WO2020007913A1

WO2020007913A1 - Pva as additive to cement formulations to inhibit ice recrystallization and protect against freeze-thaw damage

Info

Publication number: WO2020007913A1
Application number: PCT/EP2019/067850
Authority: WO
Inventors: Ilja Karina VOETS; Shuaiqi GUO; Zhengyao QU; Qingliang YU
Original assignee: Technische Universiteit Eindhoven
Priority date: 2018-07-03
Filing date: 2019-07-03
Publication date: 2020-01-09

Abstract

Cement formulations, materials or products and methods of use are provided that contain small amounts of Poly (Vinyl Alcohol) (PVA) to inhibit ice recrystallization and thereby protect cement against free-thaw damage. In one example, the cement formulations have a flexural strength of 3.5-5.0 MPa after 1-28 days and a compressive strength of 20-35MPa for 1-28 days.

Description

PVA AS ADDITIVE TO CEMENT FORMULATIONS TO

INHIBIT ICE RECRYSTALLIZATION AND PROTECT AGAINST FREEZE-THAW DAMAGE FIELD OF THE INVENTION

This invention relates to technology to inhibit ice recrystallization and protect cement against freeze-thaw damage.

BACKGROUND OF THE INVENTION

Frost action, freezing and thawing cycles, is known to impair the service life of cementitious materials in cold regions as it can cause both the surface and internal damage. Many studies have been conducted to investigate the mechanism of frost damage, but until to date, the mechanism which dominates the damage during the freeze thaw cycles is still in debate and unclear. However, one common ground is that the volume expansion caused by the transfer of water into ice plays a key role to determine the frost resistance of the cementitious materials as the pore space among the cement paste is fixed and pore structure is stiff. The present invention, provides new insight where Poly (Vinyl Alcohol) (PVA) is proposed as an additive to cement formulations to inhibit ice recrystallization and thereby protect cement against free-thaw damage. SUMMARY OF THE INVENTION

It has been discovered that the addition of a small amount of the soluble polymer Poly (Vinyl Alcohol) (PVA) to concrete mix design can substantially improve the durability of concrete in freezing environments. In one example, application of PVA in a mortar material was provided with a water to cement mass ratio of 0.5 containing cement (e.g. CEM III/A 52.5 N), sand, and PVA at a composition of 22 wt.% cement, 66 wt.% sand, 11 wt.% water and 0.018 wt.% PVA. Specifically, the invention includes the following features:

• PVA inhibits ice recrystallization at vinyl alcohol monomer concentrations CVA > 0.3 x 10 ³ M. (CVA ₌ concentration of vinyl alcohol monomer).

• PVA architecture hardly impacts ice recrystallization, thus PVA can be applied in many ways as (linear, bottlebrush, block, or the like) (co)polymer solution, or as micellar solution without loss of ice recrystallization activity.

• PVA is a water-soluble polymer that can be formulated in aqueous solution under basic conditions (e.g. pH of 12.5).

· PVA (Mw=l46, 000-186, 000) at low concentrations (< 0.02wt%) does not deteriorate the mechanical stability of the concrete structure. • Adding PVA to the concrete formulation at low concentration results in the retention of a porous concrete structure with PVA inside, which inhibits ice recrystallization and therefore prevents freeze-thaw damage.

Prior to the discovery of this invention, freeze-thaw durability of cement was improved by the following methods:

1. Air entraining agent,

2. Porous aggregates,

3. Improvement of the microstructure, and

4. Application of protection coating.

However, these methods either reduce the durability of cement or elevate the production cost of cement impeding its large-scale industrial adoption. To the knowledge of the inventors, PVA has never been added to cement to improve its durability in freeze-thaw environments. The mortar material according to this invention present more resistance against freeze-thaw damage, namely, the total amount of the surface-scaled material after 56 cycles is less than 100 g/m².

PVA at low concentrations (< 0.02wt%) does not deteriorate the mechanical stability of the concrete structure. The microstructure of the concrete material is not altered by the PVA and a flexural strength of 5.0 MPa at the age of 28 days and a compressive strength of 35MPa at the age of 28 days are observed, which is very comparable to the control samples.

