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Article

The Effect of Superplasticizer Addition on the Properties of Calcium Sulfoaluminate Mortars

by
Małgorzata Gołaszewska
* and
Jacek Gołaszewski
Faculty of Civil Engineering, Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(18), 8460; https://doi.org/10.3390/su17188460
Submission received: 22 August 2025 / Revised: 14 September 2025 / Accepted: 17 September 2025 / Published: 20 September 2025
(This article belongs to the Section Sustainable Chemical Engineering and Technology)
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Abstract

Practical use of calcium sulfoaluminate cements (CSAs) is dependent on their compatibility with admixtures. The following paper presents research into the effects of three different superplasticizers (SPs) (polycarboxylate ethers, modified polycarboxylates, and polynaphthalene sulfonate), and the effect of a w/c ratio in a range of 0.45–0.35 in mortars containing superplasticizer on the chosen mortar properties. The conducted tests related to consistency, initial setting time, hydration heat, flexural and compressive strength, early shrinkage (first 20 h), and drying shrinkage. The results indicate that the superplasticizer type has significant effect on the properties of CSA mortars. All superplasticizers prolonged the initial setting and induction phase of hydration in relation to CSA mortar which did not contain superplasticizer by up to 109%; however, their effect on compressive and flexural strength, drying shrinkage, and early shrinkage was dependent on the type of superplasticizer involved. Polycarboxylate ether SP provided the best results for mortar properties, as it did not affect compressive strength significantly, but reduced plastic shrinkage. Polynapthalene-based SP decreased strength and increased shrinkage more than other superplasticizers, making it the least compatible. Decreasing the w/c ratio for mortar containing superplasticizer allowed us to mitigate some of the issues, as mortars with SP1 in a low w/c ratio exhibited higher compressive and flexural strength by, respectively, 41% and 80% in the case of a w/c ratio of 0.35, and lower shrinkage.

1. Introduction

Calcium sulfoaluminate cements (CSAs) are among the most promising binders that provide a more sustainable alternative to Portland cement [1]. CSA clinker production emits significantly lower amounts of CO2 than Portland clinker due to the lower heat necessary to obtain proper phases and the lower amount of limestone undergoing burning, as around 35% of the feedstock used for CSA production is bauxite instead of limestone. This provides a significant benefit for the sustainability of the current concrete industry, as the cement production process remains one of the main emitters of CO2, to the point that it accounts for up to 8% of all anthropogenic CO2 emissions [2,3,4]. The main reason for this is the difference in composition of the raw materials, as instead of limestone (which, when burnt, is the main source of CO2 emissions), the main constituents of clinker input are bauxite or other alumina-rich materials, which do not emit as much CO2 during burning, significantly decreasing the carbon footprint of the CSAs [2,3,4]. Additionally, due to the low availability of alumina-rich raw material, which is more likely to be used in the production of aluminum, a significant portion of the research into CSAs is concentrated on methods of production using wastes and byproducts of aluminum production as well as iron and steel industries, often with very promising results [5]. This indicates that the use of CSAs may provide a way to utilize metallurgical industry waste that would otherwise be landfilled, which is an additional benefit in terms of sustainability. One of the barriers preventing more widespread use of CSAs is that it requires a different approach than Portland cement due to its different composition, which results in different properties.
CSA clinker consists mainly of ye’elimite (C4A3Ŝ) in an amount of 50–80%; belite (C2S) in an amount of 5–20%; and small amounts of other phases such as calcium sulfate (CŜ) or ferrite (C4AF), while in Portland cement, the main phases are alite C3S (comprising 50–90% of the cement mass) and belite C2S (comprising 10–40% of the cement mass) and up to 12% of other phases such as ferrite (C4AF) and tricalcium sulfate (C3A) [6,7,8,9,10]. This, in turn, causes the products of the hydration to differ significantly. Instead of calcium silicate hydrates (C-S-H), the main products of hydration are ettringite (C6AŜ3H32), strätlingite, and monosulfate, with only small amounts of C-S-H phase present due to the belite reaction [11,12]. This leads to CSAs exhibiting faster initial setting than Portland cements and very high early strength, low early drying shrinkage, and low resistance to carbonation, all of which are mostly related to their high ettrigite content [3,7,13,14,15,16,17,18].
Due to the difference in cement composition, the cement admixtures developed for Portland cement may have different effectiveness for CSA, especially since alumina phases in Portland cement have been found to react chemically with superplasticizers that significantly affect the hydration of cement [19,20,21,22]. With the abundance of alumina phases, mostly ettringite, it stands to reason that effects not usually observed in Portland cement can be present in CSAs.
Some studies have explored compatibility issues between CSAs and superplasticizers; however, the amount of in-depth studies on this topic is very limited. As could be predicted, the majority of studies have concentrated on the workability and early properties of CSA mortars in the presence of superplasticizers and a significant majority of them concentrated on the use of polycarboxylate (PCE) superplasticizers. Testing the effect of superplasticizers on the properties of CSAs is also important from a sustainability standpoint, due to the fact that the use of the superplasticizer can aid in decreasing the use of water and cement in concrete. Superplasticizers allow us to use less water by maintaining workability at low w/c ratios, thus providing means for concretes to reach higher strength and durability. This enables the use of concretes with lower cement content, as a low w/c ratio can offset the decreased amount of cement when it comes to maintaining basic properties [23,24]. While the production of superplasticizers can use raw materials which are derived from oil processing, due to the low amount of superplasticizer used in concrete, the environmental effect is limited, as in ordinary Portland cement concrete, it contributes only to around 0.4% of its Global Warming Potential (GWP) [25,26]. Additionally, to mitigate the issue of production input, a lot of research is being also conducted to provide more environmentally friendly alternatives such as plant-based or biopolymer-based superplasticizers [27,28]. The currently available superplasticizers, however, can also be considered to be means of achieving more sustainable concrete production.
Although the majority of the existing research has found that addition of PCE superplasticizers can increase the fluidity of CSA mortars [22,29,30,31,32] in a similar way to that observed in mortars containing Portland cements, in some cases a decrease in workability was observed after several minutes, possibly due to fast hydration and absorption of PCE in the hydration products [22,29,31,32]. Wu et al. [33] noted that for naphthalene-based superplasticizer, after 15 min of mixing, CSA paste was not fluid.
A significant delay in hydration was observed in CSA mortars with added PCE in studies by Sun et al. [30], Govin et al. [29] and Winnefeld [22]. It is unclear whether the mechanism of this delay is similar to that observed in Portland cement: namely, a decrease in solution undersaturation and steric issues that affect the growth of hydration products [34]. The length of the side-chain density has been shown to affect the retardation effect, with longer chains decreasing the retardation [30]. Interestingly, research that included the use of sodium naphthalene sulfonate (SNF) superplasticizer in CSA mortars [22,35] indicates that, while it also provides an increase in the initial setting time, the increase is significantly smaller than in the case of the use of PCE superplasticizer.
Outside of fresh mortar properties, the compressive strength of CSA mortars with superplasticizers was also a subject of research. Tests by Wu et al. [33] and Yuan et al. [35] have shown that the addition of SNF superplasticizer can increase the initial strength of the mortar (first 24 h); however, the effect of this type of superplasticizer on the subsequent strength was not clear, as in [33], the compressive strength remained higher than the reference sample for up to 7 days, while in [35], the strength was comparable to the reference sample. Discrepancies were also observed between different studies on compressive strength in the case of the use of PCE superplasticizer. Although some of the research found indicated a decrease in strength compared to the reference sample [35,36], in other cases a significant increase was observed [37]. Properties such as plastic and drying shrinkage—while part of many investigations on CSA properties, such as [15,38,39,40,41]—to the authors’ knowledge were explored not in relation to the effect of superplasticizers.
Therefore, a noticeable gap in the knowledge of the effect of superplasticizers on CSA properties can be observed, as there is a very limited number of studies comparing the effect of different superplasticizers when using the same CSA. Additionally, shrinkage testing was not a subject of inquiry when it comes to possible compatibility issues between superplasticizers and CSA. One of the main advantages of using CSAs over Portland cements is their low shrinkage, as was mentioned before; therefore, the effect of superplasticizers on shrinkage is very important for the possible widespread use of CSAs, providing information about possible issues in this topic.
In light of the limited knowledge on this topic, the aim of the presented research is to provide a systematic insight into the effects of different superplasticizers on the basic properties of CSA mortars, namely consistency, initial setting time, hydration heat, compressive and flexural strength, as well as plastic and drying shrinkage. The novelty of the work lies in its comprehensive comparison between the effects of several superplasticizers on the properties of CSA, and in the research into the effects of different superplasticizers on the shrinkage of the CSAs. First, to test compatibility with CSA, three different superplasticizers with different bases (polycarboxylate ether, modified polycarboxylates, and polynaphthalene sulfonate) were used for mortar with a w/c ratio of 0.45, in amounts chosen to obtain the same consistency. Second, mortars with a different w/c ratio in the range of 0.45–0.35 and the same superplasticizer were tested to better gauge the effects of the polycarboxylate ether superplasticizer on the CSA mortars.

