Due to the gradual diffusion of CO₂ under natural exposure, areas with varied degrees of carbonation exist at different depths from the surface of slag-rich cement paste. While extensive research has been dedicated to investigating the fully carbonated zone as identified by phenolphthalein spray, the transitional zone, located between the fully carbonated and the uncarbonated regions, has received comparatively less attention. This study thus aims to address this research gap by exploring its microstructural, micromechanical, and mineralogical properties. The results reveal that carbonation-induced damage extends beyond the fully carbonated zone as identified by phenolphthalein. Particularly in the transitional area close to the carbonated zone, nanoindentations results reveal that micromechanical properties of this area are even lower to that of the fully carbonated zone. In addition, mineralogical investigation suggest that the depth of carbonation stays within the range where slag-containing blends loses its green coloration. By comparing specimens with different slag composition, it was found that the depth of this faded green area can be an important indicator to assess the carbonation resistance of slag-containing blends.

Assessing ASR damage through the elastic modulus (E-mod) is complicated due to the non-linear behaviour of cracked concrete, which can introduce testing artifacts. To address this, acoustic emission (AE) monitoring was applied during cyclic compression tests. AE data was used to define a critical load level, i.e. the maximum load at which no additional cracking occurred. By tracking high peak frequency events, it was possible to prevent high-energy AE signals associated with cracking. Limiting the loading below this threshold minimized test-induced damage. The stabilized secant E-modulus was then determined from subsequent cycles. This method resulted in E-modulus values 5–10% higher than after conventional testing. The load threshold varied with damage degree and crack orientation and is considerably lower than the commonly used load level at 40% of the compressive strength. These findings highlight the importance of AE-guided, damage-controlled testing protocols for reliable concrete mechanical performance assessment.

The concrete recycling industry faces significant challenges due to the uncontrolled mixing of parent concretes with varying properties during demolition, resulting in inconsistent recycled concrete aggregate (RCA) quality and limiting its potential for use in new concrete production. Existing literature typically characterizes parent concrete solely based on compressive strength, neglecting other critical parameters. Consequently, RCA is often labelled as inherently heterogeneous, without fully considering the variability introduced by mixed-source demolition. This study introduces an novel protocol for systematic parent concrete characterization, combining an Artificial Intelligence (AI)-based segmentation approach with complementary techniques like polarized light and fluorescence microscopy (PFM). The proposed methodology quantifies critical properties of parent concrete, including water-to-cement (W/C) ratio, cement and aggregate content, air void content, and aggregate gradation. This enables a detailed evaluation of parent concrete variability, providing the basis for selective demolition strategies that reduce RCA heterogeneity and enhance the predictability of its properties. To illustrate its applicability, the protocol was applied to structural components of a Dutch viaduct, including prestressed beams, heavily reinforced columns, foundations, and abutment-wall. Results revealed significant differences in estimated water-to-cement ratios, ranging from 0.29 ± 0.03 in beams to 0.38 ± 0.03 in abutment walls, while hydration degrees varied between 0.80 ± 0.08 and 0.93 ± 0.02, indicating differences in cement maturity. Cement content ranged from 316 ± 11 kg/m³ in foundations to 390 ± 10 kg/m³ in beams, and air void content ranged significantly from 0.9 % in abutment walls to 4.3 % in foundations. Microstructural composition also differed substantially: paste volume ranged from 17 % to 27 %, and coarse aggregate content from 37 % to 52 % depending on the component. These quantitative differences confirm that structural concretes, even within the same structure, exhibit substantial internal variation. As such, uncontrolled demolition leads to the mixing of materials with fundamentally different properties—supporting the argument that RCA heterogeneity is not intrinsic, but largely a result of conventional demolition. This reinforces the value of the proposed protocol in identifying material variability and informing targeted demolition to enhance the predictability and uniformity of RCA.

Calcined clay plays a critical role in controlling the rheology of limestone-calcined clay cement (LC³). This study shows that the yield stress of LC³ mixtures with ultra-high volumes of calcined clay and limestone evolves significantly faster within the first 90 min than that of plain cement pastes at the same solid volume fraction. Two key mechanisms were identified: (1) calcined clay alters particle packing through its layered structure and micro-cavities, promoting physical water absorption; and (2) it enhances interparticle attractive forces via “bridge” formation (C-(A)-S-H bridges and colloidal attractive forces) between cement and calcined clay particles in the fresh state, accelerating rigidification. However, this bridging effect can be hindered by the addition of PCE-based superplasticizers.

