Ordinary Portland Cement Binder Research Articles

Shrinkage characteristics of sustainable mortar and concrete with alkali-activated slag and fly ash Binders: A focused review

Alkali-activated binders are emerging as promising sustainable alternatives to ordinary Portland cement (OPC) due to their lower environmental footprint. They offer competitive mechanical properties and enhanced durability, making them attractive options for a wide range of construction and infrastructure applications. This paper is a focused review of key findings related to the shrinkage characteristics of mortar and concrete with alkali-activated slag and fly ash binders. Shrinkage induces nonuniform deformations in mortar/concrete, leading to the development of internal tensile stresses and the formation of cracks. Such cracks may be sufficiently large to facilitate the penetration of harmful substances, which may compromise the integrity of structural elements. Several studies indicated that the combination of fly ash and slag precursors in appropriate proportions produces mortar with favorable shrinkage cracks when compared to mortar with individual binders. This article discusses the influence of the following factors on shrinkage characteristics: binder type, binder proportions, water/binder ratio, binder content, aggregate content, alkaline activator type, and concentration. The article compares the shrinkage of mortar/concrete prepared using mostly alkali-activated slag and fly ash binders and compares their shrinkage response to OPC. Shrinkage types evaluated include chemical shrinkage, autogenous shrinkage, and drying shrinkage. Shrinkage responses reported in this article include the sample curing method and environment as they are known to influence durability, strength, and shrinkage characteristics. In general, mortar/concrete with alkali-activated fly ash and slag experience higher shrinkage rates than conventional concrete with OPC binders. The study further explores shrinkage mechanisms and methods to mitigate shrinkage in alkali-activated binders.

Microstructure and mechanical properties of geopolymers utilizing excavated soils, metakaolin and slags

This paper presents a study on the potential of geopolymer (GP) as a substitute for ordinary Portland cement (OPC) in construction applications. OPC is widely used in construction due to its high strength and durability. However, manufacturing is extremely energy-intensive and produces large amounts of carbon dioxide (CO2) emissions. Also, In France, 130 million tons of excavated soils are generated each year [1] and classified as waste material without proper disposal methods. Therefore, the use of excavated clays coming from construction projects in GP formulations will have a positive impact on decreasing waste materials. Moreover, flash calcination used as heat thermal technique eliminate the organic matter and enhance the geopolymerization reaction with the alkaline reagent used in GP. On the other hand, GP are a type of alternative material made from industrial by-products such slags. This study used various methods to optimize GP mix design by evaluating the fresh and hardened properties. The results has shown that GP has a higher compressive strength than OPC in short term (between +5% and 76% at 1 day) and present a comparable strength in long term. The analysis of the microstructure throughout nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), X-ray diffraction (XRD) and Thermogravimetric analysis (TGA) has shown the GP designed has a stable reticulated polymeric structure. Mercury porosity has shown a finer nonometric porosity that corresponds to a compacted and fitted micelle structure. This study provides a solid contribution to the sustainability of the construction industry by developing a GP binder composed of flash-calcined excavated clay (FCC), metakaolin (MK), and slag (GBFS) that can achieve better results than OPC binder.

Fresh Properties, Strength, and Durability of Fiber-Reinforced Geopolymer and Conventional Concrete: A Review.

Reducing the environmental footprint of the construction industry in general and concrete in particular is essential. The addition of synthetic and natural fibers to concrete mixes at appropriate dosages enhances durability and strength and extends the lifespan of concrete infrastructures. This study reviews the geometric and mechanical properties of selected fibers such as steel, basalt, polypropylene, polyvinyl alcohol, polyethylene, glass, carbon, and natural fibers and their impact on concrete fresh, mechanical, and durability properties when combined in different configurations. The study focuses on the effect of blending fibers with concrete mixes that use alkali-activated binders based on recycled industrial byproducts such as slag and fly ash and thereby contribute to reduction of CO2 contribution through complete or partial replacement of Ordinary Portland cement (OPC). As a result, the effect of binder content, binder composition, alkaline activator concentration, and water-to-binder (w/b) ratio on fresh properties, mechanical strength, and durability of concrete with blended fibers is also evaluated in this study. The properties of fiber-reinforced concrete with alkali-activated binder and conventional OPC binders are compared. Fiber-reinforced concrete with alkali-activated binders that are based on industrial byproducts may represent sustainable alternatives to conventional concrete and offers competitive fresh and mechanical properties when fiber properties, fiber content, w/b ratio, binder type, and dosage are carefully considered in concrete mix design.

