 slag (CEM III/B) and 10% silica fume (CEM I + 10 % SF), for 18 weeks in an aggressive solution of Synthetic Liquid Manure (SLM-pH 4) composed of acetic, butyric, propionic, isobutyric and valeric acids [11]. Figure 3 represents the variation in mass loss and altered depth of different systems exposed to SLM.
(a) (b) Figure 3. Evolution of durability criteria: a) mass losses and b) altered depth, according to the immersion time. [11] Results show that, the mass losses of CEM I specimens were twice higher than CEM III/B and 1.15 times higher than CEM I + 10 % SF. On the other hand, altered depths of CEM I pastes were 1.14 and 1.26 times higher than CEM I + 10 % SF and CEM III/B respectively. Although the mass loss variation for CEM I seemed higher than other systems, altered depth variation was found to have lesser variations. Therefore, the acceptance of mass loss and altered depth parameters for measuring kinetic degradation of various systems on exposure to acids needs to be ascertained.

Special cement system
High alumina cements (HACs) such as calcium aluminate cements (CACs)/ calcium sulphoaluminate cement (CSA) are called the refractory cements that are most suitable for high temperature applications. In these systems, various crystalline and amorphous hydrates are formed in the process of hydration, which depends on the water to cement ratio, curing time and temperature [12]. Dyerexamined the performance of three types of cement paste namely, Ordinary Portland Cement, combination of Portland cement with fly ash and Calcium sulphoaluminate cement (PC, PC/FA and CSA) using laboratory experiments when exposed to acetic and butyric acids [8]. CSA cement exhibited least mass loss at lower acid concentrations and high mass loss at higher concentrations as depicted in figure 4.
(a) (b) Figure 4. Mass loss from paste specimens exposed to (a) butyric acid (b) acetic acid. [8] The high mass loss is not due to increased rate of degradation, but due to complete dissolution of silica gel from the degraded surface to a certain depth into the specimen. Abrupt erosion of degraded layers and significant cracking due to drying shrinkage has been observed. So, the depth of the degraded layer attached to the specimen will be comparatively lower. Hence, not only calcium compounds, but silica and other materials also leach out of the system, leading to a heavy mass loss. Also, CSA exhibits larger cross sectional area of inner core indicating superior performance of special cement systems at lower concentrations of acids [8].

Alkali activated system
Recently, alkali-activated binders became a promising alternative for highly durable concretes due to their high resistance within aggressive environments. Alkali activation is the generic term which is applied to the reaction of a solid aluminosilicate (termed the 'precursor') under alkaline conditions (induced by the 'alkali activator'), to produce a hardened binder [13]. Geopolymers are considered to be a subset of alkali activated binder system. Sodium aluminosilicate hydrate (N-A-S-H) gel is the main reaction product of the alkali-activated aluminosilicates.
Koenig et al. examined the resistance of alkali-activated binders under organic acid attack (acetic, propionic, lactic acid with pH 3) [5]. Despite having the lowest strength class and highest porosity, Alkali activated fly ash (AAFA) exhibited lowest mass loss and degradation depth. They also displayed a steady increase in strength over time. The results ascertain that the acid resistance increases with decrease in CaO content i.e., calcium silicate hydrate (C-S-H) < calcium alumino silicate hydrate (C-A-S-H) < sodium alumina silicate hydrate (N-A-S-H) and there is a continuous decrease in strength over time for all specimens immersed in acid solution, except for AAFA. According to Reju, fly ash based geopolymer (FGP) specimens exhibited superior performance in acid resistance with respect to mass loss in mortar specimens (only 0.8%) exposed to 0.5 M acetic acid [14]. Mass loss for slag based geopolymer (SGP) specimen was found to be higher when immersed in acetic acid solution. These findings are in alignment with the findings depicted in Koenig et al. [5]. The durability performance of FGP specimens was found to be better against acetic acid attack. The altered depth variation in the specimens was experimented by spraying phenolphthalein to the freshly fractured surfaces. Figure 5 depicts the altered depth variation of mortar specimens before and after acid attack exposed to acetic and citric acids. Based on altered depth, as a pink colour is found towards the core (pink colour indicates that the core is not attacked) it is inferred that SGP specimens have lower rate of degradation on exposure to both acetic and citric acid. However, results from the microstructural analysis showed that, FGP specimens has a better performance in resisting citric acid as no expansive products were formed unlike OPC and SGP.

