Effect of limestone filler on the deterioration of mortars and pastes exposed to sulfate solutions at ambient temperature

This paper presents data on engineering properties such as compressive strength, visual change and expansion of mortar specimens incorporating limestone filler subjected to severe sulfate attack at ambient temperature. Specimens with four replacement levels of limestone filler (0, 10, 20 and 30% of cement by mass) were immersed in sodium and magnesium sulfate solutions with 33,800 ppm of SO₄²⁻ concentration. In order to identify the products formed by sulfate attack, microstructural analyses such as XRD and SEM were also performed on the paste samples with similar replacement levels of limestone filler.The test results demonstrated that mortar and paste samples incorporating higher replacement levels of limestone filler were more susceptible to sulfate attack irrespective of types of attacking sources. However, the deterioration modes were significantly dependent on the types of sulfate solutions. Additionally, although the samples were exposed to sulfate solutions at 20 ± 1 °C, the deterioration was strongly associated with thaumasite formation in both sulfate solutions.The deterioration mechanism and resistance to sulfate attack of cement matrix incorporating limestone filler at ambient temperature is discussed in the light of the test results obtained.     

Microstructure of 5-year-old mortars containing limestone filler damaged by thaumasite

This paper presents the findings of a long-term study on the microstructure of Portland cement mortar specimens containing 5%, 15% and 35% limestone filler, as cement replacement, after exposure to a solution of magnesium sulfate at a concentration of 1.44% SO₄, for 5 years at 5 °C. The findings are compared to results reported earlier, obtained from the same systems but after 1-year exposure. It was found that the deterioration due to thaumasite advanced with the increased exposure period and limestone content. Thaumasite solid solutions (Tss) formed as the dominant phases within the deteriorated cement matrix and at the paste-aggregate interface resulting in cracks and delamination. Thaumasite was also found as an inner product in various clinker grains. Interestingly, the control specimens, with no limestone filler, were found to exhibit cracks due to the formation of Tss, with atmospheric carbon dioxide being the most likely source for the carbonates.     

Sulfate attack on cementitious materials containing limestone filler — A review

This review summarizes the results of sulfate performance in laboratory and field tests where limestone is used as a constituent of cement (PLC) or as a sand replacement where it is particularly beneficial to the properties of self compacting concretes (SCC).Laboratory studies on paste, mortar or concrete specimens exposed to Na₂SO₄ and MgSO₄ solutions in a wide range of concentrations at different temperatures as well as mixtures with different compositions, cement compositions and limestone proportions are considered in a conceptual analysis as for the resistance to external sulfate attack and, especially, thaumasite sulfate attack.A detailed analysis of environmental aggressiveness (concentration, temperature and pH), mixture composition and cement composition used in each study are presented for PLC and SCC. Reported field studies are also shown, only a few cases have used limestone filler in their composition. A conceptual graphical analysis is then proposed to relate the degree of surface deterioration and mineralogical composition of attacked surface to the main variables of external sulfate attack: water/cementitious material ratio, limestone content and C₃A content of the cement. Observation of graphical analysis clearly shows that deterioration by ESA is mainly governed by effective w/c ratio and C₃A content of the cement. Surface damage is controlled when low effective w/c ratio and low C₃A are used. In MgSO₄ solution, low temperatures increase the degree of deterioration. Thaumasite is the last attack stage in the different sulfate environments.     

Rheological properties of highly flowable mortar containing limestone filler-effect of powder content and W/C ratio

The effect of a limestone filler addition in superplasticized cement mortar was investigated. The mixtures considered in this study are highly fluid, yet stable mortars that can be used to proportion self-consolidating concrete (SCC). All the mixtures were proportioned with a fixed unit water content of 250 l and various water-cement ratios varying from 0.35 to 0.45. A limestone filler with a specific surface area of 480 m²/kg was used at different addition percentages.This paper reports test results leading to the recommendation of suitable powder contents that can be used to proportion mortar mixtures containing a limestone filler and achieving adequate rheological properties. Test results show that the effect of limestone filler is mainly affected by the W/C and the limestone filler content in use. For a given W/C, the addition of a limestone filler within a certain range did not affect fluidity. However, beyond a critical dosage, the incorporation of some limestone filler resulted in a substantial increase of the viscosity of mortar. An accurate model that can be used to predict the viscosity of such mixtures is proposed and validated on various mixtures.     