2. Materials and Methods

The tests were carried out on commercially available CSA, the composition of which is presented in Table 1, with its basic properties shown in Table 2. The chemical composition was obtained from the manufacturer, while the properties of cement were tested in the laboratory. Standard sand in a quantity of up to 2.00 mm was used (EN 196-1 [42]), with a strictly repeatable grain size distribution. The three superplasticizers used in the research are commercially available and have different chemical bases: SP1—polycarboxylate ether; SP 2—modified polycarboxylates; and SP3—polynaphthalene sulfonate, and were used in amounts that fit the range advised by the manufacturers, in the amounts necessary to obtain the same flow diameter tested on the flow table according to standard EN 1015-3 [43]. The consistency was set so that the amount of superplasticizer would be close to the middle range advised by the manufacturers, so as to provide the most viable comparison of their effects on the properties of mortars, without the issue of comparing their effectiveness. The properties of each type of superplasticizer, as declared by the manufacturer, are shown in Table 3.
The research was carried out mainly on mortars which were prepared according to EN 196-1 [42]. Two distinct blocks of tests were conducted—one for comparison of the effect of superplasticizer, in which the w/c ratios for all samples were the same and amounted to w/c = 0.45; in the case of the second research block, tests with different w/c ratios—0.35, 0.40, and 0.45—were employed with the same amount of superplasticizer. All mortar compositions are shown in Table 4.
To investigate the effects of superplasticizer types on CSA properties, fresh paste and fresh mortar, as well as hard mortar, were tested. The tests focused on the initial setting time, hydration heat, plastic shrinkage, drying shrinkage, and flexural and compressive strength. For all tests, if not specified otherwise, the mean of three samples was used. The initial setting time of the mortars was tested using the automated Vicat Apparatus according to the procedure of standard EN 196-3:2016 [44]. The change in procedure was used because the w/c ratio in the mixes was not adjusted to standard consistency but was set to be the same as for other tests. This was implemented to better gauge the effect of water in the mortar and its effect on setting.
Hydration heat was tested using the TAM Air isothermal calorimeter according to standard PN-EN 196-11 [45], with internal mixing and a temperature of 20 °C maintained during the measurement. Reference samples were prepared using quartz sand. The test sample was prepared as 5 g cement with 2.5 g distilled water and superplasticizer. This amount was chosen to reflect the composition of the paste in the mortars in Table 4. The measurement lasted 168 h (7 days) from the moment the water was added to the cement.
The shrinkage of the mortars during the first 20 h after mixing was measured using the shrinkage cone method. The method, described in [46,47], consists of a cone-shaped sample and a laser measuring device that logs changes in cone height during the first 20 h after the mortar is placed inside. The measurement setup is presented in Figure 1. Due to its shape, the difference in cone height is a good indicator of volumetric shrinkage. Mortar samples were prepared and mixed according to standard EN 196-1:2016 [42], and then immediately placed in the apparatus, after which the measurement was started. For the duration of the measurement, the shrinkage cone was placed in a climatic chamber that maintained a constant temperature of 20 °C and a relative humidity of 60%.
Drying shrinkage was tested according to standard EN 12617-4 [48] using Graf Kaufman apparatus. Mortars were prepared and mixed according to standard EN 196-1:2016 [42], and poured into 40 × 40 × 160 mm3 moulds with aluminum measurement caps embedded. For the first 24 h, the samples were kept at 20 ° C and covered with foil to maintain constant humidity. After that the samples were kept in a climatic chamber maintained at a constant temperature of 20 °C and 60% relative humidity. Measurements were taken after 1 day to set the baseline, and then after 2, 3, 7, and 28 days, and the change in the length of the sample was calculated.
The flexural and compressive strengths of the mortars were measured according to standard EN 196-1:2016 [42], with the following modifications: the w/c ratio was changed in the research from w/c = 0.5 in the standard to one set in the composition (Table 4), and superplasticizer was added at the same time as water. Prismatic samples measuring 40 × 40 × 160 mm were formed, and for the first 24 h they were covered with foil and kept at 20 °C and then, after demoulding, kept in water at a temperature of 20 °C until testing. Tests were performed after 1, 2, 3, 7, and 28 days after the mixing of the mortars. For each result of flexural strength, 3 samples were tested, and compressive strength tests were conducted on the 6 resulting samples.