Multiscale performance and environmental impact assessment of slag and Portland blended cement for optimum carbonation curing

This article presents an investigation into the potential use of ground granulated blast-furnace slag (addressed as Slag cement or ‘SC’) as a replacement to Ordinary Portland Cement (OPC) in hybrid (carbonation and hydration) cured cement-based materials. To investigate the effects of carbonation on mechanical performances and microstructures, 0 %–100 % OPC was replaced with slag cement (SC). Thermogravimetric analysis (TGA) and Fourier transformed infrared (FTIR) spectra were utilized to investigate the carbonation reaction extent, rate, and microstructural phase formations. Slag cement was found to improve the efficiency and rate of carbonation. This study revealed that a minimum of 72 h of carbonation in a CO2-containing environment yields better mechanical performance compared to the traditional curing method. Specifically, the incorporation of 72 h of carbonation curing was observed to increase the strength of concrete up to 30 % after 28 days of total curing duration (carbonation and hydration). The chloride permeability of the carbonation cured samples was observed to reduce by 80 % due to the addition of SC. Finally, it was observed that, the carbonated concrete sample with slag has nearly 60 % lower global warming potential compared to the carbonated and non-carbonated concrete sample with 100 % OPC binder.

Eco-Sustainable Recycling of Cement Kiln Dust (CKD) and Copper Tailings (CT) in the Cemented Paste Backfill

Cement global demand shows continued growth and a significant increase in the production volume, which may negatively impact the non-renewable natural resources and the environment, which is incompatible with sustainability goals. Cement kiln dust (CKD) is a primary concern associated with clinker manufacturing as a waste byproduct. Similarly, the mining industry produces copper tailing as unwanted material while beneficiating the ore, creating environmental problems due to difficulty in managing worldwide generated quantities that reach billions of metric tons. This study investigated the beneficial utilization of cement kiln dust and copper tailing as undesirable wastes in industrial applications through underground mines’ cemented paste backfill (CPB). Sixty different mixtures were prepared with three types of CKD collected from various cement manufacturers and were accordingly used with a proportion of 5, 10, and 15% to partially replace ordinary Portland cement (OPC) and pozzolan Portland cement (PPC) binders, represented in hundreds of CPB samples. The hardened specimens were subjected to density, uniaxial compressive strength (UCS), and axial deformation measurements to evaluate the physical and mechanical properties at curing up to 90 days. Meanwhile, X-ray powder diffraction (XRD) was extensively applied to chemically investigate the hydration products of CPB-hardened mixtures. Moreover, we developed a UCS predictive model applying two techniques: multiple variables regression analysis and artificial neural network (ANN). The results showed that the tricalcium silicate (Alite) and dicalcium silicate (Belite) phases form C-S-H upon hydrations and provide high strength in the binary mixtures. Meanwhile, the CKD’s lime saturation factor (LSF) governed the strength value in the ternary mixtures that utilized copper tailings. That makes CKD practical in the CPB mixture when partially replacing the OPC and PPC binders, with a proportion of up to 15%. In addition, the ANN technique’s predictive model exhibited a significant positive correlation with excellent statistical parameters that achieved 0.995, 0.065, and 0.911 for R2, RMSE, and MAE, respectively.

Effects of belite incorporation on mechanical and microstructural properties of OPC mortars cured under carbonation condition

This study investigates the effects of various dosages of belite replacing ordinary Portland cement (OPC) on properties of OPC binders using carbonation curing. Mechanical strength was examined, pore-related test and microstructural/mineralogical analyses including water absorption, scanning electron microscopy (SEM), thermogravimetric analysis and 29Si nuclear magnetic resonance (29Si-NMR) were carried out. Extra attention was paid on the changes in the thickness and porosity of interfacial transition zone (ITZ) of mortars. Compared to normal curing, carbonation curing increased the compressive strength of all mortar mixes, especially given a longer period of curing and/or with belite addition. The greatest increase (73.18%) in the compressive strength was seen in the specimen with 35% of belite after 28 days of carbonation curing compared its peer under normal curing condition. Besides, specimens with 5% and 15% belite obtained higher flexural strength than that of the control after 7 and 28 days of carbonation curing. Carbonation curing led to about 92.75% increment in the flexural strength of the specimen with 15% belite. Regardless of curing conditions, in most cases, belite modified mortars had higher fractural toughness, probably due to the formation of more C–S–H gels instead of Portlandite. Compressive and flexural strength are more sensitive towards the thickness of ITZ than its porosity and carbonation curing strengthened this sensitivity. Carbonation curing densified the bulk OPC binders, especially after a long period of curing incorporated with belite. Microstructural analyses indicate that although in general belite hinders the hydration process of OPC, a suitable amount (5 wt%) improved the hydration degree of OPC when cured in carbonation environment. The polymerization degree of C–S–H gels could be enhanced when belite is added irrespective of curing conditions.

