Hybrid Alkaline Cement Research Articles

Influence of salt aggregate on the degradation of hybrid alkaline cement (HAC) concretes in magnesium chloride-rich saline solution simulating evaporite rock

Concretes produced from salt aggregate and hybrid alkaline cements, an alkali-activated slag/fly ash blend, or a Portland cement were exposed to a magnesium chloride-rich saline solution ([Mg2+] = 3.6 m, [Cl−] = 8.3 m), representing a solution formed after contact of surface water with evaporite rock (rock salt) in a nuclear waste repository. The hydration and deterioration of the concretes were studied with X-ray diffraction, thermogravimetric analysis, pH mapping and permeability measurements. The results show that calcium silicate hydrate (C-S-H) or sodium-substituted calcium aluminium silicate hydrate (C-N-A-S-H) and Friedel's salt were the major reaction products in the concretes prior to exposure to the saline solution. During exposure to the saline solution, increasing amounts of C-S-H/C-N-A-S-H dissolved, and gypsum and a secondary AFm phase formed. The durability of the concretes improved with increasing amounts of Portland clinker in the cements, due to the associated differences in permeability and chemical resistance. Nevertheless, a massive increase of permeability occurred for all concretes, likely caused by crack formation due to the formation of gypsum from anhydrite in the salt aggregate. Thus, the behavior of the concretes differed from, and was more complex than, the behavior of plain cement pastes.

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.

Corrosion of hybrid alkaline cements in saline solution simulating evaporite rock – Effect of the Portland clinker content

Two hybrid alkaline cements (HAC) based on Portland clinker, ground granulated blast furnace slag (GGBFS), fly ash and sodium sulfate, as well as an alkali-activated GGBFS/fly ash blend and a Portland cement paste were exposed to a saturated saline solution for 70 days. The combined chemical attack of chloride, magnesium and sulfate ions and the associated changes of the phase assemblage of the materials were studied by X-ray diffraction, thermal analysis and spatially resolved X-ray fluorescence spectroscopy. The experimental results revealed dissolution of ettringite, C-N-A-S-H and calcite, and the formation of gypsum, Kuzel's salt and Friedel's salt; thermodynamic modeling indicated the formation of M-S-H. The resistance of the HAC against attack by the saline solution increased with Portland clinker fraction. The capacity of portlandite to maintain pH at values above 10 is found to be a major factor controlling the resistance of HAC against corrosion in the saline solution.

Chemical and physical characteristics of hybrid alkaline cement composite after laser interaction

This study investigated the resistance of a hybrid alkaline cement composite (HACC) to laser cutting. HACC, synthesized using geopolymer and cement paste, can be useful for rapid construction or repair due to its faster setting and higher compressive strength than cement paste. Also, laser cutting is a recently popular fabrication method because of low dust and noise production as well as fast speed. Although cement paste was proven to enhance the compressive strength of the geopolymer, its effect on the resistance against laser cutting has not been studied. Laser-cutting experiments were performed in this study on HACCs with five different cement paste contents. Thermogravimetric analysis, X-ray diffraction, and scanning electron microscope with energy dispersive microscope were conducted to investigate the structural changes after the laser treatment. Lastly, the microhardness of HACC was measured to reveal the correlation between physical hardness and thermal resistance. The results demonstrate that sodium aluminosilicate hydrate gels can resist laser cutting more than calcium (alkali) aluminosilicate hydrate and calcium silicate hydrate gels because of their chemical and physical characteristics.

Use of sodium/potassium citrate to enhance strength development in carbonate-activated hybrid cement

• Sodium/potassium citrate enhances flow of hybrid alkaline cement (HAC). • Sodium/potassium citrate enhances strength development in carbonate-activated HAC. • Sodium or potassium citrate promotes the formation of denser C-S-H structure. • Compared with OPC, HACs’ greenhouse gas emissions are reduced by up to 73%. • Compared with OPC, HACs’ costs can be reduced by up to 28%. Hybrid alkaline cement (HAC) typically has a high amount of fly ash (70–80 wt%) and a low amount of clinker (20–30 wt%). A small amount of activator is commonly incorporated in HAC to promote its strength development. Compared with those highly corrosive and hygroscopic alkaline activators (e.g. sodium hydroxide), sodium and potassium carbonate are much preferred because of their lower cost and easier handling and storage. Carbonate-activated HAC, however, usually has low early strength and slow strength development (usually <40 MPa at 28 days) because of the slow reaction of fly ash. This paper investigates the strength development of HAC activated by sodium or potassium carbonate, where a small amount of sodium or potassium citrate is added to promote strength development. It was found that the 28-day compressive strength of the HAC could be improved from 38.5 MPa up to 72.4 MPa by adding 1 wt% sodium or potassium citrate to the mixture. Microstructure characterisation revealed that the inclusion of sodium or potassium citrate led to the formation of denser C-S-H structure in the paste. The environmental impact and cost of the developed HACs were further analysed. Compared with ordinary Portland cement, the HACs’ greenhouse gas emissions can be reduced by up to 73 %, and their costs can be reduced by up to 28 %.

