# Compressive strength of cement pastes and mortars with superabsorbent polymers

Pietro Lura, Félix Durand, Ahmed Loukili, Konstantin Kovler, Ole Mejlhede
Jensen

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Pietro Lura, Félix Durand, Ahmed Loukili, Konstantin Kovler, Ole Mejlhede Jensen. Compressive strength of cement pastes and mortars with superabsorbent polymers. International RILEM Conference on Volume Changes of Hardening Concrete: Testing and Mitigation, Aug 2006, Lyngby, Denmark. hal-05406701

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# COMPRESSIVE STRENGTH OF CEMENT PASTES AND MORTARS WITH SUPERABSORBENT POLYMERS

Pietro Lura<sup>(1)</sup>, Félix Durand<sup>(2)</sup>, Ahmed Loukili<sup>(2)</sup>, Konstantin Kovler<sup>(3)</sup>, and Ole Mejlhede Jensen<sup>(1)</sup>

(1) Technical University of Denmark, Lyngby, Denmark

(2) Ecole Centrale de Nantes, Nantes, France

(3) Technion – Israel Institute of Technology, Israel

## Abstract

This paper deals with the compressive strength development of cement pastes and mortars with superabsorbent polymers, SAP. Addition of SAP to high performance cementitious mixtures is done to reduce or avoid self-desiccation and self-desiccation shrinkage, but also influences the compressive strength. This study shows that whereas the compressive strength of high-strength cement pastes is slightly reduced by SAP addition, the strength of mortars is almost unaffected. Additionally, the paper examines briefly the effect on strength of degree of hydration and porosity, moisture conditions and pore structure of the SAP composites.

## 1. INTRODUCTION

Modern, high-performance concrete with a low water/cement ratio (w/c) and silica fume is prone to early-age cracking caused by autogenous shrinkage [1]. A possible solution to this problem is water entrainment by means of superabsorbent polymer (SAP) [2]. The SAP form water-filled macropore inclusions in the fresh concrete, which prevent self-desiccation shrinkage during hardening. A paper [3] showed that addition of SAP to cement pastes and mortars reduced or eliminated self-desiccation and shrinkage, and avoided early-age cracking.

Availability of extra water for hydration and high water activity lead to increased cement hydration in internally water cured cement pastes with low w/c [2]. An increased degree of hydration is expected to improve strength, especially at later ages. An additional beneficial effect on mechanical properties is obtained by avoiding internal microcracking, which may occur due to autogenous shrinkage and aggregate restraint.

Evaluation of the influence of internal water curing on mechanical properties is complicated by the fact that the SAP is introduced in the system and will influence its performance [4]. An increasing amount of SAP, while avoiding early-age shrinkage, may reduce the strength and counteract the beneficial effects of the further hydration. Moreover, strength is also sensitive to the moisture state of the specimens, with moist concrete showing lower strength than partially dry concrete. Also the pre-compression of the aggregates, which
might take place in high strength concrete due to autogenous shrinkage [5], plays a role in the strength of the concrete.

Higher or equal later-age compressive strength compared to normal concrete was measured in concrete with LWA [6-8] and in mortar with SAP [9]. In these studies, the early-age strength was generally equal to or slightly lower than in the reference mixtures.

In this study, compressive strength of cement pastes and mortars with and without addition of SAP was measured [10]. Efforts were made to isolate different contributions to the strength of the composites, and some theoretical analysis about SAP influence on strength was developed.

# 2. MATERIALS AND METHODS

## 2.1 Cement pastes and mortars

A series of cement pastes and mortars were studied in this project. Table 1 shows the mixture compositions, while the materials used, cement, silica fume, superplasticizer, and SAP, are explained in detail in the following

<table><caption>Table 1: Mixture composition of cement pastes and mortars. The given w/c do not include silica fume.</caption><thead><tr><th>Name</th><th>w/c</th><th>w/c entrained</th><th>SF/cement (%-weight)</th><th>Aggregate (%-volume)</th></tr></thead><tbody><tr><td>Paste_Plain</td><td>0.30</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Paste_Plain_SAP</td><td>0.30</td><td>0.05</td><td>-</td><td>-</td></tr><tr><td>Paste_SF</td><td>0.30</td><td>-</td><td>20</td><td>-</td></tr><tr><td>Paste_SF_SAP</td><td>0.30</td><td>0.05</td><td>20</td><td>-</td></tr><tr><td>Mortar_Plain</td><td>0.315</td><td>-</td><td>-</td><td>50</td></tr><tr><td>Mortar_Plain_SAP</td><td>0.315</td><td>0.05</td><td>-</td><td>50</td></tr><tr><td>Mortar_SF</td><td>0.32</td><td>-</td><td>20</td><td>50</td></tr><tr><td>Mortar_SF_SAP</td><td>0.32</td><td>0.05</td><td>20</td><td>50</td></tr></tbody></table>