Blending Plastics Waste with Highly Available Jordanian Kaolin for Preparation of Alkali-Activated Mortars

Due to their lower energy demand, alkali-activated materials or geopolymers (GPs) are strong candidates to replace ordinary Portland cement binders in some applications. The present article attempts to investigate the possibility of incorporating plastics waste in place of silica sand filler in the GP mortars. The compressive strength, density, FTIR, XRD, BET and SEM of the prepared mortars were evaluated. Satisfactory compressive strength was achieved for metakaolin (MK)-based GP mortars containing plastic fillers (polyvinylchloride (PV), polystyrene (PS), polypropylene (PP) and polyethylene (PE)) which was dependent on the nature of plastic incorporated in the GP matrix: MKPV (30.3 MPa) > MKPS (15.8 MPa)~MKPP (15.9 MPa) > MKPE (9.1 MPa). The 28-day compressive strength of GP containing polyvinyl chloride was almost equivalent to that containing silica sand. Much lower values were obtained for the corresponding kaolin-based GPs (2.5, 2.8, 1.8 and 2.2 MPa, respectively). As reflected by FTIR absorption bands at 1447 and 1400 cm−1, the addition of plastic filler reduces absorption of CO2 from the atmosphere which enhanced dissolution of Al species from metakaolin. The morphology of GPs containing plastic fillers exhibited amorphous tissue-like structure compared to crystalline structure in the case of GP containing silica sand. However, both types of GPs exhibited lower porosity than previously prepared metakaolin-based GPs. Remarkably, poor adhesion of plastic filler to the GP binder was observed due to the inertness of plastic fillers toward alkali activators.

Impact of trace metals Zn, Cu, Cd and Ni on the reactivity of OPC and GGBS-based hydraulic binders at early age for sediment stabilization

The solidification/stabilization method was demonstrated to be an effective remediation technology for hazardous waste treatment. Three types of binding agents were tested in the present study in view of their potential use in solidification of contaminated sediments and soils – ordinary Portland cement (OPC), granulated ground blast furnace slag (GGBS) activated by OPC and a Supersulfated (SSC) binders. An investigation into the impact of trace metals Zn, Cu, Ni and Cd on the hydration of the binders was performed by adding them into pure binders’ systems. Hydration heat evolution of the binders was assessed through isothermal calorimetry and hydration products were analysed through XRD, NMR and XAFS analysis. Copper and zinc present the most perturbing effect on the early hydration delaying the main hydration peak occurrence while nickel and cadmium ions do not perturb or accelerate it. For example, the GGBS/OPC formulation without any metal addition has its MHP at 13.5 h, with the addition of 0.5%Zn the MHP appears 37 h later, with 0.5%Cu 11.5 h later, but with Ni and Cd 5 h earlier. The SSC formulation was shown to be the most sensitive on the trace metals (TM) addition. The electrokinetic and conductivity measurements of the binders using Zeta Potential apparatus showed a decrease in obtained values in the presence of Cu and Zn and an increase in the presence of Ni and Cd confirming the different behaviours of TM in the considered systems.

Correlated strength enhancement mechanisms in carbon nanotube based geopolymer and OPC binders

The environmental impact of the cement industry can be controlled by substituting a part of the cement with supplementary cementitious materials such as carbon nanotubes (CNTs). The CNTs alter the physical and mechanical properties of the binders. However, a poor understanding of their strength enhancing mechanism in the binders obstructs their industrial applications. This report reveals a correlation between improved compressive strengths of the ordinary Portland cement (OPC) and geopolymer matrices with CNTs indicating that the strength enhancing mechanism in both the binders must be the same. Based on the XRD, FTIR and SEM/EDS measurements, there are strong indications that the structural and morphological defects in the CNTs facilitate an ion–dipole interaction with the compounds of the binders. This facilitates the growth of the large crystallites in the matrix resulting into a compressive strength increased by 21% to 24%.11Corresponding author: Muhammad Azeem (mazeem@sharjah.ac.ae)

A novel titania/graphene composite applied in reinforcing microstructural and mechanical properties of alkali-activated slag

