Alkali-activated Cement Research Articles

Development of highly porous alkaline cements from industrial waste for thermal insulation of building envelops

Alkali activated cements (AAC) are becoming an increasingly promising alternative to traditional Portland cement. There are still some significant barriers which have yet to be addressed, by all the agents involved in the construction industry. However, it appears that some AAC applications can be introduced in current applications at a faster rate than, for instance, structural concrete. One of these applications are materials with thermal performance, based on the capacity of some types of AAC to expand during initial reactions. The present paper aims to assess the viability of producing porous AAC for application in thermally effective façade panels. A wide range of industrial by-products were activated with sodium hydroxide and sodium silicate. Two additional by-products were incorporated to increase the thermal damping of the pastes, with the intention of taking advantage of the large pore volume. Flexural and compressive strength and density was measured, and selected pastes were then submitted to microstructural characterisation (SEM/EDX), environmental behaviour (heavy metal leaching), porosity (SEM) and thermal performance (thermal coefficient). The results showed maximum compressive strengths of 2.5 MPa, while the majority of the pastes tested showed conductivity values below the threshold of 0.2 W/m.°C, which is the requirement for obtaining a classification of ‘thermal’. Therefore, the production of panels for thermal insulation based on alkali activated industrial by-products is competitive with traditional materials based on current commercial solutions, and, given the non-structural nature of this application, can be very close to reach industrial production.

Curing conditions effect on the stabilization of recycled asphalt pavement with alkali-activated metakaolin and rice husk ash-derived activator

The stabilization of recycled asphalt pavement (RAP) with alkali-activated cement (AAC) is a topic of growing interest for sustainable engineering, especially those containing alternative activators produced from waste. This study evaluated the effect of curing temperature on the stabilization of RAP with a metakaolin AAC and rice husk ash-derived activator for potential use in base and subbase layers in flexible pavement systems. Unconfined compressive strength (UCS), X-ray diffraction, and scanning electron microscopy tests were performed. Higher strength values were associated with higher temperatures and curing times. Curing oven time presented no influence over UCS and mineralogy. Blends cured at 20°C exhibited efflorescence formation and prolonged curing time at high temperatures negatively affected the mechanical performance. Curing temperature of 80°C up to 24 h promoted the formation and uniform distribution of cementing gels and a dense and compact structure, improving the compressive strength.

Influence of alkaline activator and precursor on the foam characterization and alkali-activated foamed concrete properties

This study proposes a solution by adding recycled brick powder in the alkali-activated foamed concrete to improve the fresh stability and thermal insulation. The roles of activator modulus and Na2O concentration on the fresh and hardened properties of alkali-activated foamed concrete with slag-fly ash based and slag-recycled brick powder based were investigated. The presence of alkaline activator was detrimental to the foamability and foam stability, and these characteristics were deteriorated with an increase in activator modulus and Na2O concentration. This primarily caused an inferior stability of fresh foamed concrete but achieved short initial setting time and high fluidity loss, owing to the synergistic effect of foam stability and alkali-activated cements slurry. Microstructure results shown that with the increasing of activator modulus and Na2O concentration, a growth tendency on the amount of pores with larger size and irregular shape led to increase the mean pore diameter and mean roundness. Overall, incorporation of recycled brick powder in the low alkalinity was unfavourable to the water absorption and compressive strength of foamed concrete, but it performed better in enhancing stability, water resistance, thermal insulation and strength development under higher alkaline environment and achieved spherical shape, thin pore wall and better pore refinement.