3. Results and Discussion

3.1. Setting Time of CSA Mortars with Superplasticizers

The test results for the initial setting time of the CSA with the SP tests are presented in Figure 2. It can be observed that the presence of superplasticizers significantly increased the initial setting time, delaying the start of setting by more than 50 min (an increase of more than 109%) in the case of all samples with a 0.45 w/c ratio and superplasticizer addition. This effect is consistent with the general effect of superplasticizers on Portland cements, as due to their mechanism of action, superplasticizers can disrupt the hydration process by inhibiting the availability of water to cement grains. As superplasticizers, through either an ionic effect (as is the case for naphthalene-based superplasticizers (SP3)) or steric effects (as observed for polycarboxylate-based superplasticizers (SP1 and SP2), decrease the absorption of water on the grains of cement to increase its availability for consistency, the water availability for cement grains and hydration decreases significantly [49,50,51]. Furthermore, Jolicoeur and Simard [52] indicated that the particles of polycarboxylate-based admixtures may adsorb on the aluminate phases of hydrating cement, and thus impede hydration by blocking the growth of hydration products. This may be more significant in CSAs, which have significantly more alumina phases, including ettringite, than ordinary Portland cements. Additionally, admixtures’ particles were found to bind Ca2+ ions [53,54], negatively affecting the hydration speed of Portland clinker. Due to differences in the composition of the cements, this effect may not be as prominent as in the case of Portland cement.
It should be noted, however, that the setting measured by the Vicat apparatus is a rheological test, which is affected by the initial fluidity of the mixture, which is why standard consistency is employed. However in this case, to better show the effect of superplasticizers, consistency was not adjusted to the standard, and thus mixtures containing superplasticizer and the same w/c ratio as reference samples were characterized by higher consistency (which will be discussed in Section 3.3), which can also lead to an increase in the setting time of those samples [55]. This effect can be better seen in case of the samples with different w/c ratios and SP1 addition, where, while the amount of superplasticizer and cement were the same, the decrease in the amount of water from the w/c ratio of 0.45 to 0.35 led to a 15% decrease in the setting time. The effect of the w/c ratio on the initial setting time of CSA is consistent with previous research on this topic [40,56].
In terms of differences between the effects of the superplasticizer, SP1 and SP3 did not show significant differences in the effect on setting time (we noted a ~3% difference in the setting time), while SP2 prolonged the setting time by ~10% in comparison to the use of SP1 and SP3, indicating a significant difference. Differences in the effect of superplasticizers on the setting time can be expected, as the different types of superplasticizers have different effects on the hydration process and its delay. In this case, it is possible that the steric effect of SP2 is more significant than the steric effect of SP1 and the mostly electrostatic effect of SP3 (which is based on polynaphthalene).