A low-alkali Danish white portland cement produced by Aalborg Portland was used, with a Blaine fineness of 420 m²/kg. The Bogue-calculated phase composition (in wt.%) is: C₃S: 66.1, C₂S: 21.2, C₃A: 4.3, C₄AF: 1.1, C₅S: 3.5, free CaO: 1.96, Na₂O eq.: 0.17.

Silica fume was added as a dry powder at a rate of 20 wt.% of cement. The specific surface of the silica fume is 17.5 m²/g (BET method). The chemical composition (in wt.%) is: SiO₂: 94.1, Fe₂O₃: 1.00, Al₂O₃: 0.13, MgO: 0.71, SO₃: 0.43, and Na₂O eq.: 1.09.

A naphtalene-based dry powder superplasticizer was added at a rate of 1.0 wt.% of cement+silica fume.

Quartz sand with a particle size smaller than 2 mm and a water absorption of 0.1% by mass was used for the mortars. By volume, the mortars contain 50% aggregate.

The SAP used is a suspension-polymerized covalently cross-linked acrylamide/acrylic acid copolymer [3]. The spherical particles have diameters varying from 50 to 250 µm in the dry state. The size of the swollen SAP particles in the cement pastes and mortars is two to three times larger due to pore fluid absorption (Figure 1). During mixing of the cement pastes, the SAP absorb about 12.5 g of pore fluid per g of SAP [3]. The SAP was added at a rate of 0.4
wt.% of cement. Additional mixing water was added to the mixtures with SAP in an amount sufficient to saturate the SAP particles. This amount corresponds to an entrained w/c of 0.05.

<figure><img src="image_2.png" /><figcaption>Figure 1: Optical microscope image of superabsorbent polymers in the dry state (left) and in the wet state (right). Numbers in the picture refer to mm.</figcaption></figure>

## 2.2 Mixing and sample preparation

The cement pastes were mixed in a 5-l epicyclic mixer. All dry components, including the SAP, were mixed dry for 1 minute. Mixing proceeded at low speed while gradually adding the demineralised water then the speed was increased. The water was added in two steps to assure the homogeneity of the mix and the dispersion of the silica fume. Total mixing time from first water addition was 5 minutes for the cement pastes and 10 minutes for the mortars. The temperature of the ingredients was approximately 20°C at mixing.

The cement paste and mortar samples were cast in cylindrical moulds, applying a short vibration to expel encapsulated air bubbles. Efforts were made to avoid air bubbles in the samples, since this has a major effect on strength. Despite this, there may have been small uncontrolled amounts of air on the samples.

Cement paste was cast into cylinders 150 mm high and 15 mm in diameter. Mortar was cast into cylinders 150 mm high and 24 mm in diameter (Figure 2). The samples were rotated until setting to avoid inhomogeneities due to bleeding. After setting, they were stored in sealed conditions in a desiccator at 20°C until testing.

Starting from the second month of hardening, some samples were demoulded and stored in an evacuated desiccator over a saturated NaCl solution to equilibrate them at 75% RH. Samples were weighed every week during the second month to ensure moisture equilibrium at the end of the second month. Additional tests performed with Rotronic hygroscope relative humidity stations confirmed that the samples had equilibrated at 75% RH [10].

Before testing the compressive strength, the samples were dry-cut with a diamond saw into three pieces for the cement paste and into two pieces for the mortars, obtaining cylinders with a height to diameter ratio of about 2. From the centre of the same cylinders, samples for measurement of degree of hydration were obtained. The sample division is illustrated in Figure 2.
<figure><img src="image_3.png" /><figcaption>Figure 2: Samples for testing compressive strength and degree of hydration.</figcaption></figure>

## 2.3 Measurements of compressive strength

Compressive strength was measured on cylindrical specimens according to the ASTM C39 standard [11]. Specimens were tested a few minutes after demoulding and sample preparation. Samples were 15 mm in diameter and ~30 mm in height for the cement pastes, 24 mm in diameter and ~50 mm in height for the mortars (see Figure 2). The loading rate was 0.5 MPa/s, slightly higher than suggested in [11].

