Properties and durability of alkali-activated slag pastes immersed in sea water

This paper represents the experimental trials to activate blast-furnace slag to produce cementless binding materials. The aims of the work is to study the properties of activated slag mixed with sodium hydroxide and sodium silicate liquid 6 wt% of granulated slag. Also, studying the effect of mixing water (tap and sea water) on the kinetic of activation. The rate of activation of the alkali activated slag (AAS) has been studied by FTIR, TGA, DTG and SEM techniques. The results revealed that the increase of NaOH content and mixing with sea water increase the combined water up to 90 days. On the other hand, the bulk density and compressive strength was increased by increasing Na₂SiO₃ content in presence of NaOH. The activated granulated slag showed good durability in sea water, i.e., the compressive strength increased gradually with immersing time up to 12 months. Whereas, the strength of sulfate resisting cement (SRC) pastes immersed in sea water increases up to 6 months then decreases up to one year. Therefore, it can be concluded that alkali activated slag are more durable in sea water than SRC pastes.     

Use of glass waste as an activator in the preparation of alkali-activated slag. Mechanical strength and paste characterisation

The alkaline activation of aluminosilicates yields alkaline cements, eco-efficient alternatives to ordinary Portland cements. Alkaline cements and concretes exhibit highest strength and longest durability when activated with a solution of alkaline silicate hydrates (waterglass). To obtain these alkaline silicates, however, an aqueous solution of the proper proportion of carbonate and silica salts must be heated to temperatures of around 1300 °C. The present paper explores the feasibility of using urban and industrial glass waste as a potential alkaline activator for blast furnace slag (AAS).AAS pastes were prepared with three activators: waterglass, a NaOH/Na₂CO₃ mix and the solutions resulting from dissolving glass waste in NaOH/Na₂CO₃. Mechanical, mineralogical (XRD, FTIR) and microstructural (porosimetry, NMR and SEM/EDX) trials were conducted to characterise the pastes obtained.The findings proved the feasibility of using glass waste to alkali activate slag. Treating glass waste with NaOH/Na₂CO₃ (pH = 13.6) favours the partial dissolution of the Si in the glass into its most reactive monomeric form.The solutions resulting from the treatment of glass waste act as alkaline activators, partially dissolving vitreous blast furnace slag. The composition and microstructure of the reaction products identified in the two types of paste were similar. Strength and microstructural development in the pastes activated with glass waste were also comparable to the parameters observed in AAS pastes prepared with conventional activators.     

Chemical acceleration of a neutral granulated blast-furnace slag activated by sodium carbonate

This paper presents results of a study on chemical acceleration of a neutral granulated blast-furnace slag activated using sodium carbonate. As strength development of alkali-activated slag cements containing neutral GBFS and sodium carbonate as activator at room temperature is known to be slow, three accelerators were investigated: sodium hydroxide, ordinary Portland cement and a combination of silica fume and slaked lime. In all cements, the main hydration product is C–(A)–S–H, but its structure varies between tobermorite and riversideite depending on the accelerator used. Calcite and gaylussite are present in all systems and they were formed due to either cation exchange reaction between the slag and the activator, or carbonation. With accelerators, compressive strength up to 15 MPa can be achieved within 24 h in comparison to 2.5 MPa after 48 h for a mix without an accelerator.     

Mechanical performance and microstructure of the calcium carbonate binders produced by carbonating steel slag paste under CO2 curing

Calcium carbonate binders were prepared via carbonating the paste specimens cast with steel slag alone or the steel slag blends incorporating 20% of Portland cement (PC) under CO₂ curing (0.1 MPa gas pressure) for up to 14 d. The carbonate products, mechanical strengths, and microstructures were quantitatively investigated. Results showed that, after accelerated carbonation, the compressive strengths of both steel slag pastes and slag-PC pastes were increased remarkably, being 44.1 and 72.0 MPa respectively after 14 d of CO₂ curing. The longer carbonation duration, the greater quantity of calcium carbonates formed and hence the higher compressive strength gained. The mechanical strength augments were mainly attributed to the formation of calcium carbonate, which caused microstructure densification associated with reducing pore size and pore volume in the carbonated pastes. In addition, the aggregated calcium carbonates exhibited good micromechanical properties with a mean nanoindentation modulus of 38.9 GPa and a mean hardness of 1.79 GPa.     