Concrete formulation - example of compositional range in a more concrete way as for the PVA (< 0.02wt%)

Embodiments of PVA have been tested in a mortar formulations/materials/specimens with a waterxement weight ratio of 0.5 containing cement (e.g. CEM III 52.5 N), sand, and PVA at a composition of 22 wt% cement, 66% sand, 11% water and 0.018 wt% PVA. The compositional range (waterxement) can be 0.4-0.6; PVA <0.02wt%.

Freeze thaw durability

Embodiments result in mortar specimens more resistant against freeze-thaw damage, namely, the total amount of the surface-scaled material after 56 cycles is less than l00g/m².

Mechanical property

The addition of PVA presents no deterioration of the mechanical property and pore structure of the concrete, namely with unchanged accumulative pore volume distribution, a flexural strength of 3.5-5.0 MPa after 1-28 days and a compressive strength of 20-35MPa for 1-28 days. How is the mechanics being tested?

The fresh mortar is cast in molds with the dimensions of 40 mm 40 mm 160 mm. The prisms are demolded approximately 24 h after casting and then cured at 100% RH at about 21 degrees Celsius. After curing for 7 and 28 days, the flexural and compressive strengths of the materials/compositions/formulations are tested according to EN 196-1. At least three formulations/materials/specimens are tested at each age to compute the average strength. What is found upon addition of PVA in this invention (0.01%.wt), atactic poly(vinyl alcohol), Mw= 146000- 18600; degree of hydrolysis of 99+%?

• No change in accumulative pore volume distribution curves for PVA modified sample relative to the reference.

• Concrete is classified as resistant against freeze-thaw damage [see G.

Quercia, P. Spiesz, G. Husken, J. Brouwers, Effects of amorphous nano silica additions on mechanical and durability performance of SCC mixtures, in: Proc. Int. Congr. Durab. Concr. (ICDC 2012), 2012: pp. 18- 21] if the total amount of the surface-scaled material after 56 cycles is less than l00g/m². Surface scaling of the PVA-modified sample (with 0.0l%.wt PVA) is less than in the reference sample (tested up to 56 freeze- thaw cycles), namely reduced from 241 g/m² to 99 g/m².

• The flexural and compressive strength of the reference and PVA modified mortars are shown in FIGs. 6A-B, respectively. In this invention the following was found: a flexural strength of 3.5-5.0 MPa after 1-28 days and a compressive strength of 20-35MPa for 1-28 days.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows according to an exemplary embodiment of the invention a temperature profile of the freeze-thaw test.

FIGs. 2A-G show according to an exemplary embodiment of the invention microphotographs of frozen pure water (FIGs. 2A-B) and simulated pore solution (FIGs. 2C-D), and with the presence of 4mM PVA (FIG. 2E) and (FIG. 2F). (FIG. 2A), (FIG. 2C) and (FIG. 2E) indicate the beginning of the experiments (t=0 min at -7 degrees Celsius), (FIG. 2C) and (FIG. 2D) indicate the end of the experiments (t=60 min at -7 degrees Celsius). The scale bar is 10 pm. The effect of different solution in tuning the ice crystal size is shown in (FIG. 2G).

FIG. 3 shows according to an exemplary embodiment of the invention differential and accumulative pore volume distribution curves for the reference and PVA modified samples

FIG. 4 shows according to an exemplary embodiment of the invention surface scaling of the reference sample and PVA modified sample due to the freeze-thaw cycles. FIGs. 5A-C show according to an exemplary embodiment of the invention heat evolution FIG. 5A), cumulative heat evolution FIG. 5B) and XRD patterns FIG. 5C) of the reference and PVA modified samples. E indicates ettringite, C indicates calcium aluminate hydrate, CH indicates portlandite, M indicates melilite, Q indicates quartz, CSH indicates calcium silicate hydrate, C2S indicates dicalcium silicate, C3S indicates tricalcium silicate.

FIGs. 6A-B show according to an exemplary embodiment of the invention the flexural FIG. 6A) and compressive FIG. 6B) strength of the mortars.

