Mechanism of expansion associated with ettringite formation

Ettringite formation in portland cement concretes can be responsible for both deleterious and beneficial phenomena. Several hypotheses on the mechanism of expansion associated with ettringite formation are reviewed, and a new hypothesis is proposed. Experimental evidence is presented in support of the new hypothesis. It is shown that in the presence of lime the nature of ettringite formed is colloidal, and not long lath-like crystals. It is proposed that colloidal ettringite is able to attract a large number of water molecules which cause interparticle repulsion, thus causing an overall expansion of the system.     

On the role of free calcium oxide in expansive cements

Results of research, tending to elucidate the effect of free CaO content in portland cement clinker upon the expansion of specimens of expansive cements, prepared in semi-commercial scale by intergrinding of portland cement clinker, C₄A₃$\text{S}$-phase containing special clinker and gypsum, by means of examination of liquid phase composition and porosity of mortars, have been presented. Special clinker was obtained by burning a mix of limestone, fly ash with high Al₂O₃ content and gypsum. Obtained results confirm the advantageous effect of free lime upon the hydration process and properties of expansive cements. The concentration of CaO in the liquid phase seems to influence rather the rate of ettringite formation than the size of its crystals.     

Hydration of fly ash-portland cements

Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis.     

纳米SiO_2和ZrO_2共同作用硅酸盐水泥水化机理

利用XRD和SEM表征了纳米S iO2和ZrO2对硅酸盐水泥砂浆改性后的试样。研究发现,在固定水灰比为0.5时,复掺2%纳米S iO2和0.25%ZrO2共同发挥纳米诱导水化作用,生成更多的C—S—H凝胶并向着针柱状形貌生长,细化了Ca(OH)2,使其镶嵌在C—S—H凝胶和钙钒石中,使得C—S—H与凝胶与钙矾石紧密交织,减少水泥石中总孔隙率,水泥石的结构更加细化密实,使抗压强度明显高于同龄期标准试样。     

Hydration Characteristics of Portland Cement after Heat Curing: I, Degree of Hydration of the Anhydrous Cement Phases

The degree of hydration of the four major anhydrous cement phases in three U.K. portland cement mortars has been observed during the period of water storage at room temperature after an initial short-term heat cure. Such a heat cure at 85° or 100°C for 12 h generally accelerated the initial hydration of the four major anhydrous minerals in portland cement. Subsequent retardation of the degree of hydration of the alite, tricalcium aluminate, and ferrite phases was observed when these heat-cured mortars were stored at ambient temperature. General similarity but some differences in hydration behavior were observed between the three cements. The hydration of belite in the heat-cured mortars during storage at room temperature produced porous inner products that favored deposition of ettringite and reduced the risk of expansive ettringite formation. The substantial retardation in hydration of the aluminate-bearing phases, especially the ferrite phase, during the storage at room temperature raised the overall SO₃/Al₂O₃ ratio of the cement hydrates formed, bringing about a potential for ettringite formation and hence the risk of expansion through delayed ettringite formation.     

Hydration Characteristics of Portland Cement after Heat Curing: II, Evolution of Crystalline Aluminate-Bearing Hydrates

The development of crystalline aluminate-bearing hydrates in portland cement mortars during water storage at room temperature for periods of up to 1 year after an initial heat cure for 12 h has been observed by quantitative X-ray diffraction analysis and backscattered electron imaging. Ettringite was present in the mortars immediately after a short-term cure at 20° and 60°C, calcium carboaluminate (C₄ĀH₁₁) at 60°C, monosulfate at 85°C, and hydrogarnet at 85°C and above. Ettringite started to form after an induction period ranging from several days to several months after the initial heat cure at 85/100°C, and developed substantially during the period of expansion of the mortar associated with delayed ettringite formation (DEF). Ettringite growth was also observed in the nonexpansive cement mortars. Development of the ettringite bands occurred exclusively in the expansive mortars. Although monosulfate observed in the mortars that had been heat cured at 85°C sometimes increased in amount on initial storage at room temperature, it appeared to vary little in amount for up to 1 year. The amount of hydrogarnet in the heat-cured cement product did not change significantly during storage at room temperature for more than 1 year. DEF expansion of the heat-cured mortars was attributed to ettringite band formation, which started to form at the surface of the cement product and gradually developed inwards.     

