Cement mortar and concrete with a reduced water absorption and a method for manufacture thereof

A cement mortar and concrete having a reduced water absorption and with a density of 2000-2500 kg/m.sup.3, at which it calculated on the amount of cement contains 0.2-10% by weight of a polymer in dispersion form consisting of polymeric particles containing hydropholic monomers from one or more of the groups acrylic monomers, methacrylic monomers and styrene and 1-15% calculated on the total amount of monomers of a methacrylate having an aliphatic chain of 12 or more carbons, preferably stearyl methacrylate.

BACKGROUND OF THE INVENTION 
The present invention refers to admixture of a hydrophobic polymeric 
dispersion for providing a cement mortar which after curing has a 
considerably reduced water absorption and thus an improved permanence than 
cement mortar manufactured in the conventional way. 
A fresh cement based binding agent mixture (cement mortar or concrete mass) 
consists of solid particles (aggregate), water and air. A cement-bound 
concrete substantially consists of about 1 volume part of cement, 2 parts 
of water and 6,5 parts of aggregate; in addition to this there will be 50 
l air in about 1000 l of fresh concrete mass. The cement particles react 
with a part of the mixing water to a hydration product which forms a 
mineral adhesive, the water-cement paste. 
The fresh concrete or mortar contains water in order to on one hand 
together with the cement form the chemically hardening binding agent and 
on the other hand give the mass the consistency and homogeneity that is 
required for the compaction. In most cases the water admixture is 
considerably larger than required for the chemical reactions even if the 
degree of hydration is complete. 
The water that is not chemically bound, the so called free water, forms 
capillary pores, which are continuous at the beginning of the hydration 
process and at high water-cement ratios. The volume of the capillary pores 
is therefore influenced by the water-cement ratio and the degree of 
hydration. The water in the capillary pores causes shinkage and swelling 
of hardened concrete, mortar and cement paste at drying-out and moistening 
resp. The considerably smaller pores, the paste pores, are contained in 
the water-cement paste, which makes the principle part of the hydration 
products. The volume of these pores are only determined by the volume of 
the paste and is completely independent of the initial water-cement ratio. 
The paste pores are above all determining of the shrinkage. 
Besides the said pores air pores are contained, which are formed at the 
mixing. They can be natural so called comprimation pores and intentionally 
added pores. The natural air pore content in concrete, which is obtained 
when no special measures are taken is 1.0-3.5% by volume. Intentionally 
created air pores can be provided in many different ways, as well 
mechanisms as principles are described in the literature. 
During the curing a number of chemical reactions take place, at which i.a. 
water disappears through the capillary system. The cured concrete thus has 
a capillary system, which can take up water by the fact that the walls are 
hydrophilic. In certain cases this can be a drawback, e.g. by the fact 
that water in the capillaries freezes at low temperatures and causes 
disintegration of the concrete. This gives problems with late strength of 
the concrete especially in cold and moist environments and at marine 
applications. 
It is previously known through SE-B-Nos. 7614518-4 (publ. No. 418.736), 
7614519-2 (publ. No. 418.852) and 8100489-7 (publ. No. 420.595) to prepare 
a cement mortar with low density (1200-2000 kg/m.sup.2) by adding small 
amounts (0.2-5%) of polymeric particles. This depends on that polymeric 
particles added to the mortar stabilizes air pores. The polymeric 
particles used contains a hydrophobic component consisting of e.g. acrylic 
or methacrylic monomers, styrene or butadiene and a non-ionic tenside or a 
hydrophilic component consisting of an ethylenically unsaturated 
polymerizable compound. 
In the British patent specification 1.159.377 there is described a cement 
mortar for joining structural elements. Manufacture of concrete is not 
mentioned. In the cement mortar there is added alkyl acrylates having an 
alkyl chain of up to 18 carbons, at which ethyl acrylate is preferred. The 
effect of this admixture is told to be an improvement of the adhesion of 
the mortar. 
