Residual strength properties of sodium silicate alkali activated slag paste exposed to elevated temperatures

The residual compressive strength behavior of alkali activated slag paste (AASP) after temperature exposures up to 1,200°C was investigated. Strength loss of approximately 60% occurred between 100 and 200°C and a further strength loss in the order of 30% at 800°C. Total loss of strength occurred at 1,200°C. Thermogravimetric studies (TGA/DTG) verified AASP contained no Ca(OH)₂ which governs the chemical mechanism of strength loss for ordinary Portland cement (OPC) and blended slag cement pastes. However, the TGA results showed that AASP had a higher water loss than the other binders between 100 and 200°C and higher thermal shrinkage as indicated by the dilatometry studies. The high thermal shrinkage led to a differential thermal shrinkage gradient within the AASP and induced micro stresses and cracking which was more prominent for larger samples. Differential thermal shrinkage caused by the higher thermal shrinkage of the AAS material was concluded as the mechanism which gives lower residual strength in AASP compared to OPCP.     

Effect of polypropylene fibers on shrinkage and cracking of concretes

The effects of admixed polypropylene (PP) fibers on the drying shrinkage of hardened concrete are presented in this paper. Concrete mixtures made with Ordinary Portland cement (OPC) and OPC/Slag blended cements containing various volume fractions of PP fiber were tested. The results show small but consistently higher drying shrinkages in concretes incorporating PP fibers than that without fiber. The effect is more pronounced in slag concretes and in concretes cured for only 1 day. An attempt to explain this phenomenon was made by water loss, nitrogen adsorption, sorptivity and scanning electron microscopy tests on the same concretes. Additional moisture loss and porosity are proposed as possible reasons. The results of early-age restrained shrinkage tests on slag concretes show that PP fiber concrete had higher cracking tendency than the concrete without fiber. This was found to be due to higher shrinkage and elastic modulus of PP fiber concrete.     

Mechanical properties,drying and autogenous shrinkage of blast furnace slag activated with hydrated lime and gypsum

This article reports the characteristics of blast furnace slag (BFS) pastes activated with hydrated lime (5%) and hydrated lime (2%) plus gypsum (6%) in relation to compressive strength, shrinkage (autogenous and drying) and microstructure (porosity, hydrated products). The paste mixtures were characterized using powder X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and thermogravimetric analysis (TG/DTG). BSF activated with lime and gypsum (LG) results in larger amounts of ettringite when compared with BFS activated with lime (L). Although the porosities of the L and LG mixtures were about the same, there was a greater pore refinement for the BFS activated with lime, with an increase in mesopores volume with age. The presence of ettringite and the higher volumes of macropores cause the compressive strength of BSF activated with hydrated lime plus gypsum to be smaller than that of BFS activated with lime. For both chemical activators, compressive strength developed slowly at early ages. Autogenous and drying shrinkage were greater for the BFS activated with lime, believed to result from the more refined porous structure in comparison with the mixture activated with gypsum plus lime.     

Enhancing the performance of calcium sulfoaluminate blended cements with shrinkage reducing admixture or lightweight sand

This study investigates the effect of using shrinkage reducing admixture (SRA) or lightweight sand (LWS) on enhancing the performance of calcium sulfoaluminate (CSA) cement in combination with ordinary Portland cement (OPC). Of special interest is the efficacy of the SRA or LWS in modifying the expansion/shrinkage and compressive strength characteristics of OPC-CSA systems in the absence of adequate duration of water curing, which is critical for the expansive reaction of CSA cement and its ability to mitigate shrinkage. Hydration kinetics, autogenous and drying deformation, thermogravimetry, and scanning electron microscopy (SEM) are used to evaluate the effect of SRA or LWS on the performance of the OPC-CSA systems. Test results indicate that the OPC-CSA system can exhibit similar drying shrinkage to that of the plain OPC mixture when no moist curing is applied. In the presence of LWS or SRA, the OPC-CSA systems exhibited lower shrinkage or higher extent of expansion compared to the corresponding OPC-CSA mixture alone. This is attributed to delay of the drop in internal relative humidity and promoting hydration of the OPC-CSA system which can enhance the ettringite-generating potential of CSA cement. The use of LWS was found to be highly effective in enhancing compressive strength of OPC-CSA system. SEM results at 91 days confirm the higher density and lower porosity for the paste surrounding LWS particles compared to the corresponding mixture made without LWS. In the case of inadequate moist curing, the presence of LWS or SRA is shown to enhance the overall performance of OPC-CSA system. For a given overall desirability value of 0.65 determined by multi-objective optimization, the incorporation of 1% SRA or 10% LWS was found to enable the reduction the required period of moist curing from 6 days to 5 and 3 days, respectively.     

