The Role of MWCNTs in Enhancing the Foam Stability and Rheological Behavior of Cement Pastes That Contain Air-Entraining and Superplasticizer Admixtures.

This research delves into the intricate dynamics between multi-walled carbon nanotubes (MWCNTs), air-entraining admixtures (AEAs), and a range of superplasticizers (SPs) in cementitious systems, shedding light on key aspects of construction material innovation. The focus is on how MWCNTs, AEAs, and specific SPs-namely, lignosulfonate (LS), polycarboxylate (PCE), and polyacrylate (PA)-influence the stability of foams and the viscosity and setting times of cement pastes. To assess the impacts of these components, we employed foam stability assessments, viscosity measurement techniques, electrical conductivity analysis, and evaluations of dispersion and setting times. Results indicate that MWCNTs enhance foam stability and viscosity, with the degree of improvement contingent on the type and concentration of SPs and the presence of AEAs. Notably, SPs, particularly PCE and PA, markedly influence the properties of cement paste, including increasing dispersion values and modulating setting times, especially when combined with MWCNTs and AEAs. The study concludes that strategically combining MWCNTs with specific SPs and AEAs alters the physical properties of cementitious materials significantly, underscoring the potential for customizing material design in the construction sector.

Study on the Pore Structure of Lightweight Mortar with Nano-Additives.

With the development of nanotechnology, nanomaterials have been introduced to improve the engineering properties of cement-based building materials. An abundant number of studies have been carried out on normal-weight concrete using multi-walled carbon nanotubes (MWCNTs) or nano-silica (NS) and have proven their effectiveness. Nevertheless, still very few investigations are available in terms of lightweight cement-based materials, especially when MWCNTs and NS are binarily incorporated. Thus, in this study, fly ash cenospheres (FACs) according to cement weight were applied as lightweight fine aggregates to produce lightweight mortar (LWM). MWCNTs at 0.05, 0.15, and 0.45% and NS at 0.2 and 1.0% were binarily added as modifiers. Compressive and flexural strengths were tested to investigate mechanical behaviors. A water absorption test was conducted, together with scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP), to identify the impacts of the nano-additives on the pore structure of LWM. The following results were obtained: MWCNTs and NS demonstrated synergic effects on enhancing the mechanical properties of LWM. MWCNTs exerted positive impacts on reducing the porosity and improving the pore distribution at low dosages of 0.05 and 0.15%. The hybrid addition of NS further transformed large voids into small ones and introduced closed pores.

The Effect of Natural and Synthesised Zeolites on Cement-Based Materials Hydration and Hardened State Properties.

The synthesis of zeolites from difficult-to-utilise waste materials facilitates the creation of more financially attractive and efficient synthetic zeolites. These can be incorporated into construction materials, resulting in a reduction in cement usage and the production of superior, clean, and sustainable construction materials. The potential to enhance the hydration rate of fresh cement paste by substituting up to 10% of the cement with two synthetic zeolites-one commercially produced and the other synthesised from waste and natural zeolite-was explored. Due to a higher Al/Na ratio, newly sintered waste-based zeolite possesses six times higher electrical conductivity compared to industrially produced 4A zeolite and more than 20 times higher electrical conductivity compared to natural zeolite. As the sequence of this fact, substituting up to 10% of the cement with AX zeolite cement paste accelerates the maximum heat release rate time and increases the total heat by 8.5% after 48 h of hydration. The structure, compressive strength, and water absorption of the hardened cement paste depends on the Al/Na ratio, pH, and electrical conductivity values of the zeolite used. The findings revealed that AX zeolite, due to presence of mineral gibbsite, which speeds up hydration products, such as CSH development, increases the compressive strength up to 28.6% after 28 days of curing and reduces the water absorption by up to 1.5%. Newly synthesised waste-based AX zeolite is cheap because its production is based on waste materials and is mostly promising due to superior properties of created construction materials compared to the other presented zeolites.

Effect of Mechanically Activated Nepheline-Syenite Additive on the Physical-Mechanical Properties and Frost Resistance of Ceramic Materials Composed of Illite Clay and Mineral Wool Waste.

