Establishment of a preconditioning regime for air permeability and sorptivity of alkali-activated slag concrete

This study reports an experimental investigation designed to assess the influence of near-surface moisture contents on permeation properties of alkali-activated slag concrete (AASC). Five different drying periods (5, 10, 15, 20 and 25 days) and three AASC and normal concretes with compressive strength grades ranging from C30 to C60 were considered. Assessment of moisture distribution was achieved using 100 mm diameter cores with drilled cavities. Results indicate that air permeability of AASC is very sensitive to the moisture content and its spatial distribution, especially at relative humidity above 65%. To control the influence of moisture on permeation testing, the recommendation of this paper is that AASC specimens should be dried in controlled conditions at 40 °C for 10 days prior to testing. It was also concluded from this study that AASC tends to perform less well, in terms of air permeability and sorptivity, than normal concrete for a given strength grade. This conclusion reinforces the need to further examine AASC properties prior to its widespread practical use.     

Performance of blended metakaolin/blastfurnace slag alkali-activated mortars

This paper studies the effect of silicate content on the mechanical and durability-related properties of metakaolin (MK) and metakaolin/blastfurnace slag (BFS) alkaline activated mortars. A reference mortar based on the alkaline activated MK was compared to 60/40 MK/BFS mortars containing different SiO₂/Na₂O molar ratios in the activator. The properties assessed were compressive strength, porosity (water saturation), porosity and pore size distribution by Mercury Intrusion Porosimetry (MIP) and water capillary sorption. The microstructure was assessed using SEM and x-ray computerized micro-tomography (μ-CT). Results show that the addition of BFS significantly alters the microstructure of alkali-activated mortars, promoting a reduction of porosity and capillary sorption. In addition, an optimum SiO₂/Na₂O molar ratio in the activator is required to produce better durability mortars, which however do not necessarily present the highest mechanical strength.     

Shrinkage characteristics of alkali-activated fly ash/slag paste and mortar at early ages

The purpose of this study is to investigate the shrinkage characteristics of alkali-activated fly ash/slag (henceforth simply AFS) and the factors affecting it. A series of tests were conducted to determine the chemical shrinkage, autogenous shrinkage and drying shrinkage. The microstructures and reaction products were also characterized through XRD and SEM/EDS analyses. An increase in the slag content from 10% to 30% resulted in a denser matrix and showed a higher Ca/Si ratio of C–N–A–S–H in the microstructure. Higher sodium silicate and slag contents in a mixture caused more chemical, autogenous, and drying shrinkage, but led to a higher compressive strength. From the test results, it can be concluded that the autogenous shrinkage of AFS mortar occurs mainly due to self-desiccation in hardened state rather than volume contraction by chemical shrinkage in fresh state. The AFS paste showed higher drying shrinkage than ordinary Portland cement (OPC), which may be caused by the higher mesopore volume of the AFS paste compared to that of OPC paste.     

Micromechanical properties of alkali-activated slag cement binders

An experimental investigation into the micromechanical properties of alkali-activated slag cement (AASC) binders was carried out using targeted and grid nanoindentation. The results of grid indentation techniques were deconvolved using Gaussian mixture modeling with Bayesian model selection to determine the appropriate number of component phases for the model. The information given by the resulting mixture models and from targeted indentation experiments was disseminated in the context of existing information about the composition and development of the microstructure in AASC binders. The microstructure of sodium silicate-activated slag cement contains only two components (ground mass gel and unreacted slag cement) upon microscopic examination, but indentation data suggest that it is much more complex and varied. The microstructure of sodium hydroxide-activated slag cement contains ground mass gel, unreacted slag cement, and an inner product ring surround the unreacted slag. The inner product is denser, harder, and stiffer than the surrounding product phases. The micromechanical properties in sodium hydroxide-activated slag cement are not affected by activator molarity; the macroscale strength is similarly unaffected. Conversely, the micromechanical properties of sodium silicate-activated slag show a slight improvement with increased silica modulus, while the macroscale strength shows a significant improvement. The macroscale improvement is likely due to the increased size of unreacted slag cement grains, which are shown to be very hard and stiff.     

Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution

An understanding of the rheological behaviour of OPC-based products has been widely studied, for it is essential to determining and predicting the fresh and hardened characteristics and properties of pastes, mortars and concretes. The rheology of alkali-activated material (AAM) systems has been much less intensely researched, however.The present study aimed to ascertain the effect of factors such as the nature and concentration of the alkaline activator on the rheological behaviour of alkali-activated slag (AAS) pastes, with a comparison between the rheological parameters and fluidity of these pastes to the same parameters in OPC. More specifically, the study explored how paste rheology was affected by the nature of the alkaline activator (NaOH, 50/50 wt% NaOH/Na₂CO₃ or waterglass – Wg), its concentration (3–5% Na₂CO₃ of slag weight) and, in the waterglass solution, the SiO₂/Na₂O ratio.The findings showed that AAS paste rheology is affected by the nature of the activator. The rheological behaviour in AAS pastes activated with NaOH alone or combined with Na₂CO₃ was similar to the rheology observed in OPC pastes, and fit the Bingham model. Conversely, the AAS pastes activated with waterglass fit the Herschel–Bulkley model and their rheology proved to depend on both the SiO₂/Na₂O ratio and the Na₂O concentration. Moreover, regardless of the activator used (NaOH, Na₂CO₃ or waterglass), an increase in Na₂O concentration implies a raise of shear stress.The formation of primary C–S–H gel in Wg–AAS and its effect on paste rheology were confirmed. Gel formation was likewise shown to be related to the SiO₂/Na₂O ratio and activator concentration.     

橡胶对水泥基材料干燥收缩性能的影响

为了考察橡胶增加水泥基材料干燥收缩量的机制,以橡胶水泥砂浆作为研究对象,采用毛细管张力理论分析了造成水泥砂浆干燥收缩的因素。使用压汞试验研究橡胶/水泥砂浆的孔结构,并进行了弹性模量和干燥收缩试验。研究结果表明,橡胶掺入会降低水泥砂浆的弹性模量,增加其孔隙率和干燥收缩量,且相同掺量条件下,小粒径橡胶的作用效果更明显。基于试验数据,考虑橡胶掺入对砂浆弹性模量的折减系数KE和橡胶掺入对毛细孔(孔径50nm的孔隙)数量的增加系数K_h,拟合了橡胶对水泥砂浆干燥收缩的影响参数δ_(mr)。     

Characterisation of ground hydrated Portland cement-based mortar as an additive to alkali-activated slag cement

The sustainable development of cement manufacturing requires extension of the raw material base, including large-tonnage waste. Hydrated mortar waste is a promising mineral resource for the production of Portland cements and alternative binders, such as alkali-activated slag cement. The influences of ground-hydrated mortar aged for 3 months on the properties of alkali-activated slag fresh and hardened pastes were performed. The results show that the properties are dependent on the concentration (2.5–60%), cement:sand ratio (1:1–3) and fineness (200–600 m²/kg) of the ground hydrated mortar; the alkali activator (sodium carbonate and sodium silicate); and the curing conditions (normal conditions and steam curing). The fresh paste properties that we considered in this study included the water requirement and the setting time; the hardened paste properties we considered were the water absorption, the density, and the compressive strength after 2, 7, 14, 28, 180 and 360 days of ageing. The ground hydrated mortar improved the early strength and the long-term strength of the alkali-activated slag paste and replaced the slag up to 50%. The factors that affecting the strength of the alkali-activated slag cement with ground hydrated mortar as an additive were, in order of influence, alkali activator type > curing conditions > cement:sand ratio > ground-hydrated mortar fineness.     

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.     

