Cementitious Gels Research Articles

Effect of alkaline carbonate-based activators on the properties of one-part ternary hybrid cement

This paper investigates the use of solid alkaline carbonates (Na2CO3 and K2CO3) as activators for the production of one-part hybrid cement from a combination of three precursors: blast furnace slag, metakaolin, and clinker. The impact of activator dosage and the nature of the alkaline cation (Na+ or K+) on the compressive strength (Cs) and microstructure of the elaborated materials was studied. The results indicate that the highest Cs values are achieved with 5 % Na2CO3. The analysis of the reaction products’ microstructure reveals that the inclusion of Na2CO3 mainly produces (N,C)-A-S-H and C-A-S-H cementitious gels, along with metastable carboaluminates and different types of calcium carbonate. Additionally, the presence of K2CO3 results in the creation of hemicarboaluminate.

Valorisation of "La Palma" Volcanic Ash for Making Portland-Blended, Alkaline and Hybrid Portland-Alkaline Cements.

The present work evaluates the feasibility of using volcanic fly ash (VFA) generated by the eruption of the Tajogaite volcano on the island of La Palma (Spain) in 2021, as a precursor in the preparation of cementitious materials with different Portland cement (PC) replacement levels (0%, 30%, 70% and 100%), in the absence (Blended Cement, BC) and presence of an alkaline activator (Hybrid Alkaline Cement, HAC, and Alkaline Cements, AC). Hydration kinetics (isothermal conduction calorimetry), paste mechanical strengths and reaction products were characterised by XRD, FTIR, TG/DTG and BSEM/EDX. The results obtained indicate that the strengths developed by the hybrid alkaline cements (HAC) are higher than those of the blended cements (BC), especially at the age of 2 days, where 25 MPa were obtained with the replacement of 70% PC by VFA. Alkaline cements (AC, 100% VFA) that were prepared with 8 M NaOH solution as the activator reached 40 MPa after 2 days. It was observed that in all the binders, depending on the initial composition of the binder mixture and the percentage of replacement and/or activator, VFA reacts to form cementitious gels, C-A-S-H and N-A-S-H type, which supports its use as a mineral addition to blended cement or as a precursor in the preparation of alkaline and hybrid alkaline cements.

Hybrid binders based on volcanic pumice: Effect of the chemical composition on strength and microstructures

This research focused on pastes of hybrid binders (HB-VP) based on precursors of Volcanic pumice (VP, 25–75 %) and portland cement (PC, 25–75 %), activated with 4–12 wt% Na2SO4 and Ca(OH)2 in various molar ratios Na2SO4:Ca(OH)2. The experimental work and results were based on the surface response method and statistical tools, which indicated that the %VP influenced the early strength for up to 28 days and that the activators influenced the 90 and 180-day strength. SEM, EDS, XRD, FTIR and NMR indicated heterogeneous microstructures, with complex mixtures of cementitious gels like C-S-H with ettringite; HB-VP with 50 % VP showed Al into bridging positions in C-A-S-H, while for 75 % crosslinked C-(A)-S-H structures were noted. The chemical composition of the reaction products varied depending on the surrounding precursor. The CO2 emissions and strength of the HB-VP are competitive relative to other reported binders, and appear to be a suitable sustainable replacement of the Portland Cement.

NaOH molarity influence on mechanical and durability properties of geopolymer concrete made with fly ash and phosphogypsum