THE OBJECT AND MOST IMPORTANT FEATURES OF THE INVENTION 
The object of the present invention is to provide a cement mortar and/or a 
standard concrete which after curing has a considerably reduced water 
absorption and by that an improved permanence than cement mortar 
manufactured in the ordinary way. Admixture of extra air in the mortar 
should hereby be avoided. 
This has according to the invention been achieved by the fact the cement 
mortar and/or the concrete calculated on the amount of cement contains 
0.2-10% by weight of a polymer in dispersion form containing one or more 
monomers from the groups acrylic monomers, methacrylic monomers and 
styrene and 1-15% calculated on the total amount of monomers of a 
methacrylate with an aliphatic side chain of 12 or more carbons. 
DESCRIPTION OF THE INVENTION 
The admixture of the polymeric dispersion results in that small hydrophobic 
polymeric particles from the dispersion are accumulated on the walls of 
capillaries and pores and gives them hydrophobic properties at the same 
time as the capillary and pore system is stabilized ano the construction 
is modified. In this way the water absorption is reduced and by that the 
risk for damages caused by water, salt and ice crystals. Additives for 
reducing formation of air pores known up to now usually do not provide a 
reproducable capillary and pore structure, which is of great importance 
for the resistance of concrete. 
The polymers according to the invention is manufactured by known techniques 
for emulsifying polymerization. In order to have a low admixture of air in 
the cement mortar the amount of tenside has to be kept low, preferably 
below 2%. In order to obtain small particle sizes a hydrophilic monomer 
such as acrylic acid, methacrylic acid or acrylic amide is added to the 
polymerized monomer mixture. These monomers tend to accumulate on the 
surface of the dispersion particles and stabilize the particles in this 
way. An appropriate amount of hydrophilic monomer is 1-10%. Combinations 
of hydrophilic monomers may of course also be used. 
The hydrophobic part of the polymer particles consists of polymeric chains 
obtained by polymerization of acrylates, methacrylates, styrene and 
similar commersially available monomers. Particularly good results are 
obtained when methacrylate with a long aliphatic side chain is used. A 
preferred example is stearyl methacrylate, which in an amount of 1-5% or 
preferably 2-10% calculated on the total amount of monomers gives a 
considerable reduction of the water absorption and stabilization of the 
pore system.

EXAMPLE 1 
In a 1 liter polymerization flask with a cut lid there was added 490 ml 
water, 1 g ammonium persulphate, 1 g sodium bisulphite as initiator system 
and half of a monomer mixture consisting of 40% methyl methacrylate, 50% 
butylacrylate, 5% acrylic acid and 5% stearyl methacrylate, together 230 
ml. The polymerization was carried out under nitrogen gas, under stirring 
and at a temperature of 50.degree. C. After half an hour reaction the 
other half of the monomer mixture was added drop by drop during half an 
hour. After 4 hours the reaction was completed. 
Test specimens were prepared according to standard ANSI/ASTM 305-65 of 
cement mortar containing 1% polymer of the cement weight (calculated as 
dry polymer). After curing under water during 28 days strength, air 
content, water absorption and frost resistance were measured. The tensile 
strength in bending was 7.5 MPa, the compressive strength was 49.5 MPa, 
which should be compered to 7.4 and 43.5 MPa resp. for the same cement 
mortar without polymer admixture. Immersion in water resulted after 8 
hours in 1.8% water absorption and after 144 hours 5.48. Corresponding 
values for specimens not containing polymers were 3.0% and 9.3% resp. 
Three types of concrete, K 30, K 40, K 50 were manufactured according to 
Swedish standard BBK 79 with and without resp. polymer admixture. The 
frost resistance for cylindrical concrete test specimens (10 cm diameter, 
5 cm height) was tested. They were placed in a plastic container so that 
the bottom surface of the test specimens were in contact with a 30% 
NaCl-solution. The specimens were exerted to cyclic temperature 
variations: 16 hours in -20.degree. C. (freezer) and 8 hours at room 
temperature. Crack formations in the specimens were observed after the 
following number of cycles: 
______________________________________ 
normal 
with polymer admixture 
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K 30 16 65 
K 40 11 43 
K 50 13 45 
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EXAMPLE 2 
The polymeric dispersion was the same as in example 1. Test specimens 
prepared with 3% dispersion (30% polymer) gave the following strength 
values: tensile strength in bending 7.4 MPa and compressive strength 44 
MPa. The water absorption was 1.43% after hours and 4.52% after 144 hours. 
