Effects of alkali addition on the mechanical properties and durability of concrete

https://doi.org/10.1016/j.cemconres.2004.05.007Get rights and content

Abstract

Increasing the concrete alkali content from 0.6% to 1.25% of Na2Oe of the cement mass by adding NaOH to the mixture water has harmful effects on most mechanical properties (compressive, splitting, direct tensile, and flexure strengths) of concrete made with a water-to-cement (w/c) ratio of 0.41 and limestone aggregates not susceptible to alkali–silica reaction (ASR), however not on the elasticity modulus measured under compression or direct tension. Shrinkage tests at 50% RH and 23 °C started after 7 days at 100% RH and 23 °C show that the low-alkali concrete shrinks more than the high-alkali one, despite similar water losses. Freeze–thaw tests performed on air-entrained concretes show that the two concretes resist well to freezing and thawing while showing similar air–void systems. When examined under the scanning electron microscope (SEM), the hydrates in the two concretes present similar microstructure; however, the high-alkali concrete shows a more reticular and porous microtexture, which could explain the reduction in strength.

Introduction

The effects of alkalies on various properties of cement paste, mortar, and concrete, including early hydration and setting, bleeding, strength development and ultimate strength, drying shrinkage, susceptibility to cracking, microtexture of cement hydrates, and durability, have been a subject of concern since many decades. Reviews on the subject can be found in Burrows [1] and Jawed and Skalny [2]. From the literature then available, the latter [2] concluded that significant proportions of alkalies in clinker can cause a quick setting, reduce the ultimate strength of concrete, and increase expansion under water and shrinkage under drying conditions. As discussed hereafter, based on limited or conflicting information in many respects, the present paper addresses the effects of alkalies on strength development, drying shrinkage, freeze–thaw durability, and microstructure of concrete.

Burrows [1] participated in a long-term test program that consisted in the fabrication of 104 concrete panels made with 27 different cements and that were placed in 1943 at Green Mountain dam in Colorado. Fifty-three years later, he found that the amount of cracking observed on these panels was directly proportional to the alkali content in the cements used, thus suggesting that high-alkali cements are more susceptible to drying shrinkage. Blaine et al. [3] performed shrinkage tests on cement pastes made with 199 different cements. They found that cement pastes containing more alkalies were undergoing more shrinkage; however, they did not observe any correlation between the alkali content and the amount of drying shrinkage for their companion concrete specimens. Based on the work by Carlson [4], Burrows [1] attributes this behaviour to the development of microcracking within the concrete subjected to drying, which decreases the bulk shrinkage (since the measurements account for combined effects of contraction due to shrinkage and expansion due to microcracking).

So, it has been well demonstrated that a higher cement alkali content tends to increase the susceptibility to shrinkage under drying conditions in the case of cement pastes and mortars. However, this effect is not so clear in the case of concrete.

A number of experimental studies showed that the higher the alkali content in the cement, the lower the ultimate strength of the corresponding test specimens [2], [5]. From their study with 199 commercial portland cements, Blaine et al. [3] concluded that a higher cement alkali content statistically resulted in a higher dynamic modulus of elasticity measured on corresponding concrete specimens after 14 days. Osbaeck [6] concluded that a higher alkali content in cement accelerates the strength development in the short term but decreases the ultimate strength. From their literature review, Jawed and Skalny [2] also concluded that a high alkali content in cement generally results in a higher strength at early age, but to a lower strength after 28 days. Gouda [7] observed a similar trend for concrete specimens made with low-alkali (0.58% Na2Oe) and very high-alkali (1.76% Na2Oe) cement except that the turning point was at 7 days.

A very low cement alkali content can result in abnormally low strength developments at early ages [8], which was also noted by other researchers [9], [10]. On the other hand, Odler and Wonnemann [11] observed that the alkalies incorporated into the cement clinker did not affect the compressive strength, whereas an external addition of alkali sulfate considerably reduces the strength at any age up to 28 days [10]. Alexander and Davies [12] observed that the higher the alkali increase, the lower the compressive strength of cement pastes; however, intermediate alkali additions increased the modulus of rupture whereas smaller and larger additions decreased it.