Adding hydrated lime (CH) into blended cement incorporating high volume of Supplementary Cementitious Materials (SCMs) is a viable method to provide the necessary calcium hydroxide for the pozzolanic reaction, thereby improving the mechanical performance at later stages. However, the effects of relatively small dosages of CH on the rheological properties and resulting microstructure of limestone-calcined clay cement (LC3) remain unclear. This paper aims to investigate the influence of a small CH addition on the fresh and hardened properties of LC3 systems, in which Portland cement is largely replaced (80 wt%) by limestone and calcined clay. The results indicate that the additional CH notably reduces the water film thickness, leading to increased dynamic yield stress, plastic viscosity and re-flocculation. A delay in the elasticity development and static yield stress evolution within the first 1.5 h was observed with the addition of 2.5 wt% CH, attributed to the initial dissolution of CH, which is mitigated by using 10 wt% CH. Furthermore, additional CH accelerated early-age hydration and facilitated long-term pozzolanic reactions, resulting in the increased amount of C-(A)-S-H gel and AFm phases, and reduced porosities after 7 and 28 days. These chemical effects could well compensate the high air void content caused by the high viscosity, and therefore contributes to mortars with higher compressive strengths than plain LC3 at later ages.

Accurate evaluation of air-void content in hardened concrete is important for assessing durability and long-term performance. Traditional methods often require time-consuming surface preparation and depend on imaging techniques that are sensitive to surface texture. This study investigates the potential of a high-resolution tactile sensing approach for non-destructive void detection with minimal surface preparation. To understand the effect of surface roughness, void detection was first performed on paired concrete specimens with identical internal surfaces, while one was left unpolished and the other polished. Despite the presence of surface irregularities, the tactile sensor was still able to identify most of the voids larger than a defined threshold, demonstrating its effectiveness even under challenging surface conditions. In the second phase, the sensor’s void quantification performance was benchmarked on a polished specimen using a standardized digital microscopy-based method. The tactile system estimated porosity at 4.58% with an average void area of 0.013 mm², closely matching the reference digital microscopy-based results of 5.05% and 0.011 mm². These results highlight the sensor’s capability to produce consistent and quantitative measurements on both rough and smooth surfaces. The approach offers a practical alternative to traditional void analysis methods, with the potential to simplify inspection workflows and support future automation in concrete surface evaluation.

Sample preparation is of utmost importance for any microscopy and microstructural analysis. Correct preparation will allow accurate interpretation of microstructural features. A well-polished section is essential when scanning electron microscopy (SEM) is used in backscattering electron (BSE) mode and characteristic X-rays are to be quantified using an energy-dispersive spectroscopy (EDS) detector. However, obtaining a well-polished section, especially for cementitious materials containing aggregates, is considered to be challenging and requires experience. A sample preparation procedure consists of cutting, grinding and polishing. Undercutting of soft and brittle paste between harder aggregates can be overcome by vacuum epoxy impregnation offering mechanical support in the matrix. Furthermore, most of the attention during the sample preparation is given to the polishing of the sample. There is a wide range of suggestions on polishing steps, ranging from grain sizes, time and applied force; however, the final assessment of a polish surface is often subjective and qualitative. Therefore, a quantitative, reproducible guidance on the grinding steps, effect of experimental parameters and the influence of different grinding steps on the surface quality are required. In this paper, the influence of grinding was quantitatively evaluated by a digital microscope equipped with optical profilometry tools, through a step-wise procedure, including sample orientation, grinding time and the difference between cement paste and concrete. Throughout the grinding procedure, the surface profiles were determined after each grinding step. This showed the step-wise change in surface roughness and quality during the grinding procedure. Finally, the surface qualities were evaluated using optical and electron microscopy, which show the importance of the grinding/prepolishing steps during sample preparation.

Three-dimensional (3D) concrete printing necessitates a balance between various ingredients of the mix composition. This study investigated the effect of hydroxypropyl methylcellulose (HPMC) dosage at various aggregate volumes on fresh, rheological, and mechanical properties of 3D printable concrete (3DPC). Accordingly, 3DPC mixtures having three aggregate volumes, namely 44, 41, and 38 %, were produced at a constant water-to-binder ratio. For each aggregate volume, three HPMC dosages, namely 0, 0.14, and 0.28 % by weight of cement, were studied. A mini-slump flow table and a manual printing gun were used to assess the flow diameter and printability. Rheological properties were determined using a rotational rheometer and extrusion device. The buildability was assessed through green strength testing. Results showed a directly proportional relationship between HPMC dosage and fresh and rheological properties of these mixtures. At a constant w/c ratio, increasing the aggregate volume led to higher green strength and extrusion pressure at all piston-moving velocities. Overall, at an early age, the effect of HPMC dosage was more significant on the static yield stress of mixtures with lower paste volume while being more accentuated on the green strength for mixes with higher paste volume. The positive impact of increasing HPMC dosage on the green strength becomes insignificant at later ages. The effect of increasing HPMC dosage, however, was more pronounced on the extrudability of mixtures with higher paste volume by preserving their extrudability at later ages. Finally, HPMC addition led to strength losses of up to 28.63 and 32.7 % for flexural and compressive strength, respectively.

