Alkali-activated Slag Mortars Research Articles

Partial replacement of copper slag with treated crumb rubber aggregates in alkali-activated slag mortar

Copper slag (CS) is an industrial waste, which is generally disposed of in landfills, posing threats to the environment. Alternatively, it can be used as natural aggregate replacement in concrete/mortar, however, it results in a significant increase in material density, production and handling cost. The use of crumb rubber in combination with CS can overcome these problems, but there was no prior study on effect of such combination on the properties of alkali activated mortars (AAMs). The primary aim of this study is to address the problem stated above by replacing CS with 5, 10, 15% (by volume) crumb rubber in alkali-activated slag mortars. Furthermore, the effect of pre-treatment of crumb rubber aggregates with sodium hydroxide (NaOH) solution for 1 h and 24 h on workability, compressive strength, flexural strength, water absorption, thermal conductivity, and microstructural properties of mortar mixes was investigated. Based on the test results, the use of crumb rubber resulted in inferior workability and mechanical strength, but better thermal insulating capability as compared to the control mix. Alkali treatment promoted the rubber-binder bond and effectively enhanced the strength of mixes, especially at an early-age (up to 59%), however, alkali treatment for 24 h drastically reduced the workability of fresh mix. Thus, it is recommended to use 10% crumb rubber pre-soaked in NaOH solution for 1 h to achieve acceptable performance. Finally, the economic and ecological analysis results revealed that the produced mortar is beneficial to the environment.

Long-Term Strength of Alkali-Activated Mortars with Steel Fibres Cured in Various Conditions

The long-term effect of extreme conditions, such as high concentrations of CO2, a combination of chloride and air, and sulfuric acid, on the performance of steel fibre reinforced alkali-activated fly ash and slag (AAFS) mortars was investigated. The selected conditions simulated the long-term exposure to the marine environment and had an influence on both the matrix and the fibres. Four AAFS mixes were analysed alongside a control ordinary Portland cement (OPC) mix. Mechanical properties such as the compressive strength, elastic moduli and ductility indices, as well as microscopic analyses were carried out. It was found that the AAFS was stable in most of the conditions. The primary way for its reduction in strength was through the neutralisation of pore fluids and the leaching of sodium cations. The addition of the short fibres could reduce the ingress of deleterious materials by limiting the development of cracks and allowing for the efficient use of higher activator ratios. The fibres were susceptible to corrosion by chloride and acid attacks. The relatively chemically stable environment of the AAFS provided protection to the embedded fibres. Based on this study, in a very aggressive environment, a combination of 1%–2% fibre by volume, with a high activator content in the AAFS mortar, could be the most suitable.

Glass powder as a partial precursor in Portland cement and alkali-activated slag mortar: A comprehensive comparative study

Prior studies were conducted in an isolated manner for the use of glass powder (GP) either as ordinary Portland cement (OPC) replacement material in blended cement or as a precursor in AAM material. There was no reported literature that made a direct comparison on the properties of blended cement and AAM containing GP with an equivalent mix composition. The present study was aimed to evaluate and compare the effects of GP as a partial precursor on properties of both OPC and alkali-activated slag (AAS) mortars. For this purpose, cement and ground granulated blast furnace slag (GGBFS) was partially replaced with 10%, 20%, 30%, and 40% (by weight) GP in OPC and AAS mortars, respectively. The mortar mixes were prepared for a fixed water-to-solid (W/S) ratio to make them equivalent. The evaluation was carried out on the workability, mechanical strength, water absorption, alkali-silica reaction (ASR) response and ecological analysis of the mortar. According to the test results, the flow of the fresh mortar increased monotonically with GP content, and AAS mortars showed a higher flow value than OPC mortars. Based on the mechanical properties, the optimum GP content in OPC and AAS mortars was found to be 20% and 30% by the total weight of the binder, respectively. In general, the inclusion of GP was more effective in enhancing the strength properties of AAS mortars as compared to OPC counterparts. Furthermore, the use of GP led to a systematic increase in the water absorption of mixes due to moisture instability of the sodium silicate gel produced by GP activation. According to the results of SEM analysis, AAS mortars showed a higher degree of micro-cracking, which resulted in higher water absorption and a lower flexural strength as compared to OPC mortars. Also, the incorporation of GP densified the pore system of the paste. The potential of ASR was evaluated via the accelerated mortar bar test. The OPC mortars showed up to a 55% reduction in the ASR expansion at 40% GP content. The AAS mortars, on the other hand, showed shrinkage during the first days and then started to show expansion with increasing GP content, which was beneficial as it compensated the shrinkage. The ecological analysis of mixes revealed that the carbon footprint of AAS mortars was about 60% lower than that of OPC counterpart mixes, and the use of GP reduced the CO2 emissions and energy demand of mixes.