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.     

Some factors affecting delayed ettringite formation in heat-cured mortars

Although more than 10 years of studies on delayed ettringite formation (DEF) have led to consensus in numerous areas of past disagreements, some questions remain experimental work is needed to complete the knowledge of this pathology. Following this objective, this paper studies the influence of pre-existing microcracking, wetting/drying cycles and the type of sulfated addition on DEF in steam cured mortars. The mortar specimens were prepared using an Ordinary Portland Cement and two types of sulfate were added to the mixtures: calcium sulfate (CaSO₄) or sodium sulfate (Na₂SO₄). The results confirm the well-known effect of temperature: no expansion was observed in any of the mixtures cured at room temperature. Moreover, no expansion was observed after 800 days for the reference mortar or for the mortar containing calcium sulfate but all the specimens of heat-cured mortars containing sodium sulfate expanded markedly after about 50 days whatever the supplementary treatments applied (thermal shrinkage or wetting/drying cycles). These results show the significant role played by alkalis in the occurrence of delayed ettringite. The supplementary treatments intended to cause prelimiray microcracking of the specimens did not promote expansion but contributed to a slight acceleration of the reaction. The ultimate values of expansion were similar to those obtained with sound mortars.     

The role of carbon dioxide in the formation of thaumasite

The development of the thaumasite form of sulfate attack (TSA) has received considerable attention since its discovery in the foundations of motorway bridges in England in 1998. When TSA occurs in siliceous aggregate concrete and mortars, particularly in samples from the field, it is normally assumed that the carbonate source necessary for the formation of thaumasite was either present as a minor component in the aggregate, as a limestone filler in the binder, or from the groundwater. Recent laboratory studies carried out by the authors have identified a further source of carbonate ions, and that is from atmospheric carbonation. However, in other studies, it appears that an initial air cure can improve the resistance of concretes to TSA. This apparent dichotomy suggests that there is insufficient understanding of the relationship between atmospheric carbonation and TSA.The performances of small mortar cubes made using fine aggregates of either high quality silica sand or limestone under different curing regimes have been compared. Detailed analyses of the nature of the thaumasite-ettringite solid solutions that formed have been carried out, and the mechanism of thaumasite formation, in particular, the role of calcium bicarbonate, is discussed in the light of the results obtained.     

Effect of wheat straw ash on mechanical properties of autoclaved mortar

This study investigates the effect of wheat straw ash (WSA) on the mechanical strength of autoclaved mortar. The mechanical properties studied include compressive, tensile, and flexural strengths of mortar. Mortar mixes were prepared using natural silica, wadi (local sand), and crushed limestone fine aggregates at a w/c ratio of 0.6. Mortar specimens were exposed to autoclave for 2.5 h at a pressure of 2 MPa. Three percentages of WSA replacement levels (3.6%, 7.3%, and 10.9%) by weight of sand were utilized in the study. The study showed that the replacement of sand by WSA increases the mechanical strength of autoclaved mortar. Mortar specimens containing limestone aggregate with 10.9% WSA replacement level showed an average increase in compressive, tensile, and flexural strength by 87%, 67%, and 71%, respectively, compared to control mortar specimens. Scanning electron micrographs for autoclaved paste specimens containing 7.3% WSA replacement level revealed a more packed structure compared to control paste specimens.     

Cement composition and sulfate attack: Part I

Four cements were used to address the effect of tricalcium silicate content of cement on external sulfate attack in sodium sulfate solution. The selected cements had similar fineness and Bogue-calculated tricalcium aluminate content but variable tricalcium silicates. Durability was assessed using linear expansion and compressive strength. Phases associated with deterioration were examined using scanning electron microscopy and X-ray diffraction. Mineralogical phase content of the as-received cements was studied by X-ray diffraction using two methods: internal standard and Rietveld analysis.The results indicate that phase content of cements determined by X-ray mineralogical analysis correlates better with the mortar performance in sulfate environment than Bogue content. Additionally, it was found that in cements containing triclacium aluminate only in the cubic form, the observed deterioration is affected by tricalcium silicate content. Morphological similarities between hydration products of high tricalcium aluminate and high tricalcium silicate cements exposed to sodium sulfate environment were also observed.     

