Cement-sand mortars are used in many applications in the construction industry such as tiles adhesive, interior and exterior coatings, texture, and leveling.
Mortars in tile adhesive applications have high compression strength; to improve mortar mechanical properties several solutions have been proposed in micro and macro fibers like Dramix® fibers and glass fibers. In nanoscale, solutions have been used mainly in tubular form like Multi-wall Carbon nanotubes and Halloysite nanotubes, also in the form of flakes like Graphene  .
In actuality, MWCNTs usage as engineering reinforcement is expanding due to their mechanical properties. MWCNTs have a Young modulus ranging from 270 to 950 GPa and a tensile strength ranging from 11 to 63 GPA  , making this an ideal reinforcement for cement mortar.
As an alternative for MWCNTs, HNTs are proposed base on mechanical properties, lower cost and present no health risk compared to CNTs. HNTs have a Young modulus ranging from 1 to 1000 GPa and a tensile strength ranging from 0.03 to 4.0 GPa  .
Studies of MWCNTs on cement mortar reported an improvement in mechanical properties; MWCNT reported in  showed 15% improvement in compression strength and 36% in tensile strength at concentrations of 0.5%. HNTs reported in  showed an improvement in compressive strength of 24% at concentrations of 3%
To measure the improvement in mechanical properties a mix design was proposed. It consists of a base formula without any nanoparticle addition and two samples with two concentrations of MWCNTs and HNTs. Mechanical properties of a cement mortar modified with nanoparticles were evaluated with compression and bending tests at 7 days; three specimens were performed for each concentration.
Cement mortar specimens were prepared using: Portland Cement CPO 30 RB, sand, water and chemical additives.
MWCNTs-Ni were provided by Dr. Oxana Vasilievna Kharissova and Dr. Beatriz Ortega Garcia from UANL. HNTs were provided by Dr. Laura Peña Paras from UDEM, physical properties for both nanoparticles shown in Table 1.
For an appropriate dispersion, a super plasticizer Sika Viscocrete 7300 HE and a 500 W and 20 KHZ Cole-Parmer Tip Ultrasonic processor were used.
3. Dispersion of Nanoparticles
A common problem with MWCNTs is agglomerations, nanoparticles present a high ratio of surface area/volume coupled with a relatively high Van der Waals energy. To avoid this problem was proposed the first sample of MWCNTs was dissolved in 100 gr of water and 2.45 gr of super plasticizer, and then it was so-
Table 1. Physical properties of nanoparticles.
Figure 1. MWCNTs and HNTs sonicated dispersions.
nicated for intervals of 5 minutes, with the ultrasonic processor using a power of 100 W at 20 KHz.
Samples were taken every 5 minutes and observed under an optical microscope at 20× zoom and observed looking for agglomerations of MWCNTs, continued this process until a similar manner as mentioned dispersion in  , was determined a time of 30 minutes the ideal, same time used for dispersion is mentioned in  . On this basis specimens with two concentrations of MWCNTs and HNTs were sonicated as shown in Figure 1.
4. Mixing and Specimens Preparation
The base formula comprises Portland white cement CPO 30 RB, sand, and chemical additives. For the preparation of the specimens shown in Table 2, were added in a planetary mixer first water, then the powder mortar, in the case of plain cement mortar. For mixtures with nanoparticles water is added first, then the solution of water + SP and nanoparticles, at last, dust mortar is added as shown in Figure 2. The mixing procedure is according to NMX-C-420, in all mixtures, 26% water is retained powder mortar. After the mixing is allowed to stand for 10 minutes to remove the air to each mixture. Mix density is reviewed in a bronze glass with a specific volume, the glass is according to ASTMC185 for each formula shown in Table 3. Later proceed to prepare the specimens in the molds for mechanical testing as shown in Figure 2. Specimens cured in a room controlled conditions for 7 days at 45% relative humidity and a temperature of 23˚C according to NMX-C-420.
5. Mechanical Testing Equipment
The scope of this article had the purpose of investigating the mechanical properties of a cement mortar modified with nanoparticles such as compression and bending tests. Specimens were removed from metal molds at 72 hours and then tested at the age of 7 days. The equipment used for testing is a machine by company Controls Dual 15 KN for bending 300 KN compression. Automatic digital model 65-L27D1x, shown on Figure 3 is equipped with two load cells to measure forces with high precision The machine has 2 devices, the first consists of a fixture for standard bending for beams 4 × 4 × 16 cm mortar dimensions according to EN 196-1 the second it is for testing standard compression
Table 2. Mixture proportions.
Figure 2. Mixture and specimen preparation.
Table 3. Densities of cement mortar paste.
Figure 3. Bending/Compression equipment and specimens.
Table 4. Mechanical properties at 7 days.
Figure 4. A Chart of compressive strength of specimens at 7 days.
according to EN 196-1, bending previously broken.
Bending tests and conducted over an area of mortar 640 mm2 at a loading rate of 15 kg/s using the bending device, for compression test contact area of 1600 mm2 is used, and a load speed 15 kg/s, results shown in Table 4 and Charts of Compressive and Flexural strength shown in Figure 4 and Figure 5.
7. Discussion of Results
・ Sample MWCNTs mortar with 0.10% on cement mortar, exhibits an increase in the mechanical properties of compression at the age of 7 days of 56% compared with the control formula, in terms of flexion, there is an improvement of 4% with respect to control formula.
・ Sample MWCNTs mortar with 0.25% on cement mortar, showed an increase in compressive strength of 36% at the age of 7 days with relation to the control formula, but a decrease of 12% in flexural strength at the age of 7 days.
・ Sample mortar with 0.10% HNTs on cement, exhibits a 31% increase in compressive strength at the age of 7 days, regarding flexural strength at the
Figure 5. A Chart of flexural strength of specimens at 7 days.
age of 7 days, there was a small improvement of 0.8% compared with control formula.
・ Sample with 0.25% HNTs mortar on cement, showed a 28% increase in compressive strength at the age of 7 days and an increase of 3% in terms of flexion at the age of 7 days.
Based on the results obtained, we can determine that the lower the concentration of nanoparticles was in cement mortar, the better performance in mechanical properties was achieved. According to the observed standard deviation, in HNTs it was more reduced than with MWCNTs and plain cement mortar; this was due to a better dispersion within the cement matrix.
The impact on the cost as a reference was from Sigma-Aldrich  ; HNTs cost $59.10 USD/100g or $0.59 USD/g. and for MWCNTs $116 USD/gram.
The 0.1% specimens of HNTs gave an improvement in compressive strength 16% lower than MWCNTs with same concentration, but it was noticed that the cost in the industry played an important role in decision-making. An evaluation based on the cost/performance ratio showed that HNTs were a better overall solution for cement mortar performance increase.
Studies are needed in order to review the properties at the age of 28 days and also to evaluate lower concentrations to find the optimal minimum for HNTs.