3.2. Hydration Heat of CSA Mortars with Superplasticizers

The results of hydration heat tests of pastes with different superplasticizers are shown in Figure 3.
The addition of all superplasticizers prolonged the induction phase, with the effect being strongest in the case of SP3 (Figure 3a,b). Moreover, it can also be observed that the addition of superplasticizers decreased the heat flow rate during the induction period, providing further indication that the hydration slowed down. The effect in question has been previously observed in CSA pastes and mortars by Govin et al. [29] and Belhadi et al. [31], whose research showed prolongation of the induction phase and a delay in the main hydration peak of CSA pastes in the presence of PCE superplasticizer. The cause for the setting retardation in pastes containing superplasticizers has been linked to several mechanisms, such as adsorption on the surface of nucleating particles, thus inhibiting development of the hydration products; lowering the Ca2+ ions’ concentration in the paste inhibiting the hydration process; or introducing organic compounds into the paste [49,50,57]. In case of SP3, the polynaphthalene-based superplasticizer, this effect can most likely be attributed to the adsorption on hydrate particles and intercalation into existing ettringite phases, which significantly affects the hydration process [52]. For SP1 and SP2, the main mechanisms of prolongation of the induction phase are related to the steric effect of the long chains of carboxylic groups absorbed on the surface of cement particles [19,58]. It should be noted that there is a disconnect between the setting time (Figure 2) and the length of the induction phase. This can be explained by the fact that the initial setting time test is not strictly related to the hydration process, but the loss of workability over time [59], and thus is dependent on physical factors such as consistency and water absorption.
The effects of the superplasticizers on the hydration process can also be observed in case of the main hydration peak, which is significantly delayed in the case of both SP1 and SP2 by 4.4 h, and 2.5 h, respectively. It should also be noted that paste containing SP2 was characterized by a slightly higher main peak (by 10%), while pastes containing SP1 exhibited a lower main peak (by 17%) compared to the paste with no superplasticizer. SP2 is also characterized by very rapid deacceleration of hydration, while SP1’s deacceleration period is longer and smoother, indicating a slower reaction. This shows that while the main mechanism of the effect may be similar for both superplasticizers, SP1 causes prolonged slowdown of hydration, while for SP2, even if it is added in a higher amount (1.1% c.m. to 0.8% c.m. of SP1), its negative effect on hydration wears off faster.
The addition of SP3 affects the hydration process to a greater extent than SP1 and SP2. While the induction period is prolonged, the acceleration period is shorter, indicating a faster reaction. Moreover, in the case of paste which does not contain a superplasticizer, the first main peak is lower than the second main peak, and during the deacceleration period, there is an indication of a third peak. For the paste containing SP3, the first and second main peaks are similar and higher than the peaks of paste without SP addition; moreover, the third peak can be easily observed during the deacceleration period. This effect can be explained by the fact that ettringite production in CSAs is strongly promoted by the presence of polynaphthalene [60]. Furthermore, this type of superplasticizer contains sulfates, which can improve the kinetics of the ye’elemite reaction, and therefore may be responsible for higher peaks of main hydration [22,61].
Interestingly, the delay in the hydration process was not related to a lower cumulative hydration heat (Figure 3c). All pastes had similar heat evolution after 72 h, as pastes containing SP2 and SP3 had higher heat evolution by 6% and 2%, respectively, in comparison to paste with no SP, which is bordering on measurement error [45], while the paste containing SP1 had 9% higher heat evolution. This may be explained by the fact that slower hydration can increase the hydration rate, thus allowing fuller hydration and therefore higher hydration heat after 72 h [62].
In pastes containing SP1 superplasticizer and different w/c ratios (Figure 4), the w/c ratio has a significant effect on hydration heat in acceleration and deacceleration phases.
The induction phase was significantly prolonged in the presence of SP1 (Figure 4a,b) for all pastes; it was similar for all pastes containing superplasticizer, and the w/c ratio did not appear to have any significant effect. However, during the acceleration phase, the paste with a w/c ratio of 0.35 had a significantly higher heat evolution than other pastes containing SP. In the case of the sample w/c = 0.35 SP1, the delay in the occurrence of the main hydration peak was 2.6 h, while for samples w/c = 0.45 SP1 and w/c = 0.4 SP1, it was 4.1 h and 5.7 h, respectively. Furthermore, while for sample w/c = 0.35 SP1 the main hydration peak was not significantly higher than the main peak of the heat evolution rate of the sample which did not contain superplasticizer (only by 4%), the main hydration peak for samples w/c = 0.45 SP1 and w/c = 0.4 SP1 was lower by 17% and 24%, respectively, showing a significant change. However, in the deacceleration phase, it could be observed that the sample w/c = 0.35 SP1 had a very steep deacceleration curve, while the samples w/c = 0.45 SP1 and w/c = 0.4 SP1 showed signs of prolonged hydration.
This effect can be attributed to the very low free water content in sample w/c = 0.35 SP1. Due to the small amount of water, the cement particles are close together and, therefore, the hydration may be accelerated [63]. This effect is in line with the results of the tests performed on the CSA with different w/c ratios [40]. Interestingly, the sample w/c = 0.4 SP1 did not follow the same rule. When the water content was slightly higher, the effect of the proximity of cement particles was not as prevalent as the steric effect of the superplasticizer, which could have impeded the formation of ettringite. It may be possible that for this amount of water, due to the effect of the PCE superplasticizer, nano-ettringite is formed and provides a further steric barrier for hydration; however, further testing is required to confirm this theory [64]. For greater availability of w/c ratios, higher water content may render this issue less debilitating than in the case of w/c = 0.4. Observed effects are generally consistent with existing research, as the w/c ratio was found to significantly affect early hydration, especially for low w/c ratios < 0.35, which were associated with different behaviour than when a higher w/c ratio was used [65].
However, it should be noted that for heat evolution, the fast acceleration period of w/c = 0.35 SP1 can have negative effects, while no such effect was observed for other samples. The cumulative hydration heat after 72 h (Figure 4c) was slightly higher than that of the reference sample for sample w/c = 0.45 SP1 (higher by 9%), comparable to the reference sample for the sample w/c = 0.40 SP1 (which was only 2% lower, which is within the error of the measurement), and significantly lower than the reference sample for w/c = 0.35 SP1 (which was 17% lower). This significant decrease in heat evolution for a sample with low water content can be explained by two mechanisms: one, that the fast hydration provided a barrier for water to penetrate the cement particles further, and second, that with such a low water content, there was not enough water to sustain further hydration of the CSA clinker.