Mechanism of sulfate attack: a fresh look: Part 2. Proposed mechanisms

The first paper in this two-part series [Cem. Concr. Res. 32 (2002) 915] summarized the experimental results from a comprehensive research study on sulfate attack. The current paper utilizes these results to develop models for the mechanism of attack resulting from sodium and magnesium sulfate solutions. Implications of changing the binder constituents or the experimental variables, such as concentration and temperature of the solution on the proposed mechanism, are also discussed. The potential of these mechanistic models for use in service life prediction models has also been identified.According to the proposed mechanism, the attack due to sodium sulfate solution progresses in stages. The expansion of an outer skin of the specimen leads to the formation of cracks in the interior region, which is chemically unaltered. With continued immersion, the surface skin disintegrates, and the sulfate solution is able to react with the hydration products in the cracked interior zone leading to the deposition of attack products in this zone. Now, this zone becomes the expanding zone, leading to further cracking of the interior of the mortar.In the case of magnesium sulfate solution, a layer of brucite (magnesium hydroxide) forms on the surface of the mortar specimen. The penetration of the sulfate solution then occurs by diffusion across this surface layer. As the attack progresses, the formation of attack products such as gypsum and ettringite in the paste under the surface leads to expansion and strength loss. The expansion also causes cracking in the surface brucite layer, and this leaves the mortar susceptible to direct attack by the magnesium sulfate solution. Conditions favorable for the decalcification of calcium silicate hydrate (C-S-H) are thus created, and the ultimate destruction of the mortar occurs as a result of the conversion of C-S-H to the noncementitious magnesium silicate hydrate (M-S-H).     

An evaluation of the sulfate resistance of cements in a controlled environment

It was initially established that the maintenance of the pH of a sulfate solution, in which mortar specimens were immersed, at a constant and predetermined value through controlled sulfuric acid additions ensured that the sulfate ion concentration in solution remained invariant with time. The rates of sulfate attack of mortar specimens exposed under typical immersion and environmentally controlled conditions were then compared. It was observed that environmental control significantly increased the rate of sulfate attack as measured either by strength loss or linear expansion. However, the strength changes and the expansions observed occurred in a manner consistent with the severity of the test conditions imposed.     

Thaumasite-ettringite solid solutions in degraded mortars

The thaumasite form of sulfate attack (TSA) has been observed in mortar prisms made from Portland-limestone cements after laboratory storage in 1.8% magnesium sulfate solution at 5 °C for 5 years. The prisms all showed evident signs of degradation, which increased with increasing limestone content. X-ray powder diffraction indicated that a solid solution was formed in all the prisms, which was based on the crystal structure of thaumasite, but extended towards the chemical composition of ettringite. The prism made from the cement with the highest level (35%) of limestone replacement gave the greatest amount of the thaumasite solid solution and had a composition close to the thaumasite end member, whereas those cements with lower (15%, 5% and 0%) levels of replacement gave reduced amounts of the solid solution and had greater lattice parameters. The solid solutions that have been observed are compared with those reported by Barnett et al. for synthetic thaumasite samples grown from sucrose solution.     

硫酸钠对水泥砂浆的物理侵蚀作用

采用150 g/L的硫酸钠溶液半浸泡的试验方法,以砂浆抗蚀系数为指标,研究了相对湿度和温度变化条件下,硫酸钠溶液的不同结晶产物Na2SO4(无水芒硝)与Na2SO4·10H2O(芒硝)对砂浆试件的物理侵蚀作用和不同条件下硫酸钠溶液的物理侵蚀特性及相关机理.结果表明:与环境温度变化条件相比,硫酸钠溶液在相对湿度变化条件下对砂浆试件造成物理侵蚀作用更为严重,这主要是在相对湿度变化条件下,硫酸钠溶液的结晶产物Na2SO4与Na2SO4·10H2O发生相互转化造成的.硫酸钠溶液形成Na2SO4结晶产物时,其对砂浆造成的物理侵蚀破坏作用较形成的Na2SO4·10H2O结晶产物更为显著.     