Fly ash (FA), rich in aluminosilicates, and phosphogypsum (PG), rich in free calcium, could be excellent precursor materials to synthesize geopolymer concrete (GPC). In this paper, by using PG as feedstock to FA as per circular economy model, geopolymer concrete is developed. PG from 10 to 50% and molar concentration of NaOH from 8 to 14 M are varied to create the GPC. Further mechanical and durability properties are evaluated to ascertain the impact of NaOH molarity and PG content. The maximum strength of 47.97 MPa and durability results demonstrate that 30% PG and 12 M NaOH concentration are optimum dosages for successfully synthesis of GPC with the combination of FA and PG. The study pertinent to leaching illustrates that the optimal dosage of binder materials is efficacious in preventing release of potentially toxic elements (PTEs) from GPC, endorsing that its usage in all practical applications do not endanger people or the environment. Micro-characterization discloses that free available calcium ions exquisitely participated in hydraulic reaction to form affluent cementitious gels such as calcium alumino-silicate hydrate (C-A-S-H) and sodium alumino-silicate hydrate (N-A-S-H). The interpretation of various results portrays that both NaOH molarity and amount of free calcium has significant influence on GPC and played a crucial role in high strength attainment. A linear relationship for compressive strength versus durability properties imply that the developed GPC is more durable and sustainable exhibiting overall performance in all practical applications.

Incorporating potassium-rich waste material in a sustainable way to stabilize dispersive clay: A novel practical approach for the construction industry

The inappropriate use of dispersive clay in constructing roads/highways, embankments, and other structures causes significant challenges to the construction industry. The dispersive clay treatment in this work was carried out for the first time using eco-friendly and sustainable potassium-based material rather than calcium-based, which makes it particularly innovative. This study investigated the impact of potassium-rich wood ash (KRWA) on dispersion, index, unconfined compression strength (UCS), consolidation, physio-chemical, mineralogical and microstructure properties. KRWA amount varied up to 35% and cured for up to 60 days at about 20 to 25 °C. The results revealed that 10% KRWA is the optimum content, decreasing the dispersion and sodium percentage by 82% and 57%, respectively, with 28 days of curing and changing the soil class to non-dispersive. A 56% reduction in plasticity was observed with optimum content, which caused the soil to become more friable, enhancing the treated clay's workability. Moreover, a 575% raise in UCS and a 60% reduction in compressibility were witnessed, which indicates improved load-bearing capacity and the capability of the stabilized clay to bear external loads better. The physio-chemical study revealed that the ion exchange, agglomeration, and flocculation take place immediately with minor/no curing that decreased the repulsion and diffuse double layer thickness, followed by the initiation of the pozzolanic activity, which formed different cementitious gels. X-ray diffraction and scanning electron microscopy experiments confirmed the formation of gels and produced a considerably thicker microstructure. Moreover, cost-based analysis shows that KRWA is more economical than conventional additives for highway/road applications.

Enhancing the mechanical and hydromechanical behaviors of mudstone soils using sugarcane press mud

Geological hazards, such as landslides and soil erosion, are frequently found in regions where mudstone soils are prevalent. These soils can also present challenges for the construction of infrastructure, including roads, railways, and other facilities. While various methods exist for addressing the mechanical and hydromechanical properties of mudstone soils, the reuse of waste materials may be a cost-effective alternative. This study presents the effect of press mud, a by-product of sugar purification, as an alternative material for improving the performance of mudstone soils. Unconfined compressive strength tests were conducted to evaluate the mechanical properties of untreated and press mud-treated mudstone soils. In addition, free swell tests, one-dimensional consolidation tests, and three-dimensional volumetric shrinkage tests were performed to investigate the hydromechanical properties of the soils. These tests provided insights into the behavior of the mudstone soils under various conditions. In addition, microstructural analysis such as X-ray diffraction, X-ray fluorescence, Fourier-transform infrared spectroscopy, and scanning electron microscopy were performed to identify the soil improvement mechanisms. The results demonstrated that press mud can act as a binder and help enhance the peak and postpeak strengths of mudstone soils. The swelling and shrinkage characteristics improved when it was treated with press mud; however, the inclusion of press mud slightly increased the compressibility of mudstone soils. According to microstructural analysis, the formation of extra cementitious gels helped enhance the mudstone soils performance. These findings offer promising and environmentally friendly options for effectively treating mudstone soils and address the needs of engineers seeking sustainable solutions.