Creep and drying shrinkage of concrete containing GGBFS

Experiments were conducted on 150 × 600 mm cylindrical specimens to investigate creep and drying shrinkage of concrete containing ground granulated blast furnace slag (GGBFS). The creep strain was measured for 150 days under a constant sustained load. The creep strain recovery was measured for one subsequent month after the removal of the sustained load. The shrinkage strain was also measured for 180 days. The amount of cement replacement by GGBFS was 20%, 40% and 60% by weight of cement. The test results indicate that higher GGBFS percentage exhibits higher creep and shrinkage strains. At 150 days of sustained loading, the average creep coefficients of 20%, 40% and 60% GGBFS concrete are 16.3%, 33.3% and 55.2% higher than plain concrete. In the absence of a creep and shrinkage prediction model for GGBFS concrete, a modification factor is suggested for incorporating the effect of GGBFS proportion in the existing models. The available models for predicting creep and shrinkage strain of plain concrete are compared.     

The use of microfine cement to enhance the efficacy of carbon nanofibers with respect to drying shrinkage crack resistance of portland cement mortars

Significant research has recently been aimed at quantifying the effects of carbon nanofibers and carbon nanotubes in portland cement pastes and mortars. Such efforts have shown that mechanical properties can increase with low concentrations of carbon nanofibers but have marginal improvement or are negatively affected with high concentrations. The objective of this paper is to evaluate the use of a microfine cement to enhance the efficacy of carbon nanofibers in portland cement mortar with respect to cracking resistance via enabling higher nanofiber concentrations. Experiments are performed with concentrations of carbon nanofibers up to 3% by weight of cement using either Type I/II or microfine cement. The primary test implemented was a restrained ring drying shrinkage test; unrestrained drying shrinkage tests, elastic modulus tests, and scanning electron microscopy imaging were performed to provide supplemental data to explain the observations from the restrained ring drying shrinkage tests. It was found that Type I/II cement mortars either lost performance or had insignificant gains with respect to cracking resistance, and all Type I/II mortar mixtures had losses in stiffness with the addition of high concentrations of carbon nanofibers. In contrast, microfine cement mortars had increased shrinkage cracking resistance and no loss in stiffness with increasing amounts of carbon nanofibers (up to the 3% by weight of cement tested in this research). The microfine cement mortar with 3% carbon nanofibers by weight of cement delayed the experimentally measured time of cracking in the ring test by a factor of up to 3.89. The delay in visible cracking time was attributed to microcrack bridging by the carbon nanofibers as imaged by scanning electron microscopy.     

Experimental investigation of drying shrinkage cracking of composite mortars incorporating crushed tile fine aggregate

Drying shrinkage is generally classified as an important hardened concrete property. It expresses the strain occurring in hardened concrete due to the loss of water. During the drying process, free and absorbed water is lost from the concrete. When the drying shrinkage is restrained, cracks can occur, depending on the internal stresses in the concrete. The ingress of deleterious materials through these cracks can cause decrease in the compressive strength and the durability of concrete. In this study, being as a fine aggregate in mortars, crushed tile (CT) effect on drying shrinkage and drying shrinkage cracking is investigated. Thus, compressive and flexural strength, modulus of elasticity, and free and restrained drying shrinkage tests are conducted on mortar specimens produced with and without crushed tile fine aggregate. The ring test has been used in order to investigate the cracks induced by restrained drying shrinkage. In this way, free drying shrinkage strain, along with the number and development of drying shrinkage cracks, of the crushed tile fine aggregate mortar composites are quantified and observed.     

Effects of nano-TiO2 on strength,shrinkage and microstructure of alkali activated slag pastes

For alkali-activated slag (AAS), high drying shrinkage is an obstacle which impedes its application as a construction material. In this investigation, nano-TiO₂ was added to AAS, and its mechanical properties and shrinkage were tested to examine its effect on hardened alkali-activated slag paste (AASP). To understand the impact of nano-TiO₂ on AASP at micro scale, FTIR, MIP and SEM were carried out. Experimental results indicate that the addition of nano-TiO₂ to AAS enhances the mechanical strength, and decreases the shrinkage of AASP. FTIR and SEM results demonstrated that the addition of nano-TiO₂ into the AASP accelerates its hydration process, resulting in more hydration products and denser structure. MIP results showed that the addition of nano-TiO₂ reduces the total porosity of AASP and changes the pore structure. The porosity of 1.25–25 nm mesopores, which is believed to be responsible for the high shrinkage of AASP, is remarkably reduced due to the addition of nano-TiO₂.     

