Effect of water curing conditions on the hydration degree and compressive strengths of fly ash–cement paste

This paper explains the effect of water curing condition on compressive strengths of fly ash–cement paste by quantitative data of hydration degree. Hydration of fly ash–cement paste was estimated by Rietveld analysis and selective dissolution. The result shows that the hydration degree of belite is affected by water curing conditions, more so than that of fly ash and alite. Fly ash still continues to hydrate even without an extra, external supply of water. The strong dependence of fly ash–cement concrete on curing conditions does not come from the hydration degree of fly ash, but rather comes from the hydration degree of cement, especially belite. When the water to binder ratio is low enough, the hydration of cement plus small hydration of fly ash are considered to be enough for adequate compressive strength at the beginning. Then, compressive strength of fly ash–cement paste becomes less sensitive to the water curing period.     

Fluid transport in high volume fly ash mixtures with and without internal curing

The transport of fluid and ions in concrete mixtures is central to many aspects of concrete deterioration. As a result, transport properties are frequently measured as an indication of the durability that a concrete mixture may be expected to have. This paper is the second in a series investigating the performance of high volume fly ash (HVFA) mixtures with low water-to-cementitious ratios (w/cm) that are internally cured. While the first paper focused on strength and shrinkage, this paper presents the evaluation of the transport properties of these mixtures. Specifically, the paper presents results from: rapid chloride migration (RCM), rapid chloride penetration test (RCPT), apparent chloride diffusion coefficient, surface electrical resistivity, and water absorption. The test matrix consisted of mortar samples with two levels of class C fly ash replacement (40% and 60% by volume) with and without internal curing provided with pre-wetted lightweight fine aggregates (LWA). These mixtures are compared to plain ordinary portland cement (OPC) mortars. The results indicate that HVFA mixtures with and without internal curing provide benefits in terms of reduced transport coefficients compared to the OPC mixtures.     

Effect of internal curing by using superabsorbent polymers (SAP) on autogenous shrinkage and other properties of a high-performance fine-grained concrete: results of a RILEM round-robin test

The article presents the results of a round-robin test performed by 13 international research groups (representing fifteen institutions) in the framework of the activities of the RILEM Technical Committee 225-SAP “Applications of Superabsorbent Polymers in Concrete Construction”. Two commercially available SAP materials were used for internal curing of a high-performance, fine-grained concrete in combination with the addition of extra water. The concrete had the same mix composition in all laboratories involved but was composed of local materials. All found a considerable decrease in autogenous shrinkage attributable to internal curing. Also, with regard to the shrinkage-mitigating effect of both particular SAP materials, the results were consistent. This demonstrates that internal curing using SAP is a robust approach, working independently of some variations in the concretes’ raw materials, production process, or measuring technique. Furthermore, the effects of internal curing on other properties of concrete in its fresh and hardened states were investigated. These are consistent as well and expand considerably the existing data basis on properties of concrete materials containing SAP.     

高吸水树脂对混凝土水化及强度的影响

高吸水树脂(Super-absorbent polymer,SAP)作为混凝土内养护材料可有效抑制混凝土自收缩,提高混凝土抗裂性,但其对混凝土是否具有负面影响有待研究。利用XRD和DTA-TG研究了不同SAP掺量净浆在不同养护龄期的水化产物量,并测试其抗压强度,定量分析高吸水树脂对混凝土水化和强度的影响。实验结果表明:掺加SAP会延缓混凝土早期(0~7d)的水化反应,降低混凝土的抗压强度,但对混凝土中后期(7~28d)水化的进行及强度发展的影响不大。当高吸水树脂的掺量为1kg/m~3(占胶凝材料的质量分数为0.2%)和1.5kg/m~3(占胶凝材料的质量分数为0.3%)时,混凝土28d抗压强度可达基准组的100%和96%,56d抗压强度可达基准组的107%和96%。针对C50混凝土,推荐掺量为1kg/m~3。     

Hydration kinetics of high-performance cementitious systems under different curing conditions

Curing plays an essential role in the modern concrete technology, since it has a crucial effect on the development of concrete properties. High-performance cementitious systems are especially sensitive to the applied curing methods because of self-desiccation and high sensitivity to early-age cracking. Thus, it is of particular interest to compare the efficiency of internal curing and traditional curing techniques such as sealing and water ponding. In this study, the efficiency of different types of curing was estimated by means of isothermal calorimetry. Four different water to cement (w/c) ratios in the range of 0.21–0.45 and four types of curing were studied, including sealing, water ponding with different amount of water, internal curing by saturated lightweight aggregate and super-absorbent polymer. The hydration degree was determined using heat of hydration data. Compressive strength of the tested specimens was measured and analyzed. The results indicate that efficiency of different types of curing strongly depends on w/c ratio.     