Elaboration of geopolymers based on clays by-products from phosphate mines for construction applications

Yellow clays from Moroccan phosphate mines are extracted with other waste rocks lithologies and stored in large dumps within the mine site. The present paper investigates the use of this by-product as a new aluminosilicate source for geopolymers elaboration at the laboratory scale. Yellow clays studied are mainly composed of montmorillonite, dolomite and quartz as major phases. The Toxicity Characteristic Leaching Procedure (TCLP) showed no existence of contaminant. In order to reach thermal activation, the raw clays were first calcined at different temperatures between 500 and 900 °C. Raw and calcined clays were characterized using thermogravimetric analysis (TGA), X-Ray Diffraction (XRD) and Fourier transformed infrared (FTIR) spectroscopy. For the synthesis of geopolymers, calcined powder was mixed with an alkaline solution presenting different NaOH/Na2SiO3 ratios. The compressive strength was measured at 7, 14 and 28 days and material porosity was determined using Mercury Intrusion Porosimetry method in order to explain the mechanical behavior. XRD, 27Al and 29Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy, Fourier transformed infrared and TGA have been used to investigate the gel structure.The thermal treatment at 900 °C allowed the destruction of crystalline structure of montmorillonite clays, as well as the formation of new phases such as periclase (MgO) and gehlenite (Ca2Al2SiO7). Furthermore, the 27Al - 29Si MAS-NMR and the XRD analysis of the elaborated materials confirmed the formation of two cementitious gels C-A-S-H (Calcium aluminum silicate hydrate) and N-A-S-H (Sodium aluminum silicate hydrate). The compressive strength was around 25 MPa at 28 days. The obtained results are promising; they have shown that elaborated materials can be adapted for construction applications. In this case, it is recommended that economic and commercial study be developed.

Investigating the reinforcing mechanism and optimized dosage of pristine graphene for enhancing mechanical strengths of cementitious composites.

The proposed reinforcing mechanism and optimized dosage of pristine graphene (PRG) for enhancing mechanical, physicochemical and microstructural properties of cementitious mortar composites are presented. Five concentrations of PRG and two particle sizes are explored in this study. The results confirmed that the strength of the mortars depends on the dosage of PRG. The PRG sizes have a significant influence on the enhancement rate of mechanical strengths of the mortars, whereas they do not have a significant influence on the optimized PRG dosage for mechanical strengths. The PRG dosage of 0.07% is identified as the optimized content of PRG for enhancing mechanical strengths. The reinforcing mechanism of PRG for cement-based composites is mostly attributed to adhesion friction forces between PRG sheets and cementitious gels, which highly depends on the surface area of PRG sheets. The larger surface area of PRG sheets has a larger friction area associated with cementitious gels suggested to be one of favorable parameters for enhancing mechanical strengths with graphene additives.

Strength and micro-mechanism of MK-blended alkaline cement treated high plasticity clay

This paper focuses on the strength behavior and micro-mechanisms of treated high plasticity clay-mixtures with Metakaolin- (MK-) blended alkaline cement. The MK-blended alkaline cement was prepared by the alkaline activation of Ordinary Portland Cement (OPC) and MK blends, while the MK-based geopolymer and MK-blended cement were also prepared for comparison purpose. The results show that the unconfined compressive strength (UCS) of the clay treated by MK-blended alkaline cement increases with MK replacement at ranges of 30%. Meanwhile, the pore structure was significantly improved in terms of large pores transferring to the smaller size. The increase of MK replacement from 10% to 20% leads to the increase of UCS of the MK-blended alkaline cement treated clay mixture (L/S = 53.8%, 3.2 M) by 21.8%, and increase of pores ranging from 0.01 to 0.1 μm by 47.5% along with the decrease of pores ranging from 0.1 to 1 μm by 28.1%. This attributes to the notable bonding and filling effect by the coexistence of cementitious gels such as (N,C)-A-S-H and C-A-S-H gels, which differ to the hydration products from traditional OPC hydration. More significantly, the treated clay mixture with 60% MK replacement has the potential to reduce large portion of OPC, due to its appreciable strength of approximately 31.0% increase over the OPC treated clay-mixture.