Synergistic effect of a small amount of silica aerogel powder and scrap rubber addition on properties of alkali-activated slag mortars

The combined use of smart and waste – based materials for energy efficiency in buildings seems as a promising solution. Due to its unmatched insulation properties, silica aerogel is an attractive smart material in cement technology compared to other conventional insulation materials. However, the high manufacturing cost of silica aerogel limits its application in the construction industry. In this study, the combined effect of silica aerogel (0%, 0.3% and 0.6% of binder weight) and scrap rubber (0%, 1.5% and 3.0% of binder weight) addition in alkali-activated slag (AAS) mortars was investigated. A small amount of silica aerogel and scrap rubber addition into mixtures was analyzed with the aim of minimizing reductions in mechanical properties with enhanced thermal insulation. Experimental results demonstrated that in the case of 3.0% scrap rubber loading at 0.3% aerogel content ratio, a remarkable synergistic effect could be observed in terms of thermal insulation up to 33%. Besides, after 2 days of curing, 48.35% and 77.55% increases in compressive toughness and post – peak compressive toughness of samples were detected, respectively. Synergistic effect of a small amount of silica aerogel and scrap rubber in calcium-silicate structure indicates a great potential for energy conservation.

Effect of alkali contents, moduli and curing time on engineering properties of alkali activated slag

Cement industry has become one of important sources of greenhouse gas emission. Compared with the cement, alkali activated slag (AAS) possesses comparable mechanical properties and lower carbon emission during its production. It can thus be employed as an alternative binder for low-carbon construction. This paper investigated the effect of alkali contents, moduli, and curing time on flexural strength, compressive strength, drying shrinkage, setting time, and resistance to chloride penetration of AAS. Alkali contents ranging from 2% to 6%, moduli of 0.6, 0.9, and 1.2, and curing time of 7, 28, 90 and 180 days were adopted in the test. Test results have shown that high alkali contents shorten setting and accelerate development of compressive strength, but cause large drying shrinkage, deterioration in flexural strength and resistance to chloride penetration of AAS. Alkali contents ranging from 3% to 4% in AAS can achieve the setting time, flexural and compressive strength comparable to cement. Increasing moduli can accelerate setting, improve compressive strength and lower chloride penetrability, but increase drying shrinkage and reduce flexural strength of AAS. Continuous curing after 28 days can significantly improve flexural strength, compressive strength and resistance to chloride penetration of AAS mortar with alkali contents of 2–3%. Using 28-day compressive strength as an index for classification will underestimate the AAS with low alkali contents. Analytical models considering effect of alkali contents and moduli are proposed to predict relationships between flexural strength and compressive strength of AAS mortar and achieve a good correlation with experimental results.

Curing Conditions of Alkali-Activated Fly Ash and Slag Mortar

AbstractThis study compared the effect of seven common curing conditions on the compressive strength, pore alkalinity, efflorescence, and shrinkage of five alkali-activated fly ash and slag (AAFS) ...

Ferrochrome ash – Its usage potential in alkali activated slag mortars

This study is an attempt to develop a sustainable construction material, i.e., alkali activated slag (AAS) in combination with ferrochrome ash (FCA) as a replacement to ordinary Portland cement (OPC). The effect of the various levels of FCA (0, 25, and 50%) replacing ground granulated blast furnace slag (GGBS) in AAS mortars with 4% of Na2O dosage is studied. Further, five levels of the modulus of silica (Ms = 0.75, 1.00, 1.25, 1.5, and 1.75) are chosen to achieve targeted compressive strength at 28 days under ambient temperature curing conditions. The compressive strength decreases with the increase in level of the FCA replacement. The targeted design compressive strength is achieved with 25% FCA replacement to GGBS in the AAS mortar system with Ms = 1.25. In addition, microstructure and mineralogical studies are undertaken to ascertain the formation of different hydration products with the aid of the scanning electron microscope (SEM) and the X-ray diffractometer (XRD). Gismondine and calcium aluminate silicate hydrate (C-A-S-H) are the major hydration products in the AAS mortar mixes. Sodium aluminate silicate hydrate phases (N-A-S-H) are also observed prominently as the FCA replacement level increases in the AAS mortar mixes. The Fourier-transform infrared spectroscopy (FTIR) confirms the presence of the Si–O-(Si or Al) functional group. The addition of FCA in the AAS system is of vital significance in the reduction of the embodied carbon dioxide (ECO2eq), embodied energy (EEeq) and cost.