Egyptian volcanic glass powder as a modifier agent for alkali-activated slag cement

Egypt is a rich country with natural volcanic glass, which can be utilized to produce possibility volcanic glass powder (VGP). This VGP can then be employed as a pozzolanic material for the Portland cement (PC) system. In this investigation, for the first time, the possibility to employ Egyptian volcanic glass powder (VGP) as a modifier agent for alkali-activated slag (AAS) cement was studied. The volcanic glass (VG) was delivered from a quarry in the form of lumps pieces. After crushing and powdering, it was blended with slag in proportions of up to 30 wt% of slag (as a partial replacement). The slag free from VGP and those containing VGP were activated by an alkaline activator to produce alkali-activated cement. The effect of various ratios of VGP on flowability, setting time, compressive strength, transport properties, environmental conditions measured by wetting/drying cycles, crystalline phases, hydration products, and microstructures of AAS cement were investigated. The results confirmed that VGP is an amorphous material rich in silica and has a suitable ratio of alumina. The incorporation of 10% VGP was the optimal ratio. Higher ratios of VGP than 10% showed an adverse effect on the AAS cement properties.

Influence of NaOH molarity and Portland cement addition on performance of alkali activated cements based in silicomanganese slags

The aim of this research is to explore the potential of recycling the waste of silicomanganese slag (SiMnS) from the ferroalloy industry in the synthesis of alkali-activated cements. The influence of the concentration of the activator NaOH (4, 6, 8, and 10 M) and the partial substitution of the waste with Portland cement (PC) (10–30 wt%) on the technological properties and microstructure of binders cured at room temperature has been studied. The SiMnS binders and the hybrid cements incorporating PC have been characterized using X-ray diffraction analysis (XRD), infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS). The results indicate that compressive strength improve with an increase in molarity from 4 to 6 M from 7.8 up to 14.0 MPa, respectively, at 28 curing days. Similar mechanical properties are observed for higher molarity increments (8 M and 10 M) (12.9 and 13.6 MPa, respectively). The main reaction product formed is a (C,N)-A-S-H gel with different morphologies. Partial substitution of SiMn slag with PC led to hybrid cements with a denser structure due to a greater amount of C-S-H gel, coexisting with C,N-A-S-H gel. Therefore, the optimal combination of SiMn slag (70 wt%) and PC (30 wt%) significantly enhances the technological properties of the SiMn slag binders obtaining optimum compressive strengths of 24.7 MPa. The results of this study demonstrate that silicomanganese slag can be utilized as an environmentally friendly solution in the production of binders and hybrid cements that meet the required technical specifications and standards for use and application in the construction sector in the line of current circular economy policies and Sustainable Development Goals.

Effect of low-calcium and high-magnesium ferronickel slag on the microstructure and micromechanical properties of alkali-activated blended ground granulated blast furnace slag

In recent years, low-calcium and high-magnesium ferronickel slag (FNS) has been gradually used in the preparation of alkali-activated cements (AACs). An in-depth exploration of the microstructural characteristics is crucial for a thorough understanding and enhancement of the macroscopic behaviors of AACs. In this study, the microstructural composition and micromechanical properties of the alkali-activated ground granulated blast furnace slag (GGBS)-FNS cements (AASF) prepared with sodium silicate and sodium hydroxide as alkali activators were quantitatively studied. Backscattered electron image analysis (BSE-IA) and nanoindentation were selected to reveal the micromechanical properties of different phases in FNS particles and AASF pastes. The results show that the elastic modulus of sodium aluminosilicate hydrate (N-A-S-H) in alkali-activated FNS cement is about 17 GPa, which is similar to that of the gel phase in alkali-activated fly ash cement (AAFA) and alkali-activated metakaolin cement (AAMK), confirming that the micromechanical property of N-A-S-H gel is independent of the kind of precursor materials. The Ca/Si and Al/Si of the hydration products in AASF gradually decline as FNS is incorporated, whereas the Mg/Al steadily rises, increasing the elastic modulus of the inner hydration product (IP) phase in AASF. The reaction degree of GGBS is higher in the sodium silicate-activated AASF system, while more N-A-S-H gels are formed in the sodium hydroxide-activated AASF system. The incorporation of FNS is conducive to improving the reaction degree of GGBS, resulting in the increase of calcium silicoaluminate hydrate (C-A-S-H) content and a significant decrease in IP content in AASF.