DETAILED DESCRIPTION

The following is an exemplary description of the ice recrystallization inhibition ability of PVA in the simulated pore solution of concrete and the freeze-thaw resistance of the PVA modified concrete. The influence of Ca²⁺ in the pore solution to the ice recrystallization under the freeze thaw circles and the recrystallization inhibition ability of PVA in high pH environment are also provided. The pore structure of the reference sample and modified sample are characterized due to their primary role on the freeze-thaw behavior of concrete. The influences of PVA on the mechanical property, hydration process and phase changes of concrete have been investigated. Experiments

Materials

One type of atactic Poly (Vinyl Alcohol) was purchased from Aldrich with a weight-average molar mass of Mw=l46000-l8600 and a degree of hydrolysis of 99+%. Then the PVA was dissolved in tap water at 90 °C for 2 hours with 4mmol. Mortar specimens/formulations/materials with a water cement ratio of 0.5 and cement type of CEM III 52.5 N and normal sand are used. The composition of each series is shown in Table 1. Group Ref are the reference samples without the addition of PVA and group PVA modified are the samples modified by PVA adding into the water.

Table 1. Composition of the formulations/materials/specimens in each series

Sample Cement Sand Water PVA

(kg/m³) (kg/m³) (kg/m³) (mmol)

Ref 450 1350 225 0

PVA 450 1350 225 5

modified

Anti-ice-recrystallization in simulated pore solution

The simulated pore solution was a saturated calcium hydroxide (pH~l2.5) solution at room temperature and was prepared using double distilled water and analytical pure reagents. The IRI experiments were performed as described previously ( Olijve et al, Influence of polymer chain architecture of poly (vinyl alcohol) on the inhibition of ice recrystallization, Macromol.

Chem. Phys. 217 (2016) 951-958 ). Briefly, a 1 pL sample droplet of the water dissolved in 30% sucrose is sandwiched between two microscope slides and flash frozen to form a thin ice wafer. The ice wafer is held at -7 °C and the recrystallization process monitored for 60 min. An image analysis software that is able to extract circular features from the images is used to determine the grain boundary migration processes of the ice crystals. Mechanical property

The fresh mortar is cast in molds with the dimensions of 40 mm 40 mm 160 mm. The prisms are de-mo lded approximately 24 h after casting and then cured at 100% RH at about 21 degrees Celsius. After curing for 7 and 28 days, the flexural and compressive strengths of the formulations/materials/specimens are tested according to EN 196-1 (T.S. EN, 196-1. Methods of testing cement-Part 1: Determination of strength, Eur. Comm. Stand. 26 (2005)). At least three formulations/materials/specimens are tested at each age to compute the average strength.

Influence to the hydration kinetics

The influences of PVA on the hydration process of cement and products are investigated by isothermal calorimetry and X-ray diffractometry separately. The heat release of the reference and PVA modified samples were measured by a calorimetry instrument set at 20 degrees Celsius (TAM AIR Calorimeter). It should be noted that the initial 4-6 min after mixing could not be measured due to the sample preparation procedure and the initial 0.5-1 h of the recorded data could be inaccurate because of the instability of instrument disturbed by the loading process. The results were normalized by the mass of solid. X-ray diffractometry (XRD) analysis was performed by using a Cu tube (40 kV, 30 mA) with a scanning range from 5° to 65° 20, applying a step 0.02° and 5 s/step measuring time. The qualitative analysis was carried out by using the Diffracplus Software (Bruker AXS) and the PDF database of ICDD.

Pore structure of the paste

The pore size distribution was measured using the mercury intrusion porosimetry (MIP) technique (Autopore IV, Micromeretics). The maximum applied pressure of mercury was 228 MPa, the mercury contact angle was 130 degrees and the equilibrium time was 20 s. The pore size range of 0.0063- 900 pm was investigated.