This study investigates the coupling effect of mechanically activated nepheline-syenite (NS) and mineral wool melt waste (MWMW) on the physical-mechanical properties of a ceramic body. The results indicate that an optimal amount (10-20%) of NS additive promotes the formation of the smallest pore size from 0.001 to 0.01 µm, as well as improves physical, mechanical, and durability properties of the ceramic samples with MWMW, when fired at temperatures between 1000 and 1080 °C. As the NS content increases, the composition becomes more alkaline, leading to enhanced vitrification and the formation of a glass phase during firing. This reduces open porosity, modifies pore size distribution, and enhances compressive strength and frost resistance. An NS content of 15% produces the best results, increasing the smallest pore fraction and yielding favourable properties, such as reduced open porosity, water absorption and density, increased compressive strength, and does not affect the linear shrinkage. The frost resistance test demonstrates that the coupling effect of NS additive and MWMW improves the samples' resistance to freeze-thaw cycles, with the best performance observed at 15% NS content. The study also highlights the usefulness of structural parameters and ultrasound testing for assessing and predicting the frost resistance of ceramic samples.

The Effect of Mechanical Activation of Fly Ash on Cement-Based Materials Hydration and Hardened State Properties.

Fly ash from coal represents the foremost waste product of fossil fuel combustion. These waste materials are most widely utilised in the cement and concrete industries, but the extent of their use is insufficient. This study investigated the physical, mineralogical, and morphological characteristics of non-treated and mechanically activated fly ash. The possibility of enhancing the hydration rate of the fresh cement paste by replacing part of the cement with non-treated and mechanically activated fly ash, and the hardened cement paste's structure and early compressive strength performance, were evaluated. At the first stage of the study, up to 20% mass of cement was replaced by untreated and mechanically activated fly ash to understand the impact of the mechanical activation on the hydration course; rheological properties, such as spread and setting time; hydration products; mechanical properties; and microstructure of fresh and hardened cement paste. The results show that a higher amount of untreated fly ash significantly prolongs the cement hydration process, decreases hydration temperature, deteriorates the structure and decreases compressive strength. Mechanical activation caused the breakdown of large porous aggregates in fly ash, enhancing the physical properties and reactivity of fly ash particles. Due to increased fineness and pozzolanic activity by up to 15%, mechanically activated fly ash shortens the time of maximum exothermic temperature and increases this temperature by up to 16%. Due to nanosized particles and higher pozzolanic activity, mechanically activated fly ash facilitates a denser structure, improves the contact zone between the cement matrix, and increases compressive strength up to 30%.

The Influence of Concrete Sludge from Residual Concrete on Fresh and Hardened Cement Paste Properties.

In the concrete manufacturing industry, a large amount of waste is generated. Such waste can be utilised in the production of more sustainable products with a low carbon footprint. In this study, concrete sludge, a difficult-to-utilise waste that is obtained from residual concrete by washing a concrete truck, was investigated. During washing, aggregates from the concrete mixture are separated, and the remaining insoluble fine particles combine with water to form concrete sludge. Dried and wet concrete sludge were used in the tests. Samples with different compositions were produced with dried and wet concrete sludge, cement, superplasticiser, and tap water. Seven cement pastes with different compositions were made by partially replacing cement with dried concrete sludge (0%, 5%, 10%, 15%, 20%, 25%, and 30%). In compositions with wet concrete sludge, cement was replaced by the same amounts as in the case of dried concrete sludge. The slump, setting time, and their changes with different amounts of concrete sludge were determined for fresh cement pastes. It was found that with different forms of concrete sludge, the technological properties of the mixtures change, and the setting time decreases. The density and compressive and flexural strength results were confirmed by SEM and XRD tests. The research results show that dry concrete sludge causes the deterioration of the mechanical properties of cement stone, while wet concrete sludge improves the mechanical properties of cement stone. However, it was found that replacing 5% cement with dry concrete sludge does not significantly affect the properties of hardened cement stone. In mixes with wet concrete sludge, the recommended amount of replaced cement is 10%, because the technological properties of the mixture are strongly influenced by larger amounts.

Effects of Na2CO3/Na2SiO3 Ratio and Curing Temperature on the Structure Formation of Alkali-Activated High-Carbon Biomass Fly Ash Pastes.

This study explored unprocessed high-carbon biomass fly ash (BFA) in alkali-activated materials (AAM) with less alkaline Na2CO3 as the activator. In this paper, the effects of the Na2CO3/Na2SiO3 (C/S) ratio and curing temperature (40 °C and 20 °C) on the setting time, structure formation, product synthesis, and physical-mechanical properties of alkali-activated BFA pastes were systematically investigated. Regardless of curing temperature, increasing the C/S ratio increased the density and compressive strength of the sample while a decrease in water absorption. The higher the curing temperature, the faster the structure evolution during the BFA-based alkaline activation synthesis process and the higher the sample's compressive strength. According to XRD and TG/DTA analyses, the synthesis of gaylussite and C-S-H were observed in the sample with an increasing C/S ratio. The formation of the mentioned minerals contributes to the compressive strength growth of alkali-activated BFA pastes with higher C/S ratios. The findings of this study contribute to the applicability of difficult-to-recycle waste materials such as BFA and the development of sustainable BFA-based AAM.