Chloride resistance of concrete and its binding capacity – Comparison between experimental results and thermodynamic modeling

The chloride resistance of concrete mixtures produced with different binders and water-to-binder ratios is determined by three different methods (natural chloride diffusion, accelerated chloride migration and conductivity measurement). The influence of mix design and type of binder are evaluated and related to porosity. The effect of chloride binding on chloride resistance is assessed by thermodynamic modeling and compared with chloride content measured with acid and water extraction.Chloride resistance depends on the type of binder and on water-to-binder ratio. Chloride content measurements and thermodynamic modeling both show that chloride binding is strongly related to the hydration degree of the cement and of the mineral admixtures. However, the decisive parameter for chloride resistance in all the tests is the permeability while the influence of chloride binding is less important.     

NMR relaxometry during internal curing of Portland cements by lightweight aggregates

The hydration of an ordinary Portland cement of low water-to-cement ratio under the influence of internal curing by addition of water-saturated lightweight aggregates (LWA) is investigated by non-destructive low-field NMR relaxometry. The methodology, which was recently developed to investigate internal curing by addition of a water-saturated polyelectrolyte gel, is applied to follow the transition of water from the mineral aggregates into hydrating cement paste and to detect the time dependence of the water consumption by the hydration reaction. By the changes of the transverse relaxation times of the physically bound water, the compaction of the pore space of the hardening cement is estimated qualitatively. The water retention potential of two types of LWA and the temporal moisture requirement of the cement during internal curing are determined without and with superplasticizer.     

Recommendation of RILEM TC 260-RSC: using superabsorbent polymers (SAP) to mitigate autogenous shrinkage

This recommendation is devoted to the use of superabsorbent polymers (SAP) in mitigating autogenous shrinkage of concrete and has been prepared by the working group acting within RILEM TC 260-RSC. The recommended procedure for designing mix compositions of concrete with SAP is given. Dry SAP particles of small size should be added to concrete along with additional mixing water that SAP absorb upon mixing. The SAP particles release water during hardening of concrete to compensate for chemical shrinkage and consequently reduce autogenous shrinkage. The procedure for designing mix composition is based on finding a trade-off between mitigation of autogenous shrinkage and possible negative effects on concrete properties (e.g., mechanical properties, workability). A theoretical guideline is provided based on compensating the volume of chemical shrinkage with (additional) internal curing water to be absorbed by the SAP and based on the measured absorption capacity of the SAP.     

Internal curing of concrete using lightweight aggregates

Internal cured concrete (ICC) has been recently used in the local and international construction markets. ICC contains surplus amount of water to compensate the shrinkage of the mix and the volumetric changes which result in early-age cracking of concrete. Concrete cracking is a direct result of the shrinkage of the water–cement paste during early stages of the hydration process and continues for a significant amount of time during the life span of the concrete section. Early-stage shrinkage, prior to the concrete hardening, is associated with volumetric changes, until final setting is achieved. Afterward, the reduction in cement paste particle size results in increased voids within the concrete structure. These voids result in increased permeability, additional sulfate and chloride attacks on steel reinforcement, and internal tensile stresses in concrete, which result in significant cracking. ICC uses the additional water added to the mix in counteracting the reduced volume of the concrete. Several techniques are used for internal curing (IC). In this research, water-saturated lightweight aggregates (LWAs) are used in partial replacement of normal weight aggregate as a source of additional water. LWA is submerged in water prior to concrete mixing to absorb a significant amount of water, which is stored within the LWA particles. Once added to the mix, the water is gradually desorbed and compensates the water losses during hydration. Hence, it counteracts the shrinkage induced. Different ICC mixes are developed in this research using two different sizes of LWA, and supplementary binding materials are used to improve compressive strength. ICC compressive strength and reduced shrinkage attained are presented. ICC mixes developed in this research can be successfully used in pouring highway segments and bridge decks with lower cracks and reduced life cycle cost due to reduced maintenance.     

Effect of further water curing on compressive strength and microstructure of CO2-cured concrete

This study aims to investigate the effects of further water curing on the compressive strength and microstructure of CO₂-cured concrete. The results showed that concrete with a residual w/c ratio of 0.25 showed the most rapid strength development rate upon further water curing due to hydration of uncarbonated cement particles. Thermogravimetric, IR-spectrophotometric and scanning electron microscope examinations indicated that further hydration of the cement particles could form C-S-H gel and ettringite crystals. The results showed that the calcite formed during the initial CO₂ curing was consumed during the further hydration of C₃A, and produced calcium monocarbonaluminate hydrate. Also, Ca(OH)₂ was not detected due to its reaction with the formed silica gel. Mercury intrusion porosimetry test results indicated that the porosity and pore size of the CO₂ cured mortar decreased further after water curing.     

