**Victoria Eugenia García-Vera 1, Antonio José Tenza-Abril 2,\*, José Miguel Saval <sup>2</sup> and Marcos Lanzón <sup>1</sup>**


Received: 7 December 2018; Accepted: 22 December 2018; Published: 27 December 2018

**Abstract:** Using durable materials is a sustainable solution for extending the lifetime of constructions. The use of crystalline admixtures makes cementitious materials more durable. They plug pores, capillary tracts and microcracks, blocking the entrance of water due to the formation of crystals that prevent the penetration of liquids. The literature has covered the performance of these admixtures on concrete, but studies on mortars are still scarce. The aim of this study is to investigate the effect of an aggressive environment (sulphuric acid solution—3 wt%) on mortars produced with different percentages of a crystalline admixture (1%, 1.5% and 2% by weight of cement content). Physical and mechanical properties were studied after immersing the mortars in a H2SO4 solution for 90 days. It was found that, after a 90-day sulphuric acid exposure, mortars with the crystalline admixture showed greater compressive strength than the control mortar, besides exhibiting lower mass loss. However, the crystalline admixture did not produce any significant effect on the capillary water absorption coefficient. In a nonaggressive environment, and in the short term, the crystalline admixture did not have a significant effect on the compressive strength, the capillary water absorption coefficient or the ultrasonic pulse velocity.

**Keywords:** crystalline admixture; chemical exposure; sulphuric acid attack; durability; Xypex

#### **1. Introduction**

Nowadays, cement-based materials are exposed to acids because of environmental pollution, contact with some soils, ground waters, industrial waste-waters or sewers. These conditions can be found in some foundations, sanitation networks, treatment plants, agricultural and farm facilities, etc. [1–3]. Materials that are designed to work in aggressive chemical conditions must be durable materials, that is, materials with the capacity to withstand, for its service life, the physical and chemical conditions to which they are exposed [4]. Architects and engineers are vested with the responsibility of designing and building constructions by optimising the existing but limited resources in nature. Working with durable materials contributes to sustainability since they help to maintain the existing resources and avoid causing harm to the environment.

Concrete is susceptible to acid attack because of its alkaline nature. Sulphuric acid is particularly corrosive due to the sulphate ion in addition to the dissolution caused by the hydrogen ion causing the deterioration of cement-based materials. The reaction of the sulphuric acid (H2SO4) with the chemical compounds of cement hydration (calcium silicate hydrate, portlandite and calcium sulphoaluminate hydrate) generates gypsum, amorphous hydrous silica and aluminum sulphate. [3]. These chemical reactions, located in the regions close to the surface, provoke a profound degradation of the hydrated

cement paste in terms of physical and mechanical aspects, such as compressive strength loss, mass variation, cracking, softening and decohesion. This process leads to spalling and exposure of the interior of the concrete structure [5,6].

To improve acid attack resistance in cement-based materials, the matrix must be dense, with low permeability, and it is advisable to use in the production a sulphate-resistant cement, high alumina cement, etc. [7–9]. Some authors state that effective strategies for manufacturing cementitious materials that are durable when exposed to acid attacks involve the following: (i) controlling the materials used during the manufacturing process; (ii) producing them with a low water/cement ratio; and (iii) guaranteeing suitable curing conditions [10,11].

A very popular strategy used to produce durable cement-based material is adding admixtures and additives [12–15]. Some studies utilise nanomaterials to reduce the porosity and water absorption of cement-based materials [16,17], which leads to enhanced durability and improved performance when these materials are exposed to acid attacks. Examples of such nanotechnologies are nano silica, nano alumina, titanium oxide, carbon nanotubes and polycarboxylates [18–22]. Water-repellent materials are also used to increase durability in acid attacks, since they reduce the affinity of capillary pore surfaces to moisture [23,24]. Examples of hydrophobic compounds used as admixtures are powdered stearates, oleates and products based on silanes and silicones [25–27]. When adequate recycling materials are available, another strategy to enhance the concrete durability is the use of aggregates derived from sanitary ceramics. Such concretes are characterised by low water absorption, higher mechanical strength and higher water-resistance compared to conventional concrete. Moreover, they have more resistance to chemically aggressive environments compared to conventional concretes [28–31].

Crystalline admixtures can also help increase the durability of cement-based materials, especially when cementitious materials are in acid environments. According to various manufacturers of products for crystalline waterproofings for concrete—Xypex, Kryton, Penetron, etc.—there are three ways of applying the waterproofing systems: (i) a coating applied as a cementitious slurry to the surface of existing concrete structures; (ii) an admixture added to the concrete at the time of batching; and (iii) a dry-shake to the fresh concrete surface. In all cases, the protection system is a blend of Portland cement, fine treated silica sand and undisclosed chemicals that are the intellectual property of the manufacturer. The catalytic reaction of the chemicals in the admixture occurs as long as there is moisture in the cement-based materials. These chemicals react with calcium hydroxide and other products resulting from cement hydration. The reaction generates nonsoluble crystalline formations that fill the pores and capillary tracks of cement-based materials. The crystalline structure becomes a permanent and integral part of the concrete matrix. Thus, the concrete becomes permanently sealed against the penetration of water or liquids from any direction [32].