Effects of pretreated recycled powder substitution on mechanical properties and microstructures of alkali-activated cement

In this study, recycled powder (RP) was used to replace alkali-activated cement (AAC)’s precursors [i.e., fly ash (FA) and ground granulated blast furnace slag (GGBFS)] to produce recycled AAC (RAAC). Two pretreatment methods [i.e., calcination or carbonation] were used to improve the reactivity of RP. This study investigated the effects of the replacement ratio [i.e., the ratio of RP to FA/GGBFS], pretreatment methods, and alkali modulus on the fresh and hardened properties, phase assembly, and microstructure of RAAC. The mechanical properties of RAAC gradually decreased with increasing replacement ratio, as alkaline solution reacted with the RP less than with FA/GGBFS. Meanwhile, both calcination and carbonation pretreatment improved the properties of RP and thus enhanced the mechanical properties of RAAC. Compared with carbonation, calcination of RP at 600 °C exhibited better mechanical properties and lower critical pore size, because calcination at this temperature improved the reactivity of RP by decomposing calcite and clay minerals, and the filler effect of prepared RP enhanced the pore size distribution of the RAAC paste. Moreover, increasing the RP carbonation time increased the solid [i.e., calcite] content of the RP and moderately improved the strength properties of RAAC although it increased the critical pore size of RAAC.

Application of hybrid cement in passive fire protection of steel structures

PurposeThe aim of this paper is to determine the thermal conductivity of a protective layer of alkali-activated cement and the possibility of performing fire protection with fireclay sand and Lightweight mortar. Unprotected steel structures have generally low fire resistance and require surface protection. The design of passive protection of a steel element must consider the service life of the structure and the possible need to replace the fire protection layer. Currently, conventional passive protection options include intumescent coatings, which are subject to frequent inspection and renewal, gypsum and cement-based fire coatings and gypsum and cement board fire protection.Design/methodology/approachAlkali-activated cements provide an alternative to traditional Portland clinker-based materials for specific areas. This paper presents the properties of hybrid cement, its manufacturability for conventional mortars and the development of passive fire protection. Fire experiments were conducted with mortar with alkali-activated and fireclay sand and lightweight mortar with alkali-activated cement and expanded perlite. Fire experiment FE modelling.FindingsThe temperatures of the protected steel and the formation of cracks in the protective layer were investigated. Based on the experiments, the thermal conductivities of the two protective layers were determined. Conclusions are presented on the applicability of alkaline-activated cement mortars and the possibilities of applicability for the protection of steel structures. The functionality of the passive fire layer was confirmed and the strengths of the mortar used were determined. The use of alkali-activated cements was shown to be a suitable option for sustainable passive fire protection of steel structures.Originality/valueEco-friendly fire protection based on hybrid alkali-activated cement of steel members.

Fiber-Reinforced Alkali-Activated Cements from Ceramic Waste and Ladle Furnace Slag without Thermal Curing

Alkaline-activated cement, as an alternative to conventional portland cement, is being increasingly studied due to its environmental advantages and engineering properties. However, research on the feasibility of using both uncommon precursors and curing at ambient temperature is still limited. This study aims to investigate the potential of ceramic wastes, specifically from brick and tile production and ladle furnace slag, as precursors in alkaline activated cement reinforced with polyacrylonitrile fibers cured at 20°C. Sodium silicate, in solution form, was used to activate the precursors, and three different fiber contents were tested, namely 0%, 0.5%, and 1%, by volume. Physical properties, such as capillarity and porosity, were assessed. Moreover, the mechanical behavior was thoroughly characterized by uniaxial compressive, flexural, and elasticity modulus tests. In addition, a thorough microstructural characterization, including scanning electron microscopy, X-ray energy dispersive analyzer, X-ray diffraction, and Fourier transform infrared spectroscopy was conducted at 14, 28, and 90 days. The results revealed that environmentally friendly alkali-activated binders were produced from wastes with limited industrial recycling possibilities. The mixture with 0.5% fibers was the one that presented better results, i.e., a flexural strength of 8.84 N/mm2 and compressive strength of ∼29 MPa at 90 days. The mechanical performance of this material is relevant, especially considering that a relatively low curing temperature was applied. The results also showed that calcium aluminum silicate hydrate (C-A-S-H) was detected as the main reaction product.Practical ApplicationsAlkali-activated cement has been identified as a potential alternative to traditional portland cement due to advantages such as the reduction of CO2 emissions to the atmosphere, and the conservation of natural resources, because wastes can be transformed into useful products. All over the world, ceramic materials are widely used in different types of construction, and significant amount (between 30% and 45%), end up as waste. Therefore, this study aims to show the possibility of reusing ceramic waste for the development of a fiber-reinforced alkaline activated cement composite with acceptable physical and mechanical properties that allow it to be used in non-structural applications; for example, bricks, tiles, partition walls, and other building materials. It also is a low-energy consumption process, because curing can be done at ambient temperatures. Finally, the obtained results, e.g., flexural strength of 8.84 N/mm2 and compressive strength of 29 MPa, open the door for several applications in the construction industry.