Quick lime-gypsum interactions in stabilized soil bases for concrete highways

Lime stabilization of clayey soils is very common throughout many parts of the world. Typically if heaving is observed after quick lime (CaO) stabilization, then one may think that insufficient lime and/or poor slaking and mixing techniques may be the root of the problem. Actually, there are several other reasons for the observed heaving. One explanation is that the lime may have been hard burnt, thus rendering the lime inactive until months later. Another explanation is due to the interaction of the quick lime with gypsum (CaSO₄ . 2 H₂O) in the soil to be stabilized.One may conclude that lime, Type I portland cement, or Class C fly ash stabilization of high gypsum bearing soils would at best produce poor results because of the possibility of expansion due to the formation of ettringite.     

高掺量混合材复合水泥的水化性能

通过水化微量热、化学结合水测定和X射线衍射、热重-差热分析、扫描电镜等测试方法研究了3种高掺量矿渣、粉煤灰、石灰石复合水泥的水化性能,并与硅酸盐水泥的水化进行了对比。结果表明：高掺混合材复合水泥的水化放热特征与硅酸盐水泥有明显不同,早期水化反应速度低于硅酸盐水泥,但后期由于矿渣、粉煤灰的二次水化反应使其水化速度增长较快。主要的水化产物亦为水化硅酸钙凝胶、钙钒石和Ca(OH)2晶体,但Ca(OH)2含量明显低于硅酸盐水泥浆体中的Ca(OH)2含量。     

水泥熟料中氧化镁的水化和膨胀性能

采用ＸＲＤ,ＤＳＣ等方法研究了熟料中氧化镁的水化及其膨胀性能,结果表明,料中氧化镁是一种制造膨胀型水泥的好材料,但它的缺陷在于水化及膨胀十分缓慢,利用钙矾石在早期产生膨胀的性能对熟料中氧化镁的延缓性能膨胀加以改进。可以为制造微膨胀型中热水泥和低热矿渣水泥提供依据。     

水化产物钙矾石在软土地基加固中的增强作用

通常加固软土地基是使用第一的水泥用为固化剂。研究与实践表明,由于软土中孔隙量很高,采用工业废石膏与水泥配合加固软土,其中产生的水化产物钙矾石可以高效率的填充孔隙,对固化工的强度增长有显著的增强作用,从而得固人疆土工与单纯用水泥加固相比有大幅度的提高,钙矾石形成过程中,液相ＣａＯ,ＯＨ浓度决定钙矾石的形貌和膨胀性能,不同土样对ＣａＯ,ＯＨ消耗量不同,导致固化上液相中ＣａＨ,ＯＨ,浓度不同,因而,对不     

游离氧化钙对水泥浆体体积膨胀的影响机制

硬化水泥桨体由于水泥中游高氧化钙水化会导致体积膨胀。研究发现,浆体的体积膨胀除与ｆＣａＯ的含量和活性有关外,还与浆体的结构和性能密切ｆＣａＯ水化形成Ｃａ（ＯＨ）２时,不仅固相体积增加,而且空隙体积亦增加,而且空隙体积亦增加。ｆＣａＯ在浆体硬化结构形成之后水化,且其水化产物成堆聚集时才会导致浆体体积膨胀。     

Role of delayed release of sulphates from clinker in DEF

Two clinkers rich in sulphate burned in the pilot plant rotary kiln and cements prepared from them were investigated. Clinker richer in sulphate (SO₃=3.6%) contained independent anhydrite grains as well as inclusions of anhydrite in belite. The mortar from it expanded after heat treatment at 90 °C and the addition of Na₂SO₄ or NaOH accelerated and increased this expansion. The expansion occurred irrespective of the fact that the clinker contained only 3% of C₃A, although the C₄AF content was 13%. The second clinker with 2.6% SO₃ contained mainly calcium langbeinite and expanded only when 2% of Na₂SO₄ was added. The SEM examination of the mortars revealed the presence of numerous bands of massive ettringite around sand grains. Agglomerates of cracked ettringite in cement gel were also present. In addition, microcracks were seen inside the darker C-S-H gel. The conclusion is that anhydrite forming inclusions in belite gives an expanding mortar after heat treatment at 90 °C independently of the tricalcium aluminate content. Such clinkers are not typical of industrial conditions. The expansion is caused by the bands of massive ettringite as well as its agglomerates present in the cement gel and nanometric ettringite in the C-S-H phase.     