3.3. Consistency of CSA Mortars with Superplasticizers

The results of the consistency measurements are presented in Figure 5.
The flow results are as expected—with the addition of superplasticizer, mortars exhibited quite significant free flow, almost reaching the diameter of the flow table surface for the w/c ratio of 0.45. For all mortars with a 0.45 w/c ratio and superplasticizer, the flow measured after 15 impacts of the flow table was the same, as it reached the size of the flow table. As the w/c ratio in mortars decreased with the addition of SP1 superplasticizer, it could be observed that flow and free flow decreased to a higher extent when the w/c ratio was lowered from 0.4 to 0.35 than from 0.45 to 0.4, as the flow of w/c = 0.35 SP1 was reduced by 17% and the free flow was reduced by 43% in comparison to the flow and free flow of w/c = 0.40 SP1, while the differences between the w/c ratio of 0.45 and 0.40 were 6% and 17%, respectively. This can be attributed to the high viscosity of the mix resulting from SP addition and very low water content.

3.4. Flexural and Compressive Strength of CSA Mortars with Superplasticizers

The results of the flexural and compressive strength tests of mortars with different superplasticizers are presented in Figure 6, while the statistical ANOVA is presented in Figure 7. As can be noticed, the effect of the type of superplasticizer is significant, as the p-value is lower than 0.05, which is the generally accepted level of statistical significance.
For all the tested CSA mortars with different superplasticizers, over 90% of the flexural strength (91–109%) was obtained during the first 48 h of maturing. Interestingly, the flexural strength after 28 days was lower than the strength after 2 or 3 days by up to 10% for three of four samples. Only in the case of mortar containing SP3 did the early flexural strength not at any point outperform the 28-day flexural strength. This effect has previously been observed by Brien et al. [13], Burris and Kurtis [66] and in previous research by the authors [40]. The explanation for this phenomenon may be related to the appearance of expansive ettringite when there is excess water available, which, in turn, damages the internal structure [67] or the dehydration process and subsequent change of ettringite into metaettringite [68,69]. In this case, the fact that the presence of SP3 in the mixture can affect ettringite production [60] while SP1 and SP2 have more physical effects on hydration may explain why the sample with SP3 has not exhibited such an effect, while it is very clear in both the reference sample and the SP1 and SP2 samples. The exact reason for this effect requires further study.
Outside of the decrease in the flexural strength of CSA, it can also be noticed that the presence and type of SP affect the development of flexural and compressive strength.
Only in the case of SP2 use was the flexural strength similar to the strength of mortar without SP, as the difference after 3 to 28 days was <3%, while for SP1 and SP2 the difference in 28-day flexural strength at 28 days was, respectively, 10% and 21% in relation to the reference sample. Therefore, the selection of a suitable superplasticizer is crucial with respect to the flexural strength of CSA mortars.
Each of the superplasticizers has different effects on compressive strength: SP1 accelerates strength gain up to day 7 of curing, but the 28-day strength is slightly lower than that of mortars without SP; SP2 slows strength gain between 3 and 7 days of maturation but does not significantly affect strength after 28 days. The difference can be attributed to the different levels of steric effects, which have different effects on hydration and strength development. This indicates that the type of superplasticizer can affect the development of the strength and other properties of the mortars, and also that significant insight is necessary when choosing the superplasticizer, so that the desired properties are obtained. The obtained results are compliant with the results that can be found in the literature. In their paper, Yuan et al. [35] tested the compressive strength of mortars containing two different superplasticizers and obtained different strength results. SP3 decreases compressive strength, especially in the period up to day 7 of maturation, further demonstrating the possible negative effect of polynaphthalene superplasticizer on the strength of mortars containing CSA. This is consistent with previous research on this topic, in which polynaphthalene-based superplasticizer was used in large amounts [33]. This may be attributed to the structural differences that occur in the presence of polynaphthalene-based superplasticizer, and the increased ettringite production rate and the presence of ye’elimite. This can weaken the structure of the paste, and thus lead to lower strength.
When comparing the results of compressive strength testing to the hydration heat test results, it can be noticed that while hydration was previously shown to be slower in the first 24 h (Figure 3), no significant difference in cumulative hydration heat could be identified at 72 h (3 days), corresponding with the generally similar compressive strengths of mortars containing superplasticizer and the reference sample.
In Figure 7, the effect of w/c ratio for mortars with SP1 on flexural and compressive strength is shown, with the ANOVA of the results being shown in Figure 8.
As the w/c ratio for mortars containing superplasticizer decreases, both the flexural and compressive strength increase significantly. This effect is consistent with the existing literature, as Wang and Song [56] and Zhang et al. [65] observed a similar increase in strength with the w/c decrease for CSAs. It should be noted that in the case of flexural strength, for the sample with w/c = 0.35 and SP, the gain of flexural strength over time was different than that for samples with a higher w/c ratio, which showed a significant drop in flexural strength after the third day of curing, with only a small increase afterwards, resulting in the 28-day flexural strength being lower than that recorded on day 3. For the sample with w/c = 0.35 and SP, the increase in compressive strength was not obstructed and was almost twice (75–96%) as high as the flexural strength of other samples after 28 days. This effect can be attributed to the fact that in samples with a 0.35 water–cement ratio, the amount of water can be low enough that the main possible reason for the decrease in strength—the expansiveness of ettringite in the presence of excess water [67]—cannot occur. This provides a good basis for the low w/c amount as a way of controlling the loss of flexural strength in CSA samples.
The compressive strength of the samples increases with time. It should be noted that for all samples, the compressive strength after the first day is in the range of the compressive strength at 40–50% of 28 days, demonstrating the fast development of the compressive strength of CSAs. After one day of curing, the strength is extremely similar for all of the samples containing superplasticizer, with the difference not exceeding 10% between the highest 1-day strength (for w/c = 0.35 + SP1) and the lowest (for w/c = 0.45 + SP1), showing that the w/c ratio has no significant effect on the strength, as confirmed by ANOVA. The w/c ratio is shown to have no substantial effect, with the p-value being 0.121, which is significantly higher than the usually assumed confidence interval of 0.05 (Figure 9). For the next few test dates, the difference in compressive strength between samples with different w/c ratios increased—after 28 days, for the decrease from 0.45 to 0.40, the increase in compressive strength was 19.2% and there was a decrease from 0.4 to 0.35–16.0%. This effect is agreement with other research on this topic, such as [40], which was performed on the same CSA. It should be noted that for sample w/c = 0.45 + SP1 and the reference without SP (w/c = 0.45), the strength development was significantly different, with the sample with superplasticizer having higher early strength up to day 7; however, the 28-day strength was higher for the sample without superplasticizer. A similar effect was not observed for other superplasticizers (as can be seen in Figure 5), which leads to the conclusion that the superplasticizer can have a significant effect on the strength. It should be noted, however, that with the decrease in the w/c ratio, not only did the strength increase, but the strength development between the 7th and 28th day of curing came closer to the development without the superplasticizer, as the difference between 7- and 28-day compressive strength was 27%, and for samples w/c = 0.45 + SP1, w/c = 0.40 + SP1, and w/c = 0.35 + SP1 it was, respectively, 10%, 15%, and 20%. This indicates that the negative effect of SP addition on late strength development may be offset by lowering the w/c ratio.