The crystallisation process of these admixtures is not immediate, and the complete waterproofing effect is not achieved until approximately 12 days after the concrete creation [33], as long as there is enough moisture for chemical reactions to occur. Moreover, it is especially important to have a moist curing environment to obtain better performance [34]. There are several studies that focused on testing the waterproofing effect of these admixtures [33,35], as well as the impact of an aggressive medium on concretes made with crystalline-based waterproofing products [36,37].

However, it is still a challenge to find high durability mortars manufactured with these types of admixtures in aggressive environments. In this study, a crystalline protection product (applied as an admixture) was used to make mortars with different percentages of admixture (1 wt%, 1.5 wt% and 2 wt%). Their behaviour when they were exposed to an aggressive environment (sulphuric acid solution—3 wt%) was evaluated. Physical (mass loss, ultrasonic pulse velocity and capillary water absorption coefficient) and mechanical (compressive strength) properties were studied after the mortars were immersed in an H2SO4 solution for 28 and 90 days. Moreover, the textural alterations due to the acid attack were evaluated using microscopy.

#### **2. Materials and Methods**

#### *2.1. Materials*

The cement used for the manufacture of the mortars was a high early strength type belonging to the European cement class CEM I 52.5 R [38]. Limestone sand was used as a fine aggregate (Di/di = 0/4). The particle-size distribution was calculated according to the procedure of UNE-EN 933-1 standard [39]. The distribution was obtained for three samples of sand and the mean values were calculated (Figure 1a). Xypex Admix C-1000 NF (Xypex Chemical Corporation, Richmond, BC, Canada) was added in three different concentrations (1 wt%, 1.5 wt% and 2 wt% of the cement weight) depending on the kind of mortar. The specific surface areas of the cement and crystalline admixture (Xypex Admix) used were 1.48 and 0.642 g/m2, respectively (determined by the Brunauer–Emmett–Teller—BET—method), and their particle-size distributions are plotted in Figure 1b. In total, four types of mortars were made: (i) control mortar (A mortar); (ii) mortar with 1 wt% of Xypex (B Mortar); (iii) mortar with 1.5 wt% of Xypex (C Mortar); and (iv) mortar with 2% of Xypex (D Mortar). The dosage used for all mortars was 1 part cement, 0.65 parts water and 3 parts limestone sand.

**Figure 1.** (**a**) Particle-size distribution of the limestone sand. (**b**) Particle-size distribution (done by laser granulometry) of the cement (CEM I 52.5 R) and the admixture (Xypex Admix).

#### Chemical–Physical Characterisation

The crystallographic phases present in the composition of the cement and the admixture (Xypex Admix C-1000 NF) were analysed with X-ray diffraction (XRD) using a Bruker D8-Advance diffractometer (Bruker Española S.A., Madrid, Spain) with mirror Goebel, and a generator of X-ray KRISTALLOFLEX K 760-80F (power: 3000W, voltage: 20–60 KV and current: 5–80 mA) equipped with a X-ray tube with copper anode. The spectra were registered with angles from 4◦ to 60◦ at 0.05◦ stepping intervals in Ө-Ө mode, and the X-ray tube was utilised at 40 kV and 40 mA. The diffraction patterns were evaluated with HighScore software (Malvern Panalytical, Madrid, Spain) and powder diffraction database PDF4+. Moreover, the chemical compositions of the binder and the admixture were analysed with X-ray fluorescence (XRF) using an X-ray sequential spectrometer PHILIPS MAGIX PRO (Philips Ibérica, Madrid, Spain) equipped with a rhodium X-ray tube and beryllium window. The spectrometer was controlled with the software package SuperQ (Malvern Panalytical, Madrid, Spain) that also stored the measurements and results.

The XRF results (Table 1) show that both materials were mainly composed of CaO (cement: 55.4%; Xypex: 59.8%) and SiO2 (cement: 14.9%; Xypex: 8.1%), containing moderate proportions of Al2O3, SO3 and Fe2O3 (percentages between 1.9% and 4.6%). Only the cement contained moderate proportions of MgO (3.3%). The XRD spectra (Figure 2) revealed that both materials had common crystalline compounds, such as alite (C3S), belite (C2S), tetracalcium aluminoferrite (C4AF) and calcite (CaCO3). Gypsum was only detected in the cement, and portlandite and quartz were only detected in the Xypex

admixture. The peak of portlandite in Xypex Admix may indicate that this admixture was partially constituted by hydrated cement.

**Figure 2.** X-ray diffraction (XRD) spectra of cement type CEM I 52.5 R and Xypex Admix. **Table 1.** X-ray fluorescence (XRF) characterisation of CEM I 52.5 R and Xypex Admix.