Alkali-activated slag (AAS) binder with low carbon footprint is a promising alternative to high energy-embodied ordinary Portland cement binders. However, wider applications are hindered by their ceramic-type fractural behaviours. This study involves improving the microstructure and mechanical properties of alkali-activated slag pastes by using an in-situ fabricated titania/graphene composite assisted with a novel titania intercalating method. The result indicates that titania located in between the layers of graphene could make graphene homogeneously dispersed in an aqueous solution, which is conducive to obtaining well-dispersed graphene nanoparticles in AAS pastes. Besides, the highly dispersed titania/graphene composites lead to a less disordered microstructure of AAS paste compared to the normal counterpart. Moreover, both the compressive strength, flexural strength, Young's modulus were improved as the content of titania/graphene composites (0%-0.03 wt%) increased. In addition, the fractural behaviours were also improved remarkably evidenced by a 89.7% increase of flexural toughness for AAS pastes with 0.03% titania/graphene composite. Scanning electron microscopy observations confirmed that the microstructure of the AAS pastes with titania/graphene composites was denser compared to normal AAS which is probably due to the effects of crack bridging and branching. Overall, the developed new intercalating method could be used to improve the dispersibility of graphene which is then employed effectively for reinforcing microstructure and mechanical properties of AAS binders.

Mechanical and durability-related performance of graphene/epoxy resin and epoxy resin enhanced OPC mortar

Abstract In this research, normal epoxy resin was reinforced by graphene nanosheets which was then used for improving mechanical properties and durability of ordinary Portland cement (OPC) mortars in terms of water sorptivity, resistance towards chloride penetration and freeze–thaw actions. Results showed that epoxy resin modified OPC mortar had a similar or lower compressive strength in the early curing ages whereas a higher one was obtained after 56 days of curing, especially for the mortar with the graphene addition. Flexural strength was all improved after the addition of epoxy resin, particularly for the graphene incorporated one. Besides, epoxy resin modified mortars were endowed with higher durability evidenced by lower capillary sorptivity and negligible mass loss and changes in relative dynamic modulus during freeze–thaw cycles, as compared to the normal OPC counterpart. With the addition of graphene, epoxy resin modified OPC mortar displayed an obvious lower chloride contents (especially at deeper depth) and capillary sorptivity. Microstructure analysis revealed that epoxy resin could refine the porous structure of the OPC mortar and graphene is able to enhance this benefit even further by densifying the mortar binding matrix. Considering the extremely small dosage of graphene but significant improvements in resistance against water ingress via capillary forces and chloride penetration, this novel method of combing graphene as a nanofiller with epoxy resin could be further utilised for OPC binders in practical aggressive engineering environments.

Resistance of Alkali-Activated Binders to Organic Acids Found in Agri-Food Effluents

Organic acids, such as acetic and lactic acids, are prevalent in agricultural and food effluents. They pose a considerable pollution threat and must be collected and stored safely before treatment and release. They cause significant damage to cementitious materials, reducing the service life of structures. In this study, the resistance of alkali-activated fly ash and slag-blended binders to organic acids was studied, and a comparison with ordinary portland cement binders was carried out. The findings demonstrate that alkali-activated binders with increased fly ash content have marginally better resistance to acetic acid, but mixes with increased slag content have better resistance to lactic acid. This is due to the solubility of the calcium and aluminum salts of acetic and lactic acids. Overall, the performance of the alkali-activated binders was better than that of the ordinary portland cement binder, with a lower mass and strength losses observed. This was attributed to their lower calcium content with less vulnerable phases, such as calcium hydroxide and ettringite. Instead, the calcium-silicate-hydrate (C-S-H) type gels in alkali-activated binders suffered decalcification and dealumination but left behind a silicon-rich gel, which helped to resist further acid attack.

Modifying the pore size distribution in Fe-rich inorganic polymer mortars: An effective shrinkage mitigation strategy

Inorganic polymer (IP) binder is formed upon alkali-activation of Fe-rich (41 wt%) metallurgical slag leading to materials with mechanical properties comparable to ordinary Portland cement binders. Crack formation is reported in IPs, which can be related to volumetric stability, which is to date not thoroughly understood in IPs. This study determined the autogenous and drying shrinkage of IP mortars. Shrinkage mitigation strategies, such as the addition of 2-methyl-2,4-pentanediol (2MPL), blast furnace slag (GGBFS), and applying heat curing, were used separately or in combination. IP mortars exhibited autogenous expansion and high drying shrinkage. The addition of 2MPL led to a porosity increase, as air was entrained, decreasing the drying shrinkage with 64%. Additional heat curing had no effect on samples with 2MPL. Introducing GGBFS resulted in smaller pores, increasing drying shrinkage. Shrinkage in IP mortars is driven by its pore size distribution and higher shrinkage resulted in lower flexural strength.