Effects of distribution of lightweight aggregates on internal curing of concrete

This study investigates the effects of spatial distribution of lightweight aggregates (LWAs) on internal curing of concrete. As replacements for normal aggregates, different sizes and amounts of natural pumice LWAs were used as water reservoirs to provide internal curing in mitigating autogenous deformation. Water in the pre-soaked LWAs flows into cement paste during hydration and provides internal curing to counteract the RH loss due to self-desiccation of binding paste. The results show that variations in the autogenous strain of concrete can be evaluated in terms of LWA–LWA proximity. The protected paste volume approach, previously used for air-entrained concrete, is applied to calculate the internally-cured volume of paste. The results show that the experimental rate of mitigation of autogenous strain for different series of concrete specimens, with respect to the reference concrete, gave the best-fitted values at water flow distance of 1 mm. The results indicate that the protected paste volume in internal curing can be determined by calculating the water-entrained volume using image analysis.     

Influence of aggregate content on the migration coefficient of concrete materials using electrochemical method

In order to investigate the effect of coarse aggregate content on the chloride ion migration coefficient of concrete, specimens with different coarse aggregate volume fractions and two water/cement (w/c) ratios of mortar matrix were used. The chloride ion migration coefficient of concrete is obtained using the electrochemical technique to accelerate chloride ion migration in cement-based material and the experimental results were plotted as a function of the fine aggregate volume fraction. The results are analyzed comparing experimental results and theoretical models that represent the concretes as three-phase composite materials. The three phases are the mortar matrix, the coarse aggregate, and the interfacial transition zone between the two. The chloride ion migration coefficient is used to assess the dilution, tortuosity, interfacial transition zone (ITZ) and, percolation effects of coarse aggregate in concrete. It appears that the dilution and tortuosity effects are a dominant factor affecting the chloride ion migration coefficient of concrete in the low volume fraction of coarse aggregate. As the volume fraction of coarse aggregate increases to 40 and 20% in concrete of w/c ratio 0.35 and 0.45, respectively, the ITZ with percolation effects are significantly.     

Evaluation of compressive strength of hardening silica fume blended concrete

Silica fume (SF) is a byproduct of induction arc furnaces and has long been used as a mineral admixture to produce high-strength and high-performance concrete. Owing to the pozzolanic reaction between calcium hydroxide and SF, compared with Portland cement, the hydration of concrete containing SF is much more complex. In this paper, by considering the production of calcium hydroxide in cement hydration and its consumption in the pozzolanic reaction, a numerical model is proposed to simulate the hydration of concrete containing SF. The degree of hydration of cement and degree of reaction of SF are obtained as accompanied results from the proposed hydration model. Furthermore, on the basis of the volume stoichiometries, mixing proportions and the degree of reactions of cement and SF, the gel–space ratio of hydrating blended concrete is calculated. Finally, the development of compressive strength of SF blended concrete is evaluated through Powers’ strength theory considering the contributions of cement hydration and SF reaction. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and SF substitution ratios.     

Shrinkage characteristics of heat-treated ultra-high performance concrete and its mitigation using superabsorbent polymer based internal curing method

The shrinkage and cracking risk of heat-treated ultra-high performance concrete (UHPC) can be mitigated by using the superabsorbent polymer (SAP)-based internal curing method. The heat treatment (HT) accelerates the hydration reaction and resulting self-desiccation of UHPC; consequently, the UHPC experiences severe shrinkage during the HT. This study experimentally demonstrates that the shrinkage is effectively resolved by adopting the SAP-based internal curing method during the HT period as well as early-ages. This method also reduces the strain rate resulting from dimensional change, without showing an increase in drying shrinkage. The accurately conducted experiments herein can help to better understand the shrinkage characteristics of heat-treated UHPC and broaden the application of various internal curing agents.     

Correlation between compressive strength and certain durability indices of plain and blended cement concretes

In this study, plain, silica fume and fly ash cement concrete specimens prepared with varying water to cementitious materials ratio and cementitious materials content were tested for compressive strength, water permeability, chloride permeability, and coefficient of chloride diffusion after 28 days of water curing. The data so developed were statistically analyzed to develop correlations between the compressive strength and the selected durability indices of concrete. Very good correlations were noted between the compressive strength and the selected durability indices, particularly chloride permeability and coefficient of chloride diffusion, irrespective of the mix design parameters. However, these correlations were observed to be dependent on the type of cement.     