The use of building insulation materials is an effective measure to reduce building energy consumption. To improve the sustainability of insulation materials, desert sand (DS) was used to replace part of the binder, and rice husk ash (RHA) was incorporated to further improve the performance of foamed concrete. The fresh properties, strengths, thermal properties and thermal insulation function of DS-based foamed concrete (DSFC) were systematically investigated. The use of DS and RHA to replace part of Portland cement (PC) and fly ash reduces the flowability of the mixture when the water/binder (PC, fly ash, DS and RHA) ratio is constant. Although the incorporation of DS into foamed concrete increases its density and thermal conductivity, it improves the volume stability of the sample. The strength of specimen with DS decreases due to the low reactivity of DS, which also reduces the content of hydration products. Further incorporation of RHA not only improves the matrix strength by increasing the C-S-H content but also improves the pore structure of the DSFC by increasing the yield stress of the paste. The joint application of DS and RHA effectively reduces the heat storage coefficient and thermal inertia index of DSFC, which is beneficial to improve the thermal insulation capacity of buildings and reduce energy consumption. Incorporating DS and RHA can effectively improve the environmental and economic benefits of the foamed mixture, and the unit strength cost and carbon emission per cubic meter of the 5%–10% RHA-modified samples are reduced by 20.3%–39.1% and 20.2%–38.9%, respectively, compared with the DS35. This research provides a new approach and theoretical basis for building energy saving and external wall insulation.

The Stiffness Damage Test (SDT), a cyclic test in compression, is considered as a reliable tool for assessing concrete structures affected by ASR. Depending on the extent of ASR damage in concrete, loading levels up to 40% of the compressive strength may contribute to increasing internal damage during testing. Nevertheless, previous research found that no additional damage was induced by the SDT. This confirmed the non-destructive character of the SDT making it valid to determine the compressive strength on the same test specimens following the SDT. However, other research suggests that loading levels above 15% of the compressive strength could lead to load-induced damage in the first load cycle. The implication of the non-destructive character and the loading level of the SDT needs more attention, especially when testing anisotropically ASR-damaged concrete structures. This paper thus presents a critical evaluation of the non-destructive character of the SDT by utilizing Acoustic Emission (AE) measurements. The SDT was used to evaluate an ASR affected concrete structure after 60 years in use. Several cores from cantilever slabs were extracted enabling damage assessment of the concrete structure in use. AE allowed to measure crack occurrence with a higher accuracy. Therefore, the critical load level could be more accurately identified using AE. The magnitude of enhancing internal damage during the SDT is related to the extent of ASR. From this study it can be concluded that the non-destructive character of the stiffness damage test depends the critical load level in relation to the internal degree of damage, which can be determined by means of Acoustic Emission.

Municipal solid waste incineration (MSWI) bottom ash, due to its high mineral content, presents great potential as supplementary cementitious material (SCM). Weathering, also known as aging, is a treatment process commonly employed in waste management to minimize the risk of heavy metal leaching from MSWI bottom ash. Using weathered MSWI bottom ash to produce blended cement pastes is considered as a high-value-added and sustainable waste disposal solution. However, a critical challenge arises from the metallic aluminum (Al) in weathered MSWI bottom ash, which is known to induce detrimental effects such as volume expansion and strength loss of blended cement pastes. While most metallic Al in weathered MSWI bottom ash can be removed with eddy current separators in metal recovery plants, the residual metallic Al, owing to its small particle size, cannot be removed with the same technique. This study is dedicated to addressing this issue. An in-depth analysis was conducted on residual metallic Al embedded in weathered MSWI bottom ash particles, aiming to guide the removal of this metal. This analysis revealed that mechanical removal was the most suitable method for extracting metallic Al. The specific processes and mechanisms underlying this method were elucidated. After reducing metallic Al content in weathered MSWI bottom ash by 77 %, a significant improvement in the quality of blended cement pastes was observed. This work contributes to the broader adoption of mechanical treatments for removing residual metallic Al from weathered MSWI bottom ash and facilitates the application of treated ash as SCM.