Mechanism of sulfate attack: A fresh look: Part 1: Summary of experimental results

This paper reports the results of an investigation on the effects of sodium and magnesium sulfate solutions on expansion and microstructure of different types of Portland cement mortars. The effects of using various sulfate concentrations and of using different temperatures are also reported. The results suggest that the expansion of mortars in sodium sulfate solution follows a two-stage process. In the initial stage, Stage 1, there is little expansion. This is followed by a sudden and rapid increase in the expansion in Stage 2. Microstructural studies suggest that the onset of expansion in Stage 2 corresponds to the appearance of cracks in the chemically unaltered interior of the mortar. Beyond this point, the expansion proceeds at an almost constant rate until the complete deterioration of the mortar specimen. In the case of magnesium sulfate attack, expansion occurs at a continually increasing rate. Microstructural studies suggest that a layer of brucite (magnesium hydroxide) on the surface forms almost immediately after the introduction of the specimens into the solution. The attack is then governed by the steady diffusion of sulfate ions across the brucite surface barrier. The ultimate failure of the specimen occurs as a result of the decalcification of the calcium silicate hydrate (C-S-H), and its conversion to magnesium silicate hydrate (M-S-H), after prolonged exposure to the solution. The effects of using various admixtures, and of changing the experimental variables such as the temperature and concentration of the solution, are also summarized in this paper. Models for the mechanism of the attack resulting from sodium and magnesium sulfate solutions will be presented in Part 2.     

用原位合成吸水性树脂表层处理方法提高混凝土耐硫酸盐腐蚀性

硫酸盐腐蚀是导致混凝土耐久性劣化的一个重要因素.用原位合成吸水性树脂对混凝土表面处理,为改善混凝土耐硫酸腐蚀提出了一种新方法,即利用高吸水性树脂预聚溶液浸渍混凝土,经红外辐射引发自由基共聚反应原位合成吸水性树脂,通过合成的吸水性树脂吸水膨胀、密实微裂缝或孔隙,截断硫酸盐侵蚀性介质的传输路径提高混凝土抗硫酸盐腐蚀能力.通过试验,研究了硫酸钠溶液在水泥砂浆(处理前后)中的渗透状况,并利用扫描电镜、X射线衍射分析了混凝土受硫酸盐腐蚀的特征.结果表明:用原位合成高吸水性树脂处理砂浆后,能显著降低50 g/L硫酸钠溶液在水泥砂浆内部的渗透速度;经50次硫酸钠溶液干/湿循环处理后的水泥砂浆表现出良好的抗硫酸盐腐蚀性能.     

固废填料对改性沥青老化过程中的疲劳特性影响研究

为了探究老化过程中固废填料的加入对改性沥青抗疲劳性能的影响,进而分析固废填料对改性沥青老化行为特征的影响。本研究基于赤泥、粉煤灰、硅藻土以及常规矿粉填料制备了4种改性沥青胶浆,在未老化、长期老化两个阶段分别进行线性振幅扫描试验,采用4种疲劳评价指标(累计疲劳损伤D(t)、完整性指数C(t)、应变敏感程度B值和疲劳寿命)评价疲劳性能,并提出老化-疲劳性能衰变指数来表征老化对沥青胶浆带来的负面作用。研究表明:填料的加入会降低原样沥青疲劳性能,但可以改善沥青胶浆经历老化历程后的疲劳性能,即可以提升胶浆的抗老化性能,改善效果由高到低为硅藻土、赤泥、矿粉、粉煤灰;填料的加入同时具有硬化脆化和微粒填充作用,两者此消彼长,共同决定影响效果。     

