In Mexico, 2.8x109 [m.sup.3] [year.sup.-1] (average flow rate) of raw water are treated in approximately 307 drinking water facilities (1). From the total flow rate, 22.5% (6.3x108 [m.sup.3] [year.sup.-1]) is processed in Los Berros drinking water facility (PPLB), located at the north of Mexico City. PPLB starts in 1985 and the sludges generated have been traditionally stored in protected sites located in the PPLB. Nevertheless, these sites have already reached their maximum capacity making necessary to find a solution to avoid their final disposal into the environment (1). One of the most feasible alternatives to solve this situation is their valorization as raw materials in the elaboration of different products.
Drinking water sludges have traditionally been classified as wastes. However, they present chemical compounds, such as calcium, silicon, etc., similar to those reported in literature as raw materials used to elaborate ceramic products (2-9), for the preparation of adsorbents and catalysts (10) and as soil substitute for land application and elaboration of fire-breaks in woods (11-15). Moreover, due to the residual content of aluminum, drinking water sludges can be used to clarify wastewaters. Also, these residues have shown similar physicochemical characteristics to the raw materials used to elaborate sun-dried bricks, despite that this use has not been reported in literature. However, considering that this type of brick is widely used in Mexico to build houses and recently the government has authorized the construction of apartments and houses near to the PPLB, the application of sludge as raw material to elaborate mortars and concretes could be the most feasible valorization option to be considered.
Based on this situation, this work was intended to assess the technical feasibility to valorize the sludge, generated and stored at the Los Berros drinking water facility (PPLB), as raw material to produce building products (mortar and concrete) used in the construction industry.
MATERIALS AND METHODS
The experimental study was divided into three stages: 1) sampling and characterization of PPLB sludges to determine their potential as raw material (sand substitute and supplementary cementing material) to produce mortar and concrete; 2) production and characterization of specimens, using sludge in different ratios of mixtures with cement, lime, commercial mortar and plaster to prepare mortars and concretes and 3) comparison between results obtained in compressive strength and drying contraction tests for each specimen and the Mexican criteria to build mortars and concretes.
Sampling and characterization of PPLB sludges: Samples of the PPLB sludge were collected in duplicate in the stored sites at three different points and at three different depths (1, 3 and 5 m) during the rainy season (August 2005). For each sample collected, a same amount of sample (100 g) was weighted and mixed for analysis. The parameters measured were: pH, humidity, texture, aluminum, iron, heavy metals and oxides content. All samples were analyzed according to the Standard Methods for the Examination of Water and Wastewater (16) and the Standard Methods for Water and Effluents Analysis (17). Where analysis was not immediately possible, the samples were preserved to inhibit biodegradation. All the reagents used for the analysis were of analytical grade
Elaboration of specimens with drinking water sludges: Sludge was mixed with cement, lime, plaster and a commercial mortar to elaborate cube specimens of 50 mm according to the C 109ASTM standardized technique for Compressive Strength of Hydraulic Cement Mortars (Using 50 mm Cube Specimens). Table 1 shows the composition of the binary and ternary mixtures prepared with PPLB sludges to elaborate cube specimens.
Table 1: Composition of binary and ternary specimens using PPLB sludges Composition (%, v/v) Binary Mixture 100-0 90-10 75-25 40-60 25-75 0-100 Sludge [check] Sludge-cement [check] [check] [check] [check] [check] Sludge-lime [check] [check] [check] [check] [check] Sludge-plaster [check] [check] [check] [check] [check] Sludge-mortar [check] [check] [check] [check] [check] Composition (%, v/v) Ternary mixture 90-5-5 75-12.5-12.5 40-30-30 25-37.5-37.5 Sludge-plaster-cement [check] [check] [check] [check] Sludge-plaster-lime [check] [check] [check] [check] Sludge-lime-cement [check] [check] [check] [check] The technique applied in the elaboration of the cube specimens consisted in weighting and...