Effects of slag and cooling method on the progressive deterioration of concrete after exposure to elevated temperatures as in a fire event

In the present work OPC and OPC/slag concretes were exposed to elevated temperatures, 400 and 800°C. The critical temperature of 400°C has been reported for OPC paste. Above 400°C, the paste hydrate Ca(OH)₂ dehydrates into CaO causing the OPC paste to shrink and crack. After cooling and in the presence of air moisture, CaO rehydrates into Ca(OH)₂, resulting in disintegration due to re-expansion of OPC paste. Therefore, the present work assessed whether this also applies to OPC concretes. Two cooling methods were used: furnace and water cooling. Following the heat treatment/cooling method, compressive tests and Infrared (IR) spectroscopic studies were conducted. Results showed that after 400°C, water cooling caused all concrete, regardless of the type of blended cement binder, a further 20% loss in the residual strength. After 800°C, water cooling caused OPC concrete a further 14% loss while slag blends presented around 5% loss. IR indicated that the further loss observed in the OPC concrete is due to the accelerated CaO rehydration into Ca(OH)₂. Afterward, the non-wetted furnace cooled specimens were exposed to air moisture for one week, resulting in further strength loss of 13%. IR results suggested that slow rehydration of CaO occur with exposure to air moisture. In conclusion, water cooling caused more damage in OPC concrete, while the concrete that has not been wetted undergoes progressive deterioration. This indicates a need to monitor the non-wetted concrete after a fire event has occurred for potential further deterioration.     

Effect of metakaolin on creep and shrinkage of concrete

The effect of metakaolin (MK) on the creep and shrinkage of concrete mixes containing 0%, 5%, 10%, and 15% MK has been investigated. The results showed that the early age autogenous shrinkage measured from the time of initial set of the concrete was reduced with the inclusion of MK, but the long-term autogenous shrinkage measured from the age of 24 h was increased. At 5% replacement level, the effect of MK was to increase the total autogenous shrinkage considered from the time of initial set. While at replacement levels of 10% and 15%, it reduced the total autogenous shrinkage. The total shrinkage (autogenous plus drying shrinkage) measured from 24 h was reduced by the use of MK, while drying shrinkage was significantly less for the MK concretes than for the control concrete. The total creep, basic creep as well as drying creep were significantly reduced particularly at higher MK replacement levels. Compared with estimated values by the CEB 90 model, total creep of all concretes was overestimated, especially in the mixes containing the higher levels of MK. For basic creep, estimates for low levels of MK were acceptable but, for the higher levels, creep was overestimated.     

自密实混凝土圆环约束收缩试验研究

进行了密封与沿环外侧面干燥两种条件下的自密实混凝土圆环约束收缩试验,分别研究自密实混凝土试件在自生收缩单独作用下,以及在干燥与自生收缩共同作用下钢环应变随混凝土龄期的发展规律及开裂性能,并与普通混凝土进行比较。配合比参数包括粉煤灰掺量、粉煤灰和矿渣复掺掺量、水胶比。揭示了配合比参数在两种不同条件下对钢环应变和开裂龄期的影响规律,并提出以“标准化”的钢环应变曲线作为开裂趋势曲线,以开裂系数为0.95时的龄期为“名义开裂龄期”作为圆环试验中混凝土开裂指标的建议。研究结果还表明,同一配合比参数对干燥约束收缩与自生约束收缩的作用有可能一致,也有可能完全相反,因此有必要对两种收缩分别研究;该文提出的“开裂趋势曲线”和“名义开裂龄期”能综合混凝土开裂中“作用”和“抗力”两个方面的影响,从而对混凝土的开裂性能进行动态、定量、综合的评价。     

Early-age autogenous cracking of cementitious matrices: physico-chemical analysis and micro/macro investigations

High-performance cement-based materials, characterized by low water-to-cement (W/C) ratio and high cement content, are sensitive to early-age cracking because their autogenous shrinkage rate and magnitude are particularly high during this period. This article firstly presents experimental tools especially designed for the measurement of free and restrained autogenous shrinkage at early-age. Then, the results of a multi-parameter experimental study conducted on three different types of binder are analyzed. The physico-chemical deformations of cement pastes and mortars were measured from the very early-age up to several days in saturated and autogenous conditions to investigate the effects of binder, water-to-binder ratio, presence of aggregates and temperature on the driving-mechanisms leading to early-age autogenous cracking. Complementary tests such as hydration rate measurement and microscopic observations were also performed. Among the three binders used, the blast furnace slag cement shows higher chemical strain, for a given quantity of chemically-bound water, and higher early-age autogenous shrinkage. The presence of aggregates generates a local restraining effect of cement paste deformations, leading to the formation of microcracks in the surrounding cement paste. Ring test results reveal that the first through crack of cement pastes systematically appears for maximal internal stress values lower than the material tensile strength, estimated with three-point flexural tests. This phenomenon may be due to diffuse damage of the cementitious matrix, whose deformations are partially restrained.     