Effect of activators on hybrid alkaline binder based on tungsten mining waste and ground granulated blast furnace slag

This article illustrates the effects of different combinations of alkaline activator solutions (sodium silicate “SS”, sodium hydroxide, potassium hydroxide and dissolved waste glass powder in sodium hydroxide) as well as their concentrations and the Solid/Liquid ratios on the compressive strength development, and the microstructure of hybrid alkaline binder. The current study used a combination of tungsten mining waste mud (TMWM) (90 vt.%) and ground granulated blast furnace slag (GGBFS) (10 vt.%) as precursors. The obtained hybrid alkaline binder specimens were submitted by compressive strength tests. It was found out that the alkaline activator (KOH when mixed with SS) improves the compressive strength of the hybrid alkaline binders compared to the alkaline activator (NaOH mixed with SS), which depends on the presence of alkali metal K+ and Na+ cations. Moreover, the samples prepared by the alkaline activator (dissolved waste glass powder in NaOH mixed with SS) giving the highest compressive strengthof about 32.8 MPa at 28 days. From the results of XRD, TGA-DTGA, FTIR analyses, and the obtained microstructures of the hybrid alkaline binders it indicates the existence of a different reaction product formed during the alkaline activation such as C-S-H, N-A-S-H, C-A-S-H, and K-A-S-H type cementitious gels. In addition to these conventional gels, there was a formation of a complex mixture of (C, M)-A-S-H cementitious gels were (M = K, Na). The pore structure of hybrid alkaline binder was evaluated through mercury intrusion porosimetry (MIP). The pore structure of specimens was characterised by the total porosity, average pore diameter, pore size distribution as well as characteristic pore sizes. The MIP results indicate that the type of alkaline activators, solid/liquid ratios and the alkaline activators’ concentration have a specific impact on all characteristics of the pore structure of all the hybrid alkaline binders. Finally, the cost impact on the compressive strength for different activators was also analysed.

Studies About the Hydration of Hybrid “Alkaline-Belite” Cement

The mechanical performance of a hybrid cement containing 48 % fly ash, 48 % mineralised belite clinker, 1.5 % gypsum and 2.5 % Na2SO4 was analysed and the hydration products were identified. The findings showed that the newly designed cement met European standard EN 197-1 compositional, mechanical and setting time requirements (cement type IVB). The hydration products forming in this blended ‘alkaline-belite’ cement (as it was determined through XRD, 29Si and 27Al MAS NMR and electron microscopy) consisted in a mix of cementitious gels: C-(A)-S-H and N-(C)-A-S-H, which interacted and over time evolved toward the latter with no detriment to the mechanical strength developed by the cement.

Synthesis and characterization of geopolymers derived from coal gangue, fly ash and red mud

Abstract There is a growing need to utilize large stockpiles of the industrial byproducts including coal gangue (CG), fly ash (FA) and red mud (RM) which can pose potential environmental problems. The CG, FA and RM contain aluminosilicates which can be precursors for the synthesis of geopolymers. This research studied the utilization of CG, FA and RM by silicate activation into geopolymers. Binary mixes of CG and RM and ternary mixes of CG, FA and RM were prepared and cured at temperatures ranging from 80 to 800 °C. The unconfined compressive strength tests were performed on the specimens to determine their mechanical strength. Nitrogen adsorption, mercury intrusion porosimetry, Fourier transform infrared spectroscopy and scanning electron microscopy were used to study the textural, structural and morphological characteristics of the end products. At lower curing temperatures, the ternary mixtures of CG, FA and RM showed higher strength gains when compared to the binary mixtures of CG and RM. The textural characteristics revealed that the reduction in pores and specific surface area do not necessarily result in higher compressive strength values. Furthermore, the cementitious gels formed at high temperatures resulted in well-bonded geopolymers with homogenous structures.