Influence of Dosage and Modulus on Soluble Sodium Silicate for Early Strength Development of Alkali-Activated Slag Cements

In world practice, the need for high-strength concrete with an intensive gain of early strength is due to an increase in requirements for characteristics of concrete and the desire to shorten the construction period. Alkali-activated cement, based on soluble sodium silicates (SSS), can demonstrate high strength and rapid gain due to the nano-modifying effect of amorphous silica present in SSS. However, the problem with the effective use of such cement compositions is unsatisfactory short setting times. This work investigates the effect of modifying admixtures on the structure formation of alkali-activated slag cement (AASC), its physical and mechanical properties depending on characteristics of SSS and the basicity of the aluminosilicate component (precursor), which was changed by the ratio of the ordinary Portland cement (OPC) clinker and granulated blast furnace slag (GBFS). A positive synergistic effect was noticed from glycerol and trisodium phosphate, as the components of a complex admixture, to control the setting of AASC. This resulted in extending the initial setting time from 1 to 5 min to the values of 21–72 min. The compressive strength of 21–26.3 MPa by 3 h, 36.5–43.4 MPa by 1 day, and 84.7–117.1 MPa by 28 days was obtained. Proper shrinkage deformations were equal to 0.47–0.6 mm/m. It was shown that with an increase in the basicity of the aluminosilicate component, the properties of AASC increased both in the early and late stages of hardening.

Mechanisms and differences between sodium and magnesium sulfate attacks on alkali-activated phosphorus slag

To promote the application of alkali-activated phosphorus slag (AAPS), a new type of alkali-activated cement (AAC), this study revealed the mechanisms and differences between Na2SO4 and MgSO4 attacks. During Na2SO4 exposure, the deterioration of AAPS was caused by the expansion of calcium-sodium aluminosilicate hydrate (C-N-A-S-H). The dealkalization of C-N-A-S-H was accelerated by the formation of (NaOH)0, which promoted the polymerization shrinkage of C-N-A-S-H and improved the volume stability of AAPS. During MgSO4 exposure, the deterioration of AAPS was caused by C-N-A-S-H deterioration and Mg(OH)2 expansion, but the Mg(OH)2 layer protected the AAPS from MgSO4 attack. The dealkalization, decalcification and dealumination of C-N-A-S-H was accelerated by the formation of Mg(OH)2 and (NaSO4)−, transforming C-N-A-S-H into magnesium-sodium aluminosilicate hydrate (M-N-A-S-H). The resistance of AAPS to Na2SO4 was excellent, but its resistance to MgSO4 was poorer. These differences were caused by the different binding capacities of Mg2+ and Na+ with OH− and the different degrees of forward movement of the dissolution equilibrium of C-N-A-S-H caused by Na2SO4 and MgSO4.

