Jan Olek

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.

Sulfate attack research — whither now?

Sulfate attack research is at a critical stage. In spite of meaningful advances in the past few years, this problem is still not well understood. Due to its complicated mechanism, the reaction between cement hydration products and sulfate-bearing solutions manifests itself in a variety of ways. In order to provide adequate means for selection of materials for concrete exposed to such aggressive environments, additional research is necessary to further clarify the interaction between concrete and sulfate-bearing solutions. Specifically, the role of the cation in the sulfate solution, and the effects of formation of various products like gypsum, ettringite, and thaumasite, on the extent of damage need to be investigated. The available testing methods for sulfate attack have been subject to some criticism lately. Although these test methods can give an indication of the mechanisms involved in sulfate attack, prediction of field performance using lab studies is difficult. Efforts are needed to introduce appropriate changes in the tests in order to obtain field-like conditions in the laboratory. Combined with good monitoring methods, this would enable the prediction of service life of structures exposed to sulfate solutions. Recent advances in nondestructive testing techniques can be applied to the task of monitoring field structures, although there is a significant effort necessary to calibrate these methods for sulfate attack-related scenarios. In order to produce efficient concrete designs for service in aggressive environments, it is imperative to develop reliable models. Modeling can help in selecting the appropriate materials and their proportions, as well as in determining service life parameters. As a first step towards modeling, critical parameters, which serve as an indicator of deterioration, need to be recognized and established. This paper discusses these issues, and cites some interesting recent developments. Finally, some recommendations for future studies are provided.

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

Abstract 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).

INFLUENCE OF AGGREGATE SIZE, WATER CEMENT RATIO AND AGE ON THE MICROSTRUCTURE OF THE INTERFACIAL TRANSITION ZONE

Abstract This paper presents the results of an investigation on the effect of water–cement ratio (w/c), aggregate size, and age on the microstructure of the interfacial transition zone (ITZ) between normal weight aggregate and the bulk cement paste. Backscattered electron images (BSE) obtained by scanning electron microscope were used to characterize the ITZ microstructure. The results suggest that the w/c plays an important role in controlling the microstructure of the ITZ and its thickness. Reducing w/c from 0.55 to 0.40 resulted in an ITZ with characteristics that are not distinguishable from those of the bulk paste as demonstrated by BSE images. Aggregate size appears to have an important influence on the ITZ characteristics. Reducing the aggregate size tends to reduce the ITZ porosity. The evolution of the ITZ microstructure relative to that of the bulk paste appears to depend on the initial content of the unhydrated cement grains (UH). The results suggest that the presence of a relatively low amount of UH in the ITZ at early age may cause the porosity of the ITZ, relative to that of the bulk paste, to increase with time. The presence of relatively large amount of UH in the ITZ at early ages may cause its porosity, relative to that of the bulk paste, to decrease with time.

MECHANISM OF PLASTIC SHRINKAGE CRACKING IN PORTLAND CEMENT AND PORTLAND CEMENT-SILICA FUME PASTE AND MORTAR

Abstract The objective of this paper is to discuss the mechanism of plastic shrinkage in portland cement and portland cement-silica fume paste and mortar. The effects of the three delivery forms of silica fume; asreceived powder, densified powder, and slurry, on plastic shrinkage are presented. In paste, plastic shrinkage is primarily related to development of tensile capillary pressure during drying. The higher the surface area of particles, the higher the tensile capillary pressure, and consequently, the more vulnerable the system would be to plastic shrinkage cracking. In mortar, plastic shrinkage is controlled by both capillary pressure in the paste and presence of fine aggregate particles. These particles serve to reduce cracking by (a) arresting cracks, and (b) refining the size and distribution of capillary pores. Data of rate of evaporation of water can only indicate degree of drying. Charts developed by Portland Cement Association to calculate the rate of evaporation can not predict whether or not plastic shrinkage cracks occur. A quantitative method could perhaps be developed to predict plastic shrinkage cracking.

The influence of metakaolin and silica fume on the chemistry of alkali–silica reaction products

Abstract This investigation studies the influence of two mineral admixtures, silica fume (SF) and high-reactivity metakaolin (HRM), on the chemistry of alkali–silica reaction (ASR) products. Four different mortar bar mixes containing different combinations of high-alkali cement, alkali–inert dolomitic limestone, reactive Beltane opal, HRM, and SF were prepared and stored in a 1 N NaOH solution at 80°C (ASTM C 1260) for 21 days. Expansion of bar specimens was measured, and chemical analysis was performed at different ages using X-ray spectra and maps. Test results confirmed that HRM and SF significantly reduce expansion due to ASR. In addition, X-ray microanalysis showed that calcium content increases with time in ASR products. Furthermore, it was found that as ASR proceeded the calcium content of reaction products increased proportionally as the silica content decreased.