Clay reactivity: Production of alkali activated cements

This study has assessed the suitability of dehydroxylated (5 h at 750 °C) red, white and ball clays for the use as prime materials in the production of alkaline cements. The analytical methodology applied to quantify their potentially reactive phases included selective chemical attack, which was also used in conjunction with subsequent ICP analysis of the resulting leachate to determine their reactive SiO₂/Al₂O₃ ratios. These results were compared with compressive strength values of the respective pastes activated with an 8-M NaOH solution and cured at 85 °C and 90% RH for 20 h. It was observed that when the reactive phase content was above 50%, the reactive SiO₂/Al₂O₃ ratio in the starting materials had a larger impact than the amount of reactive phase on the developed strength of the cement material. In this context, fly ash was used as the reference material. Finally, to verify the accuracy of the results, a binder consisting of 70 wt.% fly ash and 30 wt.% dehydroxylated clay was activated with an 8-M NaOH solution. The reactivity of this cement was determined by chemical attack with 1:20 HCl (v/v) and the reaction products were characterised by powder X-ray diffraction and ²⁹Si MAS NMR spectroscopy.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

CO2 binding capacity of alkali-activated fly ash and slag pastes

Quantification of the CO₂ binding capacity of reinforced concrete is of high importance for predicting the carbonation potential and service life of concrete structures. Such information is still not available for alkali activated materials that have received extensive attention as a sustainable substitute for ordinary Portland cement (OPC)-based concrete. To address this gap, this paper evaluates the CO₂ binding capacity of ground powders of alkali activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes under accelerated carbonation conditions (1% v/v CO₂, 60% RH, 20?°C) for up to 180 days. The CO₂ binding capacity, the gel phase changes, and the carbonate phases are investigated with complementary TG-DTG-MS, FT-IR and QXRD techniques.Five mixtures with different FA/GBFS ratio are considered. CEM I and CEM III/B pastes are also studied to provide a baseline for comparisons. The results showed that the alkali-activated pastes have a lower CO₂ binding capacity in comparison to cement-based pastes. Furthermore, alkali-activated pastes have similar CO₂ binding capacity regardless of the FA/GBFS ratio. It was observed that the silicate functional groups corresponding to the reaction products in the pastes were progressively changing during the first 7 days, after which only carbonate groups changed. It was also found that the CO₂ bound in the alkali-activated pastes occurs to a substantial extent in amorphous form.

Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part I: Effect of MgO

The hydration and the microstructure of three alkali activated slags (AAS) with MgO contents between 8 and 13 wt.% are investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O (WG). Higher MgO content of the slag resulted in a faster reaction and higher compressive strengths during the first days. The formation of C(− A)–S–H and of a hydrotalcite-like phase was observed in all samples by X-ray diffraction (XRD), thermal analysis (TGA) and scanning electron microscopy (SEM) techniques. Increasing the MgO content of the slag from 8 to 13% increased the amount of hydrotalcite and lowered the Al uptake by C–S–H resulting in 9% higher volume of the hydrates and a 50 to 80% increase of the compressive strength after 28 days and longer for WG activated slag pastes. For NaOH activated slags only a slight increase of the compressive strength was measured.     

Low temperature depolymerization and polycondensation of a slag-based inorganic polymer

In this paper, the depolymerization and polycondensation process, compressive strength, gel content and conductance of an alkali-activated slag (AAS) (an inorganic polymer or high-calcium geopolymer) paste cured at low temperature (−25 °C~25 °C) were studied. The results showed that gel content of the AAS inorganic polymer sample first increased and then decreased with the decreasing of the curing temperature for 24 h, reaching a maximum value of 37.8 wt% in the samples cured at 0 °C. Chemical structural data are obtained by nuclear magnetic resonance (NMR) spectroscopy for 6-coordinated (octahedral) aluminum can be observed at low temperature (<0 °C) and the gel contains primarily Q₁(1Al), Q₂(₁Al) and Q₃ at 0 °C. The inductively coupled plasma emission spectrometry (ICP) analysis of the products showed depolymerization of a great amount of slag at low temperature. The impedance analysis indicated that low temperature (＜0 °C) will inhibit the polycondensation process but almost not affect the depolymerization process so as to make a distinction between the depolymerization and polycondensation process in the AAS inorganic polymer system.     