3.5. Early Shrinkage of CSA Mortars with Superplasticizers

Early shrinkage during the first 20 h of CSA mortars with different superplasticizers in shown in Figure 10, while for mortars with SP1 and different w/c ratios it is shown in Figure 11.
As can be noticed, in mortars containing superplasticizers, the shrinkage in the first 20 h differs up to 1 mm/m. Early shrinkage, during the first 0.5 h, is significantly increased for mortars with SP in relation to mortars without SP. This effect is consistent with existing research on the effects of superplasticizers on the shrinkage of Portland cement mortars [70,71]. It is generally stated that the increase in shrinkage during the first few hours after mixing is related to the change in capillary pore distribution, with the increase in the amount of smaller capillary pore volume [70,72], while changes in the surface tension can also affect this [72]. It should be noted that while all of the mortars with different superplasticizers showed higher rates of shrinkage in the first 0.5 h, after that, only mortars containing SP2 and SP3 had higher rates of shrinkage after first 20 h (by 30% and 26%, respectively), while the shrinkage of mortar containing SP1 did not reach the same level as the shrinkage of the reference sample and remained 18% lower, showing that the type of superplasticizer can significantly affect the shrinkage of CSA mortars.
The w/c ratio affects the early shrinkage of CSA mortars containing SP1. For all the w/c ratios, the shrinkage after the first 24 h after mixing is lower than that of the reference sample. In case of w/c = 0.45 + SP1 and w/c = 0.40 + SP1, the shrinkage is higher in the first 0.5 h and similar during the first 24 h (with a 2% difference after 24 h), but in the case of w/c = 0.35 + SP1 the shrinkage is always lower, and significantly lower than that of samples with a higher w/c ratio. The shrinkage of the sample w/c = 0.35 + SP1 is ~40% lower than that of samples w/c = 0.45 + SP1 and w/c = 0.40 + SP1. The effect of the w/c ratio on the early shrinkage of Portland cements is well described [62], and it is generally agreed that with the increase in the w/c ratio of Portland cement mortars, early shrinkage also increases due to the large volume of excess water. For CSA, the same mechanism is present.

3.6. Drying Shrinkage of CSA Mortars with Superplasticizers

The results of drying shrinkage testing of CSA mortars with different superplasticizers are shown in Figure 12, while the drying shrinkage of CSA mortars with SP1 and different w/c ratio is shown in Figure 13. Samples were measured after 24 h to set the baseline, and then subsequent measurements showed the differences between measurements on a given day of curing and the size of the samples 24 h after their initial preparation. The measurement was conducted separately from the measurement of early shrinkage.
All of the CSA mortars containing superplasticizers have comparable or higher drying shrinkage after 2 days of curing, and significantly higher drying shrinkage after 28 days of curing (up to 100%). Interestingly, for all of the mortars tested that contained SPs, the development of shrinkage was different from that of the reference sample without SPs, as the shrinkage development was slower for the samples with SPs. The shrinkage was the same for w/c = 0.45 + SP3 and the reference after 7 days, while for w/c = 0.45 + SP1 and w/c = 0.45 + SP2, the shrinkage remained lower than that of the reference sample for up to 17 days of curing. This effect can be explained by the fact that all superplasticizers can affect ettringite production, and thus limit shrinkage through an increased ettringite production rate, causing possible expansion (SP1, SP2) and changes in the morphology of ettringite, which can also affect the expansion (SP3) [33,38,69]. It should also be noted that the significantly lower shrinkage of CSA mortars with SP1 and SP2 lends credibility to the theory that the decrease in flexural strength after 3 and 7 days of curing for w/c = 0.45 + SP1 and w/c = 0.45 + SP2 was caused by expansive ettringite and the resulting microstructural damage, since the drying shrinkage at this stage was 50–100% lower than that of the reference sample, which points to some possible expansion offsetting the drying shrinkage at this point in time. After this brief expansion, it is possible that the different structure that emerged allowed for an increased water evaporation rate and thus increased shrinkage between the 7th and 28th day.
For mortar w/c = 0.45 + SP3, the shrinkage development was different than in the case of other tested mortars, as between the 1st and 28th day of curing, there was no moment in which the development of shrinkage slowed down. This may be attributed to the structural changes brought about by chemical effects of the use of polynaphthalene superplasticizer, which, as was previously stated (Section 3.4), changes the products of CSA hydration [33]. It is possible that the different compositions of the hydration products are responsible for this difference; however, the exact cause of this effect requires further study.
For CSA mortars with SP1 and changing w/c ratios, it must be stated that the lower amount of water in the system led to a lower shrinkage than the reference sample by 33% in the case of w/c = 0.40 + SP1 and 49% in the case of w/c = 0.35 + SP1, as would be expected with the lower amount of water in the mortar. The development of shrinkage is, however, very different for CSA mortars w/c = 040 + SP1 and w/c = 0.35 + SP1 compared to reference sample w/c = 0.45 and w/c = 0.45 + SP1, indicating that with a lower amount of water, the shrinkage process undergoes a significant change.
It can be noticed that, initially, the shrinkage of w/c = 0.40 + SP1 and w/c = 0.35 + SP1 was over two times higher (219% and 298%, respectively) than that of the reference sample, and the shrinkage was higher for the sample with a lower w/c ratio. In case of lower w/c ratios, due to a lack of water available for reaction, instead of ettringite, monosulfate is produced [73]. Monosulfate shows a higher shrinkage rate and thus the drying process can have a more visible effect for mortars with a low w/c ratio than in the case of mortars with a higher w/c ratio. Next, expansion occurs, decreasing the shrinkage from the 2nd to the 7th day by over 80% for w/c = 0.40 + SP1 and w/c = 0.35 + SP1. This indicates further disruptions of the hydration process in the presence of SP1 in low w/c ratios; however, the exact cause requires further research into the products of CSA reaction in the presence of SP1. However, it must be noted that the change in microstructure which occurred between the 2nd and 7th day in the presence of SP1 did not affect the flexural and compressive strength to a significant degree, especially in the case of and w/c = 0.35 + SP1, and the drying shrinkage was extremely low, indicating good grounds for using SP1 with low w/c mortars with a low w/c ratio.

