Effect of Anti-freezing Admixtures on Alkali-silica Reaction in Mortars

The influence of anti-freezing admixture on the alkali aggregate reaction in mortar was analyzed with accelerated methods. It is confirmed that the addition of sodium salt ingredients of anti-freezing admixture accelerates the alkali silica reaction to some extent, whereas calcium salt ingredient of anti-freezing admixture reduces the expansion of alkali silica reaction caused by high alkali cement. It is found that the addition of the fly ash considerably suppresses the expansion of alkali silica reaction induced by the anti-freezing admixtures.

Alkali-Silica Reaction Inhibited by LiOH and Its Mechanism

A high alkali reactive aggregate zeolitization perlite was used to test the long term effectiveness of LiOH in inhibiting alkali silica reaction.In this paper,the rigorous conditions were designed that the mortar bars had been cured at 80℃ for 3 years after autoclaved 24 hours at 150℃.Under this condition,LiOH was able to inhibit the alkali silica reaction long term effectiveness.Not only the relationship between the molar ratio of n(Li)/(Na) and the alkali contents in systems was established, but also the governing mechanism of such effects was also studied by SEM.

Calculation of Alkali Silica Reaction (ASR) Induced Expansion before Cracking of Concrete

A calculation method for predicting the formation of alkali-silica gel and analyzing the relationship of ASR induced expansion and aggregate size was proposed. The complicated chemistry of alkali silica reaction was simplified to be controlled by the diffusion process of chemical ions into reactive aggregates. The transport of chemical ions was described by the Fick＇s law. The ASR induced expansion was assumed to be directly related to the volume of produced alkali-silica gel. The finally expansion of a representative volume element （RVE） of concrete was then calculated according to the ratio of volume of alkali-silica gel and RVE. The input parameters of the model contains radius of reactive aggregate, volume fraction of reactive aggregate, initial concentration of chemical ions and porosity of cement paste. The applicability of the model was validated by an experiment of ASR-affected concrete specimens containing glass aggregate. It is shown that the amount of alkali-silica gel and ASR induced expansion can be well predicted. The expansion increasing with the decreasing aggregate size can be reproduced by the proposed model.

Multiscale Homogenization Analysis of Alkali–Silica Reaction (ASR) Effect in Concrete

The alkali silica reaction (ASR) is one of the major long-term deterioration mechanisms occurring in con- crete structures subjected to high humidity levels, such as bridges and dams. ASR is a chemical reaction between the silica existing inside the aggregate pieces and the alkali ions from the cement paste. This chemical reaction produces ASR gel, which imbibes additional water, leading to gel swelling. Damage and cracking are subsequently generated in concrete, resulting in degradation of its mechanical proper- ties. In this study, ASR damage in concrete is considered within the lattice discrete particle model (LDPM), a mesoscale mechanical model that simulates concrete at the scale of the coarse aggregate pieces. The authors have already modeled successfully ASR within the LDPM framework and they have calibrated and validated the resulting model, entitled ASR-LDPM, against several experimental data sets. In the pre- sent work, a recently developed multiscale homogenization framework is employed to simulate the macroscale effects of ASR, while ASR-LDPM is utilized as the mesoscale model. First, the homogenized behavior of the representative volume element (RVE) of concrete simulated by ASR-LDPM is studied under both tension and compression, and the degradation of effective mechanical properties due to ASR over time is investigated. Next, the developed homogenization framework is utilized to reproduce experimental data reported on the free volumetric expansion of concrete prisms. Finally, the strength degradation of prisms in compression and four-point bending beams is evaluated by both the mesoscale model and the proposed multiscale approach in order to analyze the accuracy and computational ef - ciency of the latter. In all the numerical analyses, different RVE sizes with different inner particle realiza- tions are considered in order to explore their effects on the homogenized response.

Efficacy of Aluminum Hydroxides as Inhibitors of Alkali-Silica Reactions

A comparative study of amorphous and crystalline forms of commercial aluminum hydroxides as inhibitors of alkalisilica reactions in Portland cement mortars has been performed. It was found that at dosages of 1% to 3%, amorphous aluminum hydroxide can efficiently inhibit alkali-silica expansion of Portland cement compositions. High inhibiting activity of amorphous Al(OH)3 additives may be explained by their ability to actively bind Ca(OH)2 formed by the hydration of silicate phases of cement, to form ettringite (with participation of gypsum). Crystalline Al(OH)3 additives that do not possess the ability to interact with Ca(OH)2 even after additional grinding, however, demonstrate week properties to inhibit alkali-silica expansion. This may indicate that the inhibitory effect of Al(OH)3 at least—partly, may be given by its influence on the concentration of Al3+ ions in the pore solution. Some expansion of the samples with admixtures of Al(OH)3 observed during the alkaline expansion accelerated test procedure is not associated with the formation of ettringite and is only due to alkali-silicate reactions.