Freeze-thaw test

The surface scaling freeze-thaw test was performed following NEN-EN 12390-9 ( C.E.N . CEN, TS 12390-9: 2006. Testing hardened concrete-Part 9: Freezethaw resistance-Scaling, Beuth, Berlin. (2006)). The mortar formulation/material/specimen is cast in PVC tubes with a diameter of 100 mm and height of 60 mm. After one day, the mortar is demoulded and cured at 100% RH until the age of 28 days. Due to the limited volume of the climate chamber, three formulations/materials/specimens were tested for each mix, resulting in a total exposed surface area of 0.024 m² (the area recommended in [32] is 0.08 m² ). After the saturation, the freeze-thaw test was carried out with a 3 mm layer of de-mineralized water poured on the top surface. The temperature profile in the chamber followed the recommendations given in (C.E.N. CEN, TS 12390-9: 2006. Testing hardened concrete-Part 9: Freezethaw resistance-Scaling, Beuth, Berlin. (2006)) as shown in FIG. 1. The level of water on the surface of the samples was adjusted regularly. In total, 56 freeze-thaw cycles were applied, during which the surface scaling was measured every week.

The addition of 4mM PVA inhibits ice-recrystallization of the representative cement pore solution with a pH of 12.5

The ice recrystallization behaviour of the different solution is evaluated as shown in FIGs. 2A-F. The mean size of the ice crystals increases while the total number of ice crystals decreases during the annealing process. However, the mean sizes and shapes of the ice crystals are different. It can be seen in FIGs. 2C-D in the pore solution, after 60 mins, small ice crystals of tens nanometers grow into bigger ones from several hundred nano meters to 10-20 micro meters in average size. The microphotographs in FIGs. 2E-F show greatly reduced mean ice crystal sizes after the addition of PVA in the simulated pore solution compared to the reference. A large amount of ice crystals with tens nanometers can be found. This is attributed to ice recrystallization inhibition behaviour of the PVA as the conformation of the OH groups of an atactic PVA segment adsorbed on the primary and secondary prism planes of ice matches well with the ice lattice via multiple hydrogen bonds. It is also can be confirmed by the shape of the ice crystals shown in FIGs. D-F. With PVA, the ice crystals present shape edges and rectangle shape while the pore solution present round shape. Based on the size evolvement of the ice crystals, the ice growth rate can be calculated by the LSW theory), which suggests that the temporal increase in mean radius r follows: r\t) = ^ro +k_dt 0)

Where /_¾ (pm) is the initial mean radius at time t = 0 min and k _\ (pmVmin) the observed rate constant of recrystallization. Base on the theory work by Olijve et al. ( Olijve et al. A simple and quantitative method to evaluate ice recrystallization kinetics using the circle Hough Transform algorithm, Cryst. Growth Des. 16 (2016) 4190-4195 ) a radical number-average R_n , of the growing ice crystals should be applied as Ostwald ripening involves the growth of large ice crystals at the expense of small ice crystals. Therefore, R_n is calculated by

Where r, is the radius of ice crystal i. From the evolution of the mean ice crystal radius, the rate constant of recrystallization k_L\ is calculated from the slope of a fit of Eq. 1 to the experimental data. The calculated ice growth rate (Ka ) of the PVA containing solution is only 0.30% of that for the reference solution. The results indicate that PVA can inhibit the recrystallization of ice in the cement pore solution environment which demonstrates its potential for application in concrete which will be further discussed with the pore structure character. It should be noted that mean particle size of the crystal in the Ca(OH)₂ simulated solution is larger than that in the pure water. As shown in FIG. 2G, the average of ice crystal size of pure water in the freeze-thaw circles is 9.71 pm while it increased to 11.8 pm for the simulated pore solution and ice growth rate in pore solution is also higher than that in the pure water. One possible reason is that the addition of Ca²⁺ changed the net potential energy of the bulk ice. Based on the above results, it can be concluded that the pore solution of cementitious materials has a positive effect on the increase of the mean particle size of the crystals which in turn brings more harm to the pore structure of the concrete because of the Ca²⁺-rich property. Therefore, increasing the freeze thaw resistance of cementitious materials by adding ice recrystallization inhibitors has a great potential.