Mechanism of sulfate attack on portland cement concrete — Another look

Sulfate-generated deteriorations in normal portlant cement concretes include expansion, cracking, loss of strength and stiffness, and sometimes disintegration. The chemical phenomenon of ettringite formation as a result of reaction between sulfate water and hydration products of portland cement does not adequately explain all the physical manifestations of the sulfate attack. Furthermore, ettringite which causes expansions in some cases is apparently responsible for high strength in other cases. The published literature does not contain satisfactory explanations for this anomalous behavior of ettringite. In this paper, the author has attempted to provide answers to some of the questions.     

The alkali-silica reaction in alkali-activated granulated slag mortars with reactive aggregate

The expansion of alkali-activated granulated blast furnace slag (AAS) cement mortars with reactive aggregate due to alkali-silica reaction (ASR) was investigated. The alkaline activator used was NaOH solution with 4% Na₂O (by mass of slag). These results were compared to those of ordinary portland cement (OPC) mortars. The ASTM C1260-94 Standard Test Method based on the NBRI Accelerated Test Method was followed. The nature of the ASR products was also studied by SEM/EDX. The results obtained show that the AAS cement mortars experienced expansion due to the ASR, but expansion occurs at slower rate than with OPC mortars under similar conditions. The cause of the expansion in AAS cement mortars is the formation of sodium and calcium silicate hydrate reaction products with rosette-type morphology. Finally, in order to determine potential expansion due to ASR, the Accelerated Test Method is not suitable for AAS mortars because the reaction rate is initially slow and a longer period of testing is required.     

Chemistry of the Aqueous Phase of Ordinary Portland Cement Pastes at Early Reaction Times

The chemistry of the aqueous phase of ordinary portland cement paste at early ages (<2 h) has been analyzed in terms of the concentrations of the elemental components in the pore fluid. The concentrations of calcium, sulfur, aluminum, and silicon are rationalized by plotting the data on "phase diagrams." To simplify the analysis, the portland cement system is described using two subsystems: (i) CaO-Al₂O₃-CaSO₄-H₂O, modified by the presence of sodium and potassium, and (ii) CaO-SiO₂-H₂O. During the first 10 min of hydration, the calcium, sulfur, and aluminum concentrations all decrease, roughly in proportion, which suggests a precipitation process, a conversion of calcium sulfate hemihydrate to gypsum, and the initial formation of ettringite. The CaO-Al₂O₃-CaSO₄-H₂O subsystem seems to move from a phase assemblage of gypsum, Al₂O₃·3H₂O, and ettringite to an assemblage of gypsum, calcium hydroxide, and ettringite during the first 15-30 min after the water and the cement are mixed. The silicate equilibrium is approached more slowly. The intensity of mixing has relatively little effect on the concentrations beyond the first few minutes.     

石灰石粉对水泥基材料抗硫酸盐侵蚀性的影响及其机理

用天然石灰石粉等质量取代水泥20%和30%,将制备的水泥净浆和砂浆试件常温浸泡在0.35 mol/L Na2SO4溶液中,测量试件的线长度和抗折强度随浸泡时间的变化.结果表明:石灰石粉对水泥基材料的抗硫酸盐性有严重的影响,它们使水泥基材料在硫酸盐环境中的强度急剧下降并导致水泥基材料产生较大体积膨胀,引起开裂.掺石灰石粉的水泥基材料主要因形成大量较大尺寸的石膏晶体而膨胀开裂.石膏的形成导致硫酸盐侵蚀水泥基材料产生膨胀开裂.因此,在硫酸盐侵蚀环境下,不宜采用含石灰石粉的复合水泥或将石灰石粉作为矿物掺合料制备的混凝土.     

Interpretation of Compositional Patterns Found by Quantitative Energy Dispersive X-ray Analysis for Cement Paste Constituents.

