The successful implementation of nanomaterials in the production of electronic devices, biochemical sensors and biotechnology systems, pharmaceuticals, catalysts, polymers, ceramics allows you to count on their successful use in the building industry. In particular, results of the directed improvement of characteristics of concrete using nanoparticles of different chemical composition (TiO2, Fe2O3, CuO, and others) are known. One of the promising directions of increasing the compressive strength of heavy concrete – adding in their composition the nanoparticles of silica SiO2 extracted from hydrothermal solutions. For research of this problem was obtained a sol of silica and performed a series of experiments on easily workable heavy concretes with the same water-cement ratio WCR.
Getting of silica sol
The aqueous medium containing orthosilicic acid H4SiO4 in concentration range of 600–800 mg/dm3, was guided from the separators of geothermal power plant in reinforced concrete tank (cooler), where at a temperature 63°С occur a polycondensation of H4SiO4 with the formation of the silica particles. After cooling the separat was filed in baromembrane installation for concentration and to obtain a stable aqueous silica sol. Initial separat had the following characteristics:
•salinity – 702 mg/dm3; •pH = 9,73; •the total content of SiO2 Сt = 716 mg/dm3; •the concentration of dissolved silicate Cs = 160 mg/dm3. The pressure difference across the membrane layer was 0.14 MPa, flow rate of solution passing through the installation – 1.2 m3/h. At the first stage of concentration was obtained silica sol with a density of 1015–1022 g/dm3, Сt = 28–40 g/dm3. In the second stage the density of sol increased to 1070 g/dm3, and Сt – up to 115 g/dm3.
The concrete composition
Characteristics of the silica sol are shown in table 1.
As a binder used South Korean Portland cement (PC) of 42,5 R class, corresponding to the Russian standards for ordinary Portland cement. According to GOST 31108-2003 it is classified as CEM-I type based on the clinker contains С3S = 55–58%, С3А = 8.2–8.5% with conventional chemical-mineralogical indicators of the quality (lime saturation factor KN = 90–91%, alumina modulus p = 1,7, silicate modulus n = 2,3). Physico-mechanical characteristics (brand, residue on sieve No.008, setting time, compressive strength) are within the requirements of the standard for rapid hardening Portland cement PC 500-D0, class 42,5 B.
As fillers were used diorite rubble of fraction from 5 to 20 mm according to GOST 8267 (bulk density of 1300 kg/m3, true density of 2,73 g/cm3) and quartz-feldspar sand according to GOST 8736 (Μr=3.4 and 2.9, true density of 2,62 g/cm3) in a mixture with standard quartz monofuctional sand.
Additive was polycarboxylate (PCX) superplasticizer with high efficient water-reducing ability, the density of the aqueous solution of 1082 g/dm3 and the dry matter content of 412 mg/g.
The effectiveness of silica sol additive was determined by the strength of concretes with WCR=0.61–0.71, slump of standard cone CS=12–19 cm, content of SiO2 = 2.0% of the weight of cement and content of additives PCX = 2.2–2.6% of cement mass.
Concrete testing carried out according to GOST 30459-2003 p.7. Material consumption, kg/m3:
•cement (PC 550) – 345±5; •quartz-feldspar sand – 400; •standard quartz sand – 400; •diorite rubble – 1060. Dosage of sol was calculated taking into account the fact that the batch of 10 litre in addition to diorite rubble and sand contains 3500 g of cement and 2250 g of water.
Volume of sol was calculated by the formula:
where: C is the cement consumption, g; SiO2 – silica concentration, %; KS – SiO2 content in the ash, g/dm3.
Thus, the volume of sol per 10l batch is:
This volume of sol contains:
0,609 [dm3] × 115 [g/dm3] = 70 g of SiO2.
The mobility of the concrete was controlled by means of an appropriate dosage of PCX.
Technological and structural parameters of quality of mixes and concrete were determined according to the methods of the following standards:
•mobility, the density of the concrete mix – GOST 10181; •density of concrete – GOST 12730.0; •compressive strength of concrete at the age of 1 day, 2 days and 28 days of normal storage and after heat-moisture treatment (HMT) – GOST 10180. Performance criteria was calculated by the formula 2 of GOST 30459-2003:
where: Rtb – the strength of the concrete composition in the equivalent age, MPa; Rtк – the strength of the control concrete composition in the equivalent age, MPa.
The results of tests of concrete with the addition of silica sol in combination with the PCX are presented in table.2.
Evaluation of the efficacy of the additive
As shown in the graph, despite the higher value of WCR, the strength of the composition with the addition of silica sol is significantly higher than the strength of the control composition with a smaller WCR.
Table 2 shows that the addition of sol in conjunction with the PCX significantly increases the strength of concrete in all periods and in all modes of hardening. For example, the effectiveness of strength after 28 days of hardening was 37–40% compared to compounds without additives, while in the initial stages of hardening (1 day) this indicator reaches 90–128 %. It can be associated with presumably very high pozzolanic activity of nanosilica sol in the cement, probably many times higher than the activity of microsilica sol .
Other important conclusions:
•Increased values of early strength of concrete R1/R28 under normal conditions of curing also testifies to the hardening effect of sol. •Strength after HMT is consistent passed with data of Far Eastern research, design and technological Institute of construction of RAACS , although the sol in these experiments did not show the expected ability to thermo-activation. •The density of the concrete mixtures are consistent with the data of The Research Institute of concrete  for concretes of similar composition with CS>8. •Strength in the period of 28 days for the concrete without additives (No.66 and No.69) is consistent with the WCR law for concretes of normal curing and after steaming . The strength of concrete with the addition of sol (No.67) is significantly higher than the strength of control concrete with a lower WCR (No.66). •Achieved performance criteria is 2-3 times higher than the maximum required values of this indicator according to the standard GOST 24211-2008 for all modes of testing (1 day, 28 days normal curing and 1 day after HMT). Thus, the additive of silica sol at the dosage of 2% by weight of cement in combination with PCX for concretes with slump 10–18 cm (WCR=0.61–0.71) according to the criteria of effectiveness corresponds according to GOST 24211-2008 two main classes:
•the hardening accelerator; •additive that increases durability. High values of the performance criteria during the hardening period of 1 day (incl. after HMT) allows to use this additive to obtain concrete with immediate dismantling.