Development of a new rapid,relevant and reliable (R3) test method to evaluate the pozzolanic reactivity of calcined kaolinitic clays

The present paper introduces a new rapid, relevant and reliable (R³) test to predict the pozzolanic activity of calcined clays with kaolinite contents ranging from 0 to 95%. The test is based on the correlation between the chemical reactivity of calcined clays in a simplified system and the compressive strength of blends in standard mortar. The simplified system consists of calcined clay portlandite and limestone pastes with sulfate and alkali levels adjusted to reproduce the reaction environment of hydrating blended cements. The pastes were hydrated for 6 days at 20 °C or for 1 day at 40 °C. The chemical reactivity of the calcined clay can be obtained first by measurement of the heat release during reaction using isothermal calorimetry and second by bound water determination in a heating step between 110 °C and 400 °C.Very good correlations were found between the mortar compressive strength and both measures of chemical reactivity.     

Characterisation of magnesium potassium phosphate cements blended with fly ash and ground granulated blast furnace slag

Magnesium potassium phosphate cements (MKPCs), blended with 50 wt.% fly ash (FA) or ground granulated blast furnace slag (GBFS) to reduce heat evolution, water demand and cost, were assessed using compressive strength, X-ray diffraction (XRD), scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) spectroscopy on ²⁵Mg, ²⁷Al, ²⁹Si, ³¹P and ³⁹K nuclei. We present the first definitive evidence that dissolution of the glassy aluminosilicate phases of both FA and GBFS occurred under the pH conditions of MKPC. In addition to the main binder phase, struvite-K, an amorphous orthophosphate phase was detected in FA/MKPC and GBFS/MKPC systems. It was postulated that an aluminium phosphate phase was formed, however, no significant Al–O–P interactions were identified. High-field NMR analysis of the GBFS/MKPC system indicated the potential formation of a potassium-aluminosilicate phase. This study demonstrates the need for further research on these binders, as both FA and GBFS are generally regarded as inert fillers within MKPC.     

Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part II: Effect of Al2O3

The hydration and microstructural evolution of three alkali activated slags (AAS) with Al₂O₃ contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O. The formation of C(A)–S–H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al₂O₃ content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(A)-S-H and led to the formation of strätlingite. Increasing Al₂O₃ content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al₂O₃ content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.     

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH₁₀ and C₄AH₁₃ in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C₃AH₆, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.     

Novel high-resistance clinkers with 1.10 < CaO/SiO2 < 1.25: production route and preliminary hydration characterization

New amorphous calcium silicate binders, hydraulically active, were produced by a process consisting in fully melting and rapid cooling of a mixture of typical raw materials (limestone, sand, fly-ash and electric furnace slag) with overall CaO/SiO₂ molar ratios (C/S) comprised between 1.1 and 1.25. Pastes were produced from these materials by mixing them with water in a water/binder ratio of 0.375. Compressive strength was determined at the ages of 7, 28 and 90 days and the hydration of these pastes was followed during this period by XRD, FTIR and ²⁹Si MAS-NMR. Tobermorite-like structures with low C/S and semi-crystalline character were observed to develop upon hydration of these new amorphous calcium silicate hydraulic binders. Moreover, no Portlandite was formed during hydration of these materials. The maximum compressive strength after 90 days is above 40 MPa. TGA was performed in order to determine the amount of structural water present in the pastes and their content related to the amount of hydrated products obtained. The relation between compressive strength and the amount of hydration products was investigated and some considerations about the mechanical properties of the hydration products and paste microstructure were inferred.     