Conclusion
Acid attack results in rapid deterioration of concrete structures. OPC is hardly resistant to acids and hence, more studies are to be conducted on the effect of other binder systems on exposure to various organic acids. Also, the parameters to express the degradation kinetics are to be explored further. The rate of degradation is dependent on the type of acid, concentration, chemical composition of binders and solubility of the salts formed. Also the degradation kinetics caused by acids depends on the type of binders used and their properties. Based on the aggressiveness of acids, citric acid is found to be the extremely aggressive from most studies due to the high molar volume of expansive calcium citrate tetrahydrate salts. Acetic acid was also found to be aggressive as the attack leads to considerable mass loss and increase in porosity.
In general, blended cementitious systems with OPC partially replaced with supplementary cementing materials (SCM) perform better on exposure to lower concentrations of organic acids. The performance of SF (10%) was superior than other binders on exposure to 0.5M citric acid. Fly ash based alkali activated system showed highly improved performance against acetic and citric acid attack. The acid resistance of the strength-forming phases was linked to the binder composition. Thus, acid resistance increased with decrease in CaO content i.e. C-S-H < C-A-S-H < N-A-S-H. Alkali activated systems and CSA are found to be more durable in contrast with OPC and blended systems. However, there is no actual comparison between the performance of alkali activated systems and CAC or CSA. The ranking of acid resistance could vary based on the parameters chosen. Therefore, further researches are required to ascertain the correlation between mass loss and altered depth parameters in studying the kinetics of degradation.My Hero Academia Chapter 248 spoilers are here and this week, we’ll finally be seeing a bit of a time-skip. Shoto, Midoriya, and Bakugo continue their training with Endeavor but we also get to see more than that. In this post, we’ll talk about My Hero Academia Chapter 248 spoilers, so let’s get started!
Might just a year and a half ago. He’s experienced a lot since entering UA. It’s overwhelming how fast everything is moving. He wonders how fast he can go.
We switch to Endeavor talking to them and telling them not to worry since they can mess up. The successes and failures of the three won’t affect his work at all.
We skip to one week later, and we see Fuyumi talk to Endeavor. Later on, Endeavor teaches Midoriya about using his powers better and using more than what he’s capable of right now.
My Hero Academia Chapter 248 will be out officially on October 28, 2019. The raw and scans will be out on October 25, 2019.One of a key roles of a VC and investor is to help portfolio companies and startups recruit quality talent. Especially with the current competitive hiring climate, recruiting can be a challenging task for companies both big and small. Today, VC firm Greylock is announcing that it has created a dedicated talent team to help its portfolio companies recruit effectively and succeed in the current battle for talent.
Jeff Markowitz, former managing director of the venture capital practice at executive search firm Heidrick & Struggles, has joined Greylock as a ‘Talent Partner.’ Markowitz will focused on executive level talent, and will help Greylock’s portfolio companies as well as the VC firm itself to build a network of high-level talent.
Dan Portillo, formerly an HR and recruiting exec at Rypple and Mozilla, has joined as VP of Talent focusing on hiring in product and engineering for Greylock’s portfolio companies.
Clearly, Greylock is taking the battle for technical talent in Silicon Valley and the greater tech world seriously, and is staffing up to help startups wage this war. This is a trend that other top-tier VC firms are also aggressively pursuing. The firm says that it will be working on several initiatives to help companies build recruiting skills, source talent, leverage networks and more.
Greylock also recently added new talent to its own roster; bringing on Howcast co-founder and former Googler Sanjay Raman as a Senior Associate on the consumer team. He joins other recent hires including former Twitter and Facebook product lead Josh Elman and LinkedIn’s former VP of Search and Platform Adam Nash.