EXAMPLE 3 
A polymer having the monomer composition 68% styrene, 25% ethyl hexyl 
acrylate, 5% stearyl methacrylate and 2% acrylic acid was prepared in the 
corresponding way is in example 1. In this case a tenside mixture 
consisting of 1.5% B733 and 1% B09 from Berol Kemi AB was used. The 
polymerization temperature was 80.degree. C. After 4 hours the reaction 
was completed. It had a dry content of 29%. Test specimens prepared in the 
corresponding way as in example 1 with 3% polymer dispersion calculated on 
the weight of the cement gave the following strength values: bending 
strength 7.45 and compressive strength 44.8 MPa. The water absorption was 
after 8 hours 2% and after 144 hours 4.94%. 
EXAMPLE 4 
The polymer dispersion was prepared in the same way as in example 3 with 
the difference that the tenside content was lowered to one fourth. Test 
specimens prepared as in example 1 gave the same air content as without 
additive, i.e., no extra air entrainment was obtained. 
As a comparison tests were performed according to the examples 5 and 6 
below with a polymer dispersion which did not contain stearyl 
methacrylate. 
EXAMPLE 5 
In a corresponding way as in example 1 a dispersion having the monomer 
composition 48% methyl methacrylate, 50% butyl acrylate and 2% acrylic 
acid was prepared. Besides 0.2% silicon dioxide was added. After 4 hours 
at 50.degree. C. a dispersion having the dry content 28.3% was obtained. 
Test specimens prepared as in example 1 gave the following strength values 
bending strength 5.8 MPa and compressive strength 35 MPa. The water 
absorption was 3.2% after 8 hours and 7.9% after 144 hours. 
EXAMPLE 6 
In a corresponding way as in example 1 a dispersion having the monomer 
composition 48% methyl methacrylate, 50% butyl acrylate, 2% acrylic acid 
was prepared. After 4 hours at 50.degree. C. a dispersion havng a dry 
content of 29% was obtained. Test specimens prepared as in example 1 gave 
the following values: bending strength 7.4 MPa and compressive strength 
43.7 MPa. The vater absorption was 3.8% after 8 hours and 6.2% after 144 
hours. 
As can be seen from the two examples 5 and 6 above the water absorption was 
higher as compared to polymers containing stearyl methacrylate. 
EXAMPLE 7 
Tests have also been performed with lauryl methacrylate, at which 
dispersions having the following monomer composition were prepared: 
(1) 42% methyl methacrylate, 52% butyl acrylate, 3% acrylic acid and 3% 
lauryl methacrylate. 
(2) 41% methyl methacrylate, 51% butyl acrylate, 3% acrylic acid and 5% 
lauryl methacrylate. 
The same initiating agents were used as in example 1. The dispersions were 
prepared in exactly the same way as in the above described examples with 
stearyl methacrylate. 
The air content of 1:3 cement mortar (1 part of Portland cement and 3 parts 
of standard sand) was examined with an admixture of 3% by weight 
(calculated as dry polymer) of the dispersion. 
______________________________________ 
Air content % Water-cement ratio 
Density kg/m.sup.3 
______________________________________ 
(1) 10.3 0.45 2098 
13.2 0.50 2010 
(2) 6.7 0.45 2185 
9.7 0.50 2085 
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Thus it is possible to use lauryl methacrylate without tenside, but it has 
a certain air entraining effect, which possibly can be reduced by varying 
the monomer composition of the dispersion. The frost resistance was 
determined in the same way as in example 1 to 30 cycles. 
The invention is of course not limited to the above described examples but 
can be varied within the scope of the claims. It is for example likly that 
also methacrylates having an aliphatic side chain of 13-17 and more than 
18 carbons have a corresponding effect.