4. Conclusions

This paper presented the results of tests on the properties of fresh and hardened CSA mortars with three different superplasticizers as well as mortars with one superplasticizer and different w/c ratios. Comparisons were drawn based on tests of consistency, initial setting time, hydration, heat, compressive and flexural strength, as well as plastic and drying shrinkage. This led us to draw the following conclusions:
  • Adding polycarboxylate ether-based superplasticizer (SP1) to CSA mortar w/c = 0.45 caused a delay in initial setting, as well as prolonging the induction phase of hydration; however, the cumulative hydration heat remained similar to that of the reference sample. SP1 addition caused an increase in early compressive strength in the first 7 days, and did not significantly affect early flexural strength; however, both flexural and compressive strength decreased after 28 days. The addition of SP1 to mortars with w/c = 0.45 decreased early shrinkage; however, it increased shrinkage after 28 days of curing.
  • Modified polycarboxylate-based superplasticizer (SP2) had similar effect on the strength and shrinkage of CSA mortars as SP1, decreasing early shrinkage and increasing early strength at 28 days. SP2 addition decreased the compressive strength and increased shrinkage. SP2 prolonged the initial setting time to a high degree, and prolonged the induction phase significantly, indicating that it had a negative effect on the hydration process; however, after 72 h the cumulative hydration heat was not affected.
  • When both polycarboxylate-based superplasticizers (SP1 and SP2) were used, the flexural strength decreased between the 3rd and 7th day, which was possibly caused by the appearance of expansive ettringite, or the ettringite dehydration process and subsequent change of ettringite into metaettringite occurring in their presence.
  • The polynaphthalene-based superplasticizer (SP3) decreased compressive strength and increased both early and drying shrinkage, and the presence of SP3 led to a prolonged induction period and a delayed initial setting time, as well as prolonging setting. However, the cumulative hydration heat did not differ significantly from that of the sample with no superplasticizer. Those effects were possibly due to the adsorption on hydrate particles affecting the hydration process.
  • A decrease in the w/c ratio mitigated the majority of the issues connected with the use of superplasticizer-based CSA mortars, with w/c = 0.35 and SP1 being associated with significant increases in flexural and compressive strength, lower early shrinkage and drying shrinkage, and greater consistency; however, the initial setting time was prolonged and the cumulative heat after 72 h decreased in comparison to the reference sample, indicating only partial hydration.
In summation, the tested superplasticizers used for Portland cement can be used in CSAs, as they significantly increase the consistency, and their possible negative effects on other properties of CSAs can be potentially mitigated by decreasing the w/c ratio. Additionally, polynaphthalene-based superplasticizer has been proved to decrease strength and increase the shrinkage of CSA mortar more than polycarboxylate-based superplasticizers, and thus its use should be carefully considered.

