Internal relative humidity distribution in high-performance cement paste due to moisture diffusion and self-desiccation

This investigation was carried out to study internal relative humidity (IRH) distribution of cement paste made with different water / cement ratios (w / c) and mineral admixtures in isothermal drying conditions. IRH changes in cement paste resulting from self-desiccation and moisture diffusion, respectively, at different ages were studied. The change laws of IRH in cement paste resulting from combining moisture diffusion with self-desiccation were discussed. The results indicate that IRH reduction of cement paste with w / c higher than 0.4 is mainly affected by moisture diffusion. However, IRH reduction of cement paste with w / c no higher than 0.4 is controlled by both moisture diffusion and self-desiccation. With the decrease of w / c, IRH reduction of cement paste resulting from self-desiccation increases, and IRH reduction resulting from moisture diffusion decreases at a given age. IRH decrement of cement paste incorporated with silica fume and ground blast-furnace slag is higher than that of control paste. w / c and the distance to the exposed surface play a significant role in IRH change resulting from moisture diffusion in isothermal drying condition. Change laws of IRH in cement paste with silica fume due to moisture diffusion considering self-desiccation are different from those in cement paste without silica fume.     

预湿水泥浆颗粒内养护材料对混凝土早期自收缩及抗压强度的影响

制备不同粒径和水灰比的水泥浆颗粒作为低水灰比混凝土内养护材料.以最佳内养护水灰比原则,设计了使用三种水灰比分别为0.6、0.7和0.8的同粒径水泥浆颗粒等体积取代砂子的混凝土.研究了不同水灰比水泥浆颗粒对混凝土早期自收缩、抗压强度和内部微结构的影响.结果表明:颗粒吸水率与其水灰比正相关、与其粒径负相关;预湿水泥浆颗粒可显著降低混凝土早期的自收缩,颗粒水灰比越大,自收缩降低效果越明显;但是掺入水泥浆颗粒也会降低混凝土的抗压强度,颗粒水灰比越高抗压强度降低越多,应用中应优化选择预湿颗粒的水灰比;水泥浆颗粒作为高性能混凝土内养护材料,可改善微观界面的孔隙结构,提高界面的密实性,减少混凝土早期的收缩和开裂.     

Autogenous relative humidity change and autogenous shrinkage of high-performance cement pastes

In this paper, the effects of water to cementitious material ratio (w/cm), silica fume (SF) and ground blast-furnace slag (GBFS) on autogenous relative humidity (RH) change and autogenous shrinkage (AS) of high-performance cement pastes were studied. The mechanism of self-desiccation caused by mineral admixture and reduction of w/cm were studied by the parameters of mineral admixture self-desiccation-effect coefficient k and efficient w/cm r_e proposed. Furthermore, the relationship between autogenous RH and AS of high-performance paste was established. The results indicate that w/cm is a chief factor that affects autogenous RH change and AS of cement pastes. The lower the w/cm of paste is, the more reduction the autogenous RH and the increment of AS are. SF increases autogenous RH reduction and AS increment of cement paste at early ages, and GBFS increases autogenous RH reduction and AS increment at later ages. The effect of mineral admixtures on autogenous RH change of paste resulting from self-desiccation can be reflected effectively by the nonlinear equation with the parameters of k and r_e. There exists a good linear correlation between autogenous RH change and AS of cement pastes.     

基于毛细管负压技术测试混凝土最早期的自干燥效应

混凝土自干燥收缩与其内部相对湿度的下降密切相关.低水胶比混凝土在结构形成以后的初始阶段存在较大的自干燥收缩,但通常此时混凝土内部相对湿度较高,采用传统的湿度计难以测试出其自干燥效应.借鉴了土壤物理学中张力计的基本原理,研制开发了最早期毛细管负压自动测试系统,结合Laplace方程和Kelvin定律,实现了从浇筑成型开始至终凝后混凝土内部自干燥效应的自动化测试,提出了自干燥收缩零点判定的新方法.     

