Search Results (1,449)

In the last 10–15 years, the use of waste refractory materials has significantly increased because it is not economically justified to transport such expensive materials to landfills. This work compared the physical and mechanical properties of refractory concrete samples with those of individual deflocculants—polycarboxilate, sodium tripolyphosphate, and a deflocculant mixture. Three refractory concrete compositions with individual deflocculants and a deflocculant mix were created to choose the best main properties of refractory concrete. Five compositions of refractory concrete specimens were created by partial replacement of chamotte aggregate (CA) by refractory concrete waste (RCW) (100, 75, 50, and 25%). Exothermal profile, structure development and spread were determined for fresh refractory concrete pastes. It was found that with an increase in CA replacement level to RCW, the EXO maximum temperature, spread and structure evolution speed decreases. SEM and porosity tests confirmed density, compressive strength results and structural parameters. The study shows that RCW replacement slows the hydration process, particularly at replacement levels above 33%. However, replacement levels of up to 25% improve compressive strength by 13% due to the additional amount of cement minerals in RCW aggregates, which can participate in the hydration process, making it a viable option for applications where enhanced durability is required, such as in non-critical zones of industrial refractory linings. Full article

For the exploration and development of oil and gas reservoirs in shallow, cold regions and deep oceans, oil well cement (OWC) pastes face the challenge of slow cement hydration reactions and the low early-strength development of cement stone at low temperatures, which can cause the risk of fluid channeling and the defective isolation of the sealing section during the cementing construction process. To address the above challenges, a nanoscale hydrated calcium silicate (C-S-H) crystal nucleus, DRA-1L, was synthesized. Its application performance and action mechanism were studied. The structural characterization of DRA-1L revealed that its crystal structure resembles that of amorphous C-S-H gel, with a size distribution ranging from 20 to 200 nm. The addition of DRA-1L significantly shortens the transition time of static gel strength, preventing the channeling of OWC paste and promoting the strength development of cement stone at low temperatures. Moreover, the mechanism by which DRA-1L enhances the early strength of cement stone was studied. Results indicated that the nanoscale DRA-1L with nucleation effect reduces the barrier to C-S-H gel formation and accelerates cement hydration, which leads to the increased compactness and early strength of cement stone. Full article

C-S-H/PCE suspension can boost the hydration degree and strength of cement composite binding. However, the suspension will inevitably precipitate after a period of time, which is not conducive to its preservation, and its low solid content increases transportation costs in practical applications. In this study, utilizing synthetic PCE as a template, C-S-H/PCE suspension was synthesized using a co-precipitation method. Subsequently, powder seeds were produced via the spray-drying technique, and these prepared powder seeds were analyzed via microscopic characterization. The impact of these powder nucleating agents on cement hydration kinetics was evaluated through hydration heat measurements and hydration degree, fluidity, and compressive strength testing. The results indicated that these powder seeds exhibited a nano-film morphology. Their nucleation effect significantly enhanced the cement hydration rate, increased the degree of hydration, and improved strength. The hydration kinetics showed that the hydration of cement mixed with nucleating agents was not governed by a single reaction mechanism, but rather constitutes a complex, multi-component reaction process. As the content of nucleating agents increased, higher dosages of nucleating agents accelerated the production of more products within a short period, causing the system to rapidly transition to phase boundary reaction control. When the dosage of nucleating agents reached 2%, the cement hydration process bypassed the phase boundary reaction control stage and transitioned directly from the crystallization nucleation and crystal growth control process to the diffusion-controlled phase. Although the influence of powder seeds on the enhancement of the early-stage strength of mortar was slightly lower than that of the suspension, the powder was beneficial to its storage and transportation. Therefore, it has the potential to replace the suspension. Full article

LC3 (limestone calcined clay cement) is poised to become the construction industry’s future as a so-called low-carbon-footprint cement. Research into this subject has determined the minimum kaolinite content in calcined clays to guarantee good mechanical performance. This study examines the use of clay from the Valencian Community (Spain), which has a lower kaolinite content than the recommended amount (around 30%) for use in LC3 and how its performance can be enhanced by replacing part of that clay with metakaolin. This study begins with a physico-chemical characterisation of the starting materials. This is followed by a microstructural analysis of cement pastes, which includes isothermal calorimetry, thermogravimetry, and X-ray diffraction tests at different curing ages. Finally, this study analyses the mechanical performance of standard mortars under compression to observe the evolution of the control mortars and the mortars with calcined clay and metakaolin over time. The results show that the LC3 mortars exhibited higher compressive strength in the mixtures with higher calcined kaolinite contents, achieved by adding metakaolin. Adding 6% metakaolin increased the compressive strength after 90 days, while 10% additions surpassed the control mortar’s compressive strength after 28 days. Mortars with 15% metakaolin exceeded the control mortar’s compressive strength after just 7 curing days. The hydration kinetics showed an acceleration of LC3 hydration with metakaolin additions due to the nucleation effect and the formation of monocarboaluminate and hemicarboaluminate (both AFm phases). The results suggest the potential for combining less reactive materials blended with highly reactive materials. Full article