Author Contributions

Conceptualization, M.G. and J.G.; methodology, M.G. and J.G.; investigation, M.G.; writing—original draft preparation, M.G.; writing—review and editing, M.G. and J.G.; supervision, J.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Early shrinkage measurement setup in the climatic chamber, with a plastic cover added to decrease the water evaporation.
Figure 1. Early shrinkage measurement setup in the climatic chamber, with a plastic cover added to decrease the water evaporation.
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Figure 2. Setting time of CSA mortars with superplasticizer.
Figure 2. Setting time of CSA mortars with superplasticizer.
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Figure 3. Hydration heat of CSA pastes containing three different superplasticizers: (a) the rate of heat evolution during the first 72 h, (b) the rate of heat evolution in the early stages of hydration (first 6 h), and the heat evolution during a 6 h period, (c) the rate of heat evolution during a full testing period (72 h).
Figure 3. Hydration heat of CSA pastes containing three different superplasticizers: (a) the rate of heat evolution during the first 72 h, (b) the rate of heat evolution in the early stages of hydration (first 6 h), and the heat evolution during a 6 h period, (c) the rate of heat evolution during a full testing period (72 h).
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Figure 4. Hydration heat of the CSA pastes containing superplasticizer SP1 and different w/c ratios: (a) rate of heat evolution during the first 72 h, (b) rate of evolution in the early stages of hydration (the first 6 h), and heat evolution during a 6 h period, (c) the rate of heat evolution during a full testing period (72 h).
Figure 4. Hydration heat of the CSA pastes containing superplasticizer SP1 and different w/c ratios: (a) rate of heat evolution during the first 72 h, (b) rate of evolution in the early stages of hydration (the first 6 h), and heat evolution during a 6 h period, (c) the rate of heat evolution during a full testing period (72 h).
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Figure 5. Consistency of CSA mortars with different superplasticizers and a changing w/c ratio.
Figure 5. Consistency of CSA mortars with different superplasticizers and a changing w/c ratio.
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Figure 6. Influence of SP type on (a) flexural and (b) compressive strength of CSA mortars. Vertical bars indicate standard deviation.
Figure 6. Influence of SP type on (a) flexural and (b) compressive strength of CSA mortars. Vertical bars indicate standard deviation.
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Figure 7. ANOVA of influence of SP type on the flexural and compressive strength of CSA mortars with the F-value and p-value calculated. Symbol * denotes a p-value < 0.05, indicating a significant effect of superplasticizer type on the property.
Figure 7. ANOVA of influence of SP type on the flexural and compressive strength of CSA mortars with the F-value and p-value calculated. Symbol * denotes a p-value < 0.05, indicating a significant effect of superplasticizer type on the property.
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Figure 8. Influence of w/c ratio on (a) flexural and (b) compressive strength of CSA mortars modified with SP1. Vertical bars indicate standard deviation.
Figure 8. Influence of w/c ratio on (a) flexural and (b) compressive strength of CSA mortars modified with SP1. Vertical bars indicate standard deviation.
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Figure 9. ANOVA of influence of w/c ratio on flexural and compressive strength of CSA mortars modified with SP1, with the F-value and p-value calculated. Symbol * denotes a p-value < 0.05, indicating a significant effect of superplasticizer type on the property.
Figure 9. ANOVA of influence of w/c ratio on flexural and compressive strength of CSA mortars modified with SP1, with the F-value and p-value calculated. Symbol * denotes a p-value < 0.05, indicating a significant effect of superplasticizer type on the property.
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Figure 10. The early shrinkage of CSA mortar with different superplasticizers, (a) in the first 2 h and (b) in the first 20 h after mixing.
Figure 10. The early shrinkage of CSA mortar with different superplasticizers, (a) in the first 2 h and (b) in the first 20 h after mixing.
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Figure 11. The early shrinkage of CSA mortars with SP1 and differing w/c ratios. (a) In the first 2 h and (b) in the first 20 h after mixing.
Figure 11. The early shrinkage of CSA mortars with SP1 and differing w/c ratios. (a) In the first 2 h and (b) in the first 20 h after mixing.
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Figure 12. Drying shrinkage of CSA mortars with three different superplasticizers after 2, 3, 7, and 28 days of curing.
Figure 12. Drying shrinkage of CSA mortars with three different superplasticizers after 2, 3, 7, and 28 days of curing.
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Figure 13. Drying shrinkage of CSA mortars with SP1 and w/c = 0.45–0.35, after 2, 3, 7, and 28 days of curing.
Figure 13. Drying shrinkage of CSA mortars with SP1 and w/c = 0.45–0.35, after 2, 3, 7, and 28 days of curing.
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Table 1. Composition of CSA.
Table 1. Composition of CSA.
Constituent [%]
Oxide composition Phase composition
SiO2Al2O3Fe2O3CaOMgOSO3Na2OK2OLOIC4 A3ŜC2S3C2S 3CŜ CaF2CŜMgO
9.228.11.5239.23.511.40.080.350.466510.49.42.64.9
Phase composition symbols: C—CaO, A—Al2O3, Ŝ—SO3, S—SiO2.
Table 2. Basic properties of CSA used in the research. Adapted from Ref. [40].
Table 2. Basic properties of CSA used in the research. Adapted from Ref. [40].
Cement PropertyUnitValue
Initial setting timemin30
Soundness of cement, by Le Chatelier’s methodmm1
Compressive strength: After 2 daysMPa42.0
After 28 daysMPa67.7
Specific surface areacm2/g5500
Table 3. Properties of superplasticizers used in the research.
Table 3. Properties of superplasticizers used in the research.
PropertyUnitSP1SP2SP3
Base-polycarboxylate ethermodified polycarboxylatespolynaphthalene sulfonate
Densityg/cm31.071.011.20
Colour-amberyellowbrown
pH-657
Chloride content% mass≤0.1≤0.1≤0.1
Table 4. Compositions of mortars used in the research.
Table 4. Compositions of mortars used in the research.
Mortar TypeCement [g]Water [g]Sand [g]Superplasticizer [g]
SP1SP2SP3
CSA w/c = 0.45450202.51350---
CSA w/c = 0.45 + SP1202.53.6--
CSA w/c = 0.45 + SP2202.5-4.95-
CSA w/c = 0.45 + SP3202.5--4.95
CSA w/c = 0.40 + SP11803.6--
CSA w/c = 0.35 + SP1157.53.6--
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Gołaszewska, M.; Gołaszewski, J. The Effect of Superplasticizer Addition on the Properties of Calcium Sulfoaluminate Mortars. Sustainability 2025, 17, 8460. https://doi.org/10.3390/su17188460

AMA Style

Gołaszewska M, Gołaszewski J. The Effect of Superplasticizer Addition on the Properties of Calcium Sulfoaluminate Mortars. Sustainability. 2025; 17(18):8460. https://doi.org/10.3390/su17188460

Chicago/Turabian Style

Gołaszewska, Małgorzata, and Jacek Gołaszewski. 2025. "The Effect of Superplasticizer Addition on the Properties of Calcium Sulfoaluminate Mortars" Sustainability 17, no. 18: 8460. https://doi.org/10.3390/su17188460

APA Style

Gołaszewska, M., & Gołaszewski, J. (2025). The Effect of Superplasticizer Addition on the Properties of Calcium Sulfoaluminate Mortars. Sustainability, 17(18), 8460. https://doi.org/10.3390/su17188460

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