Effects of gypsum formation on the performance of cement mortars during external sulfate attack

Abstract Sodium sulfate attack was studied on C 3 S mortars, along with ASTM Type I Portland cement (PC) mortars, in an attempt to independently evaluate the effect of gypsum formation on the performance. The quantity of gypsum and ettringite, as measured by differential scanning calorimetry (DSC), increased with the time of immersion in the sulfate solution. An increase in length of the mortar specimens was also registered along with the increase in the quantity of gypsum. This result suggests that the formation of gypsum could be expansive. Indeed, considerable expansion, although delayed compared to PC mortars, was observed in the C 3 S mortars. Thus, it can be concluded that the expansion of the PC mortars occurred due to the combined effect of gypsum and ettringite formation, while the expansion of C 3 S mortars occurred as a result of gypsum formation. Thaumasite formation as small inclusions was also detected in both the C 3 S and the PC mortars, especially in regions of high gypsum deposition. The formation of thaumasite, despite the absence of carbonate bearing minerals and low temperatures, could be because of the carbonation of the surface zones of the mortars. However, it would be speculative to attribute any expansion to the formation of thaumasite, since it was detected only in minute amounts in the microstructural investigation.

Acoustic performance and damping behavior of cellulose-cement composites

Abstract This paper describes the influence of morphologically altered cellulose fibers on the acoustic and mechanical properties of cellulose–cement composites. Three fiber morphologies were considered (macro-nodules, discrete fibers, and petite nodules). The main parameters studied include the normal incident acoustic absorption coefficient (α), specific damping capacity (ψ), loss tangent (tanδ), storage modulus (E′), and loss modulus (E′′=E′tanδ). The acoustic absorption coefficient was found to increase with an increase in fiber volume for all three fiber types investigated, though “macro-nodule” fibers were found to be the most effective. Stiffness–loss relationships are reported for these composites and the behavior of cellulose–cement composites with soft cellulose fiber inclusions was found to be similar to a Voigt (series) composite model. Low volumes of fibers had a minimal effect on the loss tangent; however the stiffness was considerably reduced. Predictive equations for loss modulus as a function of fiber volume at different moisture conditions were developed. These relations compare well with the experimental values as well as the idealized Voigt composite behavior. This suggests that there is an optimum fiber volume, which maximizes the loss modulus for saturated composites while the loss modulus is practically independent of fiber volume for dry composites.

Effects of Sample Preparation and Interpretation of Thermogravimetric Curves on Calcium Hydroxide in Hydrated Pastes and Mortars

Calcium hydroxide [Ca(OH)2] is one of the major constituents of hydrated portland cement paste. Its content can be used to trace the progress of cement hydration or serve as an indicator of the extent of pozzolanic reaction. The thermogravimetric analysis (TGA) method is often used to determine the Ca(OH)2 content because it is a relatively easy and fast procedure. However, no universally accepted method exists for the preparation of TGA specimens and for the interpretation of the resulting TGA curves. This paper presents an investigation on the contents of Ca(OH)2 in samples subjected to different preparation techniques. The results showed that a certain amount of calcium carbonate (CaCO3) was produced as a result of carbonation during the sample preparation process. The degree of carbonation was dependent on the sample preparation, and carbonated Ca(OH)2 was considered to determine the accurate total Ca(OH)2 content. In addition, a modified interpretation of the TGA curve for Ca(OH)2 was suggested. In this interpretation, the mass losses caused by the other hydration products, except for the Ca(OH)2 and the carbonated Ca(OH)2, were considered so that the accurate content of Ca(OH)2 could be determined. The interpretation technique was verified by comparing the results with those obtained by differential scanning calorimetry. Ultimately, the actual contents of Ca(OH)2 in pastes undergoing different sample preparation techniques were determined by using the modified interpretation of the TGA curve for the Ca(OH)2. The results showed that this interpretation yielded comparable contents of Ca(OH)2 in most of the sample preparation techniques used in this study.

Studies on delayed ettringite formation in heat-cured mortars: II. Characteristics of cement that may be susceptible to DEF

Abstract Expansions of mortar bars, stored over (but not in) water after simulated steam curing to 85 °C, were related to certain cement compositional parameters. The relationship is expressed in the form of a “delayed ettringite formation (DEF) index.” The DEF index is computed as the joint product of the SO 3 /Al 2 O 3 molar ratio of the cement, the sum of its SO 3 and Bogue C 3 A percentages divided by 10 and the square root of the alkali content expressed as equivalent % Na 2 O. The mortars studied were made with 18 different cements, prepared from a set of six representative clinkers by incorporating Terra Alba gypsum to total SO 3 contents that were 1% below optimum, at optimum and 1% above optimum (as defined in ASTM C 563). Measurements of expansion were recorded at intervals for up to 1400 days. Severe cracking and prominent DEF-induced expansions were observed in mortar bars derived from four of the six ‘oversulfated’ cements and lesser expansions from three of the six cements prepared at optimum SO 3 contents. No expansion was found for cements of DEF index below a threshold value; above this value expansions were approximately proportional to the difference between DEF index and its threshold value. The relationship confirms the significance of all three compositional parameters making up the index, e.g., the SO 3 /Al 2 O 3 molar ratio, the joint contents of SO 3 and C 3 A, and the alkali content, in influencing the extent of DEF-induced expansion. In these measurements, the apparent pessimum effect for SO 3 content previously reported by others was not found, although SO 3 contents examined spanned the supposed pessimum value of 4%. Rather, expansion increased with increasing SO 3 content for mortars made with all clinkers exhibiting expansion.