Sulfate Resistance of Rice Husk Ash Concrete

. Durability of concrete is defined as its ability to resist deterioration after exposure to the environment of its use. This work investigates the performance of Rice Husk Ash (RHA) concrete in sodium sulfate (Na 2 SO 4 ), magnesium sulfate (MgSO 4 ) and combined Na 2 SO 4 and MgSO 4 solutions. Concrete bar specimens and cubes were prepared for expansion and strength deterioration tests respectively using RHA replacement at the 7.5% replacement by volume, which had achieved the highest compressive strength, as well as at the 30% replacement by volume, which was the highest replacement for the study. Strength deterioration tests were performed on the 7.5% replacement by the weight of cement. From the expansion test findings, it was concluded that at the 7.5% replacement, RHA could be used with an advantage over 100% cement concrete in MgSO 4 environments, whereas at the 30% replacement, RHA could be used with an advantage over 100% cement concrete in both the Na 2 SO 4 and mixed sulfate environments. RHA was also found to be more effective in resisting surface deterioration in all the sulfate solutions. The RHA specimens also exhibited superior strength deterioration resistance in comparison to the 100% cement specimens. mass loss and some disintegration; 3, extensive spalling and softening; 4, wider cracks and extensive spalling; 5, Complete disintegration


Introduction
Durability of concrete is defined as its ability to resist deterioration, thereby being capable of maintaining its original quality and form once it has been exposed to the environment of its use [1]. The deterioration of concrete can be caused by either internal chemical reactions from the constituents of concrete or external attacks from chemicals such as sulfates [1]. This study investigates sulfate attack, which is a major cause of the lack of durability in concrete.
It has been postulated by some that a possible result of sulfate attack on concrete is the loss of strength by affecting calcium hydroxide [Ca(OH)2], a product of the hydration of cement, and the strength giving Calcium Silicate Hydrate (C-S-H), a product of the reaction between Ca(OH)2 and silicone dioxide (SiO2) [2][3][4][5][6][7][8][9]. Gypsum and expansive ettringite are formed when Sodium sulfate (Na2SO4) attacks Ca(OH)2 [2]. Ettringite, which grows as needle shaped crystals causes volume increases of up to 126% depending on exposure conditions, and can generate very high stresses, which if higher than the tensile strength of concrete can bring about cracking [2].
Decalcification of C-S-H in Na2SO4 attack to cause loss of strength is negligible, and for this reason, it has been suggested that Na2SO4 attack manifests and should be evaluated through expansion [2]. On the other hand, Magnesium sulfate (MgSO4) attack has been reported to affect C-S-H, converting it to Magnesium Silicate Hydrate (M-S-H), which is not cementitious [2]; thus, a possible consequence of this, which has been suggested, is a loss of strength of concrete [2][3][4][5][6][7][8][9].
It has been posited that low sulfate resistance is caused by low levels of silicone dioxide (SiO2), and high levels of sulfate (SO4), iron (Fe2SO3), Ca(OH)2, and aluminate (C3A) [3]. It has also been reported that a high molar ratio of sulfite (SO3) to aluminium oxide (Al2O3) enhances the formation of monosulfate, which leads to the formation of ettringite and gypsum on exposure to sulfate attack [2]. It has also been suggested that the reaction between Supplementary Cementitious Materials (SCMS) such as RHA and cement, which is also known as the pozzolanic reaction helps to dilute C3A and remove Ca(OH)2 by converting it into C-S-H, hence reducing the quantities of gypsum formed [2]. A poor performance of SCMs in Mg(SO)4 solutions has however been reported, since Mg(SO)4 mainly affects C-S-H, which may result in the loss of strength [2][3][4][5][6][7][8][9][10][11][12].
Elsewhere, literature has it that permeability, which is defined as the rate at which pressured water can flow through interconnected voids within concrete, or the measure of how easily a liquid or gas can get through concrete, is the most important aspect of durability, since it slows down the flow of harmful substances into concrete [4], [5]. In as much as controlling the chemistry of concrete is vital as discussed above, it is more important to maintain low permeability [6,[13][14][15][16]. SCMs reduce the permeability of concrete by the packaging MATEC Web of Conferences 199, 02006 (2018) https://doi.org/10.1051/matecconf/201819902006 ICCRRR 2018 effect of their unreacted particles, as well as by the help of the C-S-H that is formed as a result of their use, whose benefit is a less well interconnected capillary pore structure, that leads to lower permeability [7,2,17,18].
It has further been suggested that the compressive strength of concrete is directly proportional to its durability, with low compressive strengths spelling low durability and vice versa [2][3][4][5][6][7][8]. This postulation is however, disputed by a number of concrete technologists. Nevertheless, it is vital to investigate any strength deterioration after prolonged exposure to a sulfate rich atmosphere.
RHA was defined by Bapat [2] as the product of incinerated rice husk, which is the outer shell that covers the rice kernel, the product of threshed paddy to separate rice grain and the husk. Its suitability as a SCM was investigated by the authors of this paper [9], who evaluated the strength of concrete using untreated rice husk ash as a partial cement replacement up to 30% substitution; their findings found each mix satisfied the C20/25 strength class at 91 days thus proving the potential pozzolanic qualities of RHA. Over 2 million tonnes of rice are produced every year all over the world, with Asia being the largest producer as is shown in Table  I [9]. Rice husk, the outer shell that covers the rice kernel, is a product of threshed paddy to separate rice grain and the husk; over 600 million tonnes of paddy were produced in the year 2008 [9]. Paddy is of very low nutrition to even be suitable for animal feed, but of all plant residues, it contains the highest amount of silica. RHA is obtained from either controlled or uncontrolled incineration of rice husks [9].