An investigation into the feasibility of formulating ‘self-cure’ concrete

To achieve good cure, excessive evaporation of water from a freshly cast concrete surface should be prevented. Failure to do this will lead to the degree of cement hydration being lowered and the concrete developing unsatisfactory properties. Curing can be performed in a number of ways to ensure that an adequate amount of water is available for cement hydration to occur. However, it is not alway possible to cure concrete satisfactorily. This paper is concerned with achieving optimum cure of concrete without the need for applying external curing methods. The feasibility of curing concrete by adding water-soluble chemicals during mixing that reduce water evaporation in the set concrete, making it ‘self-curing’ is discussed. The chemicals' abilities to reduce evaporation from solution and to improve water retention in ordinary Portand cement was monitored by measuring weight-loss. x-Ray powder diffraction and thermogravimetry measurements were made to assess whether any improvement in water retention was matched by an increase in degree of cement hydration. Initial surface absorption tests and compressive strength measurements were made to determine surface permeability and strength development, respectively. The scanning electron microscope was used to determine the influence of the admixtures on cement paste microstructure. It was found that two of the chemicals studied had a significant ‘self-curing’ effect. One of these chemicals enhanced hydration further than simply by means of water retention. A possible explanation of this behaviour is given.     

Experimental observation of internal water curing of concrete

Internal water curing has a significant effect on concrete. In addition to affecting hydration and moisture distribution, it influences most concrete properties, such as strength, shrinkage, cracking, and durability. The following paper is an overview of experimental methods to study internal water curing of concrete and its consequences. The special techniques needed to study internal water curing are dealt with along with the consequences of this process. Examples of applications are given and new measuring techniques that may potentially be applied to this field are addressed.     

Potential applications of phase change materials in concrete technology

In internal curing, pre-wetted lightweight aggregates (LWA) serve as internal reservoirs to supply the extra water needed by the cementitious and pozzolanic components of the concrete during their hydration processes. Due to their porous nature and reasonably high absorption capacity, the LWA can also be filled with other materials, such as phase change materials (PCMs). In this paper, three potential applications of PCM-filled LWA in concrete technology are presented. In addition to the previously explored application of increasing the energy storage capacity of concrete in residential and commercial construction by using a PCM with a transition temperature near room temperature, applications for higher and lower temperature PCMs also exist. In the former case, a PCM can be used to reduce the temperature rise (and subsequent rate of temperature decrease) of a large concrete section during (semi)adiabatic curing, to minimize thermal cracking, etc. In the latter case, a PCM can perhaps reduce the number or intensity of freeze/thaw cycles experienced by a bridge deck or other concrete exposed to a winter environment. In this paper, these latter two applications are preliminarily explored from both experimental and modeling viewpoints.     

Transient plane source measurements of the thermal properties of hydrating cement pastes

A transient plane source measurement technique is applied to assessing the heat capacity and thermal conductivity of hydrating cement pastes in their fresh state and during the course of 28 d of hydration at 20°C. Variables investigated include water-to-cement mass ratio (w/c – 0.3 or 0.4) and curing conditions (sealed or saturated curing). The heat capacity data for the fresh cement pastes are compared to a simple law of mixtures, and analytical expressions are developed to estimate the heat capacity as a function of degree of hydration for the two curing conditions. The measured thermal conductivities of the fresh pastes along with the known thermal conductivity of water are used to estimate the thermal conductivity of the original cement powder via application of the Hashin-Shtrikman (H-S) bounds. Hydration is seen to have only a minor influence on the measured thermal conductivity. Extension of the law of mixtures for heat capacity and the H-S bounds for thermal conductivity to predicting the corresponding properties of concretes are discussed.     

Influence of internal curing and viscosity modifiers on resistance to sulfate attack

Sulfate attack is one of the common degradation mechanisms for concrete in severe environments. While various strategies for minimizing sulfate attack are well recognized, including using an ASTM C150 Type V cement, employing supplementary cementitious materials, and/or reducing water-to-cementitious materials ratio, this paper explores two new approaches for increasing a mortar’s resistance to sulfate attack. In internal curing, fine lightweight aggregates (LWAs) are pre-wetted to provide additional curing water to maximize cement hydration and enhance the microstructure of the interfacial transition zone. The concurrent reductions in connected porosity should contribute to a reduction in the transport rates of sulfate from the environment into the concrete, while the isolated pores present in the LWA may help to accommodate the formation of expansive degradation products, such as ettringite, without creating substantial stresses and subsequent cracking. In the second approach, previously verified for its efficacy to reduce chloride ingress, a viscosity modifier is added to the concrete mixture to increase the viscosity of the pore solution and thus slow down the ingress of sulfates from the environment. While each approach is observed to significantly reduce the measured expansion of mortar bars in standard ASTM C1012 testing, the best performance is observed when the two are combined together by pre-wetting the LWA with a 50:50 solution of the viscosity modifier in water. With the combined approach, the time for the mortar bars to reach a critical expansion level of 0.05 % was over 80 % longer than that measured for the control mortar specimens. The expansion measurements are supported by accompanying measurements of mortar bar mass and surface resistivity throughout their exposure to the sulfate solution, along with micro X-ray fluorescence imaging and X-ray microtomography analysis of specimens extracted from the mortar bars after 9 months of exposure to the sulfate solution.