This study investigates the rheological behavior of oil well cement pastes (OWCPs) modified with core/shell TiO₂@SiO₂ (nTS) nanoparticles and polycarboxylate-ether (PCE) superplasticizers at different temperatures (25, 45, and 60 °C). Results show that nTS particles increased static and dynamic yield stresses and the apparent viscosity of the cement slurries due to an increased solid volume fraction and reduced free water availability. The increase in the slurry dispersion by adding PCE superplasticizers enhanced the effect of the nanoparticles on the rheological parameters. Oscillation rheometry demonstrated that nTS nanoparticles enhanced the structural buildup, while PCE retarded hydration. Furthermore, slurries hydrated at 60 °C experienced higher initial values of the elastic modulus and a faster exponential increase in this rheological parameter due to the acceleration of the cement hydration. Full article

Hydraulic structures are frequently subjected to soft-water or acidic environments, necessitating serious consideration of the long-term effects of calcium leaching on the durability of concrete structures. Three types of common Portland cement (ordinary Portland cement, moderate-heat cement, and low-heat cement) paste samples widely applied to hydraulic concrete were immersed in a 6 mol/L NH₄Cl solution to simulate accelerated calcium leaching behavior. The mass loss, porosity, leaching depth, compressive strength, and Ca/Si ratio of the three types of pastes were measured at different immersion stages (0, 14, 28, 56, 91, 140, and 180 days). The Vickers hardness index was employed to compare cement samples subjected to erosion for 30 and 180 days. The microstructure and composition of the mineralogical phases of the leached samples were also determined by X-ray diffraction, thermogravimetric analysis, and scanning electronic microscopy. Accordingly, the time-varying behavior and deterioration mechanism of the different cements subjected to leaching were contrastively revealed. The results showed that the calcium leaching resistance of the low-heat cement was the best, followed by the moderate-heat cement and ordinary Portland cement, proving that the content and structure of Ca(OH)₂ and C-S-H gels were closely related to the leaching performance of the cement. The less Ca(OH)₂ and more aggregated C-H-S gels produced by C₂S led to better calcium leaching resistance in the cement. Therefore, the leaching performance of Portland cement could be effectively improved by reducing the content of C₃S and increasing the content of C₂S, and the dissolution rate of calcium ions under leaching could be reduced by controlling the low initial calcium content in cementitious materials. This paper offers theoretical guidance for mitigating the long-term effects of calcium leaching on hydraulic concrete structures by conducting a comprehensive comparative analysis of the damage behavior and deterioration mechanisms of various types of Portland cement under identical erosion conditions. Full article

In order to study the effect of the crushing process on the fine separation of reclaimed asphalt pavement (RAP) and the mechanical properties of cement-stabilised aggregate mixed with RAP, four crushing processes, namely small mesh hammer crushing, hammer crushing, jaw crushing, and double roller crushing, were used to separate the aggregate from asphalt in RAP materials. The effect of crushing on the grading characteristics and agglomeration condition of RAP material was investigated. RAP cement-stabilised aggregates were prepared and analysed for their mechanical properties and micro-morphology using RAP materials obtained from fine separation. The relationship between the RAP material properties and the mechanical properties of the RAP-added cement-stabilised aggregate was analysed on the basis of the tests. The results showed that crushing breaks down large-size RAP materials, leading to grade refinement, and that hammer crushing was the most effective in reducing the grade variability. The highest agglomerate dissociation rate of RAP material above 4.75 mm after small mesh hammer crushing treatment was 96.9%, and the residual mass ratios of RAP material in two grades of 0~3 mm and 3~5 mm after hammer crushing were lower than 90%. The unconfined compressive strength, splitting strength, and compressive resilience modulus of RAP cement-stabilised aggregate after crushing were greater than those of the uncrushed RAP cement-stabilised aggregate, and the crushing increased the amount of RAP in the mix to 60%. Compared with the unadulterated RAP cement-stabilised aggregate, the hydration products of the RAP cement-stabilised aggregate were reduced after crushing, and there were obvious gaps and discontinuities between the RAP material and the cement paste. The RAP gradation and agglomeration condition correlated strongly with the mechanical properties of the mixes, with RAP coarse aggregate agglomerates being the main cause of gradation variability. This paper provides theoretical support for the proposal of a pretreatment process to reduce the variability of RAP-doped cement-stabilised aggregate and improve the mechanical properties, and the research results are conducive to the recycling of high-volume RAP materials in the base. Full article