Research significance
Some work has been carried out on the performance of RHA mortar in sulfate solutions. However, no work was found on the durability performance of RHA-replaced concrete. The work reported in this paper is a review of previous work by the authors investigating the mechanical properties and durability (sulfate resistance) of RHA concrete.

Experimental
The rice husk ash was sourced from the Indian subcontinent; chemical analysis was undertaken using XRD method. The cement used was type CEM 1 52.5 N. Using a mix proportion of 1:2:3, Cubes used were 100mm x 100mm x 100mm in line with dimensional guidelines of BS EN 12390 [19,20]. The mix used was strength class C25/30. Cement was substituted with PCRs by weight in percentages of 0, 5, 7.5, 10, 15, 20, 25 and 30. The 0% replacement was used as the control specimen. A water cement ratio (WCR) of 0.5 was used for all mixes. A total of three cubes were cast for each testing age and the average compressive strength was used. The cubes were left in the molds for 24 hours, before being stripped, marked and submerged in a water tank at temperatures of 200 ±2 until their testing age.
Sulfate tests were carried out as specified in ASTM 1012 [22]. The cubes, which were used to test for strength deterioration were made using 7.5% RHA replacement by the weight of cement, whereas the bars for expansion were made using 7.5% RHA replacement by the volume of cement.
The specimens were demolded after having been placed in an oven for 23½ hours at 35 0 C. Compressive tests were then carried out on two cubes to ensure that the concrete had achieved strengths of not less than 20 N/mm 2 ± 1.0 N/mm 2 . Sulfate solutions were prepared by mixing water with 5% Na2SO4, 5% MgSO4 and mixed 2.5% Na2SO4 + 2.5% MgSO4. The lengths of the bars were taken after which both the bars and cubes were fully immersed in the solutions. A pH of between seven and eight was maintained on the solutions throughout the period of immersion.
Tests for elongation were performed on specimens that had achieved the highest compressive strength from Table 3 above, which was at the 7.5% replacement, and the 30% replacement by the volume of cement. This choice of specimens was informed by [8] and [4]'s assumption that sulfate resistance may be governed by the compressive strength more than it may depend on the amount of SCMs used to improve the chemistry of concrete.
Length change was calculated by using equation (1), which conformed to [12]. (1) Where: L = Percentage change in length at measuring age, Observations for surface deterioration were done at the end of the 270 days of immersion.
Strength Deterioration Factors (SDFs) were used to asses strength deterioration and were calculated by using equation (2) after [13].

(2)
Where fcw' is the compressive strength of cube specimens that were immersed in water and fcs' is the compressive strength of cubes immersed in sulfate solutions. Table I shows the chemical composition of cement and RHA. As levels of Fe2O3 are low, and those of SiO2 are high, it may be concluded that RHA could have a high resistance to sulfate attack, since also the ratio of SO3 to Al2O3, which was also reported by [2] to enhance sulfate attack when high was also low.  Table 3 and Fig. 1 show the mean compressive strengths. The characteristic compressive strengths obtained were among those that are listed by Eurocode 2 [10] as being suitable for structural applications and durable.

Permeability & Sulfate Resistance
The permeability of RHA replaced specimens is shown in Table 4. From the results, a conclusion from the assumptions of [4], and [8] that compressive strength is directly proportional to durability could be made, as lower permeability was reported at highest compressive strength as opposed to at the highest replacement; this postulation, however, has been questioned by some investigators thus due caution needs to be applied if directly correlating strength to durability.   Table 5 and Fig. 2 show the expansion of RHA specimens in Na2SO4, MgSO4 and mixed Na2SO4 and MgSO4 solutions at the highest compressive strength (the optimum mix 7.5% RHA). From the findings, the performance of the RHA specimens was below that of the 0% RHA specimens (control) in the Na2SO4 and mixed Na2SO4 and MgSO4 solutions, whereas in the MgSO4 solution, their performance was above that of the control specimens.