Influence of environmental stresses on the durability of slag-based alkali-activated cement crusts for wind erosion control

Wind erosion is a significant environmental challenge in arid and semi-arid regions, and artificial crust creation on the soil surface has emerged as an effective approach to mitigate this phenomenon. Various methods of crust formation have been proposed to combat wind erosion in these regions. However, a comprehensive study assessing the durability of these crusts against environmental stresses has been lacking. Hence, the primary objective of the present study is to address this critical issue by evaluating the erodibility and surface strength of alkali-activated slag crusts in response to various environmental stressors. These stressors encompass ultraviolet radiation, heating and cooling cycles, wetting and drying cycles, and freezing and thawing cycles. Through wind tunnel tests, erosion rates were measured under different wind velocities and saltation bombardment conditions, while penetrometer tests were conducted to analyze surface strength. The results demonstrate that alkali-activated cementation produced robust crusts, exhibiting an impressive reduction of over 99.9 % in erosion rates compared to untreated samples. However, the introduction of environmental stresses led to a fivefold increase in erosion rates. Freeze and thaw cycles had the most detrimental effect on the alkali-activated cement crusts while heating and cooling cycles had a relatively minor impact. The wetting and drying cycles and UV radiation ranked second and third, respectively, in terms of their destructive effects on crust erodibility. Despite the observed effects, the crusts maintained their efficiency even when subjected to severe environmental stresses. Notably, the erosion rate of the treated crusts after enduring the most severe studied stress, that is five freeze and thaw cycles, was over 250 times lower than that of the untreated samples.

Alkali-activated cement manufactured by the alkaline activation of demolition and construction waste using brick and concrete wastes

This study examines the mechanical and materials science properties of a novel Alkali-activated cement (AAC) cured at room temperature and using construction and demolition waste (CDW), which was obtained from brick and concrete waste from a landfill in Medellín, Colombia. The cement was activated with an aqueous solution of sodium hydroxide and sodium silicate. Different mixtures were tested to determine the optimum amount of CDW and the proportion of alkaline activator. The formulations were characterized by compressive and flexural strength tests, X-ray diffraction, scanning electron microscopy, and water absorption tests. Results indicate that the mechanical properties of the developed cement are strongly influenced by both the type of activated residue and the activator ratio. The AAC with the best mechanical properties is the one based on brick waste, which has a mean compressive strength of 33 MPa.

Mechanical and durability properties of alkali-activated slag/waste basalt powder mixtures

There are several studies investigating the feasibility of basalt powder as a precursor in alkali-activated cement production. The originality of this research lies in investigating the mechanical and durability features of alkali-activated mortars manufactured by partially substituting granulated blast furnace slag (GBFS) with waste basalt powder (WBP). In this study, 20% of GBFS was replaced with WBP, and high temperature (300 °C, 600 °C, and 900 °C) and magnesium sulfate (5% concentration) resistance of the manufactured mortars were investigated. The deteriorations of the mortars subjected to high temperature and magnesium sulfate were examined by compressive strength test, ultrasonic pulse velocity test, scanning electron microscopy analysis, and Fourier transform infrared spectroscopy analysis. The compressive strength of the mortar produced with 20% WBP substitution was 9.9% lower compared to the mortar containing 100% GBFS as the precursor. However, the performance of alkali-activated slag (AAS), known for its good resistance to high temperatures, did not decrease with the WBP substitution. The reduction in strength of both mortars after 900 °C was approximately 86%. No excessive changes were observed in the visual appearance and the compressive strength of the mortars after 3 months of exposure to magnesium sulfate. Microstructure analyses also revealed that WBP substitution had a limited effect on the high temperature and sulfate resistance of AAS.

Evaluation of copper slag and stainless steel slag as replacements for blast furnace slag in binary and ternary alkali-activated cements