One-part geopolymer cement from slag and pretreated paper sludge

Abstract The aim of this study was the valorization of paper sludge waste from paper industry in designing one-part geopolymer cement using ground granulated blast furnace slag as main precursor. The effects of increasing the amount of paper sludge pretreated with a constant amount of sodium hydroxide (2% of slag) on mechanical strength, heat evolution, setting time and durability were analyzed. The reaction products were characterized using X-ray diffraction and thermogravimetric analysis. The findings showed that paper sludge can successfully be used as a secondary source of calcium carbonate in the one-part (“just add water”) geopolymer. The compressive strength of the cured geopolymer increased with increasing the amount of paper sludge, and the non-reacted sludge acted as a filler. The maximum strength (42 MPa, 28 d) was reached at 18 wt-% paper sludge addition. The main reaction products were calcium aluminate silicate hydrate and hydrotalcite-like cementitious gels. From the perspective of its blast-furnace slag content, strength properties, water absorption and setting time, the one-part geopolymer cement could be classified as a CEM IIIC and Type 32.5N class cement under the European standard EN-197.

Elastic Response of Cementitious Gels to Polycation Addition.

The high compressive strength of cementitious materials stems from the creation of a percolated network of calcium silicate hydrate (C–S–H) nanoparticles glued together by strong Ca2+–Ca2+ correlation forces. Although strong, the ion correlation force is short range and yields poor elastic properties (elastic limit and resilience). Here, the use of polycations to partially replace Ca2+ counterions and enhance the resilience of cementitious materials is reported. Adsorption isotherms, electrophoretic mobility, as well as small angle X-ray scattering and dynamic rheometry measurements, are performed on C–S–H gels, used as nonreactive models of cementitious systems, in the presence of different linear and branched polycations for various electrostatic coupling, that is, surface charge densities (pH) and Ca2+ concentrations. The critical strain of the C–S–H gels was found to be improved by up to 1 order of magnitude as a result of bridging forces. At high electrostatic coupling (real cement conditions), only branched polycations are found to improve the deformation at the elastic limit. The results were corroborated by Monte Carlo simulations.

Characterisation of pre-industrial hybrid cement and effect of pre-curing temperature

This study aimed to determine the physical-mechanical, mineralogical and microstructural properties of a pre-industrially manufactured hybrid cement (HYC) containing 5% alkaline activator and less than 30% clinker. The effect of the initial curing temperature (25 ± 1 or 85 °C for 20 h) on hydration kinetics and the development of compressive strength were also explored. The hydration products formed were characterised using XRD, SEM/EDX and 27Al and 29Si MAS-NMR. The findings showed that pre-industrial hybrid cement sets when hydrated with water and hardens to a 28-day mechanical strength of 35 MPa. The main reaction product formed was a mix of cementitious gels: C-(A)-S-H and C-A-S-H. Curing at 85 °C for 20 h, shows a behaviour similar to OPC, inhibited ettringite formation and generated more polymerised gels, enhancing 3-day but not 28- or 90-day mechanical strength.