Creep and drying shrinkage characteristics of concrete produced with coarse recycled concrete aggregate

Laboratory tests are performed to investigate the effects of a new method of mixture proportioning on the creep and shrinkage characteristics of concrete made with recycled concrete aggregate (RCA). In this method, RCA is treated as a two component composite material consisting of residual mortar and natural aggregate; accordingly, when proportioning the concrete mixture, the relative amount and properties of each component are individually considered. The test variables include the mixture proportioning method, and the aggregate type. The results show that the amounts of creep and shrinkage in concretes made with coarse RCA, and proportioned by the new method, are comparable to, or even lower than, those in similar concretes made entirely with natural aggregates. Furthermore, it is demonstrated that by applying the proposed “residual mortar factor” to the existing ACI and CEB methods for calculating creep or shrinkage of conventional concrete, these methods could be also applied to predict the creep and shrinkage of RCA-concrete.     

3D full field study of drying shrinkage of foam concrete

Drying shrinkage (DS) of concrete is important. The graded and heterogeneous DS inside the concrete may lead to cracking and further deteriorate the mechanical and durability properties. To elaborate the drying gradient and deformation heterogeneity, the full field DS distributions of foam concrete have been studied using an expanded Digital Volume Correlation method, which has a high precision of 0.01 voxel (about 0.6 μm) in displacement. The effectiveness of DS in local sub-volume is verified from bulk shrinkage of the whole specimen. The DS gradient due to drying is clearly revealed, and DS heterogeneity in spatial domain and in frequency domain is identified. A full view of foam concrete's drying processes is built. At the middle drying stage, three different states exist simultaneously, especially a drying front arises with high drying shrinkage.     

Characterisation of pore structure development of alkali-activated slag cement during early hydration using electrical responses

This paper describes the results of a study investigating early age changes in pore structure of alkali-activated slag cement (AASC)-based paste. Capillary porosity, pore solution electrical conductivity and electrical resistivity of hardened paste samples were examined and the tortuosity determined using Archie's law. X-ray computed micro-tomography (X-ray μCT) and Scanning electron microscope (SEM) analysis were also carried out to explain conclusions based on electrical resistivity measurements. AASC pastes with 0.35 and 0.50 water-binder ratios (w/b) were tested at 3, 7, 14 and 28 days and benchmarked against Portland cement (PC) controls. Results indicated that for a given w/b, the electrical resistivity and capillary porosity of the AASC paste were lower than that of the PC control, whilst an opposite trend was observed for the pore solution conductivity, which is due to AASC paste's significantly higher ionic concentration.Further, capillary pores in AASC paste were found to be less tortuous than that in the PC control according to estimations using Archie's law and from the results of X-ray μCT and SEM analysis. In order to achieve comparable levels of tortuosity, therefore, AASC-based materials are likely to require longer periods of curing. The work confirms that the electrical resistivity measurement offers an effective way to investigate pore structure changes in AASC-based materials, despite threshold values differing significantly from PC controls due to intrinsic differences in pore solution composition and microstructure.     

Specific surface area of aggregate and its relation to concrete drying shrinkage

This paper deals with the influence of aggregate properties on the shrinkage of concrete during drying. The drying shrinkage strains of concretes with various types of aggregates were measured and their influences on the fundamental properties of the different types of aggregates were investigated. Furthermore, the specific surface areas (SSAs) of aggregates were obtained by the BET method using both nitrogen (N₂) and water vapour (H₂O). The SSAs determined by using H₂O exhibited higher values than those by using N₂. The drying shrinkage strains of concretes increased with the H₂O SSAs of the aggregates used. Our results suggest that the SSA determined by using H₂O is an effective index for evaluating the influence of the aggregate type on the drying shrinkage of concrete.     