## 2.4 Measurements of degree of hydration

The degree of hydration was determined based on non-evaporable water content. Discs were cut from the cylindrical specimens shown in Figure 2. Immediately after cutting, the samples were vacuum dried to stop hydration. The samples were then weighed and kept for 24 hours in an oven at 105°C. After weighing a second time, they were crushed with a hammer and ground to a powder. Grinding to a powder was necessary to avoid explosion of the very dense samples during ignition at 1050°C. The powder was weighed and placed in a furnace at 1050°C for 1 hour. The non-evaporable water content by mass of cement, Wn (g water/g cement), was calculated from the difference between the 105°C and 1050°C mass measurements and corrected for the loss on ignition of the cement, the silica fume and the quartz sand. The degree of hydration was determined by dividing the non-evaporable water content by the non-evaporable water content at full hydration, 0.222 g water/g cement, which was calculated based on the phase composition of the cement. More details are available in [10].

# 3. RESULTS

## 3.1 Compressive strength

Figure 3 shows the compressive strength of the four cement pastes, while Figure 4 shows the corresponding results for the four mortars. SAP addition reduces the early age strength of the pastes by about 20%, while at later ages the strength reduction is about 10%. In the mortars, however, no influence of the SAP on the strength is observed.
<figure><img src="image_4.png" /><figcaption>Figure 3: Compressive strength of cement pastes with and without superabsorbent polymers, measured on cylinders with diameter 15 mm and height ~ 30 mm. The specimens were cured in sealed conditions at 20°C. The graph shows the average and the standard deviation of 3-5 specimens tested at each age.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4: Compressive strength of mortars with and without superabsorbent polymers, measured on cylinders with diameter 24 mm and height ~ 50 mm. The specimens were cured in sealed conditions at 20°C. The graph shows the average and the standard deviation of 3-5 specimens tested at each age.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5: Compressive strength of cement pastes with and without superabsorbent polymers.</figcaption></figure>

Full lines refer to specimens stored in sealed conditions at 20°C. Dashed lines refer to specimens stored at 75% RH and 20°C starting at 28 days. The graph shows the average and the standard deviation of 3-5 specimens tested at each age.

Figure 5 shows the change in the compressive strength of cement pastes with different storage conditions. Samples stored in sealed conditions are compared to samples stored at 75% RH between 28 days and the age of testing, 55 or 56 days. The results appear inconsistent: whereas the plain cement paste samples showed an increase of compressive strength after drying, the samples with silica fume showed a strength decrease, rather high in the sample with silica fume and no SAP.

## 3.2 Degree of hydration

Table 2 shows the measured late-age degree of hydration of the cement pastes. The measured degree of hydration of the mortars, not shown in Table 2, is consistent with the cement pastes. The ultimate degree of hydration achievable by the cement pastes, calculated with Powers model [12], is also reported in the table. The ultimate degree of hydration was calculated taking into account both the water entrained in the SAP and the influence of silica fume on hydration [2, 12]. Within the uncertainties of this calculation method, the two pastes with SAP reached a substantially higher degree of hydration than the other pastes, showing the beneficial effect of internal curing on hydration. The ultimate calculated degree of hydration of the plain cement pastes is higher than the measured valued, as expected. However, the ultimate degree of hydration of the pastes with silica fume and no SAP is lower than the value measured at 28 days. This might be due to some incorrect assumption made in applying Powers model for silica-fume cement pastes. For example, the model explained in [12] assumes that hydration of cement and the pozzolanic reaction of silica fume with calcium hydroxide proceed at the same speed. In fact, if the pozzolanic reaction is slower, a higher degree of hydration of the cement can be achieved.
<table><caption>Table 1: Degree of hydration of cement pastes with and without superabsorbent polymers.</caption><thead><tr><td>Name</td><td>Measured degree of hydration</td><td>Calculated ultimate degree of hydration</td></tr></thead><tbody><tr><td>Paste_Plain</td><td>62% (56 days)</td><td>72%</td></tr><tr><td>Paste_Plain_SAP</td><td>68% (55 days)</td><td>83%</td></tr><tr><td>Paste_SF</td><td>63% (28 days)</td><td>58%</td></tr><tr><td>Paste_SF_SAP</td><td>70% (28 days)</td><td>73%</td></tr></tbody></table>

# 4. DISCUSSION

## 4.1 Influence of moisture conditions on strength

The moisture conditions of a sample influence its compressive strength. Bartlett and MacGregor [13] observed a 22% strength reduction when a specimen was soaked in water after drying at RH<60%. The higher strength at lower RH can be attributed to the strengthening action of the capillary forces in the pore fluid on the solid structure.