Characteristics of hybrid alkaline cement composites with high cement content: flash set and high compressive strength

This paper explores hybrid alkaline cement composites with high cement content over 50%. Calcium (Ca) accelerates the strength development and improves the compressive strength of geopolymer. However, the mixing proportion of Ca in geopolymer has been limited because Ca significantly decreases the flow of fresh geopolymer. This study resolves the flow decline issue by mixing a part of the initial mixing water with cement before adding it into a metakaolin-based geopolymer, which is named hybrid alkaline cement composites (HACC) in this study. The cement/(metakaolin + cement) ratio investigated is up to 85%. Based on the initial setting time and compressive strength results, some unique characteristics of the HACC are observed: i) the material is set within 10 min and the 28-day compressive strength is over 80 MPa with 1:1 mixing ratio of geopolymer and cement paste, and ii) the compressive strength of HACC is not always higher with a lower w/c ratio of cement paste.

Thermodynamic modeling and mechanical properties of hybrid alkaline cement composites

Hybrid alkaline cement composites (HACC) were developed to overcome the low compressive strength of hybrid alkaline cement. Synthesized with geopolymer and cement paste, HACC contains a larger amount of calcium and alkali-activator than general hybrid alkaline cement does. Although HACC can achieve higher compressive strength than geopolymer and cement paste, the phase assemblage and microstructure of the HACC remains elusive. The characteristics of HACC with various cement paste contents are examined by employing thermodynamic calculations and experimental techniques such as X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, and compressive strength tests. The obtained results provide new insight into the strength-developing mechanism of HACC by deriving microstructural information and phase assemblages. In particular, it is revealed that the ratio of N-A-S-H gels to C-(A)-S-H gels can affect the homogeneity of the HACC structure. As the HACC structure becomes more homogeneous, the porosity decreases resulting in higher compressive strength.

Microstructural, Mechanical and Radiological Characterization of Mortars Made with Granite Sand.

The study reported the effect of granite sand on strength and microstructural developments in mortars prepare from OPC with a high coal fly ash (FA) content or from hybrid alkaline cements. The radiological behaviour of the resulting mortars was compared to materials prepared with siliceous sand (with particles sizes of <2 mm) and the relationship between such radiological findings and mortar microstructure and strength was explored. A new method for determining natural radionuclides and their activity concentration Index (ACI) on cement mortars (specifically to solid 5-cm cubic specimens) was applied and validated. The microstructural changes associated in mortars have no effect on mortar radiological content measurements. The mortars with granite sand exhibited very high ACI > 0.96, which would ultimately limit their use. A conclusion of interest is that where information is at hand on the starting materials (OPC, FA, sand, admixtures), their proportions in the mortar and the mixing liquid content (water or alkaline activators) their radiological content is accurately predicted. The inference is that a mortar’s radiological content and ACI can be known prior to mixing, providing a criterion for determining its viability. That in turn lowers environmental risks and the health hazards for people in contact with such materials.

Microstructural characterisation of hybrid cement after exposure to high temperatures

This study was designed to determine the mineralogical and microstructural changes taking place in a hybrid alkaline cement (HYC = 30% Portland cement clinker + 70% (slag + fly ash + alkaline activator)) exposed to high temperatures. The cement pastes were heated for 2 h at 400 °C, 600 °C, 800 °C or 1000 °C and subsequently air- or water-cooled. Their mechanical strength was determined after cooling. The HYC pastes had higher residual strength than the reference porland cement (OPC). That better post-thermal performance in HYC and its failure to collapse when water-cooled after reaching temperatures of over 800 °C were attributed to the recrystallisation of scantly hydraulic phases such as gehlenite and rankinite.

X-ray Mapping Characterisation of Hybrid Alkaline Cement Discolouration Area

X-ray Mapping Characterisation of Hybrid Alkaline Cement Discolouration Area

Effect of high temperatures on the mechanical behaviour of hybrid cement

The high-temperature mechanical behaviour of a pre-industrial hybrid alkaline cement (HYC) was studied. The HYC in question contained 30 % Portland clinker and 70 % of a blend of slag, fly ash and a solid activator (mix of alkaline salts with a predominance of Na2SO4). The material was tested during exposure to high temperatures to establish its compressive and bending strength and elastic modulus, as well as fracture toughness, analysed using an innovative methodology to notch the hydrated cement paste specimens. Post-thermal treatment tests were also run to assess residual mechanical strength after 2 h of exposure to temperatures ranging from 400 °C to 1000 °C. TG/DTA, MIP and SEM were deployed to ascertain heat-induced physical-chemical changes in the structure. The higher mechanical strength during and after treatment exhibited by the hardened hybrid alkaline cement than the CEM I 42.5R ordinary portland cement (OPC) paste used as a reference was associated with the lower water and portlandite content found in HYC. Pseudo-plastic behaviour was observed at high temperatures in the loaded HYC in the tests conducted during exposure.