The Impact of Aluminum- and Iron-Bearing Admixtures on the Resistance of Portland Cement Mortars to Alkali-Silica Reaction and Sulfate Attack

Study of sulfate resistance of mortars with aluminum- and iron-bearing admixtures (Al(OH)3, Al2(SO4)3, FeSO4, Fe2(SO4)3) in conditions close to those established in ASTM C 1012, and the study of the mitigation effect of these admixtures on alkali-silica reaction in accordance with accelerated “mortar bar” test ( GOST 8269.0, ASTM C 1260) were performed. Iron (II) and (III) sulfates show ability for mitigation alkali-silica reaction, while also, in contrast with Al-bearing substances, do not induce the drastic reducing of the initial setting time and do not promote the progress of sulfate corrosion. Compared with FeSO4, iron (III) sulfate has moderate deleterious impact on the early strength of cement paste and can be of interest alone as an inhibitor of ASR. Iron (II) sulfate may be used together with aluminum sulfate to offset the accelerating effect of the latter on the setting of cement paste and to reduce a risk of sulfate corrosion. During prolonged water storage, the mortar elongation and secondary ettringite formation do not occur, even when Al2(SO4)3 is available. Therefore, the investigated admixtures cannot act as agents of internal sulfate attack, however, Al2(SO4)3 can enhance the outer sulfate attack.

Effect of Pozzolanic Reaction Products on Alkali-silica Reaction

The effect of fly ash on controlling alkail-silica rection （ASR） in simudated alkali solution was studied. The expausion of mortar bars and the content of Ca（ OH）2 in cement paste cured at 80 °G for 91 d were measured. Traasmission electron microscopy （TEM） and high-resolution transmission electron microscot9 （HRTEM） were employed to study the microstructure of C-S-H. TEM/ energy dispersive spectroscopy （EDS） leas then used to determine the composition of C-S-H. The pore structure of the paste was analyzed by mercury intntsion porosimetry （MIP）. The results show that the contents of fly ash of 30% and 45% can well inhibit ASR. And the content of Ca（OH） 2 decreases with the increase of fly ash. That fly ash reacted with Ca（OH）2 to produce C-S-H with a low Ca/Si molar ratio could bind more Na^＋ and K^＋ ious, and produce a reduction in the amount of soluble alkali available for ASR. At the same time, the C- S- H produced by pozzolanic reaction converted large pores to snudler ones （ gel pores smaller than 10 nm ） to deusify the pore structure. Perhaps that could inhibit alkali trausport to aggregate for ASR.

Effect of Mineral Admixtures on Alkali-Silica Reaction

The influence of silica fume, slag and fly ash on alkali-silica reaction under the condition of 70 ℃ is studied. The results show that silica, slag and fly ash may inhibit alkali-silica reaction only under suitable content. When the content is less than 10%, silica fume does not markedly influence the expansion of alkali- silica reaction. When the content is 15%-20%, silica fume only may delay the expansion of alkali-silica reaction. When the content is 30%-70%, slag may only delay the expansion of alkali-silica reaction, but cannot inhibit the expansion of alkali-silica reaction. When the content is 10%, fly ash does not markedly influence the expansion of alkali-silica reaction. When the content is 20%-30%, fly ash may only delay the expansion of alkali-silica reaction, but cannot inhibit the expansion of alkali-silica reaction. When the content is over 50%, it is possible that fly ash can inhibit effectively alkali-silica reaction.

Effect of the Composite of Natural Zeolite and Fly Ash on Alkali-Silica Reaction

The effect of the composite of natural zeolite and fly ash on alkali-silica reaction (ASR) was studied with natural alkali-reactive aggregate and quartz glass aggregate respectively.The expansive experiment of mortar bar and concrete prism was completed.The results show that ASR can be suppressed effectively by the composite of natural zeolite and fly ash.