Durability of the Reinforced One-Part Alkali-Activated Slag Mortars with Different Fibers

This paper investigates the effects of reinforcing one-part alkali-activated slag binders (OAASs) with different types of fiber (steel, polyvinyl alcohol [PVA], basalt, and cellulose) and fiber combinations (single and hybrid) on the mechanical and durability properties of OAASs. All OAASs were reinforced by a 1% fiber volume fraction. Compressive and flexural strengths were the mechanical properties, which were addressed. The durability of the reinforced OAASs was examined based on water absorption by immersion and capillary, acid resistance, high temperature resistance, carbonation resistance, and freeze/thaw resistance. The experimental results showed that the fiber type and combination greatly affects the mechanical and durability properties of OAASs. Moreover, the influence of fiber type and combination on high temperature resistance and freeze/thaw resistance is greater than the influence on acid resistance and carbonation resistance.Graphic

Passivation and corrosion behavior of 2304 duplex stainless steel in alkali-activated slag materials

The passivation capability and chloride-induced corrosion behavior of 2304 duplex stainless steel exposed to alkali-activated slag (AAS) extract solution and mortar were investigated. Ordinary Portland cement (OPC) samples were also prepared for comparison purpose. Compared with OPC extract solution and mortar, much lower passivation capability was evident for steel in both AAS extract solution and mortar. Furthermore, although the steel surface was partially covered with precipitates in AAS extract solution and more compact interfacial microstructure was highlighted for AAS mortar when exposure to chlorides, the corrosion resistance of 2304 duplex stainless steel in AAS extract solution and mortar was significantly lower than that in OPC ones, which is primarily due to the formation of unstable and less protective passive film.

Durability Properties of Geopolymer Mortars Containing Slag

In this study, some durability properties of alkali-activated slag (geopolymer) mortars were investigated. In determining these durability properties, a series of test procedures such as high temperature, abrasion, rapid chloride permeability and wetting and drying cycles were performed. For the slag activation, sodium metasilicate was used at three sodium concentrations, 4%, 8% and 12% by mass of slag. Prismatic specimens with 40 × 40 × 160 mm dimensions were prepared from geopolymer mortars for the tests. After 24 h, the specimens were demolded and cured in three different ways until the test time: curing in an oven at 75 °C for 1 day, curing in air for 28 days and curing in water at 22 ± 1 °C. The results obtained from the tests were compared with the control mortar made with ordinary Portland cement (OPC).The findings revealed that the resistance of alkali-activated slag mortars under aggressive media investigated within the scope of this work was higher than that of OPC mortar.

Mechanical fracture characterization of alkali-activated slag mortars with standardized and natural sand

This paper is focused on the development of the physical, mechanical and fracture characteristics of alkali-activated slag (AAS) mortars during ageing. The two AAS mortars were selected for the purpose of the experiment. The difference was in a type of used sand: first mortar was made using the standardized siliceous sand with the maximum nominal grain size of 2 mm and the second mortar using the natural sand with a maximum grain size of 4 mm. The results of the elastic, fracture and strength parameters determined within the time interval of 3 to 330 days of ageing are summarized in the paper. All experimental results were evaluated for the prismatic specimens with dimensions of 40 × 40 × 160 mm and were expressed by the average value and sample standard deviation calculated from three independent measurements.

Effect of Polypropylene Fiber on Properties of Alkali‐Activated Slag Mortar

Alkali‐activated slag (AAS) is becoming an increasingly popular building material due to its excellent engineering properties and low CO2 emissions, but its large shrinkage is an important reason to restrict its application and popularization. This work is aimed to study the possibility of inhibiting the shrinkage of AAS mortar by incorporating polypropylene fiber (PPF). For this, an experimental study was carried out to evaluate the effects of PPF content on setting time, fluidity, physical properties, mechanical properties, impact resistance, and microstructure of AAS mortar. The volume content of PPF is 0.05%, 0.1%, 0.15%, and 0.2%. The working, physical (porosity, water absorption, and bulk density), mechanical, shrinkage, and impact resistance properties of the AAS mortars were evaluated. The results show that incorporating PPF effectively reduces the shrinkage deformation of AAS mortar, significantly improves its impact resistance, enhances its toughness, and slightly improves its compressive strength in the later stage. At the same time, PPF delays the initial setting time of AAS mortar and reduces the fluidity, density, porosity, and water absorption of AAS mortar. SEM results show that the bridging effect of PPF between AAS mortars can inhibit the generation and propagation of cracks, improve the internal microstructure, and enhance the performance of AAS mortar.