Spalling is the main problem concerning safety in concrete structures under rapid heating. Despite being studied for nearly a century, its prediction remains a challenge. In addition, the main recommendations for its prevention are merely the addition of a fixed amount of polypropylene fibres, without accounting for mix design. This paper presents an experimental study focusing on the thermo-hygral mechanism contribution to spalling. The use of mercury intrusion porosimetry and X-ray microtomography for the determination of pore connectivity is considered a key parameter. The results of the experiment are used for the development and calibration of an analytical equation capable of predicting spalling in concrete samples. The equation is then validated using over 100 examples extracted from literature, achieving an accuracy of 91.2%. Based on the validation, the proposed equation is a step forward in the prediction of concrete spalling based on the mix design parameters.

Autogenous shrinkage may be a critical issue concerning the use of limestone-calcined clay-cement (LC3) in high-performance concrete and 3D printable cementitious materials, which have relatively low water to binder (W/B) ratio. Adding an internal curing agent, i.e., superabsorbent polymer (SAP), could be a viable solution in this context. However, employing SAP (without adding additional water) may also influence the fresh properties of LC3 composites by increasing yield stress and viscosity, which may be beneficial for 3D printability. Therefore, this study attempts to use SAP as a rheology modifying admixture with the aim of investigating the impact of SAP on flow behavior, structural build-up, hydration kinetics, compressive strength, and autogenous shrinkage of LC3 pastes with a fixed W/B (0.3). In addition, hydroxypropyl methylcellulose (a typical rheology/viscosity modifier in 3D printable cementitious materials) was also employed in two mixtures to compare their effects. Results show that adding SAP increases the dynamic yield stress and the apparent viscosity, as well as structural build-up and hydration, but decreases the compressive strength at 3, 7 and 28 days. Furthermore, using SAP (especially 0.2 wt% SAP) not only promotes the early-age expansion but also effectively mitigates the autogenous shrinkage of LC3 pastes for up to 7 days. Overall, the obtained results indicated that SAP could act as a promising rheology modifier for the development of 3D printable cementitious materials.

To understand the influence of slag chemistry on the carbonation resistance of slag-rich cement, this paper explored the carbonation characteristics of blended cement systems with different Al₂O₃ contents in slag through accelerated carbonation test. Irrespective of slag chemistry, three main CO₂ binding phases were identified during accelerated carbonation test, i.e. carbonated Ca-Al AFm phases (amorphous or nano-crystalline), carbonated hydrotalcite-like phase, and calcium carbonate (amorphous calcium carbonate, vaterite, and calcite). Additionally, it was noted that the classification employed for slag reactivity (based on slag chemistry) cannot be extended to predict carbonation resistance of slag-rich cement directly. The main challenge occurred for slag with high alumina content. The experimental results showed that Al₂O₃-rich slag exhibited a high reactivity and can be considered as a reactive component in the blended mixture; however, it did not contribute to carbonation resistance of the mixture. Especially for CO₂ binding capacity, it was similar for systems with varied alumina content in slag (from 3.69 to 18.19 wt.%) in the completely carbonated area.

The implementation of extrusion-based 3D concrete printing (3DCP) in large-scale constructions is currently limited by concerns regarding rheology control and the sustainability of this process. To address these issues, this study presents an approach to develop limestone-calcined clay-based cementitious (LC3) materials accelerated by Ca(NO₃)₂ solution in an inline static mixer-based 3DCP setup. Using this approach, a printable mixture containing only about 275 kg/m³ of Portland cement was formulated that can exhibit a good buildability performance and a 28-day compressive strength of over 30 MPa. Additionally, the effects of adding Ca(NO₃)₂ solution on the initial setting time, structural build-up, inline buildability, early-age hydration, and compressive strength of LC3 materials were investigated and discussed. Results show that the addition of Ca(NO₃)₂ solution improved the buildability and accelerated initial setting as well as the structuration due to the promoted ettringite precipitation and C–S–H nucleation. Furthermore, compressive strength at 7 and 28 days was improved through increasing the Ca(NO₃)₂ dosage, which can be attributed to the formation of NO₃-AFm and the increase in the amount of C–S–H gels.