Physico-chemical deformations of solidifying cementitious systems: multiscale modelling

At early stages of hydration and in autogenous conditions (no mass transfer with the outside), solidifying cementitious systems exhibit dimensional variations following two main processes: Le Chatelier contraction (also called chemical shrinkage) and self-desiccation shrinkage causing autogenous shrinkage. Chemical shrinkage results from the difference between the specific volumes of reactants (anhydrous cement and water) and hydration products. Early-age autogenous shrinkage is generally attributed to the development of a negative capillary pressure in the porous network related to the water consumption by the hydration reactions. If restrained, deformations associated to these shrinkages can induce the development of internal stresses high enough to generate cracking of the hardening material. The purpose of this study is to propose a multiscale approach to model the rate of self-desiccation shrinkage of cementitious materials at very early-age, between 0 and 48 h. Within the first hours, Le Chatelier contraction is computed from a formulation suggested in a later work which is based on the chemical equations of hydration and the specific volume of each phase. Then, when the setting of the cement paste takes place, the autogenous shrinkage is calculated according to the evolution of the capillary pressure and the stiffness of the cement paste. The stiffness is calculated by applying a classical homogenization method. Computed results are discussed and analyzed. Good agreements between experiments and simulations are achieved and a sensitivity study is performed to assess the influence of the cement fineness and the aggregate volume fraction on early-age autogenous strain.     

Effect of gypsum on free and restrained shrinkage behaviour of slag-concretes subjected to various curing conditions

The effect of gypsum in slag-blended cement on free and restrained shrinkage of concrete subjected to various curing conditions is presented in this paper. Added gypsum in slag-blended cements was found to increase the autogenous shrinkage of concrete up to 56 days. However, added gypsum caused small reduction in the long-term shrinkage when the concrete was exposed to drying. Slag concretes with 3% added gypsum content, when exposed to drying at the age of 24 h, exhibited more cracking tendency than comparable concrete with 0% added gypsum. This is attributed to the increased shrinkage evolution of slag concrete with 3% gypsum content at early ages. However, if moist cured for 7 days, increasing the amount of gypsum from 3 to 5% in slag-blended cement reduced the cracking tendency. It is concluded that the beneficial effect of increasing gypsum in reducing cracking tendency in slag concrete is only favourable if moist cured for 7 days.     

Understanding the drying shrinkage performance of alkali-activated slag mortars

In this work, drying shrinkage of four alkali-activated slag (AAS) mortars, prepared using various types/dosages of activator, was characterized at four different levels of relative humidity (RH) and two drying regimes (i.e. direct and step-wise drying). The results show that drying shrinkage values of AAS are significantly dependent on the drying rate, as AAS shrinks more when the RH is decreased gradually, instead of directly. At high RH, the drying shrinkage of AAS exhibits a considerable visco-elastic/visco-plastic behavior, in comparison to ordinary portland cement (OPC). It is concluded that the cause of high-magnitude shrinkage in AAS mortar is due to the high visco-elastic/visco-plastic compliance (low creep modulus) of its solid skeleton. Furthermore, the activator affects the shrinkage behaviors of AAS by influencing the pore structure and mechanical properties.     

Cracking in cement paste induced by autogenous shrinkage

Detection and quantification of microcracks caused by restrained autogenous shrinkage in high-performance concrete is a difficult task. Available techniques either lack the required resolution or may produce additional cracks that are indistinguishable from the original ones. A recently developed technique allows identification of microcracks while avoiding artefacts induced by unwanted restraint, drying, or temperature variations during sample preparation. Small cylindrical samples of cement paste are cast with steel rods of different diameters in their centre. The rods restrain the autogenous shrinkage of the paste and may cause crack formation. The crack pattern is identified by impregnation with gallium and analyzed by optical and scanning electron microscopy. In this study, a non-linear numerical analysis of the samples was performed. Autogenous strain, elastic modulus, fracture energy, and creep as a function of hydration time were used as inputs in the analysis. The experimental results and the numerical analysis showed that samples with larger steel rods had the highest probability of developing microcracks. In addition, the pattern and the width of the observed microcracks showed good agreement with the simulation results.     