Behaviour of the waste steel fibre reinforced alkali-activated slag mortars exposed to high temperatures

Behaviour of the waste steel fibre reinforced alkali-activated slag mortars exposed to high temperatures

The analysis of cracking risk by shrinkage restraint of an alkali-activated slag mortar

Alkali-activated slag (AAS) binders show in general larger autogeneous shrinkage strains than ordinary Portland cement (OPC) based binders. However, AAS can be a relevant alternative to OPC, if, for example low hydration heat release and fine pores, are required. This study proposes an evaluation of the advantage of using AAS materials in small-sized or massive structures with regard to cracking risk by autogeneous shrinkage and thermal strains. A cracking risk index is calculated; this risk is defined as the ratio between stress generated by full restraint and tensile strength. All required experimental data were investigated in an OPC and AAS mortar, these are: heat release, autogeneous shrinkage, Young’s modulus, tensile strength and basic creep evolutions. The material parameters of a rate-dependent model developed in 1D were then identified. Numerical simulations were then performed for different thicknesses in full-restraint conditions. These show that, as expected, basic creep is a very important material parameter to assess. Indeed, basic creep enables the significant reduction of the generated stresses. Besides, it is found that the more the structure is large (and sensitive to cracking by risk by thermal strain), the more the AAS material is becoming appropriate compared to the OPC material.

Influence of different activators on microstructure and strength of alkali-activated nickel slag cementitious materials

Comparisons between three alkali-activators (NaOH, NaOH/Na2CO3, water glass) on the performance of alkali-activated nickel slag cementitious materials (ANC) were made. The properties of ANC, including setting time, mechanical properties, morphology of hydration products and pore structure were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) and nitrogen adsorption test etc. The average pore size and total pore volume of water glass-activated system (NS6) are the smallest, and then the NaOH/Na2CO3-activated system (NN42), and the average pore size and total pore volume of NaOH-activated system (NH6) are the largest. Interfacial transition zone (ITZ) between aggregate and hardened cement paste (HCP) decreased from large to small is in the order of NS6 > NN42 > NH6. The microcracks of NS6 in HCP is most obvious, and then the NH6, and the microcracks of NN42 in HCP is least obvious. The pore structure and ITZ play dominated role on mechanical properties, and therefore the compressive and flexural strengths of alkali-activated nickel slag mortar are ranked from large to small in the order of NS6 > NN42 > NH6.

Shrinkage of the alkali-activated slag mortars containing alternative activator

The paper deals with the experimental determination of the shrinkage process of the alkali-activated slag mortars (AASM). Two mortars which differed especially in the type of used aggregate were mixed for the experiment. The slag-aggregate ratio was 1:3 for both mortars. The waste sludge, obtained from the water glass production, was used as an activator. The desired workability of the fresh mixtures was reached by addition of water. Because of the high water demand of waste sludge, the water-slag coefficient (w/s) was relatively high – 0.98 for the mortar with the natural sand and 0.73 for the mortar with the standard sand. All test specimens were removed from the moulds at the age of 24 hours and left to dry freely in the laboratory conditions with a temperature of 21 ± 2°C and relative humidity of 50 ± 10% during the whole time of measurement. The results show that AASMs which contain waste sludge exhibit rapid shrinkage with a high magnitude independent on the type of aggregate. During the first 40 hours of ageing, both mortars reached approx. 50% of the final value of shrinkage. The high shrinkage was strengthened by a high w/s coefficient needed to maintain the workability of the fresh mixtures.

Influence of sodium silicate powder silica modulus for mechanical and chemical properties of dry-mix alkali-activated slag mortar

Sodium silicate powders with different SiO2/Na2O (silica modulus) were characterized by solubility rate, pH, and chemical structure (by 29Si MAS-NMR) and compared in the preparation of one-part (or dry-mix) alkali-activated blast furnace slag mortar. The low SiO2/Na2O indicated the beneficial presence of less-polymerized silica (Q1 and Q2 Si environments) and thus faster dissolution. Consequently, using sodium silicates with SiO2/Na2O of 0.9, 2.1, and 3.4 resulted 28 d compressive strengths of 103, 80, and 2 MPa, respectively, with increasing setting time and decreasing heat release in isothermal calorimetry. Adjustment of activator SiO2/Na2O from 2.1 or 3.4 to 0.9 by adding NaOH powder resulted increased or decreased mechanical properties of mortar, respectively, depending on the initial silica modulus. These properties were not, however, similar to those obtained with sodium silicate having SiO2/Na2O of 0.9 originally. Depending on the case, the added NaOH can be consumed for dissolving sodium silicate activator, slag, or the forming (C,N)-(A)-S-H gel.