The Effect of Dry Mix Sodium Hydroxide onto Workability and Compressive Strength of Geopolymer Paste

The production of ordinary Portland cement (OPC) consumes considerable amount of natural resources, energy and at the same time contribute in high emission of CO2 to the atmosphere. A new material replacing cement as binder called geopolymer is alkali-activated concrete which are made from fly ash, sodium silicate and sodium hydroxide (NaOH). The alkaline solution mixed with fly ash producing alternative binder to OPC binder in concrete named geopolymer paste. In the process, NaOH was fully dissolved in water and cooled to room temperature. This study aims to eliminate this process by using NaOH in solid form together with fly ash before sodium silicate liquid and water poured into the mixture. The amount of NaOH solids were based on 10M concentration. The workability test is in accordance to ASTM C230. Fifty cubic mm of the geopolymer paste were prepared which consists of fly ash to alkaline solution ratio of 1: 0.5 and the curing regime of 80℃ for 24 hours with 100% humidity were implemented. From laboratory test, the workability of dry method geopolymer paste were decreased. The compressive strength of the dry mix of NaOH showed 55% and the workability has dropped to 58.4%, it showed strength reduction compared to the wet mix method.

Evaluation of alkalinity changes and carbonation of geopolymer concrete exposed to wetting and drying

The investigation of alkalinity changes of geopolymer concrete (GPC) in various environments is important to enhance the application of GPC in the construction field. This study investigates the alkalinity changes of GPC with different proportions of fly ash and slag constituents under three different exposure conditions, such as 1% CO2, cyclic exposure to 1% CO2 and water, and cyclic exposure to 1% CO2 and chloride solution. The changes in pH value, carbonation depth, compressive strength variation, and electrical resistance were determined after a six-month exposure to the three-above-mentioned media. The pH measurements indicate that the geopolymer mix with a high fly ash content has a low initial pH (10.8), and the alkalinity reduction was small after the exposure (reaching 8.4 in 1% CO2 and chloride media). In contrast, the alkalinity of the geopolymer mix with high slag content changed from 12.2 to 7.6, which is similar to the Ordinary Portland Cement (OPC) concrete test results. Compared to the OPC binder, a geopolymer binder is more vulnerable to carbonation and corrosion. However, the resistance to carbonation increased with the addition of slag to the mix. The compressive strength loss of fly ash-based GPC is higher than the OPC concrete. However, it can be reduced with the increment of slag in the mix. Finally, the study on the corrosion of reinforcement bar revealed that the fly ash-based GPC is more prone to corrosion even with higher alkalinity values after the exposure.

Physical and mechanical properties and micro characteristics of fly ash-based geopolymer paste incorporated with waste Granulated Blast Furnace Slag (GBFS) and functionalized Multi-Walled Carbon Nanotubes (MWCNTs)

Geopolymers are highly durable and have favorable mechanical properties, and are thus regarded as an eco-friendly alternative to traditional ordinary Portland cement binder. In this study, MWCNTs are obtained through a modification method using a compound of nitric acid and sulfuric acid, and are then dispersed using three types of dispersants. Fly ash-based geopolymers are prepared to validate the effectiveness and feasibility of adopting 0.05 wt.%, 0.10 wt.%, and 0.15 wt.% functionalized MWCNTs and substitution ratios of 10 %–40 % of fly ash with GBFS. The structure and dispersity of the functionalized MWCNTs in aqueous solutions are characterized using FT-IR and TEM, respectively. Then, the setting time, water absorption capacity, and mechanical behaviors are evaluated. In addition, SEM-EDS, FT-IR, TG-DSC, 29Si NMR, and XRD are employed to investigate the morphology, elemental components, mineralogical phases, and geopolymerization degree of the gel products. The experimental results show that the functionalized MWCNTs comprise −COOH and −OH groups and can be uniformly dispersed in aqueous solution containing SDS dispersant. Furthermore, geopolymer paste incorporated with 0.1 wt.% functionalized MWCNTs and having 30 % substitution of fly ash with GBFS exhibits a higher compressive and flexural strength and a lower water absorption capacity compared with all other geopolymer pastes.