Commonly used alkali activation precursors such as blast furnace slag and fly ash will soon become less available due to resource competition, and may cease to be produced in certain regions. This limitation in future supply is a main driving force for the investigation of alternative precursor sources, such as non-blast furnace slags and non-ferrous slags, to produce alkali-activated binders. The current study investigates the incorporation of copper slag (CS) and stainless steel slag resulting from electric arc furnace operations (EAFSS) as partial replacements for ground granulated blast furnace slag (GGBFS) in producing alkali-activated materials (AAMs), at paste level. Five binary alkali-activated mixtures with different replacement levels of GGBFS with CS, and three ternary mixtures with both CS and EAFSS as partial and total replacements for GGBFS, are activated by a sodium silicate solution. Replacing GGBFS with CS and EAFSS retards the reaction kinetics, resulting in improved fresh-state properties of the investigated AAMs, better retention of workability and longer setting times. The reaction of alkali-activated 100% CS shows minimal initial exothermic activity until 3.5 h, when a single intense peak appears, representing delayed dissolution and subsequent polycondensation. X-ray diffraction (XRD) data indicate that the main crystalline phases of CS and EAFSS are stable in these alkaline systems; it is the glassy components that react. The use of CS and EAFSS in blended AAMs causes a minor increase in porosity of ~ 1–3% with respect to GGBFS only, and a small reduction in compressive and flexural strengths, although these reach 80 MPa and 8 MPa, respectively, after 28 days, even at a replacement level over 65 wt. %. Conversely, the 100% CS mixture exhibits a one-day compressive strength of 23 MPa, with a negligible increase thereafter. This result agrees with both FTIR and SEM analysis which highlight only minor changes in binder development after two days. It is believed that the unusual behaviour of CS in the investigated mixtures is related to the low availability of calcium in this precursor material.

The ambient and elevated temperature performance of hemp fibre reinforced alkali-activated cement foam: Effects of fibre dosage and alkali treatment

Hemp fibre is gaining attention as a sustainable alternative to synthetic fibres in cementitious composites. This study investigates the effects of dosage and alkali treatment of hemp fibre (HF) on the strength, shrinkage, and fire resistance of alkali-activated cement foam (AACF). Raw untreated and NaOH-treated HFs were added at 0.3%, 0.7%, and 1% of the volume of precursor slurry. The findings reveal that alkali-treated HF increases the compressive strength, which increased with dosage up to 0.7% and then dropped slightly at 1%. Whereas raw HF lowered compressive strength except for 0.7%. Flexural strength was enhanced by both treated and untreated fibre, which increased with the dosage up to 0.7% and then dropped slightly at 1%. The flexural strength enhancement (up to 260%) was higher than compressive strength (up to 54%) at all dosages. Moreover, alkali-treated HF exhibited higher strength compared to raw HF (54% vs 32% under compression and 260% vs 220% under flexure). Residual compressive strength after exposure was measured as an indicator of fire resistance. It followed a similar variation with fibre dosage to their ambient compressive strength when exposed to 100°C and 200°C, where the optimum dosage of 0.7% had up to 65% higher strength for treated fibre and 16% higher strength for raw fibre. However, HF did not improve the residual strength for higher temperatures (400°C and 800°C). In contrast, they caused a higher proportion of strength loss by up to 83–91% of the original strength compared to 67–84% in the control.

Investigating the retarding effect of CAC in alkali-activated cements

This paper discusses the design of hybrid alkali-activated binders at ambient temperature, specifically the development of mixes based on ground granulated blast-furnace slag (GGBFS) with calcium aluminate cement (CAC) as an additive, aiming to obtain high early strength binders with optimised Al incorporation, increased crosslinking and degree of polymerisation. The effects of 10 wt% CAC replacement of GGBFS, activated with sodium silicate with varying modulus (SiO2/Na2O molar ratio) or with sodium hydroxide, on fresh and hardened properties are investigated. The inclusion of CAC in GGBFS mixtures activated using sodium silicate results in an unexpected retardation of the polycondensation reactions required to form the main calcium aluminosilicate hydrate gel phase. This is due to incomplete dissolution of the GGBFS precursor, inhibited by the rapid formation of additional reaction products (especially cubic aluminate hydrates, C3AH6) resulting in lowered reaction kinetics, and thus delayed setting and hardening times. For sodium silicate activators, this retarding effect appears to be only slightly dependent on the activator solution modulus. When activating with 4 M NaOH, the retarding effect is reduced, along with the incorporation of CAC in the C-A-S-H gel, increasing the amount of reactive aluminium present in the binder to form a more compact gel product. Increasing the molarity of NaOH solutions results in a similar delay in reaction kinetics. The results suggest the existence of only a limited pH range in which the addition of CAC can promote the formation of a binding gel with enhanced mechanical properties, whilst other factors including the effect of sodium silicate inclusion in CAC systems are still unclear. The typical hydration products of CAC were not detected in this study.