Shrinkage and restrained shrinkage cracking of self-compacting concrete compared to conventionally vibrated concrete

Self-compacting concrete (SCC) used in Switzerland contains about 80 l/m³ more volume of paste than conventionally vibrated concrete (CVC). Consequently, there are some systematic differences in the properties of the hardened concrete. Normally, shrinkage of SCC is higher than shrinkage of CVC. Therefore, risk of cracking in case of restrained deformations can be increased for SCC. In this study shrinkage of thirteen different SCC mixtures using volume of paste, water content, type of binder, grain size distribution or content of shrinkage reducing admixture (SRA) as variables was compared with shrinkage of three different CVC mixtures with constant volume of paste but variable w/b. Furthermore, the risk of cracking of the different SCC- and CVC-mixtures in restrained conditions was studied under constant and varying curing conditions. The results show that shrinkage is mainly depending on volume of paste. Due to the higher volume of paste, SCC displayed higher shrinkage than CVC. Adding an SRA was the only measure to reduce shrinkage of SCC to values of CVC. Restrained shrinkage cracking is depending on shrinkage rate, mechanical properties and drying velocity. For slow shrinkage stress development, cracking risk of SCC can be lower compared to CVC despite the higher shrinkage rate.     

Effects of gypsum and phosphoric acid on the properties of sodium silicate-based alkali-activated slag pastes

In this paper, the effects of gypsum and phosphoric acid on the properties of sodium silicate-based alkali-activated slag paste were examined. Alkali-activated slag is recognized as a good performing binder. However, it suffers from fast setting and drying shrinkage. Phosphoric acid has been used as a retarder and gypsum has been used as a drying-shrinkage inhibitor in previous studies. However, it is not known what will happen if one adds both of them to alkali-activated slag. In this study, the effects of gypsum and/or phosphoric acid on the compressive strength development, setting time and drying shrinkage of alkali-activated slag were examined. Experimental results indicated that adding gypsum shortened the setting time, and at the same age, the compressive strength reached a higher value when the amount of used gypsum is higher. Drying-shrinkage decreased when the amount of used gypsum increased; however, when 0.82 M phosphoric acid was added in the activator with gypsum the results were somewhat different. With added phosphoric acid, adding more gypsum shortened the setting time as well. The compressive strength of specimens with added gypsum was lower than that of control specimens under 28 days. Drying-shrinkage increased with phosphoric acid as the amount of used gypsum increased.     

NMR,XRD, IR and synchrotron NEXAFS spectroscopic studies of OPC and OPC/slag cement paste hydrates

This work aims to determine the fundamental similarities and/or differences between OPC and OPC/slag paste hydrates. OPC and 35% slag pastes are investigated using five techniques: ²⁹Si NMR, ²⁷Al NMR, X-ray diffraction (XRD), infrared (IR) and synchrotron near edge X-ray absorption fine structure (NEXAFS) spectroscopy. ²⁹Si NMR provides valuable information related to the formation of the C–S–H gel, the main hydrated phase of the cement paste. ²⁷Al NMR is a useful tool to characterize calcium aluminates and aluminate hydrates such as ettringite and monosulphate hydrate. XRD identifies polycrystalline phases of the hardened cement paste, including ettringite, monosulphate and CaOH₂. Vibrational frequencies in IR assist in identifying the silicate, sulphate and carbonate phases of the cement paste. As far as we are aware, Si K-edge NEXAFS has never been applied in cement research and its advantages and disadvantages are discussed. Using these techniques, a comparison between OPC and 35% slag paste hydrates is made, shedding light on differences in the amount and form of hydrated phases present, especially the absence of ettringite in the 35% slag paste.     

Influence of entrained air voids on pore water pressure and volume change of concrete before and during setting

The paper discusses the role of entrained air voids on the transformation from liquid to solid of cement based materials (paste, mortar, concrete). The discussion is based on pore water pressure and volume change measurements. Air pores seem to contribute to increased rate of autogenous shrinkage in the time before the “knee-point” (i.e. in the few hours before autogenous shrinkage rate becomes significantly lower than the chemical shrinkage rate), and an earlier appearance of the knee-point. Through setting and shortly after, air pores alter the pore water pressure evolution in that they act as a buffer, and thereby reduce the pressure decrease and the subsequent autogenous shrinkage, as well as friction against panels in slipforming. The influence on plastic shrinkage does not seem to be significant.     