The Mechanism of the Eeffect of Mineral Admixtures on the Expansion of Aalkali-silica Reaction

On the base of the influence rule of silica fume, slag and fly ash on alkali-silica reaction under the condition of 70 ℃, the mechanism of the effect of mineral admixtures on alkali-silica reaction is studied further in the paper. The results show that the effects of mineral admixtures on alkali-silica reaction are mainly chemistry effect and surface physichemistry effect. Under suitable condition, the chemistry effect may make alkali-silica reaction to be inhibited effectively, but the physichemistry effect only make alkali-silica reaction to be delayed. The chemistry effect and the physichemistry effect of minerals admixture are relative to the content of Ca（OH）2 in system. Under the condition that there is a large quantity of Ca（OH）2, mineral admixture cannot inhibit alkali-silica reaction effectively. Only when Ca（OH）2 in the system is very less, it is possible that mineral admixture inhibits alkali-silica reaction effectively.

Calcined Clay Pozzolan as an Admixture to Mitigate the Alkali-Silica Reaction in Concrete

Calcined clay pozzolan has been used to replace varying portions of high alkali Portland limestone cement in order to study its effect on the alkali-silica reaction (ASR). Portland limestone cement used for the study had a total Na2Oeq of 4.32. Mortar-bar expansion decreased as pozzolan content in the cement increased. The highest expansion was recorded for reference bars with no pozzolan, reaching a maximum of 0.35% at 42 days whilst the expansion was reduced by between 42.5% and 107.8% at 14 days and between 9.4% and 16.4% at 84 days with increasing calcined clay pozzolan content. Mortar bars with 25% pozzolan were the least expansive recording expansion less than 0.1% at all test ages. X-ray diffractometry of the hydrated blended cement paste powders showed the formation of stable calcium silicates in increasing quantities whilst the presence of expansive alkali-silica gel, responsible for ASR expansion, decreased as pozzolan content increased. The study confirms that calcined clay pozzolan has an influence on ASR in mortar bars and causes a significant reduction in expansion at a replacement level of 25%.

Ultrafine Silica Additives Behavior during Alkali-Silica Reaction Long-Term Expansion Test

A silica fume, precipitated silica, metakaolin and siliceous fly ash behavior as constituents of mortars was studied, while mortar samples have been tested for long-term alkali-silica reaction expansion in accordance to the GOST 8269.0 specification. Solid-state 29Si-MAS NMR spectroscopy and thermogravimetric analysis were used to describe Portland cement hydration, supplementary cementitious material pozzolanic reaction and to establish a structure of products of those processes. It was found that long-term test conditions, in contrast to the accelerated test, do not affect the composition of products formed too much, compared to normal conditions. This allows results obtained with long-term test to be expected as more relevant in terms of predicting of supplementary cementitious materials inhibiting properties.

The Mitigation of Alkali-Silica Reactions by Aluminum-Bearing Substances

An ability of aluminum-bearing substances-amorphous aluminum hydroxide, aluminum sulphate and basic aluminum sulphate to mitigate alkali-silica reactions in Portland cement mortars has been studied. At equivalent dosages in terms of Al2O3, these substances are ranged in the following order in respect of inhibiting effect: Al(OH)1.78(SO4)0.61 ≥ Al2(SO4)3 > Al(OH)3. It is found that the plasticizing agents of the main types used in cement compositions have no influence on the inhibiting effect of aluminum-bearing admixtures. To control the setting time of cement paste, iron(II) sulphate may be used for partial substitution of Al2SO4·18H2O, and this operation is not influence on the results of ASR expansion test.

The Mechanism of Ground Granulated Blastfurnace Slag Preventing Alkali Aggregate Reaction

Three different methods were applied to study the alkali content of gelpores in cement. In the closed system, the concentration of K＋, Na＋ and OH - have not reduced with the increase of age. In the open system, the diffusion and transferring of K＋ and Na＋ towards free space leads to the de-crease of total alkali content. In the micro-analysis system, the contents of K＋ and Na＋ in the first hy- drated layer of ground granulated blastfurnace slag （GBFS） are very low, while the contents of calcium and magnesium are relatively high. This phenomenon shows that the mechanism of GBFS preventing alkali aggregate reaction （AAR） is： when GBFS is dissolved by alkali medium, SiO2 and Al2O3 are dissolved into the cement matrix, then around GBFS particles form reaction rings rich in Ca2＋ and Mg^2＋, and the C-S-H gel of positive charges formed in the area repulses K＋ and Na＋, which are forced to transfer to the mortar＇s matrix, pore or mortar sample surface. The transferred K ^＋ and Na^＋ form alkali gel products with other dissolved ions, then become evenly distributed in the mortar sample and react with Ca（OH）2 in pore solutions to form （Na,K）x-2z·zCa·（SiO2）y·（OH）x gel products; and thus changes the AAR gel products＇ structure. The gel products will not expand, and so they can delay expansion destruction.