Hybrid Alkaline Cements: Bentonite-Opc Binders

Moderately alkaline activators can be used to formulate cementitious binders with a high Supplemetary Cementitious Materials (SCMs) and a low portland cement content (hybrid alkaline cements). This study aimed to prepare hybrid alkaline cements containing large percentages of dehydroxylated bentonite (BT) and small Portland cement (OPC) fractions, with 5% Na2SO4 as a solid alkaline activator. The hydration kinetics of the pastes hydrated in water in the presence and absence of the solid activator were assessed by isothermal conduction calorimetry, whilst the reaction products were characterised with X-Ray Powder Diffraction (XRD) and Fourier-transform Infrared Spectroscopy (FTIR). The presence of the alkaline activator hastened OPC and BT/OPC hydration: more heat of hydration was released, favouring greater initial bentonite reactivity. The portlandite forming during cement hydration reacted readily with the Na2SO4, raising medium alkalinity and enhancing bentonite dissolution and with it reaction product precipitation (primarily (N,C)-A-S-H-like gels that co-exist with C-S-H- or C-A-S-H-like gels). The presence of sulfate ions favoured the formation of AFm-like phases. Preceding aspects accelerated the hydration reactions, with the formation of more reaction product and matrix densification. As a result, the 28 days Na2SO4 activated systems developed greater mechanical strength than the water-hydrated systems, with the 60% BT/40% OPC blends exhibiting higher compressive strength than the 100% OPC pastes.

Effect of Activators on Strength of Hybrid Alkaline Cement

Hybrid alkaline cement is a class of alkaline cement resulted from alkali activation of the medium calcium content of aluminosilicate materials. This paper presents an experimental analysis of alkali activators effect on strength of hybrid alkaline cement produced from 80% fly ash and 20% ordinary Portland cement. Two alkali activators were observed i.e. 5% sodium sulfate and a combination of 5% of sodium sulfate-1.1 M SiO2 of sodium silicate solution. Compressive strength tests were performed on 20mmx 40mm cylinder paste specimens while setting time tests were conducted by Vicat needle. Scanning electron microscopy analysis and measurement of fly ash reaction degree were performed to explain the compressive strength of paste. It is concluded that addition of soluble silicate on the dry mix of hybrid cement-sodium sulfate activator reduce compressive strength and shorten the setting time. Both of activators give relative low fly ash reaction degree.

Hydration of Hybrid Alkaline Cement Containing a Very Large Proportion of Fly Ash: A Descriptive Model.

In hybrid alkaline fly ash cements, a new generation of binders, hydration, is characterized by features found in both ordinary portland cement (OPC) hydration and the alkali activation of fly ash (AAFA). Hybrid alkaline fly ash cements typically have a high fly ash (70 wt % to 80 wt %) and low clinker (20 wt % to 30 wt %) content. The clinker component favors curing at ambient temperature. A hydration mechanism is proposed based on the authors’ research on these hybrid binders over the last five years. The mechanisms for OPC hydration and FA alkaline activation are summarized by way of reference. In hybrid systems, fly ash activity is visible at very early ages, when two types of gel are formed: C–S–H from the OPC and N–A–S–H from the fly ash. In their mutual presence, these gels tend to evolve, respectively, into C–A–S–H and (N,C)–A–S–H. The use of activators with different degrees of alkalinity has a direct impact on reaction kinetics but does not modify the main final products, a mixture of C–A–S–H and (N,C)–A–S–H gels. The proportion of each gel in the mix does, however, depend on the alkalinity generated in the medium.

Cements with low Clinker Content

Hybrid alkaline cements are multi-component systems containing a high percentage of mineral additions (fly ash, blast furnace slag), low proportions (<30%) of Portland clinker and scarce amounts of alkaline activators. The substantially lower amount of clinker needed to manufacture these binders in comparison to ordinary Portland cement is both economically and ecologically beneficial. Their enormous versatility in terms of the raw materials used has made them the object of considerable interest. The present study explored the mechanical strength of binary blends mixes; B1= 20% clinker (CK) + 80% fly ash (FA) and B2=20% clinker + 80% blast furnace slag (BFS), both hydrated in the presence and absence of an alkaline activator specifically designed for this purpose. The use of the activator enhanced the development of early age strength considerably. All the hydrated matrices were characterised with XRD, SEM/EDX and (29Si and 27Al) NMR. The use of the alkaline activator generated reaction products consisting primarily of a mix of gels ((N,C)-A-S-H and C-A-S-H) whose respective proportions were found to depend upon system composition and initial reactivity.