In the case of concrete specimens placed outdoor by Burrows [1] at the Green Mountain dam, the least cracked of all the 104 concrete panels made after 53 years of field exposure was the one made with a very low alkali cement (0.23% Na2Oe) [1]. On the other hand, the same concrete was also presenting the lowest 7-day compressive strength of all ASTM Type I cements investigated, which is coherent with a lower early rate of hydration.

Vivian [13] subjected to flexure tests mortar specimens containing alkali–silica reactive and nonreactive aggregates. The mortars were made with a cement containing 0.59% Na2Oe, and NaOH additions of 0%, 0.5%, 2%, and 4.5% (by mass of cement), which correspond to total alkali contents of 0.59%, 0.98%, 2.14%, and 4.08% Na2Oe, respectively (by mass of cement). The author observed that the addition of NaOH did not reduce the tensile strength of the mortars containing nonreactive aggregates, except for mixtures containing 4.08% Na2Oe, which content is, however, unrealistic. It must be mentioned that all mortars with a lower alkali content were made with a water-to-cement (w/c) ratio of 0.50 or 0.52, whereas a higher ratio (0.60) was used for the 4.08% Na2Oe mixture, which could explain the decrease in strength by itself. Moreover, the study was limited to mortar specimens.

Shayan and Ivanusec [9] studied the changes in the mechanical properties (compressive strength and modulus of rupture) and microstructural characteristics of cement pastes and mortars of various alkali contents. The two cementitious systems were made with w/c ratios of 0.4 and 0.6, a cement containing 0.84% Na2Oe, with and without reactive aggregates (in the case of mortars). The alkali content of the different systems was adjusted to the level desired by using mixture waters made of 0, 0.5, 1.0, 2.0, and 4.5 M NaOH solutions. The corresponding total alkali contents (by mass of cement) were 0.84%, 1.9%, 3.0%, 5.1%, and 10.5% Na2Oe for the pastes or mortars with a w/c of 0.6; they were 0.84, 1.6, 2.3, 3.8, and 7.4% Na2Oe for those with a w/c of 0.4. The authors found that the higher the alkali content, the lower the compressive strength and modulus of rupture at any age (i.e., 7, 28, and 90 days); however, the highest reduction in strength was obtained for alkali contents comprised between the minimum tested and about 3% Na2Oe. Unfortunately, just one of all alkali contents investigated by the authors falls between the range of 0.5% to 1.3% Na2Oe (by mass of cement) used for most field concretes. Moreover, the study was limited to cement pastes and mortar specimens.

Unfortunately, most studies on the effect of alkalies on the mechanical properties of hydraulic systems were performed on cement pastes and/or mortar specimens. Moreover, the alkali content was adjusted to various levels by using different cements of different alkali contents; consequently, the alkali content was not the single parameter involved, except maybe in a few studies [9], [13], also on cement pastes and/or mortars, for which the results are conflicting, at least within a realistic range of alkali content. In addition, in most studies, the mechanical properties involved were usually limited to compressive strength.

Burrows [1] reports that in the case of the concrete specimens placed at the Green Mountain dam, the least cracked concrete panel after 53 years, made with a very low alkali cement (0.23% Na2Oe), was also presenting the lowest resistance in the ASTM C 666 freezing–thawing test; it must be mentioned that no panel was undergoing alkali–silica reaction (ASR). Backstrom and Burrows [14] measured the number of freezing–thawing cycles to reach 0.1% expansion for concretes exposed outside and made with five cements of different alkali contents (0.25%, 0.41%, 0.60%, 0.74%, and 0.93% Na2Oe), each ground at five different finenesses. For each cement, they observed that the finer the grind, the less durable the concrete. Moreover, for a given cement fineness, the cement with the lowest alkali content (0.25% Na2Oe) always gave the most resistant concrete, followed far behind by the one with 0.41% Na2Oe. Burrows [1] suggested that the better performance in the freezing–thawing test (conducted outside) of low-alkali, coarse ground cements, is attributed to less microcracking from drying and weathering.

However, the effect of alkalies on freeze–thaw durability was not so clear for the three other alkali contents investigated by Backstrom and Burrows [14]. Indeed, a reverse trend was observed, with the cement of 0.93% Na2Oe resulting in the best durability (just a little worse than the cement with 0.41%), followed by the cement with 0.74%, then by the one with 0.60%. Moreover, the results by Plante et al. [15] suggest that a higher alkali content in concrete could be positive for the stability of the air–void spacing factor, which could then improve the freezing–thawing durability of concrete made with a higher alkali content.