Improving compressive strength of fly ash-based geopolymer composites by basalt fibers addition

In this study, ASTM Class C fly ash used as an alumino-silicate source was activated by metal alkali and cured at low temperature. Basalt fibers which have excellent physical and mechanical properties were added to fly ash-based geopolymers for 10–30% solid content to act as a reinforced material, and its influence on the compressive strength of geopolymer composites has been investigated. XRD study of synthesized geopolymers showed an amorphous phase of geopolymeric gel in the 2θ region of 23°–38° including calcium-silicate-hydrate (C-S-H) phase, some crystalline phases of magnesioferrite, and un-reacted quartz. The microstructure investigation illustrated fly ash particles and basalt fibers were embedded in a dense alumino-silicate matrix, though there was some un-reacted phase occurred. The compressive strength of fly ash-based geopolymer matrix without basalt fibers added samples aged 28 days was 35 MPa which significantly increased 37% when the 10 wt%. basalt fibers were added. However, the addition of basalt fibers from 15 to 30 wt% has not shown a major improvement in compressive strength. In addition, it was found that the compressive strength was strong relevant to the Ca/Si ratio and the C-S-H phase in the geopolymer matrix as high compressive strength was found in the samples with high Ca/Si ratio. It is suggested that basalt fibers are one of the potential candidates as reinforcements for geopolymer composites development.     

The effect of SCMs and curing time on resistance of mortars subjected to organic acids

Agricultural effluents such as liquid manure and ensilage effluents contain organic acids that constitute a severe chemical threat toward the concrete of agricultural structures. In contact with an acidic solution, the chemical equilibrium of the hydrates in cement paste is destabilized, causing negative effects on porosity, reinforcement corrosion, mechanical strength, and, in the long term, may result in the collapse of the structure. More durable concrete in this environment is needed. The purpose of this study is to examine the effect of the nature of the supplementary cementing materials (SCMs) as well as the curing time on the chemical and the physical modifications of cement pastes and on the compressive strength, mass loss, altered depth and microstructure of mortars immersed in acetic acid at a pH of 4.This study concentrated on three types of hardened cement pastes or mortars made with ordinary Portland cement (OPC), slag and metakaolin cements, cured for period varying from 28 days to 1 year. The results show the beneficial effect of the curing time before the acid immersion, the better durability of metakaolin cement and the good chemical resistance of the slag cement against acid attack. The latter develops low compressive strength and is more sensitive to the curing time but the drop of its resistance due to the acid immersion is minimal due to its strong chemical resistance.     

Hardened properties of high-performance printing concrete

This paper presents the hardened properties of a high-performance fibre-reinforced fine-aggregate concrete extruded through a 9 mm diameter nozzle to build layer-by-layer structural components in a printing process. The printing process is a digitally controlled additive method capable of manufacturing architectural and structural components without formwork, unlike conventional concrete construction methods. The effects of the layering process on density, compressive strength, flexural strength, tensile bond strength and drying shrinkage are presented together with the implication for mix proportions. A control concrete (mould-cast specimens) had a density of approximately 2250 kg/m³, high strength (107 MPa in compression, 11 MPa in flexure) and 3 MPa in direct tension, together with a relatively low drying shrinkage of 175 μm (cured in water) and 855 μm (cured in a chamber at 20 °C and 60% relative humidity) at 184 days. In contrast well printed concrete had a density of 2350 kg/m³, compressive strength of 75–102 MPa, flexural strength of 6–17 MPa depending on testing direction, and tensile bond strength between layers varying from 2.3 to 0.7 MPa, reducing as the printing time gap between layers increased. The well printed concrete had significantly fewer voids greater than 0.2 mm diameter (1.0%) when compared with the mould-cast control (3.8%), whilst samples of poorly printed material had more voids (4.8%) mainly formed in the interstices between filaments. The additive extrusion process was thus shown to retain the intrinsic high performance of the material.     