水泥净浆早期热膨胀系数试验研究

利用设计开发的水泥净浆早期热膨胀系数测量装置，测量了不同水灰比的水泥净浆从初凝后开始的热膨胀系数，总结了水泥净浆热膨胀系数在早期随龄期的发展规律，并给出了拟合公式。     

Internal curing by superabsorbent polymers in ultra-high performance concrete

To limit self-desiccation and autogenous shrinkage that may lead to early-age cracking of ultra-high performance concrete (UHPC), internal curing by means of superabsorbent polymers (SAP) may be employed. Cement pastes and UHPC with water-to-cement ratio below 0.25, with or without SAP, were studied. The absorption capacity of a solution-polymerized SAP was first determined on hardened cement pastes by SEM image analysis. It was observed that the SAP cavities become partially filled with portlandite during cement hydration. Isothermal calorimetry showed that water entrainment with SAP delays the main hydration peak, while after a couple of days it increases the degree of hydration in a manner similar to increasing the water-to-cement ratio. Internal curing by SAP is effective in reducing the internal relative humidity decrease and the autogenous shrinkage. Although the mechanical properties are affected by SAP addition, it is possible to reach compressive strengths of almost 150 MPa at 28 days.     

Will wind always boost early-age shrinkage of cement paste?

Immediately after casting, the occurrence of early age shrinkage is sensitive to internal stresses, degree of hydration, and tensile strength, which is of concern to engineers. This study investigated the influences of wind speeds on early-age shrinkage and tensile strength (strain) of cement paste, and the results were further interpreted by an investigation of hydration degree and capillary water. The outcomes show that as the wind speed increased, the shrinkages (i.e., drying, chemical, and autogenous shrinkage) experienced various trends depending on several factors. Meanwhile, a variety of wind speeds altered the proportion of the evaporation in the capillary water, the inner moisture distribution, and the humidity difference between specimens’ surface and their interior sections. Further, tensile strength and elastic modulus were deteriorated by wind, whereas with the growth of the wind speed, the ultimate tensile strain of specimens exhibited various scenarios within and after 24 h.     

Internal curing of blended cement pastes with ultra-low water-to-cement ratio: Absorption/desorption kinetics of superabsorbent polymer

Internal curing by superabsorbent polymer (SAP) is an effective method to mitigate the autogenous shrinkage of cement-based materials with low water-to-cement ratio (w/c). In this study, the water absorption/desorption kinetics of SAP were studied quantitatively in blended cement pastes with ultra-low w/c. An absorption process at a rate of 0 to 6 g/(g h) was calculated at early ages. After that, SAPs showed mainly two distinct water desorption behaviors with a rate of 0 to 1.1 g/(g h), which was mainly governed by the osmotic pressure and capillary pressure triggered by the drop of internal relative humidity (IRH). The size and amount of SAP played a predominant role in controlling its absorption and desorption kinetics in the cement paste. Compared with ordinary Portland cement, a different desorption process with a higher release rate was noticed in binary and ternary cement pastes, primarily due to the changes in osmotic pressure resulting from the acceleration of cement hydration by silica fume at early ages. Overall, the mitigation of autogenous shrinkage is found to be highly dependent on SAP's absorption and desorption kinetics.     

掺无机纳米矿粉水泥复合净浆的化学收缩与自收缩

依据ASTM试验标准,改良其试验方法,测量水泥复合净浆化学收缩与自收缩,研究单掺不同粒度无定型纳米二氧化硅、亲水型和表面预湿型纳米碳酸钙、不同晶型和粒度纳米二氧化钛三种无机纳米矿粉对其早期收缩的影响。研究表明：一定掺量范围内,无机纳米矿粉的掺入显著提高复合净浆化学收缩与自收缩,且随龄期增长,差异渐大,其中尤以掺纳米二氧化硅最甚,且纳米粒度越小,早期收缩越大。与不亲水但表面预湿型纳米碳酸钙相比,同掺量条件下掺亲水型纳米碳酸钙的净浆早期收缩更大。纳米氧化钛掺量3．0％～5．0％,净浆化学收缩最大,自收缩值增幅不大,远远低于掺纳米二氧化硅和掺碳酸钙的。无机纳米矿粉对水泥基材料早期收缩行为的影响与所掺纳米矿粉种类、晶型、颗粒尺度、表面状态以及掺量等密切相关。     

Autogenous shrinkage in high-performance cement paste: An evaluation of basic mechanisms

In this paper, various mechanisms suggested to cause autogenous shrinkage are presented. The mechanisms are evaluated from the point of view of their soundness and applicability to quantitative modeling of autogenous shrinkage. The capillary tension approach is advantageous, because it has a sound mechanical and thermodynamical basis. Furthermore, this mechanism is easily applicable in a numerical model when dealing with a continuously changing microstructure. In order to test the numerical model, autogenous deformation and internal relative humidity (RH) of a Portland cement paste were measured during the first week of hardening. The isothermal heat evolution was also recorded to monitor the progress of hydration and the elastic modulus in compression was measured. RH change, degree of hydration and elastic modulus were used as input data for the calculation of autogenous deformation based on the capillary tension approach. Because a part of the RH drop in the cement paste is due to dissolved salts in the pore solution, a method is suggested to separate this effect from self-desiccation and to calculate the actual stress in the pore fluid associated with menisci formation.     