Expansion
These findings spell that at highest compressive strength, RHA could be used with an advantage over 100% concrete in MgSO4 environments. This may not however necessarily signify high durability in MgSO4 environment since as discussed earlier, deterioration in MgSO4 environments is evaluated through the loss of strength [2][3][4][5][6][7][8]. Moon, et al. [13] attributed the slight increase in length in the MgSO4 solution to the formation of brucite, even though [14] reported higher expansions on Silica Fume (SF) replaced specimens immersed in the MgSO4 solution.
Consistent with [13] the RHA specimen's performance in the mixed sulfate solution was poor, a factor which the authors attributed to the predominance of the more aggressive MgSO4 attack. Table 6 and Fig. 3 show expansion of RHA specimens in Na2SO4, MgSO4 and mixed Na2SO4 and MgSO4 solutions at the 30% replacement. From the findings, RHA showed a high performance in the Na2SO4 solution and a better performance than the control in the MgSO4 solution, even though its performance in the mixed sulfate solution was below that of the control. These findings are consistent with [15] and [13], who also reported a better performance than the control on RHA in reducing gypsum and ettringite.
As discussed earlier, SCMs aid in resisting sulfate attack as they refine pores, dilute C3A and remove Ca(OH)2 by converting it into C-S-H, thereby reducing the quantities of gypsum formed [2].
The results are not however consistent with findings in the literature that MgSO4 attack can only manifest in the loss of strength and not in expansion [2], since as stated earlier, [14] also reported expansion on bars that were immersed in the MgSO4 solution.
Even though [4] reported that low permeability is important as it inhibits the diffusion of harmful substances into the concrete matric, the findings of this study call into question this assumption since from Table  4, [11] reported a lower coefficient of water absorption at the 7.5% replacement than at the 30% replacement, and yet the results show a lower expansion at the 30% replacement than at the 7.5% replacement in the Na2SO4 and mixed sulfate solutions.
The results are also not consistent with [2]'s assumptions that the filler effect of unreacted particles improves permeability. Adesanya & Raheem [1] however attributed the high permeability at high replacements to low levels of Ca(OH)2 available to react with excess SCMs for the formation of the less permeable C-S-H.

Strength deterioration factors (SDFs)
As discussed in the methods section, the loss of strength was assessed using Strength Deterioration Factors (SDFs) after [13]. Table 7 shows the SDFs of the RHA specimens that were immersed in Na2SO4, MgSO4 and mixed Na2SO4, and MgSO4 solutions. The RHA specimens showed lower SDFs than the control specimens in the Na2SO4 and mixed sulfate solutions. The findings confirmed findings in the literature that MgSO4 attacks C-S-H in SCMs to form the noncementitious M-S-H, and hence the higher SDFs recorded for the RHA specimens than those of the control specimens in the MgSO4 solution [2].
These results were also consistent with the findings of Kamau et al [16] and [17] who reported lower SDFs than those of the control on maize cob ash specimens in the Na2SO4 and mixed sulfate solutions, but higher than those of the control in the MgSO4 solution.
The low SDFs of the RHA specimens in the Na2SO4 and mixed sulfate solutions spell the possibility of using RHA with an advantage over 100% cement to improve the performance of concrete in these environments. Although there are contradictory notions regarding correlating strength to durability, the RHA samples clearly have greater strength deterioration resistance in comparison to 100% cement concrete.  Table 8 shows surface deterioration observed on the RHA specimens that were immersed in Na2SO4, MgSO4 and mixed Na2SO4 and MgSO4 solutions. The method used by [18] to assess strength deterioration was employed. RHA was observed to improve the surface deterioration of specimens in all the three sulfate solutions over the control specimens. The findings were not consistent with [13] who reported higher surface deterioration on the control specimens than on the SF specimens in the Na2SO4 solution,

Conclusion
This work investigated the performance of RHA replaced concrete in sodium sulfate (Na2SO4), magnesium sulfate (MgSO4) and mixed Na2SO4 and MgSO4 environments. From the findings, the following conclusions were drawn: -1. At the highest compressive strength (7.5% replacement) in expansion, RHA could be used with an advantage over 100% cement in MgSO4 environments 2. At the 30% replacement, RHA could be used with an advantage over 100% cement in Na2SO4 and MgSO4 environments 3. Strength deterioration results indicate that RHA could be used with an advantage over 100% cement in Na2SO4 and MgSO4 environments 4. Surface deterioration results show that RHA could be used with an advantage over 100% cement in Na2SO4, MgSO4 and mixed sulfate environments.