Ефективність використання червоного шламу при розробці кислотостійких гібридних лужних цементів

The use of secondary industrial products has long been an integral part of the construction industry. The most common man-made raw materials used in the production of building materials, in addition to slag, are fly ash and red mud.
 This work is devoted to the study of the effectiveness of using red mud as an additional source of an alumina silicate component of man-made origin in the production of alkali-activated cements with increased acid resistance.
 In conventional cement systems, the use of significant amounts of red mud is not possible due to the high content of alkali metal compounds and heavy elements, but this is not a problem for alkali-activated binders.
 Red mud obtained by processing bauxite ore using the Bayer process is highly alkaline, and its disposal and reusability are hampered by high sodium concentrations and high pH. That is why, according to the analysis of literature sources, research on the integrated use of red mud is mainly found in only a few areas, among which construction occupies a leading position.
 In the course of research, the parameters of dough of normal density, hardening terms and strength for the developed systems were determined. Direct determination of the acid resistance index was also carried out using two methods: accelerated and by long-term aging of samples. In this way, the main physical and mechanical characteristics of the developed compounds were determined and the results were analyzed.
 The obtained results of the study confirmed the possibility of partially replacing alumina silicate components with red mud to obtain acid-resistant alkaline cements. It was found that the introduction of up to 30% of red mud makes it possible to obtain material with an acid resistance coefficient (Ks) in the range of 0.8-0.95.

Development of Alkali Activated Inorganic Foams Based on Construction and Demolition Wastes for Thermal Insulation Applications.

Nowadays, the construction industry is challenged not only by increasingly strict environmental regulations, but also by a shortage of raw materials and additives. It is critical to find new sources with which the circular economy and zero waste approach can be achieved. Promising candidates are alkali activated cements (AAC), which offer the potential to convert industrial wastes into higher added value products. The aim of the present research is to develop waste-based AAC foams with thermal insulation properties. During the experiments, pozzolanic materials (blast furnace slag, fly ash, and metakaolin) and waste concrete powder were used to produce first dense and then foamed structural materials. The effects of the concrete fractions, the relative proportions of each fraction, the liquid/solid ratio, and the amount of foaming agents on the physical properties were investigated. A correlation between macroscopic properties (strength, porosity, and thermal conductivity) and micro/macro structure was examined. It was found that concrete waste itself is suitable for the production of AACs, but when combined with other aluminosilicate source, the strength can be increased from 10 MPa up to 47 MPa. The thermal conductivity (0.049 W/mK) of the produced non-flammable foams is comparable to commercially available insulating materials.

Retarding effect of gypsum for hybrid alkali-activated cements (HAACs) at ambient temperature

Hybrid alkali-activated cement (HAAC) has become known as a new alternative cementitious material for Portland cement in recent years due to its excellent mechanical properties and low carbon emissions. However, rapid setting is a critical issue that restricts application of HAAC, especially at high alkali dosage. In this study, the industrial by-product gypsum is used to retard the setting of a typical HAAC that consists of 75 wt% slag and 25 wt % Portland cement, with addition of 5 wt% (relative to cementitious materials) alkaline activator. The influence of gypsum on hydration products and microstructure are investigated to explore the retarding mechanism. Both the initial and final setting time of HAAC are prolonged with the increase of gypsum dosage and reach their maximum when the dosage is 20 wt%. X-ray diffraction and thermogravimetric analysis show that monosulfate and thaumasite are formed, rather than ettringite. The incorporation of a large quantify of gypsum leads to the reduction of hydration products and a loose microstructure. The retarding effect of gypsum is considered to be the declined ion dissolution rate caused by chemical reaction instead of traditional coverage theory in conventional Portland cement.