More than 200 individual particles within a portland scement paste were morphologically categorized by backscatter SEM and analyzed systematically by EDXA. Nearly all of the analyses fall either in a dense cluster around a mean C-S-H gel composition or along tie lines connecting this composition to theoretical compositions for calcium hydroxide, monosulfate, ettringite, and ferrite. "Phenograin" C-S-H gel (formed as a direct replacement product within large cement grains) contains small amounts of Al, Fe, and S, but otherwise shows almost negligible compositional variation. In contrast, smaller groundmass particles and shells of gapped phenograins show a slightly differentc S-H gel composition. Some of these, despite being morphologically homogeneous, show compositions falling along tie lines to the other recognized phases. This is attributed to extensive occurrence of intimate mixtures of C-S-H gel with the other components below the scale of SEM observation. Ionic substitutions appear to be minor except for the monosulfate and ettringite components in such mixtures. These show a variable but usually high deficiency in the sulfate content.     

石灰-水泥-粉煤灰改性磷石膏对水泥物理性能的影响研究

采用石灰、水泥、粉煤灰对磷石膏进行改性处理,测定了改性磷石膏中硫酸根的溶解性能,对比了原状磷石膏与改性磷石膏对水泥物理性能的影响,并结合X射线衍射（XRD）和扫描电镜（SEM）分析了改性前后磷石膏对水泥不同龄期水化产物的影响。结果表明：随着石灰掺量的增加改性磷石膏的pH逐渐增大,当石灰掺量为4%（质量分数）时磷石膏的pH达到12.22,此时磷石膏中的可溶性磷、氟转化成难溶性的磷酸盐、氟化钙;随着水泥和粉煤灰掺量的增加,改性磷石膏的溶解性能呈现降低趋势。当石灰掺量为4%、水泥掺量为10%（质量分数）、粉煤灰掺量为10%（质量分数）时,改性磷石膏经过7 d养护在水中浸泡8 h所得滤液中硫酸根的质量浓度为0.30 g/L,比未改性磷石膏在水中浸泡8 h所得滤液中硫酸根的质量浓度降低了81.8%。与掺加未改性磷石膏的水泥浆体相比,掺加改性磷石膏的水泥浆体的水灰质量比由0.41降低到0.38、初凝时间和终凝时间分别缩短34.6%和27.2%、28 d抗压强度提高21.1%。石灰、水泥、粉煤灰改性处理磷石膏后,生成的水化硅酸钙和钙矾石等水硬性产物包裹在石膏颗粒表面,使硫酸根在水中的溶出速率降低,减少了对水泥中铝酸三钙的影响,使得硬化体内部结构变得致密、力学性能显著提高。     

掺煅烧石膏水泥早期水化过程的研究

利用DTA，XRD，IR测定水泥水化浆体的化学结合水和Ca(OH)2的生成量，研究了煅烧石膏，二水石膏对硅酸盐水泥早期水化过程的影响。结果表明：在水化龄期相同时，掺煅烧石膏水泥浆体中水化产物同掺二水石膏相比，Ca(OH)2生成量大；在1d前无钙钒石（AFt)生成，结合水量在1d前，前者高于后者，而1d后则相反。指出了煅烧石膏加快水泥水化产物形成的机理在于：由于它的溶解度较低，在水泥水化初期（1d前），存在于水泥中的铝酸盐相不能形成AFt，从而减缓了AFt对水泥水化的延缓作用，加速了整个熟料矿物相的水化。     

The kinetics of ettringite formation and dilatation in a blended cement with β-hemihydrate and anhydrite as calcium sulfate

The phase formation, heat of hydration and dilatation in a blended cement consisting of 50 wt.% calcium aluminate cement, 25 wt.% Portland cement and 25 wt.% calcium sulfate were studied (w/c=1). The calcium sulfate was β-hemihydrate, anhydrite and mixes of the two. Kinetic expressions describing the ettringite formation in the pastes with the pure calcium sulfates were found. Hydration reactions were suggested and the phase development was compared to the hydration heat by mass and heat balances. When the calcium sulfate was 75 and 50 wt.% β-hemihydrate, the systems behaved as a linear combination of the 100 and 0 wt.% blends. At 25 wt.%, the hydration kinetics differed from the other blends. With only β-hemihydrate, the last 50% of ettringite formation was accompanied by expansion, mainly caused by interaction of crystals growing radially on cement grains. In the paste with only anhydrite, ettringite crystals grew in solution and produced no expansion.