Effect of Glass Powder on Chloride Ion Transport and Alkali-aggregate Reaction Expansion of Lightweight Aggregate Concrete

The effects of glass powder on the strength development, chloride permeability and potential alkali-aggregate reaction expansion of lightweight aggregate concrete were investigated. Ground blast furnace slag, coal fly ash and silica fume were used as reference materials. The re- placement of cement with 25% glass powder slightly decreases the strengthes at ？ and 28 d, but shows no effect on 90 d＇s. Silica fume is very effective in improving both the strength and chloride penetration resistance, while ground glass powder is much more effective than blast furnace slag and fly ash in improving chloride penetration resistance of the concrete. When expanded shale or clay is used as coarse aggregate, the concrete containing glass powder does not exhibit deleterious expansion even if alkali-reactive sand is used as fine aggregate of the concrete.

Alkali Aggregate Reaction in Alkali Slag Cement Mortars

By means of 'Mortar Bar Method',the ratio of cement to aggregate was kept as a constant 1∶2.25,the water cement ratio of the mixture was 0.40,and six prism specimens were prepared for each batch of mixing proportions with dimensions of 10×10×60mm 3 at 38±2℃ and RH≥95%, the influences of content and particle size of active aggregate, sort and content of alkali component and type of slag on the expansion ratios of alkali activated slag cement(ASC) mortars due to alkali aggregate reaction(AAR) were studied. According to atomic absorption spectrometry,the amount of free alkali was measured in ASC mortars at 90d.The results show above factors affect AAR remarkably,but no dangerous AAR will occur in ASC system when the amount of active aggregate is below 15% and the mass fraction of alkali is not more than 5% (Na 2O).Alkali participated in reaction as an independent component, and some hydrates containing alkali cations were produced, free alkalis in ASC system can be reduced enormously.Moreover,slag is an effective inhibitor, the possibility of generating dangerous AAR in ASC system is much lower at same conditions than that in ordinary Portland cement system.

Solid State Reaction Yielding a Mineral Utilizing Silica Obtained from an Agricultural Waste

Wollastonite, a mineral of wide industrial applications was synthesised from rice husk ash silica and limestone. A number of raw batches consisting of these starting materials, in 1:1 molar ratio, were heat treated to produce it through solid state reaction from 900℃ to 1300℃. The conducted reaction was monitored by XRD step by step. Amount of Wollastonite formed at every temperature was also studied to some extent. Analyses of the obtained data indicated that the target mineral formation was quite effective and almost proportional to a rise in temperature up to 1200℃. The results from both, XRD and chemical analysis were found in fair agreement with one another

Preparation of Lead-free Base Glaze Suitable for Pearlescent Pigments by Low-temperature Solid-phase Reaction with Alkali Waste

A lead-free base glaze suitable for pearlescent pigments was prepared by a low-temperature solid-phase reaction with alkali waste.Tests were performed to evaluate the effects of the sintering conditions and alkali waste composition on the prepared base glaze and pearlescent glaze.The experimental results show that partially replacing SiO_(2) with B_(2)O_(3) effectively reduced the sintering temperature and time to form a glass network,but the network structure becomes disconnected as the B_(2)O_(3) content increases.An amorphous base glaze was obtained when soda ash was replaced with a small amount of alkali waste,but increasing the addition of NaCl further was adverse to base glaze formation by resulting in crystallization of the base glaze and a decrease in the bridging oxygen content.The pearlescent pigment was thermally stable in the glaze at 750℃,while higher temperatures caused the crystalline phase of NaAlSiO_(4) to appear and adhere to the surface of pigment granules,which degraded the pearlescent effect of the glaze.

Chemical Kinetic Aspects of Solid State Reaction Producing Wollastonite from Rice Husk Silica and Limestone

An industrial mineral wollastonite (CaSiO3) was produced under solid state conditions from rice husk silica and limestone. Reaction was carried out at 900'C to 1300'C for 1 h. The product batches were subjected to XRD and chemical analysis techniques specific for wollastonite. Mole fractions of different product batches were calculated on the basis of accumulated data to study the kinetics. Specific rate constants and reaction rate were also found out. Various probable models of mechanism for reaction were considered and testified with the laid down criterion for suggesting the suitable one. The resulting data were treated with Arrhenius equation as well and activation energy was calculated--therefrom. In addition to finding it's value from the slope of Arrhenius curve, an alternate method was also applied for this purpose. Both of the values were observed to be comparable. The activation energy required for performed reaction was found to be almost one third of that reported for synthesizing CaSiO3 by using quartz. This referred to the economical preparation of wollastonite by using rice husk as a source of silica instead of quartz.