreases to 4.2 MPa, the thermal conductivity can be reduced to 0.27 W·(m·K)-1. Furthermore, there is no significant change within 180 days, demonstrating long-term thermal insulation stability. These developed insulation materials can effectively improve the thermal insulation performance of the cement sheath, thereby maintaining the stability of the upper natural gas hydrate layer in the oil and gas production process of deepwater wells, providing an innovative solution for the long-term operation of deepwater oil and gas wells.

Photocatalytic NOx degradation and [formula omitted] adsorption by mortar modified with S-g-C3N4/MgAl-CLDH: Influence factors and stability

NOx is a harmful gas that causes respiratory diseases and acid rain. Commonly used photocatalysts for degrading NOx such as UV responsive TiO2 show weak photocatalytic degradation efficiency under visible light. Furthermore, the great concern is that the degradation product, nitrates (NO3-), from NOx could be washed away and increase the nitrogen deposition and eutrophication in the surrounding soil and aquatic environments, resulting in serious secondary pollution. To address these issues, this study proposes a novel solution by incorporating S-g-C3N4/MgAl-CLDH nanocomposites into a cementitious system which features high NOx degradation efficiency under visible light condition and the efficient capture and adsorption of NO3- in situ. The NOx degradation was comprehensively studied, encompassing the influencing factors and stability. The mechanism for NO3- adsorption process was expounded using the isothermal model. The results show that the amount of exposed S-g-C3N4/MgAl-CLDH and the contact area between S-g-C3N4/MgAl-CLDH and NOx are the essential reasons affecting the NOx degradation. The NOx degradation ability of photocatalytic mortar exhibits excellent stability with the NOx removal ratio decreasing by only 4.8 % and 14.5 % after ten consecutive tests and 120 min of ultrasonic washing, respectively. Furthermore, S-g-C3N4/MgAl-CLDH increases the NO3- adsorption capacity of photocatalytic mortar by about 1.5 times, and the adsorption process of NO3- follows the Freundlich isotherm. The study offers an alternate method for designing innovative nanocomposites in cement-based materials to control environmental pollution.

Rapid CO2 catalytic activation of binary cementing system of CSA and Portland cement

This study presents a novel approach to activating a binary cementing system composed of calcium sulfoaluminate (CSA) cement and ordinary Portland cement (OPC) using an instant CO2 catalysis. The activation led to significant and rapid strength enhancement, with the binary cement achieving a strength of 15.42 MPa immediately after activation, all within 1 min. The rapid CO2 activation catalyzed a notable heat release, accelerating the hydration of ye'elimite to form ettringite, which is a crucial component capable of bridging gaps and imparting high initial strength. Simultaneously, the CO2 activation catalyzed the increase in sulfur concentration, which in turn, also facilitated the formation of ettringite at an early age. Subsequent strength development was attributed to belite hydration. Apart from the rapid strength gain, employing CO2 activation facilitated control over the pH value of the pore solution, thus enabling the manipulation of ettringite's crystal morphology to strategically enhance the microstructure. Samples with lower pH values exhibited needle-like ettringite formations, whereas samples with higher pH values yielded rod-like and column-like ettringite crystal structures. Comprehensive analytical investigations were analyzed using XRD, FTIR, TGA, 27Al NMR, MIP, SEM, and ICP-OES. The present study provides a new perspective on the potential application of CSA-based cement, such as precast concrete element, instant concrete product delivery, and urgent reconstruction.

Effect of fly ash and nano Silica on the formation and evolution of Calcium Silicate Hydrate in Portland Cement during hydration process

Recently, the trend of replacing cement with mineral admixtures in concrete has witnessed increasing interest in the construction industry due to the reduction in embodied energy and CO2 emissions. However, the addition of these admixtures in Portland Cement (PC) affected the formation as well as the structure of the hydration product Calcium-Silicate-Hydrate (C-S-H), which is reflected in the mechanical and durability properties of cement concrete. In the present study, nano-silica (NS) and a mixture of fly ash (FA) and NS were replaced into PC (PC + 0.5 % NS, PC + 1 % NS, PC + 0.5 % NS + 20 % FA and PC + 1 % NS + 40 % FA) to systematically study the formation and structural evolution of hydration products, namely C-S-H, at atomic and micro-scales at the early stage of hydration (3 days) and a later stage of hydration (90 days). The formation and structure of C-S-H were analyzed to correlate the microstructure development by using XRD, Rietveld technique, FESEM, TEM, FTIR and nano-scale mechanical properties by employing the nanoindentation techniques. Atomic Pair Distribution Function (PDF) is used to understand the effect of admixtures on the C-S-H structure from atomic point of view. PDF shows the structural modification of C-S-H such as different polyhedral, separation of intralayer/interlayer, pair/bond length, coordination number of atoms corresponding to the addition of additives with hydration age. Also, porosity measurements show increasing or decreasing porosity during hydration with additives. It is observed that the contribution of low-density C-S-H (LD C-S-H) is greater than that of high-density C-S-H (HD C-S-H) in the beginning days, whereas HD C-S-H more than LD C-S-H in the latter days of hydration. The elastic modulus increased from 15.44 GPa to 19.82 GPa and the hardness decreased from 0.41 GPa to 0.35 GPa of LD C-S-H in early days, whereas elastic modulus decreased from 34.84 GPa to 29.33 GPa and the hardness increased from 0.75 GPa to 0.83 GPa of HD C-S-H in the latter stage due to addition of additives. However, both increased for LD C-S-H and HD C-S-H for each corresponding mixture during hydration. The present study reported the formation and microstructural modifications of C-S-H and the correlation with nano-mechanical properties due to addition of nano or micro-sized admixtures (nano silica and fly ash) in the cement system. This correlation helps to develop low-energy cement composites by tailoring cement systems with desired properties.

The role of ettringite seeds in enhancing the ultra-early age strength of Portland cement containing aluminum sulfate accelerator

The ultra-early age strength of shotcrete with Portland cement-based materials in various supporting constructions is crucial for safety and engineering efficiency. However, concerns exist about the low strength and uncertain mechanism when using aluminum sulfate, the main component of most-used alkali-free accelerators. This study addresses these concerns by introducing ettringite seeds into the Portland cement system with aluminum sulfate. Significant improvement in ultra-early age compressive strength of mortar, i.e., 242 % at 6 h and 201 % at 8 h, was achieved by mere 1 % seed addition. Analyses of hydration heat, composition and microstructure demonstrate that the ettringite seeds mainly affect the mechanical performance by forming a more prolonged and coarser ettringite skeleton, rather than directly accelerating cement hydration. Such an enhanced skeleton was proved to establish stronger interactions between particles in the Monte Carlo simulations. Additionally, the synergistic effect of the ettringite skeleton and C–S–H gel on ultra-early age strength was also explored. These proposed strengthening mechanisms were verified by the C3S and equivalent CaCO3 systems.