Pore size distribution is unchanged with the addition of 4mM PV A

FIG. 3 shows the differential and total pore size distribution curves of the reference and PVA modified samples. The region under the curve represents the concentration of the pores. It was observed that the total volume of the pores from reference and PVA modified samples are very close which means total porosity of the cement paste is not influenced by the addition of PVA. Differential intrusion curves in FIG. 3 provide similar pattern on pore structure distribution with two main peaks which both belong to the capillary pores. The first peak located in the region around 10-40 nm and the second peak located in the region around 0.05-0.1 pm which are the remnants of the original water-filled space among the cement particles. As water associate in these pores and freezes in the freeze-thaw circles, this class of pores are regard as the biggest threaten to the freeze-thaw resistance of concrete structures. With the help of PVA, the mean particles of the ice crystals decrease after the ice recrystallization which is close to second peak of the pore distribution. Therefore, the pressure generated from the volume expansion of ice to the pore structure is decreased as the pores have more space to tolerate. It should be noted that as the pore distribution of the reference and PVA modified concrete present similar pattern, the freeze-thaw resistance of the two samples should be the same. However, based on the result of the next section, the PVA modified possess better freeze-thaw durability and this can be attributed to the ice recrystallization inhibition ability of the PVA.

PVA significantly improves the durability of the concrete during the freeze thaw circles

The freeze-thaw test was performed for 56 cycles, during which the surface scaling was measured every 7 days and is shown in FIG. 4. After the modification by PVA, the surface scaling after 56 freeze-thaw circles decreased significantly from 241 g/m² to 99 g/m² comparing with the reference sample. Following the classification of resistance of concrete against freeze-thaw damage given in Quercia et al. ( Quercia et ah, Effects of amorphous nano-silica additions on mechanical and durability performance of SCC mixtures, in: Proc. Int. Congr. Durab. Conor . (ICDC 2012), 2012: pp. 18-21), as the total amount of the surface-scaled material after 56 cycles is less than l00g/m². Cai et.al concluded that the pore structure has a close relationship of the freeze-thaw durability of concrete ( Cai et al, Freeze-thaw durability of concrete: ice formation process in pores, Cem. Conor. Res. 28 (1998) 1281-1287). In this invention, however, at such a low addition amount, the pore structure did not change after the modification by PVA. Hence, it can be concluded that enhanced freeze-thaw resistance of concrete is solely caused by the PVA. This can be explained by the ice-recrystallization inhibition activity of PVA in the pore solution herein. With the help of the PVA, the ice recrystallization process is largely inhibited and the mean size of the ice crystals is therefore decreased. It can be seen from the FIG. 2F, large amount of the small ice crystals ranging from tens of nanometer to several micrometer exists. These ice crystals can distribute easily in the second peak of pores as shown in FIG. 3 with sizes ranging from 0.05-0.1 pm resulting in less tension to the pore structure of the mortar. However, without the PVA, large ice crystals will generate as shown in FIG. 2D which largely beyond the pore size of the mortar and definitely bring mechanical damage to the original pore structure. It should also be noted that, even with PVA, the mean ice crystal size is beyond the size of the pores located in the first peak. This illustrates that why surface scaling still happens with the addition of PVA. Reaction kinetics

The normalized heat flow of the reference cement and PVA modified blends within the first 80 h are shown in FIG. 5A. The presented calorimetric curves are in accordance with the previous studies on CEM III 52.5 N ( Yuan et ah, Reaction kinetics, reaction products and compressive strength of ternary activators activated slag designed by Taguchi method, Mater. Des. 86 (2015) 878-886 ), which show four typical reaction stages including initial dissolution, induction, acceleration/deceleration and a stable period. The initial dissolution stage corresponds to the initial wetting and dissolution of the raw materials. It usually occurs within the first few minutes of mixing and is generally regarded as a physical process, although some chemical reactions are possibly involved. The calorimetric peaks of this stage are not shown in the figure due to its much higher magnitude. The acceleration peak is located at around 16 h after mixing, which is assigned to the massive formation of reaction products from dissolved Ca, Si and Al units. FIG. 5B shows the cumulative heat evolution of the samples. It can be seen that the addition of the PVA did not change the hydration process and heat release amount of the cement. It has also further been proven by the XRD results as shown in FIG. 5C. The addition of the PVA did not change the phase organization of the hydrated cement. Therefore, based on the results provided herein, it can be concluded that the addition of PVA did not influence the reaction kinetics of the cement. Mechanical properties