5℃环境下水泥砂浆抗硫酸盐侵蚀性能试验研究

研究了不同水泥品种、矿物掺合料对水泥基材料在5℃下抗硫酸盐侵蚀的性能的影响,分别采用普通硅酸盐水泥、中抗硫水泥以及加入矿粉与硅灰的水泥砂浆试件,测试各试样在(5±1)℃的3%Na2 SO4溶液浸泡后的强度变化情况,综合考虑砂浆强度与抗蚀系数对砂浆抗硫酸盐侵蚀性能进行评价,并运用SEM、EDS、XRD分析方法对腐蚀机理进行了分析.结果表明:在5℃环境下,砂浆试样的强度普遍低于常温环境下,砂浆抗硫酸盐侵蚀能力15%矿粉+3%硅灰>中抗硫水泥>15%矿粉+1%硅灰>普通硅酸盐水泥;加入矿物掺合料明显改善了水泥砂浆抗硫酸盐侵蚀性能,并且硅灰的含量越高效果越明显;低温下腐蚀产物不仅有石膏,还有碳硫硅钙石的生成.     

Effect of solution concentrations and replacement levels of metakaolin on the resistance of mortars exposed to magnesium sulfate solutions

This paper reports an experimental study on the magnesium sulfate resistance of mortar and paste specimens incorporating 0%, 5%, 10% and 15% metakaolin (MK). The resistance of mortar specimens was evaluated using visual examination, reduction in compressive strength and expansion measurements.Results confirmed that mortar specimens with a high replacement level of metakaolin showed lower resistance to a higher sulfate concentration of magnesium solution. However, in a lower concentration, there were no visibly remarkable differences in the deterioration of mortar specimens, even up to 360 days of exposure, regardless of replacement levels of metakaolin.The negative effect of metakaolin on the magnesium sulfate resistance is partially attributed to the formation of gypsum but not ettringite and thaumasite. In addition, the reduction of calcium hydroxide and the increase of secondary C-S-H in the cement matrix due to pozzolanic reaction of metakaolin provided an opportunity to lead to the conversion of primary and secondary C-S-H gel into the M-S-H gel.It is concluded that it is necessary to pay special attention when using metakaolin in concrete exposed to highly concentrated magnesium sulfate solution.     

10℃下不同水灰比水泥基材料抗硫酸盐侵蚀性能试验研究

研究了不同水灰比对水泥砂浆试件在10℃下抗硫酸盐侵蚀的性能的影响,采用0.36与0.5两种水灰比的普通硅酸盐水泥、中抗硫水泥以及加入矿粉与硅灰的水泥砂浆试件,测试各试样在(10±1)℃的3％Na2SO4溶液中浸泡后的强度变化情况,综合考虑强度与抗蚀系数对砂浆抗硫酸盐侵蚀性能进行评价.结果表明:在10℃下0.36水灰比试件强度高于0.5水灰比试件,抗硫酸盐侵蚀性能随着水灰比的降低而提高;加入矿物掺合料明显改善了水泥砂浆抗硫酸盐侵蚀性能,并且硅灰的含量越高效果越明显.砂浆抗硫酸盐侵蚀性能15％矿粉+3％硅灰>15％矿粉+1％硅灰>中抗硫水泥>普通硅酸盐水泥.     

特性环境对水泥砂浆硫酸盐侵蚀类型的影响

研究了硫酸盐种类(Na2SO4,MgSO4)及温度(5 ℃,20 ℃)等影响因素对水泥砂浆硫酸盐侵蚀类型的影响,以明确碳硫硅酸钙型硫酸盐侵蚀的特性环境条件.试验结果表明:掺加石灰石粉的水泥砂浆试件置于5% MgSO4溶液、5 ℃及20 ℃温度环境下浸泡450 d后,均能生成碳硫硅酸钙.而一般水泥砂浆试件置于5% Na2SO4溶液、5 ℃及20 ℃温度环境浸泡侵蚀后,未生成碳硫硅酸钙.证明水泥混凝土在＞15 ℃的硫酸盐侵蚀环境下亦可生成碳硫硅酸钙,而Mg2 的存在对碳硫硅酸钙的形成过程具有加速催化作用.