Dispersive soil is highly susceptible to water erosion, leading to significant engineering challenges, such as slope instability and canal damage. Common modifiers such as lime are effective but cause environmental pollution. Therefore, it is important to explore eco-friendly modifiers. This study investigates the effects of sticky rice and calcium chloride (SRC) on dispersive soil. Dispersivity tests identified an optimal ratio of sticky rice to calcium chloride of 3:1. To analyze the effects of different SRC contents and curing times on the soil properties, tests of dispersivity, hydraulic, mechanical, chemical, and microscopic mechanisms were conducted based on this optimal ratio. The results indicated that 1.5% SRC effectively eliminated soil dispersivity even without curing, and its effectiveness improved with an extended curing time. After 28 days of curing, the water stability increased significantly, permeability decreased by an order of magnitude, and cohesion improved by approximately 85.97%. SRC reduced soil dispersivity through three primary mechanisms: lowering the pH, promoting ion exchange between Ca²⁺ and Na⁺, and the cementing effect of the sticky rice paste. Additionally, Ca²⁺ acted as a bridge between organic colloids and clay particles, further strengthening the structural stability of microaggregates. Overall, SRC proved to be an effective eco-friendly modifier for improving physicochemically dispersive soil. Full article

Gasification slag is the solid waste produced in the process of coal gasification. China produces approximately 30 million tons of gasification slag every year, which urgently needs to be recycled in an efficient and sustainable way. This paper discusses the feasibility of using gasification slag as a supplementary cementitious material (SCM). The working properties, mechanical properties, and microstructure of cement paste after the addition of gasification slag were studied and compared with those of pure cement paste. The results indicate that the hydration products of the composite paste contain a significant amount of Ca(OH)₂ and C-S-H gel when the content of gasification slag is less than 30%. However, when the gasification slag content exceeds 30%, the primary hydration product shifts to the C-A-S-H gel. Furthermore, the C-(A)-S-H gel tends to exhibit a lower calcium–silicon ratio and a higher degree of polymerization as the gasification slag content increases. Specifically, the Ca/Si ratio of the 60% C-A-S-H gel is 1.66, with a degree of polymerization of 0.77. When the gasification slag content is maintained at or below 30%, the compressive strength of the gasification slag cement paste decreases by approximately 3.7% to 9.3% compared with that of Portland cement (PC). Nevertheless, the composite cement meets the design requirements of 42.5 composite Portland cement. Thus, gasification slag has emerged as a promising supplementary cementitious material (SCM), with significant potential for widespread application. Full article

Advancements in mine tailings treatment technology have increased the use of superfine tailings, but their extremely fine particle size and high specific surface area limit the performance of superfine tailings cemented paste backfill (STCPB). This study investigates the effects of using superfine cement as a binder to enhance the fluidity, strength, and pore structure of STCPB. The influence of water film thickness (WFT) on STCPB performance is also examined. The results show that the cement-to-tailings ratio (CTR) and solid content (SC) significantly affect the spread diameter (SD) and unconfined compressive strength (UCS), following distinct linear/logarithmic and exponential trends, respectively. WFT has an exponential impact on SD and a non-linear effect on UCS, enhancing strength at low levels (0 μm < WFT < 0.0071 μm) and balancing hydration and flowability at moderate levels (0.0071 μm < WFT < 0.0193 μm) but reducing strength at high levels (WFT > 0.0193 μm). Additionally, superfine cement significantly improves the pore structure of STCPB by reducing porosity and macropore content. These findings provide valuable insights into optimizing STCPB for enhanced performance and sustainability in mine backfilling applications. Full article

Cemented paste backfill (CPB) is a cemented void filling method gaining popularity over traditional hydraulic or rockfill methods. As mining depth increases, CPB-filled stopes are subjected to higher confining pressures. Due to the soil triaxial apparatus limitations, as the conventional method of triaxial testing on CPB, no confining pressures higher than 5 MPa can be applied to CPB over a range of curing time. This lack of data introduces uncertainty in predicting CPB behavior, potentially leading to an overestimation of the required strength. To address this, this study introduces a new testing method that allows for higher confinement beyond traditional limitations by modifying the Hoek triaxial cell to accommodate low-strength materials. This study then investigates the coupled influence of confining pressure and curing time (hydration) on CPB characteristics, specifically examining the impacts of different curing times and confining pressures on the mechanical and rheological properties of CPB. A total of 75 triaxial tests were conducted using 42 mm cylinder shape samples at five various curing times from 7 to 96 days, and applied at low and high confinement condition levels (0.5 to 30 MPa). The results reveal that hydration and confinement positively impact the CPB strength. The modified structured Cam-Clay model was selected to predict the behavior, and its yield surface was updated using the experimental results. The proposed yield model can be utilized to describe CPB material subjected to various curing and pressure conditions underground. Full article