The shrinkage of alkali activated fly ash

In this study, the autogenous and drying shrinkage of alkali activated fly ash (AAFA) pastes prepared with different contents of sodium silicate solution are reported. A higher amount of both Na₂O and SiO₂ resulted in a larger autogenous and drying shrinkage. Although a large autogenous shrinkage was obtained during the first 1–3 days, cracking was not observed in the ellipse ring tests. In AAFA pastes, water was not a reactant, but mainly acted as a medium. The experiment results indicate that the autogenous shrinkage of AAFA is not caused by the well-known self-desiccation process that happened in cement paste, but related to the continuous reorganization and polymerization of the aluminosilicate gel structure. AAFA pastes with a larger drying shrinkage exhibited a higher weight loss. The different microstructures lead to the different drying shrinkage of these AAFA mixtures.     

水泥浆体早期收缩的试验研究

开发了一套采用高精度LVDT位移传感器测定水泥浆体长度变化的仪器,并用该仪器对不同水灰比和不同混合材的水泥浆体的收缩行为进行了研究。结果表明,该仪器能够准确、连续地测量水泥浆体终凝后的自收缩和干燥收缩的发展过程。早期自收缩和干燥收缩随着水灰比降低而明显增加,在20℃相对湿度50%的环境下,水泥浆体的自收缩占干燥收缩的10%左右;掺粉煤灰可以少量降低水泥浆体的自收缩和干燥收缩,而掺入石灰石和矿渣则不同程度的增大了水泥浆体早期收缩率。     

玻璃粉水泥浆的初期水化产物和浆体结构

采用SEM、XRD研究了玻璃粉水泥浆的初期水化产物、浆体结构.并用化学结合水量和有效结合水法来定性和定量分析玻璃粉对水化初期复合体系及水泥的促进或抑制作用以及作用程度.研究表明:在水化反应初期(1d内),因为玻璃粉的掺入既由此而产生的稀释作用使有效水灰比增加而产生的对水泥熟料水化的促进作用,因此,硅酸盐水泥熟料的水化程度较高,但从整体来看,大掺量(50%)的玻璃粉延缓了复合胶凝材料总水化程度;水化开始(6 h~1 d)时,水化反应开始加速进行,水化产物的数量迅速增加,主要为纤维状CSH凝胶、针棒状钙矾石晶体和Ca(OH)2,这些水化产物彼此间相互搭接、交错生长,部分未水化的水泥颗粒镶嵌其中,并将玻璃粉粘结成整体,构成体系骨架.     

Autogenous shrinkage of concrete: a balance between autogenous swelling and self-desiccation

According to physical analyses, the driving force of autogenous shrinkage of concrete is the change in the capillary pressure induced by self-desiccation in its cement matrix. Self-desiccation is caused by the balance between the absolute volume reduction (chemical shrinkage) and the building up of the capillary network. The aim of this study was to quantify the influence of the cement characteristics on the chain of mechanisms leading from hydration to autogenous deformations. Four parameters were selected: (i) for clinker, the amount of C₃A and free lime and the SO₃/K₂O ratio; (ii) for cement, the fineness. To master the experimental area, 16 cements were prepared at the laboratory from pure raw materials. An important number of characterizing techniques were used in the experimental study. Their choice was based on the important parameters drawn from the physical analysis: setting time, suspension-solid transition, hydration kinetics through isothermal calorimetry and nonevaporable water, chemical shrinkage, evolution of relative humidity, capillary porosity and autogenous shrinkage. Using different techniques allowed to determine the precise mechanism of action of each parameter. Results showed that these mechanisms are generally different, even if their macroscopic consequences may be identical. This point will probably be useful for modeling and determining the industrial keys reducing the autogenous shrinkage. The physical mechanisms involved in autogenous deformations were further understood. In particular, this study shows that initial autogenous shrinkage should be considered as a balance between the self-desiccation and an initial swelling phase. The influence of the four parameters considered on this last phenomenon were also characterized.     