Formulation of quaternary blended system containing ordinary Portland cement or clinker, slag, limestone and calcined clay (LC2) appeared to be a viable approach to developing low-clinker cements without severely sacrificing mechanical performance at later ages. This paper investigates the effect of two material parameters, i.e., LC2-to-slag ratio and gypsum content, on fresh properties, hydration, and compressive strength of quaternary blended cement pastes (about 65 wt% of LC2 and slag in the binder). Results show that the increase in LC2 proportion decreased flowability and increased water retention capacity, yield stress, and plastic viscosity, as well as accelerated the evolution of stiffness with time (G′ growth). On the other hand, the addition of 2–4 wt% gypsum had little effect on most of the fresh properties. A new metric, the free water indicator, was proposed to describe the effect of free water content and the total specific surface area of binding materials. It correlated strongly with the growth of structural build-up metrics. Finally, adding gypsum delayed the aluminate peak and enhanced compressive strength only at 3 days, whereas increasing slag content reduced accumulated heat of hydration (7 days) but improved 28-day compressive strength. Therefore, adjusting LC2-to-slag ratio of the quaternary blended cement is a feasible way to meet requirements for fresh properties and compressive strength.

In 3D concrete printing, fast structuration is a prerequisite for ideal buildability. This paper aims to study the impact of inorganic additives, i.e., CaCl₂ and gypsum, on structural build-up and very early-age hydration of limestone-calcined clay-cement (LC3) pastes within the first 70–80 min. Results show that, increasing the dosage of CaCl₂ or gypsum can accelerate storage modulus G' and static yield stress evolution with time, as well as increase chemically bound water (H) content and total specific surface area (SSA_total). Furthermore, good correlations were found between G' and H content, as well as static yield stress and the ratio of free water content to SSA_total. The acceleration by CaCl₂ can be attributed to stimulating C₃S and C₃A hydration and promoting crystal formation, i.e., ettringite, portlandite, and Friedel's salt. Additionally, the increase in gypsum percentage led to a large amount of unreacted gypsum in the system, resulting in an increase in SSA_total.

The properties of slag-rich cement paste are fundamentally associated with slag chemistry. In the present research, 10 slags covering the common chemistry range, including eight synthetic slags of CaO-SiO2-Al2O3-MgO system and two commercial slags, were adopted to evaluate the influence of slag composition on the early (7 days) and later (3 months) age properties of blended paste. Mixture containing Al2O3-rich slag performed better at 7 days as it favored the formation of ettringite and/or monosulfate. The MgO-rich slag cement paste exhibited good properties at both early and later ages, and it effectively promoted the precipitation of Mg-Al hydrotalcite-like phase. It was also noted that the Mg/Al atomic ratio of hydrotalcite-like phase was positively related to the Mg/Al atomic ratio of slag itself. Conversely, with the increasing MgO content in slag, the Al/Si atomic ratio of C-S(A)-H gel phase decreased. High Al2O3 and/or MgO contents can compensate the negative effect of reduced CaO/SiO2 ratio at early age while not at later age. Overall, attention should be paid to aluminum- and sulfur-rich slags. These two elements in slag promoted the formation of ettringite and/or monosulfate at an early age; however, this positive effect disappeared at later ages.

This paper presents the influence of P₂O₅ incorporated in slag on the hydration characteristics of cement-slag system. It was found that the gradual addition of phosphorus oxide in slag did not change overall mineralogy of the hydration products. Except hydration retardation in the dormant stage, chemically bound water and portlandite contents, hydration degree of slag, and pore structure at all investigated ages were similar among cement-slag pastes with different P₂O₅ percentages. Furthermore, significantly higher amount of monosulfate was observed as the P₂O₅ content in slag increased. In addition, a higher Al/Si atomic ratio was measured in the C-S(A)-H gel phase formed in the cement matrix. However, similar Ca/Si atomic ratio of C-S(A)-H gel phase and Mg/Al atomic ratio of hydrotalcite-like phase were determined in all slag pastes, irrespective of the addition of P₂O_5. In contrast to magnesium ion which was retained within the original slag boundary, phosphorus ions could migrate into cement matrix. Therefore, P/Si atomic ratio of the C-S-H gel phase increased with the increasing phosphorus oxide content in slag, reaching up to ∼0.08.

In this paper, the authors characterized two types of zonation within slag rims in aged alkali-activated slag (AAS) systems through SEM and TEM technology. These two elemental zonation were closely related to the pore structure of AAS pastes, thus providing strong implication for the transport- and durability-related performance of AAS systems. The first type of zonation occurred in the rims of AAS specimens under sealed curing. It was found that lath-like hydrotalcite-like phase accumulated near the boundary while a generally homogeneous and foil-like C-(N-)A-S-H gel phases precipitated in the following sub-zone. When slag rims were thick enough, a new Mg-rich region occurred. The second type of zonation was noticed in the carbonated AAS pastes. For this kind of distribution pattern, C-(N-)A-S-H gel phases were observed near the boundary. Following, the accumulation of Mg and Ca occurred alternatively. Additionally, transformation mechanism between these two types of zonation was also proposed.