Non-saturated ion diffusion in concrete – A new approach to evaluate conductivity measurements

Non-saturated ion diffusion properties of cementitious materials were evaluated in an experimental study. To assess these properties, resistivity measurements have been performed on mortars with different binders (ordinary Portland cement – OPC, OPC with 5% silica fume, 40% slag and 70% slag, respectively) and different water-to-binder ratios (w/b, 0.38 and 0.53). Specimens have been conditioned to eight different climates with relative humidity (RH) from 100% to 33% RH in order to assess an effective diffusion coefficient. The results from the resistivity measurements have been corrected for changes of the conductivity of the pore solution when drying to different degrees of saturation.The diffusion coefficients for Portland cement binders within the range 100–59% RH are presented. They showed that the diffusion coefficient of the mortar with high w/b ratio was higher at high RH, but at low RH the opposite trend was found. By comparing these results with the corresponding desorption isotherms, it is shown that the diffusion coefficient for the two w/b ratios have the same dependency on the degree of saturation.     

A Stochastic Multi-Scale Model for Prediction of the Autogenous Shrinkage Deformations of Early-age Concrete

Autogenous shrinkage is defined as the bulk deformation of a closed, isothermal, cement-based material system, which is not subjected to external forces. It is associated with the hydration process of the cement paste. From the viewpoint of engineering practice, autogenous shrinkage deformations result in an increase of tensile stresses, which may lead to cracking of early-age concrete. Since concrete is a multi-phase composite with different material compositions and microscopic configurations at different scales, autogenous shrinkage does not only depend on the hydration of the cement paste, but also on the mechanical properties of the constituents and of their distribution. In this paper, a stochastic multi-scale model for early-age concrete is presented, which focuses on the prediction of autogenous shrinkage deformations. In this model, concrete is divided into three different levels according to the requirement of separation of scales. These levels are the cement paste, the mortar, and the concrete. A specific representative volume element (RVE) for each scale is described by introducing stochastic parameters. Different scales are linked by means of the asymptotic expansion theory. A set of autogenous shrinkage experiments on the cement paste, the mortar, and the concrete is conducted and used for validation of the developed multi-scale model. Furthermore, the influence of the type and the volume fraction of the aggregate on autogenous shrinkage is studied. Besides, a combined optimum of fine and coarse aggregates is determined. The analysis results show that the proposed model can effectively estimate the autogenous shrinkage deformations of concrete at early-age by taking the influence of the material composition and configuration into consideration.     

Optimization of ternary cementitious mortar blends using factorial experimental plans

Producing cements incorporating high-volume replacement of ordinary portland cement (OPC) by recycled industrial by-products is perceived as the most promising venture for the cement and concrete industry to meet its environmental obligations. However, the two-component (binary) cements thus produced are often associated with shortcomings such as the need for extended moist-curing, increased use of chemical admixtures, low early age strength, increased cracking tendency due to drying shrinkage, and de-icing salt scaling problems. There is need for research to investigate whether high-volume replacement multi-component (ternary and quaternary) cements could be optimized with synergistic effects allowing component ingredients to compensate for any mutual shortcomings. This study uses factorial experimental plans to investigate the performance of OPC-silica fume (SF)-class F fly ash (FA) and OPC-SF-ground granulated blast furnace slag (GBFS) ternary cementitious blends. Response surfaces for the superplasticizer requirement to achieve a constant flow, setting time, drying shrinkage up to 112 days, compressive strength at 1, 7, 28 and 56 days, and for the sulfate expansion up to 9-months were obtained for up to 20%, 60%, and 60% replacement levels of OPC by SF, FA and GBFS, respectively. A multiparametric optimization is used to establish response surfaces for a desirability function, which is used to rate ternary cementitious blends. Results indicate that when rheological, mechanical, durability and cost requirements are combined; the use of costly mineral admixtures such as silica fume is not economic in ternary OPC-SF-FA or OPC-SF-GBFS blends beyond levels of about 3 to 5% Moreover, it is shown that the major hurdle for high-volume replacement of OPC with class F fly ash is compromising the early age performance. Results also indicate that a good quality high-fineness GBFS can be used at replacement levels of OPC up to 60% without major disadvantages.     

Self-healing ability of fly ash–cement systems

Concrete is susceptible to cracking due to both autogenous and drying shrinkage. Nevertheless, most of these types of cracks occur before 28 days. Because fly ash continues to hydrate after 28 days, it is likely that hydrated products from fly ash may modify microstructure, seal these cracks, and prolong the service life. This research investigates the self-healing ability of fly ash–cement paste. Compressive strength, porosity, chloride diffusion coefficients, hydration reactions and hydrated products were studied. The research focuses on behavior after 28 days. According to the experimental results, the fly ash–cement system has the self-healing ability for cracks that occur from shrinkage. The self-healing ability increased when the fraction of fly ash increased.