Use of Magnesium Silicate Contaminated with Organic Compounds in Ceramic Materials as a Pore Modifier.

This study investigated the use of organic compound waste (OCW) contaminated magnesium silicate/diatomite in ceramics. Substituting part of the clay (between 5 and 20 wt.%) with OCW modifies a pore structure and enhances the ceramic product's thermal conductivity, density, and frost resistance. Prepared samples were tested at 1000-1060 °C temperatures and their structural parameters and Maage factor, useful for frost resistance prediction, were evaluated. Results show that OCW modifies the porous structure and improves the insulating properties of the ceramic body. Increasing OCW content up to 15% in the ceramic body decreases density by up to 15.0%, and thermal conductivity by up to 42.5%, because of the modified pore structure. According to structural parameters calculation, the higher frost resistance can be predicted for ceramic bodies containing 5-10% of OCW, according to Maage factor calculation ceramic bodies containing 5-20% of OCW are frost resistant. Designed ceramic products can be attractive for use in construction due to improved energy efficiency and reduced energy consumption in buildings due to their low thermal conductivity, satisfactory mechanical strength, and sustainability based on predicted frost resistance.

Study of the Course of Cement Hydration in the Presence of Waste Metal Particles and Pozzolanic Additives.

As the construction of hydrotechnical and energy facilities grows worldwide, so does the need for special heavyweight concrete. This study presents the analysis of the influence of waste-metal particle filler (WMP) on Portland cement (PC) paste and mortars with pozzolanic (microsilica and metakaolin) additives in terms of the hydration process, structure development, and physical-mechanical properties during 28 days of hardening. Results have shown that waste-metal particle fillers prolong the course of PC hydration. The addition of pozzolanic additives by 37% increased the total heat value and the ultrasound propagation velocity (UPV) in WMP-containing paste by 16%; however, in the paste with only WMP, the UPV is 4% lower than in the WMP-free paste. The density of waste-metal particle fillers in the free mortar was about two times lower than waste-metal particle fillers containing mortar. Due to the lower water absorption, the compressive strength of WMP-free mortar after 28 days of hardening achieved 42.1 MPa, which is about 14% higher than in mortar with waste-metal particle filler. The addition of pozzolanic additives decreased water absorption and increased the compressive strength of waste-metal particle filler containing mortar by 22%, compared to pozzolanic additive-free waste-metal particle fillers containing mortar. The pozzolanic additives facilitated a less porous matrix and improved the contact zone between the cement matrix and waste-metal particle fillers. The results of the study showed that pozzolanic additives can solve difficulties in local waste-metal particle fillers application in heavyweight concrete. The successful development of heavyweight concrete with waste-metal particle fillers and pozzolanic additives can significantly expand the possibility of creating special concrete using different local waste. The heavyweight concrete developed by using waste-metal particle fillers is suitable for being used in load balancing and in hydrotechnical foundations.

Effect of Calcium Nitrate on the Properties of Portland-Limestone Cement-Based Concrete Cured at Low Temperature.

The effect of calcium nitrate (CN) dosages from 0 to 3% (of cement mass) on the properties of fresh cement paste rheology and hardening processes and on the strength of hardened concrete with two types of limestone-blended composite cements (CEM II A-LL 42.5 R and 42.5 N) at different initial (two-day) curing temperatures (-10 °C to +20 °C) is presented. The rheology results showed that a CN dosage up to 1.5% works as a plasticizing admixture, while higher amounts demonstrate the effect of increasing viscosity. At higher CN content, the viscosity growth in normal early strength (N type) cement pastes is much slower than in high early strength (R type) cement pastes. For both cement-type pastes, shortening the initial and final setting times is more effective when using 3% at +5 °C and 0 °C. At these temperatures, the use of 3% CN reduces the initial setting time for high early strength paste by 7.4 and 5.4 times and for normal early strength cement paste by 3.5 and 3.4 times when compared to a CN-free cement paste. The most efficient use of CN is achieved at -5 °C for compressive strength enlargement; a 1% CN dosage ensures the compressive strength of samples at a -5 °C initial curing temperature, with high early strength cement exceeding 3.5 MPa but being less than the required 3.5 MPa in samples with normal early strength cement.