Samples with SAP have a final internal RH of about 95%, much higher than samples without SAP, which self-desiccate to RH values close to 75% [3]. For a fair comparison between strength of mixtures with and without SAP, the samples should be tested at identical moisture conditions. For this reason, samples with and without SAP were conditioned to 75% RH between 28 and 56 days of age.

It was expected that conditioning of the samples at 75% RH would lead to a strength increase in the pastes with SAP. The strength of the pastes without SAP, on the contrary, was expected to remain about constant, since they would have self-desiccated during sealed hydration and reached an internal RH close to 75%. However, the trends observed in Figure 5 are different: while cement pastes without silica fume increased their strength after drying, the mixtures with silica fume showed a substantial strength loss. The strength loss shown by the pastes with silica fume might be explained by stress gradients or microcracking in the samples caused by to moisture gradients [14]. The high scatter of the experimental results might be another indication that microcracking has indeed occurred.

## 4.2 Influence of porosity on strength (gel-space ratio concept)

Powers and Brownyard [15] proposed the following formula to calculate the compressive strength of a cementitious material as a function of hydration time:

$$f_c(t) = f_i \cdot X(t)^n \quad (1)$$

where $f_i$ (MPa) represents the intrinsic strength of the cement and $n$ takes on values between 2.6 and 3.0. The gel-space ratio $X$ is the ratio of the cement gel formed to the space currently available for it. It can be calculated for plain pastes and pastes with silica fume addition based on mix composition and degree of hydration according to the models presented in [2, 12]. The cement pastes with SAP have initially a much higher porosity, which results in a lower gel-space ratio. This difference in porosity is reduced in time, since hydration proceeds further in pastes with SAP due to availability of internal curing water.

The gel space ratio concept can be used to estimate the influence of SAP addition on the compressive strength of the cement pastes examined in this study. The intrinsic strength of the cement is the same for mixtures with and without SAP, while it might vary with silica fume addition. Assuming an average value for the exponent $n$, eq. 1 can be rewritten as:
$$ \frac{f_{c,SAP}(t)}{f_c(t)} = \left( \frac{X_{SAP}(t)}{X(t)} \right)^{2.8} \qquad (2) $$

where $f_{c,SAP}$ and $f_c$ and (MPa) are the compressive strength of a cement paste with and without SAP addition, respectively, and $X_{SAP}$ and $X$ are the corresponding gel-space ratios for the two cement pastes.

Using the measured values for the degree of hydration (Table 2) to calculate the gel-space ratio, a 10% strength reduction due to SAP addition is calculated for the plain cement paste at 2 months of age, while a 7% strength reduction is calculated for the silica fume cement paste at 1 month of age. The corresponding measured values are a 9% reduction in the plain cement paste and a 12% reduction is the silica fume modified cement paste. There is good agreement between measured and calculated values, especially considering the scatter in the compressive strength results (Figure 4).

## 4.2 Influence of pore structure on strength

The strength of a material is influenced by the size of the largest defect present in its solid matrix. In cement pastes, the voids left by the largest SAP, about 0.5 mm across, are the largest defects. In the mortars, however, the largest defects tend to be the interfaces between paste and aggregates and weak or pre-cracked aggregates. These defects are of the size of the aggregates [16], up to 2 mm, and larger than the SAP. This may explain why no influence of the SAP on the compressive strength of the mortars can be detected. It may also explain the lower strength of the mortars compared to the cement pastes (Figure 4 and 5). In other words, since the aggregate or the interfacial transition zones, and not the cement paste, are the limiting factor for the strength of the mortars, the influence of the SAP on the strength is negligible.

<figure><img src="image_7.png" /><figcaption>Figure 6: Optical microscope picture of cement paste with superabsorbent polymers, where a microcrack has propagated between two inclusions left by the polymers. The microcrack has been intruded with gallium according to a technique detailed in [4].</figcaption></figure>
Some additional considerations can be made about influence of pore shape on mechanical properties. In pastes that experience self-desiccation, a system of partially connected and irregular capillary pores, with maximum size of a few µm, is left when hydration stops. In pastes with internal water curing due to SAP inclusion, the initial capillary pores will be filled by hydration products, and the porosity is concentrated in isolated spherical inclusions about 100 µm across on average. These inclusions might have a toughening effect on the cement paste, acting like crack arrestors. In a normal cement paste without internal curing, a crack would propagate through the capillary network. In a cement paste with SAP, a crack reaching the surface of a spherical SAP inclusion would need to form a new crack surface before propagating through the inclusion (Figure 6). Creating new crack surfaces, instead of propagating the crack through an existing capillary network, is an energy-consuming process, so it is expected that the fracture toughness of concrete might be increased by SAP addition.