Development of shrinkage limit specification for high performance concrete used in bridge decks

Early-age cracking of high performance concrete (HPC) structures, in particular bridge decks, results in additional maintenance costs, burden on serviceability, and reduced long-term performance and durability. The causes behind cracking in HPC are well known and documented in the existing literature. However, appropriate shrinkage limits and standard laboratory/field tests are not clearly established in either the technical literature or in specifications. The purpose of this research was to provide shrinkage threshold limits for specifications which allow proper criteria to ensure crack-free or highly cracking-resistant HPC. The restrained ring test (ASTM C1581) was used to identify the cracking potential of 14 different HPC mixtures. By comparing free shrinkage (ASTM C157, 75 × 75 × 285 mm specimen) and restrained shrinkage tests results, a free shrinkage limit of 450 microstrain at 28 days was proposed to ensure satisfactory cracking resistance.     

Shrinkage and strength of alkaline activated ground steel slag/ultrafine palm oil fuel ash pastes and mortars

This study evaluated the contributions of steel slag and activators ratio to the shrinkage of the alkali-activated ground steel slag (G)/ultrafine palm oil fuel ash (U) or AAGU pastes and mortars. The base materials were combined such that G/U+G varied from 0 to 0.8 (pastes) and 0–0.6 (mortars) with the use of 10M-NaOH_aq and Na₂SiO_3aq (Ms = SiO₂/Na₂O of 3.3) as activators whose ratios (Na₂SiO_3aq/10M NaOH_aq) were varied as 1.0/1.0 and 2.5/1.0. The findings revealed that steel slag reduced the AAGU shrinkage through pore-refinement, elimination of microcracks, and improvement in the microstructural density and strength. The changing of Na₂SiO₃/10NaOH ratio in the synthesis of AAGU products from 2.5 to 1.0 slightly reduced the shrinkage through the modification of amorphousity and nature of the products (C-A-S-H/C-S-H). The maximum 90-day slag-free AAGU paste and mortar shrinkages were 60.80 × 10³ με and 11.82 × 10³ με but reduced to 25.88 × 10³ and 2.71 × 10³ με, respectively as G/(U+G) = 0.4 in AAGU_0.4.     

Influence of clays on the shrinkage and cracking tendency of SCC

The influence of different types of clay on the shrinkage and cracking tendency of fly ash modified self-consolidating concrete (SCCF) for the application of slipform paving were investigated in this study. The mortar phase of each mix was tested for autogenous shrinkage, total free shrinkage under drying and restrained shrinkage cracking. The mechanical properties (flexural strength, compressive strength, and modulus) were studied to supplement the results of the shrinkage and cracking tests. The plain SCCF mix was compared against the clay-modified SCCF mixes, as well as conventional SCC and slipform concrete (SFC) mixes. The results showed that the very early-age autogenous shrinkage of SCCF mortar was increased by the addition of clays due to adsorption effects. The effects of the clays on total shrinkage under long-term drying were found to depend mainly on the pozzolanic reactivity, but these effects were very slight at low dosages of about 1% by mass of binder. The early-age cracking tendency was aggravated by the clays composed of purified magnesium alumino silicate and metakaolin, but little influenced by the clay composed of kaolinite, illite and silica. Overall, the SCC mixture modified with both fly ash and a small amount of clay showed comparable shrinkage and early-age cracking performances as conventional SFC.     

Influence of limestone content,fineness, and composition on the properties and microstructure of alkali-activated slag cement

The influence of the fineness, concentration, and chemico-mineralogical composition of limestone on the workability, reaction kinetics, compressive strength, microstructure, and binder gel characteristics of sodium carbonate–based waste-activated waste slag cement pastes was investigated in this work. Alkali-activated slag cements incorporated with limestone, containing 33–100% of calcite, at a content of up to 60% with a 28-day compressive strength of 26.2–48.8 MPa were proposed. The main reaction products of hardened alkali-activated cement pastes and those incorporated with limestone are CSH, CaCO₃, Na₂Ca(CO₃)₂·5H₂O, and Na₂CaSiO₄. “Physically active” limestone does not chemically react with the binder gel but it can improve the physical structure. The higher packing density of mixed cement, without an increase in the water demand, the satisfactory binding strength of limestone with the binder gel lead to the improvement in the physical structure and compressive strength of alkali-activated slag paste.