Towards sustainable bricks made with fiber-reinforced alkali-activated desulfurization slag mortars incorporating carbonated basic oxygen furnace aggregates

This study examines the development of bricks made with fiber-reinforced alkali-activated desulfurization slag based mortars (AADS) with carbonated basic oxygen furnace (BOF) slag aggregates. This paper was executed in stages, including: 1) parametrical assessment was conducted to identify the impacts of alkali properties and the content of carbonated BOF aggregates on compressive and flexural strengths and efflorescence; 2) different fiber types (polyvinyl alcohol, basalt, polypropylene, and cellulose), and contents were added to improve the mechanical and durability (water absorption by immersion and capillary action, carbonation resistance, and high temperature) properties, and reduce both efflorescence and drying shrinkage; and 3) an environmental analysis was conducted to correlate global warming potential with the mechanical strength and to indicate the participation of each mix ingredient in carbon dioxide emissions into the atmosphere. The results showed that increasing both molarity of sodium hydroxide and the ratio of carbonated BOF aggregates to binder (desulfurization slag) above 8 M and 3, respectively, degraded the mechanical strength and intensified the efflorescence in the plain AADS. The addition of basalt and cellulose fibers increased the mechanical strength and reduced the efflorescence in AADS. Moreover, it was found that using basalt fibers produced better performance than cellulose fibers in minimizing the drying shrinkage and strength loss in carbonation, while adding cellulose fibers had a greater impact on increasing the mechanical strength.

Preparation, characterization of highly dispersed reduced graphene oxide/epoxy resin and its application in alkali-activated slag composites

A highly dispersed reduced graphene oxide/epoxy resin composites (HD-RGO/EP) were synthesized and applied in alkali-activated slag based composites firstly. The synthesis, characterization and application in alkali-activated slag cementitious materials of this synthesized graphene are reported in this paper. The dispersion states of the reduced graphene oxide in epoxy resin and alkali activated slag paste are proved to be sound by observation by TEM and Metallographic Microscope respectively. Both FTIR and Raman spectrograms verifies that there is no new reaction products generated under HD-RGO dispersing in the epoxy resin. It is confirmed that HD-RGO/EP increases the fracture toughness of alkali-activated slag mortar 75 times the most compared with the one blend with EP, the bonding behavior of the alkali-activated slag composite blended with HD-RGO/EP is enhanced significantly as well. HD-RGO/EP alkali-activated slag mortar is believed to have broad application prospects in repair and rehabilitation of reinforced concrete structures.

Hydration properties of alkali-activated quick cooled copper slag and slow cooled copper slag

The hydration properties of quick cooled copper slag (QCS) and slow cooled copper slag (SCS) activated with 4% and 8% (by Na2O mass) sodium hydroxide (NA) and water glass (NS) were investigated by hydration heat. Meanwhile, the hydration degrees and hydration products of alkali-activated copper slags were determined by X-ray diffraction, Fourier transform infrared, scanning electron microscope, energy-dispersive X-ray and backscattered electron tests. In addition, the compressive strengths of alkali-activated copper slag mortars were tested. Results show that alkali-activated QCS has significantly higher activities than alkali-activated SCS. At the concentration of 4%, the QCS activated with NA has a higher hydration heat than the QCS activated with NS, and the cumulative hydration heat of QCS activated with NS increases with the silica modulus at early ages. But this regularity generally reverses when the activator concentration is 8%. Meanwhile, elevated activator concentration could increase the cumulative hydration heat of QCS. In addition, with the increase in silica modulus, the second exothermic peak value decreases, the time of the second exothermic peak delays and the peak width increases. The hydration degree of QCS and the polymerization of amorphous hydration products increase with the silica modulus of activator. What’s more, the compressive strengths of alkali-activated QCS mortars and SCS mortars increase with the silica modulus of activator. When the silica modulus is more than 1, the compressive strength of the QCS mortar activated with NS could be qualified for common constructions.