**5. CONCLUSIONS**

- Internal curing by means of SAP had almost no influence on the compressive strength of high performance mortars. The compressive strength of cement pastes was reduced 20% at early ages and 10% at later ages.

- The influence of moisture conditions on strength is unclear from the experiments performed, probably due to occurrence of microcracking in the samples.

- The gel-space ratio concept could explain the strength reduction of cement pastes with SAP. Mortars might be less sensitive to the strength reduction because their strength is limited by the presence of interfacial transition zones and weak aggregates.

- It is speculated whether SAP could increase the fracture toughness of concrete due to SAP inclusions acting as crack arrestors and energy dissipaters.

**ACKNOWLEDGEMENTS**

Funding of this project by the Danish Technical Scientific Research Council (STVF) is gratefully acknowledged. Thanks to Dale Bentz and Jason Weiss for useful discussion.

**REFERENCES**

[1] Jensen, O.M. and Hansen, P.F., 'Autogenous Deformation and RH-Change in Perspective', Cem. Concr. Res. 31 (12) (2001) 1859-1865.

[2] Jensen, O.M. and Hansen, P.F., 'Water-entrained cement-based materials – I. Principles and theoretical background', Cem. Concr. Res. 31 (4) (2001) 647-654.

[3] Jensen, O.M. and Hansen, P.F., 'Water-entrained cement-based materials – II. Experimental observations', Cem. Concr. Res. 32 (6) (2002) 973-978.

[4] Lura, P., Ye, G., Tanaka, K. and Jensen, O.M., 'Microcrack detection in high-performance cementitious materials', Proc. 4th Int. Sem. on Self-desiccation and its Importance in Concrete Technology, Gaithersburg, MD, 20 June 2005, pp. 165-178.

[5] Dela, B.F. and Stang, H., 'Two-dimensional analysis of crack formation around aggregates in high-shrinkage cement paste', Eng. Fract. Mech. 65 (2-3) (2000) 149-164.

[6] Weber, S. and Reinhardt, H.W., 'A New Generation of High Performance Concrete: Concrete with Autogenous Curing', Adv. Cem. Based Mater. 6 (2) (1997) 59-68.

[7] Lura, P., 'Autogenous deformation and internal curing of concrete', Ph.D. thesis, Delft University of Technology, Delft, 2003
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[9] Bentz, D.P., Geiker, M. and Jensen, O.M., 'On the mitigation of early age cracking', Proc. 3rd Int. Sem. on Self-desiccation and Its Importance in Concrete Technology, Lund, Sweden, 2002, 195-204.

[10] Durand, F., 'Water entrainment of cement mortar. Experimental investigation of strength effects and inclusion size', MSc thesis, Ecole Centrale de Nantes and Technical University of Denmark, Lyngby, Denmark, Sept. 2005, 51 pp.

[11] ASTM C 39, Standard test method for compressive strength of cylindrical concrete specimens, The American Society of Testing and Materials, Philadelphia, USA, 1986, 20-23.

[12] Lura, P., Jensen, O.M and Breugel, K. van, 'Autogenous shrinkage in high-performance cement paste: An evaluation of basic mechanisms', *Cem. Concr. Res.* **33** (2) (2003) 223-232.

[13] Bartlett, F.M. and MacGregor, J.G., 'Cores from high-performance concrete beams', *ACI Mater. J.* **91** (6) (1994) 567-576.

[14] De Larrard, F. and Bostvironnois, J.-L., 'On the long-term strength losses of silica-fume high-strength concretes', *Mag. Concr. Res.* **43** (155) (1991) 109-119.

[15] Powers, T.C. and Brownyard, T.L., 'Studies of the Physical Properties of Hardened Portland Cement Paste', Bulletin 22, Research Laboratories of the Portland Cement Association, Chicago, 1948.

[16] Weiss, W.J., 'Prediction of Early age Cracking in Thick Concrete Structures', PhD Thesis, Northwestern University, Evanston, IL, 1999.