Thus, the effect of alkalies on freezing–thawing durability is still not clear at least within the range of cement alkali contents normally used in field concrete (i.e., 0.5% to 1.3% Na2Oe).

According to Jawed and Skalny [2], “when alkali-containing cement is mixed with water, the alkali metal ions readily go into the liquid phase of the hydrating system and influence the rate of cement hydration and the morphology of the hydration products. This, in turn, affects the strength and other engineering properties of the concrete.” The effect of alkali on microstructure is reviewed by Burrows [1] and Jawed and Skalny [2]. The second authors report from the literature that the microstructure of CSH becomes coarser and heterogeneous in alkali solutions and the lower strength development at later age is due to this heterogeneous structure. Moreover, the shape of hydrates formed depends of the alkali anion (i.e., sulfate or carbonate or hydroxide). Vivian [16] reported that in the presence of high alkali contents, the products of hydration tend to be gelatinous rather than crystalline; however, such changes in the microtexture of cement hydrates were observed only when the alkali content of the mortar tested exceeded 4.5% Na2Oe, which is quite unrealistic.

Under the scanning electron microscope (SEM), Shayan and Ivanusec [9] observed that high-alkali cement pastes had a less dense microstructure compared to low-alkali pastes. From another study where they investigated the nature of solid phases formed during early hydration of a portland cement in solutions of various NaOH concentrations, Way and Shayan [17] did not observe any change for concentrations up to 1 M; however, for concentrations of 2 and 4.5 M, the CSH contained significant amounts of sodium and new phases were even formed. Shayan and Ivanusec [9] then attributed the cracking of cement pastes made with high NaOH concentrations to the formation of these new phases. It must be recalled that such alkali concentrations correspond to total alkali contents that are unrealistic. Since the sharpest decrease in strength observed by the authors arose at low levels of alkali addition, i.e., before new phases were formed, the authors then suggested that substitution of Na in CSH likely also occurs at lower alkali addition, which has a significant detrimental effect on the development of the mechanical properties of the hydrating cement phases. Suzuki et al. [18] also showed that sodium was incorporated into CSH in significant amounts in basic NaOH solution.

The following statement by Jawed and Skalny [2] still represents the present state of the knowledge about the effects of alkalies: “The effects of alkalies on the role of hydration and on the nature and morphology of hydration products influences the engineering properties of concrete such as setting time, strength development, shrinkage, volume expansion, etc. However, one finds conflicting reports in the literature regarding the effects of alkalies on these properties, this presumably reflecting the use of cements produced from different raw materials and of technical processes involving different parameters. The strength development of a given cement is controlled by several factors (e.g., clinker composition, cement fineness, gypsum content, and its degree of dehydration), and it may not be easy to single out one factor responsible for changes in strength of cement paste or concrete produced from a given cement”.

In fact, the effects of alkalies on most properties of concrete are still not well understood, for the following reasons: (1) Most studies involved cement paste and/or mortar specimens. (2) The different alkali contents were usually achieved by using cements with different alkali contents, such as the alkali content is not the single parameter involved. (3) The compressive strength was often the only mechanical property investigated in most studies. The available information is also conflicting as concerns the effect of alkali on drying shrinkage (i.e., cement paste and mortar vs. concrete), whereas very limited as concerns freezing–thawing durability.

It is well known that concrete, in the presence of reactive aggregates and sufficient quantities of alkalies and moisture, can undergo deleterious expansion and cracking due to ASR. Testing for ASR commonly requires raising the alkali content of concrete to accelerate the reaction/expansion processes. This may in itself significantly affect the mechanical properties of concrete, i.e., induce reduction in strength that is not due to ASR only. Some authors effectively pointed out that the strength reductions observed for concrete affected by ASR can be partly attributed to the alkalies present in significant amounts in the concrete.

As concerns cracking, ASR-affected concrete can also exhibit map cracking that may have originated from drying shrinkage and this may sometimes cause misleading in the diagnosis of the cause of cracking. In his review paper, Burrows [1] discusses a number of cases where drying shrinkage cracking due to high-alkali cement was incorrectly diagnosed as ASR.