Mineralogy and microstructure effects on the stiffness of activated slag treated alluvium

Alkali-activated alumino-silicate-based industrial waste products have recently proven to be beneficial as low-carbon alternatives to ordinary Portland cement binders for strengthening soft soils. This paper examines the small-strain stiffness behaviour of a UK silty alluvial soil in its natural state and artificially cemented using sodium hydroxide-activated ground-granulated blast-furnace slag (GGBS). Undrained triaxial testing with bender element measurements revealed that the initial small strain shear stiffness (Gmax), shear strength and hydraulic conductivity of the alluvium were all significantly enhanced after 28 d curing. Microstructural and mineralogical analyses were carried out on stabilised soil to understand the mechanisms better through which the enhanced engineering performances were achieved. Through hydration and pozzolanic reactions, a significant proportion of the clay minerals within the original soil had been converted into new cementitious hydrates. These were observed to infill pore spaces, coating soil and GGBS particle surfaces and increased interparticle bonding throughout the matrix of the material. The outcomes from this study have the potential to contribute towards improving current practices for modelling cemented soils and ultimately making geotechnical designs involving deep soil mixing less conservative.

Influence of type of binder and size of plant aggregate on the hygrothermal properties of bio-concrete

This study investigated the two innovant and high energy efficient corn stalk concretes based on magnesium phosphate cement (MPC) and ordinary Portland cement (OPC) as binders and corn stalk (CS) as plant aggregate. Three different sizes of corn stalks were employed to produce the concrete. The effect of type of binder and size of corn stalk on the hygrothermal, mechanical and microstructure properties of corn stalk concrete were studied. The results revealed that MPC-CS concrete exhibited higher sorption and water vapor permeability as compared to OPC-CS concrete and both binders showed excellent thermal performance. The size of corn stalk was more influential in changing the water absorption, capillary saturation, and thermal properties. Whereas, MPC-CS concrete showed better mechanical performance as compared to OPC-CS concrete. Microstructure analysis of concrete showed that MPC binder had superior cohesion properties and compatibility with corn stalk as compared to OPC binder, which also validated the mechanical results.

Drying shrinkage of alkali-activated cements: effect of humidity and curing temperature

Alkali-activated concrete (AAC) is a recycled and low-CO2 alternative to ordinary portland cement (OPC) concrete. One challenge with AACs is their propensity to shrinkage and cracking. In this work, drying shrinkage of four different structural-grade AACs was studied, namely: an activated class F fly ash, an activated slag, and two activated fly ash/slag blends. All four binders and a control OPC binder had the same (liquid/solid)vol and initial porosity. Drying shrinkage and mass loss of paste prisms were monitored as a function of time, relative humidity (RH), and the preceding moist-curing temperature. The results show that all AACs moist cured at 23 °C (73 °F) showed high shrinkage in comparison with the OPC binder and the shrinkage increased with higher proportions of fly ash. Steam curing at 60 °C (140 °F) substantially reduced the shrinkage of fly ash-rich mixtures, but it was less effective for slag-rich mixtures. Slag mixtures showed a significant time dependent response (creep), where the pastes continued to shrink after their mass loss had reached equilibrium.

Monitoring the Carbonation-Induced Microcracking in Alkali-Activated Slag (AAS) by Nonlinear Resonant Acoustic Spectroscopy (NRAS)

Abstract This article presents the potential of nonlinear resonant acoustic spectroscopy (NRAS) for noninvasive monitoring of the carbonation progress in alkali-activated slag (AAS) mortars. In the search for sustainable concrete, AAS has emerged as a potential substitute for ordinary portland cement binder. However, carbonation is reported to be an important durability concern for AAS due to the absence of portlandite. In this study, the correspondence between the physical properties and microstructural evolution of sodium silicate-activated slag (SS-AS) and sodium hydroxide-activated slag (SH-AS) mortars were studied over the full course of accelerated carbonation. The measured properties include the following: compressive strength, carbonation depth, porosity, pore size distribution, and phase assemblage. In addition, NRAS was used to track the changes in materials stiffness (linear resonance frequency) and hysteretic nonlinearity (amplitude dependency of resonance frequency). Scanning electron microscopy (SEM) images and porosimetry results showed the formation of microcracks and increased micrometer porosity in a carbonated AAS binder caused by calcium aluminum silicate hydrate decalcification; the cracking was more severe in the SS-AS than in the SH-AS. The NRAS results revealed a close correspondence between the observed microscopic changes in the samples and measured macroscopic test parameters, indicating the potential of acoustic techniques for monitoring the advancement of carbonation fronts in AAS mortars.