In order to better understand the effect of compositional parameters on the properties of magnesium phosphate cement (MPC) mortar, the relationship between the thickness of paste film and the workability and strength of MPC mortar is revealed. A three-parameter filling density prediction model is adopted to study the filling density of sand with different gradations. The validity of the three-parameter filling density prediction model is validated by experimental results. The thickness of the paste film of MPC mortar is calculated with different sand gradations. The results show that the thickness of paste film has a great influence on the slump flow and strength of MPC mortar. The linear positive relationship between paste film thickness and slump flow of MPC mortar. At different sand-to-binder ratios, there is no significant linear relationship between the thickness of the paste film and the mechanical properties. But under the same sand-to-binder ratio, there is an optional thickness of paste film for the strength of the MPC mortar. Comprehensively considering the workability and mechanical properties, magnesium phosphate cement mortar’s optimal paste film thickness ranges from 73 µm to 74 µm. When designing the proportion of magnesium phosphate cement, the appropriate thickness of the paste film can be selected according to the different engineering types and construction environments. Full article

Alite calcium sulfoaluminate (ACSA) cement is an innovative and environmentally friendly cement compared to ordinary Portland cement (OPC). The synthesis and hydration of ACSA clinkers doped with gradient sulfur were investigated. The clinker compositions and hydrated pastes were characterized by X-ray diffraction (XRD), isothermal calorimetry, mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) to analyze its mineral contents, hydration products, heat release, pore structure, and microstructure. The compressive strength and linear expansion of ACSA mortars were tested for their mechanical properties. Results showed that clinkers doped with 2 wt.% MgO can offset the hurdle that SO₃ caused to the formation of C₃S (tricalcium silicate). Clinkers with varying ratios of C₃S and C₄A₃$ (calcium sulfoaluminate) were obtained, achieving 58–70% C₃S and 2.0–5.6% C₄A₃$ in ACSA through adjusting the KH (lime saturation factor) values and SO₃ dosage. ACSA cement showed better early mechanical properties. The 0.93 KH value with 3% SO₃ dosage in the raw meal, which contains 63.9% C₃S and 2.98% C₄A₃$ in the clinker, reached an optimal compressive strength level at 1d (26.35 MPa) and at 3d (39.41 MPa), marking 30.45% and 18.70% increases compared to PII 52.5. The excellent early strength of ACSA cement may offer promising applications t increasing the incorporation of supplementary cementitious materials, thereby reduce pollution and carbon emissions. Full article

Conventional cement production is a major source of carbon dioxide emissions, which creates a significant environmental challenge. This research addresses the problem of how to reduce the carbon footprint of cement paste production using agricultural and industrial waste by-products, namely wheat straw ash (WSA) and silica fume (SF). Currently, accurate models that can predict the mechanical properties of cement pastes incorporating these waste materials are lacking. To fill this gap, our study proposes a model based on response surface methodology and Box-Behnken design, designed to predict the strength of cement pastes with partial substitutions of WSA and SF. Through mechanical and characterization tests, the model demonstrated high accuracy in predicting the strength of the pastes, validated with three mixes, which showed maximum errors of less than 6% at different ages (7, 28, and 56 days). Response surface analysis revealed that replacing cement with 0–20% WSA and more than 5% SF can effectively reduce the carbon footprint by maximizing waste incorporation. This model allows for the calculation of optimal cement substitution levels based on the required strength, thus promoting sustainability in the construction industry through the use of local waste/resources. Full article

This study evaluated the properties and processing of cemented paste backfills (CPBs) for potash mining through loop tests. The CPBs were made with steel slags as the binder, granulated potash tailings as the aggregate, and waste brine water as the liquid phase. The effects of solid concentration and steel slag dosage on the transport and mechanical properties of CPBs were assessed. The loop test demonstrated that all CPB slurries performed well, exhibiting strong long-distance pipeline transport capabilities. The 28-day compressive strength of the backfills exceeded 1 MPa, meeting the design requirements for backfill strength. The key rheological parameters, including yield stress (τ₀) and viscosity coefficient (η), were comprehensively and theoretically analyzed based on the variations in pressure loss per unit distance of the filling slurry measured during the loop test. The empirical formulas for CPB pressure loss, accounting for varying flow rates and pipeline diameters, were derived with an error margin under 2%. The response surface analysis showed that the affecting extents of factors on pressure loss in CPB slurry were ranked as follows: solid concentration > cementing agent content > flow rate. This study offered valuable guidance for the processing of potash mine backfill operations. Full article