With the main objective to better interpret the reduction in the mechanical and durability properties of field and laboratory concretes affected by ASR, which are usually alkali rich, this study was undertaken to isolate the effect of alkalies on a number of properties of “nonreactive” concrete, which includes strength development (compressive, indirect tension, direct traction, modulus of rupture, modulus of elasticity under compression and traction), drying shrinkage, air–void system, and freezing–thawing durability. All tests are performed on concrete specimens with two alkali contents (0.6% and 1.25% Na2Oe with respect to the cement), which cover the range of alkali content used in most field concretes. All specimens are made with the same (low alkali) cement, however, adding NaOH to the mixture water in the case of the high-alkali concrete. This is not usual for field concrete; however, it is current practice when testing aggregates for potential alkali reactivity or when investigating in the laboratory the effect of ASR on the mechanical properties of concrete, while being the only way to isolate the effect of alkalies.

Section snippets

Materials and characteristics of concrete mixtures

Low-alkali (0.6% Na2Oe by mass of cement) and high-alkali (1.25% Na2Oe) concrete mixtures were made with nonreactive coarse (limestone) and fine (granitic sand) aggregates, with and without air entrainment. The basic cement used was a 50–50 blend of two normal (CSA Type 10 or ASTM Type I) cements with alkali contents of 0.4% and 0.8% Na2Oe, respectively (Table 1). The high-alkali concrete was obtained by adding reagent-grade NaOH pellets dissolved in the mixture water to raise the alkali level

Making, curing, and testing of concrete specimens

A series of concrete prisms, 50×75×350 mm in size, and cylinders, 100×200 mm in size, were made from the concrete mixtures described before and subjected to various mechanical tests at different ages. All the test specimens were stored in a moist curing room at 23 °C and 100% RH until due for testing. Considering the small size of the specimens, attention was given to try minimizing the leaching of alkalies. Specimens were cured in boxes with no direct contact with water till the last testing

Results of mechanical testing

The results of the mechanical tests performed in this study are given in Table 3. All results presented are the mean for three specimens.

Results of drying shrinkage testing

Fig. 3 illustrates the average mass loss and drying shrinkage of test prisms cast from the low- and high-alkali concretes and subjected to drying shrinkage. Fig. 3A shows that both low- and high-alkali specimens suffered quite similar mass losses over time; however, the test specimens cast from the high-alkali concrete mixture showed slightly less drying shrinkage compared to those cast from the low-alkali mixture (Fig. 3B). Picket [19] found that adding external alkalies as potassium sulphate

Results of freezing and thawing testing

Backstrom and Burrows [14] found that a very low alkali concrete can perform badly in the ASTM C 666 durability test without suffering major deterioration in the field. On the other hand, Plante et al. [15] observed that the use of high-alkali cements could improve the stability of the air–void system of air-entrained concrete. However, since the high-alkali concrete tested in our investigation shows relatively lower tensile strength and modulus of rupture, it could potentially be less

Microstructure investigation

Cement pastes of w/c of 0.4 and 0.5 were cast at the 0.6% and 1.25% Na2Oe alkali levels. The paste specimens were cast and stored at 23°C and 100% RH. They were analyzed by X-ray diffraction and examined under the SEM at various ages to determine if the addition of alkalies caused differences in the nature and morphology of the cement hydrates.

X-ray diffraction showed the same peaks for the two systems with no indication of different phases formed. However, SEM observations showed that the

Discussion

The various tests performed in this study indicate that the addition of alkalies in the form of NaOH, which is commonly done when evaluating aggregates for potential alkali–silica reactivity and the effect of ASR on the mechanical properties of concrete, can cause reductions in compressive, tensile, splitting, and flexure (modulus of rupture) strengths. SEM observations suggest that this could be related to the formation of hydration products with a weaker (reticular) microtexture and a

Conclusion

The present study on the effect of alkalies on a number of concrete properties was undertaken due to limited or conflicting information in the literature in many respects, particularly as concerns strength development, ultimate strength, drying shrinkage, and durability against freezing and thawing. We can conclude the following from the results obtained:

  • The addition of alkali in the form of NaOH causes significant reductions in the compressive, splitting, direct tensile, and flexure (modulus

References (20)

There are more references available in the full text version of this article.

Cited by (137)

View all citing articles on Scopus
View full text