Preliminary studies on hydrated cement for its reuse in geopolymers

<p>The carbonation of hydrated ordinary Portland cement (OPC) allows the transformation of hydrated calcium aluminates and silicates into calcium carbonate and amorphous silica/ alumina. These carbonated materials are appropriate to being used as inorganic precursors for alkaline activation. The use of sodium silicate and hydroxide solutions enables the production of cementitious gels. Two ways of carbonation of hydrated cement were studied: in a dry environment, where relative humidity did not exceed 70% and in an aqueous medium at a temperature of 5 ± 1 ° C. Both materials were micro-structurally characterised and they have been used for the manufacturing of geopolymeric mortars. These mortars reached mechanical strength between 10 and 20 MPa, depending on the activating solution, the water/binder ratio and curing time/temperature. These results show the feasibility of reusing hydrated cement contained in construction and demolition wastes.</p>

Development of New Cementitious Caterials by Alkaline Activating Industrial by-Products

The alkaline activation of aluminosiliceous industrial by-products such as blast furnace slag and fly ash is widely known to yield binders whose properties make them comparable to or even stronger and more durable than ordinary Portland cement. The present paper discusses activation fundamentals (such as the type and concentration of alkaline activator and curing conditions) as well as the structure of the cementitious gels formed (C-A-S-H, N-A-S-H). The durability and strength of these systems make these materials apt for use in many industrial applications, such as precast concrete elements (masonery blocks, railroad sleepers), protective coatings for materials with low fire ratings and lightweight elements.

Specific Examples of Hybrid Alkaline Cement

Hybrid alkaline cements are obtained by alkali-activating cementitious blends in the Na 2 O-CaO-SiO 2 -Al 2 O 3 -H 2 O system. The present paper discusses the results of activating different cementitious blends containing a low OPC clinker content ( 15MPa a 2 days) different alkaline activators were used (liquid and solid). The reaction products obtained were also characterised by XRD, SEM/EDX and 27 Al and 29 Si NMRMAS. The results showed that the main reaction product was a mix of cementitious gels C-A-S-H and (N,C)-A-S-H, and that their relative proportions were strongly influenced by the calcium content in the initial binder

“Metakaolin‐Slag‐Clinker Blends.” The Role of Na+ or K+ as Alkaline Activators of Theses Ternary Blends

This article reports on research into the use of solid alkalis (Na2CO3 and K2CO3) as activators to obtain hybrid cement (cement whose hydration generates a mix of C–A–S–H and (N,C)–A–S–H gels) from a blend of 20% clínker + 40% blast furnace slag + 40% metakaolin. More specifically, the study aimed to determine the effect of activator dosage (5 and 8 wt%) and type of alkaline cation (Na+ or K+) on the 2‐ and 28‐d mechanical strength of the end materials. The findings showed that the highest mechanical strength values were obtained with 5% Na2CO3. According to the XRD, NMR, and SEM/EDX analyses conducted on the reaction products, the alkalinity and solubilized chemical species generated by adding 5% Na2CO3 to the system yielded a mix of (N,C)–A–S–H and C–A–S–H cementitious gels as the main reaction products. The secondary reaction products included metastable (3CaO·Al2O3·CaCO3·11H2O‐type) carboaluminates that evolved into the calcite or vaterite forms of calcium carbonate. When K2CO3 was used (instead of Na2CO3), a (3CaO·Al2O3·0.5Ca(OH)2·0.5CaCO3·11H2O‐type) hemicarboaluminate also formed. The study also revealed that Na+ favors coagulation/precipitation more effectively than K+, generating gels with a wider range of Qn species.

Effect of Calcium Additions on N–A–S–H Cementitious Gels

This paper reports a research on the properties of synthetic sodium aluminosilicate hydrate gels as models for the gels arising in alkali‐activated aluminosilicate systems. The interaction between these N–A–S–H gels and aqueous calcium was studied to explore a potential consequence of mixing conventional Portland cements and alkali‐activated systems. The properties of two N–A–S–H gels (with Si/Al=1 and 2) synthesized in highly alkaline media, in the presence of calcium are reported. N–A–S–H gels were synthesized using aluminum nitrate, sodium hydroxide, and a sodium silicate. FTIR, NMR, and TEM/EDX techniques were used to characterize N–A–S–H gels before and after adding calcium. The results obtained show that calcium is uptaken in N–A–S–H gels resulting in a modification of the chemical composition due to the substitution of sodium per calcium.