The effect of drying on early-age morphology of C–S–H as observed in environmental SEM

The morphology of early-age C–S–H, often referred to as outer product or low-density C–S–H, is generally accepted to be fibrillar and forms mainly during the early stages of hydration. This paper reports the effect of drying on the microstructure in young tricalcium silicate paste, which provides insight into the mechanism that leads to the fibrillar morphology. During the first few days after C₃S is mixed with water, the morphology of C–S–H is more affected by drying rate than by relative humidity. This sensitivity is most apparent at partial pressures greater than 85%. During this time, the fibrillar C–S–H product can be suppressed by drying C₃S paste samples very slowly prior to imaging. This approach is interpreted as evidence that this fibrillar morphology, which naturally form over time, grow as tiny colloidal particles that rearrange during the early stages of hydration, leading to well-established larger scale morphologies.     

Early-age acoustic emission measurements in hydrating cement paste: Evidence for cavitation during solidification due to self-desiccation

In this study, the acoustic emission activity of cement pastes was investigated during the first day of hydration. Deaired, fresh cement pastes were cast in sealed sample holders designed to minimize friction and restraint. The majority of acoustic emission events occurred in lower water to cement ratio pastes, while cement pastes with higher water to cement ratios showed significantly less acoustic activity. These acoustic events occurred around the time of setting. A layer of water on the surface of the cement pastes substantially reduced acoustic emission activity at the time of setting. According to these experimental results, the acoustic emission measured around setting time was attributed to cavitation events occurring in the pores of the cement paste due to self-desiccation. This paper shows how acoustic emission might be used to indicate the time when the fluid–solid transition occurs in a cement paste, often referred to as time-zero. Knowledge of time-zero is fundamental for determining when mechanical properties develop and in calculations of residual stresses.     

Effect of internal curing on durability-related properties of high performance concrete

Internal curing of high performance concrete (HPC) by pre-saturated lightweight aggregates is a well-established method of counteracting self-desiccation and autogenous shrinkage. However, by introducing the internal water reservoirs strength and durability properties can be injured. Tests by the widely accepted methods of durability assessment, such as resistance to chloride penetration, air permeability, water absorption, autogenous and drying shrinkage and mass loss, were conducted on HPC mixes made at water to cement ratios in the range of 0.21–0.33. The effect of internal curing on the durability related properties of high-performance concretes as a function of water to cement ratio is reported.     

Impact of time-dependant thermal expansion coefficient on the early-age volume changes in cement pastes

Volume changes and time-dependent thermal expansion coefficient were determined at early stages and the measured total strain was separated into thermal strain and autogenous strain. Cement paste specimens were subjected to temperature histories that imitated hydration-induced temperature rise of the mass concrete. It was shown that the thermal expansion coefficient increased significantly with the development of hydration and became more conspicuous when the ground granulated blast furnace slag was added. The time-dependant increase of thermal expansion coefficient, due to self-desiccation, could result in considerable shrinkage strain at the end of the temperature history. The impact of the time-dependant increase of thermal expansion coefficient might be taken into account as one of the necessary factors in the crack control design from now and cannot even be neglected within the range of the water to binder ratio of this study, because the shrinkage originated in that effect sometimes exceed the autogenous shrinkage.     

Mechanism of the carbonatation shrinkage of lime and hydrated cement

Carbonatation of hydrated lime compacts produces shrinkage and changes in mechanical properties comparable with those found in similarly treated compacts of bottle-hydrated cement and samples of hardened Portland cement paste. From experimental results obtained with the simpler lime system a mechanism for carbonatation shrinkage is proposed that postulates a through-solution reaction and induced cycles of wetting and drying. Experiments on compacts of bottle-hydrated cement show that carbonatation of the ‘combined lime’ is at least as rapid and extensive as the carbonatation of the ‘free lime,’ and that contrary to current opinion the former reaction results in even greater carbonatation shrinkage. The mechanism for carbonatation shrinkage of this system is considered to be connected with dehydration and polymerisation of the hydrous silica product of carbonatation.     

The origin of early age expansions induced in cementitious materials containing shrinkage reducing admixtures

Studies on the early-age shrinkage behavior of cement pastes, mortars, and concretes containing shrinkage reducing admixtures (SRAs) have indicated these mixtures frequently exhibit an expansion shortly after setting. While the magnitude of the expansion has been noted to be a function of the chemistry of the cement and the admixture dosage; the cause of the expansion is not clearly understood. This investigation uses measurements of autogenous deformation, X-ray diffraction, pore solution analysis, thermogravimetry, and scanning electron microscopy to study the early-age properties and describe the mechanism of the expansion in OPC pastes made with and without SRA. The composition of the pore solution indicates that the presence of the SRA increases the portlandite oversaturation level in solution which can result in higher crystallization stresses which could lead to an expansion. This observation is supported by deformation calculations for the systems examined.