Cements with a low clinker content: versatile use of raw materials

A model for alkaline activation has recently been designed to develop new cementitious systems known as hybrid alkaline cements. These multi-component systems contain a high percentage of mineral additions (fly ashes, slags (BFS), metakaolin (MK), bentonites (BT) …), low proportions (<30%) of Portland clinker and moderately alkaline solid activators. This study explored the mechanical performance of two sodium carbonate-activated hybrid cements, CEM MK (20% CK + 40% BFS + 40% MK) and CEM BT (20% CK + 40% BFS + 40% BT). The reaction kinetics of the two systems were analyzed by isothermal conduction calorimetry. The hydrated matrices were characterized with XRD, SEM/EDX and 29Si and 27Al NMR. Both systems developed high mechanical strength, for both generated a mix of (N,C)-A-S-H and C-A-S-H gels. The proportions of the gels precipitating vary with system composition and initial reactivity, however.

Compressive strength, microstructure and hydration products of hybrid alkaline cements

Ordinary Portland cement (OPC) is the dominant binder in the construction industry with a global production that currently reaches a total of 3 Gt per year. As a consequence, the cement industry's contribution to the total worldwide CO2 emissions is of about 7% of the total emissions. Publications on the field of alkali-activated binders (also termed geopolymers), state that this new material is, potentially, likely to fbecome an alternative to Portland cement. However, recent LCA studies show that the environmental performance of alkali-activated binders depends, to great extent, of their composition. Also, researchers report that these binders can be produced in a more eco-efficient manner if the use of sodium silicate is avoided. This is due to the fact that the referred component is associated to a high carbon footprint. Besides, most alkali-activated cements suffer from severe efflorescence, a reaction originated by the fact that the alkaline and/or soluble silicates that are added during processing cannot be totally consumed. This paper presents experimental results on hybrid alkaline cements. The compressive strength results and the efflorescence observations show that some of the new mixes already exhibit a promising performance.

Some durability aspects of hybrid alkaline cements

Blended cements that contain a high content of fly ash and a low content of Portland cement typically suffer from low early strength development and long setting times. Recently, one method of overcoming these problems has been to use an alkali activator to enhance the reactivity of fly ash particles at early ages. Such cements can be grouped under the generic term “hybrid alkaline cements”, where both cement clinker and fly ash, encouraged by the presence of alkalis, are expected to contribute to cementitious gel formation. The work presented here examines some of the durability aspects of high fly ash content hybrid alkaline cement. Specifically, the aspects investigated were: exposure at high temperatures (up to 1000°C), resistance to immersion in aggressive solutions and susceptibility to the alkali aggregate reaction. All tests were repeated with a commercially available sulfate resistant Portland cement for comparison. When exposed to high temperatures, the hybrid alkaline cement showed strikingly different behaviour compared to the control Portland cement, showing fewer micro-cracks and maintaining residual compressive strengths at least equal to original strengths. Beyond 700°C, the hybrid alkaline cement began to sinter, which resulted in shrinkage of around 5% and a 100% increase in residual compressive strengths. No such sintering event was noted in the control Portland cement, which showed a drastic loss in residual compressive strengths upon heating. In immersion tests, the hybrid alkaline cement possessed excellent resistance to sulfate and seawater attack, similar to the control sulfate resistant cement. Both cements were however severely degraded by immersion in 0.1M HCl for 90 days. Both binders complied with the accelerated alkali-aggregate test but when this test was extended, the hybrid alkaline binder showed much greater dimensional stability. Possible reasons for the differences in durability behaviour in both cements are discussed, based on experimental evidence provided.

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

The Early Age Hydration Reactions of a Hybrid Cement Containing a Very High Content of Coal Bottom Ash

A study examining the complex hydration chemistry of a hybrid alkaline cement containing a high content of coal bottom ash (BA) (&gt;70%) and a low content of portland cement clinker in the presence of an alkaline activator is presented. The use of a water reducing additive was found to significantly delay the overall hydration process, allowing an opportunity to more clearly distinguish the hydration reactions that take place. The results presented showed that both the cement clinker phases and the ash glassy phases are highly reactive for the first 3 d of hydration. In situ formed reaction products portlandite and gypsum were shown to be metastable and had disappeared within 3 d of hydration. Ettringite stability was limited in the hybrid system but unlike gypsum and portlandite, remained detectable for the first 3 d of hydration at least. SEM‐EDX and subtracted Fourier transform infrared evidence suggest the development of three different gel bond environments, tentatively attributed to C–(A)–S–H, C–A–S–H, and (N,C)–A–S–H type gels.