Compressive strength and microstructure of alkali-activated mortars with high ceramic waste content

The present work investigated alkali-activated mortars with high ceramic waste contents. Tile ceramic waste (TCW) was used as both a recycled aggregate (TCWA) and a precursor (TCWP) to obtain a binding matrix by the alkali-activation process. Mortars with natural siliceous (quartz) and calcareous (limestone) aggregates, and with other ceramic waste materials (red clay brick RCB and ceramic sanitaryware CSW waste), were also prepared for comparison purposes. Given the lower density and higher water absorption values of the ceramic aggregates, compared to the natural ones, it was necessary to adapt the preparation process of the recycled mortars by presaturating the aggregate with water before mixing with the TCWP alkali-activated paste. Aggregate type considerably determined the mechanical behaviour of the samples cured at 65 °C for 3 days. The mortars prepared with the siliceous aggregate presented poor mechanical properties, even when cured at 65 °C. The behaviour of the limestone aggregate mortars depended heavily on the applied curing temperature and, although they presented the best mechanical properties of all those cured at room temperature, their compressive strength reached a maximum when cured at 65 °C, and then decreased. The mechanical properties of the mortars prepared with TCWA progressively increased with curing time (53 MPa at 65 °C for 28 days). An optimum 50 wt% proportion was observed for the limestone/TCWA mortars (≈43 MPa, 3 days at 65 °C), whereas the mechanical properties of that prepared with siliceous particles (10 MPa) progressively increased with the TCWA content, up to 100 wt% substitution (23 MPa). Limestone particles interacted with the binding matrix, and played an interesting beneficial role at the 20 °C curing temperature, with a slight reduction when cured long term (28 days) at 65 °C. The results demonstrated a potential added value for these ceramic waste materials.     

Development of a High Strength Geopolymer by Novel Solar Curing

Geopolymer is a popular construction material derived from different sources of aluminosilicates known for its environmental benefits and excellent durability in harsh conditions. However, the curing of fly-ash based geopolymer normally requires a thermal treatment that increases the manufacturing cost and carbon footprint. This paper explored a new economical and environmentally-friendly alternative, i.e. solar curing, that harnesses solar radiation to achieve accelerated geopolymerization process. Geopolymer mortars coated in two different greyscales namely solar curing black (SCB) and 40% black (grey, SCG) were prepared to study the effect of solar radiation absorption ability on the strength of the specimens, along with ambient cured specimens (ATC) for comparison. Mechanical properties such as workability, compressive strength, stress-strain relationship from 1 day to 28 days were tested. The SCB specimens that can easily reach 65 °C under the sun showed a substantial improvement of the compressive strength especially at the early age, i.e. 49.2 MPa at 1-day compared with 25.5 MPa for the ATC ones. At 28-day, SCB reached 92 MPa in compressive strength which is 17.8% (13.9 MPa) higher than that of ATC. SCG showed a moderate enhancement in strength. Through in-depth physical and chemical characterizations, the structure and morphology of geopolymers were identified through X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). It was found that geopolymer cured by solar radiation had more calcium aluminate silicate content hence leading to a higher mechanical strength. Furthermore, a titration study that determines the conversion rate of the activators inside geopolymers suggested a faster geopolymerization process in the solar cured specimens.     

In-situ fabrication of TiC-Fe3Al cermet

Titanium carbide has high hardness, resistance to oxidation and abrasion while iron aluminide has proper ductility as well as good strength and excellent oxidation resistance up to high temperatures. Therefore, it can be expected TiC-iron aluminide cermet to have excellent mechanical properties as a cutting tool and a wear-resistance material. In this study, mechanical milling and hot press sintering processes were used to manufacture in-situ TiC-Fe₃Al cermet, whose microstructure and mechanical properties were examined according to the changes in volume fraction of TiC and milling time. After 48 h of milling each mechanically alloyed powder crystallized in a TiC and Fe₃Al biphasic material. The milled powder was hot-pressed at 1250 ℃ and 50 MPa for 30 min to obtain sintered bodies also consisting of only TiC and Fe₃Al phases. The hard phase, TiC, had a size of 100–300 nm with overall uniform distribution decreasing as the volume fraction of TiC increased. The hardness of each sintered body showed a linearly increasing tendency according to the increase in TiC content, the hardness for 90 vol% TiC cermet being as high as 1813Hv. On the other hand, the bending strength was 1800 MPa and 1780 MPa when TiC volume fraction was 50% and 70%, respectively, while it showed an abrupt decrease up to 580 MPa at 90% TiC volume fraction. Fe₃Al phase is effective to toughening of TiC-Fe₃Al cermet and the volume fraction of Fe₃Al phase significantly influences the bending strength of the cermet.