The flexural and compressive strength of the reference and PVA modified mortars are shown in FIGs. 6A-B, respectively. Although some have found that the addition of PVA in concrete mix design will decrease the mechanical properties due to more pores will generate after the hydration of the cement paste, there is almost no difference that can be found between the reference and PVA modified samples which is in line with the results of mechanical properties ( Kim et al, Structure and properties of poly (vinyl alcohol- modified mortar and concretel, Cem. Conor . Res. 29 (1999) 407-415 ). This may be attributed to the very small application amount of PVA in this study (0.0l%.wt) while a dosage of 2%.wt was used previously Kim et al. (1999). This result shows that PVA can be applied with a concentration at which ice recrystallization is inhibited without any compromise on the mechanical strength, which is of high importance to cementitious materials.

Conclusions

The results presented herein lead to the following conclusions:

1. The simulated pore solution will increase the mean size of the ice crystal which means more harm from ice recrystallization to the micro structure of the concrete during the freeze-thaw circles. PVA is still active as an ice recrystallization inhibitor in the simulated pore solution with a pH value of 12.5.

The freeze thaw resistance of the modified concrete significantly improved and the surface scaling after 56 freeze-thaw circles decreased significantly from 241 g/m² to 99 g/m² comparing with the reference sample.

Because of the low addition amount (0.1 wt%), the macro property of the modified concrete did not change including the mechanical property, pore distribution. Meanwhile, the PVA did not influence the hydration of the cement.

Claims

CLAIMS What is claimed is:

1. A mortar material to inhibit ice recrystallization and protect against freeze-thaw damage, comprising: a composition of cement, water and Poly (Vinyl Alcohol) (PVA), wherein the composition comprises a water to cement ratio of 0.4 to 0.6 and a PVA concentration of less than 0.02 wt%

2. The mortar material as set forth in claim 1 , wherein the PVA has a molecular weight of 146,000-186,000 at concentration of less than

0.02 wt%

3. The mortar material as set forth in claim 1, wherein the PVA inhibits ice recrystallization at vinyl alcohol monomer concentrations defined as CVA > 0.3 x 10 ³ M, where CVA is defined as concentration of vinyl monomer.

4. The mortar material as set forth in claim 1, wherein the mortar material comprises a flexural strength of 3.5-5.0 MPa after 1-28 days and a compressive strength of 20-35 MPa for 1-28 days.

5. A concrete product to inhibit ice recrystallization, comprising a composition of cement, water and PVA, wherein the composition comprises a water to cement ratio of 0.4 to 0.6 and a PVA concentration of less than 0.02 wt%.

6. The concrete product as set forth in claim 5, wherein the PVA has a molecular weight of 146,000-186,000 at concentration of less than 0.02 wt%.

7. The concrete product as set forth in claim 5, wherein the PVA inhibits ice recrystallization at vinyl alcohol monomer concentrations defined as CVA > 0.3 x 10 ³ M, where CVA is defined as concentration of vinyl monomer.

8. The concrete product as set forth in claim 5, wherein the concrete product comprises a flexural strength of 3.5-5.0 MPa after 1-28 days and a compressive strength of 20-35 MPa for 1-28 days.

9. A method of using Poly (Vinyl Alcohol) (PVA) to inhibit ice recrystallization, comprising: using a composition of cement, water and

PVA, wherein the composition comprises a water to cement ratio of 0.4 to 0.6 and a PVA concentration of less than 0.02 wt%.

10. The method as set forth in claim 9, wherein the PYA has a molecular weight of 146,000-186,000 at concentration of less than 0.02 wt%.

11. The method as set forth in claim 9, wherein the PVA inhibits ice recrystallization at vinyl alcohol monomer concentrations defined as C_VA > 0.3 x 10 ³ M, where C_VA is defined as concentration of vinyl monomer.

12. The method as set forth in claim 9, wherein the composition comprises a flexural strength of 3.5-5.0 MPa after 1-28 days and a compressive strength of 20-35 MPa for 1-28 days.